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 enviro nmental 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 h ave three different forms of GnRH regionally distributed in the brain, in most species investigated only one form is present in the pituitar y and apparently involved in GtH-II secretion. In nearly all species i nvestigated, dopamine (DA) inhibits GtH-II secretion by direct actions on gonadotrophs, as well as by inhibition of GnRH release, Sex steroi ds 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 re sponsiveness of the pituitary to GnRH and an important negative feedba ck action is to increase DA turnover, thereby increasing the overall D A inhibitory tone on GtH-II secretion. The preovulatory surge of relea se of GtH-II is stimulated by a surge release of GnRH. A decrease in D A turnover also occurs to disinhibit GnRH and GtH-II release. Environm ental factors including photoperiod temperature and spawning substrate may cue ovulation and spawning. Social and pheromonal interactions pl ay a very important role in synchronizing preovulatory endocrine chang es: ovulation and spawning behaviour in many species. A widely used te chnique for inducing ovulation of cultured fishes is injection of the combination of a GnRH superactive analogue? to stimulate GtH-II releas e, and a DA receptor antagonist, to block the inhibitory actions of DA . This is termed the Linpe technique and has proven particularly usefu l with those species having synchronous or group synchronous follicula r development and a large preovulatory surge of GtH-II. In other group s 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 hig hly successful in inducing multiple ovulations and spawnings. However, the lack of specific radioimmunoassays for GtH-II in many of these sp ecies has hindered progress, as the precise pattern of GtH-II release necessary for the recruitment of vitellogenic oocytes into final matur ation and ovulation in these multiple spawners remains an intriguing n euroendocrine question.