Stimulation of Serotonin2 Receptors in the Ventrolateral Medulla of the Cat Results in Nonuniform Increases in Sympathetic Outflow
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1267
Stimulation of Serotonin2 Receptors in the
Ventrolateral Medulla of the Cat Results in
Nonuniform Increases in Sympathetic Outflow
Aloke K. Mandal, Kenneth J. Kellar, Wesley P. Norman, and Richard A. Gillis
Topical application of the serotonin2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane
or DOI, in a dose of 30 jig/side to the intermediate area of the ventrolateral surface of the
medulla produced a significant increase in mean arterial pressure with no significant change
in heart rate both in intact animals (n=8) and in cervically vagotomized animals (n=3). The
pressor response of DOI was blocked by pretreatment of the intermediate area with ketanserin,
a serotonin2 antagonist (n =7). Pretreatment with intravenous phentolamine did not block the
pressor response of DOI (n=3). However, this pressor response could be counteracted by
intravenous propranolol (n=5) or by bilateral stellate ganglionectomy (n=3). These data
suggest that sympathoexcitation by centrally applied DOI selectively increased cardiac inotropy
but not chronotropy. Further studies indicate that DOI increased contractile force without
increasing heart rate and that the positive inotropic effect of DOI could be counteracted by
bilateral stellate ganglionectomy. Bilateral microinjections of DOI into the subretrofacial
nucleus in a dose of 100 ng (n=3) and a dose of 300 ng (n=3) increased mean arterial blood
pressure by 23±2 and 44±6 mm Hg, respectively, without producing any changes in heart rate.
These data suggest that DOI has a central site of action in the ventrolateral medulla,
presumably at the subretrofacial nucleus, which leads to selective sympathoexcitation of the
cardiac ventricles. (Circulation Research 1990;67:1267-1280)
W e recently reported evidence that sug- receptors.12 At the time of these studies, McCall et
gested that serotonin (5 -HT) applied top- al3,4 reported that intravenous administration of
ically to the intermediate area of the 5-HT2 agonist drugs raised blood pressure and in-
ventrolateral medulla of the cat could stimulate creased central sympathetic outflow. These data sug-
serotonin2 (5-HT2) receptors, resulting in a rise in gested to us that within the central nervous system
arterial blood pressure.' Indeed, our evidence for 5-HT simultaneously activates 5-HTlA receptors,
making such a statement was very indirect and con- which lower blood pressure, and 5-HT2 receptors,
sisted of demonstrating that blockade of 5-HT2 re- which increase blood pressure.
ceptors at the intermediate area with topical appli- In testing the statement that activation of central
cation of the 5 -HT2 receptor antagonist drug nervous system 5-HT2 receptors causes an increase in
ketanserin, at the same site, augmented the hypoten- sympathetic outflow, resulting in an increase in blood
sive effect of 5 -HT. Presumably, the hypotensive pressure, we first sought to determine whether activa-
effect of 5 -HT was a result of activation of 5 -HT1A tion of 5 -HT2 receptors at the level of the intermediate
area of the ventrolateral surface of the medulla would
From the Departments of Pharmacology (R.A.G., K.J.K., cause an increase in blood pressure. We chose to look
A.K.M.) and Anatomy (W.P.N.), Georgetown University School
of Medicine, Washington, D.C. at this area initially because it has been shown that
A portion of this work was presented at the 19th Annual many drugs that have a central action on cardiovascular
Meeting of the Society for Neuroscience, October 29-November 3, activity and sympathetic outflow often act at the inter-
1989, Phoenix, Arizona. mediate area.5,6 To this end, we evaluated the effects of
Supported by a grant to R.A.G. from Byk-Gulden Pharmazeu-
tica, Konstanz, FRG. A.K.M. is a recipient of a Medical Student topical application to the intermediate area of the
Research Fellowship in Pharnacology-Clinical Pharmacology selective 5-HT2 agonist agent 1-(2,5-dimethoxy-4-io-
from the Pharmaceutical Manufacturers Association Foundation, dophenyl)-2-aminopropane, commonly referred to as
Washington, D.C. DOI. This compound has been shown to selectively
Address for correspondence: Richard A. Gillis, PhD, Depart- label 5-HT2 receptor binding sites and has been pro-
ment of Pharmacology, Georgetown University School of Medi-
cine, 3900 Reservoir Rd., NW, Washington, DC 20007. posed as a selective 5-HT2 receptor agonist.7-9 Respi-
Received February 28, 1990; accepted July 19, 1990. ratory activity also was monitored because neurons
Downloaded from http://circres.ahajournals.org/ by guest on September 7, 20151268 Circulation Research Vol 67, No 5, November 1990
reached by diffusion of drugs topically applied to the pledgets. In each experiment, control solutions of 2
intermediate area not only affect cardiovascular func- ,ul of the solvents used to dissolve DOI, ketanserin,
tion but also affect breathing.6 N-methyl-D-aspartic acid (NMDA), and prazosin
were examined. Pledgets soaked in the solvents used
Materials and Methods to dissolve these drugs did not produce significant
General changes in cardiorespiratory activity.
Experiments were performed on adult cats of Stellectomy
either sex, weighing between 1.7 and 4.3 kg. Anes- Animals were ventilated artificially with room air.
thesia was induced and maintained with a single Crescent-shaped incisions were made bilaterally ipto
intravenous dose of a-chloralose (75 mg/kg). Rectal the chest wall from the manubrium to the posterior tip
temperature was monitored and maintained between of the scapula. The various muscle layers were excised,
37.00 and 38.00C with an infrared heating lamp revealing the rib cage. On each side, the second rib was
connected to a thermistor. The femoral artery and isolated from the intercostal muscles and excised. Both
vein were cannulated for the measurement of arterial stellate ganglia were then dissected away from the
blood pressure and for the systemic administration of surrounding fascia and removed. To determine the
drugs, respectively. Lead II of the electrocardiogram effectiveness of bilateral stellectomy in abolishing sym-
was monitored, and heart rate was determined by pathetic neural effects on the heart, the degree of
measurement of the RR interval. The trachea was reflex-induced sinus tachycardia produced by sodium
cannulated and fitted with a Fleisch No. 0 pneumo- nitroprusside infused intravenously at a rate of 200
tachograph connected to a respiratory flow trans- ,g/mVmin for 15 seconds was observed before and
ducer (HP 4730A, Hewlett-Packard Co., Waltham, after extirpation of the stellate ganglia. With both
Mass.). The airflow signal obtained from the trans- stellate ganglia intact, sodium nitroprusside infusion
ducer was integrated with a Hewlett-Packard respi- lowered mean blood pressure -31±5 mm Hg and
ratory integrator (HP 8815A) to obtain tidal volume. raised heart rate +39±7 beats/min. After bilateral
Respiratory rate was obtained from fast tracings of stellate ganglionectomy, sodium nitroprusside infusion
the flow signal, and respiratory minute volume was lowered mean arterial pressure -36+ 10 mm Hg with-
calculated as the product of tidal volume and respi- out appreciably changing heart rate (-2±+1 beats/min).
ratory rate. All respiratory and cardiovascular pa- Hence, the lack of reflex-induced sinus tachycardia
rameters were recorded simultaneously on a Hew- with sodium nitroprusside indicated successful stellate
lett-Packard eight-channel recorder (HP 7758B). ganglionectomy.
Control measurements were taken at approxi-
mately 5-minute intervals for at least 30 minutes Experiments of Contractile Force
before the administration of drugs in each experi- In some animals, the effects of drugs applied to the
ment. Once a drug was administered, measurements intermediate area on contractile force of the right
were made at the time at which peak responses ventricle were examined in the absence and presence
occurred and then at 5-minute intervals thereafter of bilateral stellate ganglionectomy. These animals
until cardiorespiratory activity had returned to the were ventilated artificially with room air. A cali-
predrug level or had stabilized at a new level. brated strain gauge arch (Warren Research Prod-
ucts, Charleston, S.C.) was sewn onto the right
Ventral Surface Application of Drugs ventricle and stretched by 50% of its initial length.
To apply drugs on the intermediate area of the Right ventricular force was measured because
ventrolateral surface of the medulla, the following changes in systemic pressures have relatively less
procedures were performed. After a longitudinal effect on right than on left ventricular diastolic fiber
midline incision was made in the neck, the trachea length.11,12 The leads of the arch were connected to a
and esophagus were transected and retracted ceph- preamplifier (model 7P1A, Grass Instrument Co.,
alad. The prevertebral muscles were bluntly dis- Quincy, Mass.), which was coupled to a dynograph
sected away from the base of the skull, which then recorder (model R611, Sensormedic, Anaheim,
was removed. The dura and arachnoid membranes Calif.). A bipolar electrode was sutured onto the
then were removed. Cottonoid wicks were placed on right atrial appendage. The electrode served as a
all edges of the craniotomy to prevent excessive pacing electrode and was connected to a stimulator
pooling of cerebrospinal fluid during the experiment (Grass Instruments). Blood pressure and lead II of
and, thereby, prevented excessive dilution and spread the electrocardiogram were monitored on a recorder
of the drug solutions. (model 220, Gould Instruments, Cleveland).
Drugs were applied to the intermediate area of the
ventral surface of the medulla with 2-mm-diameter Microinjection Technique
pledgets of No. 2 filter paper that were soaked with 2 The ventral surface of the medulla was exposed in
,ul of drug solutions. Additional details of the proce- the same manner described above. Double-barreled
dure and location of drug application have been micropipettes (i.d., 0.3 mm; FHC, Inc., Brunswick,
described previously.10 Administration of 2 ,ul of drug Me.) were pulled with a vertical pipette-puller (model
or control solutions resulted in saturation of the 700C, David Kopf Instruments, Tujunga, Calif.), and
Downloaded from http://circres.ahajournals.org/ by guest on September 7, 2015Mandal et al 5-HT2 Receptors and Sympathetic Outflow 1269 the tips were cut to approximately 15 ,um i.d. The a bright green area; in some cases, tissue damage also pipette was then mounted on a David Kopf electrode marked the micropipette track. manipulator. Each barrel of the pipette was connected to polyethylene tubing and filled using negative pres- Drugs Used and Drug Preparation sure. One barrel was filled with a 100 mM solution of a-Chloralose was obtained from ICN Biomedical, L-glUtamic acid, while the other was filled with DOI, Cleveland, and dissolved in 30 ml heated 0.9% saline. which also contained a 1% solution of fast green DOI was obtained from Research Biochemicals, Inc., (Sigma Chemical Co., St. Louis) for subsequent histo- Natick, Mass., and was dissolved in 0.9% saline. Ket- logical verification of the injection site. anserin tartrate was obtained from Research Biochem- Two microinjection sites were selected in evaluat- icals and was dissolved in distilled water. N-Methyl-D- ing the site of action of DOI. One of these sites was aspartic acid was obtained from Sigma Chemical Co., the locus where microinjections of L-glutamic acid St. Louis, and was dissolved in 0.9% saline. Prazosin produce an increase in blood pressure, namely, the was obtained from Sigma and was dissolved in ethanol nucleus reticularis rostroventrolateralis.13 In that and dimethyl sulfoxide and diluted with distilled water. study, a volume of 200 nl made up the microinjectate. Phentolamine (Regitine) was obtained from CIBA Since then, our laboratory has observed effects with Pharmaceutical Co., Edison, N.J., and was dissolved in volumes of microinjectate as low as 10 nl. With this 0.9% saline. Propranolol was obtained from Ayerst reduced volume, we have been able to localize better Laboratories, New York, and was dissolved in 0.9% this "L-glutamic acid-sensitive site." This site also saline. Sodium nitroprusside was obtained from Elkins- has been described by McAllen14 as the subretrofa- Sinn, Inc., Cherry Hill, N.J., and was reconstituted and cial nucleus (SRFn). The coordinates used for micro- diluted in 5% dextrose. injection into the SRFn were 6.0-7.0 mm caudal to the foramen cecum, 4.0 mm lateral to the midline, Statistical Analysis and 1.0-1.5 mm below the ventral surface. With Data were analyzed using the two-tailed paired t these coordinates, a micropipette was placed on each test, the two-tailed grouped t test, or one-way analysis side. Then, using a series of pressure injections (30 of variance with Duncan's multiple range test. The psi, 400-1,000 msec in duration) with a pneumatic criterion for significance was p
1270 Circulation Research Vol 67, No 5, November 1990
TABLE 1. Effect of Bilateral Application of 1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on the Intermediate Area of the
Ventral Surface of the Medulla on Cardiorespiratory Function
Mean
blood Tidal Respiratory Respiratory
Experimental Experimental pressure Heart rate volume rate minute volume
group n condition (mm Hg) (beats/min) (ml) (breaths/min) (ml/min)
Control 8 Control (initial 110+11 209± 12 42+4 12±1 536+65
(DOI 30 values)
,ug/side) Maximal change +39+7* +8+5 +12±5* -5+0* -163+38*
Ketanserin 7 Baseline values 116+7 172+12 27+1 12+2 316+44
pretreatment after ketanserin
(25 rg/side) Maximal change +10±3*t 0+2 +1±1 -1±It -31---22t
produced by
DOI (30
,g.g/side)
Values are expressed as mean+SEM.
*pMandal et al 5-HT2 Receptors and Sympathetic Outflow 1271
Tidal
30
P1
Volume / / 1,!!
(ml) 11
11 1
1 aec 1 whi
Tracheal
Air Flow
200_
Blood L NNNNNNNN K>NNNKN1272 Circulation Research Vol 67, No 5, November 1990
TABLE 2. Effect of Ketanserin Pretreatment on the Cardiorespiratory Effects Elicited by Bilateral Application of N-Methyl-D-aspartic Acid
(NMDA) to the Intermediate Area of the Ventral Surface of the Medulla
Mean blood Tidal Respiratory Respiratory
Experimental Experimental pressure Heart rate volume rate minute volume
group n condition (mm Hg) (beats/min) (ml) (breaths/min) (ml/min)
Control 3 Control (initial 101+3 195+9 23+2 14+2 319+53
(NMDA 11 values)
,rg/side) Maximal change +45 +2* +31±+ 6* -1±+-2 0+1 -17+13
Ketanserin 4 Baseline values after 101+1 172+12 27+1 16+2 433+56
pretreatment ketanserin
(25 pg/side) Maximal change +42±12* +25+16 +12±6 +1+1 +239±130
produced by
NMDA
Values are expressed as mean±SEM.
*pMandal et al 5-HT2 Receptors and Sympathetic Outflow 1273
TABLE 4. Effect of Various Pharmacological and Surgical Interventions on Cardiorespiratory Effects of 1-(2,5-Dimethoxy-4-iodophenyl)-
2-aminopropane (DOI) Applied to the Intermediate Area of the Ventral Surface of the Medulla
Mean
blood Tidal Respiratory Respiratory
Experimental Experimental pressure Heart rate volume rate minute volume
group n condition (mm Hg) (beats/mia) (ml) (breaths/min) (ml/min)
Bilateral cervical 3 Baseline values after 87±11 174±7 34±8 11±2 317±39
vagotomy vagotomy
Maximal change +41±6* -6±9 +1±2 -1±1t -40±32
produced by DOI
(30 pg/side)
Phentolamine 3 Baseline values after 68±13 250±0 38±2 19±4 690±134
pretreatment phentolamine
(5 mg/kg) Maximal change +31±5* -18±8 +1±2 -8±1* -290±64*
produced by DOI
(30 pg/side)
Propranolol 5 Baseline values after 128±12 179±7 33±4 16±2 560±131
pretreatment propranolol
(1 mg/kg i.v.) Maximal change +11±3*t -13±7 -2±2 -7±2 -271±112
produced by DOI
(30 ,g/side)
Bilateral stellate 3 Baseline values after 76±11 156±13
ganglionectomy ganglionectomy
Maximal change +18±4*t +4±3
produced by DOI
(30 ,g/side)
Values are expressed as mean±SEM.
*p1274 Circulation Research Vol 67, No 5, November 1990
Stellate Gaanglia Intact.
* Stollate Ganglia Removed.
6- pMandal et al 5-HT2 Receptors and Sympathetic Outflow 1275
TABLE 5. Effect of Ketanserin Pretreatment (i.e., Ketanserin Applied to the Intermediate Area) and Propranolol Pretreatment (i.e.,
Propranolol Administered Intravenously) on Cardiorespiratory Effects of 1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI) Admin-
istered Intravenously
Mean blood Tidal Respiratory Respiratory
Experimental pressure Heart rate volume rate minute volume
Experimental group n condition (mm Hg) (beats/min) (ml) (breaths/min) (ml/min)
DOI (100 ,g/kg i.v.) 3 Control (initial 142±4 210±+16 36±+10 12±1 449± 124
values)
Maximal change +33±2* -8+12 -11±10 +1±1 -116±146
Animals pretreated 2 Control (values after 64±4 159±20 31±0 13±0 382±7
with ketanserin ketanserin)
(25 pg/side) and
givenDOI (100 Maximalchange +47+2 -9±2 -7±2 +6±1 +80±10
,ug/kg i.v.)
Animals pretreated 3 Control (values after 98±30 127±13 31±8 13±5 312±58
with propranolol propranolol)
(1 mg/kg i.v.) and
given DOI (100 Maximal change +38±8* -12±10 -8±0* 0±1 -80±54
pg/kg i.v.)
Values are expressed as mean±SEM.
*p1276 Circulation Research Vol 67, No 5, November 1990
LL 200i
-
1 00 _
_Oco EEE
m1°0cc
Lu
leIn
1 sec.
0 1 min.
HR: 194 HR: 200
(D
01 Mm-my~
20 ni OF L-GLUTAMIC ACID 20 nl OF L-GLUTAMIC ACID
ON LEFT SIDE ON RIGHT SIDE
200
U) 1_
O u) E
H : 200
w E
mX CC _
HR: 200 HR: 2 14
llJ
.i, -J, ,,, - ._ 1 .- - .-.
1
1 ug OF DOI ON LEFT SIDE 1 ug OF DOI ON RIGHT SIDE
FIGURE 5. Representative experiment showing the use of microinjection ofL-glutamic acid to locate the subretrofacial nucleus on
each side (top panel) and the effects of bilateral microinjection of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI, 1
pg/side) into the subretrofacial nucleus on blood pressure and heart rate (HR, bottom panel).
would counteract the pressor effect of intravenously sequence of microinjections is shown in Figure 5. As
administered DOI. Data obtained from three animals can be seen, microinjection of L-glutamic acid on
are shown in Table 5 and indicate that propranolol each side evoked an immediate increase in blood
pretreatnent does not counteract the pressor effect pressure, which was brief in duration. When arterial
produced by a bolus intravenous injection of DOI. blood pressure had stabilized, DOI was microin-
jected bilaterally into the same site in a dose of 1
Microinjections of DOI Into the gg/site. Immediately after microinjection of DOI,
Subretrofacial Nucleus there was a striking increase in arterial pressure:
In an attempt to better localize the site of the mean blood pressure increased from 78 to 112
pressor effect of DOI within the ventrolateral me- mm Hg, producing an increase of 34 mm Hg. The
dulla, this agent was microinjected into a major location of the micropipette tips in this experiment is
nucleus that influences central sympathetic outflow illustrated in Figure 6. As can be seen, the injection
and that can be reached by diffusion of drug initially sites were within the SRFn.
placed on the intermediate area of the ventrolateral In subsequent experiments, lower doses of DOI
medulla. This nucleus is the SRFn.14 Coordinates were microinjected into the SRFn of cats that had
have been established for locating this nucleus (see undergone cervical vagotomy. The doses were 100
"Materials and Methods"). Along with these coordi- and 300 ng/site, and the data obtained are shown in
nates, an additional guide for locating the SRFn is to Table 6. These two doses of DOI appeared to
find the site where L-glutamic acid microinjection produce a dose-related increase in mean arterial
produces a rapid and marked increase in arterial pressure. The response observed with the 300 ng/site
blood pressure. Hence, these experiments were con- microinjection was similar in magnitude to that ob-
ducted first by microinjecting 20-60 nl of a 100 mM served after topical application of 30 jig/side DOI to
solution of L-glutamic acid as a test agent for locating the intermediate area (compare data in Table 6 with
the SRFn on each side. After the blood pressure and data in Table 1). The increases in mean arterial
heart rate had returned to baseline values, the effects pressure produced by microinjection of 100 and 300
of bilateral microinjections of DOI were then deter- ng/site were not associated with significant changes
mined. A representative experiment illustrating this in heart rate (Table 6).
Downloaded from http://circres.ahajournals.org/ by guest on September 7, 2015.t it.t
1278 Circulation Research Vol 67, No 5, November 1990
applied topically to the intermediate area of the ventral with the intermediate area. The major nucleus asso-
surface of the medulla. These data also are summarized ciated with the intermediate area and shown to
in Table 6 and indicate that topical application of 100, influence sympathetic outflow in the cat is the
300, and 1,000 ng/side (i.e., 1 ,ug/side) had no signifi- SRFn.14 Hence, experiments were performed to de-
cant effect on mean arterial blood pressure. termine whether the pressor effect of topically ap-
plied DOI was due to diffusion to this nucleus. This
Discussion was tested by microinjection of DOI bilaterally into
The major new findings from our study are as the SRFn in doses that were inactive after topical
follows: 1) stimulation of 5-HT2 receptors in the application to the intermediate area and then observ-
ventrolateral medulla, presumably in the SRFn, in- ing the same spectrum of cardiovascular effects that
creases sympathetic outflow, resulting in an increase were obtained after topical application of DOI to the
in arterial blood pressure; and 2) activation of 5 -HT2 intermediate area in a dose of 30 ,ug/side. In contrast,
receptors in the ventrolateral medulla results in a bilateral microinjections of 300 ng/site DOI into the
nonuniform increase in sympathetic outflow. The parapyramidal region had no effect on blood pres-
outflow appears to be increased primarily to ventric- sure. As mentioned earlier, the parapyramidal region
ular muscle with either no or relatively minor also has been shown to influence sympathetic out-
changes in outflow to the sinus node and vascular flow.17-19 Our data strongly indicate that the site of
beds. action of DOI to increase sympathetic outflow and
Evidence that activation of 5-HT2 receptors in the raise arterial blood pressure is the subretrofacial
ventrolateral medulla increases sympathetic outflow, nucleus, not the parapyramidal region.
resulting in an increase in arterial blood pressure, was Evidence that activation of 5-HT2 receptors in the
obtained initially in our previous studies with 5 -HT ventrolateral medulla results in a nonuniform in-
applied to the intermediate area of the ventral sur- crease in sympathetic outflow is based on several
face of the medulla.1 In that study, topical applica- findings, and they are as follows. 1) Both topical
tion of 5-HT to the intermediate area produced a application of DOI to the intermediate area and
small but statistically significant hypotensive effect. microinjection of DOI into the subretrofacial nucleus
By blocking 5 -HT2 receptors at this site, hypotension in vagotomized animals cause an increase in arterial
resulting from topical application of 5-HT was en- blood pressure without any significant change in
hanced significantly. These data implied that 5-HT heart rate, indicating that DOI increases sympathetic
was activating 5-HT2 receptors in the intermediate outflow but not to the sinus node. 2) The pressor
area, resulting in an increase in arterial blood pres- effect of DOI applied to the intermediate area is
sure. With the introduction of a specifically acting counteracted either by systemic propranolol pre-
agonist for the 5 -HT2 receptor, namely DOI, we were treatment or by bilateral stellate ganglionectomy but
able to test directly whether activation of this recep- not by systemic phentolamine pretreatment. These
tor subtype would result in an increase in arterial observations indicate that DOI increases sympathetic
blood pressure. Indeed, topical application of DOI to outflow to the heart (with the exception of the
the intermediate area did produce a consistent rise in sinoatrial node) but not to the vasculature. 3) The
arterial blood pressure. Similar results of DOI in pressor effect of topically applied DOI is associated
anesthetized cats recently have been reported by with an increase in cardiac contractile force, and
King and Holtman.21 bilateral stellate ganglionectomy counteracts both
Evidence that the effect of DOI was mediated DOI-induced increases in cardiac contractile force
through activation of 5 -HT2 receptors was obtained and in arterial blood pressure. This latter result
using the 5-HT2 receptor antagonist drug ketanserin.28 provides confirmatory evidence that DOI is selec-
Topical application of ketanserin to the intermediate tively increasing sympathetic outflow to the heart
area prevented the pressor effects of subsequent ad- (with the exception of the sinoatrial node). 4) In
ministration of DOI to the same site. The selectivity of contrast to the pressor effect of DOI, the pressor
ketanserin for blocking the 5-HT2 receptor was dem- effect of NMDA topically applied to the intermediate
onstrated by showing that a dose of ketanserin that area is associated with both an increase in heart rate
blocked the pressor effect of DOI did not block the and an increase in cardiac contractile force that is not
pressor effect of NMDA applied topically to the inter- counteracted by bilateral stellate ganglionectomy.
mediate area. Because ketanserin also blocks a1- This latter finding in addition to the earlier findings
adrenoceptors,24-26,28 it was important to show that the of McAllen29 indicates that activation of neurons in
interaction between ketanserin and DOI was not due the ventrolateral medulla (intermediate area) causes
to the a1-adrenoceptor blocking property of ketanserin. widespread increases in sympathetic outflow,
To this end, it was noted that the prototypic a1- whereas activation of 5-HT2 at the same site causes a
adrenoceptor antagonist, prazosin, did not antagonize selective increase in sympathetic outflow to the heart
the DOI-induced pressor response. (i.e., to the ventricles). Based on these findings, we
The lack of a pressor effect of DOI topically postulate that one reason neurons in the central
applied to either the rostral area or the caudal area nervous system can be selectively engaged to perform
of the ventral surface of the medulla indicated that a physiological function is because of the pattern of
DOI was exerting its effect at a nucleus associated distribution of neurotransmitter receptors on neu-
Downloaded from http://circres.ahajournals.org/ by guest on September 7, 2015Mandal et al 5-HT2 Receptors and Sympathetic Outtlow 1279
rons, such as the 5-HT2 receptor on certain neurons that this central hypotensive effect is due to the
within the SRFn. actions of ketanserin as an a,-adrenoceptor antago-
Other investigators have obtained data indicating nist rather than as a 5-HT2 receptor antagonist.36'37
that systemic administration of DOI can increase In addition, ketanserin has been shown to have
central sympathetic outflow3,4'30 and arterial blood relatively high affinity for receptors other than the
pressure.3,4,29-32 Heart rate was increased slightly in 5-HT2 receptor and the a1-adrenoceptor,2' and these
the initial study by McCall and colleagues,3 but not in other receptor effects might contribute to its central
the later study4 in which a similar dose range of DOI hypotensive action. Further studies with newer and
was used. Other investigators have reported that more selective 5 -HT2 antagonists, such as LY-
DOI, in doses that exert a pressor effect, does not 53857,25,39 and their effects at the ventrolateral me-
increase the heart rate.30-32 Most important, McCall dulla need to be performed to resolve this important
and colleagues3 reported that DOI increases sympa- question.
thetic outflow through a central nervous system site In our study, we also observed changes in respira-
of action. Their evidence was based on the finding tory activity after topical application of DOI to the
that DOI given intravenously increases sympathetic
discharge in preganglionic splanchnic fibers in intermediate area. Specifically, these changes were an
baroreceptor-denervated cats. In our study, we ob- increase in tidal volume and a decrease in respiratory
served that intravenously administered DOI in- rate. This decrease in respiratory rate has been de-
creased the arterial pressure. This finding raises the scribed in the study by King and Holtman.21 Previ-
question as to whether intravenously administered ously, we reported that topical application of drugs
drug acts at a site in the ventrolateral medulla to that activate 5--HT1A receptors to the intermediate
exert a pressor effect. To test this point, we repeated area decreases tidal volume and increases respiratory
our intravenous studies under conditions for which rate.1'40A4l Therefore, just as activation of 5-HT2 recep-
the pressor effect of topically applied DOI to the tors and activation of 5-HT1A receptors in the inter-
ventrolateral medulla was inhibited. These condi- mediate area have opposing effects on blood pressure,
tions were after topical application of ketanserin to activation of these two subtypes of serotonergic recep-
the intermediate area of the ventrolateral medulla tors also has opposing actions on respiratory activity.
and after intravenous administration of propranolol. Furthermore, the respiratory effects of topically ap-
Under both conditions, intravenously administered plied DOI were lost after microinjection of DOI in the
DOI produced a striking increase in arterial blood SRFn. Therefore, it appears that the central site of
pressure. Based on these findings, we conclude that action for the respiratory effects of DOI is not the
intravenous DOI is not exerting a pressor effect by SRFn but some other area. More studies need to be
stimulating 5 -HT2 receptors in the region of the performed to elucidate the site of DOI's respiratory
ventrolateral medulla. effects.
According to Dabire and colleagues,31 DOI pro- In summary, our data indicate that there is a central
duces a greater increase in arterial blood pressure in pathway for selective sympathoexcitation of the heart,
the pithed rat as compared with the intact rat. This leading to an increase in cardiac inotropy without an
result indicates that the central nervous system is not increase in cardiac chronotropy. The pathway for this
required for the action of intravenous DOI to raise selective sympathetic outflow to the cardiac ventricles is
arterial blood pressure. Indeed, it has been shown that mediated by activation of 5-HT2 receptors on neurons
there are 5-HT2 receptors on vascular tissue and that located within the ventrolateral medulla, most likely
activation of these receptors results in vasoconstric- within the SRFn. Although it has been shown that it is
tion.33-35 The question we have not addressed in our possible to selectively increase sympathetic outflow
study is whether the DOI-induced increase in central from the SRFn to different vascular beds,'4 to our
sympathetic outflow after systemic administration is knowledge, our report is the first that documents
due to an interaction of DOI with 5-HT2 receptors selective sympathoexcitation to different areas within a
located in the ventrolateral medulla. We suspect that specific organ, namely, the heart.
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cardiovascular activity and respiratory motor outflow to the
diaphragm and larynx. J Pharmacol Exp Ther 1990;252: KEY WoRDs * central sympathetic outflow * blood pressure
665-674 heart rate * serotonin receptors * subretrofacial nucleus
Downloaded from http://circres.ahajournals.org/ by guest on September 7, 2015Stimulation of serotonin2 receptors in the ventrolateral medulla of the cat results in
nonuniform increases in sympathetic outflow.
A K Mandal, K J Kellar, W P Norman and R A Gillis
Circ Res. 1990;67:1267-1280
doi: 10.1161/01.RES.67.5.1267
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