Is Ethanol a Pro-Drug? Acetaldehyde Contribution to Brain Ethanol Effects
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0145-6008/05/2908-1514$03.00/0 Vol. 29, No. 8
ALCOHOLISM: CLINICAL AND EXPERIMENTAL RESEARCH August 2005
Is Ethanol a Pro-Drug? Acetaldehyde Contribution to
Brain Ethanol Effects
Etienne Quertemont, C. J. Peter Eriksson, Sergey M. Zimatkin, Pavel S. Pronko, Marco Diana, Milena Pisano,
Zachary A. Rodd, Richard R. Bell, and Roberta J. Ward
This article presents the proceedings of a symposium at the 2004 meeting of the International Society for
Biomedical Research on Alcoholism, held in Mannheim, Germany. The symposium was organized by
Etienne Quertemont and chaired by C. J. Peter Eriksson. The presentations were (1) Brain ethanol
metabolism and its behavior consequences, by Sergey M. Zimatkin and P. S. Pronko; (2) Acetaldehyde
increases dopaminergic neuronal activity: a possible mechanism for acetaldehyde reinforcing effects, by
Marco Diana and Milena Pisano; (3) Contrasting the reinforcing actions of acetaldehyde and ethanol
within the ventral tegmental area (VTA) of alcohol-preferring (P) rats, by Zachary A. Rodd and Richard
R. Bell; (4) Molecular and biochemical changes associated with acetaldehyde toxicity, by Roberta J. Ward;
and (5) Role of acetaldehyde in human alcoholism and alcohol abuse, by C. J. Peter Eriksson.
Key Words: Acetaldehyde, Ethanol, Brain, Catalase, Self-Administration, Dopamine.
A cetaldehyde, the first product of ethanol metabolism,
plays a major role in the toxic effects of ethanol
controversial (Quertemont, 2004). In recent years, a num-
ber of studies in both humans and animals have investi-
(Eriksson, 2001). However, acetaldehyde is also a possible gated the role of acetaldehyde in various ethanol effects
psychoactive compound. It has long been suggested that the (Deitrich, 2004; Quertemont and Tambour, 2004). How-
conversion of ethanol into acetaldehyde is accountable for ever, these studies have frequently yielded conflicting re-
some of the behavioral effects of alcohol consumption sults. Whereas animal studies often report the reinforcing
(Hunt, 1996; Smith et al., 1997). Indeed, acetaldehyde itself action of brain acetaldehyde (Brown et al., 1979; Rodd-
when injected to rodents induces a range of behavioral Henricks et al., 2002a), human studies generally show that
effects similar to ethanol (Aragon et al., 1986; Correa et al., acetaldehyde accumulation leads to an aversion for alcohol
2003; Quertemont et al., 2004), including reinforcing prop- consumption (Quertemont, 2004). The main reason for
erties (Brown et al., 1979; Quertemont and De Witte, 2001; such discrepancies might be the localization of acetalde-
Rodd-Henricks et al., 2002a). However, the precise role of hyde accumulation. Although peripheral acetaldehyde ac-
acetaldehyde in alcohol abuse and alcoholism remains a cumulation would be predominantly aversive, its action
matter of intense debate. Although some authors have within the brain seems to be mainly reinforcing, at least in
suggested that acetaldehyde mediates most of the behav- rodents. Very few studies have been able to investigate the
ioral effects of ethanol, this assumption remains extremely effects of brain acetaldehyde in humans. The aim of the
present symposium was to bring together scientists from
From Neuroscience Comportementale et Psychopharmacologie, University both human and animal research fields to discuss the role
of Liège, Liège, Belgium (EQ); the Department of Mental Health and Alcohol of acetaldehyde in ethanol neurobehavioral effects and its
Research, National Public Health Institute, Helsinki, Finland (PCJE); potential mechanisms of action. In particular, the hedonic
Grodno State Medical University, Grodno, Belarus (SMZ, PSP); Laboratory
of Cognitive Neuroscience, Department of Drug Sciences, University of Sas-
properties of acetaldehyde, rewarding versus aversive, is
sari, Italy (MD, MP); the Institute of Psychiatric Research, Department of debated.
Psychiatry, Indiana University School of Medicine, Indianapolis, Indiana
(ZAR, RLB); and Unité de Biochimie, Université catholique de Louvain,
Louvain-la-Neuve, Belgium (RJW). BRAIN ETHANOL METABOLISM AND ITS BEHAVIOR
Received for publication January 27, 2005; accepted March 10, 2005. CONSEQUENCES
Supported by National Institute on Alcohol Abuse and Alcoholism grants
1R21AA12284-01, KO5AA00093, P50AA03527, RO1-AA12650, and 5 R21 Sergey M. Zimatkin and Pavel S. Pronko
AA12284-02 to R. A. Deitrich, University of Colorado Alcohol Research
It is often postulated that many neuropharmacological,
Center.
Etienne Quertemont, Neuroscience Comportementale et Psychopharma- neurochemical, neurotoxic, and behavioral effects of etha-
cologie, University of Liege, Boulevard du Rectorat 5/B32, B-4000, Liege, nol are mediated by the first metabolite of ethanol, acetal-
Belgium; E-mail: equertemont@ulg.ac.be dehyde (Hunt, 1996; Smith et al., 1997; Zimatkin and Dei-
Copyright © 2005 by the Research Society on Alcoholism. trich, 1997). Such acetaldehyde-mediated effects require at
DOI: 10.1097/01.alc.0000175015.51329.45 least the presence of ethanol-derived acetaldehyde inside
1514 Alcohol Clin Exp Res, Vol 29, No 8, 2005: pp 1514–1521ACETALDEHYDE AND BRAIN ETHANOL EFFECTS 1515 the brain. There are two possible origins for brain acetal- and 66%, respectively) and acetate (by 63% and 47%, dehyde: 1) the penetration from the blood of acetaldehyde respectively). A pretreatment with the CYP2E1 inhibitor derived from the peripheral ethanol metabolism or 2) the diallylsulfid (2 mM) significantly reduced acetaldehyde production of acetaldehyde inside the brain during ethanol concentration (by 67%) and acetate accumulation (by metabolism in situ. The first option is hardly possible be- 74%). Another CYP2E1 inhibitor, -phenylethyl isothio- cause of the high aldehyde dehydrogenase (ALDH) activity cyanate (100 M), reduced acetaldehyde and acetate accu- in the liver and the powerful blood-brain barrier for alde- mulations (by 34% and 53%, respectively). The accumula- hydes represented by the same enzyme (Zimatkin, 1991). tion of ethanol-derived acetaldehyde in brain homogenates The second option was confirmed in the early nineties in of acatalasemic mice was 47% of control values (p ⬍ 0.01). several laboratories (Aragon et al., 1992; Gill et al., 1992; In mice with genetic deficiency of CYP2E1, it was 91% of Hamby-Mason et al., 1997; Zimatkin et al., 1998). Those control values and in double mutants with genetic deficien- studies found that catalase is the main enzyme oxidizing cies of both catalase and CYP2E1, it was at 24% (p ⬍ 0.01). ethanol within the brain and discarded the possible contri- The data obtained in the present study confirm the crucial bution of alcohol dehydrogenase (ADH) and cytochrome role of catalase in ethanol oxidation within the brain, where P-450 – dependent system in brain tissue ethanol oxidation. it can be responsible for about 50% to 70% of ethanol However, an almost complete inhibition of catalase in brain oxidation. The results also indicate the possible role of homogenates by the addition of catalase inhibitors such as CYP2E1 in this process (responsible for about 20% to 30% 3-amino-1,2,4-triazole or sodium azide, as well as genetic of ethanol oxidation). Brain ethanol oxidizing properties catalase deficiency (in acatalasemic mice) preserves about might also be attributed to ADH and others unknown half of brain ethanol oxidation and acetaldehyde produc- factors. tion (Aragon et al., 1992; Gill et al., 1992; Hamby-Mason et We also examined acetaldehyde accumulation in brain al., 1997; Zimatkin et al., 1998). This suggests that some subcellular fractions in the presence of the ALDH inhibitor other enzymatic systems are involved in brain ethanol me- citral. The investigation of acetaldehyde accumulation from tabolism. The best candidate is the cytochrome P-4502E1 ethanol in the presence of citral showed that this process (CYP2E1), which is expressed in rat brain and is induced by took place mainly in the microsomal fraction (p ⬍ 0.01), ethanol (Aragon et al., 1992; Hansson et al., 1990; Upadhya where it was significantly higher than in other fractions. A et al., 2000). In addition, a contribution of alcohol dehy- high acetaldehyde accumulation was also observed in the drogenase in brain ethanol oxidation cannot be ruled out fraction enriched with peroxisomes, and it was significantly because of the demonstrated translation of ADH I in the higher there compared with nuclear, mitochondrial, and rat brain (Martinez et al., 2001). cytosol fractions (p ⬍ 0.01). The high level of ethanol In the present study, we examined ethanol oxidation in oxidation in the peroxisomal fraction confirms an impor- perfused brain homogenates from outbred, heterogeneous, tant role of catalase in the oxidation of ethanol in the brain. and various inbred lines of rats and mice by measuring the However, the pronounced ethanol oxidation in the micro- accumulation of both acetaldehyde and the subsequent somal fraction also suggests the participation of CYP2E1 in ethanol metabolite, acetate, by gas chromatography. The rat brain ethanol metabolism. The accumulation of acetal- levels of acetaldehyde and acetate correlated significantly dehyde increased two times after the addition of hydrogen (r ⫽ 0.971; p ⬍ 0.01) in the course of ethanol oxidation in peroxide to the incubation medium of the peroxisomal rat brain homogenates. The levels of acetaldehyde dose- fraction during ethanol oxidation in the presence of citral. dependently increased and those of acetate decreased after This further confirms the role of catalase in brain tissue the administration of the ALDH inhibitor cytral. We used ethanol oxidation. After the addition of NADPH to the several approaches to investigate the enzymatic mecha- incubation medium, the rate of ethanol oxidation in the nisms of ethanol metabolism in the brain: 1) the preincu- microsomal fraction increased of 70%. Furthermore, a pre- bation of brain homogenates with various inhibitors of treatment with the CYP2E1 inhibitor phenylethylisothio- ethanol-metabolizing enzymes (catalase, CYP2E1, ADH, cyanate decreased ethanol oxidation to control levels. and ALDH), 2) the use of animals with genetic deficiencies These latter observations support the notion that CYP2E1 in ethanol-metabolizing enzymes, 3) the assessment of eth- is involved in ethanol oxidation within the brain tissues. anol oxidation in brain subcellular fractions with known The data obtained in the present study clearly demonstrate high and low activities of ethanol-metabolizing enzymes. the major role of catalase and CYP2E1 and the minor role Brain homogenates were preincubated for 20 minutes of ADH in brain ethanol metabolism and show substantial with inhibitors of ethanol-metabolizing enzymes or water. differences between the brain and the liver in the main sites It was found that the ADH inhibitor 4-methyl-pyrazole (5 of ethanol oxidation. This opens the way for specific ma- mM) slightly but nonsignificantly reduced acetaldehyde ac- nipulations of brain ethanol oxidation. cumulation (by 22%) but significantly decreased acetate A number of previous studies from the groups of Z. Amit accumulation (by 66%). The catalase inhibitors sodium and C. Aragon showed that various pharmacological ma- azide (5 mM) and 3-amino-1,2,4-triazole (5 mM) signifi- nipulations of ethanol metabolism that theoretically altered cantly lowered the accumulation of acetaldehyde (by 74% brain acetaldehyde levels induced changes in several
1516 QUERTEMONT ET AL.
alcohol-related behaviors. However, in these studies, brain projecting to the nucleus accumbens (Pulvirenti and Diana,
acetaldehyde concentrations were never determined. 2001).
Therefore, there was no direct evidence linking brain acet- In contrast, studies suggest that acetaldehyde may par-
aldehyde levels with any behavioral effect of ethanol. In ticipate in the motivational properties of ethanol (Aragon
several in vitro and in vivo studies with randomly outbred, et al., 1986; Eriksson, 2001; Smith et al., 1997). Indeed,
heterogeneous, inbred and selectively bred rats and mice, acetaldehyde is self-administered directly into the VTA of
we have demonstrated a positive correlation between the alcohol-preferring rats (Rodd-Henricks et al., 2002a) and
sensitivity of animals to the hypnotic effects of ethanol (as into the cerebral ventricles (Brown et al., 1979) of uns-
assessed by the duration of ethanol-induced sleep) and the elected rodents. Further, when administered intracerebrov-
accumulation of brain ethanol-derived acetaldehyde but no entricularly, acetaldehyde is able to induce place-
correlation with their brain catalase activity. We also found preference in rats (Smith et al., 1984) and to produce a
significant differences in the ethanol-oxidative capacity in conditioned stimulus preference even when administered
the brain homogenates of lines of mice with high and low peripherally (Quertemont et al., 2001). All these studies
acute functional tolerance to ethanol. These results are lend support to the hypothesis that central actions of eth-
direct indications of the role of brain acetaldehyde in the anol might be mediated by its metabolite acetaldehyde
behavioral effects of alcohol (Zimatkin et al., 2001a; Zi- instead.
matkin et al., 2001b). In the present study, we sought to determine directly if
acetaldehyde administration alters DA neuronal activity in
the VTA and if this action bears any relationship with
exogenously administered ethanol. To this aim, we blocked
ACETALDEHYDE INCREASES DOPAMINERGIC NEURONAL
ACTIVITY: A POSSIBLE MECHANISM FOR ethanol metabolism with the alcohol dehydrogenase inhib-
ACETALDEHYDE REINFORCING EFFECTS itor 4-methyl-pyrazole (4-MP) and studied the effect of
ethanol and acetaldehyde on the electrophysiological prop-
Milena Pisano and Marco Diana erties of DA-containing VTA neurons.
Alcoholism is a major addictive disorder with profound Male Sprague-Dawley albino rats (200/300 g) were used
reflections on the individual and society. Among the vari- in all experiments. Rats were divided into subgroups as
ous pharmacological treatments available for this disorder, follows: 1) acetaldehyde (n ⫽ 19), which received exponen-
disulfiram (Antabuse) is the oldest (Chick et al., 1992; tially increasing doses (5 to 40 mg/kg IV) of acetaldehyde;.
Fuller et al., 1986; Litten et al., 1996) and perhaps the most 2) ethanol (n ⫽ 10), which received exponentially increas-
widely used. Its mechanism of action is thought to reside on ing doses of ethanol (250 to 1000 mg/kg IV); 3) pretreated
the property to inhibit aldehyde dehydrogenase, through ethanol (n ⫽ 5), which received a single dose of the alcohol
which it should raise acetaldehyde blood levels, produced dehydrogenase inhibitor 4-MP (90 mg/kg IP) dissolved in
by ethanol ingested and metabolized by the alcohol dehy- saline and ethanol (250 to 1000 mg/kg iv) 48 hours later; 4)
drogenase normally found in gastric and hepatic tissue of pretreated acetaldehyde (n ⫽ 5), which received a single
human beings (Baraona et al., 1991). In turn, the aug- dose of the alcohol dehydrogenase inhibitor 4-MP (90
mented blood acetaldehyde concentrations are considered mg/kg IP) dissolved in saline and acetaldehyde (5 to 40
to be aversive (Eriksson, 2001; Litten et al., 1996) and to mg/kg IV) 48 hours later; 5) control rats (n ⫽ 9), which
form the basis for the well known “flushing syndrome,” received an equal volume (0.1 ml/kg body weight) of vehicle
commonly observed in many Asians, an ethnic group with (saline IP) and 48 hours later, ethanol (n ⫽ 4) or acetal-
low incidence of alcoholism after ethanol ingestion. dehyde (n ⫽ 5). All groups underwent identical surgical
On the other hand, at least some of the motivational procedure. Subjects were anesthetized with urethane (1.3
properties of ethanol are thought to be mediated by the g/kg ip), the femoral vein was exposed, and a catheter was
mesolimbic dopamine (DA) system, whose cell bodies are inserted into the lumen to allow intravenous administration
located in the ventrotegmental area (VTA) in the mid- of pharmacological agents. Rats were then mounted on a
brain. Accordingly, acute ethanol administration increases stereotaxic apparatus (Kopf, Tujunga CA USA) for the
electrophysiological activity of these neurons (Brodie et al., placement of a recording electrode filled with 0.5 M NaCl,
1990; Gessa et al., 1985) and augments DA extracellular above the VTA (AP 1.8/2.0 from lambda; L 0.2/0.5 from
concentrations in terminal areas (Imperato et al., 1986). midline). Presumptive dopaminergic neurons were identi-
Conversely, ethanol withdrawal decreases dopaminergic fied according to well-established electrophysiological
neuronal activity (Diana et al., 1993) and reduces DA characteristics, that is, action potentials with biphasic or
concentrations in the nucleus accumbens (Diana et al., triphasic waveforms greater than 2.5 msec in duration—a
1993; Rossetti et al., 1992; Weiss et al., 1996). All these typically slow spontaneous firing rate (2 to 5 Hz)— occur-
studies have suggested that both positive (reinforcing) and rence of single and burst spontaneous firing pattern. The
negative (dysphoriant) properties induced by acute ethanol extracellular neuronal signal from single neurons was am-
and by its withdrawal, respectively, are mediated, at least plified (Neurolog System) and displayed on a digital oscil-
partially, by increments and decrements of DA neurons loscope (Tektronix TDS 3012) before storage on magneticACETALDEHYDE AND BRAIN ETHANOL EFFECTS 1517
tape for off-line analysis of the data. Data were logged on in normal (untreated) animals. Ethanol-stimulating capac-
a standard PC computer through a CED 1401 interface, ity on VTA neuronal activity was completely abolished in
and firing rate and pattern analysis were performed by a 4-MP rats. In contras, acetaldehyde administration in-
CED Spike2 system using firing rate histograms generated creased neuronal activity in 4-MP–pretreated rats to a de-
by CED Spike2 software. A burst was defined according to gree similar to that observed in untreated rats.
Grace and Bunney (1984) as a train of at least two spikes The results presented here strongly suggest that the en-
with the first interspike interval of 80 msec or less and a hancement of dopaminergic transmission after ethanol ad-
termination interval greater than or equal to 160 msec. ministration is, in fact, produced by acetaldehyde. Accord-
Burst activity was analyzed according to the total percent of ingly, acetaldehyde administration readily and dose-
firing occurring in bursts called percentage of bursts and by dependently increased firing rate, spikes/burst, and burst
the mean number of spikes within a burst (Diana et al., firing of DA-containing neurons of the VTA, the brain
1989). The analysis of these parameters (spikes/sec, spikes/
region that is known to be involved in the positive motiva-
burst, and percentage of burst firing) is an important index
tional properties of drugs of abuse in general, including
for the activity of DA cells and allows one to evaluate the
ethanol. In addition, acetaldehyde stimulated electrophys-
influences that a putative drug exerts in the cell pattern.
iological parameters of DA neurons in animals in which
After five minutes of stable neuronal recording (basal ac-
ethanol metabolism was pharmacologically blocked by the
tivity), exponentially increasing doses of ethanol (0.25/0.25/
0.5 g/kg) or acetaldehyde (5/5/10/20 mg/kg) were injected alcohol dehydrogenase inhibitor 4-MP, whereas ethanol
intravenously every two minutes, so that last administered was totally ineffective under this condition. This experi-
dose was equal to the sum of the drug already received and ment indicates that conversion of ethanol into acetaldehyde
cell activity was recorded. Only one cell was recorded per is essential to observe an enhancement of DA transmission
rat. Drug-induced modifications of the basal activity were after ethanol administration. Further, acetaldehyde (5 M)
calculated in percentage for the two-minute period after produces an inward current in DA neurons recorded in
each administration and compared with the predrug base- vitro in the whole-cell configuration of the patch-clamp
line. Statistical significance of the data were evaluated by technique (Melis and Bonci, unpublished results), suggest-
one-way analysis of variance for repeated measures. At the ing a direct effect on the membrane of DA neurons.
end of each recording section, DC current (5 A for 15 These results add significantly to a growing body of
minutes) was passed trough the recording electrode to eject evidence that lends support to the hypothesis that acetal-
Pontamine sky blue, which allowed the identification of the dehyde might be an active metabolite of ethanol in the
recorded cells. Brains were removed and fixed in 8% for- euphoriant properties of alcoholic beverages. Indeed, acet-
malin solution. The position of the electrodes was micro- aldehyde is self-administered directly into the VTA (Rodd-
scopically verified on sections (60 m) stained with Cresyl Henricks et al., 2002a) and into the cerebral ventricles
violet. (Brown et al., 1979), produces place preference when ad-
The effect of acetaldehyde on VTA dopaminergic neu- ministered intracerebroventricularly (Smith et al., 1984),
ronal activity was studied in a total of 19 VTA neurons. In and produces a conditioned stimulus preference even when
13 cases, acetaldehyde was administered up to the cumu- administered peripherally (Quertemont et al., 2001).
lative dose of 20 mg/kg IV, and in the remaining six neu- These results may also bear important consequences on
rons, a cumulative dose of 40 mg/kg was reached. Since no the therapeutic side of alcoholism and drug addiction, in
statistical difference was found, basal activity values were general. Indeed, according to the present results, blockade
pooled and analyzed for differences between before and
of ethanol metabolism should deprive ethanol of its re-
after acetaldehyde.
warding properties and, possibly, discourage individuals
Baseline firing rate was 3.08 ⫾ 0.25 (mean ⫾ SEM), and
from intake. Accordingly, 4-MP has been found to be ef-
it was increased dose-dependently by intravenous acetalde-
fective in reducing spontaneous alcohol intake in rodent
hyde administration. Acetaldehyde administration pro-
duced also an increment in the number of spikes contained lines selected for high alcohol preference (Waller et al.,
in each burst (spikes/burst) and in the percentage of spikes 1982), and similar results were recently observed in human
delivered in bursts (burst/firing). Intravenous ethanol ad- nicotine addicts with lower metabolic capacity for nicotine
ministration (0.25 to 1 g/kg) produced increments in all (Pianezza et al., 1998). This would suggest that a reduced
three parameters studied of similar magnitude. metabolism of drugs of abuse, either pharmacologically
To gain some further insight on the relative contribution obtained or genetically determined, may reduce the risk of
of the two drugs (ie, acetaldehyde and ethanol) to the addiction.
activation of VTA neurons, an additional group of rats (n In conclusion, the present results suggest that ethanol
⫽ 10) was pretreated with the alcohol dehydrogenase in- stimulates dopaminergic transmission in the limbic system
hibitor 4-MP (Waller et al., 1982). Ethanol was then ad- through its byproduct acetaldehyde, previously thought
ministered in 4-MP–pretreated rats and relative control only as an aversive compound useful in the pharmacologi-
rats (pretreated with saline) at the same doses administered cal treatment of alcoholics.1518 QUERTEMONT ET AL.
CONTRASTING THE REINFORCING ACTIONS OF ies have indicated that similar to ethanol, acetaldehyde is
ACETALDEHYDE AND ETHANOL WITHIN THE VENTRAL self-administered into the posterior VTA but not into the
TEGMENTAL AREA OF ALCOHOL-PREFERRING RATS
anterior VTA or areas surrounding the posterior VTA
Zachary A. Rodd and Richard R. Bell (Rodd et al., 2005b). Overall, the data suggest that acetal-
Acetaldehyde is the first metabolite of ethanol and is a dehyde can produce reinforcing effects within the posterior
biologically active compound. Some of the effects of etha- VTA of P rats and that acetaldehyde is a more potent
nol have been attributed to acetaldehyde. The objectives of reinforcer in this region than is ethanol.
a series of experiments were to determine the involvement The conversion of ethanol into acetaldehyde in the brain
of acetaldehyde in the reinforcing effects of ethanol within is thought to occur primarily through a catalase reaction
the ventral tegmental area (VTA). (Aragon et al., 1992; Hamby-Mason et al., 1997). Several
The intracranial self-administration (ICSA) technique pharmacological studies suggested that the effects attrib-
has been used to identify specific brain regions involved in uted to ethanol might result from the formation of acetal-
the initiation of response-contingent behaviors for the de- dehyde through the catalase pathway (Aragon and Amit,
livery of a reinforcer (Goeders and Smith, 1987). The 1992; Aragon et al., 1986). If the self-infusion of ethanol
technique allows for organisms to self-administer small within the VTA is due to its conversion to acetaldehyde,
quantities of drugs directly into discrete brain regions. then blocking acetaldehyde formation should reduce the
Studies using the ICSA procedure have successfully iso- ICSA of ethanol. P rats were allowed to acquire ethanol
lated discrete brain regions where opioids (Bozarth and self-administration into the posterior VTA and then were
Wise 1980), amphetamine (Hoebel et al., 1983), and co- given the opportunity to self-administer ethanol and a cata-
caine (Rodd et al., 2005a; Rodd-Henricks et al., 2002b) lase inhibitor (triazole). Coadministration of triazole did
may produce their rewarding effects. Gatto et al. (1994) not alter ethanol self-administration into the posterior
reported that selectively bred alcohol preferring (P) rats VTA (Rodd et al., 2005b). However, a major caveat of this
administered ethanol into the VTA at concentrations rang- experiment is that triazole was not present before the
ing from 25 to 200 mg%. In contrast, alcohol nonpreferring ethanol self-administration and thus may have been inef-
(NP) rats failed to self-administer ethanol into the VTA at fective because of the experimental procedures instead of
any of the concentrations tested. A recent study indicated actual pharmacological efficacy.
that common stock Wistar rats will also self-administer Previous success from our laboratory indicated that
ethanol into the posterior but not anterior VTA (Rodd- ethanol self-administration into the posterior VTA could
Henricks et al., 2000). be blocked by coinfusing quinpirole (Rodd et al., 2004)
The presentation focused on recent studies conducted or 5-HT3 antagonists (Rodd-Henricks et al., 2003).
that examined 1) if P rats would self-administer acetal- These studies suggested that a local pharmacological
dehyde directly into the posterior VTA, a neurological effect for blocking the actions of ethanol could be ob-
site shown to support ethanol self-administration (Rodd-
tained with the ICSA procedure. Therefore, P rats were
Henricks et al., 2000); 2) the definition of any possible
allowed to acquire ethanol or acetaldehyde self-
acetaldehyde self-administration behavior; 3) if conver-
administration into the posterior VTA and then were
sion of ethanol to acetaldehyde was required for ethanol
given the opportunity to self-administer ethanol or acet-
self-administration into the posterior VTA; 4) the deter-
aldehyde with quinpirole (a D2/3 agonist) or ICS 205,930
mination of the neurotransmitter systems that regulate
(a 5-HT3 antagonist). Coadministration of quinpirole
the reinforcing properties of acetaldehyde within the
VTA. (activation of D2 autoreceptors in the posterior VTA
In the initial series of experiments, rats were assigned to should inhibit dopamine neuronal activity) blocked both
one of five groups that self-administered either artificial ethanol and acetaldehyde self-administration into the
cerebrospinal fluid (aCSF) throughout all eight sessions posterior VTA (Rodd et al., 2005b). Thus, both ethanol
(four hours in duration), or 3 to 360 M acetaldehyde. and acetaldehyde self-administration into the posterior
Adult P rats readily self-administered 6 to 90 M acetal- VTA require functional dopamine neuronal activity. In
dehyde (1000-fold lower concentration than is required to contrast, coadministration of ICS 205,930 blocks only
observe ethanol self-administration into the posterior ethanol self-administration while having no effect on
VTA) into the posterior VTA and discriminated between acetaldehyde self-administration into the posterior VTA.
the active and inactive lever (Rodd-Henricks et al., 2002a). In summary, the present findings suggest that ethanol
Furthermore, rats self-administering 90 M acetaldehyde and acetaldehyde can produce independent reinforcing ef-
also demonstrated extinction behavior when aCSF was sub- fects within the posterior VTA, which involve activation of
stituted for acetaldehyde and gradually reinstated active local DA neurons. In addition, activation of local 5-HT3
lever responding when acetaldehyde was reintroduced. P receptors appears to be involved in mediating the reinforc-
rats maintained similar numbers of infusions and infusion ing effects of ethanol but not acetaldehyde in the posterior
patterns under both time-out schedules. Additionally, stud- VTA of the P line of rats.ACETALDEHYDE AND BRAIN ETHANOL EFFECTS 1519
MOLECULAR AND BIOCHEMICAL CHANGES ical basis of acetaldehyde-reinforcing properties probably
ASSOCIATED WITH ACETALDEHYDE TOXICITY involves the mesolimbic dopaminergic system. This conclu-
Roberta J. Ward sion is supported by two lines of evidence. First, acetalde-
hyde self-administration into the posterior VTA requires
Many of ethanol’s toxic effects may be mediated through
the activation of dopamine neurons, as demonstrated by
its major metabolite acetaldehyde. Its generation in many
the disrupting effects of quinpirole on acetaldehyde self-
tissues, including the liver and the gut, may be responsible
administration. Additionally, the study from Pisano and
for adverse changes in cellular function, while its synthesis
Diana shows that acetaldehyde injections increase the fir-
in the brain may be involved in the mediation of addiction.
ing rate of the VTA dopamine neurons. In humans, there is
The higher sensitivity of women to alcohol-related dis-
also evidence that brain acetaldehyde might exert reinforc-
ease—in that they drink less alcohol over a shorter period
ing effects, although peripheral acetaldehyde accumulation
of time before toxicity occurs— has been extensively re-
is primarily aversive and prevents alcohol drinking (Quer-
ported. Our recent studies have identified important dif-
temont, 2004). A particularly controversial question regard-
ferences between male and female subjects in ethanol phar-
ing the role of acetaldehyde in ethanol’s central effects is
macokinetics; the development of alcohol misuse in female
whether significant acetaldehyde concentrations are ob-
subjects was related to changes in the rate of ethanol
tained within the brain after alcohol consumption. Indeed,
elimination during first-pass metabolism, which increased
under normal physiological conditions, peripherally pro-
circulating levels of blood acetaldehyde (Ward and Coute-
duced acetaldehyde does not reach the brain due to the
lle, 2003).
high activity of ALDH enzymes both in the liver and in the
Significant alterations in blood acetaldehyde elimination
microvasculature of the brain. However, it has been dem-
curves were observed after ingestion of 0.6g ethanol/kg
onstrated that the brain possesses ethanol-metabolizing
body weight in control subjects with either ADH31 ADH3 1
properties and therefore that acetaldehyde is produced in
or ADH32 ADH32 genotypes (Ward and Coutelle, 2003).
situ within the central nervous system. Sergey Zimatkin
For ADH31 ADH31 genotype, a steady rise in acetaldehyde
lends further support to this notion with a demonstration
content was evident during the first 30 minutes, after which
that acetaldehyde can be produced from ethanol metabo-
time the mean concentration diminished until 105 minutes.
lism in brain homogenates. This latter study shows that
A small rise in acetaldehyde content was then observed,
brain ethanol metabolism mainly involves catalase (for the
although this value was substantially lower than the initial
most part) and the CYP2E1 and maybe other undefined
peak at 30 minutes. In contrast, the ADH32 ADH32 subjects
pathways to a minor extent. However, it remains uncertain
showed a much faster rise in acetaldehyde content, reach-
whether pharmacologically significant acetaldehyde con-
ing a peak 15 minutes after ethanol ingestion, and re-
centrations are produced within the brain in vivo after
mained higher at each time point than the corresponding
alcohol consumption. Further studies are needed to mea-
values for the ADH31 ADH31 genotype.
sure such concentrations in vivo to substantiate the theory
In vivo studies in an animal model have shown that acute
that acetaldehyde plays a significant role in ethanol’s cen-
intraperitoneal injections of acetaldehyde evoke significant
tral effects. In humans, blood acetaldehyde levels after
decreases in dopamine levels in specific brain regions
alcohol consumption are affected by many factors, includ-
(Heap et al., 1995; Ward et al., 1997), thereby counteract-
ing gender and genetic polymorphism of ethanol-
ing increases in the levels of this monoamine induced by
metabolizing enzymes (Ward and Coutelle, 2003). How-
ethanol (Blanchard and Glick, 1995). Such alterations in
ever, human brain acetaldehyde levels in various conditions
dopamine levels may, in part, be involved in the rewarding
remain to be defined in future studies to elucidate its
and aversive aspects of ethanol ingestion.
possible role in alcoholism.
Altered ethanol pharmacokinetics, inherited or induced
by excessive ethanol consumption, for example, P450IIEI
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The 2006 Jellinek Memorial Fund Award for Outstanding
Contribution to the Advancement of Knowledge
on Alcohol/Alcoholism
Call for Nominations
Nominations are being solicited for the 2006 Jellinek Memorial Fund Award for a scientist who has made an
outstanding contribution to the advancement of knowledge in the alcohol/alcoholism field. Nominated candi-
dates may come from any country. The category for the Year 2006 award, specified by the Board of Directors
of the Jellinek Memorial Fund, will be Behavioral (Clinical and Experimental) Studies. Nominees must have
contributed outstanding research in this specific (albeit broad) area, and should be someone who would provide
an example and serve as a model for others who might be attracted to work in this field. In addition to a cash
award of CDN$5,000, the recipient is presented with a bust of the late E. M. Jellinek with an appropriate
inscription. The Jellinek Memorial Fund Award is traditionally presented at a major international conference,
and if necessary, travel and accommodation expenses are provided to permit the awardee to attend the
conference for presentation of the award. To complete the nomination of a candidate, submit four copies of
the following materials: 1) a detailed letter describing the principal contribution(s) for which the candidate is
being nominated, signed by the nominator and any co-nominators; and 2) a current copy of the candidate’s
curriculum vita. Nominations must be received no later than November 1, 2005, and should be sent to the Chair
of the Selection Committee: Dr. Richard Fuller, 20 Paddock Ct., Potomac, MD 20854, USA.You can also read
