Abstract

The role of serotonin (5-hydroxytryptamine; 5-HT) 5-HT₂ receptor subtypes (5-HT_2AR, 5-HT_2BR, and 5-HT_2CR) in acute cocaine-evoked hyperactivity was compared with their contribution to the development and expression of locomotor sensitization upon repeated, intermittent treatment with cocaine (10 mg/kg/day for 5 days) in male Wistar rats. Cocaine-evoked hyperactivity was significantly enhanced by pretreatment with the preferential 5-HT_2AR agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT_2CR antagonist SDZ SER-082 [(+)-cis-4,5,7a,8,9,10,11,11a-octahydro-7H-10-methylindolo(1,7-BC)(2,6) naphthyridine fumarate]. The 5-HT_2AR antagonist SR 46349B [1(Z)-[2-(dimethylamino)ethoxyimino]-1(2-fluorophenyl)-3-(4-hydroxyphenyl)-2(E)-propene] and the preferential 5-HT_2CR agonist MK 212 [6-chloro-2-(1-piperazinyl)pyrazine HCl] (2 mg/kg) significantly attenuated acute cocaine-evoked hyperactivity; however, a lower dose of MK 212 (0.3 mg/kg) enhanced cocaine-evoked hyperactivity. The 5-HT_2BR agonist BW 723C86 (1-[5-(2-thienylmethoxy)-1H-3-indolyl]propan-2-amine HCl) and the 5-HT_2BR antagonist SB 204741 [N-(1-methyl-5-indolyl)-N′-(3-methyl-5-isothiazolyl) urea] had no effect on cocaine-evoked hyperactivity. Repeated treatment with cocaine alone resulted in a 2-fold increase in hyperactivity upon challenge with cocaine 5 days after termination of the cocaine regimen (sensitization). The 5-HT_2AR antagonist SR 46349B also blocked cocaine-evoked hyperactivity following repeated cocaine treatment, whereas the other 5-HT₂R ligands were ineffective. When any of the 5-HT₂R ligands was coadministered with cocaine during the treatment regimen (10 mg/kg/day for 5 days), the development of sensitization was unchanged as measured by the level of cocaine-evoked hyperactivity upon challenge 5 days after termination of the treatment. The present study implies that 5-HT_2AR and 5-HT_2CR exert oppositional influence upon hyperactivity evoked by acute administration of cocaine; this balance is altered following repeated cocaine administration.