Dopamine D4 receptor involvement in the discriminative stimulus effects in rats of LSD, but not the phenethylamine hallucinogen DOI

Abstract

Rationale: Lysergic acid diethylamide (LSD) differs from other types of hallucinogens in that it possesses direct dopaminergic effects. The exact nature of this component has not been elucidated. Objective: The present study sought to characterize the effects of several dopamine D4 agonists and antagonists on the discriminative stimulus effect of LSD at two pretreatment times and 2,5-dimethoxy-4-iodoamphetamine (DOI), a selective 5-HT 2A/2C agonist. Materials and methods: Male Sprague-Dawley rats were trained in a two-lever, fixed ratio (FR) 50, food-reinforced task with LSD-30 (0.08 mg/kg, i.p., 30-min pretreatment time), LSD-90 (0.16 mg/kg, i.p., 90-min pretreatment time), and DOI (0.4 mg/kg, i.p., 30-min pretreatment time) as discriminative stimuli. Substitution and combination tests with the dopamine D4 agonists, ABT-724 and WAY 100635, were performed in all groups. Combination tests were run using the dopamine D4 antagonists A-381393 and L-745,870 and two antipsychotic drugs, clozapine and olanzapine. Results: WAY 100635 produced full substitution in LSD-90 rats, partial substitution in LSD-30 rats, and saline appropriate responding in DOI-trained rats. ABT-724 partially mimicked the LSD-90 and LSD-30 cues, but produced no substitution in DOI-trained rats. In combination tests, both agonists shifted the dose-response curve of LSD leftward, most potently for the LSD-90 cue. The D4 antagonists significantly attenuated both the LSD-90 and LSD-30 cue, but had no effect on the DOI cue. Conclusion: Dopamine D4 receptor activation plays a significant modulatory role in the discriminative stimulus effects in LSD-90-trained rats, most markedly for the later temporal phase of LSD, but has no effect on the cue produced by DOI. © 2008 Springer-Verlag.