The enigmatic function of chandelier cells

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The enigmatic function of chandelier cells
FOCUSED REVIEW
                                                                                                                             published: 08 December 2010
                                                                                                                             doi: 10.3389/fnins.2010.00201

                                          The enigmatic function of chandelier cells
                                          Alan R. Woodruff 1*, Stewart A. Anderson 2 and Rafael Yuste1
                                          1
                                              Department of Biological Sciences, Howard Hughes Medical Institute, Columbia University, New York, NY, USA
                                          2
                                              Department of Psychiatry, Weill Cornell Medical College, Cornell University, New York, NY, USA

                                          Chandelier (or axo-axonic) cells are one of the most distinctive GABAergic interneurons in the
                                          brain.Their exquisite target specificity for the axon initial segment of pyramidal neurons, together
                                          with their GABAergic nature, long suggested the possibility that they provide the ultimate
                                          inhibitory control of pyramidal neuron output. Recent findings indicate that their function may
                                          be more complicated, and perhaps more interesting, than initially believed. Here we review
                                          these recent developments and their implications. We focus in particular on whether chandelier
                                          cells may provide a depolarizing, excitatory effect on pyramidal neuron output, in addition to a
                                          powerful inhibition.
                                          Keywords: GABAergic depolarization, excitation, cortex, axon initial segment

                                          Introduction                                                    arborizations of different interneurons reflects
                                          The mammalian brain is an organ of seemingly                    their subcellular target specificity, which in turn
                                          impossible complexity. There are billions of neu-               has important implications for their function.
                                          rons, trillions of synaptic connections, and prob-              Indeed, a common delineation exists between
                                          ably hundreds of distinct cell types, each of which             interneurons targeting dendritic domains, such as
                                          is presumably specialized to perform distinct                   neocortical Martinotti and double bouquet cells,
Edited by:
                                          tasks. A crucial step in addressing the underly-                and those targeting axo-somatic domains, such
David Linden, Johns Hopkins University,
USA                                       ing logic in the design of the brain is therefore               as chandelier and basket cells (Markram et al.,
Reviewed by:
                                          identifying the function, or functions, of a given              2004). Whereas dendrite-targeting neurons may
Stephen R. Williams, University of        neuronal class.                                                 be more suited to modify and gate incoming exci-
Cambridge, UK                                 The most obvious separation in neuron func-                 tatory input (Murayama et al., 2009), axo-somatic
Miles A. Whittington, Newcastle           tion within cortical structures is between exci-                interneurons are likely to exhibit a greater impact
University, UK                            tatory, glutamatergic pyramidal (or principal)                  on the direct output of the postsynaptic neuron
*Correspondence:                          neurons and inhibitory GABAergic interneurons.                  (Marr, 1970; Miles et al., 1996).
                                          Information is presumably carried and processed                    Here we focus on attempting to understand
                                          by pyramidal (Pyr) neurons, whose connections                   the chandelier cell (ChCs), an interneuron type
                                          traverse cortical layers and regions. Interneurons,             that exemplifies the target specificity of cortical
                                          by contrast, are typically considered to project                interneurons and thus potentially illustrates the
                                          only locally, and provide a means of controlling                purposeful design of neuronal circuits. ChCs
Alan R. Woodruff received his Ph.D.       the excitation provided by pyramidal neurons. It                were “missed” by Cajal and Lorente and were first
from the University of Queensland under
the mentorship of Prof. Pankaj Sah,
                                          is, of course, not that simple. It turns out that a             identified by Szentagothai and, independently, by
where he studied the local circuit        vast heterogeneity exists amongst the GABAergic                 Jones (Jones, 1975; Szentagothai, 1975). The defin-
properties of GABAergic neurons in the    interneurons, which constitute ∼20% of all cor-                 ing morphological characteristic of ChCs is the
basolateral amygdala. His current         tical neurons. This heterogeneity has long been                 array of short, vertically oriented rows of terminal
research, at Columbia University with     recognized (Ramón y Cajal, 1899; Lorente de Nó,                 boutons, which resemble candlesticks. Originally
Prof. Rafael Yuste, is focused on
elucidating the function of cortical
                                          1922), and one of the first aspects of this heteroge-           believed to contact the apical dendrite of pyrami-
chandelier cells.                         neity to be noticed, their “short axons,” is likely to          dal cells (Szentagothai, 1975), these axonal car-
aw2343@columbia.edu                       be one of the most important – the distinct axonal              tridges were later shown via electron microscopic

Frontiers in Neuroscience                                               www.frontiersin.org                              December 2010 | Volume 4 | Article 201 | 1
The enigmatic function of chandelier cells
Woodruff et al.                                                                                                                                  Chandelier cell function

Chandelier cell                           reconstructions of Golgi-stained specimens to                     nomenclature that is used interchangeably. This
GABAergic interneuron whose exclusive     exclusively contact the axon initial segment (AIS)                highly stereotyped and visually striking appear-
postsynaptic target is the axon initial
segment of pyramidal neurons.
                                          of pyramidal neurons (Figure 1C; Somogyi, 1977;                   ance of ChCs facilitated early studies regarding
                                          Fairen and Valverde, 1980). Because of this, chan-                their location, abundance, and neurochemical
                                          delier neurons were renamed “axo-axonic cells,” a                 features. ChCs are present in all cortical layers,

                                               A                                                        B

                                                                                        L1
                                                                                       L2/3

                                                                                        L4

                                                          100 µm

                                               C                                                        D                      20 mV
                                                                                                                              400 pA
                                                                                                                          100 ms
                                                                             10 µm
                                                                                                             ChC

                                                                                                              BC

                                               E                                                        F
                                                            ChC                        BC
                                               -58 mV                   -59 mV              0.5 mV          Record ChC     Record BC                         50 mV
                                                                                     20 ms                                             ChC           10 ms
                                               -66 mV                   -67 mV
                                                                                                                                       BC
                                                                                                                                                     NBQX
                                               -73 mV                   -72 mV
                                                                                                                    Pyr
                                                                                                                                                         5 pA
                                               -81 mV                   -85 mV
                                                 (rest)                  (rest)
                                                                                                                                            3.5 ms
                                               -91 mV                   -90 mV

                                           Figure 1 | Characteristic features of chandelier cells. (A) Neurolucida reconstruction of a layer 2/3 chandelier cell. Cell
                                           body and dendrites are in blue, axon in red. Several vertically oriented axonal segments are visible, and are the
                                           characteristic morphological feature of ChCs. The long descending axon (arrowhead) reached and arborized in layer 6, but
                                           has been digitally truncated. (B) Parvalbumin immunoreactivity of a ChC. A GFP-labeled ChC (right panel, arrow)
                                           co-expresses PV (left panel, arrow). (C) Biocytin-filled ChC forms a row of cartridge synapses (arrowheads) on the AIS of a
                                           biocytin-filled cortical pyramidal neuron (axon marked by white arrow). (D) Chandelier and basket cells have characteristic
                                           responses to threshold current injection. Both cells exhibit the fast-spiking phenotype at higher current intensities (right
                                           panel, 2× threshold illustrated). (E) In cortical layer 2/3 pyramids, EGABA differs at ChC–Pyr and BC–Pyr synapses. (F) ChCs
                                           can initiate polysynaptic events. Activation of a GABAergic ChC evokes a response in a simultaneously recorded basket
                                           cell. The response (left panel) is sensitive to the AMPA receptor antagonist NBQX, has a disynaptic latency, and is
                                           observed in a cell type (BC) that receives no direct synapses from ChCs. Schematic of disynaptic circuit is shown in right
                                           panel. (B, D and E) modified, from Woodruff et al. (2009).

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Woodruff et al.                                                                                                             Chandelier cell function

                                      most abundantly in layer 2/3 (DeFelipe et al.,        and resulted in ChCs being less receptive to
                                      1985; Inda et al., 2007). Although originally         ascending sensory input. However, ChCs mark-
                                      described in neocortex (Szentagothai, 1975),          edly increased their output during conditions of
                                      ChCs have also been found in the CA3 (Sik et al.,     high network activity, implying that ChCs may
                                      1993), CA1 (Somogyi et al., 1983) and dentate         be recruited to dampen excessive excitation.
                                      gyrus (Soriano and Frotscher, 1989) regions of the    Until recently, therefore, all evidence seemed to
                                      hippocampus, and in the amygdala (McDonald,           suggest that ChCs were inhibitory GABAergic
                                      1982). The expression of GABA (Somogyi et al.,        neurons, unique in their high degree of spatial
                                      1985), parvalbumin (PV; DeFelipe et al., 1989),       target selectivity, but otherwise not particularly
                                      and corticotropin-releasing factor (Lewis et al.,     remarkable in comparison to other GABAergic
                                      1989) provides a neurochemical identity to these      cell types.
                                      neurons (Figure 1B).
                                          Despite this knowledge, it has been signifi-      Are chandelier cells depolarizing?
                                      cantly more difficult to ascertain the physiologi-    In 2006, a controversial paper appeared demon-
                                      cal properties of ChCs, and more particularly         strating an excitatory effect of cortical layer 2/3
                                      their putative functions. This is in part due to      chandelier cell activation (Szabadics et al., 2006).
                                      the rarity of ChCs and the fact that common           Three pieces of evidence were presented in sup-
                                      markers label both chandelier and the much            port of this. Firstly, disynaptic, glutamatergic exci-
                                      more abundant basket cells. To date there is cur-     tatory postsynaptic potentials (EPSPs) could be
                                      rently no known unique ChC marker. Although           elicited following a single spike in the chandelier
                                      a variety of transgenic mouse lines now exist in      cell – an effect attributed to direct recruitment
                                      which various populations of interneurons are         of pyramidal neurons by ChCs (Figure 1F). But
                                      fluorescently labeled, including PV-positive neu-     for direct recruitment of a pyramidal neuron to
                                      rons (Meyer et al., 2002; Chattopadhyaya et al.,      occur with such a short latency, the ChC synapse
                                      2004), ChCs have been recorded from much less         at the AIS had to be depolarizing, a condition not
                                      frequently than have the more numerous basket         previously considered for this particular synapse.
                                      cells. Physiologically, both ChCs and BCs fire        Secondly, the authors confirmed that the ChC
                                      high frequency, minimally adapting trains of          synapse was indeed depolarizing, with the GABAA
                                      narrow action potentials – a fast-spiking pheno-      reversal potential (EGABA) significantly elevated
                                      type. In the cortex, BCs and ChCs were believed       above the neuron’s resting potential (Vrest), and
                                      indistinguishable on physiological grounds            above that of basket cell synapses (Figure 1E).
                                      (Kawaguchi, 1995; Gonzalez-Burgos et al., 2005),      Importantly, EGABA in this study was measured
                                      while in the hippocampus, ChCs were reported          using the antibiotic gramicidin in the postsynap-
                                      to exhibit a greater degree of spike frequency        tic pipette of a paired recording. Gramicidin forms
                                      adaptation than BCs (Han, 1994). Thus in most         membrane pores that allow exclusive exchange of
                                      cases, unequivocal identification of a ChC was        monovalent cations and small uncharged mol-
                                      only possible following post-recording recov-         ecules (Kyrozis and Reichling, 1995). Thus there
                                      ery of the neuron’s morphology (Figure 1A).           is no dialysis of intracellular chloride, as occurs
                                      Whilst this is not a difficult procedure, it is not   with whole-cell patching, and more physiological
                                      routinely performed, and the number of physi-         EGABA responses are recorded through the perfo-
                                      ological and functional studies on ChCs has been      rated neuronal membrane. Thirdly, immunogold
                                      somewhat limited.                                     labeling and electron microscopic reconstruction
                                          The studies hinting at a functional role for      demonstrated an apparent absence of the potas-
                                      ChCs are therefore correlative rather than causa-     sium-chloride cotransporter KCC2 from the AIS
                                      tive. In vivo recordings have revealed the firing     – an effect that would theoretically lead to less
                                      pattern of hippocampal ChCs during various            extrusion of intracellular chloride and lead to the
                                      network states (Klausberger et al., 2003; Tukker      depolarized EGABA reported.
                                      et al., 2007), showing that ChCs fire antiphase           Thus, a powerful form of excitation had been
                                      to pyramidal neurons during theta activity, and       demonstrated for a neuron previously believed –
                                      fire immediately prior to pyramidal neuron            based on the strategic location of its GABAergic
                                      activation during sharp-wave-associated ripples       synapses – to exert a powerful form of inhibition.
Gramicidin                            (Klausberger et al., 2003). Perhaps a more direct     The Szabadics data did not exclude an inhibitory
Antibiotic that forms membrane        functional role was suggested by whole-cell           action of ChCs, but rather added an excitatory
channels impermeable to chloride.
                                      recordings from ChCs in rat somatosensory cor-        role to their function. Instead of being purely
Gramicidin is useful for measuring
GABAA receptor-mediated events when   tex during whisker deflection. Whisker-evoked         inhibitory, ChCs now had the potential to be
maintaining an accurate GABAA         stimulation revealed different sequences of affer-    either inhibitory or excitatory, depending on the
reversal potential is desired.        ent input compared to BCs (Zhu et al., 2004),         membrane potential of the postsynaptic neuron.

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Woodruff et al.                                                                                                                            Chandelier cell function

                            By extension, the overall activity state of the net-                            Both the Szabadics and Khirug data were
                            work would determine whether ChCs behaved as                                 obtained primarily using the gramicidin per-
                            excitatory or inhibitory neurons.                                            forated patch technique. Although avoiding
                                Further support for a depolarizing effect fol-                           direct chloride exchange between pipette and
                            lowing AIS GABAA receptor activation came when                               cell, gramicidin recordings may still be prone to
                            it was shown, again using gramicidin-based patch                             some error in calculating EGABA. This is because the
                            recordings, that dentate granule cells exhibited an                          chloride equilibrium potential is set by the com-
                            axo-somato-dendritic gradient in EGABA (Khirug                               bined activity of the cation chloride cotransport-
                            et al., 2008). The relatively depolarized axonal                             ers KCC2 and NKCC1 (Figure 2A). Because both
                            EGABA was shown to persist, to some degree, even                             sodium and potassium permeate the membrane
                            in whole-cell recordings, an effect attributed to                            pores created by gramicidin, pipette concentra-
                            efficient import of chloride by the Na-K-2Cl                                 tions of these ions that don’t precisely match the
                            cotransporter NKCC1 (Khirug et al., 2008), rather                            physiological intracellular concentrations can
                            than, or perhaps in addition to, the absence of                              alter the activity of the transporters, and conse-
                            KCC2 suggested by Szabadics et al. (2006). A simi-                           quently shift EGABA, potentially giving a spurious
                            lar hyperpolarizing–depolarizing EGABA gradient                              reading. For example, resting [Na+]i is low relative
                            from dendrite to axon was shown for cortical                                 to resting [K+]i, so slight inaccuracies in pipette
                            pyramidal cells of layer 2/3.                                                [Na+] in particular may produce significant effects

                                 A                                                                             B
                                                                                           depolarizing

                                             Cl–                                                Cl–                            ChC

                                                                                                                      VThr
                                                                                                                    EGABA

                                                    hyperpolarizing                         K+ Cl–
                                                                          AXON
                                  SOMA                                                                               Vrest

                                                                                 2 Cl– K+/Na+
                                     K+
                                                   2 Cl–
                                            Cl–                                                                      VThr
                                                    K+/Na+               NKCC1          Nav1.6
                                                                                                               EGABA / VNa
                                                                         KCC2           GABAA

                                                                                                                   Vrest
                                                                                                                                  Na+

                                 C
                                             small Gsyn/∆V

                                                                  VA/S

                                                           Gsyn                                          +
                                                                                                      p(AP)

                                                                                                                             conductance
                                          large Gsyn/∆V                                                   –
                                                                                                                                closed
                                                                                                          +
                                                                                                      p(AP)

                                                                                                          –

                             Figure 2 | GABAergic excitation. (A) The soma and axon of a pyramidal neuron are represented. Somatic E Cl is
                             hyperpolarized due to the high expression of KCC2 and low expression of NKCC1. GABAA receptor activation leads to
                             hyperpolarization, or no net flux of ions (shunting). At the AIS, the expression of cation chloride transporters is reversed,
                             favoring chloride efflux and depolarization upon GABAA channel opening. Low-threshold NaV1.6 sodium channels may be
                             activated by the depolarization, enhancing the excitatory effect. (B) Although depolarizing, a GABAergic event may be
                             inhibitory, due to the conductance effect of channel opening. However, the inhibitory conductance effect (red) decays
                             more quickly than the excitatory membrane potential change (blue), providing at least some window of excitation (right
                             panel, bottom). The strength and duration of excitation (blue) and inhibition (red), denoted here as spike probability (p(AP)),
                             will depend on the magnitude of the conductance effect relative to the change in membrane potential. (C) The ChC
                             synapse is expected to be hyperpolarizing during high activity periods (red) and depolarizing under resting conditions.
                             Because AIS EGABA is in the range of Na+ channel activation, some depolarizations may be enhanced by sodium currents
                             (bottom panel).

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Woodruff et al.                                                                                                               Chandelier cell function

                                       on transporter activity. For this reason, regions of   of the synapse may be expected to ­dissipate if
                                       the neuron where sodium exchange via NKCC1             the recording pipette is moved hundreds of
                                       is used to set the transmembrane chloride gradi-       microns away, along the somatodendritic axis.
                                       ent, such as the axon (Khirug et al., 2008), may       Consistent with this, when the recording pipette
                                       be more prone to allowing inaccurate values of         was moved to distal dendritic sites, the authors
                                       EGABA to be recorded. Additionally, slight changes     still observed unitary fields of a polarity consist-
                                       in [Na+]i and [K+]i introduced by the recording        ent with a hyperpolarizing synapse at the AIS.
                                       pipette may alter the neuron’s resting membrane        Thus even when not subjected to the possibil-
                                       potential, causing a compensatory redistribution       ity of chloride leak from the recording pipette,
                                       of chloride ions across the membrane through           the synapse was ­hyperpolarizing. Additionally,
                                       passive diffusion.                                     the concern of Cl− leaking from the electrode
                                           These concerns were addressed using a novel,       was addressed by replacing the 3 M NaCl in
                                       non-invasive approach to recording the polarity        the recording pipette with artificial cerebrospi-
                                       of unitary GABAergic responses in hippocampal          nal fluid (ACSF). This manipulation generated
                                       CA1 (Glickfeld et al., 2009). The authors recorded     similar results in the observed amplitude of the
                                       “unitary fields” – i.e., extracellular field poten-    unitary field evoked by basket synapses (Glickfeld
                                       tials produced by activation of a single, mor-         et al., 2009), which would not be expected if chlo-
                                       phologically verified interneuron – at various         ride leak was responsible for a hyperpolarizing
                                       somatodendritic locations of the CA1 pyramidal         shift in EGABA.
                                       cell population. These recordings rely on small            These data, therefore, obtained using a non-
                                       changes in the flow of ions around the extracel-       invasive technique (unitary field recordings), are
                                       lular recording electrode. As membrane channels        at odds with the data of Szabadics et al. (2006) and
                                       open, transmembrane ion flux results in accumu-        Khirug et al. (2008), both of which were obtained
                                       lations and depletions of the permeant ions in the     using the slightly more invasive gramicidin perfo-
                                       regions proximal to the recording electrode. The       rated patch technique. The polarity of effect of a
                                       direction of current flow is indicated by the polar-   GABAergic synapse is likely a major indicator and
                                       ity of the extracellular response. Importantly, this   determinant of that synapse’s functional role. The
                                       technique does not interfere in any way with the       conflicting data described above, and the differing
                                       intracellular milieu of the postsynaptic neurons,      techniques used to obtain the different results,
                                       and in this respect represents an improvement on       prompted us to investigate whether the depolar-
                                       whole-cell and gramicidin recordings. For each         izing ChC effect may be an artifact introduced by
                                       class of interneuron recorded, targeting either        gramicidin recordings.
                                       the dendritic, somatic, or axonal compartment,
                                       the authors found only a hyperpolarizing effect.       Depolarizing chandeliers, revisited
                                       Thus the non-invasive approach taken provided          The goal of our experiments (Woodruff et al.,
                                       no evidence for a depolarizing effect of ChCs.         2009) was to re-examine the depolarizing effect
                                           A potential caveat with the technique arises,      of cortical ChCs. For this, we first found a method
                                       however, because Glickfeld et al. (2009) used very     to routinely record from ChCs, something that to
                                       high (molar) concentrations of chloride ions in        our knowledge had not previously been achieved.
                                       their extracellular recording electrodes, which        We used Nkx2.1Cre MADM mice, a strain of
                                       may have leaked into the surrounding neuropil.         genetically engineered animals that express GFP
                                       Typically, extracellular chloride concentrations       in a subset of neocortical interneurons arising
                                       are ∼130 mM. But by using 3 M [Cl−]o in their          from the medial ganglionic eminence. This subset
                                       extracellular pipette, the authors could poten-        included ChCs, which importantly were labeled
                                       tially have produced a ∼20-fold increase in            brightly, allowing the distinctive axonal cartridges
                                       local chloride concentrations, should any chlo-        of ChCs to be identified prior to recordings. ChCs
                                       ride leakage occur from the pipette solution.          could therefore be easily targeted, particularly in
                                       Increased extracellular chloride levels would          upper layer 2/3, near the layer 1 border. We rou-
                                       shift EGABA to hyperpolarized potentials, favor-       tinely recorded from ChCs in layer 2/3 and were
                                       ing chloride influx upon GABAA channel open-           able to distinguish them from basket cells based
                                       ing, and thus a hyperpolarizing response at the        on their firing pattern and passive membrane
                                       synapse. Could the hyperpolarizing response            properties (Figure 1D; Woodruff et al., 2009).
                                       of CA1 ChCs, which contrasts with the grami-           This distinction was extremely reliable, as con-
                                       cidin recordings in cortex and in hippocampal          firmed by the subsequent anatomical identifica-
Extracellular field potentials
The electrical potential produced by
                                       dentate granule cells, therefore be an artifact of     tion of recorded cells, and obviated the need for
neurons due to transmembrane ion       the recording technique? If significant chloride       morphological verification in the vast majority
flux, measured extracellularly.        leak did occur, any changes in [Cl−]o at the site      of subsequent cases.

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Woodruff et al.                                                                                                                   Chandelier cell function

                                               We first performed paired recordings from          Can cortical chandelier cells be
                                           presynaptic ChCs and gramicidin-patched                excitatory?
                                           postsynaptic pyramidal cells. Our data, taken          But does the axonal depolarization caused by corti-
                                           from mouse neocortex, closely matched that of          cal ChCs promote or inhibit action potential firing?
                                           Szabadics et al. (2006) from rat neocortex, with       To date, the only demonstration that ChCs are exci-
                                           EGABA of the ChC synapse lying ∼20 mV above            tatory – that they promote firing – is the suprath-
                                           resting membrane potential, and ∼10 mV below           reshold activation of pyramidal neurons reported
                                           action potential threshold. Similar experiments        by several groups (Szabadics et al., 2006; Woodruff
                                           performed on basket cell synapses showed a pre-        et al., 2006; Molnar et al., 2008; Glickfeld et al.,
                                           dominantly shunting synapse, in which EGABA was        2009). But even this phenomenon, while clearly
                                           typically at the neuron’s resting membrane poten-      excitatory, should be interpreted cautiously.
                                           tial. While this was consistent with the previous          Firstly, nerve injury is known to result in an
                                           gramicidin-based recordings from Szabadics             upregulation of NKCC1 and a downregulation
                                           and Khirug, the possibility remained that the          of KCC2 (Hasbargen et al., 2010), both of which
                                           depolarized axonal EGABA was simply the result         would favor a depolarized EGABA. Thus the frequent
                                           of changes in the activity of NKCC1 that may           axotomies that occur during the preparation of
                                           occur via alterations in intracellular Na+ concen-     brain slices could create artificially high axonal
                                           trations, as outlined above. We therefore com-         EGABA values, leading to hyperexcitability.
                                           plemented these recordings with a completely               Secondly, chandelier-triggered activation
                                           non-invasive technique, tight seal cell-attached       of a pyramidal neuron has never been directly
                                           recordings. Cell-attached recordings have only         observed, instead being inferred by the presence
                                           rarely been used to record synaptic potentials         of a disynaptic, glutamatergic response that is
                                           (but see Kantrowitz et al., 2005; Perkins, 2006),      time-locked to the ChC spike (Figure 1F) and
                                           and to our knowledge, only to record large-            which is sensitive to the GABAA receptor antago-
                                           amplitude, network driven events (Kantrowitz           nist bicuculline. The reason for this may be merely
                                           et al., 2005). We first showed that detecting sub-     statistical, in that the chances of recording from
                                           millivolt voltage deflections would be possible        the one or two responsive pyramidal neurons are
                                           with tight seal cell-attached recordings. Our          low. Alternatively, it may be because the active
                                           paired recordings then consistently revealed a         pyramidal neurons are unhealthy, with a conse-
                                           depolarization of the postsynaptic pyramidal cell      quent increase in NKCC1 expression responsible
                                           after ChC activation. All recordings, regardless of    for the phenomenon. These neurons would not be
                                           whether they displayed evidence of a connection        targeted for patching. Bulk loading of membrane-
                                           in the cell-attached configuration, were subse-        permeable acetoxymethyl (AM) Ca2+ indicators,
                                           quently tested in whole-cell mode for the pres-        which allows action potentials to be monitored
                                           ence of a synaptic response. We were thus able         in hundreds of neurons (Yuste and Katz, 1991;
                                           to distinguish purely shunting synapses, which         Smetters et al., 1999), should facilitate the detec-
                                           would not be detectable in cell-attached mode          tion of active neurons, although we remain una-
                                           but would be revealed upon entering whole-             ware of success with this technique. This failure
                                           cell mode, from unconnected ChC–Pyr pairs.             could be due to low signal-to-noise, to unhealthy
                                           Importantly, we never observed purely shunt-           neurons not incorporating the AM Ca2+ indica-
                                           ing or hyperpolarizing ChC–Pyr synapses. These         tors, or to saturating concentrations of indicator
                                           data supported our gramicidin recordings, and          that prevent the detection of action potential-
                                           furthermore demonstrated that the ChC-evoked           induced fluorescence changes. Alternatively, the
                                           depolarization at the AIS observed using grami-        action potential produced by chandelier activa-
                                           cidin recordings could not be disregarded as an        tion may, depending on its initiation point rela-
                                           experimental artifact.                                 tive to the ChC synapse, fail to backpropagate to
                                               These data demonstrated that at least for layer    the soma. This would prevent its detection both
                                           2/3 cortical ChCs, and seemingly also for dentate      by somatic patch recording and somatic action-
Cell-attached recordings                   granule cells, GABA at the AIS is depolarizing.        potential derived fluorescence changes.
Recordings in which the recording
pipette is placed on the exterior of the
                                           However, ChCs in CA1 produce a hyperpolariza-              It is certainly not clear that the suprathreshold
cell membrane. Loose-seal cell-attached    tion. Because the reversal potential of a GABAergic    activation described by us and others – which is
recordings are commonly used for           synapse is likely an important determinant of that     the sole excitatory ChC action reported to date – is
recording currents due to action           synapse’s function, an interesting dichotomy may       a pathological response. However, until such acti-
potentials. Tight-seal cell-attached       exist in the roles played by ChCs in different parts   vations are directly observed by recording from a
recordings can be used to measure
synaptic potentials. In the absence of
                                           of the brain, an intriguing possibility for a cell     healthy pyramidal neuron, or until it is observed
injected current, cell-attached            type previously hypothesized to be a fine example      in the intact animal, the possibility that they are
recordings are completely non-invasive.    of functional specialization.                          artifactual should be kept in mind.

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Woodruff et al.                                                                                                                         Chandelier cell function

                                                  Disregarding the suprathreshold activations           the driving force (EGABA − Vrest) is high enough,
                                              induced by ChCs, we are left with the fact that in        and EGABA is not too distant from VThr, GABA is
                                              some parts of the brain, but perhaps not in others,       proven to be excitatory in these neurons. The val-
                                              ChCs are depolarizing. This subthreshold depo-            ues reported for the three determining param-
                                              larization is, in fact, likely to be the effect felt by   eters in this study are, in fact, in line with those
                                              the vast majority of recipient pyramidal neurons          at the cortical ChC synapse, and an important
                                              upon ChC activation. A highly pertinent question,         requirement for GABAergic excitation appears
                                              then, is whether this subthreshold depolarization         to be the activation of voltage-gated sodium
                                              is excitatory. A few factors merit some thought.          channels (VGSCs), which allow the neuron to
                                              The opening of any membrane channel decreases             continue depolarizing despite being above EGABA
                                              the neuronal input resistance, causing shunting           (Figures 2A,B; Rheims et al., 2008; Valeeva et al.,
                                              inhibition – a given current then produces a              2010). Since VGSCs at the AIS have a relatively
                                              smaller voltage deflection. This shunting effect          hyperpolarized activation threshold (Colbert and
                                              is therefore by nature inhibitory, and antagonistic       Pan, 2002; Astman et al., 2006; Hu et al., 2009)
                                              to any depolarization (Figure 2C). Whether the            due to the presence of NaV1.6 channels, in the
                                              magnitude of the conductance shunt outweighs              range of axonal EGABA, it certainly seems possible
                                              the magnitude of the depolarization is therefore          that ChCs can be excitatory in a manner similar
                                              an important consideration, though it should              to that seen in immature neurons.
                                              be noted that once the conductance closes, any                It should be noted that there is considerable
                                              remaining depolarization is likely to be excitatory       neuron to neuron variability in both EGABA and
                                              (Figure 2C).                                              VThr. Both of these parameters may be plastic,
                                                  Secondly, EGABA for the ChC synapse lies below        changing according to the neuron’s activity his-
                                              threshold. Although a ChC PSP may initially be            tory (Henze and Buzsaki, 2001; Woodin et al.,
                                              depolarizing, with the pyramidal neuron below             2003; Fiumelli et al., 2005), and small changes in
                                              EGABA, continued depolarization of the pyra-              these values are likely to be critical in determining
                                              mid above EGABA, perhaps through additional               whether GABA can be excitatory (Rheims et al.,
                                              glutamatergic inputs, will result in the ChC PSP          2008). Thus, the precise interplay of the neuron’s
                                              switching to hyperpolarizing and inhibitory, pro-         membrane potential, EGABA and VThr, their modi-
                                              vided the conductance is still open. Whether the          fication by prior activity, and the activation of
                                              net effect of ChC activation under these condi-           axonal VGSCs seem to be the critical parameters
                                              tions will help or hinder firing is therefore not         that will determine whether ChCs can provide
                                              straightforward.                                          excitatory, as well as inhibitory, input.
                                                  Another point worth considering is that con-              The discussion so far has focused on experi-
                                              ductance effects aside, there still exists the theo-      ments performed in vitro. However, neurons in
                                              retical possibility for a depolarization-induced          vivo are constantly bombarded with synaptic
                                              inhibition due to inactivation of the Na+ chan-           input, producing a depolarized and fluctuating
                                              nels responsible for spike generation. However, we        membrane potential that promotes the opening
                                              believe this is unlikely to be a significant factor in    and closure of a vast array of voltage depend-
                                              the case of ChCs, given that depolarization also          ent channels (Destexhe et al., 2003). That in vivo
                                              greatly enhances Na+ channel open probability.            membrane potentials are considerably more
                                              Indeed, if this possibility is to be considered, one      depolarized than in the quiescent in vitro slice
                                              must also consider the possibility that similar           may be particularly relevant for ChCs. Axonal
                                              mechanisms can result in glutamatergic inputs             EGABA in cortical layer 2/3 neurons is in the range
                                              being inhibitory due to the depolarization they           of −65 to −55 mV, values similar to mean mem-
                                              produce – the situation is analogous. In addition,        brane potentials recorded in awake animals
                                              the AIS contains not only fast-inactivating Na+           (Brecht et al., 2004; Poulet and Petersen, 2008;
                                              channels responsible for spike generation, but            Gentet et al., 2010). Predicting the effect of ChCs
                                              also slowly inactivating channels with a negatively       in vivo therefore becomes quite a complicated
                                              shifted activation threshold (Astman et al., 2006),       exercise. If the in vivo membrane potential fluctu-
Shunting inhibition
                                              which underlie the “persistent” sodium current,           ates either side of EGABA, the ChC has the potential
A form of inhibition that results from        INAP (Fleidervish et al., 2010). This persistent cur-     to be excitatory as well as inhibitory. This may
the opening of a membrane                     rent may be important in aiding spike generation          promote the homogenization of pyramidal neu-
conductance and the decrease in               following a ChC input.                                    ron firing rates, inhibiting strongly active neu-
excitability this causes. Typically used to       A situation analogous to that in cortical ChCs, in    rons but exciting those that are less active (Vida
describe the effect of a GABAergic
interneuron when EGABA of the synapse
                                              which EGABA lies significantly above Vrest but below      et al., 2006). In this scenario, ChCs may act as
is equal to the membrane potential of         VThr, exists in immature neocortical (Rheims et al.,      activity sensors, passively adjusting their function
the neuron.                                   2008) and hippocampal pyramidal cells. Provided           according to what is required by the surrounding

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Woodruff et al.                                                                                                      Chandelier cell function

                            network. The effect will be dictated, of course, by      2010; Varga et al., 2010). Whether there is spe-
                            the pyramidal neuron membrane potential, and             cificity in which particular local and interlaminar
                            by the complex interplay of that neuron’s syn-           neurons ChCs target is unknown, although there
                            aptic inputs. Whether the membrane potential             is some evidence that they may preferentially
                            ever stays below EGABA for a sufficient period of        contact pyramidal neurons with predominantly
                            time to allow depolarization, and whether the            intracortical projections (De Carlos et al., 1985;
                            presumably small amount of depolarization (so            Farinas and DeFelipe, 1991). The presynaptic
                            close to EGABA) provided by the ChC will out-            partners of ChCs are less well documented. Laser
                            weigh the shunting effect, are critical questions.       scanning photostimulation (LSPS) was recently
                            Alternatively, perhaps the ChC synapse in vivo           applied to L2/3 ChCs in mouse primary somato-
                            should be considered predominantly shunting,             sensory cortex, demonstrating excitatory input
                            and thus purely inhibitory. Indeed, during active        predominantly from layers 2/3 and 5a, and inhibi-
                            states, the shunting effect often ascribed to basket     tory input from L2/3 and L1 (Xu and Callaway,
                            cells, based on EGABA approximating resting poten-       2009). While this laminar input information is
                            tial in a slice, may be more appropriate for ChCs,       certainly useful, a higher resolution technique
                            while BCs would instead provide hyperpolarizing          such as two-photon photostimulation (Nikolenko
                            inhibition. In this way, ChCs and BCs could have         et al., 2007) could be useful in determining more
                            quite distinct inhibitory, perisomatic functions. In     precise input specificity. In addition, because these
                            CA1, where the ChC and BC synapses presumably            mapping experiments are performed in slices, a
                            have more similar reversal potentials, any differ-       great number of longer-range, inter-areal connec-
                            ences in function must depend more critically on         tions are likely to be severed. We commonly find
                            the location of the synapse (AIS vs. soma/proxi-         ChCs at the border between layer 1 and layer 2/3,
                            mal dendrites), or on differences in their pre and       a site from which their dendrites have easy access
                            postsynaptic partners.                                   to long-range “feedback” projections from higher
                                                                                     cortical areas. A promising method for determin-
                            Future Directions                                        ing inter-areal connectivity is monosynaptic, ret-
                            While it has recently become more feasible to            rograde tracing using a deletion-mutant rabies
                            record from ChCs, significant barriers to under-         virus (Wickersham et al., 2007). These projec-
                            standing their function remain. Firstly, because of      tions are unlikely to be maintained in vitro. In
                            their sparseness, recording from and manipulat-          theory however, in vivo single-cell electroporation
                            ing more than one or two at a time is still difficult.   or whole-cell patching of a ChC at the layer 1
                            Although some studies report robust network-             border is possible and should allow the introduc-
                            level effects after manipulation of a single neuron      tion of the virus, enabling both its local and long-
                            (Brecht et al., 2004; Houweling and Brecht, 2008;        range presynaptic connections to be elucidated
                            Bonifazi et al., 2009), it would certainly be ben-       (Marshel et al., 2010). It would be interesting, for
                            eficial to selectively control several ChCs at once.     example, to determine the similarity in inter-areal
                            This sort of control has been demonstrated for           input received by the apical tuft of L2/3 pyramidal
                            PV-expressing fast-spiking cells using neurons           neurons and the layer 1 dendrites of ChCs located
                            transfected with channelrhodopsin-2 (ChR2;               in upper layer 2/3, whose major targets are those
                            Cardin et al., 2009; Sohal et al., 2009), a light-       same pyramidal neurons.
                            activated proton pump capable of depolarizing                While dissecting the synaptic and network
                            and activating neurons with high temporal reso-          effects of chandelier neurons on cortical activ-
                            lution (Boyden et al., 2005). As of now, however,        ity remains a major challenge, considering how
                            no unique genetic marker exists for ChCs, so that        these effects may change during later postnatal
                            interrogating their function in this manner is not       development adds another important twist to
                            yet possible. We expect this to happen in the near       this story. Like other PV-expressing interneurons,
                            future, however, and their currently murky role          ChCs in the mouse originate in the medial gan-
                            should become considerably clearer.                      glionic eminence from progenitors that express
                                Another important direction will be to               the fate-determining transcription factor, Nkx2.1
                            determine the synaptic partners of ChCs. Their           (Xu et al., 2008; Fazzari et al., 2010). Factors that
                            postsynaptic targets are quite well established,         specify chandelier interneuron fate from other
                            predominantly being local pyramidal neurons,             Nkx2.1+ lineages, including other PV-expressing
                            although some interlaminar connections also              interneurons as well as those that express soma-
                            exist (Somogyi et al., 1982). There are increasing       tostatin, are at this point unknown. During post-
                            numbers of reports demonstrating target specifi-         natal development, the chandelier axon cartridges
                            city in neuronal connections (Yoshimura et al.,          begin to be identifiable in mouse cortex at about
                            2005; Brown and Hestrin, 2009; Anderson et al.,          postnatal day 14, based on analysis of GFP+ cells

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Woodruff et al.                                                                                                                                                Chandelier cell function

                                                in Nkx2.1Cre MADM mice. In the cat visual cor-                         tant to determine whether the quantity as well as
                                                tex, chandelier axon terminals undergo major                           the postsynaptic and network effects of chande-
                                                refinements between the early postnatal period                         lier neuron activity change during the pubertal
                                                and adulthood, although they appear to target                          age range of cortical development.
                                                only the AIS (Somogyi et al., 1982).                                       Clearly, many questions remain regarding the
                                                    Interestingly, this postnatal refinement may                       function of ChCs. Even the quite basic question
                                                be quite protracted. In the macaque neocortex,                         of whether their synapses are depolarizing or
                                                where chandelier cartridges are identifiable by                        hyperpolarizing is still somewhat open. While
                                                immunohistochemistry for PV (DeFelipe et al.,                          they may exert different effects in different parts
                                                1989), the detectability of these cartridges is                        of the brain, this implies a different function for
                                                highly dynamic (Anderson et al., 1995). A low                          what is otherwise considered a single neuronal
                                                density is present by 3 months, but they increase                      subtype, an intriguing possibility. Secondly, if
                                                greatly to reach a peak at about 15 months.                            the synapse can be depolarizing in vitro, that
                                                However, their numbers diminish over the                               does not necessarily hold for conditions in vivo
                                                pubertal age range, falling back to the level seen                     (at least in awake animals), and thus any func-
                                                in 3-month-old brains by about 3 years of age.                         tions ascribed to ChCs recorded in the slice must
                                                Since neither the density of GABA terminals                            be confirmed in the intact brain. Nor has it been
                                                (Erickson and Lewis, 2002) nor the density of                          established experimentally that the depolariza-
                                                GABAA receptors on the AIS (in mice; Katagiri                          tion cortical ChCs can provide is excitatory. The
                                                et al., 2007) changes over this period, and since                      role that ChCs play in controlling or modulating
                                                PV expression is known to be dependent on                              neuronal communication remains an intrigu-
                                                extrinsic factors (Vogt Weisenhorn et al., 1998),                      ing mystery, hopefully to be solved in the near
                                                the reduction of PV detectability in chandelier                        future.
                                                axon terminals may reflect alterations of chande-
                                                lier neuron activity over the pubertal age range.                      Acknowledgments
                                                Given the major role of GABA maturation on                             We thank laboratory members for comments,
                                                late-developing aspects of cortical plasticity                         Yeonsook Shin and Laura McGarry for help with
                                                (Hensch, 2005), evidence that chandelier axon                          anatomical reconstructions and histological pro-
                                                marker expression changes dramatically during                          cedures, and the National Eye Institute and the
                                                postnatal development suggests that it is impor-                       NIMH for support.

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