Modulation of growth hormone, luteinizing hormone, and testosterone secretion by excitatory amino acids in boars

Ketamine hydrochloride, an n-methyl-d-aspartate (NMDA) receptor antagonist was used in an experiment that tested the hypothesis that fasting-induced increases in growth hormone (GH) secretion is mediated by excitatory amino acid (EAA) neurotransmission in boars. The effects of the drug on circulating concentrations of luteinizing hormone (LH) and testosterone were also evaluated. Blood was sampled at 15-min intervals for 8 h from 12 boars fitted with jugular vein catheters. At Hours 4 and 6, fasted boars (feed was withdrawn 48 h before the start of blood sampling) received i.m. injections of ketamine (19.9 mg/kg body weight; n=4) or .9% saline (n=4). Boars allowed feed on an ad libitum basis (n=4) received i.v. injections of n-methyl-d,l-aspartate (NMA; 2.5 mg/kg body weight), an NMDA receptor agonist, at Hours 4 and 6. Secretion of GH increased after NMA injections but was unaffected by treatment with ketamine or saline. Circulating concentrations of LH and testosterone were increased by injections of ketamine but were unaffected by injections of NMA or saline. Our results suggest that NMA is a potent GH secretagogue, but do not support the hypothesis that EAA neurotransmission drives the increased GH secretion displayed in fasted boars. Our finding that ketamine increased LH and testosterone release supports the notion that EAA have inhibitory effects on gonadotropin secretion in acutely fasted swine.