IMMUNOHISTOCHEMICAL LOCALIZAZION OF GONADOTROPIN-RELEASING HORMONE NEURONES IN THE BLUEFIN TUNA (Thunnus thynnus) BRAIN

In the last few years the growing demand of bluefin tuna (Thunnus thynnus) by the market has increased fishery pressure on this species. Thus, a more in-depth knowledge of the biology of reproduction is strictly necessary to achieve the reproduction of this species in captivity in order to avoid the overexploitation of wild stock. Gonadotropin-releasing hormone (GnRH), a decapeptide released from the hypothalamus, is central to reproduction in vertebrates because is the major neuroendocrine regulator of pituitary-gonadal axis. A clear correlation between brain GnRH contents and gonadal maturity has been observed in some species of teleosts. In recent years, it has been shown that in the Teleostes different GnRH forms are expressed within the brain of a single species. The objective of the present study was to characterize and immunolocalize the GnRH-producing neurons within the brain of the bluefin tuna. Due to the bad weather condition of the last year, only the brains from eight Mediterranean Sea bluefin tunas were collected and fixed in 10% buffered formalin and embedded in paraffin wax. Serial sections (7 µm thick) were stained either with Cresil-violet metod or incubated with suitable dilutions of antisera generated against (1) salmon GnRH, (2) chicken GnRH, and (3) mammalian GnRH. Immunoreactivity visualisation was performed using avidin-biotin-peroxidase complex (ABC)-DAB procedure. The immunostaining was found in the pericarya of some neurons as well as in some nerve fibers of the pre-optic and periventricular areas. These nuclei showed two different ellipsoidal multipolar neuron populations characterized by the following size ranges: 1) the largest with major diameter of 26.4-56.5 µm and minor diameter of 16.7-25.2 µm, 2) the smallest with major diameter of 7.8-26.4 µm and minor diameter of 5.0-8.8 µm. Further investigations are in progress.