Orexin-A suppresses the pulsatile secretion of luteinizing hormone via beta-endorphin

Orexins, the novel hypothalamic neuropeptides that stimulate feeding behavi or, have been shown to suppress the pulsatile secretion of LH in ovariectom ized rats. However, the mechanism of this action is still not clear. We exa mined the effect of naloxone, a specific opioid antagonist, on the suppress ion of the pulsatile secretion of LH by orexins to determine whether beta - endorphin is involved in this suppressive effect. We administered orexins i ntracerebroventricularly and injected naloxone intravenously in ovariectomi zed rats, and we measured the serum LH concentration to analyze the pulsati le secretion. Administration of orexin-A significantly reduced the mean LH concentration and the pulse frequency, but coadministration of naloxone sig nificantly restored the mean LH concentration and the pulse frequency. Admi nistration of orexin-B also significantly reduced the mean LH concentration and the pulse frequency, and coadministration of naloxone did not restore them. These results indicate that orexin-A, but not orexin-B, suppresses Gn RH secretion via beta -endorphin. (C) 2001 Academic Press.