Effects of N-methyl-D,L-aspartate (NMA) on growth hormone and thyroid hormone levels in steroid-primed immature rainbow trout (Oncorhynchus mykiss)

Abstract

In this study we investigate whether the stimulatory action of the glutamate agonist, N-methyl-D,L-aspartate (NMA) on growth hormone (GH) levels in rainbow trout (Oncorhynchus mykiss) is steroid hormone-dependent. NMA was administered to sexually immature juvenile rainbow trout that had been primed with testosterone (T), 17β-estradiol (E2) or 5α-dihydrotestosterone and changes in plasma growth hormone (GH) concentrations were measured. NMA had no significant effect on the plasma GH concentrations of non-steroid hormone-primed trout (controls), although in both T- and E2-primed fish plasma GH levels were elevated 6 hours following NMA administration. The response had disappeared by 24 hours in the T-primed group, but was still evident in the E2-primed fish. In comparison, in the 5αDHT-primed fish, NMA significantly depressed plasma GH levels 6 hours post-injection (P < 0.05), but this effect had disappeared 24 hours after NMA administration. NMA had no effect on plasma T4 levels in any treatment group, and although plasma T3 levels were significantly (P < 0.05) depressed in T-primed fish 6 hours post-injection, this response was not seen in any other group. These results suggest that NMA has no effect on plasma thyroid hormone levels but it does have a steroid hormone-dependent action on plasma GH levels in rainbow trout. J. Exp. Zool. 279:126-132, 1997. © 1997 Wiley-Liss, Inc.