Changes in the Release of Gamma‐Aminobutyric Acid and Catecholamines in the Preoptic/Septal Area Prior to and During the Preovulatory Surge of Luteinizing Hormone in the Ewe

The technique of intracranial microdialysis was used to monitor changes in the outflow of the catecholamines, noradrenaline, adrenaline and dopamine and the inhibitory amino‐acid γ‐aminobutyric acid (GABA) in the preoptic/septal area of the conscious ewe during an oestradiol‐induced surge of luteinizing hormone (LH). The same animals were sampled twice from an identical brain site, once in the presence of oestradiol and once in its absence, when no surge occurred and LH levels remained low. Changes in the outflow of GABA, noradrenaline and adrenaline (but not dopamine) were related to changes in LH secretion. Specifically, GABA outflow was maximal in the hours following oestradiol administration but began a sustained fall some 10 h before the surge began, to level off just before the first increment in LH secretion. Low GABA concentrations were maintained until after gonadotrophin levels had once more returned to baseline. The release of all three catecholamines was pulsatile. Noradrenergic activity was greater in the presence of oestradiol although activity did not alter over the 20 h of sampling. The pulse frequency of adrenaline was maximal in the hours immediately prior to the LH surge and minimal in the hours following its initiation. These data suggest that a decrease in GABAergic transmission in the vicinity of the LH‐releasing hormone cell bodies is a necessary component of the neural mechanism by which the oestradiol‐induced surge of LH is generated. A general increase in noradrenergic activity coupled with changes in the release of adrenaline at the time of the surge may be additional prerequisites for successful ovulation.