Identified Motoneurons Involved in Sexual and Eliminative Functions in the Rat Are Powerfully Excited by Vasopressin and Tachykinins

Abstract

The pudendal motor system is constituted by striated muscles of the pelvic floor and the spinal motoneurons that innervate them. It plays a role in eliminative functions of the bladder and intestine and in sexual function. Pudendal motoneurons are located in the ventral horn of the caudal lumbar spinal cord and send their axon into the pudendal nerve. In the rat, binding sites for vasopressin and tachykinin are present in the dorsomedial and dorsolateral pudendal nuclei, suggesting that these neuropeptides may affect pudendal motoneurons. The aim of the present study was to investigate possible effects of vasopressin and tachykinins on these motoneurons. Recordings were performed in spinal cord slices of young male rats using the whole-cell patch-clamp technique. Before recording, motoneurons were identified by 1,1Ј-dilinoleyl-3,3,3Ј,3Ј-tetramethylindocarbocyanine, 4-chlorobenzenesulfonate retrograde labeling. The identification was confirmed, a posteriori, by choline acetyltransferase immunocytochemistry. Vasopressin and tachykinins caused a powerful excitation of pudendal motoneurons. The peptide-evoked depolarization, or the peptide-evoked inward current, persisted in the presence of tetrodotoxin, indicating that these effects were mainly postsynaptic. By using selective receptor agonists and antagonist, we determined that vasopressin acted via vasopressin 1a (V1a), but not V1b, V2, or oxytocin receptors, whereas tachykinins acted via neurokinin 1 (NK1), but not NK2 or NK3, receptors. Vasopressin acted by enhancing a nonselective cationic conductance; in some motoneurons, it also probably suppressed a resting K ϩ conductance. Our data show that vasopressin and tachykinins can excite pudendal motoneurons and thus influence the force of striated perineal muscles involved in eliminative and sexual functions.

Role of vasopressin V1Areceptor in the urethral closure reflex in rats

“…VPA has relatively weak selectivity over V 1B R and oxytocinR (20). Because a highly selective V 1A R antagonist was not available for the intrathecal injection in our study, selective agonists for other vasopressin receptors were applied to confirm the responsible receptor.…”

“…Taken together, these findings suggest that V 1A R is responsible for the AVP-induced increase in urethral resistance. Ogier et al (20) indicated that the cultured motor neurons from Onuf's nucleus in neonatal rats can be potently excited by AVP. They showed that the excitation was mediated by V 1A R but not by V 1B R, V 2 R, or oxytocinR.…”

“…These studies suggest that increases in serotonin and norepinephrine at the presynaptic gaps of Onuf's nucleus would enhance the sensitivity of glutamate receptors. Ogier et al (16,20) indicated that AVP application induces both membrane depolarization and action potential discharge in motor neurons prepared in vitro from newborn rats. We consider that AVP would induce membrane depolarization but would not evoke action potentials enough to cause urethral closure responses in vivo because the V 1A R-mediated increase in urethral resistance was not observed without the urethral closure reflex.…”

“…V 1A R is identified in rat Onuf's nucleus by autoradiographic binding analysis (24,25). Additionally, AVP potentiates the neuronal activity of cultured motor neurons from Onuf's nucleus in neonatal rat via V 1A R, but not via the other vasopressin receptors, based on the findings of an electrophysiological study (16,20).We conducted a quantitative RT-PCR with total RNA from rat lamina IX containing Onuf's nucleus after identification of area-innervating urethra by using Fast Blue and identified V 1A R as the most highly expressed receptor among the vasopressin receptor family. We wondered whether V 1A R stimulation would enhance the urethral closure reflex like 5-HT 2 receptor and ␣ 1A -adrenoceptor activation .…”

“…V 1A R is identified in rat Onuf's nucleus by autoradiographic binding analysis (24,25). Additionally, AVP potentiates the neuronal activity of cultured motor neurons from Onuf's nucleus in neonatal rat via V 1A R, but not via the other vasopressin receptors, based on the findings of an electrophysiological study (16,20).…”