The effects of methadone on operant behavior maintained with and without conditioned reinforcement in the pigeon

Preclinical research on cocaine self-administration: environmental determinants and their interaction with pharmacological treatment

It has been asserted that any comprehensive understanding of cocaine abuse and its treatment will require attention to both behavioral and pharmacological variables. Although the preclinical literature evaluating the effects of pharmacological variables on cocaine self-administration has been extensively reviewed, no comprehensive review of the effects of environmental variables on cocaine self-administration has been published. The present review summarizes and critiques the preclinical findings on environmental determinants of cocaine self-administration. The influence of environmental variables on the effects of pharmacological interventions on cocaine self-administration are also described. Several environmental variables have been shown to affect cocaine self-administration, including unit dose, schedule of cocaine delivery, schedules of nondrug stimuli, behavioral history, conditioned stimuli, food deprivation, exposure to stress, and rearing environment. Among these variables, unit dose, schedule of cocaine delivery, availability of alternative nondrug reinforcers, food deprivation, and rearing environment have also been shown to alter pharmacological treatment effects on cocaine self-administration. Thus, drug effects on cocaine self-administration are malleable and dependent upon the environmental context within which they occur. Suggestions for future research on the effects of these and other environmental variables on cocaine self-administration and its pharmacological treatment are presented.

Behavioral and neurochemical mechanisms of opioid-antidepressant interactions

Twelve pigeons key-pecked under a multiple variable interval 15-s, 150-s schedule of food reinforcement. The effects of methadone were studied alone and in combination with chronic daily administration of either imipramine (IMI) or desipramine (DMI). Chronic IMI was also given following reductions in response rates by unsignaled delay-to-reinforcement (UDR). Acute administration of methadone produced dose-dependent reductions in response rates under both schedules of reinforcement. Chronic daily administration of IMI or DMI alone did not result in lasting changes in baseline responding. When administered in combination, chronic daily IMI significantly attenuated the rate-reducing effects of methadone, whereas neither a low nor a high dose of chronic daily DMI was effective. The same dose of chronic daily IMI failed to ameliorate response rate reductions under delayed reinforcement. The behavioral and neurochemical specificity of the antidepressant effect is discussed.

Changes in food-maintained progressive-ratio responding of rats following chronic buprenorphine or methadone administration

Seven rats lever pressed under a progressive ratio 3 (PR 3) schedule of food presentation; the number of responses per reinforcer systematically increased during each session. Break point (i.e., the number of responses in the last completed ratio before session termination) was measured under daily methadone (4.5 mg/kg and 3.0 mg/kg) or buprenorphine (0.3 mg/kg) administered prior to experimental sessions. Both drugs initially eliminated rats' food-maintained progressive-ratio responding. Break points during chronic methadone did not return to baseline levels after 80 drug sessions and a dose reduction. In contrast, break points during chronic buprenorphine administration were considerably above baseline control levels for two rats and returned to baseline levels for the third.

Effects of methadone on free feeding in satiated rats

A variety of opioids and opiates are known to increase short-term food intake. In the present study, we evaluated the effects of methadone on free feeding in satiated rats. We assessed the effect of methadone (0, 1.5, 3.0, 5.0, and 10.0 mg/kg) on food intake 1, 2, 4, and 6 h after injection for 3 consecutive days. Two hours after methadone administration, food intake was inversely related to dose, but after 6 h a direct relationship between dose and feeding was obtained. Food intake increased with repeated methadone administration. In Experiment 2, methadone (5.0 mg/kg) was injected and food was made available 0, 1, 2, or 3 h later. Maximal food intake occurred in the third and fourth hours following methadone administration. As in Experiment 1, food intake increased with repeated methadone administration. Increases in food intake following repeated methadone administration may have been due to the development of tolerance to effects of methadone that may interfere with feeding, such as sedation. In Experiment 3, methadone was administered daily or every fifth day, assuming that spacing injections would retard tolerance development. Repeated daily methadone administration was associated with increased food intake earlier in the session, whereas increases in food intake following spaced methadone administration occurred later in the session. These data indicate that methadone increases short-term feeding in satiated rats. This is in contrast to the reported decrease in food-reinforced behavior noted in operant studies. This contrast may be due to sedating or other disabling effects of methadone.

Food-paired stimuli as conditioned reinforcers: effects of d-amphetamine

Seven pigeons were studied in two experiments in which key pecks were reinforced under a second-order schedule wherein satisfaction of variable-interval schedule requirements produced food or a brief stimulus. In the second part of each session, responses produced only the brief stimulus according to a variable-interval schedule (food extinction). For the 4 pigeons in Experiment 1, the response key was red throughout the session. In separate phases, the brief stimulus was either paired with food, not paired with food, or not presented during extinction. d-Amphetamine (0.3 to 10.0 mg/kg) dose-dependently reduced food-maintained responding during the first part of the session and, at intermediate dosages, increased responding during the extinction portion of the session. The magnitude of these increases, however, did not consistently depend on whether the brief stimulus was paired, not paired, or not presented. It was also true that under nondrug conditions, response rates during extinction did not differ reliably depending on pairing operations for the brief stimulus. In Experiment 2, 3 different pigeons responded under a procedure wherein the key was red in the component with food presentations and blue in the extinction component (i.e., multiple schedule). Again, d-amphetamine produced dose-related decreases in responding during the first part of a session and increases in responding in the second part of the session. These increases, however, were related to the pairing operations; larger increases were observed when the brief stimulus was paired with food than when it was not or when it was not presented at all. Under nondrug conditions, the paired brief stimulus controlled higher response rates during extinction than did a nonpaired stimulus or no stimulus. These findings suggest that d-amphetamine can enhance the efficacy of conditioned reinforcers, and that this effect may be more robust if conditioned reinforcers occur in the context of a signaled period of extinction.