Effects of Phenobarbital on Hypothalamic LHRH and Catecholamine Turnover Rates in Proestrous Rats

In previous studies we observed that LHRH concentrations increase in the preoptic-suprachiasmatic tuberoinfundibular system (PSTS) during the morning hours of proestrus when norepinephrine (NE) and dopamine (DA) turnover rates are low. Concomitant with the preovulatory surge in gonadotropins, NE turnover rates increase, DA turnovers decrease, and LHRH declines in PSTS. The present studies examine the effects of phenobarbital (PB) on these proestrous events. PB, when given at 0900 hr proestrus, does not prevent the rise in median eminence (ME) LHRH which occurs between 0900 and 1200 hr. When the barbiturate is administered at 1200 hr, LH, FSH, and prolactin proestrous surges are blocked. While PB (given at 1200 hr) does not affect the increase in turnover rates of NE and DA which occur between 1200 and 1400 hr proestrus it markedly suppressed NE turnover rates in ME and suprachiasmatic nuclei but not in the medial preoptic nuclei between 1500 and 1700 hr. The decline in ME-DA which occurs between 1500 and 1700 hr proestrus is not affected by PB treatment at 1200 hr. Pituitary responsiveness to two pulse injections of LHRH (60 min apart) is not reduced by PB treatment. We conclude that PB blockade of preovulatory gonadotropin surges may be via its suppression of hypothalamic NE release during the afternoon of proestrus.