Abstract:

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-II), one of the hormones involved in the regulation of reproduction. GtH-II secretion in many teleosts is under a dual control, with release stimjulated by gonadotropin releasing hormone (GnRH), and inhibition by dopamine on the actions of GnRH, as well as spontaneous release of GtH-II. The inhibitory 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-II and ovulation. In bream and loach, dopamine inhibition is weak, injection of a high dose of GnRH analog alone is effective in stimulating GtH-II release and ovulation; however, the combination of DOM with GnRH analog results in potentiation of the GtH-II response and shortening of the response time from injection to ovulation. In seabream, sciaenids and some other marine fish, GtH-II secretion is not under dopaminergic inhibitory control, they are sensitive to GnRH analogs and can be induced to ovulate and spawn by multiple injections or chronic administration of GnRH analogs.

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Date 1996/05/01

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