Gonadotropin‐releasing hormone (GnRH) immunoreactive system in the brain of the dwarf gourami (Colisa lalia) as revealed by light microscopic immunocytochemistry using a monoclonal antibody to common amino acid sequence of GnRH

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Abstract
The present paper aims to give a morphological basis for the study of the terminal nerve system and its relation to the whole gonadotropin-releasing hormone (GnRH) immunoreactive (ir) neuronal system. We examined the GnRH-ir neuronal system of a tropical fish, the dwarf gourami, by using a recently developed monoclonal antibody against GnRH (LRH13) which recognizes the amino acid sequence common to all known variants of GnRH (Park and Wakabayashi, Endocrinol. Jpn. 33:257–272, '86). The ganglion cells of the terminal nerve (TN-ggl cells) in the transitional area between the olfactory bulb and the telencephalon reacted strongly with the LRH13. A distinct bundle of axons emanating from the TN-ggl cells ran caudally through the ventral telencephalon and the preoptic area. Some of these axons entered the optic nerve and innervated the retina. The remaining axons continued caudally to enter the hypothalamus and the midbrain. A second group of GnRH-ir cell bodies was found in the preoptic area. A distinct bundle of GnRH-ir fibers originating from these cell bodies innervated the pituitary. This pathway is equivalent to the preoptico-infundibular pathway of other vertebrates, and the GnRH in this pathway is presumed to function as hypophysiotrophic hormone to facilitate the release of gonadotropins from the pituitary. The distribution of GnRH-ir fibers in the brain was extensive. Most fibers apparently originated from the TN-ggl cells and covered various brain regions from the olfactory bulb to the spinal cord. They were especially abundant in the olfactory bulb, ventral telencephalon, preoptic area, optic tectum, and some hypothalamic areas. Thus, GnRH might function as a neuromodulator and/or neurotransmitter in these areas. The abundant GnRH-ir fibers in the ventral telencephalon and the preoptic area might affect some aspects of sexual behavior, since these areas have been suggested to be involved in the control of sexual behavior in teleosts.