Differential roles of 5-hydroxytryptamine1A and 5-hydroxytryptamine1B receptor subtypes in modulating spinal nociceptive transmission in mice.

The modulatory effect of spinal serotonin (5-HT)1 receptors on nociception was studied in mice. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and buspirone, putative 5-HT1A agonists, m-trifluoromethylphenyl-piperazine (TFMPP) and 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo(1,2-1a)quinoxaline (CGS 12066B), 5-HT1B agonists, and 5-carboxamidotryptamine (5-CT), a mixed 5-HT1A and 5HT1B agonist, were used. Intrathecal administration of 8-OH-DPAT, buspirone and 5-CT (1-12 nmol/mouse) significantly facilitated the tail-flick reflex, whereas TFMPP and CGS 12066B prolonged tail-flick latency. When administered i.t. after s.c. pretreatment (25 min) with morphine sulfate, 8-OH-DPAT, buspirone and 5-CT shifted the morphine sulfate dose-response curve 3- to 5-fold to the right. Spiperone, propranolol and pindolol (mixed 5-HT1A and 5-HT1B antagonists) effectively reversed both the tail-flick facilitation and the antagonistic effect on morphine sulfate-induced antinociception produced by 8-OH-DPAT and 5-CT. In addition, simultaneous i.t. administration of 8-OH-DPAT with substance P or N-methyl-D-aspartic acid decreased biting but increased scratching behavior, an effect which is also blocked by the 5-HT1 antagonists. These results confirm and extend other reports on the facilitory role of 5-HT1A receptor subtype on nociceptive responses and support the involvement of 5-HT1B receptor subtype in the antinociceptive action of serotonin.