The LHRH neuronal system in female rats: relation to the medial preoptic nucleus.

Previously we have found that small lesions confined to the medial preoptic nucleus (MPN) or the suprachiasmatic nucleus (SCN) blocked the cyclic release of gonadotropins in the female rat, inducing a persistent estrous state. Since the MPN is located just caudal to the organum vasculosum of the lamina terminalis (OVLT) where LHRH cell bodies are most concentrated, we applied an immunocytochemical technique to examine the possibility that the lesions had simply disrupted LHRH neurons or fibers. Using a new anti-LHRH provided by Dr. V. D. Ramirez, we found that the distribution pattern of immunoreactive LHRH cell bodies and fibers was similar to that previously reported, although the staining was more intense and extensive with low background. There was no concentration of LHRH cell bodies and fibers in the MPN or SCN and, in fact, these nuclei generally showed a lower density of stained elements than did surrounding tissue. In persistent estrous animals with lesions confined to the MPN there was no detectable reduction of stained fibers in the median eminence. These results, along with the results of other workers, suggest that persistent estrus following lesions of the MPN or SCN is not due to reduction of LHRH neurons or fibers. Rather, they support the hypothesis that these nuclei are critical for triggering the ovulatory release of LHRH.