Situational specificity of tolerance to effects of phencyclidine on responding of rats under fixed-ratio and spaced-responding schedules

Impaired DRL 30 performance during amphetamine withdrawal

Repeated administration of psychomotor stimulants may produce an impulsive state that could contribute to the cycle of drug abstinence and relapse seen in human drug addicts. We have previously reported that the inhibitory effects of dopamine (DA) on the firing rate of medial prefrontal cortex (mPFC) neurons were reduced in rats after repeated amphetamine treatment suggesting impaired mPFC DA function. Here, we used a differential reinforcement of low rates of responding (DRL) operant conditioning task, which is dependent on mPFC DA, to test impulsivity and inhibitory control. Food-restricted rats were trained to inhibit a nose poke response for 30s before a subsequent nose poke would result in a food reward (DRL 30). Once training was completed, rats received 5 days of no treatment, daily i.p. saline injections or daily i.p. injections of 5mg/kg amphetamine. Nine days of DRL 30 test performance began following a 3-day withdrawal from treatment. The percent of training active hole nose pokes was significantly increased and the percent of training efficiency was significantly decreased in rats withdrawn from repeated amphetamine administration as compared to saline or nai;ve rats. This suggests that impulsivity is increased during amphetamine withdrawal, which we hypothesize is associated with disrupted DA function in the mPFC.

Specificity of effects of chronically administered diazepam on the responding of rats under two different spaced-responding schedules

Lever pressing of rats was maintained in different chambers during two different sessions each day. At 0900 h, responding was maintained under a two-component multiple schedule in which responses initiated an interval that had to elapse before delivery of food (time delay of 20 s and 40 s). In this schedule, a 'response-pause' sequence preceded reinforcers, and acutely administered diazepam only decreased responding. At 1400 h, responding by the same subjects was maintained under a different two-component multiple schedule, in which individual responses initiated an interval that had to be terminated by another response before delivery of food (DRL 20 s and 40 s). In this second schedule, a 'response-pause-response' sequence preceded reinforcers, and acutely administered diazepam increased responding. After studying the acute behavioral effects of diazepam during each separate 'timing' schedule, animals systematically received 1.7 mg/kg per day diazepam 2-5 min prior to their different schedule components, in order to study the influence of reinforcement contingency on the chronic effects of this drug. Diminution of the initial effects of diazepam during daily drug administration prior to DRL 20 s responding did not extend to DRL 40 s responding or to time-delay responding, and tolerance did not develop at all for time-delay responding. When diazepam was again administered after all the daily schedules for approximately 1 month, and then given before the individual DRL schedules, DRL responding was increased again as it had been prior to chronic drug administration. These results suggest that the behavioral effects of acutely administered diazepam are influenced by different 'timing' requirements, and that the behavioral effects of chronically administered diazepam are influenced by 'timing' requirements and by drug- and chamber-related stimuli.

Specificity of chronic effects of diazepam on responding of rats under fixed-ratio schedules

Behavioral effects of diazepam were studied in rats responding in different daily sessions using different operant chambers and manipulanda. In one experiment, key pressing was maintained in a first session under a 40-response fixed-ratio schedule; lever pressing was maintained in a second session under a 40-response fixed-ratio schedule; and a third session consisted of a multiple schedule comprising both key and lever pressing maintained under a 40-response fixed-ratio schedule. In a second experiment, the first session consisted of a multiple schedule with both key and lever pressing maintained under a 40-response fixed-ratio schedule and the second session consisted of lever pressing maintained under a 40-response fixed-ratio schedule. After studying effects of widely spaced injections of diazepam (0.3-3.0 mg/kg) on responding for each separate schedule, animals received 1.7 mg/kg/day diazepam in order to study chronic effects of the diazepam on behavior among the different schedule-conditions. In both experiments, tolerance to rate-decreasing effects of diazepam in a particular schedule component did not extend to any other schedule component when the manipulandum or chamber was different, but did extend to other schedule components when the manipulandum or chamber was the same. These results suggest that behavioral effects of chronically administered diazepam were influenced not only by pharmacologic processes, but also by learned relationships among its interoceptive effects, reinforcement contingencies, and particular behavioral environments.

Discrimination of methadone and cocaine by pigeons without explicit discrimination training

Pigeons were trained to peck a key on a variable-interval 2-min schedule of food reinforcement. Prior to each session, either 2.0 mg/kg methadone (n = 3), 3.0 mg/kg cocaine (n = 4), or 5.6 mg/kg cocaine (n = 2) was administered. When each pigeon's rate of pecking was stable, a range of doses of the training drug and saline were administered prior to 20-min extinction sessions separated by at least four training sessions. Rate of pecking during these extinction tests was generally an increasing function of dose, with the lowest rates obtained following saline and low doses and the highest rates obtained following doses near the training doses. Dose functions from pigeons trained with 5.6 mg/kg cocaine were steeper than those from pigeons trained with 3.0 mg/kg cocaine. Pigeons trained with methadone or 3.0 mg/kg cocaine were then given discrimination training, in which food reinforcement followed drug administration and 20-min extinction sessions followed saline administration. Rates of pecking under these conditions quickly diverged until near-zero rates were obtained following saline and high rates were obtained following drug. Discrimination training steepened dose functions for the training drugs, and the effects of several other substituted drugs depended on the pharmacology of the training drug. The pigeons trained with 5.6 mg/kg cocaine were tested with d-amphetamine, methadone, and morphine prior to discrimination training. d-Amphetamine increased rates dose dependently, and methadone and morphine did not. The results suggest that discriminative control by methadone and cocaine was established without explicit discrimination training.