Sensitisation to repeated withdrawal, in mice treated chronically with diazepam, is blocked by an NMDA receptor antagonist.

Mice were treated either with diazepam (15 mg/kg s.c. in oil), for 21 days, or for 3x7-day periods interspersed with two 72-h drug-free periods. Convulsant thresholds to pentylentetrazole infused into the tail vein 72 h following the final chronic treatment were lower in multiple-withdrawal mice than in mice which had experienced the same drug load, but only a single withdrawal, consistent with sensitisation of withdrawal events following previous withdrawal experience. The increase in seizure sensitivity of repeatedly withdrawn mice was prevented by treatment with the NMDA receptor antagonist CGP 39551 (20 mg/kg, i.p.) given once daily during the 3-day breaks in diazepam treatment, suggesting a role of glutamatergic transmission in the sensitisation process.

Previous experience of diazepam withdrawal prevents the formation of a withdrawal-conditioned taste aversion: test of a blocking hypothesis.

Prior experience of withdrawal from chronic diazepam treatment reduces the aversiveness of withdrawal when precipitated withdrawal is made the unconditioned stimulus in a conditioned taste aversion (CTA) paradigm. Accounts of the mechanism by which unconditioned stimulus pre-exposure reduces its effectiveness in CTA postulate that unconditioned stimulus pre-exposure leads to the formation of associations with the environment, resulting in blocking of taste conditioning. We tested whether a blocking explanation accounted for the reduced effectiveness of withdrawal as a unconditioned stimulus in a CTA following prior exposure. Mice received chronic diazepam (15mg/kg/day, s.c. in sesame oil), or sesame oil vehicle, for three periods of 7 days, interspersed with 3-day withdrawal periods. The first two withdrawals occurred either in the home cage, or in one compartment of a place-conditioning apparatus (PCA). Animals which experienced withdrawal in the home cage were given equivalent experience of the PCA outside the withdrawal period. The third withdrawal was precipitated by i.p. administration of flumazenil (20mg/kg). Thirty minutes before injection, all animals were placed individually in the compartment of the PCA to which they had been previously exposed, allowed to drink a novel 10% sucrose solution, injected with flumazenil, and replaced in the PCA for 2 h, before being returned to the home cage. When sucrose consumption was measured 24 h later, only that group which had experienced all three withdrawals in the PCA showed evidence of a CTA. These animals (but not those that had experienced withdrawal in the home cage, or vehicle-treated mice) also showed strong avoidance of the chamber in which they had experienced withdrawal. Thus, no evidence was adduced that prior conditioning of an environment-conditioned stimulus to a withdrawal unconditioned stimulus blocked the formation of a CTA. When the CTA conditioning was repeated in the home cage, again only the mice that had experienced withdrawal in the place-conditioning apparatus showed evidence of conditioning. These observations are discussed in the context of blocking explanations of unconditioned stimulus pre-exposure.

Previous experience of withdrawal from chronic diazepam ameliorates the aversiveness of precipitated withdrawal and reduces withdrawal-induced c-fos expression in nucleus accumbens.

Flumazenil (20 mg/kg, i.p.)-precipitated withdrawal from chronic treatment with diazepam (DZP, 15 mg/kg, s.c. in sesame oil for 21 days) resulted in a decreased seizure threshold to the convulsant, pentylenetetrazole (PTZ), infused into the tail vein; withdrawal from 21-day chronic diazepam treatment, interspersed with two periods of drug withdrawal, resulted in a greater decrease in convulsant threshold. A separate experiment showed that consumption of a sucrose solution immediately prior to precipitated withdrawal resulted in a decreased subsequent consumption of the sucrose solution; no such evidence of a conditioned taste aversion (CTA) was seen in mice given prior experience of withdrawal. Thus, prior experience of withdrawal enhanced the effects of a subsequent precipitated withdrawal in increasing seizure sensitivity, but weakened the ability of this withdrawal to serve as an aversive unconditioned stimulus (US). The weakening of the aversive properties of precipitated withdrawal may reflect habituation to the withdrawal stimulus, and was accompanied by a loss of the ability of withdrawal to induce c-fos expression in the shell of the nucleus accumbens, an area sensitive to both novel, and stressful, as well as rewarding stimuli.

State-dependent behavioural sensitization: evidence from a chlordiazepoxide state.

The present study investigated the hypothesis that behavioural sensitization to psychomotor stimulants is expressed only if the internal state of the animal is the same as it was at the time of sensitization development. This state-dependency hypothesis has previously been put forward to explain the apparent blockade of sensitization by N-methyl-D-aspartate (NMDA) receptor antagonists. The present study used amphetamine (0.375 mg/kg) as the stimulant and chlordiazepoxide (CDP) as a secondary stimulus. Mice were given double injections of either amphetamine + CDP, amphetamine + saline, CDP + saline, or saline + saline daily over 8 days. On the ninth and tenth days, all mice were challenged with amphetamine + saline and with CDP + saline, in a counterbalanced design. CDP does not show evidence of locomotor sensitization and does not act at NMDA receptors, but in other studies it has been shown to give rise to state-dependent-like effects. Thus any progressive augmentation of the response to repeated amphetamine + CDP treatment would be attributable to amphetamine sensitization, and any blockade of sensitization would not be due to NMDA receptor antagonism. Both groups receiving amphetamine became sensitized over the first 8 days (shown by progressive increases in locomotion). When challenged with amphetamine alone, the amphetamine + CDP group failed to show a sensitized response. This observation supports the state-dependency hypothesis and emphasizes the importance of considering the context provided by drug-induced internal states in studies of sensitization.