Benzodiazepine receptor mediated discriminative cues: effects of GABA-ergic drugs and inverse agonists

Constitutive activity of a G protein-coupled receptor, DRD1, contributes to human cerebral organoid formation

The intricate balance of neural stem cell (NSC) amplification and neurogenesis is central to nervous system development. Dopamine D1 receptor (DRD1) is a typical G protein‐coupled receptor (GPCR) mainly expressed in neurogenic area, with high constitutive activity. The receptor appears in the embryonic period before the formation of mature synaptic contacts, which indicates that dopamine receptor and its constitutive activity play crucial roles in the embryonic brain development. Here, we found that DRD1 was enriched in human NSCs. Inhibition of the receptor activity by its inverse agonists promoted human NSCs proliferation and impeded its differentiation. These results were also mimicked by genetic knockdown of DRD1, which also blocked the effects of inverse agonists, suggesting a receptor‐dependent manner. More interestingly, knock‐in A229T mutant with reduced DRD1 constitutive activity by CRISPR‐Cas9 genome editing technology resulted into increased endogenous human NSCs proliferation. These results were well reproduced in human cerebral organoids, and inhibition of the DRD1 constitutive activity by its inverse agonists induced the expansion and folding of human cerebral organoids. The anatomic analysis uncovered that decreasing the constitutive activity of DRD1 by its inverse agonists promoted the NSCs proliferation and maintenance that led to hindered cortical neurogenesis. Further mechanistic studies revealed that the PKC‐CBP pathway was involved in the regulation by DRD1. Thus, our findings indicate that the constitutive activity of DRD1 and possibly other GPCRs plays an important role in the development of human nervous system.

Dual expression of constitutively active Gαs-protein-coupled receptors differentially establishes the resting activity of the cAMP-gated HCN2 channel in a single compartment

The hyperpolarization-activated cyclic nucleotide-gated 2 (HCN2) channel is a major subtype of the HCN channel family expressed in the nervous system that sets the membrane potential, regulates cell excitability and senses changes in the extracellular environment. Neurons express various Gα_s-protein-coupled receptors (GPCRs), many of which show ligand-independent constitutive activity. These membrane-bound proteins are expressed in various subcellular compartments of neurons. Therefore, some proportion of HCN2 channels opens in response to the basal cAMP pool size produced by constitutively active GPCRs. Here, we employed an exogenous HEK293 expression system and voltage-clamp patch-clamp recordings to investigate basal HCN2 channel activity in the presence of two GPCRs with diverse basal activities in a single compartment. We utilized the β2-adrenoceptor (β2AR) together with odorant receptors (ORs), as both GPCR families are known to show strong basal activity. Consequently, β2AR alone strongly enhanced the activity of HCN2 channels, and co-expression of ORs further diversified the HCN2 channel activity, which was totally abolished by an adenylate cyclase inhibitor. Thus, we conclude that the dual expression of constitutively active GPCRs establishes the diverse range of the basal cAMP pool size in resting cells through mutual additive or suppressive interactions, even in the absence of external stimulation.

Discriminative stimulus effects of tiagabine and related GABAergic drugs in rats

RATIONALE

Tiagabine is an anticonvulsant drug which may also have sleep-enhancing properties. It acts by inhibiting reuptake at the gamma-aminobutyric acid (GABA) transporter (GAT-1).

OBJECTIVES

The aim of the study was to determine whether tiagabine acted as a discriminative stimulus and, if so, whether other GABAergic compounds would generalise to it.

MATERIALS AND METHODS

Rats were trained to discriminate tiagabine (30 mg/kg p.o.) from vehicle, and generalisation to drugs that modulate GABA was assessed.

RESULTS

Gaboxadol (5-20 mg/kg p.o.), a selective extrasynaptic GABA A agonist, generalised to tiagabine, although the extent of the generalisation was inconclusive. Indiplon (1 mg/kg p.o.), a benzodiazepine-like hypnotic, also partially generalised to tiagabine, although zolpidem and S-zopiclone did not. Baclofen, a GABA B receptor agonist, and gabapentin, which increases synaptic GABA, did not generalise to tiagabine. (+)-Bicuculline (3 mg/kg i.p.), a GABA A receptor antagonist, blocked the tiagabine cue, but the less brain-penetrant salt form, bicuculline methochloride, had no effect.

CONCLUSIONS

These data suggest that tiagabine generates a discriminative stimulus in rats, and provides a central GABA-mediated cue, but is distinct from the other GABAergic compounds tested.

Lack of generalisation between the GABAA receptor agonist, gaboxadol, and allosteric modulators of the benzodiazepine binding site in the rat drug discrimination procedure

INTRODUCTION

The binding sites for gamma-aminobutyric acid (GABA) and GABA(A) receptor agonists are located differently from the binding sites for benzodiazepine receptor agonists. Furthermore, the major pharmacological effects of benzodiazepine receptor agonists and the GABA(A) receptor agonist gaboxadol (4,5,6,7-tetrahydrroisoxazolo(5,4-c)pyridin-3-ol, THIP) are mediated by different GABA(A) receptor subunit compositions; that is, gaboxadol may interact primarily with extra-synaptically located alpha(4)beta(2/3)delta-containing receptors and benzodiazepines with the synaptically located alpha(1)beta(2/3)gamma(2)-containing receptors.

OBJECTIVES

The aim of the present study was to address if this different receptor subtype selectivity was reflected in vivo.

MATERIALS AND METHODS

A two-lever liquid reinforced operant discrimination procedure was conducted. Three groups of rats were trained to discriminate gaboxadol, diazepam and zolpidem 5.5, 1.5 and 0.7 mg/kg i.p., respectively, from vehicle.

RESULTS

Substitution tests showed that gaboxadol-trained animals failed to recognize diazepam (0.75-1.5 mg/kg), zolpidem (0.4-0.7 mg/kg), zopiclone (2.5 mg/kg), zaleplon (1.0-1.5 mg/kg) or indiplon (0.31 mg/kg). In contrast, all benzodiazepine receptor agonists, but not gaboxadol (4.5-5.5 mg/kg), generalised to the discriminative stimulus in diazepam- and zolpidem-trained animals.

DISCUSSION

In agreement with these data, the competitive benzodiazepine receptor antagonist flumazenil (10 mg/kg s.c.) antagonised the discriminative stimulus of zolpidem but not of gaboxadol. Interaction tests showed no synergistic interaction of concomitant administration of gaboxadol and zolpidem or diazepam.

CONCLUSION

Previous studies have shown that gaboxadol and benzodiazepines interact with different receptor populations, and the present study confirms that in vivo functional consequences of this receptor selectivity exist in the form of differential behavioural responses in rats.

Plasma and CNS concentrations of Gaboxadol in rats following subcutaneous administration

Gaboxadol has been suggested to be a selective extrasynaptic GABA(A) receptor agonist. However, there is little information on Gaboxadol concentrations in the central nervous system (CNS) at therapeutically relevant doses. In order to investigate this, rats were injected subcutaneously with Gaboxadol and plasma and CNS concentrations were determined using the dynamic-no-net-flux and ultraslow microdialysis methods. Results using the 2 methods were similar and showed that Gaboxadol rapidly entered the brain and that peak CNS concentrations after 2.5, 5 and 10 mg/kg were in the range of 0.7 to 3 microM. Furthermore, a very short half-life (28 min) in both plasma and CNS was observed. It is concluded that concentrations of Gaboxadol in the CNS are in a range, which are likely to activate only extrasynaptic (nongamma subunit containing) GABA(A) receptors.

Gaboxadol, a selective extrasynaptic GABA(A) agonist, does not generalise to other sleep-enhancing drugs: a rat drug discrimination study

Gaboxadol is a selective extrasynaptic GABA(A) receptor agonist (SEGA) which enhances slow-wave sleep, and may act principally at extrasynaptic GABA(A)alpha4betadelta receptors. Drug discrimination is a very useful approach for exploring in vivo pharmacological similarities and differences between compounds and was therefore used to compare gaboxadol and zolpidem, an established hypnotic drug, against zopiclone, S-zopiclone, indiplon and tiagabine, all of which have been reported to enhance sleep. Gaboxadol generalised to itself, but not to zolpidem, zopiclone, S-zopiclone, R-zopiclone, indiplon or tiagabine. By contrast, zolpidem generalised to itself, zopiclone, S-zopiclone and indiplon, but not to R-zopiclone (the inactive enantiomer of zopiclone), gaboxadol or tiagabine. This suggests that zolpidem, zopiclone, S-zopiclone and indiplon share a discriminative stimulus, which may be mediated by their efficacy at GABA(A)alpha1betagamma receptors. Gaboxadol and tiagabine each have a different discriminative stimulus from all the other drugs tested.

Prenatal protein malnutrition enhances stimulus control by CDP, but not a CDP/THIP combination in rats

In the present study, the effects of prenatal protein malnutrition on stimulus control exerted by the benzodiazepine (BZ), chlordiazepoxide (CDP) and the GABA-A receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) were characterized. The adult, male offspring of female Sprague-Dawley rats fed either low (6% casein) or adequate (25% casein) protein diets 5 weeks prior to mating and throughout pregnancy served as subjects. Subjects were first trained to discriminate CDP (8.0 mg/kg ip) from saline using drug discrimination procedures. Once a criterion level of performance was achieved, generalization tests were performed to lower doses of CDP (4.0, 2.0, 1.0, 0.5 and 0.25 mg/kg) and then to several doses of THIP (10.0, 7.5, 5.6 and 3.2 mg/kg). Lastly, the ability of a single dose of THIP (3.0 mg/kg) to enhance discriminative control by several low doses of CDP (4.0, 2.0, 1.0 and 0.5 mg/kg) was assessed. Although both diet groups acquired the original CDP/saline discrimination at the same rate, malnourished rats exhibited significantly more generalization to low doses of CDP than their well-nourished counterparts. Neither diet group exhibited significant generalization to THIP nor a difference in THIP's ability to enhance the CDP cue. These results suggest that a subject's sensitivity to the stimulus properties of drugs can be selectively modified by prenatal malnutrition.

Effect of flumazenil and diazepam on transient actions in defensive burying elicited by the social interaction experience in rats

In the present work, we studied the effects of benzodiazepine (BZ) receptor antagonist, flumazenil, and of the agonist, diazepam, on social interaction-induced transient changes in defensive burying (DB). Enhanced defensive burying was observed after 1.5 min of social interaction experience, while a longer social interaction experience, 15 min, inhibited the expression of burying behavior. Defensive burying and social interaction paradigms have been used for the screening of compounds with anxiolytic potential and, more extensively, to study the neurobiology of anxiety. To elucidate the participation of the BZ receptor on transient changes induced by intervals of social interaction experience, its receptor antagonist, flumazenil (2.5, 5, and 10 mg/kg) was intraperitoneally injected (IP). Flumzenil enhanced in a dose-dependent manner, the blocking effect of the saline IP injection on facilitated DB in 1.5-min social interaction-experienced subjects. In addition, flumazenil enlarged in a dose-dependent manner the blocking effect of saline IP on defensive burying levels in animals exposed to social interaction experience for 15 min. To analyze the presumed participation of the BZ receptor mediating enhanced burying behavior levels in subjects exposed to 1.5 min of social interaction, a suboptimal dose of diazepam (0.25 mg/kg) was administered. Diazepam enhanced the saline IP elicited defensive burying reduction. Results are discussed in terms of the suggested BZ receptor mediation on transient changes in defensive burying elicited by social interaction experience.

Discriminative stimulus properties of the benzodiazepine receptor inverse agonist methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM)

The purpose of this study was to determine whether rats could be trained to discriminate the stimulus properties of the benzodiazepine (BZ) receptor inverse agonist DMCM from saline in a conditioned taste aversion paradigm. On a drug trial, water-deprived rats were injected with DMCM (0.55-0.6 mg/kg IP), allowed access to a 0.25% saccharin solution for 30 min, and then injected with LiCl. On non-drug trials, saline injections bracketed the drinking period. Conditioned controls were treated similarly with DMCM and saline on drug and non-drug trials, but received injections of saline instead of LiCl. At the completion of training, CMCM produced a 69% reduction of saccharin consumption on drug trials, compared with 23% for conditioned controls. The stimulus properties of DMCM were then measured by its ability to reduce the preference for saccharin over water in a two-bottle choice test. DMCM reduced saccharin preference in rats that received discrimination training from 68% to 19%, but did not alter saccharin preference in conditioned controls. Other compounds with varying activity at BZ receptors were evaluated for their ability to substitute for the discriminative stimulus effects of DMCM. Two BZ receptor inverse agonists, beta-CCE (10-18 mg/kg) and FG 7142 (3.2-18 mg/kg), substituted completely for DMCM. Partial substitution for DMCM was shown by the BZ receptor antagonist CGS 8216 (3.2-10 mg/kg) and the non-BZ convulsant pentylenetetrazol (10-20 mg/kg). The BZ receptor agonists chlordiazepoxide (0.32-5.0 mg/kg), diazepam (0.32-10 mg/kg), and alprazolam (0.1-3.2 mg/kg) and the BZ receptor antagonist flumazenil (1.0-32 mg/kg) failed to substitute for the DMCM stimulus. Pretreatment with flumazenil (1.0 mg/kg) blocked the stimulus effects of the training dose of DMCM and produced a shift to the right of the DMCM generalization curve. The pattern of compounds that substituted for the DMCM stimulus and the blockade of that stimulus by flumazenil indicate that the stimulus properties of DMCM are associated with its effects as a BZ receptor inverse agonist.

Evidence for state-dependent retrieval in conditioned place aversion

State-dependent retrieval (SDR) in conditioned place aversion (CPA) was observed using Long-Evans male rats, with three different aversive drugs injected ip: lithium chloride (31.8 mg/kg), FG 7142 (10 mg/kg), and naloxone (5 mg/kg). Experiment 1 showed that state-dependent dissociation was complete with lithium chloride following two conditioning trials, but disappeared when the number of learning sessions was increased. Experiment 2 showed that following two conditioning trials, such complete state-dependent dissociations were also observed when FG 7142 or naloxone were used during conditioning. The results of this study showed that the expression of SDR is not monotonic in CPA paradigm but depends on the stage of learning. In addition, SDR can be observed following administration of a variety of drugs known to produce CPA, but differing in their neurochemical targets and the nature of the aversive effects they induce. Finally, these results are discussed with regard to what is known about SDR in other paradigms.

Morphine attenuates ultrasonic vocalization during agonistic encounters in adult male rats

Ultrasonic vocalizations (USV) in rats may communicate "affective" states during pain, sex and aggression. This proposal was evaluated in an experiment with adult male Long-Evans rats during agonistic encounters; specifically, morphine and naltrexone effects were studied on different types of USV by intruder rats exposed to resident attacks and to "threat of attacks" (i.e., intruder residing within the home cage of the resident but prevented from physical contact by a wire mesh cage). Intruders readily emitted USV during agonistic encounters. These calls consisted primarily of two distinct distributions of pure tone whistles: 0.3-3 s, 19-32 kHz ("low") calls and 0.02-0.3 s, 32-64 kHz ("high") calls. Sonographic analysis revealed a considerable repertoire of frequency modulated calls. Different types of vocalizations proved to be differentially sensitive to the opiate treatments: morphine (1-10 mg/kg SC) dose-dependently decreased the rate, duration and pitch of both low and high frequency USV during the threat of attack; this decrease in rate and duration measures was naltrexone-reversible (0.1 mg/kg IP). Interestingly, audible vocalizations were also emitted but were unaffected by morphine in this dose range. Concomitant with the decrease in USV after morphine was a dose-dependent decrease in rearing, walking and nasal contact behavior with increases in submissive crouch behavior and tail flick analgesia. The decreases in rate and duration of both low and high USV and the pitch of specific frequency modulated calls after morphine administration may reflect an attenuation of affective aspects of pain, and the many characteristics of US (rate, duration, pitch, frequency modulation, pre-and suffix attributes and temporal structure) point to potentially diverse functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Pentobarbital-like discriminative stimulus effects of direct GABA agonists in rats

The discriminative stimulus effects of direct and indirect-acting GABAergic drugs were investigated in rats trained to discriminate 5 mg/kg pentobarbital (PB) from saline under a two-lever fixed ratio (FR) 32 schedule of food reinforcement. PB and diazepam produced dose-dependent substitution for the training dose of PB with response rate reduction only at doses above those producing full substitution. Muscimol, thiomuscimol and 4,5,6,7-tetrahydroisoxazolo [5,4-c]-pyridin-3-ol (THIP) produced intermediate levels of pentobarbital-lever responding (40-60%), accompanied by dose-dependent decreases in rates of responding following THIP and muscimol administration. The GABAA agonist progabide and its metabolite 4-([(4-chlorophenyl) (5-fluoro-2-hydroxyphenyl)methylene]amino)] butyric acid (SL 75102) also partially substituted for PB, producing means of 39-73% PB-lever responding. The GABAB agonist, baclofen, completely failed to substitute for PB even at doses that decreased rates of responding. These results show that the discriminative stimulus effects of indirect GABAA agonists, PB and diazepam, although similar to one another, differ from those of direct GABAA receptor agonists, which produced only partial substitution for PB. The GABAB agonist, baclofen, can be distinguished by lacking any ability to substitute for PB. These results contribute to a further understanding of the similarities and differences in the behavioral effects of different types of GABA agonists.

Effects of chlordiazepoxide and putative anxiogenics on conditioned suppression in rats

This paper reports two experiments. In Experiment 1, the effects of chlordiazepoxide alone and in combination with a series of putative antagonists at various sites on the GABA/benzodiazepine receptor complex on conditioned suppression of operant behavior in rats were assessed. Response rates during presentation of a stimulus associated with shock (CS responding) and when only positive reinforcement is effective (pre-CS responding) were analysed. Chlordiazepoxide (10 mg/kg) significantly increased CS responding. This effect was significantly antagonised by Ro15-1788 (10 mg/kg) and by picrotoxin (1.5 mg/kg), but not by bicuculline (1.5 mg/kg) or by delta-amino-n-valeric acid (10 or 20 mg/kg). Chlordiazepoxide also significantly, albeit more slightly, increased pre-CS responding and none of the other drugs tested significantly antagonised this action, though Ro15-1788 plus chlordiazepoxide resulted in pre-CS response rates not significantly different from either chlordiazepoxide alone or control. These interactions are discussed in the context of the proposed GABA/benzodiazepine receptor complex with the conclusion that drug effects at the benzodiazepine- and picrotoxin-sensitive channel sites have an important role in mediating anxiolytic action. However, behavioral evidence of an important role for GABAa or GABAb receptors remains very limited. The second experiment studied the intrinsic actions of bicuculline, picrotoxin, and Ro15-1788 on conditioned suppression. Responding during a conditioned stimulus associated with a mild (0.125 to 0.15 mA) electric shock (CS responding) and a control rate of responding (pre-CS responding) were recorded. Bicuculline (1.5 mg/kg) and Ro15-1788 (10 mg/kg) did not significantly affect either response rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of tiagabine (NO-328), a new potent and selective GABA uptake inhibitor

Tiagabine (NO-328) (R(-)-N-[4,4-bis(3-methylthien-2-yl)but-3-enyl]nipecotic acid, hydrochloride) is a new centrally acting GABA uptake inhibitor. The anticonvulsant activity of tiagabine was evaluated against seizures induced by methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), pentylenetetrazol, bicuculline, maximal electrostimulation (MES), or high intensity sound. The sedative actions of tiagabine were evaluated in tests for traction, rotarod performance and exploratory behavior. Finally, interoceptive properties of tiagabine were assessed using diazepam-, CGS 9896-, pentylenetetrazol-, or amphetamine-discriminating rats. Tiagabine was an effective anticonvulsant in doses which did not produce sedation or motor debilitation, although it was not potent against MES. In a manner similar to other anti-epileptic drugs, tiagabine potentiated dopaminergic function (methylphenidate-induced gnawing in mice) although it did not substitute for amphetamine in amphetamine-trained animals. Furthermore, although tiagabine antagonized DMCM-induced convulsions, it exhibited neither CGS 9896 or diazepam-like interoceptive effects, nor did it block (or potentiate) pentylenetetrazol-discrimination. Thus, GABA uptake inhibition represents a novel rationale for a valproate-like anticonvulsant drug therapy.

Inverse agonist properties of the FG 7142 discriminative stimulus

A two-lever, food-motivated discrimination was established between the benzodiazepine receptor partial inverse agonist FG 7142 (5.0 mg/kg) and its vehicle. The FG 7142 discriminative stimulus was pharmacologically characterized by testing trained rats with a variety of benzodiazepine receptor ligands. Administration of the inverse agonist DMCM (0.15-0.30 mg/kg) dose-dependently mimicked the FG 7142 stimulus. In contrast, the benzodiazepine receptor agonist chlordiazepoxide, partial agonist ZK 91 296, mixed agonist/antagonist CGS 9896 and antagonist RO 15-1788 blocked the FG 7142 cue. These results indicate that the FG 7142 discriminative stimulus is based on its inverse agonist activity. The generalization of FG 7142 to the anxiogenic/convulsant compound pentylenetetrazole (PTZ), but not to the anorectic agent norfenfluramine, indicates that the anxiogenic properties of FG 7142, rather than its anorectic actions, may underlie the FG 7142 discriminative stimulus.

Effects of agonists and antagonists at the GABA/benzodiazepine receptor on conditioned suppression in rats

Certain drugs generally regarded as GABA agonists, such as valproate and combinations of muscimol and baclofen, have been reported to produce apparent anxiolytic effects in various animal behavioral tests. The present paper reports two experiments on the effects of these agents on conditioned suppression in rats. In the first study, muscimol (0, 1.25 micrograms/kg or 1 mg/kg), baclofen (0, 1 mg/kg) and combinations of these treatments failed to alleviate conditioned suppression. Experiment Two showed that valproate (200 mg/kg) did attenuate conditioned suppression, and that its effects were antagonised by picrotoxin (1.5 mg/kg), but not by bicuculline (1.5 mg/kg), Ro 15-1788 (10 mg/kg) or by delta-amino-n-valeric acid (10 mg/kg). The findings are discussed in the context of the proposed GABA/benzodiazepine receptor complex, with the conclusion that there is little evidence for a mediating role of GABAa or GABAb receptors in such drug actions, and that the site of valproate action is probably the chloride ion channel associated with the receptor complex.

Ethanol and chronic diazepam interactions on conflict behavior in rats

Four groups of five rats each received training on the Geller-Seifter procedure and then received 0.5, 1.0 or 2.0 mg/kg diazepam (DZ) or vehicle, respectively, 15 min prior to the behavior sessions for 14 days. On Day 14 of DZ treatment rats also were administered 2.0 g/kg of ethanol (EtOH) 30 min prior to testing. Significant dose-related decreases in lever pressing occurred between groups over the 13 days of DZ treatment during the pre- and posttone periods. However, a significant effect attributable to days occurred only in the pretone period. During the tone period, the dose-related effects were greater but the variability also increased and specific contrasts between individual means were not significant. The largest increases in lever pressing associated with an anxiolytic effect occurred with 1.0 mg/kg of DZ at Day 6. On Day 14, all animals received 2.0 g/kg of EtOH and their performance was compared to performance on Day 13. During the pre- and posttone periods, the EtOH resulted in significantly less lever pressing in the control, 0.5 and 2.0 mg/kg DZ groups, indicating a sedative effect. Rats treated with 1.0 mg/kg of DZ did not exhibit this EtOH reduction in lever pressing. During the tone or conflict period, a significant dose-related increase in lever pressing due to the EtOH was observed. The increase in the control group was not significant. During repeated exposure to DZ there was a small but significant decrease in number of reinforcements which was enhanced by the EtOH at the 0.5 and 2.0 mg/kg DZ doses but not at 1.0 mg/kg DZ.(ABSTRACT TRUNCATED AT 250 WORDS)

Chlordiazepoxide and valproate enhancement of saline drinking by nondeprived rats: effects of bicuculline, picrotoxin and Ro15-1788

Drinking of 0.85% saline by nondeprived rats was significantly enhanced by chlordiazepoxide (5 or 10 mg/kg) and by valproate (100 or 300 mg/kg), drug effects being strongest in the earlier parts of a 30-minute test. When given alone, both bicuculline and picrotoxin significantly reduced saline drinking at 2.5 mg/kg, but not 1.5 mg/kg. Administration of valproate at either dose or of chlordiazepoxide (10 mg/kg) completely prevented bicuculline action and 5 mg/kg chlordiazepoxide reduced it. Picrotoxin, however, largely prevented the actions of both chlordiazepoxide and valproate. The increase in saline drinking induced by valproate (300 mg/kg) was also blocked by RO15-1788 (10 or 25 mg/kg). These findings are discussed in the context of the three-site model of the GABA/benzodiazepine receptor complex. It is concluded that drugs acting at the benzodiazepine site or the chloride ion channel affect saline drinking, but that there is little evidence of an important functional role for the GABAa site at present.

Riluzole antagonizes the anxiogenic properties of the beta-carboline FG 7142 in rats

The possible anxiolytic activity of riluzole, a drug which interferes with glutamic acid neurotransmission, was studied in rats using operant conflict procedures. In both "anxiolytic" and "anxiogenic" procedures, riluzole alone did not possess any anticonflict or proconflict effect at doses of 2 and 4 mg/kg PO. Riluzole over the same dose-range was able to antagonize the well known proconflict effect of the beta-carboline derivative FG 7142, an inverse agonist at the GABA-benzodiazepine-chloride ionophore receptor complex. This effect could be related to the possible interaction of riluzole with glutamic acid neurotransmission, since it has been demonstrated previously that beta-carbolines such as DMCM and beta-CCM were able to deplete the levels of aspartic and glutamic acids in rodent cortex, perhaps by enhancing release of amino acid neurotransmitters. If one subscribes to the hypothesis that the anxiety induced by beta-carboline derivatives is related to depression, riluzole might be of value in the treatment of anxiety related to depression.

Ethanol and diazepam interactions on conflict behavior in rats

Twenty-four adult male rats trained on the Geller-Seifter conflict procedure received: 1.0, 2.0, or 4.0 mg/kg diazepam at 15 min, 0.25, 0.5, 1.0, 2.0, or 4.0 g/kg of ethanol (EtOH) at 30 min prior to the test session, or combinations of the same diazepam doses and 0.5 g/kg of EtOH. Ethanol administration produced a significant (p less than 0.05) dose-related decrease in lever pressing during the periods prior to the introduction of the tone stimulus, compared to lever pressing during the corresponding control periods on the day prior to drug treatment. During the posttone periods 0.25 g/kg increased lever pressing (p less than 0.05), while doses of EtOH from 0.5 to 4.0 g/kg resulted in significantly lower levels of lever pressing in periods following the discontinuation of the tone. In the presence of the conflict evoking tone stimulus, rats treated with 2.0 g/kg of EtOH increased their rate of lever pressing, while animals receiving the other EtOH doses exhibited lever pressing comparable to that observed during the corresponding control periods. Administration of EtOH had no effect on the number of reinforcements received during the entire test session, except for a significant (p less than 0.05) reduction at the highest dose of 4.0 g/kg. Diazepam increased the number of reinforcements received during the entire test session as compared to those obtained during control sessions. Lever pressing was not affected by diazepam treatment during pretone or tone periods. During the posttone periods responses were significantly (p less than 0.05) increased in rats receiving the lowest dose of 1.0 mg/kg of diazepam but the other doses were without effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus properties of CGS 9896: interactions within the GABA/benzodiazepine receptor complex

Male rats were trained to discriminate the stimulus effects of CGS 9896 (30.0 mg/kg) from its vehicle. Once trained, discriminative performance was observed to be dose-responsive in the 3.75-30.0 mg/kg range and analysis of the dose-response curve generated an ED50 of 6.44 mg/kg. Generalization testing with chlordiazepoxide and pentobarbital produced CGS 9896-appropriate responding, whereas administration of the GABA agonists SL 75 102 resulted in 75% (intermediate) generalization to the CGS 9896 discriminative stimulus. Although full antagonism of the CGS 9896 cue was obtained following administration of Ro15-1788 and pentylenetetrazole, the inverse agonist DMCM failed to provide complete antagonism. These results suggest that the discriminative properties of CGS 9896 are consistent with its activity as a benzodiazepine receptor agonist.

Discriminative stimulus control by the anxiogenic beta-carboline FG 7142: generalization to a physiological stressor

Drug discrimination was employed to investigate the similarities between FG 7142-induced anxiogenesis and the stress produced by exposure to either a novel environment or to footshock. Eight rats were trained to discriminate between the stimulus properties of the beta-carboline FG 7142 (5.0 mg/kg) and its vehicle in a two-lever, food motivated operant task. Once trained, decreasing doses of FG 7142 produced fewer FG 7142-appropriate responses and the dose-response relationship yielded an ED50 of 1.45 mg/kg. Rats were subsequently subjected to two physiological/environmental stressors, footshock and novelty, and then tested in the discriminative paradigm. Exposure to novelty resulted in partial FG 7142-appropriate responding, whereas footshock sessions produced responding predominately on the FG 7142-appropriate lever. This is the first report of stimulus control by FG 7142 and it is likely that the interoceptive cue state produced by this compound is anxiogenic in nature, as reported to occur in man. The anxiogenic nature of the FG 7142 discriminative stimulus is supported by the generalization of FG 7142 to the state produced following stressful environmental manipulation.

Comparison of discriminative stimuli produced by full and partial benzodiazepine agonists: pharmacological specificity

Benzodiazepines produce discriminative stimuli that are stereospecific and antagonized by specific benzodiazepine receptor antagonists. The potency of these stimuli correlate with the ability of these compounds to bind to the benzodiazepine receptor complex. These data indicate that benzodiazepine stimuli are transduced via the benzodiazepine receptor. The underlying basis of these stimuli is unclear. Results with novel compounds that produce preclinical anxiolytic effects without the sedation and muscle relaxation of the classical benzodiazepines suggest that muscle relaxation may contribute to these stimuli. A direct comparison of discriminative stimuli established on a classical benzodiazepine agonist with the stimuli established on a partial benzodiazepine agonist supports the possibility that the classical benzodiazepine cue is mediated by a muscle relaxant effect, while the partial agonist cue is related to the anxioselectivity of the compound.

Discriminative stimulus properties of anxiolytic and sedative drugs: pharmacological specificity

In the first set of experiments rats were trained to discriminate a dose of 5 mg/kg chlordiazepoxide from saline. The chlordiazepoxide cue was antagonized by flumazepil (Ro 15-1788) and by CGS 8216, and generalized to a variety of anxiolytic and sedative drugs including the benzodiazepine receptor ligands zopiclone, suriclone, CL 218,872, CGS 9896, and ZK 91296. The novel imidazopyridine hypnotic, zolpidem, which also displaces benzodiazepines from their binding sites, failed to produce high levels of responding on the chlordiazepoxide-associated level except at a dose which greatly reduced rates of lever pressing. In further experiments rats were trained to discriminate a dose of 2 mg/kg zolpidem from saline. This dose produced reductions in response rates but an attempt to establish a lower dose of zolpidem as a discriminative stimulus was largely unsuccessful. Zolpidem-appropriate responding was produced by pentobarbital, chlordiazepoxide, triazolam, CL 218,872, clorazepate, lorazepam, quazepam, and zopiclone but only at doses which reduced response rates. The zolpidem cue was antagonized by flumazepil, CGS 9896, and ZK 91296. While the discriminative stimulus produced by chlordiazepoxide may be related to its anxiolytic action, the zolpidem stimulus is probably more closely associated with sedation. It was also tentatively concluded that the stimulus properties of chlordiazepoxide and zolpidem are produced by activity at different subtypes of benzodiazepine receptors.

Anxiogenic properties of beta-CCE and FG 7142: a review of promises and pitfalls

The behavioral effects of the benzodiazepine (BZP)-receptor partial inverse agonists, beta-CCE and FG 7142, are reviewed and the claim that these compounds possess "anxiogenic" properties is examined. Results obtained from human studies and global observations in animals, as well as those from experiments on aggression in animals or from studies of pentylenetetrazole discrimination cannot be considered conclusive. Contradictory findings have been obtained in studies using animal testing procedures derived from BZP-sensitive models of anxiety and in newer experimental situations and these are discussed from various theoretical perspectives: (1) the ability of the models to measure increased anxiety; (2) the possible ability of the drugs to reveal latent anxiety which generalizes from a punished to an otherwise non-fearful component of a testing procedure ("spreading anxiety"); (3) anxiety produced by a pro- or pre-convulsant state. Finally, several hypotheses are considered to account for the behavioral effects of beta-CCE and FG 7142 without assuming anxiogenic properties. These include the possible existence of different forms of anxiety, rate dependency, and drug-induced motivational changes. It is concluded that available data are insufficient to strongly support the notion that FG 7142 and beta-CCE are the anxiogenic drugs "par excellence" they are often claimed to be.

Discriminative stimulus properties of oxazepam in the pigeon

Five pigeons were trained to discriminate IM injections of oxazepam (4.0 mg/kg) from vehicle with responding maintained under a fixed-ratio 30 schedule of food delivery. Under test conditions, responding increased in a dose-dependent manner in all pigeons after the administration of other benzodiazepines including diazepam (0.01-1.0 mg/kg), temazepam (0.01-3.0 mg/kg), halazepam (0.1-56.0 mg/kg), and midazolam (0.1-1.0 mg/kg) as well as the barbiturate pentobarbital (2.0-8.0 mg/kg) and the non-benzodiazepine anxiolytic CL 218,872 (1.0-8.0 mg/kg). At the higher doses of each of these compounds, over 80% of responding occurred on the oxazepam-appropriate key. Cocaine (0.5-4.0 mg/kg), bupropion (3.0-56.0 mg/kg) and nortriptyline (3.0-56.0 mg/kg) failed to substitute for oxazepam even at doses that decreased rates of responding. The discriminative stimulus (DS) effects of the lowest doses of oxazepam and CL 218,872 that produced 100% drug-appropriate responding were blocked by the benzodiazepine antagonist Ro 15-1788. This antagonism was reversed by increasing the dose of the agonists. The DS effects of diazepam were antagonized partially by Ro 15-1788 (3 of 5 pigeons), and the antagonism was reversed by higher doses of diazepam in two of these pigeons. The DS effects of pentobarbital were antagonized by Ro 15-1788 in 2 of 5 pigeons, but the blockade was not reversed by higher pentobarbital doses.

Midazolam cue in rats: effects of drugs acting on GABA and 5-hydroxytryptamine systems, anticonvulsants and sedatives

The discriminative stimulus effect of midazolam, a short-acting benzodiazepine, was used for testing the effects of related drugs including agents thought to act at different sites in the proposed benzodiazepine receptor complex. Rats were trained in a standard two- bar operant conditioning procedure with food reinforcers delivered on a tandem schedule. The 0.4 mg/kg dose of midazolam used for training was well discriminated, typically yielding about 95% correct responding. There was no generalization to the GABA agonists muscimol and THIP, to the 5-HT antagonists cyproheptadine and methergoline, to buspirone, CGS 9896, ethanol, Ro 5-4864, promethazine, phenytoin sodium or sodium valproate. Muscimol and THIP also failed to potentiate the effects of midazolam. The GABA antagonist bicuculline weakly attenuated the discriminative effect of midazolam without impairing generalization to pentobarbitone, whereas the benzodiazepine inverse agonist FG 7142 did not attenuate the effect of midazolam. The results provide additional evidence for the notable specificity of the midazolam cue but do little to link the behavioural effects of benzodiazepines to GABA or 5- HT systems. Perhaps the potency, efficacy or selectivity of the GABA agonists was inadequate to produce the expected results. Only the effects of bicuculline, and those reported previously for picrotoxin, provided some support for the hypothesis that midazolam cue is mediated by the GABA system.

The role of benzodiazepine receptors in the discriminative stimulus properties of delta-9-tetrahydrocannabinol

Twenty male Sprague-Dawley rats were trained to discriminate 3.0 mg/kg delta-9-tetrahydrocannabinol (THC) from its vehicle. Following acquisition of this discrimination animals were tested for generalization to 3.0 mg/kg diazepam. Thirteen animals showed a generalization from THC to diazepam, whereas the remaining seven animals did not. The generalization curve for diazepam was dose-dependent from 0.1 to 10.0 mg/kg in the first group; the latter group showed no generalization from THC at any dose of diazepam in this range. No differences were found between these groups in the generalization curve for THC. The benzodiazepine antagonist Ro 15-1788 (2.0 mg/kg) antagonized the generalization to diazepam in the group that discriminated diazepam as THC. In contrast, Ro 15-1788 increased THC lever responding of 10 mg/kg diazepam in the group which did not generalize from THC. Ro 15-1788 did not alter the discriminability of THC in either group. THC also showed partial generalization to pentobarbital (1 to 10 mg/kg). The generalization was again complete in one subgroup and absent in another, but there was only a 43 percent overlap between the subgroups found with testing for generalization to diazepam. The percent THC lever responding with 3.0 mg/kg pentobarbital was increased by Ro 15-1788 in the group which generalized to diazepam, but not the other group. These data suggest that the discriminative stimulus properties of THC may have some commonality with the effects of diazepam in a subpopulation of rats trained to discriminate THC. These THC-like effects of diazepam are probably mediated by benzodiazepine receptors since they are antagonized by a specific benzodiazepine receptor antagonist.

Effects of chlordiazepoxide training dose on the mixed agonist-antagonist properties of benzodiazepine receptor antagonist Ro 15-1788, in a drug discrimination procedure

In experiment 1, rats (n = 12) were trained to discriminate the benzodiazepine (BDZ) compound chlordiazepoxide (CDP, 20 mg/kg, IP) from saline in a two-lever food-reinforced procedure, and subsequently were tested for stimulus control with different doses of CDP, Ro 15-1788 (a proposed BDZ receptor antagonist) and Ro 15-1788 plus 20 mg/kg CDP. Ro 15-1788 (0.63-40 mg/kg) dose-dependently antagonized CDP, and induced predominantly saline appropriate responding when administered alone. Thereafter, the same rats were retrained by progressively decreasing the training dose, to discriminate 2.5 mg/kg CDP from saline, and were tested again with the same compounds. Ro 15-1788 (0.16-40 mg/kg) now failed to antagonize CDP (2.5 mg/kg) and increased the percentage of drug-appropriate responding in a dose-related manner when administered alone. In experiment 2, separate groups of rats (n = 10) were similarly trained to discriminate either 15 or 3 mg/kg CDP from saline. Tests with CDP, Ro 15-1788 and Ro 15-1788 plus CDP (either 15 or 3 mg/kg) yielded similar results to experiment 1, suggesting that the training dose effects on generalization and antagonism of Ro 15-1788 were not affected by the manner in which the lower CDP dose acquired drug stimulus control. It is concluded that mixed agonist-antagonist properties are apparent after variations of the BDZ training dose in a drug discrimination procedure.

Midazolam cue in rats: effects of Ro 15-1788 and picrotoxin

The discriminative stimulus effect of midazolam, a short-acting benzodiazepine, was used for testing the effects of drugs thought to act as antagonists at different sites in the proposed benzodiazepine receptor complex. Rats were trained in a standard two-bar operant conditioning procedure with food reinforcers delivered on a tandem schedule. The 0.4 mg/kg dose of midazolam used for training was well discriminated, typically yielding at least 95% correct responding. The benzodiazepine receptor antagonist Ro 15-1788 blocked the discriminative effect of midazolam but did not influence generalization to pentobarbitone (7.5 mg/kg). The indirect GABA antagonist picrotoxin attenuated both generalization to pentobarbitone and its response rate-reducing effect. Picrotoxin had no effect on the discriminative effect of midazolam at 0.4 mg/kg but it blocked the effect of 0.1 mg/kg. Even in doses which reduced overall response rates, nicotine did not block discrimination of midazolam (0.4 mg/kg). The results are consistent with models which postulate a GABA-linked ion channel which is a site of action for barbiturates and which is "downstream" of the benzodiazepine receptor itself.

Intrinsic actions of the benzodiazepine receptor antagonist Ro 15-1788

The imidazodiazepine Ro 15-1788 is a benzodiazepine receptor antagonist that was initially reported to be lacking in intrinsic activity in a variety of test situations in which benzodiazepine-like effects can be identified. However, many recent studies have shown that this compound does indeed have intrinsic activity in a variety of behavioural, neurological, electrophysiological and biochemical preparations in both animals and man. The purpose of the present review is firstly to describe these intrinsic actions, and secondly to consider to what extent these intrinsic actions of Ro 15-1788 have implications for current concepts of the functioning of the benzodiazepine receptor.

Naloxone potentiates the depressant effect of chlordiazepoxide on spontaneous activity in mice

Recent reports suggest that at least some of the behavioural effects of benzodiazepines may involve interaction with endogenous opioid mechanisms. In the present study, chlordiazepoxide (CDP, 10-20 mg/kg, i.p.) induced dose-dependent reductions in spontaneous locomotor activity and rearing in mice. Although naloxone (1 mg/kg) was itself without effect, and did not alter behavioural changes induced by the lower dose of CDP, it enhanced the depressant effects of 20 mg/kg CDP on locomotor activity. Results are discussed in relation to previous studies in which opiate antagonists have been found to block or to have no effect upon behavioural actions of benzodiazepines.

Antagonism of the amphetamine cue by both classical and atypical antipsychotic drugs

Rats were trained to discriminate the stimulus properties of 1 mg/kg of d-amphetamine sulphate (AMPH) from saline in a two-lever task in which correct responding was reinforced with water under a fixed ratio (FR 32) schedule. Classical antipsychotic drugs from different chemical classes were all able to block the AMPH cue. Doses (mg/kg) inhibiting the cueing effect to 50% (ID50) were 0.035 (haloperidol), 0.04 (spiroperidol), 0.09 (cis(Z)-flupenthixol), 0.12 (trifluperazine), 0.15 (perphenazine), 0.92 (chlorpromazine) and 1.40 (pimozide). The AMPH cue was also antagonized by antipsychotic drugs that are considered atypical due to their relative lack of activity in conventional animal models or inability to produce extrapyramidal symptoms in the clinic. The following ID50 values were obtained: 0.88 (molindone), 1.22 (clozapine), 5.48 (metoclopramide), 15.4 (thioridazine) and 52.8 [-)-sulpiride). In addition, the AMPH cue was blocked by the D-1 selective dopamine (DA) antagonist, SCH 23390 (ID50 = 0.014 mg/kg). The abilities of these drugs to block the AMPH cue were unrelated to the drugs' effect upon the rate of responding. For example, some drugs (e.g. haloperidol, spiroperidol and SCH 23390) blocked the AMPH cue completely without any effect on the response rate. Furthermore, the non-antipsychotic phenothiazine, promethazine (2.5-12.5 mg/kg) failed to affect the AMPH cue although the drug strongly suppressed the response rate. However, the potent DA agonists, apomorphine (0.05-0.33 mg/kg) and lisuride (0.02-0.08 mg/kg), and the DA and norepinephrine agonist, DPI (0.4 and 0.8 mg/kg), did not mimic the AMPH cue or did so only partially. These results suggest that the 1 mg/kg AMPH cue depends on (DA) systems other than those involved in the stereotyped motor behavior commonly produced by high doses of AMPH or DA agonists. Low-dose AMPH discrimination may thus serve as a new model for studying antipsychotic drug action.

GABA and the behavioral effects of anxiolytic drugs

Much recent research has shown that benzodiazepine binding sites in the central nervous system are associated with GABA receptors. It is therefore possible that the pharmacological and therapeutic effects of benzodiazepines and drugs with similar profiles are mediated through GABAergic mechanisms. In this paper the evidence is considered for a possible involvement of GABA in the behavioral effects of anxiolytic drugs. There are a number of reports that the behavioral actions of anxiolytics can be antagonised by GABA antagonists such as bicuculline or picrotoxin but there are many contradictory findings and these drugs are difficult to use effectively in behavioral studies. In general, GABA agonists do not exert anxiolytic-like behavioral effects after systemic injection but intracerebral administration of muscimol has been shown to produce benzodiazepine-like actions. Although a number of questions remain unanswered, current evidence does not provide strong support for a role for GABA in the behavioral effects of anxiolytic drugs.

Can drug effects on anxiety and convulsions be separated?

The effects of benzodiazepines, barbiturates, a series of novel putative anxiolytic compounds and anxiogenic compounds are reviewed in animal tests of anxiety and on experimentally-induced seizures. It is clear from the data that drug effects on anxiety and convulsions are not always in the same direction; certain compounds are apparently both anxiolytic and proconvulsant, others are anxiogenic and anticonvulsant, others have varied effects depending on the test situation. It is suggested that this work necessitates considerable revision of our traditional concepts of an "anticonvulsant." The extent to which drug-induced anxiety is correlated with weak epileptiform activity in the brain is discussed. Finally, the Discussion considers a number of possible mechanisms that could underlie the separation of drug effects on anxiety and convulsions that is observed.

Discriminative stimulus properties of methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), an inverse agonist at benzodiazepine receptors

Rats (N = 8) were trained to discriminate the stimulus properties of the potent benzodiazepine (BZ) receptor inverse agonist methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) from saline in a two-lever operant task. The initial training dose of DMCM was 0.4 mg/kg at which the discrimination developed slowly; increasing the dose to 0.8 mg/kg resulted in rapid acquisition. However, since convulsions eventually developed during further training (sensitization), the training dose was finally individualized below the convulsive threshold (0.4-0.7 mg/kg). The DMCM cue was mimicked by FG 7142 (10 mg/kg), a non-convulsant anxiogenic beta-carboline, by pentylenetrazol (20-30 mg/kg), and by the GABA antagonist bicuculline (2 mg/kg). The DMCM cue was not, or marginally, blocked by diazepam (2.5 mg/kg) or pentobarbital (10-15 mg/kg). Furthermore, the BZ receptor antagonists CGS 8216 (2.5 mg/kg), ZK 93426 (20 mg/kg), and Ro 15-1788 (20-80 mg/kg) also did not, or only marginally, block the DMCM cue. However, the receptor antagonists (alone) substituted for DMCM although Ro 15-1788 was less effective. The partial BZ receptor agonist ZK 91296 (25 mg/kg), which is structurally similar to DMCM, blocked completely the DMCM stimulus effect. THIP (4 mg/kg) did not block the DMCM cue. To explain these results, we suggest that the repeated DMCM treatment, necessary for maintaining the discrimination, shifts the balancing point ("set-point") for positive (i.e., BZ-like) agonist efficacy versus inverse agonist efficacy, towards inverse action. This hypothesis was supported by the finding of an enhanced ability of GABA to reduce 3H-DMCM binding to cortical neuronal membranes of animals treated chronically with DMCM in a regimen similar to that used to maintain the DMCM discrimination. Furthermore, this treatment did not affect baseline 3H-DMCM binding, baseline or GABA stimulated 3H-diazepam binding, or 35S-TBPS binding (to chloride channels).

The effect of drugs on the acquisition of stimulus control in a conditioned suppression procedure

Rats were trained to press a lever under a variable-interval (VI) schedule of water reinforcement. After stable responding had developed, a 4.5-KHz tone (CS) was conditioned classically to a 2.5-mA electric shock (US) in groups of animals which had been given various psychoactive drugs or saline. Twenty-four hours later, a stimulus generalization test was conducted in the absence of drug; during this session, tones that varied in frequency around 4.5 KHz were presented while the animals were responding under the VI schedule. In animals conditioned under saline, all tones (non-differentially) suppressed responding which, however, recovered gradually over time. This suppressive effect was eliminated by lysergic acid diethylamide (LSD; 0.2 and 0.32 mg/kg), cocaine (20 mg/kg), diazepam (2.5 mg/kg), lisuride (0.08 mg/kg), mescaline (20 mg/kg) and 5-methoxy-N,N-dimethyltryptamine (4 mg/kg), and was attenuated by amphetamine (4 mg/kg), pentobarbital (15 mg/kg) and morphine (4 mg/kg). Atropine (10 mg/kg), scopolamine (1 mg/kg), clonazepam (0.5 mg/kg), and chlorpromazine (4 mg/kg) did not alter the suppressive effect of the tone. The serotonin antagonist BC-105 (6 mg/kg) reversed the effect of 0.2 mg/kg of LSD. These results suggest (1) that drug-induced stimuli may "overshadow" other (e.g., external) stimuli during classical conditioning and, (2) that drugs might affect behavior by altering processes (stimulus control or others) that do not simultaneously involve response or motor control.

Diazepam-induced place preference conditioning: appetitive and antiaversive properties

The place conditioning paradigm was used to examine the reinforcing properties of diazepam. Rats were injected with diazepam (0.5-5.0 mg/kg, IP) and 30 min later were confined for 30 min to one side of a shuttle box, in which each of the two compartments had distinctive features. On alternate (control) days they received vehicle injections and were confined for 30 min to the opposite side. At almost all doses tested, diazepam produced place preference for the distinctive compartment that had been previously associated with the drug. Preference for the drug side developed regardless of whether diazepam was paired or unpaired with the least-preferred side, and regardless of whether testing was carried out in the undrugged or in the drugged state. The rats preferred the drug side over a novel compartment, but they did not change their initial preference for the side when diazepam was given after removal from the training box. Animals injected with meprobamate (70 mg/kg, PO), a non-benzodiazepine anxiolytic, also developed conditioned preference for the drug side, comparable to that seen following cocaine hydrochloride (10 mg/kg, IP). The diazepam (2.5 mg/kg)-induced place preference was antagonized by CGS 8216 (3 mg/kg, IP), picrotoxin (2 mg/kg, IP) and naloxone (0.8 mg/kg, SC), injected 3 min before and 15 and 20 min after diazepam respectively. Sodium valproate (200 mg/kg, IP) did not influence diazepam (1 mg/kg)-induced place preference. Sodium valproate by itself had marginal effects on place conditioning. Picrotoxin and naloxone, but not CGS 8816, produced place aversion which, in the case of picrotoxin, was due to state dependent learning.(ABSTRACT TRUNCATED AT 250 WORDS)