Role of medial preoptic GABA neurones in regulating luteinising hormone secretion in the ovariectomised rat

Summary The role of GABA neurones in the medial preoptic area (MPOA) in regulating the activity of the luteinising hormone-releasing hormone (LHRH) neurones projecting to the median eminence was investigated in the conscious ovariectomised rat. Plasma luteinising hormone (LH) concentrations were measured while (1) endogenous GABA release from the MPOA was monitored with the technique of microdialysis, or (2) activity at the GABA receptor was modulated by local infusions into the MPOA. Microdialysis studies revealed a fluctuating level of GABA release in the MPOA which did not correlate with pulsatile LH secretion. Infusion of 10 M GABA (n=8) or bicuculline methiodide (BMI, n=6) into the MPOA, at a rate of l 1/30 min, significantly inhibited mean LH concentrations (P< 0.05-0.001) and LH pulse frequency (P< 0.05-0.001) compared with controls (n = 8). LH pulse amplitude was not significantly altered by infusion of GABA (P> 0.05) while too few pulses were found after BMI treatment to enable statistical analysis. Infusions of GABA into the ventral half of the MPOA had a more significant inhibitory effect upon LH secretion compared with dorsal infusions (P=0.012). A similar relationship did not exist for BMI infusions. These results show that acute changes in preoptic GABA receptor occupancy result in disruption of pulsatile LH secretion in the ovariectomised rat. This suggests that GABA neurones provide a tonic input important for the functional integrity of the neural network controlling LH secretion. However, as changes in extracellular GABA concentrations in the MPOA do not correlate with pulsatile LH release, the preoptic GABA population, as a whole, is unlikely to be directly responsible for the pulsatile activity of the LHRH neurones in the ovariectomised rat.