EVIDENCE THAT LUTEINIZING-HORMONE SUPPRESSION IN RESPONSE TO INHIBITORY NEUROPEPTIDES, BETA-ENDORPHIN, INTERLEUKIN-1-BETA, AND NEUROPEPTIDE-K, MAY INVOLVE EXCITATORY AMINO-ACIDS

A large body of recent evidence suggests that a number of inhibitory and excitatory neuropeptides and amino acids may participate in the episodic secretion of hypothalamic LHRH and pituitary LH in castrated rats. However, the precise functional relationships among these messenger molecules in the control of LH secretion remain to be ascertained. The aim of this study was to test the hypothesis that inhibition of LH release by an opioid [beta-endorphin (betaEND)], cytokine [interleukin-1beta (IL-1beta)], or tachykinin [neuropeptide-K (NPK)] is a result of diminished excitatory amino acid (EAA) signaling. Adult male rats were castrated and received an intracerebroventricular cannula in the third ventricle for administration of betaEND (10 mug/rat), NPK (2.5 nmol/rat), or IL-1beta (100 ng/rat) 2 weeks postcastration. One day before the experiments, rats received an intraatrial cannula for frequent blood sampling and for iv injection of the glutamate receptor agonist N-methyl-D-aspartate (NMDA; 5 mg/kg) at 30-min intervals. Blood samples for LH measurements were withdrawn immediately before and 10 min after each NMDA injection. The results show that intracerebro-ventricular betaEND, IL-1beta, or NPK inhibited LH release. Multiple injections of NMDA did not alter the existing pattern of LH secretion in castrated control rats. However, similar NMDA injections completely prevented the decrease in LH release by betaEND, IL-1beta, or NPK. Plasma LH levels in these rats remained within the range seen in untreated control rats throughout the 120-min duration of the experiment, and NMDA injections at 30-min intervals evoked pulses of LH that resembled those seen normally in castrated rats. The blockade of the inhibitory effects of the three peptides by NMDA and previous knowledge of hypothalamic sites of NMDA action suggest that EAA systems may represent a common pathway down-stream in the hypothalamic LHRH-regulating circuitry to mediate diminution of LH release by inhibitory peptides. Further, their inhibitory influence may be exerted either directly at the level of LHRH neurons and/or by diminution in EAA efflux, leading to suppression of LHRH and LH release.