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

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.