SPINAL 5-HT2 RECEPTOR-MEDIATED FACILITATION OF PUDENDAL NERVE REFLEXES IN THE ANESTHETIZED CAT

1 5-Hydroxytryptamine (5-HT) is intimately associated with central sym pathetic and somatic control of the lower urinary tract. The sympathet ic and somatic innervation of the lower urinary tract is conveyed thro ugh efferent axons of the hypogastric and pudendal nerves, respectivel y. 2 The present study examined the effects of 2,5-dimethoxy-4-iodophe nylisopropylamine (DOI), a 5-HT2 receptor subtype-selective agonist, o n evoked potentials recorded from the central ends of the hypogastric and pudendal nerves in response to electrical stimulation of afferent fibres in the pelvic and pudendal nerves, respectively. Various spinal ization paradigms were employed to localize the site of action. All ca ts were pretreated with xylamidine (1 mg kg(-1)), a peripherally-restr icted 5-HT2 receptor antagonist. 3 In acute spinal cats, DOI (0.01-3 m g kg(-1), i.v.) reliably produced dose-dependent increases in the pude ndal nerve reflex (to 228+/-31% of control). These increases were reve rsed by the 5-HT2 receptor-selective antagonist, LY53857 (0.3-3 mg kg( -1), i.v.). On the other hand, in spinally-intact cats, DOI produced n o significant changes in the pudendal reflex. However, within minutes of spinalization of DOI-pretreated cats, a marked increase (to 221+/-1 6% of control) in the pudendal reflex was observed which could be reve rsed by LY53857. No significant effects were observed on hypogastric r eflexes in either acute spinal or spinally-intact cats following DOI a dministration. No effects were seen in either spinally-intact or acute spinal animals when LY53857 was administered as the initial drug. 4 T hese results indicate that activation of spinal 5-HT2 receptors facili tates pudendal reflexes. In spinally-intact cats, it is hypothesized t hat DOI activates supraspinal pathways that mediate inhibition of the pudendal reflexes and counteracts the facilitatory effects of spinal 5 -HT2 receptor activation.