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THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 1997 by The American Society for Pharmacology and Experimental Therapeutics
JPET 280:162–173, 1997
Vol. 280, No. 1
Printed in U.S.A.
Discriminative Stimulus Effects of Zolpidem in PentobarbitalTrained Subjects: I. Comparison with Triazolam in Rhesus
Monkeys and Rats1
JAMES K. ROWLETT and WILLIAM L. WOOLVERTON
Department of Psychiatry and Human Behavior (J.K.R., W.L.W.) and Department of Pharmacology and Toxicology (W.L.W.), University of
Mississippi Medical Center, Jackson, Mississippi
Accepted for publication September 23, 1996
Zolpidem (AMBIEN®) is an imidazopyridine hypnotic drug
that acts at the BZ recognition site on the GABAA receptor/
chloride channel complex. The BZ site recognized by zolpidem is distinct from the site recognized by other positive
allosteric modulators of GABA function, e.g., pentobarbital
(see Lüddens et al., 1995; Sanger et al., 1994, for review).
Moreover, the receptor binding profile of zolpidem is distinct
from the profile of classic BZ agonists in several respects. For
example, binding sites for zolpidem show a different pattern
of distribution in the CNS, being in greater proportion in the
cerebellum than in other regions (Benavides et al., 1988,
1992, 1993; Dennis et al., 1988). This CNS distribution is
consistent with zolpidem being a selective ligand for the BZ1
subtype of the BZ receptor (see Lüddens et al., 1995; Sanger
et al., 1994, for review). Consistent with this notion, molecReceived for publication March 15, 1996.
1
This work was supported by National Institute on Drug Abuse Grant
DA-09139 and by the College on Problems of Drug Dependence.
and no drug-appropriate responding at the 45-min pretreatment time. In contrast, triazolam occasioned $80% pentobarbital-appropriate responding at 0.10 and 0.20 mg/kg. Both
zolpidem and triazolam produced dose-dependent decreases
in the rate of responding. The rate-decreasing effects of zolpidem were maximal at the 5-min pretreatment time and had
dissipated after the 45-min pretreatment time. Further studies
were conducted in rats to equate procedural variables between
the monkey and rat studies. When the FR was reduced from 10
to 1, zolpidem occasioned 26 to 62% pentobarbital-appropriate responding over a dose range of 1.0 to 6.0 mg/kg i.p. After
i.g. administration, zolpidem occasioned 100% drug-appropriate responding at the highest dose tested (6.0 mg/kg); however, only two of seven rats responded. Taken together, these
data raise the possibility of a species difference between nonhuman primates and rats in the pentobarbital-like discriminative
stimulus effects of zolpidem.
ular biological studies have shown that the affinity of zolpidem for binding to reconstituted receptors, in contrast to
typical BZs, depends on expression of a subunits of the
GABAA receptor/chloride channel complex. Thus, zolpidem
binds with highest affinity to receptors expressing a1 subunits (Ki 5 15–25 nM), which are thought to be associated
with the BZ1 receptor subtype (Hadingham et al., 1993;
Pritchett and Seeburg, 1990). In contrast, zolpidem binds
with intermediate to low affinity (Ki values ranging from 350
to .15,000 nM) to receptors expressing a2, a3 and a5 subunits, the latter three associated with BZ2 receptor subtypes
(Hadingham et al., 1993; Pritchett and Seeburg, 1990). Interestingly, recent in vivo binding data suggest a less selective profile in terms of CNS distribution for zolpidem (Byrnes
et al., 1992; Schmid et al., 1995). Moreover, the in vivo binding characteristics of zolpidem in baboon brain are different
from in vitro binding characteristics of zolpidem in rat brain,
with less evidence of heterogeneity for zolpidem binding oc-
ABBREVIATIONS: BZ, benzodiazepine; CNS, central nervous system; FR, fixed ratio; GABA, g-aminobutyric acid; i.g., intragastric.
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ABSTRACT
The present study compared the discriminative stimulus effects
of the imidazopyridine, zolpidem, with a triazolobenzodiazepine, triazolam, in pentobarbital-trained rhesus monkeys and
rats. Rhesus monkeys (n 5 4), trained to discriminate pentobarbital (10 mg/kg intragastric [i.g.]) from saline under a FR 1
discrete-trials shock avoidance procedure, were given zolpidem (0.10 –30 mg/kg i.g.) or triazolam (0.01– 0.3 mg/kg i.g.).
Both zolpidem and triazolam produced dose-dependent increases in pentobarbital-appropriate responding that reached
80% or greater at the highest doses tested. Zolpidem, but not
triazolam, increased latency to respond in a dose-dependent
manner. Sprague-Dawley rats (n 5 12), trained to discriminate
pentobarbital (8.0 mg/kg i.p.) from saline under a FR 10 schedule of food reinforcement, were given zolpidem (0.50 – 4.0
mg/kg i.p.; 5-, 15- and 45-min pretreatment) or triazolam
(0.025– 0.20 mg/kg i.p., 15-min pretreatment). Zolpidem occasioned intermediate drug-appropriate responding (maximum
group mean 5 46%) at the 5- and 15-min pretreatment times
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163
In addition to the results in lorazepam-trained baboons,
zolpidem occasioned full drug-appropriate responding in baboons trained to discriminate the barbiturate pentobarbital
from a no-drug condition (Griffiths et al., 1992). In a previous
study (Sanger and Zivkovic, 1986), pentobarbital produced
only partial drug-appropriate responding in zolpidemtrained rats. As mentioned, it has generally been found that
BZs occasion pentobarbital-appropriate responding up to
100% in a dose-dependent manner (Ator and Griffiths, 1983;
Herling et al., 1980; Jarbe, 1976; Nader et al., 1991; Overton,
1976; Winger and Herling, 1982; Woolverton and Nader,
1995). Because of the pharmacological selectivity of drug
discrimination procedures, these results suggest a common
mechanism of action of BZs and pentobarbital (Ator and
Griffiths, 1989). Therefore, the partial pentobarbital-appropriate responding observed in zolpidem-trained rats further
suggests a mechanism of action different from typical BZs. In
addition, this finding raises the possibility of a species difference between rodents and nonhuman primates in the ability of zolpidem to occasion drug-appropriate responding in
subjects trained to discriminate a classic sedative/hypnotic
agent, such as pentobarbital, a drug generally attributed
with having discriminative stimulus effects that are nonspecific with respect to sedative/hypnotic agents (Ator and Griffiths, 1989).
The purpose of the present study was to assess whether
pentobarbital-like discriminative stimulus effects of zolpidem vary as a function of species. To do so, the discriminative
stimulus effects of zolpidem were compared with a triazolobenzodiazepine, triazolam, in pentobarbital-trained rhesus
monkeys and rats. This study is part of a series of experiments designed to make cross-species comparisons with human subjects trained to discriminate pentobarbital from placebo (Rush et al., 1997). Thus, zolpidem was tested in this
series because, as noted above, its discriminative stimulus
effects in pentobarbital-trained subjects may vary as a function of species. Triazolam was included as a positive control,
because previous drug discrimination experiments conducted
in nonhuman primates and rats have shown that triazolam
occasions pentobarbital-appropriate responding (Ator and
Griffiths, 1989). Various doses of zolpidem and triazolam
were tested in rhesus monkeys and rats involved in ongoing
experiments in our laboratory, with two different experimental protocols for two-lever drug discrimination: FR 1 discretetrials shock avoidance (rhesus monkeys) and FR 10 food
reinforcement (rats). Finally, the possibility that differences
in the discriminative stimulus effects of zolpidem between
rhesus monkeys and rats were caused by procedural differences was explored by varying parameters of the protocol
used for rats. Thus, the discriminative stimulus effects of
zolpidem in pentobarbital-trained rats were assessed after
varying pretreatment times, after varying the schedule of
reinforcement, and after varying the route of administration.
These manipulations were conducted to equate various aspects of the two protocols used.
Methods
Rhesus Monkeys
Subjects. The subjects were four adult rhesus monkeys, three
males (8236, 8106, 8814) and one female (7976), weighing between
7.0 and 9.8 kg. At the time of the study, the monkeys had been
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curring in various brain structures in baboons (Schmid et al.,
1995). Resolution of these issues may occur with the characterization of as yet undiscovered BZ receptor subtypes. Nonetheless, zolpidem’s neurochemical profile remains unique
among the BZ ligands.
The profile of behavioral effects produced by zolpidem administration in rodents also is unique among BZ ligands.
Notably, the discriminative stimulus effects of zolpidem are
distinguishable from typical BZs in rats (Depoortere et al.,
1986; Sanger, 1987; Sanger et al., 1987; Sanger and Zivkovic,
1986, 1987). For example, in rats trained to discriminate 5
mg/kg chlordiazepoxide from saline, a high dose of zolpidem
(3 mg/kg) produced only partial chlordiazepoxide-appropriate
responding (i.e., greater than 20%, but less than 80%, drugappropriate responding) and often suppressed responding
completely (Depoortere et al., 1986; Sanger et al., 1987). In
contrast, both chlordiazepoxide and triazolam occasioned full
chlordiazepoxide-appropriate responding (i.e., greater than
80% drug-appropriate responding) in all rats tested (Depoortere et al., 1986; Sanger et al., 1987). Conversely, when zolpidem (2.0 mg/kg) was trained as a discriminative stimulus,
triazolam and chlordiazepoxide occasioned only partial drugappropriate responding, even at doses that significantly suppressed responding (Sanger and Zivkovic, 1986). In further
studies, the partial agonists CGS 9896 and ZK 91296 antagonized the discriminative stimulus effects of zolpidem, but
not chlordiazepoxide (Sanger and Zivkovic, 1987), whereas
the mixed agonist-antagonists Ro 16 – 6028 and Ro 17–1812
occasioned full drug-appropriate responding in rats trained
to discriminate chlordiazepoxide, but not zolpidem (Sanger,
1987). More recently, zolpidem was shown to occasion full
drug-appropriate responding at a low (0.32 mg/kg) but not
high (3.2 mg/kg) dose of midazolam in rats trained in a
three-choice procedure (Sannerud and Ator, 1995b). Chlordiazepoxide also occasioned full drug-appropriate responding
at the low, but not high, midazolam training dose, whereas
triazolam occasioned full drug-appropriate responding at
both training doses (Sannerud and Ator, 1995b). Taken together, these results suggest that the profile of discriminative stimulus effects of zolpidem often is distinguishable from
BZs such as triazolam and chlordiazepoxide, and further
raise the possibility of a unique profile of discriminative
stimulus effects for ligands with selectivity for the BZ1 receptor subtype.
In contrast to results obtained from drug discrimination
studies in rats, results from a recent study by Griffiths et al.
(1992), with nonhuman primates as subjects, suggested that
the discriminative stimulus effects of zolpidem were similar
to typical BZs. Zolpidem occasioned $80% drug-appropriate
responding in four of five baboons trained to discriminate
lorazepam from a no-drug condition (Griffiths et al., 1992). In
addition, zolpidem suppressed response rate in only one of
the four baboons at the highest dose tested (Griffiths et al.,
1992). In previous work by these authors (Ator and Griffiths,
1989), typical BZs (e.g., triazolam), with few exceptions, have
been shown to occasion full drug-appropriate responding in
lorazepam-trained baboons and to have relatively few effects
on rate of responding. These results with baboons suggest the
possibility of a species difference between rats and baboons,
also corroborated with data from other behavioral paradigms
(e.g., tolerance and withdrawal studies; cf. Griffiths et al.,
1992; Sanger et al., 1994).
Pentobarbital Stimulus and Zolpidem
164
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tion was the pentobarbital dose-response function which was determined only once in monkeys 7976 and 8106.
Drugs. Sodium pentobarbital (Sigma Chemical Co., St. Louis,
MO) was diluted with 0.9% saline to a final concentration of 40
mg/ml, from a stock solution (400 mg/ml) with a vehicle of propylene
glycol, 95% ethanol and water (4:1:5). Zolpidem tartrate (NIDA,
Baltimore, MD) was mixed in sterile water, and triazolam (NIDA,
Baltimore, MD) was prepared in a vehicle of Emulphor EL-620
(Alkamuls EL-620, Rhone Poulenc, Cranbury, NJ):95% ethanol (1:1).
Test solutions/suspensions were prepared immediately before the
session in which they were tested. Doses were varied by varying the
volume of the standard solution. Pentobarbital and BZs were given
intragastrically via nasogastric tube followed by a flush with 1 to 2
ml of saline to clear the tube.
Data analysis. The percentage of the total trials completed on the
pentobarbital-appropriate lever and the mean latency/trial (mean
time between the onset of a trial and a lever press, averaged across
trials) were calculated for test sessions for each subject. Data are
presented as individual subjects and mean percent drug-appropriate
trials for each drug. If drug doses were tested twice, data were
analyzed and presented as the average of the two determinations.
Full pentobarbital-appropriate responding was concluded if responding on drug-appropriate trials was 80% or more of total responding.
Saline-appropriate responding was concluded if responding on drugappropriate trials was 20% or less of total responding. Between 20
and 80% was considered partial drug-appropriate responding. To
assess reliable dose effects on latency, tests were planned a priori to
compare the latency at each dose with the saline test session latencies. Dunnett’s tests were used for these comparisons. The reported
statistics are q values, which are analogous to t values, but with use
of the within-subject residual error value as the error term of the
ratio [degrees of freedom 5 (treatment level 2 1) 3 (number of
subjects 2 1)]. The alpha level for these tests was P # .05.
Rats
Subjects. Twelve naive adult male Sprague-Dawley rats (Harlan
Industries, Indianapolis, IN), weighing between 275 and 300 g before
reducing their food, were used as subjects. The rats were singly
housed, and acclimated to the laboratory for approximately 1 week,
then food was reduced until the rats were maintained at approximately 80% of their free-feeding weights. Once training started, the
rats were fed 15 g of food after an experimental session and on days
when no sessions were conducted. Over the course of the experiment,
the weights of the rats gradually increased such that weights toward
the end of the experiments were approximately 130% of their initial
free-feeding weights. Water was available continuously in the individual home cages. A 12-hr light/dark cycle was maintained, and all
experimental sessions were conducted during the light phase of the
cycle. The sessions were conducted at noon each day.
Apparatus. Experimental sessions were conducted in eight identical rat operant chambers (Gerbrands, Arlington, MA). In each
chamber, two response levers were mounted on one wall (12.5 cm
apart, center to center; 10 cm above the floor) and a food receptacle
was located between them. A minimal downward force of 0.30 N was
required to activate each lever. Each chamber was illuminated at the
onset of the session by a single 6-W light located on the wall opposite
the two levers and by a white light located above each lever. Extraneous noise was diminished by enclosing each chamber in an insulated box and by operating a ventilation fan mounted on the outside
of each box. Data collection and experimental events were controlled
by a Macintosh II microcomputer, with custom software and interfaces.
Discrimination training. Rats initially were trained to press a
lever once (i.e., FR 1) to obtain a 40-mg food pellet (Noyes Co.,
Lancaster, NH) after an injection of 8.0 mg/kg i.p. pentobarbital or
1.0 ml/kg saline. For six rats, food pellets were delivered only after
responding on the right lever in the presence of pentobarbital; this
condition was reversed for the other six rats. Drug or saline were
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subjects in the present drug discrimination paradigm for approximately 4 years. During that time, they had been tested with various
BZs and psychomotor stimulants. The monkeys were housed individually in stainless steel cages in which water was continuously
available. They were fed 120 to 150 g of monkey chow after each
session and were given a chewable multiple vitamin tablet, formulated for children, 3 days/week. A 12-hr light/dark cycle was maintained, and all experimental sessions were conducted during the
light phase of the cycle.
Apparatus. During experimental sessions each monkey was
seated in a restraining chair (Plas Labs, Lansing, MI) and placed in
a wooden cubicle (175 cm high 3 85 cm wide 3 65 cm deep) containing two response levers mounted 100 cm above the floor with a
distance between the levers of 25 cm, from center to center of the
levers. A minimal downward force of 0.30 N was required to activate
each lever. A 40-W white house light was mounted on the ceiling. The
monkey’s feet were placed into shoes, the bottoms of which were
fitted with brass plates that could deliver electric shocks using a
shock generator (Model SG-903, BRS/LVE, Beltsville, MD). Programming and recording of experimental events were accomplished
with an Aim 65 microprocessor located in an adjacent room.
Discrimination training. The monkeys had been trained previously to discriminate pentobarbital from saline in a two-lever, discrete-trials shock avoidance procedure (FR 1). Each monkey was
placed in the chair, given an intragastric infusion (via nasogastric
tube) of saline or 10 mg/kg pentobarbital, then returned to the home
cage. Fifty-five minutes later the monkey was seated again in the
chair and placed in the chamber. After 5 min, the house lights and
lever lights were illuminated (trial) and responding on one lever
(correct lever) avoided electric shock (avoidance response) and extinguished the lights. Responding on the incorrect lever started a 2-s
changeover delay during which correct lever responding had no
consequence. If a correct lever response was not made within 5 s of
onset of the lights, an electric shock (250 ms duration, 7 mA intensity) was delivered; if a correct response was made within 2 s after
the first shock (escape response), the trial was terminated. Otherwise, a second shock was delivered and the trial ended automatically. Two consecutive trials in which a monkey failed to make an
avoidance or escape response automatically ended the session. Trials
were separated by a 30-s timeout, during which all lights in the
chamber were extinguished and responding was recorded but had no
programmed consequences. The session lasted 30 trials or 20 min,
whichever came first. The correct lever was determined by the infusion that was administered before the session. For two monkeys, the
left lever was correct after drug infusion and the right lever was
correct after saline infusions. This condition was reversed for the
other monkeys. The sequence of daily (5 days/week) sessions was
SDDSS, DSSDD (S, saline pretreatment; D, drug pretreatment).
Testing. When the first response of the session and at least 90%
of the total trials were completed on the correct lever for at least
seven of eight consecutive sessions, test sessions were added to this
sequence such that the first test session each week was preceded by
two training sessions, one with saline and one with drug pretreatment and the second test session of the week was preceded by either
saline or drug pretreatment (i.e., SDTST, DSTDT, where T denotes a
test session). Test sessions were identical with training sessions
except that the drug pretreatment was novel and a response on
either lever avoided or escaped shock delivery. In the event that
either criterion for stimulus control was not met during the training
sessions, the training sequence continued without test sessions until
the criteria were met for seven of eight sessions.
In test sessions, pentobarbital (3.0–10 mg/kg i.g., 60-min presession) and saline (i.g., 60-min presession) were determined first, followed by dose-response determinations for triazolam (0.01–0.3
mg/kg i.g., 60-min presession) and zolpidem (0.10–30 mg/kg i.g.,
60-min presession). Doses were tested in a nonsystematic order. The
effects of doses were generally determined twice. The primary excep-
Vol. 280
1997
165
gavage, by use of a stainless steel gavage tube, 15 min before the
session. Thus, the rats were given the i.g. dose, returned to the home
cage, then placed in the chambers for the 5-min timeout followed by
the 15-min session. Each dose of pentobarbital and zolpidem was
determined twice.
Drugs. Pentobarbital was diluted from Nembutal® (50 mg/ml
pentobarbital, 40% propylene glycol, 10% ethanol, 50% sterile water;
Abbott Laboratories, Chicago, IL) in saline every 2 days, and injected
at a volume of 1.0 ml/kg for both i.p. and i.g. doses. Triazolam was
suspended each day of testing in a Tween 80 solution (10% Tween 80,
90% sterile, distilled water). Zolpidem tartrate was dissolved each
day of testing in saline, and injected in a volume of 2.0 ml/kg (i.p.) or
3.0 ml/kg (i.g. and 6.0 mg/kg i.p).
Data analysis. For all drug doses that were tested twice, data
were analyzed and presented as the average of the two determinations. Drug-appropriate responding was expressed as the percent of
responding on the drug-appropriate lever out of the total number of
responses. Drug lever responding was calculated in the FR 10 studies only if the rat completed a response requirement. Rate of responding was calculated by dividing the total number of responses
for a session by the session time of 900 s. These data were calculated
even if a rat did not complete a response requirement. Response rate
was not calculated in the FR 1 study; instead total number of responses in the session were calculated (latency data were not available).
For pentobarbital, triazolam and the zolpidem time course, the
data are presented as individual rats and mean percent drug lever
responding. For the FR 1 and i.g. studies, means and ranges are
presented. Full pentobarbital-appropriate responding was concluded
if responding on the drug lever was 80% or more of total responding.
Saline-appropriate responding was concluded if responding on the
drug lever was 20% or less of total responding. Between 20 and 80%
was considered partial pentobarbital-appropriate responding. To assess reliable dose effects on rate (pentobarbital, triazolam, zolpidem
time course and i.g. studies) and total number of responses (FR 1
study), tests were planned a priori to compare the rate at each dose
with the saline test session rate. Dunnett’s tests were used for these
comparisons as described above for latency measures in the rhesus
monkey studies.
Dose (milligrams per body surface area) and Relative
Dose Calculations
To compare doses across species, the doses based on body weight
were converted to doses based on body surface area, according to
Dews (1976). Area (A) was estimated as the 2/3 power of weight, and
relative doses were calculated by dividing dose in milligrams per
kilogram by dose in mg/kg2/3, according to the formula: relative
dose 5 dose/(wtzdose/kg2/3), given A } kg2/3. In addition, pentobarbital, zolpidem and triazolam doses from the companion manuscript
that used humans as subjects (Rush et al., 1997) were converted to
relative doses for comparison with rhesus monkeys and rats.
Results
Rhesus Monkeys
Pentobarbital. As can be seen in the top panel of figure 1,
pentobarbital produced a dose-dependent increase in drugappropriate responding (percent drug-appropriate trials) in
the four monkeys. That is, percent drug-appropriate trials
was at or near zero percent for the four monkeys when tested
with saline or 3.0 mg/kg pentobarbital. The mean percent
drug-appropriate trials was 50% at 5.6 mg/kg pentobarbital,
reflecting 50% responding in two monkeys, 100% responding
in one monkey, and 0% responding in the other monkey. The
50% responding in the two monkeys reflected an average of 0
and 100% responding in double determinations of this dose.
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administered 15 min before the 15-min session. Ten minutes after
the injections, the rats were placed in the chamber for the remaining
5 min of the pretreatment time, during which the lever lights were
extinguished (timeout). The trial started when the lever lights were
illuminated. There was no timeout after pellet delivery during the
trial. Lever pressing by six rats was shaped initially by differentially
reinforcing successive approximations after pentobarbital injection;
lever pressing by the remaining six was shaped after saline injection.
As soon as FR 1 responding was acquired, or the rat had received
three consecutive daily injections of drug or saline, the rat was
placed on a double alternation schedule and the FR gradually increased to FR 10. Responding on the inappropriate lever reset the
response requirement on the appropriate lever. Once FR 10 responding was obtained, rats were trained 5 days a week with a daily
injection schedule of SDDSS, DSSDD (S, saline; D, pentobarbital).
The criteria for stimulus control were: 1) 90% of responding in a
session on the appropriate lever for 7 of 8 days, and 2) 90% of
responding on the appropriate lever before the first food pellet delivery for 7 of 8 days.
Testing. Once a rat reached criteria, test sessions were run in
which 10 responses on either lever resulted in food pellet delivery.
Tests were conducted twice a week according to the schedule SDTST,
DSTDT (T, test); if responding was not on criteria during the training sessions, the rats were returned to training sessions with no test
sessions until criteria were again met for seven of eight sessions. The
first test session consisted of a pentobarbital training dose test (8.0
mg/kg pentobarbital i.p.), and the second test session consisted of a
saline test (1 ml/kg i.p.). Next, four doses of pentobarbital (1.0, 2.0,
4.0, 16 mg/kg i.p., 15-min pretreatment) were tested in nonsystematic order. The pentobarbital dose-response function was determined
first in all rats.
Once pentobarbital dose-response functions were established in
the rats, eight rats were tested with either zolpidem or triazolam.
Four rats received zolpidem doses first, followed by triazolam,
whereas for the other four rats this sequence was reversed. Triazolam (0.025, 0.050, 0.10 and 0.20 mg/kg) was administered i.p. 15 min
before the session, with the same test procedure used to test pentobarbital. Initially, zolpidem (0.50, 1.0, 2.0 and 4.0 mg/kg) also was
administered i.p. 15 min before the session. For both drugs, the doses
were tested in a nonsystematic order. Most doses of pentobarbital,
triazolam and zolpidem were tested twice, and the data are presented as the mean of two determinations or the single determination.
Zolpidem time-course study. After testing with triazolam and
zolpidem as described above, the four doses of zolpidem were retested at either 5 or 45 min before the session. For the 5-min pretreatment, the rats were injected with a zolpidem dose and placed
immediately into the chambers for the 5-min timeout, followed by
the 15-min session. For the 45-min pretreatment, the rats were
injected and returned to the home cage for 40 min, after which they
were placed in the chamber for the 5-min timeout and 15-min session. Thus, the time course consisted of 5-, 15- (obtained previously)
and 45-min pretreatment times. The doses for the 5-min pretreatment time were determined twice in eight rats, whereas the doses at
the 45-min pretreatment time were determined once in four rats.
Zolpidem FR 1 study. Five rats, four of which were tested in the
previous conditions, were tested with pentobarbital (8.0 mg/kg i.p.),
saline and zolpidem (1.0, 2.0 and 4.0 mg/kg) under a FR 1 schedule.
These tests were conducted with a 15-min pretreatment time (pentobarbital, saline) or a 5-min pretreatment time (zolpidem, saline) as
described above, except that the FR was lowered from 10 to 1 during
the test sessions only. Each zolpidem dose was determined twice.
Zolpidem i.g. study. Three doses of pentobarbital (4.0, 8.0 and 16
mg/kg) and four doses of zolpidem (1.0, 2.0, 4.0 and 6.0 mg/kg), as
well as saline, were tested via the i.g. route in the four rats not tested
with zolpidem or triazolam in the initial FR 10 studies (one rat was
tested in the FR 1 study), as well as rats R1 to R4 that were tested
with zolpidem and triazolam. The i.g. doses were administered by
Pentobarbital Stimulus and Zolpidem
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Rowlett and Woolverton
At the training dose of 10 mg/kg pentobarbital, all monkeys
responded at or near 100% drug-appropriate trials.
As can be seen in the bottom panel of figure 1, pentobarbital dose-dependently increased latency to respond in the
four monkeys. Dunnett’s tests revealed a reliable difference
between saline latencies and 10 mg/kg pentobarbital only
[q(9) 5 4.665, P , .05].
Zolpidem. As can be seen in the left top panel of figure 2,
zolpidem produced a dose-dependent increase in drug-appropriate responding (percent drug-appropriate trials) in the
four monkeys. As with pentobarbital, some intermediate
drug-appropriate responding after zolpidem represented
twice-determined points in which one determination was at
or near 0% and the second was at or near 100% (3.0 mg/kg
zolpidem in subject 8814, 17 mg/kg zolpidem in subject 7976).
Intermediate responding observed for monkey 8814 at 10
mg/kg zolpidem reflected intermediate responding on the
levers during the test sessions for both determinations,
rather than the average of near 0% and near 100% drugappropriate responding. Compared with pentobarbital, there
was more variation in the dose range of zolpidem that produced dose-dependent drug-appropriate responding. Thus, in
monkey 8106, zolpidem occasioned 0 and 100% drug-appro-
priate responding at doses of 0.30 and 1.0 mg/kg, respectively. In contrast, $80% drug-appropriate responding was
observed at 17 mg/kg in monkey 8814 and only at 30 mg/kg in
the remaining two monkeys. Because monkey 8106 showed
this 30-fold difference in doses of zolpidem that occasioned
full drug-appropriate responding, this monkey was omitted
from analyses of the zolpidem data.
As can be seen in the bottom left panel of figure 2, zolpidem
dose-dependently increased latency to respond in monkeys
8814, 7976 and 8236. Dunnett’s tests performed on the latency data of these three monkeys (doses 3.0–17 mg/kg only,
30 mg/kg was not tested in monkey 8814) revealed a reliable
difference between saline latencies and 17 mg/kg zolpidem
[q(6) 5 3.15, P , .05].
Triazolam. As can be seen in the right top panel of figure
2, triazolam produced a dose-dependent increase in drugappropriate responding (percent drug-appropriate trials) in
the four monkeys. As with pentobarbital and zolpidem, some
intermediate responding at or near 50% drug-appropriate
trials represented the average of 0% or near 0% and 100% or
near 100% drug-appropriate trials after double determinations. Intermediate responding that reflected intermediate
distribution of completed trials occurred for subjects 8106,
7976 and 8814 at 0.10 mg/kg of triazolam. Compared with
zolpidem, there was less variation in the dose range of triazolam that produced dose-dependent pentobarbital-appropriate responding. Thus, in the monkey in which zolpidem occasioned 100% pentobarbital-appropriate responding at a 30fold lower dose than the other monkeys (monkey 8106),
triazolam occasioned 40 and 96% drug-appropriate responding at doses of 0.1 and 0.3 mg/kg, respectively. This pattern
of results was similar to the other three monkeys, all of whom
showed $80% drug-appropriate responding at 0.3 mg/kg triazolam only.
As can be seen in the bottom right panel of figure 2,
triazolam appeared to slightly increase latency to respond
above saline levels at the two highest doses tested in the four
monkeys. However, Dunnett’s tests performed on the latency
data revealed no reliable differences between saline latencies
and the four doses of triazolam.
Rats
Pentobarbital. As can be seen in the top panel of figure 3,
pentobarbital produced a dose-dependent increase in drugappropriate responding (percent drug lever) in rats R1 to R8.
Pentobarbital dose-response data in rats R9 to R12 under the
FR 10/i.p. condition were not included in this study, but did
not differ from the results of rats R1 to R8. For the pentobarbital dose-response function, percent drug lever responding was at or near zero percent for all rats when tested with
saline, 1.0 and 2.0 mg/kg pentobarbital; but increased to 80 to
100% at 8.0 and 16 mg/kg pentobarbital. Intermediate responding was observed at 4.0 mg/kg (mean percent drug
lever 5 40%), largely because of five rats responding predominantly on the saline lever and three rats responding predominantly on the drug lever.
As can be seen in the bottom panel of figure 3, pentobarbital dose-dependently decreased response rate in the eight
rats. Dunnett’s tests revealed a reliable difference between
saline rate and 16 mg/kg pentobarbital only [q(35) 5 14.27,
P , .05].
Downloaded from jpet.aspetjournals.org at ASPET Journals on May 11, 2017
Fig. 1. The effects of pentobarbital in monkeys trained to discriminate
10 mg/kg pentobarbital from saline. (Top panel) The percentage of trials
completed on the pentobarbital-appropriate lever during test sessions
in which responding on either lever avoided electric shock. (Bottom
panel) The average latency (seconds) to respond in test sessions.
(Abscissae) Dose of pentobarbital in mg/kg given intragastrically via
nasogastric tube. Each point represents data from individual monkeys
identified in the symbol key. Solid lines through the points represent
lines connecting the group means. Each point represents the mean of
two determinations, except for monkeys 7976 and 8106 in which each
point represents a single determination.
Vol. 280
1997
Pentobarbital Stimulus and Zolpidem
167
Zolpidem. As can be seen in the three top panels of figure
4, zolpidem produced inconsistent drug-appropriate responding (percent drug lever) at the 5-min (top left panel) and
15-min (top middle panel) pretreatment times, and no drugappropriate responding at the 45-min pretreatment time (top
right panel). Drug-appropriate responding generally was not
dose-dependent and showed considerable variability among
subjects. For example, at the 15-min pretreatment time, rat
R3 (open triangles) responded exclusively on the saline lever
at each dose tested, whereas rat R6 (filled squares) responded exclusively on the drug lever at each dose. Other
rats demonstrated no pentobarbital-appropriate responding
at some doses and $80% pentobarbital-appropriate responding at other doses; but, in general, no consistent effects were
observed as a function of zolpidem dose. The mean percent
drug lever values at the 5- and 15-min pretreatment times
ranged from 13% to 46%, whereas drug lever responding at
all doses in the four rats tested was at 0% at the 45-min
pretreatment time.
As the dose of zolpidem was increased, the number of rats
not completing a response requirement during the 15-min
session also increased. These rats were consequently not
included in the analyses of percent drug lever. This effect was
most pronounced at the 5-min pretreatment time (fig. 4, top
left panel). Thus, 4 of 8 rats did not complete a response
requirement at 2.0 mg/kg zolpidem and 8 of 8 rats did not
complete a response requirement at 4.0 mg/kg zolpidem. At
the 15-min pretreatment time, 5 of 8 rats did not complete a
response requirement at 4.0 mg/kg, whereas all rats completed at least one response requirement at the lower doses.
At the 45-min pretreatment time, 4 of 4 rats completed at
least one response requirement at the four doses.
As can be seen in the bottom panels of figure 4, zolpidem
appeared to produce a profound dose-dependent decrease in
response rate. Dunnett’s tests revealed a reliable difference
between saline rate and doses above 0.50 mg/kg at the 5-min
pretreatment time [1.0 mg/kg, q(28) 5 10.42; 2.0 mg/kg, q(28)
Fig. 3. The effects of pentobarbital in rats (n 5 8) trained to discriminate 8.0 mg/kg pentobarbital from saline. (Top panel) The percentage
of responses completed on the pentobarbital-appropriate lever during
test sessions in which responding on either lever resulted in food pellet
delivery. (Bottom panel) The average response rate (responses/second)
of responding on either lever in test sessions. (Abscissae) Dose of
pentobarbital in mg/kg given via the i.p. route. Each point represents
data from individual rats identified in the symbol key. Solid lines through
the points represent lines connecting the group means. Each point
represents the mean of two determinations.
Downloaded from jpet.aspetjournals.org at ASPET Journals on May 11, 2017
Fig. 2. The effects of zolpidem (left panels)
and triazolam (right panels) in monkeys trained
to discriminate 10 mg/kg pentobarbital from
saline. (Top panels) The percentage of trials
completed on the pentobarbital-appropriate
lever during test sessions in which responding
on either lever avoided electric shock. (Bottom
panels) The average latency (seconds) to respond in test sessions. (Abscissae) Dose of
zolpidem or triazolam in mg/kg given intragastrically via nasogastric tube. Each point represents data from individual monkeys identified
in the symbol key. Solid lines through the
points represent lines connecting the group
means. The long-dashed line in the top and
bottom left panels represents data from one
monkey that were not included in the group
mean calculation. The horizontal short-dashed
lines represent the range of pentobarbital-appropriate responding (top panels) and latencies (bottom panels) after saline tests. Each
point represents the mean of two determinations.
168
Rowlett and Woolverton
Vol. 280
5 13.74; 4.0 mg/kg, q(28) 5 14.16; all P values , .05]. At the
15-min pretreatment time, the 2.0 and 4.0 mg/kg groups were
reliably lower than saline [q(28) 5 6.81, P , .05 and q(28) 5
9.51, P , .05, respectively], whereas the other doses were not
reliably different from saline. No doses were reliably different from saline at the 45-min pretreatment time.
Triazolam. As can be seen in the top panel of figure 5,
triazolam produced a dose-dependent increase in drug-appropriate responding (percent drug lever) in rats R1 to R8. That
is, percent drug lever responding was at or near zero percent
for all rats when tested with vehicle and 0.025 mg/kg triazolam, but increased to 70 to 100% at 0.20 mg/kg triazolam,
with all but one rat (R8) showing $80% pentobarbital-appropriate responding at the highest dose. Intermediate responding was observed at 0.050 mg/kg because of four rats responding predominantly on the saline lever and four rats
responding predominantly on the drug lever. At 0.10 mg/kg,
all rats showed full drug-appropriate responding except rat
R7, which responded entirely on the saline lever.
As can be seen in the bottom panel of figure 5, triazolam
dose-dependently decreased response rate in the eight rats.
Dunnett’s tests revealed a reliable difference comparing both
0.10 mg/kg triazolam [q(28) 5 5.06, P , .05] and 0.20 mg/kg
triazolam [q(28) 5 8.22, P , .05] with vehicle.
Pentobarbital and zolpidem, FR 1. As can be seen in
table 1, 8.0 mg/kg pentobarbital occasioned 100% drug-ap-
propriate responding (percent drug lever), when tested under
a FR 1 schedule of reinforcement. Saline occasioned 0% drugappropriate responding when administered at the same pretreatment time as pentobarbital. Mean total responding was
not reliably different for pentobarbital compared with saline.
When zolpidem was administered 5 min before the session
and tested under the FR 1 schedule, the pattern of results
observed was similar to that obtained under the FR 10 schedule. Thus, zolpidem did not produce a dose-dependent increase in drug-appropriate responding over the dose range of
1.0 to 4.0 mg/kg i.p. In contrast to the FR 10 schedule,
however, all rats responded and obtained reinforcers at every
dose. Because of this increase in response output, a higher
dose of zolpidem was tested (6.0 mg/kg i.p.). This higher dose
of zolpidem occasioned a mean percent drug lever of 62%. Of
the five rats tested, one rat did not respond, two rats responded exclusively on the drug lever and two rats responded
on both levers (93% and 13% drug-lever responding). The
total responses obtained after 4.0 and 6.0 mg/kg zolpidem
were reliably lower than obtained after saline administered 5
min before the session [q(12) 5 5.40, P , .05 and q(12) 5
5.36, P , .05, respectively; data analyzed from n 5 4 rats].
Pentobarbital and zolpidem i.g. As can be seen in table
2, pentobarbital, administered i.g. and tested under a FR 10
schedule, occasioned a dose-dependent increase in drug-appropriate responding (percent drug lever). Dunnett’s tests
Downloaded from jpet.aspetjournals.org at ASPET Journals on May 11, 2017
Fig. 4. The effects of zolpidem in rats trained to discriminate 8.0 mg/kg pentobarbital from saline. (Top panels) The percentage of responses
completed on the pentobarbital-appropriate lever during test sessions in which responding on either lever resulted in food pellet delivery. (Bottom
panels) The average response rate (responses/second) of responding on either lever in test sessions. (Left panels) Discriminative stimulus and rate
effects of zolpidem given 5 min before the session. (Middle panels) Discriminative stimulus and rate effects of zolpidem given 15 min before the
session. (Right panels) Discriminative stimulus and rate effects of zolpidem given 45 min before the session. (Abscissae) Dose of zolpidem in mg/kg
given via the i.p. route. Each point represents data from individual rats identified in the symbol key. Solid lines through the points represent lines
connecting the group means. Eight rats were tested at the 5- and 15-min pretreatment times, doses in which symbols are missing indicate that
the rat did not complete a FR (all rats were included in the rate calculations). Four rats were tested at the 45-min pretreatment time. For the 5and 15-min pretreatment times, each point represents the mean of two determinations. Each point at the 45-min pretreatment time represents a
single determination.
1997
169
Pentobarbital Stimulus and Zolpidem
TABLE 1
Discriminative stimulus effects of zolpidem, tested under a FR 1
schedule of reinforcement, in rats trained to discriminate
pentobarbital (8.0 mg/kg i.p., FR 10) from saline (1.0 ml/kg i.p.,
FR 10)
Druga Dose
mg/kg
Saline-15 min
Pentobarbital
8.0
Saline-5 min
Zolpidem
1.0
4.0
6.0
Fig. 5. The effects of triazolam in rats (n 5 8) trained to discriminate
8.0 mg/kg pentobarbital from saline. (Top panel) The percentage of
responses completed on the pentobarbital-appropriate lever during
test sessions in which responding on either lever resulted in food pellet
delivery. (Bottom panel) The average response rate (responses/second)
of responding on either lever in test sessions. (Abscissae) Dose of
triazolam in mg/kg given via the i.p. route. Each point represents data
from individual rats identified in the symbol key. Solid lines through the
points represent lines connecting the group means. Each point represents the mean of two determinations.
Dose (milligrams per body surface area) and Relative
Dose
For milligrams per body surface area and relative dose
comparisons among rhesus monkeys, rats and humans (table
3), the body weights averaged across the experiments were
Rats
Responding/
Total Rats
Testedc
%
0.00
(0.00–0.00)
194
(113–248)
4/4
100
(100–100)
6.00
(0.00–29.0)
196
(178–283)
149
(60–192)
5/5
46.0
(0.00–98.0)
26.0
(1.00–71.0)
59.0
(0.00–100)
62.0
(13.0–100)
248
(106–758)
111
(57–211)
14.0
(2.0–23)
15.0
(0.00–23)
5/5
5/5
4/4
5/5
4/5
a
Saline (saline-15 min) and pentobarbital were administered i.p. 15 min before
the session. Saline (saline-5 min) and zolpidem were administered 5 min before
the session.
b
Total responses were the total number of responses on either lever for the
15-min session.
c
“Rats responding” was defined as rats completing at least one FR 1 during
a test session. Percent drug lever data were calculated only from rats responding.
TABLE 2
Discriminative stimulus effects of zolpidem, administered i.g., in
rats trained to discriminate pentobarbital (8.0 mg/kg i.p.) from
saline (1.0 ml/kg i.p.)
Druga Dose
mg/kg i.g.
Saline
revealed no reliable difference in rates for any pentobarbital
dose compared with saline administered i.g. Similar to the
i.p. route, zolpidem occasioned partial drug-appropriate responding over a dose range of 1.0 to 4.0 mg/kg i.g. In contrast
to the i.p. route, Dunnett’s tests revealed no reliable difference in rates for zolpidem doses of 1.0, 2.0 and 4.0 mg/kg i.g.
compared with saline (because of the repeated measures
design, only n 5 4 rats were used in the rate analysis).
Because no rate effects were observed at these doses, a
higher dose of zolpidem was tested (6.0 mg/kg i.g.). This high
dose of zolpidem occasioned 100% drug-appropriate responding, but only two of the seven rats tested completed a response requirement, and of the rats not completing a response requirement, only one rat responded at all (this rat
made one response on the saline lever). When the rate data
for the four rats tested in all conditions were compared with
saline, the 6.0 mg/kg zolpidem dose was reliably lower than
saline [q(12) 5 4.0, P , .05].
Mean Total
Responsesb (Range)
Pentobarbital
4.0
8.0
16
Zolpidem
1.0
2.0
4.0
6.0
Mean Drug Leverb
(Range)
Rate-Mean
(Range)
Rats Responding/Total
Rats Testedc
%
responses/s
0.00
(0.00–0.00)
1.3
(0.91–1.7)
4/4
25.0
(0.00–100)
97.0
(88.0–100)
96.0
(84.0–100)
1.3
(1.1–1.6)
1.2
(0.79–1.6)
0.81
(0.48–1.4)
4/4
25.0
(0.00–100)
45.0
(0.00–100)
56.0
(0.100–100)
100
(100–100)
0.91
(0.070–1.3)
1.3
(1.1–1.6)
0.61
(0.090–1.6)
0.15
(0.00–0.66)
4/4
4/4
4/4
4/4
8/8
2/7
a
Saline, pentobarbital and zolpidem were administered i.g. via gavage 15 min
before the session.
b
Percent drug lever data were calculated only from rats responding, whereas
rate data were calculated from all rats tested.
c
“Rats responding” was defined as rats completing at least one FR 10 during
a test session.
used. As can be seen in table 3, the pentobarbital training
doses expressed as milligrams per body surface area were
similar for rats and humans (5.9 and 5.3 mg/A, respectively),
whereas for monkeys, the pentobarbital training dose was
approximately 4-fold higher (20 mg/A). In general, higher
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2.0
Mean Drug Lever
(Range)
170
Rowlett and Woolverton
Vol. 280
TABLE 3
Dose (mg/kg and mg/A) and relative dose comparisons of
pentobarbital, zolpidem and triazolam in rhesus monkeys, rats
and humansa trained to discriminate pentobarbital from saline
Dose
Drug and Species
Pentobarbital
Rhesus monkey
Rat
Human
Rat
Human
Triazolam
Rhesus monkey
Rat
Human
Column 2
mg/kg
mg/Ab
Relative Dose (Column
1/Column 2)
3.0
5.6
10*
1.0
2.0
4.0
8.0*
16*
0.31
0.62
1.25*
1.9*
6.0
11
20*
0.74
1.5
3.0
5.9*
12*
1.3
2.7
5.3*
8.3*
0.50
0.51
0.50
1.4
1.3
1.3
1.4
1.3
0.24
0.23
0.24
0.23
0.10
0.30
1.0**
3.0
10
17
30*
0.50
1.0
2.0
4.0
6.0
0.031
0.062*
0.12*
0.25***
0.20
0.61
2.0**
6.1
20
35
61*
0.37
0.74
1.5
3.0
4.4
0.13
0.27*
0.53*
1.1***
0.50
0.49
0.50
0.49
0.50
0.49
0.49
1.4
1.4
1.3
1.3
1.4
0.23
0.23
0.23
0.23
0.010
0.030
0.10
0.30*
0.025
0.050
0.10*
0.20*
0.00078
0.0015
0.0031
0.0062*
0.020
0.061
0.20
0.61*
0.019
0.037
0.074*
0.15*
0.0033
0.0066
0.013
0.026*
0.50
0.49
0.50
0.49
1.4
1.4
1.4
1.3
0.23
0.23
0.24
0.24
a
Data are adapted from Rush et al. (1997).
Dose based on mg per body surface area (A), assuming that A } kg2/3 (see
Dews, 1976). Doses in boldface represent training doses.
* Dose of drug that occasioned full pentobarbital-appropriate responding in all
subjects, determined as the mean pentobarbital-appropriate responding $80%.
** Occasioned 100% pentobarbital-appropriate responding in one monkey
that subsequently was not tested at higher doses.
*** Occasioned a mean of 77% pentobarbital-appropriate responding in the
four subjects.
b
doses of pentobarbital, zolpidem and triazolam were required
to obtain full drug-appropriate responding in monkeys, when
dose was expressed as milligrams per body surface area, than
for either rats or humans. To compare the relationship of
dose based on weight of the subject to dose based on body
surface area, relative dose values were computed (see table
3). As can be seen in the table, relative dose was constant
across drugs within a species. The relative dose decreased as
the weight of the subject increased, with a rank order of rat .
monkey . human.
As found in baboons (Ator and Griffiths, 1989; Griffiths et
al., 1992), both zolpidem and triazolam occasioned at least
80% drug-appropriate responding in pentobarbital-trained
rhesus monkeys. Thus, zolpidem and triazolam were similar
to other BZ agonists, such as diazepam, chlordiazepoxide,
etizolam and brotizolam, which also occasioned 80% or more
drug-appropriate responding in rhesus monkeys trained to
discriminate pentobarbital from saline (Nader et al. 1991;
Winger and Herling, 1982; Woolverton and Nader, 1995).
Additionally, Takada et al. (1986) have reported that the BZ
agonist, midazolam, engendered greater than 80% drug-appropriate responding in rhesus monkeys trained to discriminate the barbiturate, methohexital, from saline. Thus, the
present results extend those findings in rhesus monkeys to
the imidazopyridine BZ agonist zolpidem, and the triazolobenzodiazepine triazolam, and support the results found in
baboons (Griffiths et al., 1992). These results suggest that in
primates, the behavioral effects of zolpidem are similar to
pentobarbital and classic BZs. Moreover, in the companion
paper in the present series (Rush et al., 1997), in which
humans discriminated pentobarbital from placebo, both zolpidem and triazolam occasioned full pentobarbital-appropriate responding. In addition, the subject-rated and performance-impairing effects of zolpidem were similar to those of
triazolam and other classic sedative/hypnotic agents, such as
barbiturates and BZs (Rush and Griffiths, 1996; Rush et al.,
1997). BZs and barbiturates have consistently been found to
have similar subjective effects in humans (deWit and Griffiths, 1991). Taken together, these results suggest that the
discriminative stimulus effects of zolpidem in humans and
nonhuman primates are similar to classic BZs, and these
results generally support the hypothesis that discriminative
stimulus effects in nonhuman primates predict subjective
effects in humans.
Despite the similarity of discriminative stimulus profiles
between zolpidem and triazolam in pentobarbital-trained
monkeys, there were some differences between the two compounds. First, in terms of potency differences among monkeys, zolpidem was more variable than triazolam. That is,
zolpidem was approximately 17- to 30-fold more potent in one
monkey than the other three monkeys, but potencies were
similar for triazolam across monkeys. In addition, increases
in latency to respond were reliably higher than saline latency
at 10 mg/kg zolpidem, whereas triazolam did not reliably
alter latency to respond, although a trend for an increase was
evident at the higher triazolam doses. It is not clear whether
these differences in interanimal variability or performance
decrements represent a unique profile of effects for zolpidem
compared with triazolam. Previous research in our laboratory with the same procedures typically has not revealed
interanimal differences in potency such as those observed for
zolpidem in monkeys tested with diazepam, chlordiazepoxide, etizolam (Woolverton and Nader, 1995) or brotizolam
(Nader et al., 1991). However, considerable variability has
been observed with the latency measure for these compounds, with BZ effects on latency being inconsistent across
drugs and among animals (Nader et al., 1991; Woolverton
and Nader, 1995).
In contrast to the results observed in rhesus monkeys,
zolpidem occasioned only partial drug-appropriate respond-
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Zolpidem
Rhesus monkey
Column 1
Discussion
1997
171
pared with other benzodiazepines, it is possible that the
discriminative stimulus effects of zolpidem were masked in
the rat studies by the inability of the rat to respond. Thus,
one hypothesis is that zolpidem has pentobarbital-like discriminative stimulus effects that are attainable only under
conditions of low response output. Consistent with this notion, zolpidem shared discriminative stimulus effects with
pentobarbital in rhesus monkeys responding under a FR 1
schedule. Second, the discriminative stimulus effects of zolpidem may vary as a function of route of administration.
Consistent with this notion, all positive results with humans
and nonhuman primates were obtained after i.g. or perioral
administration, whereas the negative results obtained in the
present study were observed after i.p. administration.
To test the hypothesis that zolpidem’s rate effects interfere
with measurement of its discriminative stimulus effects, zolpidem was tested in the pentobarbital-trained rats under a
FR 1 schedule of reinforcement. Similar to the FR 10 schedule, partial drug-appropriate responding was observed after
zolpidem tested under a FR 1 schedule. This partial drugappropriate responding was observed even after a dose of
zolpidem was tested that was higher than could have been
tested under the FR 10 schedule. Thus, these results indicate
that relatively low response requirements, such as the FR 1
schedule used in the rhesus monkey study, likely did not
account for zolpidem showing full pentobarbital-appropriate
responding. Consistent with this, zolpidem occasioned full
pentobarbital-appropriate responding in baboons responding
under a range of FRs from 15 to 40 (Griffiths et al., 1992).
To test whether the discriminative stimulus effects of zolpidem vary as a function of route of administration, zolpidem
was administered i.g. in pentobarbital-trained rats. Interestingly, zolpidem occasioned $80% drug-appropriate responding when tested at a high dose (6.0 mg/kg) via the i.g. route,
but only two of seven rats completed a FR 10 at this dose.
These results suggest that full pentobarbital-appropriate responding may be observed only after oral doses of zolpidem
that severely impair performance, i.e., under a condition in
which only 29% of the subjects responded. Thus, it is difficult
to fully attribute the difference in discriminative stimulus
effects of zolpidem in pentobarbital-trained rats and primates to a difference in i.p. versus oral absorption, because
no evidence of a similar relationship between performance
decrements and discriminative stimulus effects was observed
in either human or nonhuman primate subjects after any
zolpidem dose.
Other factors which may have accounted for the differences
observed between rats and monkeys should be noted. One
possibility may be the result of relative differences in the
training dose of pentobarbital. In general, a larger number of
drugs, varying in such factors as pharmacological mechanism and intrinsic activity, often share discriminative stimulus effects at low compared with high training doses (e.g.,
Sannerud and Ator, 1995a,b). Thus, the training dose of
pentobarbital may have been relatively low in rhesus monkeys, increasing the probability of observing drug-appropriate responding to a greater variety of drugs, compared with
the training dose used in rats. To examine the issue of training dose, the doses of drugs were converted from mg/kg to
mg/A, according to Dews (1976). This conversion is based on
body surface area and takes into account that different species may not only vary in body weight, but in surface area as
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ing in rats trained to discriminate pentobarbital from saline,
under similar conditions in which pentobarbital was administered (15-min pretreatment i.p.). Moreover, zolpidem produced severe reductions in rate, such that many rats at the
high zolpidem doses did not complete FRs or emit any responses at all. These results with zolpidem were similar to
those reported previously in chlordiazepoxide-trained rats
(Depoortere et al., 1986; Sanger et al., 1987). In addition,
pentobarbital occasioned only partial drug-appropriate responding in rats trained to discriminate zolpidem from saline
(Sanger and Zivkovic, 1986). Finally, the triazolobenzodiazepine triazolam occasioned 80% or higher drug-appropriate
responding in the pentobarbital-trained rats, consistent with
previous findings and similar to other classic BZs (Ator and
Griffiths, 1989). Compared with the results with rhesus monkeys, baboons and humans, these data with zolpidem in
pentobarbital-trained rats raise the possibility of an important species difference between rodents and primates, including humans.
One possibility for the pattern of effects observed after
zolpidem administration in rats may be related to zolpidem’s
pharmacokinetics. Zolpidem is a relatively fast-acting drug
that is rapidly eliminated (Sanger and Zivkovic, 1986;
Trenque et al., 1994). To test this possibility, two other pretreatment times were assessed: 5 min before the session and
45 min before the session. At the 5-min pretreatment time,
zolpidem again occasioned only partial pentobarbital-appropriate responding. However, the dose-response function for
rate suppression was shifted to the left relative to the 15-min
pretreatment time. Additionally, more rats did not complete
FRs at the 5-min pretreatment time than at the 15-min
pretreatment time, with no rats completing a FR at the
highest dose tested (4.0 mg/kg). At the 45-min pretreatment
time, all responding was on the saline-appropriate lever and
no effects on rate of responding were observed. Thus, the
peak effect of zolpidem on rate of responding was observed
with the 5-min pretreatment time, whereas zolpidem occasioned primarily partial drug-appropriate responding at 5and 15-min pretreatment times. All effects of zolpidem had
dissipated at the 45-min pretreatment time. These data suggest that the lack of consistent dose-dependent pentobarbital-appropriate responding observed after zolpidem in rats
was not caused by pharmacokinetic factors, such as testing
the drug at too late or too early a pretreatment period for
sufficient levels of zolpidem to be in the CNS during the
testing period.
Although species differences may have accounted for the
difference in discriminative stimulus effects for zolpidem observed between rats and rhesus monkeys, a variety of procedural factors also may have contributed to this difference.
The difference between rats and rhesus monkeys in discriminating zolpidem may have been caused by the fact that
shock avoidance (i.e., negative reinforcement) was used as
the contingent event for rhesus monkeys, whereas food presentation (i.e., positive reinforcement) was used as the contingent event for rats. However, full drug-appropriate responding was obtained in humans (Rush et al., 1997) and
baboons (Griffiths et al., 1992) by use of positive reinforcement; furthermore, the baboon study used food presentation.
Nonetheless, at least two hypotheses based on procedural
differences may be posited. First, because zolpidem produces
rate decreases of relatively higher magnitude in rats com-
Pentobarbital Stimulus and Zolpidem
172
Rowlett and Woolverton
Acknowledgments
The animals used in this study were maintained in accordance
with the USPHS Guide for Care and Use of Laboratory Animals and
all procedures were approved by the appropriate Institutional Animal Care and Use Committees. We thank Dr. Justin English for
assistance with relative dose calculations and interpretation. We
also thank Dr. Craig R. Rush and Kristin Sonntag for comments on
an earlier version of this manuscript. For excellent technical assistance, we thank Susan Kearney, Karen Machinist and Franco Campanella.
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Send reprint requests to: William L. Woolverton, Ph.D., Dept. of Psychiatry
and Human Behavior, Univ. of Mississippi Medical Center, 2500 North State
Street, Jackson, MS 39216.
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