N-methyl-d, l-aspartate stimulates growth hormone but not luteinizing hormone secretion in the sheep

The objective of this experiment was to determine the effects of N-methyl-d, l-aspartate (NMA) on luteinizing hormone (LH) and growth hormone (GH) secretion in castrated male sheep. Blood was sampled from Hampshire wethers every 15 min for 8 hr on day 1. At 4 and 6 hr after the initiation of the experiment, wethers were treated i.v. with NMA at a dose of 12 mg/kg body weight (n = 5) or .9% saline (n = 5). The dosage of NMA was within the range of doses that was previously demonstrated to stimulate LH secretion in monkeys. Blood samples were also collected every 15 min for 1 hr on day 2, beginning 24 hr after the first injection of NMA or saline. Treatment with NMA had no effect on mean LH concentrations, LH pulse frequency or LH pulse amplitude during the 4 hr period following the first injection on day 1. On day 2, however, mean LH concentrations were lower (p less than .01) in NMA versus saline-treated wethers. Conversely, administration of NMA evoked a dramatic increase (p less than .02) in mean GH concentrations on day 1. The mechanisms responsible for the effects of NMA described herein and whether or not these effects are relevant to the physiological control of LH and GH release in the sheep warrants further scrutiny.