NEURONS IN THE PARAVENTRICULAR NUCLEUS OF THE HYPOTHALAMUS THAT PROJECT TO THE SEXUALLY DIMORPHIC LOWER LUMBAR SPINAL-CORD CONCENTRATE H-3 ESTRADIOL IN THE MALE-RAT

The location and distribution of estradiol-concentrating neurons in th e hypothalamus afferent to segments of lumbar spinal cord that contain the sexually dimorphic spinal nucleus of the bulbocavernosus (SNB) we re determined by combining retrograde fluorescent tract tracing with s teroid hormone autoradiography. Injections of Fluorogold were made int o segments of L5-L6 of the spinal cord of adult male rats and 12 days later animals were castrated. One week following castration, males rec eived injections of [H-3]estradiol and were perfused. Their brains wer e then processed for steroid hormone autoradiography. Following exposu re times of 11 to 12 months, autoradiograms were developed and the hyp othalamus was analyzed for neurons that concentrate estradiol and proj ect to the spinal cord. Numerous neurons in the hypothalamus projected to the spinal cord, specifically neurons in the paraventricular nucle us (PVN), the lateral hypothalamus and the dorsal area of the hypothal amus. Although many subnuclei of PVN, as well as lateral hypothalamus, contained Fluorogold labelled neurons and estradiol concentrating neu rons, the majority of double labeled cells were found in the lateral p arvocellular (lp) subnucleus of PVN. Approximately 30% of the neurons in the lp subnucleus that projected to spinal cord also concentrated e stradiol. Up to one half of the estradiol-concentrating neurons in lp sent axons to the lower lumbar spinal cord. These results suggest that some of the effects of gonadal steroid hormones on SNB development, p lasticity and function may in fact, be indirect, via steroid-sensitive afferents.