Effects of the CCKB antagonist L-365, 260 on benzodiazepine withdrawal-induced hypophagia in rats

The effect of the selective CCK_B antagonist L-365, 260 on chlordiazepoxide (CDP) withdrawal-induced hypophagia was assessed in two related studies in rats pretreated for 21 days with CDP at doses escalated from 10 to 30 mg/kg per day (b.i.d.). L-365, 260 was studied at doses from 0.001 to 10 mg/kg (b.i.d.). There was no evidence that L-365, 260 at any dose alleviated CDP withdrawal-induced hypophagia. These data contrast with reports that CCK_B antagonists alleviate behavioural benzodiazepine (BZ) withdrawal symptoms considered to be indicative of anxiogenesis. Presumably, such positive effects of CCK_B antagonists are due to functional antagonism, with enhanced anxiety during BZ withdrawal being attenuated by anxiolytic actions of CCK_B antagonists. Collectively, studies with CCK_B antagonists and other agents involving a number of different BZ withdrawal signs suggest that BZ withdrawal is a heterogeneous syndrome, with various different underlying mechanisms. CCK_B antagonists appear to alleviate only a subset of possible BZ withdrawal signs.     

Effects of the 5HT(1A) agonist anxiolytic gepirone on benzodiazepine withdrawal signs in rats

The effects of gepirone at 3, 9 and 27mg/kg (b.i.d.) on benzodiazepine (BZ) withdrawal signs were studied in rats pretreated with chlordiazepoxide for 21 days at doses up to 40mg/kg b.i.d. The BZ withdrawal indices studied were weight loss and anorexia. At 9 and 27, but not 3mg/kg (b.i.d.) gepirone potentiated the weight loss and anorexia seen during BZ withdrawal. These effects could not be attributed simply to high dose drug-induced "malaise" inhibiting food intake, since in drug-naive animals gepirone stimulated food intake and increased bodyweight. These data show clearly that gepirone potentiates BZ withdrawal signs. Similar findings have been reported recently in studies with ipsapirone (Goudie and Leathley, 1991). Such effects could be mediated by the a(2)-adrenoceptor antagonist actions of 1-(2-pyrimidinyl)piperazine (1-PP), an active metabolite of both gepirone and ipsapirone. Such findings may explain why prior BZ experience impairs the clinical response to buspirone-type anxiolytics acting at the 5-HT(1A) receptor.     

Evaluation of the dependence potential of the selective 5-H1A agonist ipsapirone in rats and of its effects on benzodiazepine withdrawal

Two initial studies investigated: i) the effects of withdrawal from ipsapirone [a putative non-benzodiazepine (BZ) anxiolytic] and chlordiazepoxide (CDP); and ii) effects of ipsapirone in animals withdrawn from CDP. Rats were injected b.i.d. for 21 days with saline, ipsapirone or CDP at doses up to 40 mg/kg/injection. Subsequently, controls received the treatment administered previously, other subjects received saline during withdrawal from ipsapirone or CDP. Further subjects received ipsapirone (3, 10 or 30 mg/kg b.i.d.) during CDP withdrawal. Withdrawal indices recorded were body weight and food intake. Withdrawal signs were absent after ipsapirone treatment but present after CDP treatment, when food intake and bodyweight measures fell and then recovered. At the high dose of 30 mg/kg (b.i.d.) ipsapirone potentiated CDP withdrawal signs. Potentiation of withdrawal wasnot seen in animals treated with ipsapirone at lower doses (3 and 10 mg/kg, b.i.d.). In a subsequent study we found that ipsapirone conditioned a taste aversion, a possible index of drug-induced malaise, at doses as low as 7.5 mg/kg. Therefore a possible explanation for the potentiation of BZ withdrawal in subjects treated with high doses of ipsapirone was that drug-induced malaise reduced food intake and body weight, rather than ipsapirone causing true potentiation of BZ withdrawal. However, in a further study we showed that the ipsapirone treatment regime which potentiated BZ withdrawal didnot significantly reduce food intake or body weight, suggesting that high doses of ipsapirone potentiate BZ withdrawal by a mechanism that does not simply involve malaise. The most plausible account of the observed potentiation of withdrawal by ipsapirone involves actions of the ipsapirone metabolite (1-(2-pyrimidinyl)-piperazine) on alpha₂-adrenoceptors, which are known to be implicated in BZ withdrawal. However, the precise mechanism involved remains unclear. Collectively, the studies reported show that ipsapirone does not induce the type of withdrawal signs seen with BZs. However, there was no evidence that ipsapirone attenuated BZ withdrawal. It is therefore likely that patients withdrawn from BZs will experience withdrawal if treated with ipsapirone, and that if treated with high doses withdrawal may be exacerbated.An abstract report of some of the data reported here has been published previously (Goudie A.J., Leathley M., McNally J. (1989a))     

Effects of the 5-HT3 antagonist ICS 205-930 on benzodiazepine withdrawal signs in rats

Effects of the 5-HT3 antagonist ICS 205-930 on chlordiazepoxide (CDP) withdrawal were assessed in rats treated for 21 days with doses of CDP up to 40mg/kg/day (b.i.d.). Withdrawal signs recorded were body weight and 24h food intake, which both fell during withdrawal and then recovered. ICS 205-930 (0.001-1.0mg/kg) was administered b.i.d. during withdrawal. At no dose over the wide range tested did ICS 205-930 reduce withdrawal signs. These data contrast with published findings with the 5-HT3 antagonist ondanestron, some of which indicated that ondansetron completely alleviated the "anxiogenic" suppression of exploratory behaviour observed during benzodiazepine (BZ) withdrawal. Possible reasons for these differing findings are discussed. Collectively, these findings suggest that 5-HT3 antagonists may have limited utility in the clinical treatment of BZ withdrawal.     

Effects of the 5-HT3 antagonist ondansetron on benzodiazepine-induced operant behavioural dependence in rats

This study was designed to assess whether rats made tolerant to the suppressant action on Fixed Ratio operant responding of the benzodiazepine (BZ) chlordiazepoxide (CDP) would show behavioural disruption on drug withdrawal—so-called operant behavioural dependence. In addition, the study examined the effects of the 5-HT₃ antagonist ondansetron on such operant behavioural dependence. During 42 consecutive days of CDP treatment, at deses escalated from 10 to 30 mg/kg/day, marked tolerance developed to the rate-suppressant action of CDP. On subsequent days, during spontaneous withdrawal, response rates declined significantly by around 30% in animals treated with saline, although some recovery of responding was seen over successive days of withdrawal. Similar reductions in responding followed by recovery were seen in rats treated with the 5-HT₃ antagonist ondansetron (0.01–0.1 mg/kg, b.i.d.). These findings demonstrate for the first time that it is possible to use operant procedures to detect spontaneous BZ withdrawal. They also suggest, in agreement with recent studies from this laboratory (Leathley and Goudie 1992), that 5-HT₃ antagonists may have relatively limited utility in treating some signs of BZ dependence.     

Dynorphin A (2–17) attenuates the unconditioned but not the conditioned effects of opiate withdrawal in the rat

OBJECTIVES: An unbiased place preference conditioning procedure was used to examine the influence of the non-opioid peptide, dynorphin A 2-17 (DYN 2-17), upon the conditioned and unconditioned effects of opiate withdrawal in the rat. METHODS: Rats were implanted SC with two pellets containing 75 mg morphine or placebo. Single-trial place conditioning sessions with saline and the opioid receptor antagonist naloxone (0.1-1.0 mg/kg; SC) commenced 4 days later. Ten minutes before SC injections, animals received an IV infusion of saline or DYN 2-17 (0.1-5.0 mg/kg). Additional groups of placebo- and morphine-pelleted animals were conditioned with saline and DYN 2-17. During each 30-min conditioning session, somatic signs of withdrawal were quantified. Tests of place conditioning were conducted in pelleted animals 24 h later. RESULTS: Naloxone produced wet-dog shakes, body weight loss, ptosis and diarrhea in morphine-pelleted animals. Morphine-pelleted animals also exhibited significant aversions for an environment previously associated with the administration of naloxone. These effects were not observed in placebo-pelleted animals. DYN 2-17 pretreatment resulted in a dose-related attenuation of somatic withdrawal signs. However, conditioned place aversions were still observed in morphine-pelleted animals that had received DYN 2-17 in combination with naloxone. Furthermore, the magnitude of this effect did not differ from control animals. CONCLUSIONS: These data demonstrate that the administration of DYN 2-17 attenuates the somatic, but not the conditioned aversive effects of antagonist-precipitated withdrawal from morphine in the rat. Differential effects of this peptide in modulating the conditioned and unconditioned effects of opiate withdrawal are suggested.     

L-DOPA attenuates nicotine withdrawal-induced behaviors in rats

There is some evidence that during nicotine abstinence brain dopamine levels are reduced. The hypothesis for the present study was that the precursor amino acid L-DOPA would relieve nicotine withdrawal-induced behaviors. Separate groups of adult male Sprague-Dawley rats were used. (−)-Nicotine bitartrate (9 mg/kg/day, salt content) or equimolar sod ium tartrate was infused into each rat via a subcutaneous osmotic minipump for 7 days. To assess nicotine withdrawal behaviors, locomotor activity was measured for 24 h in their home cage. Somatic signs were also counted approximately 22 h after pump removal. Moreover, depressive-like behaviors were evaluated in the forced swimming test approximately 48 h after pump removal. One day after removal of pumps, locomotor activity was suppressed in nicotine-infused rats compared to the tartrate-infused controls. Somatic signs of nicotine withdrawal were increased in nicotine-infused rats compared to the controls. Two days after removal of pumps, increased immobility in the forced swimming test was observed in abstinent nicotine-infused rats as compared with controls. The administration of L-DOPA methyl ester (equivalent to 50 mg/kg L-DOPA, s.c.) and benserazide (10 mg/kg, s.c.) attenuated somatic signs of withdrawal and reversed nicotine withdrawal-induced depressive-like behaviors in the forced swimming test. It did not mitigate nicotine withdrawal-induced locomotor suppression in the animals' home cages. These results indicate that L-DOPA could be a useful agent to alleviate some nicotine withdrawal symptoms.     

The involvement of alpha 2A-adrenoceptors in morphine analgesia, tolerance and withdrawal in mice

Alpha(2)-adrenoceptor agonists potentiate opioid analgesia and alleviate opioid withdrawal. The effects of two alpha(2)-adrenoceptor agonists, clonidine (2 mg/kg) and dexmedetomidine (20 and 100 microg/kg), and the alpha(1)-adrenoceptor antagonist prazosin (0.5 mg/kg) were tested on morphine analgesia, tolerance, and withdrawal in wild-type and alpha(2A)-adrenoceptor knock-out (KO) mice. Analgesia and tolerance were assessed with the tail-flick test. Withdrawal was precipitated with naloxone. Prazosin potentiated morphine analgesia equally in both genotypes. Clonidine and dexmedetomidine had no analgesic effects in alpha(2A)-adrenoceptor KO mice, but morphine analgesia and tolerance were similar in both genotypes. Alpha(2A)-Adrenoceptor KO mice exhibited 70% fewer naloxone-precipitated jumps than wild-type mice; weight loss was similar in both genotypes. The alpha(2)-adrenoceptor agonists reduced opioid withdrawal signs only in wild-type mice. We conclude that alpha(2A)-adrenoceptors are not directly involved in morphine analgesia and tolerance, and not critical for potentiation of morphine analgesia by prazosin, but that alpha(2A)-adrenoceptors modulate the expression of opioid withdrawal signs in mice.     

Assessment of the benzodiazepine-like dependence potential in rats of the putative 5-HT(1A) agonist anxiolytic S-20499

The dependence potential of the putative 5-HT(1A) agonist anxiolytic S-20499 was assessed in rats in a study in which the benzodiazepine chlordiazepoxide (CDP) was used as a "positive control". S-20499 was administered b.i.d. (at 10.00 and 16.00h) for 21 days, at constant doses of either 0.5, 3 or 18mg/kg i.p. Subsequently, spontaneous withdrawal from S-20499 was studied over 7 days. Acutely, S-20499 decreased body weight and food intake and complete tolerance developed to these effects. When S-20499 was withdrawn there was no evidence of any effect on body weight or food intake. CDP was also administered b.i.d. (at 10.00 and 16.00h) for 21 days at doses escalated from 10 to 30mg/kg i.p. CDP differed from S-20499 in some of its acute effects, stimulating body weight and food intake. In contrast to S-20499 withdrawal, CDP withdrawal induced weight loss and aphagia. Acutely, S-20499 resembled CDP in inducing hypothermia. Full tolerance developed to this effect of both drugs. However, only CDP withdrawal induced hyperthermia. Thus S-20499 appeared to be devoid of benzodiazepine-like dependence potential after administration for a relatively long period of time, at doses that are substantially greater than "anxiolytic doses" in rats.     

A liquid diet model of chlordiazepoxide dependence in mice

Mice fed chronically (3 to 4 weeks) a liquid diet containing chlordiazepoxide (CDP) became physically dependent on the drug as demonstrated by the occurrence of withdrawal signs precipitated by injection of the benzodiazepine antagonist Ro15-1788 (5 to 25 mg/kg) or by omitting CDP from the diet (spontaneous withdrawal). Very low blood concentrations of CDP, but medium to high levels of the active metabolites N-desmethyl CDP and demoxepam were found during the period of CDP administration. The Ro15-1788-induced withdrawal signs appeared within 1 min after the injection of the antagonist and lasted for at least 10 min. Quantifiable withdrawal signs included tail lift, tremor, impaired movement and handling-induced seizures. Mice undergoing spontaneous withdrawal had milder withdrawal signs such as weight loss, in appetite and suppression of runway and head-dipping activities on day 1 or day 2 of withdrawal. These signs were also present in Ro15-1788-induced withdrawal. A long-lasting rebound increase in runway and head-dipping activities occurred several days after CDP withdrawal.     

Physical dependence potential of an enkephalin analog, EK-399, in rats

The physical dependence potential of Tyr-D-Met(O)-Gly-EtPhe-NHNHCOCH3.AcOH (EK-399), a novel enkephalin analog with a potent analgesic effect, was assessed in rats. The animals were given EK-399 (0.008, 0.032, 0.125, or 0.5 mg/kg), morphine (0.125, 0.5, or 2 mg/kg), pethidine (2 or 8 mg/kg), or pentazocine (2 or 8 mg/kg) every hour through an implanted intravenous cannula. After 3 days of treatment, precipitated withdrawal tests were conducted: naloxone (5 mg/kg) was administered subcutaneously. Rats treated with morphine showed withdrawal signs such as hyperirritability, salivation, diarrhea, and weight loss. Rats treated with pethidine, pentazocine, or EK-399 showed similar signs, but they were less evident than those in morphine-treated rats. In abrupt withdrawal tests after 7 days of treatment, rats treated with morphine, pethidine, or pentazocine showed weight loss, whereas rats treated with EK-399 showed little or no weight loss. In substitution tests, EK-399 suppressed the withdrawal signs in morphine-dependent rats, and vice versa. These results show that EK-399 has a morphine-like physical dependence potential that is weaker than that of morphine, pethidine, or pentazocine in rats.     

Partial cross-dependence on ethanol in mice dependent on chlordiazepoxide

Mice which had been fed chronically a liquid diet containing chlordiazepoxide (CDP) showed spontaneous and Ro15-1788-induced withdrawal signs upon CDP withdrawal. Ethanol (1.5 g/kg) injected 5 min before Ro15-1788 injection almost completely suppressed the withdrawal signs induced by the benzodiazepine receptor antagonist. However, neither ethanol injection nor ethanol diet administration could prevent the loss of appetite and weight loss on day 1 of CDP withdrawal. Likewise, the addition of saccharin in the ethanol diets did not prevent the loss of appetite. Mice which had been fed the CDP diet followed by 9 days of ethanol treatment (CDP/ethanol) showed more severe hypothermia during ethanol withdrawal compared to mice which had been fed the control/ethanol diets. The CDP/ethanol mice also retained the increase in runway activity attained from the prior CDP treatment. The data indicate that CDP-dependent mice showed partial rather than full cross-dependence on ethanol.     

Effect of peripheral administration of cinnarizine and verapamil on the abstinence syndrome in diazepam-dependent rats

The effects of two calcium channel blockers (verapamil and cinnarizine) were evaluated on diazepam withdrawal symptoms. Rats were made diazepam dependent by chronic treatment with daily injections of the drug, 20 mg/kg IP for 3 weeks. On abrupt termination of the drug, animals showed withdrawal hyperactivity that was assessed by autonomic, behavioural and motor signs. The peak effect was seen 3 days after the withdrawal of diazepam. On IP administration, verapamil and cinnarizine (10, 20 and 40 mg/kg) given on eight occasions at an interval of 12 h reversed the withdrawal-induced increase in spontaneous motor activity. Cinnarizine in higher doses (20 and 40 mg/kg) was found to be effective in suppressing the behavioural signs but verapamil did not show any protective effect against startle response and irritability. These results suggest that modulation of the calcium influx in the CNS might influence withdrawal.This study was presented in XIth International Congress of Pharmacology (IUPHAR) at Amsterdam (July 1–6, 1990)     

Cocaine-induced anxiety: alleviation by diazepam,but not buspirone,dimenhydrinate or diphenhydramine

Clinical reports and animal experiments indicate that both cocaine administration and cocaine withdrawal increase anxiety. We investigated the ability of a number of putative anxiolytic agents to alleviate these anxiety states using the elevated plus-maze. Rats in the cocaine condition received either saline or cocaine (20 mg/kg) 40 min prior to testing; those in the withdrawal condition were tested 48 h following a chronic treatment regime (saline or cocaine 20 mg/kg per day for 14 days). Prior to testing, animals received a benzodiazepine (1.0 or 2.0 mg/kg diazepam), a serotonergic agonist (0.5 or 1.0 mg/kg buspirone), an antihistamine (50 mg/kg dimenhydrinate or 27 mg/kg diphenhydramine) or a saline injection. All drugs were administered intraperitoneally. Cocaine administration and cocaine withdrawal reduced the percentage time spent on and the number of entries into the open arms. Diazepam dose-dependently alleviated cocaine withdrawal-induced anxiety and non-significantly attenuated cocaine-induced anxiety. Buspirone, dimenhydrinate and diphenhydramine did not consistently alleviate the anxiety caused by either cocaine pre-treatment regime; in the saline conditions, however, each of these treatments was anxiogenic. In summary, benzodiazepines alleviated cocaine-induced anxiety, while future research on the ability of serotonergic and antihistaminergic drugs to alleviate these anxiety states is warranted.     

Isobolographic analysis of chlordiazepoxide and triazolam combinations in squirrel monkeys discriminating triazolam

RATIONALE: The discriminative stimulus (DS) effects of chlordiazepoxide (CDP) differ from those of other typical benzodiazepine (BZ) agonists in that CDP does not always occasion full substitution for a BZ agonist DS. OBJECTIVES: The present study tested the hypothesis that the unusual DS effects of CDP may result from its relatively low intrinsic efficacy by examining the combinations of CDP and triazolam using isobolographic analysis in squirrel monkeys discriminating triazolam. METHODS AND RESULTS: Squirrel monkeys were previously trained to discriminate triazolam (0.03 mg/kg, i.v.) from vehicle under a fixed-ratio 10 (FR 10) schedule of food reinforcement. CDP occasioned partial substitution for triazolam and did not alter the DS effects of triazolam, whereas single doses of triazolam enhanced the DS effects of triazolam, and bretazenil antagonized the triazolam DS. The isobolographic analysis showed that CDP and triazolam combinations resulted in additive effects in animals in which CDP substituted for triazolam, whereas infra-additive effects were obtained in animals in which CDP did not substitute for triazolam. CONCLUSIONS: The partial substitution of CDP for triazolam and the infra-additive effects obtained in animals in which CDP did not substitute for triazolam suggest that CDP may have lower intrinsic efficacy than triazolam. However, the lack of overall effect of CDP pretreatment and the lack of shift in animals in which CDP substituted for triazolam suggest that other factors, such as differential activity at BZ receptor subtypes, may play a role in the effects of CDP.     

Study of morphine-induced dependence in gonadectomized male and female mice

In this study we evaluated the effects of sex difference and also sex hormones on the naloxone-precipitated morphine withdrawal in both orchidectomized (ORC) male and ovariectomized (OVX) female mice. Morphine (50, 50 and 75 mg/kg/day for 4 days, s.c.) was administered to animals and at 5th day naloxone (4 mg/kg, i.p.)-precipitated morphine withdrawal signs, jumpings and the percentage of weight loss, were measured. There was no significant alteration in withdrawal jumpings between male and female mice, though weight loss was significantly higher in male ones. Jumpings was significantly lower in both OVX and ORC mice and percentage of weight loss was significantly higher in OVX mice than corresponding non-operated or sham animals. In OVX mice, E₂V (10 mg/kg, s.c.) increased number of jumpings and decreased percentage of weight loss. Progesterone (25 mg/kg, s.c.) had no effect on jumpings, whereas it decreased weight loss in OVX mice. Testosterone (2.5 mg/kg, s.c.) increased jumpings in ORC mice while it had no effect on percentage of weight loss. Our results demonstrated that sex hormones could play a role in the morphine withdrawal syndrome in both ORC male and OVX female mice.     

Inhibition of drinking by naltrexone in the rat: interaction with the dopamine D-1 antagonist SCH 23390 and the D-2 antagonist sulpiride

The involvement of dopamine receptors in water intake was investigated in the rat deprived of water for 24 hr. A 0.03 mg/kg dose of SCH 23390 markedly enhanced naltrexone (0.1 and 10.0 mg/kg)-induced hypodipsia, whilst the drug alone significantly decreased water intake at doses of 0.01 to 3.0 mg/kg, accompanied by marked motor dysfunction. Sulpiride (20.0 and 40.0 mg/kg) did not markedly affect water intake and naltrexone-induced hypodipsia. Consistent with previous results, apomorphine (0.3 mg/kg) alone was without marked effects, while it produced a marked potentiation of naltrexone (1.0 and 10.0 mg/kg)-induced hypodipsia. SCH 23390 (0.003 mg/kg) and sulpiride (40.0 mg/kg) completely antagonized the enhancing effects of apomorphine on naltrexone-induced hypodipsia. Similar effects were also seen in the latency to begin drinking. In contrast to the effects on naltrexone-induced hypodipsia, it appears that both dopamine D-1 and D-2 receptors play a key role in the effects of apomorphine on naltrexone-induced hypodipsia in the rat.     

Benzodiazepine receptor function in the chick social separation-stress procedure.

The role of benzodiazepine (BZ) receptors in modulating social separation-induced distress vocalizations (DVocs) and stress-induced analgesia (SIA) were examined in 8-day-old cockerels (Gallus gallus). In Experiment 1, the BZ agonist chlordiazepoxide (CDP; 5.0 mg/kg) reversed both DVocs and SIA in isolated chicks. Coadministration of the BZ antagonist flumazenil (0.01, 0.03, or 0.10 mg/kg) reversed CDP anxiolytic effects. In Experiment 2, the BZ agonists alprazolam (ALP; 0.065, 0.125, 0.25, or 1.50 mg/kg) and lorazepam (LOR; 0.125, 0.25, 0.50, or 1.0 mg/kg) dose dependently reversed social separation-induced DVocs and SIA. The ED50s for ALP and LOR in attenuating DVocs were 0.19 and 0.34 mg/kg, respectively. These data strongly support the theory that CDP anxiolytic effects are mediated by BZ receptor activity in the chick social separation procedure (Experiment 1) and that this model is sensitive to BZ agonists of different potencies (Experiment 2).     

Effect of GB-115 dipeptide on anxiety in rats with model benzodiazepine withdrawal syndrome

The effects of GB-115 dipeptide, a retroanalog of endogenous CCK-4, on the behavioral indices in "elevated plus maze" (EPM) test and on the content of biogenic amines in the brain structures after discontinuation of a chronic administration of benzodiazepine (BZ) derivatives phenazepam (2.0 mg/kg, i.p.) and diazepam (4.0 mg/kg, i.p.) have been studied in outbred and inbred MR/MNRA rats. It is established that, in 24-48 h following BZ withdrawal, GB-115 dipeptide administered in doses of 0.1 and 0.5 mg/kg, i.p., produced an anxiolytic effect in all animals, which was manifested by increasing the stay time and number of entries in EPM. In the striatum of outbred rats, GB-115 increased DOPAC (+25%) and DA (+31.6%) levels that were decreased during diazepam withdrawal syndrome. The obtained results showed the GB-115 efficiency in attenuating the anxiety caused by BZ withdrawal.     

The neurosteroid pregnanolone prevents the anxiogenic-like effect of inescapable shock in the rat

RATIONALE AND OBJECTIVES: The ability of progesterone (P4) and its neurosteroid metabolite, 3alpha-OH-5beta-pregnan-20-one (pregnanolone) in protecting against the anxiogenic-like effect of inescapable shock (IS) in male rats was examined, as these steroids exert anxiolytic, anticonvulsant, and ataxic effects similar to the benzodiazepines (BZ), drugs shown to prevent IS-induced anxiogenesis. METHODS: Adult male rats were injected with pregnanolone (8 mg/kg, SC), P4 (4 mg/rat) or its appropriate vehicle before exposure to IS. Twenty-four hours later, animals were injected with the steroid or its vehicle and then tested in the elevated plus-maze. In a control experiment, animals were injected with chlordiazepoxide (CDP, 15 mg/kg, IP) or vehicle before IS, and tested in the plus-maze 24 h later. RESULTS: Whereas CDP or pregnanolone before IS resulted in the loss of the anxiogenic-like response seen 24 h after IS, P4 before IS did not protect against the anxiogenic-like effect of IS. The acute anxiolytic-like effect of pregnanolone and P4 was lost in animals that were injected with vehicle before the IS, but was observed in animals that were injected with the steroids before IS. Moreover, P4 injection in non-shocked animals was associated with an anxiogenic-like response observed 24 h after steroid administration. CONCLUSION: The protection against the effect of IS afforded by a GABAergic neurosteroid is comparable to that observed with BZs, and thus provides further evidence of an allosteric relationship between the neurosteroid and BZ binding site on the GABA(A) receptor complex.