The Hypothalamic Peptides, β‐Endorphin, Neuropeptide K and Interleukin‐1β, and the Opiate Morphine, Enhance the Excitatory Amino Acid‐Induced LH Release under the Influence of Gonadal Steroids

Abstract

Several hypothalamic neuropeptides and amino acids are known to inhibit or excite pituitary luteinizing hormone (LH) release, but the precise interplay between these 2 classes of signals in episodic LH discharge is not known. In this study, we have evaluated the interaction between neuropeptides shown previously to inhibit LH release in castrated rats and the excitatory amino acid agonist, N-methyl-D-aspartate (NMDA), on LH release in intact male rats. Rats received a permanent intracerebroventricular (i.c.v.) cannula and 9-12 days later an intrajugular cannula for frequent blood sampling. The next day, rats received i.c.v. either saline (SAL, 3 microliters, controls) or a neuropeptide: the opioid beta-endorphin (beta-END; 2.9 nmol), the tachykinin neuropeptide K (NPK, 2.5 nmol) or the cytokine interleukin-1 beta (IL-1 beta, 5.9 pmol) in SAL. The LH response to 2 consecutive i.v. injections of NMDA (5 mg/kg) at 30 min intervals was evaluated. In control rats, each NMDA injection evoked a significant release of LH at 10 min. Quite unexpectedly, the three peptides, instead of exerting an inhibitory effect, enhanced the LH response to NMDA. The peak plasma LH levels after each NMDA injection and the cumulative LH responses were significantly higher in peptide-treated than in control rats. This peculiar ability of the peptides that inhibit LH release in castrated rats, to potentiate the NMDA-induced LH release in the presence of gonadal steroids was further validated in female rats treated with an opiate receptor agonist, morphine (MOR) which is also known to suppress LH release in ovariectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)