Effects of excitatory amino acids on in vivo and in vitro gonadotropin and growth hormone secretion in testosterone‐primed immature rainbow trout, Oncorhynchus mykiss

Abstract The present studies were conducted to evaluate the effects of excitatory amino acids (EAAs) on gonadotropin II (GtH II) and growth hormone (GH) secretion in testosteroneprimed immature rainbow trout, Oncorhynchus mykiss . Intraperitoneal injections of N‐methyl‐DL‐aspartate (NMA) elevated both plasma GtH II and GH levels. By comparison, Des Gly 10 [D‐Ala 6 ] LHRH‐ethylamide (LHRH‐A) elevated plasma GtH II levels but had no effect on plasma GH levels. The GtH II response to NMA was not evident in sexually regressing adult rainbow trout. GtH II secretion by perifused pituitary glands of testosterone‐primed immature rainbow trout was elevated following NMA or salmon GnRH analogue (sGnRH‐A) challenge. The GtH II response to NMA was eliminated in pituitaries previously exposed to either the NMA receptor antagonist, AP5, or a GnRH receptor antagonist (D‐pGlu 1 ,D‐Phe 2 ,D‐Trp 3,6 ‐LHRH), suggesting that NMA operates by stimulating the secretion of GnRH rather than by direct actions on the pituitary gonadotropic cells. The results of these studies provide evidence of the presence of NMA‐specific receptors in the hypothalamo‐hypophyseal system that when activated led to the release of GtH II. L‐glutamic acid (GLU) also stimulated GtH II secretion from perifused pituitary glands. However, the response to GLU was only partly blocked by AP5, suggesting that GLU exerted some of its effect via non‐NMA type receptors, in addition to its actions on NMA receptors. The results of these studies support the contention that EAA receptor activation may play a role in regulating the onset of sexual maturation in teleosts, as it seems to do in mammals. © 1994 Wiley‐Liss, Inc.