The role of spinal cord 5-HT1A and 5-HT1B receptors in the modulation of a spinal nociceptive reflex.

The role of the 5-hydroxytryptamine (5-HT) receptor subtypes in the spinal cord in the regulation of nociception is unknown. This study examined whether administration of different 5-HT1 receptor agonists into the spinal subarachnoid space of mice modulates the nociceptive tail-flick reflex, and whether effects on the tail-flick reflex involve changes in tail skin temperature. The tail-flick latencies (the time needed to evoke the tail-flick reflex by noxious radiant heat) were significantly increased after intrathecal (i. th.) injection of 5-HT (10-20 micrograms), the 5-HT1A/5-HT1B receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT, 10-20 micrograms), the selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 20 micrograms) and after i.th. injection of 1(m-chlorophenyl)piperazine (mCPP, 5-20 micrograms) and 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole (RU 24969, 5-20 micrograms) which have high affinity for the 5-HT1B receptors. None of the 5-HT1 receptor agonists had the ability to change the tail skin temperature. The results show that in the mouse i.th. injection of both 5-HT1A and 5-HT1B receptor agonists has the ability to inhibit the tail-flick reflex without interfering with the tail skin temperature.