Antagonism of discriminative stimuli produced by anxiogenic drugs as a novel approach to bioassay anxiolytics

Chlorpyrifos shares stimulus properties with pentylenetetrazol as evaluated by an operant drug discrimination task

Based on previous data from elevated plus-maze tests suggesting a possible anxiogenic effect of the insecticide chlorpyrifos (CPF), the experiment reported here was designed to determine whether this organophosphate (OP) caused an interoceptive discriminative stimulus (IDS) in rats similar to that produced by the anxiogenic drug pentylenetetrazol (PTZ). Rats were trained to discriminate PTZ (20 mg/kg) from saline, using a drug discrimination procedure. When appropriate lever selection was achieved, generalization tests were performed. Tests of various doses of PTZ showed that the drug exerts dose-dependent discriminative control over response. Two more generalization tests were conducted with 250 mg/kg of CPF and 76.8 mg/kg of LiCl for up to 9 days. Results revealed that CPF (250 mg/kg s.c.) produced a PTZ-like IDS that fully substituted for PTZ 24 h after injection and that subjective effects remain for at least 6 days. However, administration of LiCl did not produce any generalization to PTZ on any of the days tested. These results suggest that CPF shares a site of action, and perhaps functional properties, with PTZ that last for several days, are not due to general malaise and should be taken into account in the use of cholinesterase inhibitors in the treatment of different types of dementia.

The discriminative stimulus effects of pentylenetetrazol as a model of anxiety: recent developments

Pentylenetetrazol (PTZ), a GABA(A) receptor antagonist and prototypical anxiogenic drug, has been extensively utilized in animal models of anxiety. PTZ produces a reliable discriminative stimulus which is largely mediated by the GABA(A) receptor. Several classes of compounds can modulate the PTZ discriminative stimulus including 5-HT(1A), 5-HT(3), NMDA, glycine, and L-type calcium channel ligands. Spontaneous PTZ-lever responding is seen in trained rats during withdrawal from GABA(A) receptor compounds such as chlordiazepoxide and diazepam, and also ethanol, morphine, nicotine, cocaine, haloperidol, and phencyclidine. This effect is largely mediated by the GABA(A) receptor, which suggests that anxiety may be part of a generalized withdrawal syndrome mediated by the GABA(A) receptor. There are also important hormonal influences on PTZ. Corticosterone plays some role in mediation of its anxiogenic effects. There is a marked sex difference in response to the discriminative stimulus effects of PTZ, and estrogens appear to protect against its anxiogenic effects. Further work with the PTZ drug discrimination is warranted for characterization of anxiety during withdrawal, and the hormonal mechanisms of anxiety.

Effects of NG-nitro-L-arginine methyl ester, 7-nitro indazole, and agmatine on pentylenetetrazol-induced discriminative stimulus in Long-Evans rats

This study was undertaken to determine any role that nitric oxide (NO) may play in the discriminative stimuli produced by pentylenetetrazol (PTZ). The PTZ-induced discriminative stimulus is pharmacologically similar to anxiety in humans and is used in a behavioral assay of anxiety (the PTZ model of anxiety). In the present study, effects of L-N(G)-nitro arginine methyl ester (L-NAME), 7-nitroindazole (7-NI) and agmatine, NO synthase (NOS) inhibitors, on PTZ-induced discriminative stimulus were investigated in male Long-Evans rats (330-350 g). Rats were trained to discriminate PTZ (16 mg/kg) from saline using a two-lever, food-reinforced choice procedure (FR 10). The rats that met the training criteria were injected with L-NAME (15, 30, and 60 mg/kg), 7-NI (15 and 30 mg/kg), agmatine (20, 40, and 60 mg/kg), and saline or vehicle intraperitoneally before each test. They were tested for the PTZ-discrimination to determine if the NOS inhibitors produce discriminative stimulus similar to PTZ or if they block PTZ-induced discrimination. Treatment with the NOS inhibitory drugs neither substituted for PTZ nor altered the PTZ lever selection in any other way. These findings suggest that PTZ-induced discriminative stimulus may not be related to NO-mediated central mechanisms.

Ventilatory effects of negative GABA(A) modulators in rhesus monkeys

This study examined changes in ventilation produced by negative gamma-aminobutyric acid(A) (GABA(A)) modulators in rhesus monkeys. The effects of Ro 15-4513, beta-CCE and beta-CCM were examined in four rhesus monkeys breathing air or 5% CO2 in air. When monkeys breathed CO2, minute volume (VE) and frequency (f) increased, on average, to 158 and 140% of control (air), respectively. Ro 15-4513 did not modify ventilation in monkeys breathing either gas mixture; however, beta-CCE and beta-CCM increased VE and f in monkeys breathing air to between 123 and 141% of control and had no effect on ventilation of 5% CO2. Increased ventilation produced by the negative GABA(A) modulators appeared to be maximal, because ventilation was not further enhanced when the dose was increased three-fold. Each of the three negative GABA(A) modulators reversed the decreases in ventilation produced by diazepam, suggesting that these drugs are acting at benzodiazepine receptors; however, the increased ventilation produced by beta-CCE and beta-CCM might suggest that they have more negative efficacy than Ro 15-4513. These data extend previous findings by showing that some negative GABA(A) modulators (Ro 15-4513) do not alter ventilation and further indicate that changes in ventilation can be used to evaluate efficacy differences among GABA(A) modulators.

Discrete versus cumulative dosing in dose-response discrimination studies

This study describes the results of a 'side-by-side' comparison of two measurement techniques and two dosing regimens in a discrimination study using rats trained to either 10 mg/kg cocaine or 2 mg/kg 3,4-methylenedioxymethamphetamine (MDMA). The measurements employed were either quantal or quantitative; the former an all-or-none correct lever selection measure and the latter a measure of all responses made at the time that the criterion for selection was met. The dosing regimens were either a discrete single injection of lower doses than used in training or a cumulative dose administration sequence, in an ascending order, during one session on two separate occasions. Results indicate that the cumulative dose-response relationships, as indicated by both the slope of the curve or the generated ED50 value, for the discrete and cumulative dose response curves do not significantly differ. In addition, both the quantal and quantitative measurements yield almost identical ED50 values, thus allowing for accurate comparability of drug-discrimination data using different techniques. The present experimentation employed two drugs known to produce heightened response rates which would not allow for behavioral suppression at the highest doses used either in discrete or cumulative regimens. The pharmacokinetics of the two drugs employed in the discrimination tests are considered and discussed in light of the advantages and disadvantages of each of the two methods employed.

A behavioural and pharmacological evaluation of the discriminative stimulus induced by pentylenetetrazole in the pig

The anxiogenic nature of the interoceptive discriminative stimulus induced by pentylenetetrazole (PTZ) was investigated by examining the discriminatory behaviour of PTZ conditioned pigs during a conditioned emotional response (CER). A CER was induced in a nonoperant situation, by pairing a tone stimulus with the application of a mild, non-injurious electric shock. Subsequent presentation of the conditioned tone stimulus alone produced a generalisation to the PTZ cue. This generalisation of the conditioned emotional state (CES) to the PTZ cue was antagonised by pretreatment with diazepam (0.5 mg/kg, PO; 30 min). The PTZ stimulus was also antagonised by diazepam (0.5 mg/kg, PO; 30 min) but not by an anticonvulsant dose of ethosuximide (30 mg/kg, PO; 1-3 h), providing further confirmation of the anxiogenic nature of the PTZ cue. Our results demonstrate the validity of the PTZ discrimination paradigm in pigs as a test of anxiety.

Discriminative stimulus properties of a new anxiolytic, DN-2327, in rats

In an operant learning lever-pressing procedure on an FR10 schedule of milk reinforcement, male Wistar rats were trained to discriminate between saline and 3 mg/kg IP DN-2327, a new anxiolytic which acts on benzodiazepine receptors, 3 mg/kg IP diazepam or 15 mg/kg IP pentylenetetrazol (PTZ). More than 80% appropriate lever responding was established after 27, 38 and 44 daily training sessions with DN-2327, diazepam and PTZ, respectively, as the training drug. Although rats trained with DN-2327 dose-dependently generalized to various doses of DN-2327 and diazepam, the cue of DN-2327 was more potent than that of diazepam: ED50 values of DN-2327 and diazepam for stimulus generalization were 0.30 and 0.66 mg/kg, respectively. These animals partially generalized to pentobarbital (1-10 mg/kg) but did not generalize to buspirone (0.1-10 mg/kg). Rats trained with diazepam dose-dependently generalized to various doses of DN-2327, diazepam and pentobarbital with ED50 values of 0.51, 0.47 and 4.5 mg/kg, respectively, but did not generalize to buspirone. In rats trained with PTZ, DN-2327 and diazepam antagonized the discriminative stimulus produced by 15 mg/kg PTZ in a dose-dependent manner with ED50 values of 0.27 and 0.83 mg/kg, respectively, but buspirone neither antagonized nor was able to substitute for the PTZ-induced stimulus. The cue of DN-2327 was antagonized by flumazenil dose-dependently as was that of diazepam.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus properties of RU 33965, a benzodiazepine receptor weak partial inverse agonist

Rats were trained to discriminate the low-efficacy benzodiazepine receptor inverse agonist RU 33965 from vehicle in a two-lever discrimination task on a fixed ratio (FR) 20 schedule. Consistent discrimination was obtained at 0.5 mg/kg PO RU 33965. Both leptazol and stronger inverse agonists (FG7142, S-135, RU 34000) substituted for the cue. The weak inverse agonists/antagonists RU 33094, RU 34030, Ro 15-1788, and ZK 93426 also substituted for the cue with the latter two compounds being particularly potent. The agonist and partial agonists diazepam, RU 33203, and RU 39419 did not substitute for the RU 33965 cue but RU 39419 antagonised it. The full agonists diazepam and loprazolam only consistently antagonised the cue when given IP 5 min pretest. These data suggest that the RU 33965 cue results from its weak inverse agonist activity at benzodiazepine receptors, but kinetic factors must be considered when interpreting drug effects in discrimination studies.

Evidence for noradrenergic involvement in mediating the FG 7142 discriminative stimulus

Rats were trained to discriminate the stimulus properties of the benzodiazepine receptor partial inverse agonist beta-carboline-3-carboxylate acid methyl amide (FG 7142) (5.0 mg/kg) or the alpha 2-adrenergic receptor antagonist 17 alpha-hydroxyyohimban-16 alpha-carboxylic acid methyl ester (yohimbine) (3.0 mg/kg) from vehicle in a two-lever, food-motivated operant task. These compounds have in common a beta-carboline structure and anxiogenic behavioral profiles. The yohimbine discriminative stimulus was mimicked by the alpha 2-adrenergic receptor antagonist idazoxan and antagonized by the alpha 2-adrenergic receptor agonist clonidine, indicating that the yohimbine stimulus was mediated through the alpha 2-adrenergic receptor. The anxiogenic beta-carbolines FG 7142, 1,2,3,4-tetrahydro-beta-carboline (THBC), and norharmane, the anxiogenic/convulsant agent pentylenetetrazole (PTZ), and two physiological stressors failed to mimic the yohimbine discriminative stimulus. In contrast, both yohimbine and idazoxan dose responsively mimicked the anxiogenic FG 7142 stimulus. The present results demonstrate that an asymmetrical generalization exists between the discriminative stimuli produced by yohimbine and FG 7142. Furthermore, these data suggest that yohimbine can produce a multicomponent discriminative stimulus, part of which may be anxiogenic in nature. The ability of alpha 2-adrenergic receptor antagonists to mimic the FG 7142 cue suggests that activation of the noradrenergic system may underlie cues produced by benzodiazepine receptor inverse agonists.

The detection of changes in psychological state using a novel pharmacological conditioning procedure

Using a novel pharmacological conditioning procedure, pigs were conditioned to discriminate the effects of a subconvulsant dose of the anxiogenic drug pentylenetetrazole (PTZ; 2.8-10 mg/kg, i.v.) from saline. The operant chamber provided two levers at which pigs were trained to press at a fixed ratio of 20 presses per food reward (FR 20). The animals were conditioned to select both levers alternately following saline treatment and to select one lever only following PTZ treatment. This procedure enabled the onset and offset of the PTZ stimulus to be detected within single test sessions; infusion of PTZ to animals already selecting both levers alternately after a saline pretreatment induced a period of response exclusive to the PTZ lever followed by a return to an alternation of lever selection response. The ability of the novel procedure to detect the time course of the drug cue should improve future evaluations of the psychological states induced by centrally acting drugs. With PTZ as the training drug, the novel procedure presents a valuable means to study the neurobiology of anxiety.

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.

Aversive and compensatory classical conditioning with diazepam as conditional stimulus

The purpose of the first experiment was to investigate whether diazepam could acquire anxiogenic properties by signalling an aversive event. Rats were trained in an operant chamber in the pentylenetetrazol (PTZ) model of anxiety. Thereafter the animals were divided into groups that received classical diazepam conditioning (Group 1), and conditioning of diazepam + tone (Group 2). In the test phase diazepam was injected prior to placement in the operant chamber. Group 2 selected the PTZ-appropriate lever more often than the other groups, indicating that the tone induced anxiety, and diazepam did not. Tones and shock may therefore be more easily associated than diazepam and shock. The second experiment investigated this. Rats were trained the same way as in the first experiment. Thereafter the experimental group received injections of a small dose of diazepam prior to a second injection of a large dose of diazepam. The hypothesis was that a compensatory anxiogenic conditional response to diazepam's anxiolytic effect should be elicited by the small dose. There were no differences between the groups in lever selection, indicating that a compensatory anxiogenic response was not elicited.

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.

The effects of 5-HT1B characterizing agents in the mouse elevated plus-maze

Although the serotonergic system has been implicated in the modulation of anxiety states, the specific receptor subtypes that mediate these states require clarification. The effects of drugs that act preferentially at 5-HT1B receptors were evaluated on the behavior elicited in the elevated plus-maze, an animal model of anxiety. Variations in the intensity of light affected mouse behavior in the plus-maze; lower light intensity increased the entries to and time spent on the open arm in a manner similar to that seen with stress-attenuating circumstances. Opposite effects were observed in high light-intensity, similar to effects seen under elevated stress conditions. Chlordiazepoxide produced increased entries and time spent on the open arm, whereas pentylenetetrazol (PTZ) produced opposite effects. The preferential 5-HT1B agents TFMPP and mCPP exhibited a profile similar to PTZ. The effects of TFMPP in the plus-maze were reversed by chlordiazepoxide, but not by the benzodiazepine receptor antagonist flumazenil, which suggests that this effect is not directly mediated by benzodiazepine receptors. The decreased entries and time spent on the open arm of the maze following TFMPP or mCPP administration was possibly mediated by an antagonistic action at 5-HT1B receptors, since this effect was reversed by the selective 5-HT1B agonist CGS 12066B. The present study further demonstrates the utility of mouse behavior in the elevated plus-maze as a model for identifying anxio-modulatory substances.

Animal models of drug withdrawal symptoms

There have been few attempts to model subjective symptoms of drug withdrawal using animals as subjects. Two approaches for developing such models are reviewed. First, using drug discrimination methodology, it may be possible to train animals to detect the effects of withdrawal. This method has two difficulties: 1) the only discriminations trained to date involve precipitated withdrawal, and 2) the stimulus controlling behavior is difficult to specify. Second, withdrawal from many drugs of abuse produces the symptom of anxiety, and it seems likely that animal models of anxiety could be useful for studying drug withdrawal. This hypothesis has been explored most fully using subjects trained to detect the discriminative stimulus properties of the putative anxiogenic drug pentylenetetrazole (PTZ). Withdrawal from benzodiazepines or ethanol substitutes fully for PTZ, and withdrawal from cocaine, morphine, and nicotine substitutes partially for PTZ. Emerging data suggest that other animal models of anxiety may also be useful for detecting drug withdrawal. The final portion of this review examines a behavioral test that is very sensitive for detecting physical signs of withdrawal in animals. In subjects maintained on an operant baseline using food as a reinforcer, withdrawal from a drug of dependence frequently is associated with disruption of that operant behavior. For example, tetrahydrocannabinol and cocaine, drugs that are not traditionally seen as having significant withdrawal signs, produce disruption of operant responding when high-dose administration is terminated, and their readministration reverses this behavioral disruption. Based on the observation that withdrawal is associated with anxiogenic stimuli, we suggest a method to determine if disruption of operant behavior may be related to these stimuli.

Drug discrimination models in anxiety and depression

Drug discrimination is a technique for investigating the stimulus properties of centrally active drugs. Although many studies have employed animals to investigate the stimulus properties of substances used clinically for the treatment of anxiety and depression, it would be a mistake to consider the internal discriminative stimuli as being related specifically to the anxiolytic or antidepressant properties of these drugs. Rather drug cues are better considered as relating to the pharmacological action of classes of compounds. Thus, benzodiazepine cues generalize to other compounds acting at benzodiazepine receptors, but not to substances (anxiolytic or otherwise) acting at 5-HT1A receptors. Similarly, antidepressants with different pharmacological properties, for example the tricyclic imipramine, or the phenylaminoketone buproprion produce distinct, unrelated discriminative stimuli. For this reason, the limits of drug discrimination techniques for investigating novel anxiolytic or antidepressant drugs should be clearly recognized. Attempts to identify an anxiogenic discriminative stimulus using pentylenetetrazole have also been misguided. In this technique it has proven difficult to separate unequivocally the pharmacological proconvulsant effects of the drug from the psychological construct anxiety. Nevertheless, drug discrimination remains a valuable technique for investigating pharmacological interactions in animals and man.

Sensitivity of pentylenetetrazol discrimination increased by a stimulus fading technique

The interoceptive stimulus produced by pentylenetetrazol (PTZ) is pharmacologically similar to anxiety and is used in a behavioral assay for anxiety-related stimuli (the PTZ model of anxiety). The stimulus fading technique was tested as a method to increase the sensitivity of this assay. Rats were trained with food-reward to press one lever after injection of PTZ and an alternate lever after saline. Rats initially learned the discrimination at a PTZ dose of 20 mg/kg. They were then trained with sequentially lower doses until they reliably discriminated a PTZ dose of 10 mg/kg. Substitution tests with other doses and drugs showed that, after the fading procedure, dose-response curves were shifted to lower doses for PTZ, Ro 5-3663, and nicotine Similarly, the dose of diazepam required to block the low dose of PTZ was lower than that required to block the higher dose of PTZ. These results indicated that the sensitivity of the discrimination was enhanced in rats trained to discriminate a lower dose of PTZ. Doses of nikethamide, cocaine, and yohimbine that did not substitute for the higher dose of PTZ also did not substitute for the lower dose. These data suggest that rats can be trained to discriminate a low dose of PTZ by the stimulus fading technique. Moreover, they suggest that this training method does not compromise the specificity of the discrimination.

Ethanol and Ro 15-4513: behaviour maintained by operant procedures (DRL-72s and PTZ-drug discrimination) in rats

The proposed amethystic imidazobenzodiazepine Ro 15-4513 and ethanol (ETOH) were examined in rats using two operant procedures, differential reinforcement of low rates of responding (DRL), and drug discrimination learning (DDL). In the first bar-pressing responses occurring 72 s or longer after the last reinforcement were rewarded; responses occurring earlier reset the time schedule. In the latter, animals were trained to discriminate between the effects of the analeptic pentylenetetrazole (PTZ) and the non-drug condition; the schedule of reinforcement was FR-10. Water was the reinforcer. A dose of 1000 mg/kg ETOH decreased the rate of bar pressing in the DRL experiment; doses of 300 and 560 mg/kg ETOH did not. The decrease was not attenuated by Ro 15-4513. No significant deviations from baseline responding occurred with Ro 15-4513 (1, 3 10 mg/kg). The number of reinforcements increased significantly after ETOH (1000 mg/kg), but not after Ro 15-4513. Combinations of the two agents produced increases in the number of reinforcements. Changes in DRL behaviour induced by diazepam (1 and 10 mg/kg) were normalized by 3 mg/kg Ro 15-4513. In DDL, Ro 15-4513 (10 mg/kg) substituted for PTZ; ETOH did not. Diazepam and Ro 15-1788 attenuated the response generalization from Ro 15-4513 to PTZ; ETOH did not. There was a dose-related increase in the time to complete the DDL tests after ETOH treatment; addition of Ro 15-4513 increased the time further. In conclusion, antagonism between Ro 15-4513 and ETOH did not occur in the present studies; data are furthermore consistent with the view that Ro 15-4513 acts as a partial inverse benzodiazepine agonist.

Cueing effects of anxiolytic benzodiazepines (DZP and Ro 11-3128): stereospecificity and antagonism by the convulsant benzodiazepine Ro 5-3663

Two groups of gerbils were trained in a T-maze to discriminate between the vehicle condition (4 ml/kg) and either of the benzodiazepine (BDZ) agonists diazepam (DZP) and Ro 11-3128; administration (5.6 mg/kg) was IP 5 min prior to training onset. Once trained, novel doses and drugs were assessed in test sessions interposed between the regular training days. A dose-related generalization effect occurred with both compounds (range 0.1-5.6 mg/kg), the effect being similar at both the 5 and 15 min test intervals; the two intervals were evaluated after a single injection. The lack of generalization of Ro 11-3624 (range 5.6-56 mg/kg) indicates a stereoisomeric separation of BDZ agonist activity. Ro 5-4864 (range 17.5-56 mg/kg), an agent chemically/structurally related to DZP, did not produce DZP responding at either of the two test inverals; clear-cut convulsant activity occurred at the 15 min interval. The convulsant BDZ compound Ro 5-3663 (3 and 10 mg/kg) antagonized the DZP stimulus irrespective of whether Ro 5-3663 was given either prior to, simultaneously with, or shortly after the DZP injection.

Potential use of drugs modulating 5HT activity in the treatment of anxiety

1. It has been long suggested that central 5HT-mediated systems may be involved in modulation of anxiety and in the anxiolytic effect of benzodiazepines. However, recent evidence has questioned this hypothesis, particularly with respect to the mode of action of benzodiazepines. 2. Development of 5HT agonists and antagonists selective for different 5HT receptor sub-types (5HT1A, 5HT1B, 5HT2, 5HT3) has opened a new avenue for investigation of the potential role of 5HT in anxiety. 3. Buspirone is clinically active in the treatment of anxiety and it, and other anxiolytic candidates, gepirone and isapirone, may act as agonists (or perhaps partial agonists) on 5HT1A receptors. 4. The prototype 5HT1A agonist 8OH-DPAT may also have potential anxiolytic effects. 5HT1A agonists may act to suppress the activity of 5HT neurones as a major part of their action. 5. Although there is some supporting evidence, there is no clear indication of anxiolytic activity with agonists with some selectivity for 5HT1B sites (RU24969, mCPP, TMPP). 6. A selective 5HT2 antagonist, ritanserin, has anxiolytic effects in clinical studies but, like the 5HT1A agonists, does not show a similar profile to benzodiazepines in models of anxiety. 7. This raises the question of clinical predictivity of the various models used. 8. A recently developed 5HT3 antagonist, GR38032F, has been claimed to possess potential anxiolytic activity but its mode of action in this respect requires further elucidation.

Enhancement of a diazepam withdrawal symptom by bicuculline and yohimbine

The role of the GABA system in producing a pentylenetetrazol-like interoceptive discriminative stimulus during withdrawal from diazepam was investigated in rats by determining the sensitivity of this system to GABAergic drugs before and after chronic treatment with diazepam. Food-restricted rats were trained to obtain a reward of food by responding on one lever following an injection of pentylenetetrazol (PTZ; 20 mg/kg) and the other lever following an injection of saline (1 ml/kg). After rats had acquired this discrimination, the effectiveness of Ro 15-1788, bicuculline and yohimbine to substitute for pentylenetetrazol was determined. Prior to chronic treatment with diazepam, rats selected the appropriate lever for saline after Ro 15-1788 and the appropriate lever for pentylenetetrazol after bicuculline (0.04-2.5 mg/kg) or yohimbine (0.16-5.0 mg/kg). Although the selection of the appropriate lever for pentylenetetrazol was dose-dependent, full substitution for pentylenetetrazol was not obtained with either drug as larger doses of bicuculline produced convulsions while the rats began to select the appropriate lever for saline after larger doses of yohimbine (bell-shaped curve). Diazepam blocked the pentylenetetrazol-like interoceptive discriminative stimulus for bicuculline. The rats were then injected with diazepam (80 mg/kg/8 hr) for 24 days. Upon termination of the administration of diazepam, the animals were tested for lever-selection following the administration of saline, Ro 15-1788 (10 mg/kg), bicuculline (0.32, 0.64 and 1.25 mg/kg) or yohimbine (0.16, 0.64 and 2.5 mg/kg). After saline, 33% of the rats selected the appropriate lever for pentylenetetrazol whereas selection of this lever was enhanced after Ro 15-1788, bicuculline or yohimbine.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of the discriminative stimulus produced by pentylenetetrazol by centrally administered drugs

Pentylenetetrazol is anxiogenic in humans and produces an interoceptive discriminative stimulus in rats which is mimicked by anxiogenic drugs and other treatments and antagonized by anxiolytic drugs. It was proposed that the discriminative stimulus of pentylenetetrazol originates centrally. This hypothesis was tested by injecting small amounts of anxiogenic or anxiolytic drugs into the brain and comparing their ability to mimic or block, respectively, the response to pentylenetetrazol, observed after systemic injection. Food-restricted rats were trained in a two-lever operant task to discriminate the interoceptive discriminative stimulus produced by pentylenetetrazol. Intraperitoneal or intracerebroventricular injection of Ro 5-3663 was substituted in a dose-dependent manner for the stimulus produced by systemically administered pentylenetetrazol. Diazepam injected systemically, blocked the pentylenetetrazol-like stimulus associated with Ro 5-3663 administered systemically or centrally. Midazolam injected intracerebroventricularly and in a dose-dependent manner, antagonized the discriminative stimulus produced by systemic injection of pentylenetetrazol. When injected into the amygdala, midazolam also antagonized in a dose-dependent manner the pentylenetetrazol-induced stimulus. Thus, these data suggest that there are sites in the CNS for both the initiation of a pentylenetetrazol-like stimulus by Ro 5-3663 and the antagonism of the stimulus produced by pentylenetetrazol by midazolam.

Anxiety-like subjective effect of ethanol antagonist RO 15-4513 demonstrated in pentylenetetrazol discrimination

Ro 15-4513, a benzodiazepine-receptor ligand which antagonizes ethanol, was tested in the pentylenetetrazol discrimination, a bioassay for anxiogenic drugs. Rats were trained with food reward to discriminate pentylenetetrazol (PTZ) from saline in a two-lever operant task. In lever-selection tests, rats selected the PTZ lever both after PTZ and after Ro 15-4513. The PTZ-like stimulus produced by Ro 15-4513 was blocked by diazepam and by the benzodiazepine receptor blocker Ro 15-1788. Substitution for the anxiogenic drug PTZ, and blockade by the anxiolytic diazepam, support the hypothesis that Ro 15-4513 is anxiogenic; blockade by Ro 15-1788 suggests that the PTZ-like stimulus produced by Ro 15-4513 occurs through its action at the benzodiazepine receptor.

Pharmacology of the pyrazolo-type compounds: agonist, antagonist and inverse agonist actions

Five compounds that bind to the benzodiazepine (BZ) receptor, but show different pharmacological characteristics from the classical BZs, are profiled. CGS 8216 is a BZ antagonist/inverse agonist that reverses the effects of diazepam and also acts as a proconvulsant. CGS 9895 is also a potent BZ antagonist. In addition, this compound shows an anxiolytic profile. CGS 9896, CGS 17867A and CGS 20625 are BZ agonists (i.e., anxiolytics and anticonvulsants) which produce varying magnitudes of antagonist effect. All of these compounds are unique from the classical BZs in that each has a reduced propensity to produce the sedative and/or muscle relaxant effects characteristically associated with BZs.