COMPARATIVE DISTRIBUTION OF MAMMALIAN GNRH (GONADOTROPIN-RELEASING-HORMONE) AND CHICKEN GNRH-II IN THE BRAIN OF THE IMMATURE SIBERIAN STURGEON (ACIPENSER-BAERI)

The brain of the sturgeon has recently been shown to contain at least two forms of GnRH (gonadotropin-releasing hormone), mammalian GnRH (mG nRH) and chicken GnRH-II (cGnRH-II). In this study, we compared the di stribution of immunoreactive (ir) mGnRH and cGnRH-II in the brain of i mmature Siberian sturgeons (Acipenser baeri). The overall distribution of mGnRH was very similar to the distribution of sGnRH in teleosts su ch as salmonids or cyprinids. mGnRH-ir perikarya were observed in the olfactory nerves and bulbs, the telencephalon, the preoptic region, an d the mediobasal hypothalamus. All these cell bodies are located along a continuum of ir-fibers that could be traced from the olfactory nerv e to the hypothalamo-pituitary interface. No ir-fibers were observed i n the anterior lobe of the pituitary, but a few were seen to enter the neurointermediate lobe. mGnRH-ir fibers were detected in many parts o f the brain, particularly in the forebrain. mGnRH-ir cerebrospinal flu id-contacting cells were observed in the telencephalon, the preoptic r egion, and the mediobasal hypothalamus. In contrast, cGnRH-II was pres ent mainly in the posterior brain, although a few ir axons were seen i n the above-mentioned territories. In particular, cGnRH-II-ir cell bod ies, negative for mGnRH, were consistently observed in the nucleus of the medial longitudinal fasciculus of the midbrain tegmentum. The cGnR H-II innervation in the optic tectum, cerebellum, vagal lobe, and medu lla oblongata was more abundant than the mGnRH innervation in the same areas. This study provides evidence that the organization of the GnRH systems in a primitive bony fish is highly similar to that reported i n teleosts and further documents the differential distribution of two forms of GnRH in the brain of vertebrates. (C) 1993 Wiley-Liss, Inc.