Neuroendocrine Control of Reproduction in Teleosts

Gonadotropin releasing hormone (GnRH) activity has been demonstrated in crude extracts of the hypothalamus of several teleost species. Teleost GnRH is similar to the GnRH in elasmobranchs, reptiles, and birds. Luteinizing hormone–releasing hormone (LH–RH) or its superactive analogues stimulate gonadotropin (GtH) secretion in teleosts and can induce ovulation. The actions of LH–RH or its analogues can be self-potentiated or self-suppressed by previous exposure to low and high doses, respectively, and the fish may also be able to modulate its response by other means. There is no agreement on the distribution in the brain of LH–RH immunoreactive perikarya and nerve fibers in teleosts, but the main sites for perikarya are the nucleus lateral tuberis (NLT), preoptic region and telencephalon. Lesioning the NLT by electrical current or monosodium L-glutamate demonstrates that it has functional involvement in the stimulation of GtH release, presumably via GnRH. The NLT and pineal are involved in regulation of daily cycles of GtH secretion, which have importance in stimulating gonadal activity. A GtH release-inhibitory factor (GRIF) apparently originates in the anteroventral preoptic region. Abolition of GRIF action on the pituitary by destruction of its origin or its neural pathways to the pituitary cause very high serum levels of GtH in sexually mature females and males, and ovulation of the females; regulation of the ovulatory surge of secretion of GtH probably involves abatement of GRIF and stimulation by GnRH. Sex steroids have a negative feedback effect during gonadal recrudescence; the greatest negative feedback influence is during the spawning period. In sexually immature fish estrogens and androgens aromatizable to estrogens have a positive feedback effect to stimulate accumulation of GtH in the pituitary; this positive feedback effect may be part of the mechanism of onset of sexual maturity or gonadal recrudescence in teleosts. Binding sites in the brain for sex steroids have been investigated by autoradiography, and active locations were found in the NLT, preoptic region, area ventralis of the telencephalon, and parts of the posterior hypothalamus, as well as the pituitary. The functional significance of most of these binding sites is not known.Key words: teleosts, gonadotropin, gonadotropin releasing hormone, gonadotropin release-inhibitory hormone, steroid feedback, nucleus lateral tuberis, nucleus preopticus