Discriminative stimulus properties of the serotonin agonist MK 212

In an attempt to clarify the role of 5-hydroxytryptamine (5-HT) in the discriminative stimulus properties of MK 212 (6-chloro-2[1-piperazinyl]pyrazine), male Sprague-Dawley rats were trained to discriminate 0.5 mg/kg of this compound from saline. While the putative 5-HT agonists fenfluramine and m-chlorophenylpiperazine (MCPP) mimicked MK 212 in a dose-related manner, d-lysergic acid diethylamide (LSD), 8-hydroxy-2(di-n-propylamino)tetralin (8-OHDPAT), 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), quipazine, Ru 24969, and 1-(m-trifluoromethylphenyl)piperazine (TFMPP) failed to substitute completely. The 5-HT₁/5-HT₂ antagonists BC 105, metergoline, and methysergide completely blocked the MK 212 cue, while the selective 5-HT₂ antagonists ketanserin and pirenperone, the dopamine antagonists haloperidol and spiperone, and the beta-noradrenergic antagonist propranolol were without effect. The substitutions of fenfluramine and MCPP for MK 212 support a role for 5-HT in the MK 212 cue; however, the lack of substitution of many other 5-HT agonists is difficult to explain. The complete antagonism by 5-HT₁/5-HT₂ but not by selective 5-HT₂, antagonists suggests the possibility that 5-HT₁ receptors mediate the stimulus properties of MK 212. Further research is needed to support this hypothesis and to investigate the relative role of 5-HT and other neurotransmitters in the stimulus effects of MK 212.Portions of this research were presented at the Meeting of the Committee on Problems of Drug Dependence Satellite Session (International Study Group Interested in Drugs as Reinforcers and the Society for the Stimulus Properties of Drugs) in Baltimore, MD (1985)