In fish as in other vertebrates, the brain is actively involved in the control of reproduction, first by participating, under the influence of external factors, in the establishment of an appropriate endocrine status, but also by allowing synchronization of the partners by the time of spawning. It is now well established that the pituitary gonadotropic function is controlled by multiple stimulatory and inhibitory factors, originating mainly from the preoptic region and the mediobasal hypothalamus, both target regions for sexual steroids. Little is known about the mechanisms involved in the mediation of external and internal factors, however there is indication that internal factors, such as androgens and melatonin, known to trigger particular behavioural and endocrine responses, act both at the level of neuroendocrine territories, but also on sensorial systems, which are the actual sites of action for external factors. This paper represents an attempt to summarize and integrate the recent literature devoted to the different aspects of the brain as a major participant in the complex endocrine and behavioural mechanisms of reproduction in fish.

The brain of the sturgeon has recently been shown to contain at least two forms of GnRH (gonadotropin-releasing hormone), mammalian GnRH (mGnRH) and chicken GnRH-II (cGnRH-II). In this study, we compared the distribution of immunoreactive (ir) mGnRH and cGnRH-II in the brain of immature Siberian sturgeons (Acipenser baeri). The overall distribution of mGnRH was very similar to the distribution of sGnRH in teleosts such as salmonids or cyprinids. mGnRH-ir perikarya were observed in the olfactory nerves and bulbs the telencephalon, the preoptic region, and the mediobasal hypothalamus. All these cell bodies are located along a continuum of ir-fibers that could be traced from the olfactory nerve to the hypothalamopituitary interface. No ir-fibers were observed in the anterior lobe of the pituitary, but a few were seen to enter the neurointermediate lobe. mGnRH-ir fibers were detected in many parts of the brain, particularly in the forebrain. mGnRH-ir cerebrospinal fluid-contacting cells were observed in the telencephalon, the preoptic region, and the mediobasal hypothalamus. In contrast, cGnRH-II was present mainly in the posterior brain, although a few ir axons were seen in the above-mentioned territories. In particular, cGnRH-II-ir cells bodies, negative for mGnRH, were consistently observed in the nucleus of the medial longitudinal fasciculus of the midbrain tegmentum. The cGnRH-II innervation in the optic tectum, cerebellum, vagal lobe, and medulla 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 in teleosts and further documents the differential distribution of two forms of GnRH in the brain of vertebrates.