Sexually dimorphic distribution of a galanin‐like peptide in the central nervous system of the teleost fish Poecilia latipinna

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Abstract
Immunohistochemical techniques were used to visualize areas of the brain and spinal cord containing a galaninlike peptide in the teleost fish, the sailfin molly. Galaninlike immunoreactivity (GAL‐LI) in both males and females was identified in neurons in the nucleus preopticus periventricularis, nucleus lateralis tuberis, and nucleus commissuralis. GAL‐LI fibers had a comparable distribution in the forebrain, preoptic, hypothalamic, and visceral sensory areas of both sexes. In striking contrast to these areas, the optic tectum, torus semicircular is, brainstem tegmentum, and spinal cord of the male contained much higher levels of GAL‐LI than the female. GAL‐LI in these dimorphic areas in the female was limited to single fiber bundles in the ventromedial tegmentum and in the trigeminal system. Additionally, a population of neurons in the preoptic nucleus was found to contain GAL‐LI in the male only.Sexual dimorphism was especially prominent in the spinal cord, where extensive GAL‐LI fibers were found in the male only. These fibers were oriented in the longitudinal plane and confined largely to the gray matter. Comparative studies were performed on the goldfish spinal cord, in which GAL‐LI was localized solely in the dorsal horn and exhibited no sexual dimorphism. Further, examination of spinal cord material from neonatal mollies revealed a lack of spinal GAL‐LI at this developmental stage. As the extent of GAL‐LI in the male molly spinal cord differs from both the goldfish and from that reported for the mammalian spinal cord, and a prominent sexual dimorphism in GAL‐LI extends from the diencephalon to the caudal spinal cord, it is suggested that a galaninlike peptide may play a unique, sex‐specific role in this species.