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High-Voltage Electron Microscopy Reveals Direct Synaptic Inputs from a Spinal Gastrin-Releasing Peptide System to Neurons of the Spinal Nucleus of Bulbocavernosus

Ushimado Marine Laboratory (H.S.), Graduate School of Natural Science and Technology, Okayama University, Ushimado, Setouchi, Okayama 701-4303, Japan; Department of Anatomy and Neurobiology (H.S., M.K.), Kyoto Prefectural University of Medicine, Kawaramachi-Hirokoji, Kamigyo-ku, Kyoto 602-8566, Japan; and Section of Brain Structure Information (T.A.), Supportive Center for Brain Research, National Institute for Physiological Sciences, Nishigonaka, Myodaiji, Okazaki 444-8585, Japan

Address all correspondence and requests for reprints to: Hirotaka Sakamoto, Ph.D., Ushimado Marine Laboratory, Graduate School of Natural Science and Technology, Okayama University, Ushimado, Setouchi, Okayama 701-4303, Japan. E-mail: hsakamo{at}uml.okayama-u.ac.jp; or Mitsuhiro Kawata, M.D., Ph.D., Department of Anatomy and Neurobiology, Kyoto Prefectural University of Medicine, Kawaramachi-Hirokoji, Kamigyo-ku, Kyoto 602-8566, Japan. E-mail: mkawata{at}koto.kpu-m.ac.jp.

The spinal nucleus of bulbocavernosus (SNB) is a sexually dimorphic motor nucleus located in the anterior horn of the fifth and sixth lumbar segments of the spinal cord that plays a significant role in male sexual function. We recently found that a sexually dimorphic expression of gastrin-releasing peptide (GRP) in the lumbar spinal cord regulates male copulatory reflexes. Although it is reported that these systems are both profoundly regulated by circulating androgen levels in male rats, no direct evidence has been reported regarding GRP synaptic inputs onto SNB motoneurons. The aim of the current study was to determine the axodendritic synaptic inputs of spinal GRP neurons to SNB motoneurons. Immunoelectron microscopy, combined with a retrograde tracing technique using high-voltage electron microscopy (HVEM), provided a three-dimensional visualization of synaptic contacts from the GRP system in the lumbar spinal cord onto SNB motoneurons. HVEM analysis clearly demonstrated that GRP-immunoreactive axon terminals directly contact dendrites that extend into the dorsal gray commissure from the SNB. These HVEM findings provide an ultrastructural basis for understanding how the spinal GRP system regulates male sexual behavior.