Methallibure-induced inhibition of hypothalamo-hypophyseal-ovarian activity in the catfish Heteropneustes fossilis involves changes in hypothalamic monoamine activity

Administration of Methallibure, a non-steroidal gonadotropin (GTH) inhibito r 20 mu g g(-1) body weight; i.p., daily for 10 days, to prespawning phase female Heteropneustes fossilis inhibited the brain-pituitary-ovarian axis a s indicated by significant reductions in plasma and pituitary levels of GTH -II, and plasma levels of 17 beta-estradiol (E-2) and testosterone. Concurr ently, the treatment resulted in significant reductions in the hypothalamic content of serotonin, noradrenaline (and adrenaline) that stimulate, and a significant elevation of dopamine that inhibits GTH-II release in this spe cies. Activities of the monoamine degrading enzymes, monoamine oxidase and catechol-O-methyltransferase were significantly increased, while that of th e synthesizing enzymes, dopamine-beta-hydroxylase and phenylethanolamine-N- methyltransferase were significantly decreased. These results suggest that the mechanism of inhibition of GTH-II secretion includes, among others, dif ferential actions of the drug on hypothalamic monoamine metabolism.