Specific areas of the preoptico-suprachiasmatictuberoinfundibular system (PSTS) were microdissected at various hours on proestrus and diestrous day 1 in rats. LHRH concentrations were measured by RIA to determine if the patterns of change observed in the median eminence (ME) were paralleled by similar LHRH patterns in the remainder of the PSTS. Another group of proestrous rats received phenobarbital to block preovulatory gonadotropin surges, and LHRH concentrations in the PSTS were measured throughout the afternoon hours. Plasma concentrations of LH, FSH, estradiol, and progesterone were determined in all animals. Rats were decapitated on proestrus and diestrous day 1 at 0900, 1200, 1500, and 1800 h. Proestrous animals that received phenobarbital at noon were sacrificed at 1500 and 1800 h. The following brain areas were microdissected: the suprachiasmatic preoptic nucleus, medial preoptic nucleus, suprachiasmatic nucleus, anterior hypothalamic nucleus retrochiasmatic area, arcuate nucleus, and ME. On the day of proestrus, LHRH concentrations increased between 0900–1200 h (before the gonadotropin surge), decreased between 1200–1500 h (during the surge) and remained low at 1800 h in the medial preoptic nucleus, suprachiasmatic nucleus, retrochiasmatic area, and median eminence. A similar trend was observed in the anterior hypothalamic nucleus and suprachiasmatic preoptic nucleus, but only the drop in the suprachiasmatic preoptic nucleus, between 1200–1500 h attained statistical significance. No such patterns of change were observed on diestrous day 1. Phenobarbital blocked the LH and FSH surges but failed to prevent the afternoon decline in LHRH between 1200-1500 h in any of the brain regions examined. The morning rise and afternoon fall in LHRH may represent an increase in the synthesis and storage of the hormone which is released during the afternoon hours. The PSTS appears to function as an integrated unit, the component parts of which may be involved in synthesis, transport, storage, and the ultimate release of LHRH on proestrus.
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