Gonadal and age-related influences on NMDA-induced growth hormone secretion in male rats.

Activation of N-methyl-D,L-aspartic acid (NMDA) receptors stimulates growth hormone (GH) secretion. The mechanisms involved in this action are still a matter of debate. Present experiments were carried out to assess specifically: (1) the age-related changes in NMDA effects; (2) the physiological role of NMDA in pulsatile GH secretion; (3) the hypothalamic and/or pituitary actions of NMDA, and (4) the influence of gonadal function on NMDA-induced GH release. NMDA (15 mg/kg i.p.) stimulated GH secretion in neonatal, prepubertal and adult males, this effect being blocked by MK-801, a selective antagonist of NMDA receptors. In adult males, pulsatile GH secretion was abolished after administration of MK-801 and AP-5, antagonists of NMDA receptors. The stimulatory effect of NMDA on GH release was exerted at the hypothalamic level, since in vitro GH secretion was slightly inhibited in the presence of NMDA (0.5 mM). The increase in GH release after NMDA treatment cannot be explained through an increase in GHRH release, as the NMDA effect persisted in animals pretreated with GHRH antiserum and in those neonatally injected with mono- sodium glutamate, a drug that destroys GHRH neurons. In addition, NMDA-induced GH secretion was independent of testicular function since it remained after orchidectomy, testosterone replacement as well as after permanent damage of testicular function by neonatal administration of estrogens (500 microg on day 1 of life). We conclude that NMDA receptors play a physiological role stimulating GH secretion through a hypothalamic mechanism that is, at least partially, not GHRH-dependent, and is not modulated by testicular secretion.