Hormones and Spawning in Fish

Fish integrate their reproductive activities with seasonal environmental cycles. Certain en­vironmental factors such as temperature, photoperiod and rainfall act as cues for the approach­ing favorable season for reproduction. Signals from environmental cues and endogenous physi­ological cycles input to the neuroendocrine system, which in turn regulates pituitary and go­nadal functions. One of the main reasons for lack of ovulation and spawning in a number of cultured fish is failure of the pituitary to release gonadotropin (GtH-11), one of the hormones involved in the regulation of reproduction. GtH-U secretion in many teleosts is under a dual control, with release being stimulated by gonadotropin releasing hormone (GnRH), and inhibi­tion by dopamine on the actions of GnRH, as well as spontaneous release of GtH-H. The inhibi­tor}' actions of dopamine on GtH secretion can vary in potency between species. In goldfish, carps and catfish, dopamine inhibition is very strong: injection of a dopamine blocker, such as domperidone (DOM), results in potentiation of the actions of GnRH analogs, leading to a large release of GtH-11 and ovulation. In bream and loach, dopamine inhibition is weak, injection of a high dose of GnRH analog alone is effective in stimulating GtH-H release and ovulation; how­ever, the combination of DOM with GnRH analog results in potentiation of the GtH-11 response and shortening of the response time from injection to ovulation. In seabream, sciaenids and some other marine fish, GtH:11 secretion is not under dopaminergic inhibitory control, they are sensitive to GnRH a_haJ0gs and can be induced to ovulate and spawn by multiple injections or chronic administration of GnRH analogs.