PEPTIDERGIC INNERVATION OF THE CREMASTER NUCLEUS .1. A SEXUALLY DIMORPHIC POPULATION OF SUBSTANCE P-CONTAINING INTRASPINAL NEURONS EXISTS IN THE SUBSTANCE-P PATHWAY TO THE RAT CREMASTER NUCLEUS

The cremaster nucleus (CN) lies in the lumbar spinal cord and is sexually dimorphic: the male CN contains three times as many motoneurons as the female. The substance P (SP) innervation of the CN is also sexually dimorphic with males receiving a very prominent innervation which is greatly diminished in females. These investigations examined SP-containing neurons located in the ventral half of lamina IV and the lateral aspects of laminae V, VII, and IX, in lumbar spinal levels 1,2. SP-containing intraspinal neurons in these laminae are at least three times as numerous in males than females. This provides the first demonstration of a sexually dimorphic population of spinal neurons which is not motor or preganglionic in nature. These SP-containing interneurons are found within, or adjacent to, the SP-containing fibers which constitute the massive SP pathway to the male CN. Processes of these SP-containing neurons were observed to contribute to the formation of the SP pathway to the male CN. The immunohistochemically demonstrable presence of these lumbar 1,2 laminae IV-IX, SP-containing neurons validates former studies which suggested their existence (Gibson et al., Brain Research, 301 (1984) 243-251; Uda et al., Neurosci. Lett., 57 (1985) 185-190).