The chlordiazepoxide/pentylenetetrazol discrimination: characterization of drug interactions and homeostatic responses to drug challenges

Evidence for PTZ-like cues as a function of time following treatment with chlordiazepoxide: implications for understanding tolerance and withdrawal

The present study used a two-lever, drug-discrimination procedure to train rats to discriminate between the cues associated with 5 mg/kg of the anxiolytic, chlordiazepoxide (CDP) and 15 mg/kg of the anxiogenic, pentylenetetrazol (PTZ), to investigate the relationship between withdrawal and acute tolerance. Training doses of the two drugs were chosen so that rats responded about equally on both levers when tested on saline (SAL). Following acquisition of the discrimination, rats were injected with 10 mg/kg CDP and tested for lever choice at various intervals from 6 h to 192 h. These tests revealed that cues associated with CDP withdrawal lasted approximately three times longer than the cues associated with the drug's primary effects. At the shortest retest interval (6 h) after treatment with 10 mg/kg CDP, rats responded primarily on the CDP lever, followed by a shift to predominant responding on the PTZ lever at the 16 h and 24 h intervals before returning to predrug, baseline levels at the longer intervals (48-192 h). In order to investigate the relationship between tolerance and withdrawal to the cue properties of CDP, CDP dose-response curves were determined 24 h following treatment with SAL or 10 mg/kg CDP. Acute tolerance, as defined by a rightward, parallel shift in the dose-response function, was observed in the rats pretreated with CDP. Furthermore, it was evident that the baseline shift associated with CDP withdrawal, rather than a weaker drug cue, accounted for acute tolerance. The results from this study are relevant to evaluating the role positive and negative reinforcement play in motivating compulsive drug use.

Tolerance to the anticonflict effects of diazepam: importance of methodological considerations

The present study examines the effects of chronic diazepam treatment on conflict behavior in rats using the Geller-Seifter paradigm. A dose-response function for the effects of diazepam (DZ) on punished and unpunished responding was determined (0.0, 0.63, 1.25, 2.5, and 5.0 mg/kg DZ intraperitoneally) using five independent groups. The test doses of DZ produced an inverted U-shaped function where punished responding increased as a function of dose up to 2.5 mg/kg and then decreased at 5.0 mg/kg. All groups were then treated with 2 x 5 mg/kg DZ per day for 5 days. When the dose-response function was redetermined at 36 h post-chronic treatment, it was found that the function had shifted to the right, indicating tolerance. Because of the inverted U-shaped nature of the original function, tolerance was manifested as a decrease in responding on the ascending portion of the function and as an increase in responding on the dose (5 mg/kg) representing the descending side of the inverted U.

Drug discrimination is a continuous rather than a quantal process following training on a VI-TO schedule of reinforcement

Debate continues as to whether drug discrimination in animals is an inherently quantal or continuous process. This issue is important in determining the appropriate interpretation of results from drug discrimination studies designed to assess the nature of drug-induced interoceptive cues. The quantal approach holds that subjects perceive a drug cue in an all-or-none manner, while the continuous view proposes that when appropriate training and testing procedures are used, subjects can discriminate along a continuum of interoceptive cues. Data consistent with the quantal view have consistently been generated by animals trained to respond on schedules of reinforcement having an FR component. Since quantal responding is a characteristic of these schedules, results from drug discrimination studies using training schedules with FR components are of little value in empirically determining whether drug discrimination reflects a quantal or continuous process. Use of variable schedules of reinforcement might be more appropriate because the pattern of responding generated does not preclude results consistent with either of the competing views. Data from the following studies that trained subjects using VI schedules with a concurrent TO for incorrect lever responding were analyzed: Barrett et al. (1982): L-5-hydroxytryptophan versus saline; Smith (1990): diazepam versus pentylenetetrazol; Barrett et al. (1992): amphetamine versus haloperidol; Barrett and Steranka (1983): amphetamine versus haloperidol. In every case, when experimental conditions produced a group mean intermediate to that for the training drugs, the distribution of scores for individual animals was normally rather than bimodally distributed.

Effects of desipramine and alprazolam in the forced swim test in rats after long-lasting termination of chronic exposure to picrotoxin and pentylenetetrazol

Rats were treated for 5 weeks with three subconvulsant doses of picrotoxin (PTX) and pentylenetetrazol (PTZ) per week to induce a persistent reduction of the GABAA receptor function which results in chemical kindling. Fifteen days after termination of this treatment schedule, the effect of desipramine (DMI) and alprazolam (ALP) on immobility time in the forced swim test (FST) was evaluated. Chronic PTX and PTZ did not alter the immobility time. Acute PTX and PTZ reduced the immobility of rats chronically treated with vehicle but not of those exposed chronically to PTX and PTZ. Chronic PTX did not influence the anti-immobility effect of DMI, but blocked that of ALP. Chronic PTZ markedly potentiated the anti-immobility effect of DMI but blocked that of ALP. Concomitant administration of chlordiazepoxide prevented the effects of chronic PTX and PTZ. These findings suggest that a long-lasting reduction in GABAA receptor function, unlike acute reduction, does not play an important role in the mobility of rats in the FST and in the anti-immobility effect of DMI while it blocks that of ALP.

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.

Tolerance, withdrawal, and supersensitivity to dopamine mediated cues in a drug-drug discrimination

Rats were trained to discriminate between 0.25 mg/kg amphetamine (AMPH) and 0.03 mg/kg haloperidol (HAL) in a two-lever drug discrimination task. In order to test for a drug-induced withdrawal state, animals were assigned to one of three chronic treatment groups and given injections of AMPH, HAL, or distilled water (DW) for 10 consecutive days. Subjects from each treatment condition were then tested at 24, 48, or 72 h after the final injection. At the 24 h retest interval, subjects injected with AMPH responded as though administered an acute dose of HAL (0.028 mg/kg) and subjects injected with chronic HAL responded as though administered an acute dose of AMPH (0.15 mg/kg). By 72 h choice behavior had returned to pretreatment values. To determine whether the rebound observed after 10 days of drug treatment was present after a single injection, independent groups of subjects were injected with single doses of either 10 mg/kg AMPH or 1.0 mg/kg HAL and then retested from 4 h to 48 h later. Single doses of both AMPH and HAL produced significant rebounds that peaked between 20 h (AMPH) and 24 h (HAL) following administration. In a third experiment, animals were tested with or without acute doses of drug following pretreatment with either HAL or AMPH. Receptor supersensitivity accounts for the tolerance observed to HAL 24 h after treatment with 1.0 mg/kg HAL, whereas receptor subsensitivity accounts for the tolerance observed 20 h after treatment with 10 mg/kg AMPH.

Pentylenetetrazole can induce a conditioned place preference

Thirty-two male Sprague-Dawley rats were randomly selected into 4 groups (n = 8/group) and conditioned in a standard place preference task. The groups differed in the dose of pentylenetetrazole (PTZ) administered prior to conditioning trials. With respect to the three treatment groups, placement into the, initially, nonpreferred side of the CPP apparatus was preceded by injections of 5.6, 10, or 17.8 mg/kg PTZ. The control group was injected with hypertonic saline (1.8% w/v) on the rats', initially, nonpreferred sides and isotonic saline (0.9% w/v) on their preferred sides, to control for any irritative effects of PTZ injections in the treatment groups. Six pairs of drug-saline conditioning trials were conducted with each subject. PTZ produced a dose-dependent increase in the amount of time spent in the drug-associated environment. Saline control subjects' preference scores did not change over the course of the study. These data suggest that PTZ is not aversive in the place learning task; more importantly, the data suggest that a dose-dependent shift in the hedonic valences associated with environmental stimuli can occur when these stimuli are repeatedly paired with PTZ administration. The data are discussed in terms of the stimulus properties of PTZ and the hypothetical "anxiety" state the drug may produce.

Cross-generalization between an ethologically [correction of ecologically] relevant stimulus and a pentylenetetrazole-discriminative cue

Twelve male Sprague-Dawley rats were trained in a two-choice pentylenetetrazole vs. saline drug discrimination (DD) task under an FR10 schedule of food reinforcement. Two-minute reinforced test sessions with the two training stimuli engendered exclusive injection-appropriate responding. Rats were injected with saline and then exposed for 20 min to the presence of a domestic cat pretreated with catnip. Immediately following the predator exposure, rats were tested for stimulus generalization in the DD task. The predator/prey interaction engendered 92% PTZ-appropriate responding. These data suggest that the interoceptive state associated with species-specific defense reactions in rats is similar to the cues produced by a pharmacological agent within a behavioral assay which has been suggested as an animal model of human anxiety.

Increased drug sensitivity in the drug discrimination procedure afforded by drug versus drug training

Rats were trained to discriminate norfenfluramine (NF) 1.4 mg/kg from its vehicle or amphetamine (AMPH) 0.8 mg/kg or pentobarbital (PB) 6.0 mg/kg in order to determine the role that drug combination training plays in the rate of learning and sensitivity to lower drug doses. The results suggest that drug versus drug training can increase the rate of drug discrimination learning for some drugs that are learned slowly when trained in a drug versus vehicle training procedure, whereas drug versus drug training does not increase the rate of learning for other drugs that are learned rapidly. Drug versus drug training does, however, appear to increase the level of stimulus control of the training drug for all drugs examined in this study.

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.

Conditioned opponent processes in the development of tolerance to psychoactive drugs

1. Tolerance may involve classical conditioning processes. A conditioned drug-opponent response is thought to increase with drug exposures and summate with the unconditioned response to the drug - resulting in tolerance. 2. Support for this theory comes from reports that tolerance exhibits many basic features of classical conditioning phenomena. 3. Problems for this account of tolerance arise from the fact that some, but not all, empirical attempts to demonstrate conditioned opponent responses have failed. 4. It has consequently been suggested that tolerance might best be conceptualised in terms of theories of habituation, which do not involve opponent processes. 5. The controversy over the role of opponent processes is theoretically important, because conditioned opponent processes are believed to be involved in dependence. Resolution of the controversy requires that parametric studies are conducted, in which the optimal conditions for obtaining conditioned tolerance are defined, and an efficient "model system" developed for analysis of the possible role of opponent processes in tolerance. 6. Research in this area highlights the importance of interdisciplinary studies of tolerance and dependence.

The discriminative stimulus properties of ethanol and acute ethanol withdrawal states in rats

Twelve male Sprague-Dawley rats were trained in a standard two-choice Drug 1-Drug 2 discrimination task utilizing 3.0 mg/kg chlordiazepoxide (CDP, an anxiolytic drug) and 20 mg/kg pentylenetetrazol (PTZ, an anxiogenic drug) as discriminative stimuli under a VR 5-15 schedule of food reinforcement. Saline tests conducted at specific time points after acute high doses of ethanol (3.0 and 4.0 g/kg) indicated a delayed rebound effect, evidenced by a shift to PTZ-appropriate responding. Insofar as such a shift in lever selection indexes a delayed anxiety-like state, this acute 'withdrawal' reaction can be said to induce an affective state similar to that seen with chronic ethanol withdrawal states. Ethanol generalization tests: (1) resulted in a dose- and time-dependent biphasic generalization to CDP, (2) failed to block the PTZ stimulus and (3) failed to block the time- and dose-dependent elicitation of an ethanol-rebound effect. These data suggest that ethanol's anxiolytic effects are tenuous.

Conditioned place aversion produced by FG 7142 is attenuated by haloperidol

A place conditioning paradigm was used to examine the affective properties of FG 7142, a benzodiazepine receptor inverse agonist. At the highest dose tested (10 mg/kg, IP), FG 7142 produced a significant place aversion to the drug-paired compartment. In a second experiment, haloperidol injections were given before FG 7142. It was found that haloperidol (0.2 mg/kg) significantly reduced the measured conditioned place aversion produced by FG 7142, without exhibiting any aversive or rewarding effects by itself. These results suggest that dopamine receptors are involved in the learning or expression of conditioned place aversion induced by benzodiazepine receptor inverse agonists.