NEUROPHYSIN-CONTAINING PATHWAY FROM THE PARAVENTRICULAR NUCLEUS OF THE HYPOTHALAMUS TO A SEXUALLY DIMORPHIC MOTOR NUCLEUS IN LUMBAR SPINAL-CORD

A model that has been widely used in the study of steroid sensitive ne urons, the spinal nucleus of the bulbocavernosus (SNB) is a sexually d imorphic motor nucleus in the lower lumbar spinal cord that innervates the striated bulbocavernosus (BC) muscle. The BC is responsible for p enile reflexes in the male rat, which are important in ensuring pregna ncy in females. The characterization of afferents to the SNB aids in t he understanding of the neural circuitry involved in reproductive beha vior. We have recently identified the paraventricular nucleus (PVN) as a possible source of afferents to the SNB. Because the PVN is the maj or source of oxytocin/vasopressin within the central nervous system (C NS), the purpose of the present study was to examine and characterize a neurophysin (NP)-containing pathway from the PVN to the SNB. The res ults demonstrate that neurons of the lateral parvicellular subnucleus of PVN, which project to levels of spinal cord containing SNB motoneur ons, contain NP, the coproduct of oxytocin and vasopressin. NP-contain ing fibers and putative terminals were found in the region of the SNB and appear to contact the soma and proximal dendrites of SNB motoneuro ns which were retrogradely identified as BC-innervating. Electrolytic lesions, which destroy the lateral parvicellular subnucleus of PVN, ab olish NP-containing fibers in the region of the SNB, suggesting that t he PVN is the source of these NP fibers. The results of this study ind icate a NP-containing projection from the hypothalamus directly to SNB motoneurons. It is suggested that this pathway may play a role in the integration of penile reflexes with other aspects of male copulatory behavior that are under hypothalamic control. (C) 1993 Wiley-Liss, Inc .