Role of the preoptic brain in the regulation of preovulatory gonadotropin surges in the hamster.

These studies have examined the role of the preoptic-suprachiasmatic area (POA-SC) in brains of cyclic female hamsters in regulating proestrous preovulatory gonadotropin surges. The spontaneous release of LH and FSH which normally occurs on proestrous afternoon was blocked with phenobarbital. Temporal changes in serum LH and FSH were measured in such blocked animals after delivery of direct current (100 muA/60 s) to the POA-SC or that portion of the medial basal hypothalamus (MBH) which includes the arcuate nuclei and the median eminence. Bilateral dc treatment of MBH resulted in a 30-fold increase in serum LH and a 4-fold rise in serum FSH over basal concentrations. Unilateral MBH dc treatment produced a 12-fold increase in serum LH but FSH levels remained basal. In contrast, the delivery of dc to the POA-SC did not evoke any increase in serum LH or FSH. Sham electrode placement in the MBH or POA-SC also did not alter basal LH and FSH serum concentrations. These results suggest that, unlike the rat, passage of dc with concomitant production of an irritative lesion and deposition of ferrous ion does not activate structures responsible for preovulatory gonadotropin surges in hamsters. In a second study, discrete electrochemical lesions were produced in the basal anterior portion of the preoptic brain. These lesions did not involve the medial preoptic area or the suprachiasmatic nuclei. Following such brain destruction, spontaneous ovulation, LH surges, and 4-day vaginal cyclicity ceased. When the suprachiasmatic nuclei or extensive regions of the dorsal medial preoptic area, including the anterior commissure, were destroyed, vaginal cyclicity was disrupted for only 8-12 days. Thereafter, these animals had spontaneous preovulatory gonadotropin surges and ovulated. Seemingly, input from the medial basal anterior preoptic region (anterior to the SC) is essential for preovulatory LH and FSH surges to occur.