M.W. Emmett-Oglesby

Parameters of self-administration of cocaine in rats under a progressive-ratio schedule

Progressive-ratio (PR) schedules may provide a more direct measure of drug-reinforcing efficacy than the more traditionally used fixed-ratio schedules. Under a PR schedule, an increasing number of lever presses is required for the delivery of each successive reinforcer. However, there have been few studies of fundamental parameters of cocaine self-administration under a PR schedule. This study was undertaken to assess if PR responding using cocaine reinforcement in rats would: a) be acquired rapidly; b) be maintained on a stable baseline for long periods; and c) provide data on the effect of changing the dose of cocaine that are amenable to statistical analysis. In addition, the effects of pretreatments with SCH23390, a D1 receptor antagonist, or ondansetron, a 5-hydroxytryptamine3 (5-HT3) receptor antagonist, were tested against several doses of cocaine. Stable performance of PR cocaine self-administration (0.90 mg/kg) was acquired within 10 training sessions and was maintained for over 50 training sessions. Increasing the dose of cocaine from 0.10-2.70 mg/kg resulted in a directly related increase in a) the number of reinforcers obtained, b) the highest ratio completed, and c) the interreinforcer time (ISRT: time between each cocaine infusion). In terms of statistical analysis, the number of reinforcers obtained was found to be preferable to the highest ratio completed as a measure of breakpoint. Pretreatment with SCH23390 significantly reduced the breakpoint; this reduction was not due to a motor-incapacitating effect of SCH23390 because the ISRT showed a tendency to be shortened by SCH23390. Pretreatment with ondansetron failed to significantly affect either the number of reinforcers obtained or the ISRT. These results show that rats can readily acquire the task of self-administration of cocaine under a PR schedule and maintain a stable baseline for an extended period. Further, a PR schedule appears to be suitable for the study of pharmacological treatments that might affect cocaine self-administration. Simultaneous monitoring of the breakpoint and of the ISRT determines if a decrease in the breakpoint is the result of a motor-incapacitating side effect of the pretreatment.

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.

Parallel processing strategies in the application of microcomputers to the behavioral laboratory

The Operant Package for the Neurosciences (OPN) is a software system that runs on the TRS-80 Model I and Model III and the Apple II+ and Apple lie microcomputers. It is designed to allow a single microcomputer to control behavioral experiments in up to eight different stations, record experimental data, and provide detailed data analysis. Although segments of the program are written in BASIC, allowing users to interact with OPN in a simple English question-and-answer format, the part that provides real-time experimental control is written in Z-80 assembly language. The present report describes in detail assembly language algorithms that we found to facilitate speed of processing and flexibility of control. Particular problems dealt with how to update and change flag variables controlling the reinforcement contingencies and output conditions in each station, as well as how to decide when each station should shift contingencies. The solutions to these problems utilize byte-oriented parallel processing strategies, converting serially organized reference information for each station into a group of working flag bytes that control contingency and output variables for all stations in parallel.

A TRS-80-based system for the control of behavioral experiments ☆

A TRS-80-based system is described for controlling and recording events from operant chambers. The system features a TRS-80 microprocessor with 32 K RAM, a floppy disk drive, an LVB optical interface, a printer, and a user-oriented program (Operant Protocol System, OPS). The LVB interface system is capable of up to 128 independent input/output operations for each of up to 8 boxes. The system uses assembly language routines for real-time control and event recording of up to 8 boxes simultaneously. BASIC is used to obtain user input and deliver data output in an interactive fashion. Advantages include dependable hardware, extensively documented and tested software, relative inexpensiveness, and standard-deviation-based variable schedules.

Mediation in the nucleus accumbens of the discriminative stimulus produced by cocaine.

Rats were trained to detect an intraperitoneal (IP) administration of cocaine, 10.0 mg/kg, using a two-lever choice discrimination procedure in which food reinforcement was only delivered following 10 responses on the correct lever (FR10): one lever was correct after cocaine injection, and the other lever was correct after saline injection. Following training, cocaine was generalized to the cocaine training stimulus in a dose-dependent manner. Subsequently, guide cannulae were implanted bilaterally in the prefrontal cortex (from bregma, A = 2.7, L = 1.0, V = 3.0 mm), nucleus accumbens (A = 2.2, L = 1.5, V = 6 mm) or caudate putamen (A = 0.2, L = 2.5, V = 4). Injections were made via cannulae that extended 1 mm past the tip of the guide cannulae. Injection in the nucleus accumbens substituted for the IP training dose of cocaine in a dose-dependent manner with maximum generalization occurring with 10 micrograms of cocaine per side (87% cocaine-lever responding); in contrast, injections of cocaine in the prefrontal cortex or caudate-putamen produced only partial cocaine-lever responding (a maximum of 48 and 37% cocaine-lever responding, respectively). These data support the hypothesis of central mediation of the cocaine stimulus and show that cocaine administered in the nucleus accumbens is sufficient to produce the stimulus. The partial substitution of cocaine in the prefrontal cortex and caudate-putamen may reflect partial mediation of the cocaine stimulus in these brain areas.

Tolerance and selective cross-tolerance to the motivational effects of opioids

The issue of whether tolerance develops to the motivational effects of opioids was addressed by use of an unbiased place preference conditioning procedure. Administration of the μ-opioid agonists morphine or fentanyl produced dose-related preferences for the drug-associated place in control rats. In contrast, the κ-opioid agonist, U-69593 produced conditioned place aversions. Non-contingent administration of morphine (5.0 mg/kg/12 h) for 4 days prior to conditioning resulted in tolerance to its reinforcing effects, and cross-tolerance to the effects of fentanyl. No cross-tolerance to the motivational effects of the psychostimulantd-amphetamine or the κ-opioid agonist U-69593 was observed. Chronic administration of U-69593 prior to conditioning produced tolerance to its aversive effects. This treatment did not, however, modify the reinforcement produced by morphine. These data demonstrate that tolerance develops to both the reinforcing and aversive properties of opioids and suggest that differential cross-tolerance may provide a useful method for determining the pharmacological basis underlying drug-induced motivational effects.

Substitution and cross-tolerance profiles of anorectic drugs in rats trained to detect the discriminative stimulus properties of cocaine

Rats were trained to discriminate cocaine, 10.0 mg/kg, using a two-lever operant procedure. Dose-effect data were determined for the substitution of cocaine, diethylpropion, methylphenidate, phenmetrazine, phentermine, and fenfluramine for the cocaine stimulus. All of these drugs, except fenfluramine, substituted fully for the cocaine stimulus. Subsequently, training was halted and cocaine, 20 mg/kg/8 h, was administered for 9 days, and dose-effect data were redetermined for all of these drugs on days 7–9 of chronic administration. Chronic administration of cocaine produced tolerance to the stimulus properties of cocaine, and cross-tolerance to the stimulus properties of methylphenidate, phenmetrazine, and phentermine, such that doses approximately two-fold higher than those used acutely were necessary to reproduce the original effect; the potency for the substitution of diethylpropion for the cocaine stimulus was decreased greater than four-fold; and fenfluramine still failed to substitute for the cocaine stimulus. These data suggest that 1) a common mechanism may mediate tolerance to the discriminative stimulus properties of cocaine, methylphenidate, phenmetrazine, and phentermine, and 2) tolerance in the drug discrimination procedure may have potential for establishing a comprehensive evaluation of dependence liability of CNS stimulants.

Anxiogenic aspects of diazepam withdrawal can be detected in animals

Animals can be trained to discriminate the presence of pentylenetetrazol, and this discrimination has previously been proposed as an animal bioassay for anxiogenicity. In rats made dependent on diazepam, pentylenetetrazol-like stimuli occurred during spontaneous or precipitated (with RO 15-1788) withdrawal; these stimuli were blocked by pentobarbital. These results demonstrate that the pentylenetetrazol-based animal model of anxiety can be used to objectively quantify a subjective aspect of benzodiazepine dependence/withdrawal.

The pentylenetetrazol model of anxiety detects withdrawal from diazepam in rats

This experiment tested whether benzodiazepine withdrawal could be detected in an animal model of anxiety. Rats were trained in operant chambers using food reward to press one lever after pentylenetetrazol (PTZ), 20 mg/kg, injection and the other lever after saline injection. Previously, the PTZ cue has been shown to be simulated by anxiogenic drugs and blocked by anxiolytic drugs. After rats reliably performed this discrimination, they were injected with diazepam, 20 mg/kg, from 1 to 4 times a day for six days. For one group of subjects, on the third, fourth and sixth days, they were also injected with 40 mg/kg of RO 15-1788, a benzodiazepine receptor antagonist, and tested for lever selection: 50-80% of the subjects selected the PTZ lever; these results are in contrast to those obtained prior to chronic diazepam treatment in which RO 15-1788 did not generalize to PTZ. A second group of subjects was also injected for six days with diazepam and then allowed to withdraw spontaneously for eight days: PTZ lever selection over this period varied from 20 to 60% of rats. These data indicate that animals trained to discriminate a PTZ cue: 1) generalize the benzodiazepine withdrawal state to the PTZ cue, and 2) discriminate the withdrawal state for long periods of time, agreeing with clinical observations of long-lasting anxiety signs during benzodiazepine withdrawal.

Withdrawal from chronic nicotine substitutes partially for the interoceptive stimulus produced by pentylenetetrazol (PTZ)

Rats were trained on an FR10 schedule of food reinforcement to press one lever after pentylenetetrazol (PTZ), 20 mg/kg, IP, and an alternate lever after saline. After acute nicotine, 0.64 mg/kg, SC, 35% of the rats pressed the PTZ-lever. Diazepam, 5 mg/kg, IP, blocked the stimulus produced by PTZ, and mecamylamine, 5 mg/kg, IP, blocked the stimulus produced by nicotine. Training was then suspended and rats were treated with nicotine, at 8-h intervals, 0.64 mg/kg on the 1st day, and 1.25 mg/kg on subsequent days, for 21 days. To determine whether nicotine withdrawal substitutes for the stimulus produced by PTZ, rats were tested with saline at various times after chronic nicotine injections. Data from this part of the study were replicated in another group given nicotine for 15 days. Saline at 8 h after nicotine (five determinations each group) produced a small but stable degree of PTZ lever selection (35±4%). At 48 h after termination of nicotine treatment, the percentage of rats selecting the PTZ lever (50%) was greater than that in a control group tested after an equivalent period without training. The PTZ-like stimulus detected after chronic nicotine was not altered by mecamylamine, was additive with PTZ, and was blocked by diazepam. These data suggest that withdrawal from chronic nicotine produces a weak PTZ-like stimulus, which can be antagonized by an anxiolytic drug.