Neuroendocrine regulation of ovulation in fishes: Basic and applied aspects

This review summarizes the major neuroendocrine mechanisms regulating ovulation, thus providing a basis for understanding the various environmental and hormonal techniques for induction of ovulation of cultured teleosts. The secretion of gonadotrophin-rr (GtH-II) is stimulated by gonadotrophin-releasing: hormone (GnRH) and, although some teleosts have three different forms of GnRH regionally distributed in the brain, in most species investigated only one form is present in the pituitary and apparently involved in GtH-II secretion. In nearly all species investigated, dopamine (DA) inhibits GtH-II secretion by direct actions on gonadotrophs, as well as by inhibition of GnRH release, Sex steroids act at both brain and pituitary levels to regulate GtH-II secretion through a combination of positive and negative feedback actions; one important positive feedback action is that sex steroids enhance the responsiveness of the pituitary to GnRH and an important negative feedback action is to increase DA turnover, thereby increasing the overall DA inhibitory tone on GtH-II secretion. The preovulatory surge of release of GtH-II is stimulated by a surge release of GnRH. A decrease in DA turnover also occurs to disinhibit GnRH and GtH-II release. Environmental factors including photoperiod temperature and spawning substrate may cue ovulation and spawning. Social and pheromonal interactions play a very important role in synchronizing preovulatory endocrine changes: ovulation and spawning behaviour in many species. A widely used technique for inducing ovulation of cultured fishes is injection of the combination of a GnRH superactive analogue? to stimulate GtH-II release, and a DA receptor antagonist, to block the inhibitory actions of DA. This is termed the Linpe technique and has proven particularly useful with those species having synchronous or group synchronous follicular development and a large preovulatory surge of GtH-II. In other groups of teleosts, particularly those species having asynchronous ovarian development and multiple spawnings over an extended period, treatment with a sustained-release preparation of a GnRH superactive analogue to cause a prolonged, somewhat enhanced release of GtH-II has proven highly successful in inducing multiple ovulations and spawnings. However, the lack of specific radioimmunoassays for GtH-II in many of these species has hindered progress, as the precise pattern of GtH-II release necessary for the recruitment of vitellogenic oocytes into final maturation and ovulation in these multiple spawners remains an intriguing neuroendocrine question.