ACUTE STRESS SUPPRESSES THE N-METHYL-D-ASPARTATE-INDUCED LUTEINIZING-HORMONE RELEASE IN THE OVARIECTOMIZED ESTROGEN-PRIMED RAT

The effects of N-methyl-D-aspartate (NMDA) and luteinizing hormone releasing hormone (LH-RH) on luteinizing hormone (LH) secretion were examined in ovariectomized estrogen-primed rats under nonstressed and acutely stressed conditions. The basal LH levels were significantly elevated 15 min after the onset of acute immobilization stress, but were not altered in emotionally stressed or nonstressed rats. Intravenous injections of 10 and 40 mg/kg NMDA significantly elevated serum LH levels by 161 and 212%, respectively, from baseline within 10 min in nonstressed animals. However, the NMDA-induced LH release was significantly reduced when tested 30 min after the onset of acute immobilization stress. Acute emotional stress, which did not affect the baseline LH, also suppressed the LH release response to NMDA, suggesting that the reduced LH responses to NMDA in stressed animals was not due to the elevated baseline level. Pituitary LH release responses to LHRH were not affected by acute immobilization. We conclude from these results: (1) acute immobilization stress exerts both stimulatory and inhibitory effects on LH release, while acute emotional stress has only an inhibitory effect in estrogen-primed ovariectomized rats; (2) this inhibition occurs at the supra-pituitary level, and (3) it involves a suppression of the responsiveness of the hypothalamic LH-RH neuronal system to the excitatory amino acid input.