Isodirectional conditioning effects of d-amphetamine and pentobarbital on schedule-controlled operant behavior in pigeons

Differential involvement of dopamine receptors in conditioned suppression induced by cocaine

Cocaine-paired stimuli can suppress food-reinforced operant behavior in rats, providing an animal model of conditioned drug effects. To study the neuropharmacological basis of this phenomenon, we examined the effects of various dopamine receptor antagonists on the acquisition and expression of cocaine-induced conditioned suppression in rats. Superimposed on an ongoing baseline of food-reinforced operant responding, a stimulus was paired with response-independent cocaine (3.0 mg/kg, i.v.) during each of 8 training sessions. To study acquisition, independent groups of rats were given saline, the dopamine D(1)-like receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390) (0.001-0.03 mg/kg, i.p.), or the dopamine D(2)-like receptor antagonist eticlopride (0.001-0.03 mg/kg, i.p.) prior to each training session. To study expression, independent groups of rats were trained first, then given saline, SCH 23390, eticlopride, or N-[4-(4-(2-methoxyphenyl)piperazinyl)butyl]-2-naphthamide (BP 897) (a dopamine D(3) partial receptor agonist; 0.1-1.0 mg/kg, i.p.) before test sessions in which the stimulus was presented without cocaine. Pre-treatment with either SCH 23390 or eticlopride during acquisition reduced the direct suppressant effects of cocaine, but conditioning was blocked only in rats that were treated with SCH 23390 during acquisition training. Expression of conditioning was attenuated only by eticlopride. Thus, dopamine at least partially mediates both the acquisition and expression of cocaine-induced conditioned suppression, with activation of dopamine D(1)- and D(2)-like receptors underlying these respective processes.

Conditioned suppression with cocaine as the unconditioned stimulus

A conditioned-suppression procedure was used to study drug conditioning using cocaine as the unconditioned stimulus (UCS). Rats were first trained to nose poke for food-reinforcement during daily 60-min sessions. At least 1 week following jugular vein catheterization, a 5-min tone-light compound stimulus was presented 30 min into the food-reinforcement session. Two minutes after the onset of the stimulus, either 0 (saline), 1.0, 3.0 or 5.6 mg/kg cocaine, was administered i.v. to separate groups of rats. For another group, the stimulus was presented, and the 5.6 mg/kg dose of cocaine was injected in an unpaired fashion (i.e., at different times). After 5 days of training a test was given with the tone-light stimulus presented alone. No disruption of responding during the tone-light stimulus was observed in the saline and 1.0 mg/kg cocaine groups or for the unpaired group. When the tone-light stimulus was paired with 5.6 mg/kg cocaine; however, it produced nearly a 50% reduction in responding, which then gradually extinguished when the stimulus was presented alone for 5 days. The 3.0 mg/kg cocaine group produced intermediate suppression. When the tone-light compound stimulus was shortened to 70 s and the interstimulus interval (ISI) was 0, 30, or 60 s in three separate groups of rats, the most robust conditioned suppression was observed at the 60 s ISI. Therefore, the conditioned suppression procedure, using 3.0 or 5.6 mg/kg i.v. cocaine doses as the UCS, produced robust conditioning effects comparable to other drugs and more conventional reinforcers. The conditioned suppression procedure may be a useful model for studying the classically conditioned effects of cocaine.