Effect of chronic treatment with 5-HT1 agonist (8-OH-DPAT and RU 24969) and antagonist (isapirone) drugs on the behavioural responses of mice to 5-HT1 and 5-HT2 agonists.

The effects of chronic (14 day) administration to mice of the 5-HT1 agonists 8-hydroxy 2-(di-n-propylamino) tetralin (8-OH-DPAT) and 5-methoxy-3 (1,2,3,6-tetrahydropyridin-4-yl) IH indole (RU 24969) on the hypothermic response to 8-OH-DPAT and the locomotor response to RU 24969 have been examined. Chronic administration of 8-OH-DPAT (5 mg kg-1, s.c.) resulted in an attenuated hypothermic response to this drug given subcutaneously (s.c.) or intracerebroventricularly (i.c.v.) but did not alter the locomotor response to RU 24969. Chronic injection of RU 24969 (3 mg kg-1, i.p.) produced an attenuated locomotor response to this drug given i.p. or i.c.v. but not the hypothermic response to 8-OH-DPAT (0.5 mg kg-1, s.c.). Chronic administration of the putative presynaptic 5-HT1 antagonist isapirone (10 mg kg-1, i.p.) decreased the hypothermic response following 8-OH-DPAT injection but did not alter RU 24969-induced locomotion. Chronic treatment with 8-OH-DPAT (5 mg kg-1, s.c.) produced a modest enhancement of the 5-HT2 receptor-mediated head-twitch behaviour initiated by 5-hydroxytryptophan injection while chronic isapirone decreased this behavioural response. 5-HT2 receptor number in frontal cortex was unaltered by isapirone treatment but markedly decreased (34%) by chronic 8-OH-DPAT. These data suggest that chronic administration of the 5-HT1 agonists induces tolerance in their respective responses but not cross-tolerance, while chronic isapirone may down-regulate the 5-HT1A site in a matter analogous to that seen by 5-HT2 receptors following 5-HT2 receptor antagonists. 7 The data further demonstrate that chronic treatment with 8-OH-DPAT and isapirone alter postsynaptic 5-HT2 receptor function although 5-HT2 receptor number in the frontal cortex did not correlate with the behavioural change.