Ontogeny of gonadotropin-inhibitory hormone cells and fibers in the brain and pituitary gland of the pejerrey fish, Odontesthes bonariensis

The gonadotropin-inhibitory hormone is a neuropeptide belonging to the RF-amide family of peptides, first characterized in birds. This peptide can inhibit the synthesis and release of gonadotropins in both avians and mammals. Although gonadotropin-inhibitory hormone (Gnih) variants have also been detected in different fish species, knowledge of their physiological action is still inconclusive and controversial. In addition, there is very little information on the neuronal development of Gnih cells in early stages. In this context, the objective of the present study is to characterize Gnih-containing neurons and fibers in the brain and pituitary gland of adult fish, and during early development of the pejerrey (Odontesthes bonariensis), and the possible relationship with the sex differentiation process.

The Gnih neurons and fibers were determined by immunohistochemistry by using different antisera in the adult pejerrey brain. Gnih-immunoreactive (Gnih-ir) neurons were observed in the olfactory bulbs, the terminal nerve ganglion, the lateral nuclei of the ventral telencephalon, the posterior periventricular nucleus, the dorsal tegmentum, and some isolated neuronal bodies were detected in the secondary gustatory nucleus. Very few Gnih-ir fibers were detected innervating the pituitary gland.

The Gnih neuronal and fiber distribution was also studied from hatching to week 10, covering the sex differentiation period until the gonadal sex was observed. Gnih-immunoreactive neuronal bodies were identified from hatching in the terminal nerve ganglion and the diencephalic posterior periventricular nucleus. Gnih-ir fibers were observed in many regions, including the retina, and a profuse innervation was observed in the pituitary. From the first week post-hatching, Gnih-ir neuronal bodies were identified in the dorsal mesencephalic tegmentum and the lateral nucleus of the ventral telencephalon. In addition, Gnih-ir cells were detected in the pituitary. These Gnih-ir cells were consistently detected from 3 to 7 weeks after hatching, coinciding with onset of gonadal differentiation. At week 10 and in the adult, only a few Gnih-ir fibers were observed in the pituitary. Although the precise function of these pituitary Gnih-ir cells is unknown, the relationship between the appearance of these cells and the process of sex differentiation suggests that Gnih may influence this process.