EFFECT ON LUTEINIZING-HORMONE SECRETION OF GABA RECEPTOR MODULATION IN THE MEDIAL PREOPTIC AREA AT THE TIME OF PROESTROUS LUTEINIZING-HORMONE SURGE

Using a bilateral medial preoptic area (MPOA) infusion system in conscious female rats we have investigated the role of the GABA system in this area on the proestrous luteinizing hormone (LH) surge. Fifteen-minute blood samples for LH estimation were taken throughout the afternoon of proestrus from female rats exhibiting 4-day oestrous cycles and implanted at least 2 weeks prior with cerebral guide cannulae. Between 15:00 and 17:00 h rats received an infusion (1-mu-l/30 min) of artificial cerebrospinal fluid (n = 7), 10-mu-M GABA (n = 6) or 10-mu-M bicuculline methiodide (BMI, n = 6). Animals infused with GABA failed to exhibit an LH surge, while BMI-treated animals displayed an LH surge which was not significantly different to controls. These data show that on the afternoon of proestrus, there are no tonic modulatory actions of the GABA system, acting through the GABA(A) receptor, on neural elements controlling the LH surge in the MPOA. If, however, GABA levels are elevated in the MPOA at this time then the LH surge is blocked. In conjunction with data from correlative studies showing a decrease in endogenous GABA release prior to the LH surge, we suggest that this fall in activity is an essential component of the LH surge mechanism.