Sexual differences in the topographical distribution of serotonergic fibers in the anterior column of rat lumbar spinal cord.

Sexual dimorphism in the topographical organization of immunoreactive serotonergic fibers has been shown for the first time in the anterior column of the rat lumbar cord. A characteristic preferential arrangement of serotonergic fibers on the small cell column composed of anterior horn motoneurons, which have been proved to send their axons to the cremaster muscle, was demonstrated at the lumbar segments (L1-L2) of male rats, using the immunoperoxidase technique with antiserum against serotonin. A similar finding was also observed in female rats, but was less prominent than in males. Comparative analysis of the cell numbers and the size of neuronal somata of the aforementioned nucleus, done using retrograde transport of fluorescent dye (DAPI) via axonal fibers coursing down the genitofemoral nerve to the cremaster muscle, gave significantly larger values in males. The sexual difference in the serotonergic innervation pattern was, in consequence, surmised to be caused by the cytoarchitectonic contrast ascertained in the lumbar anterior column. Furthermore, there may be a striated muscle endowed with some active functions homologous to those of the male cremaster muscle in the female rat.