Effects of age on GABA turnover rates in specific hypothalamic areas in female rats.

During aging in female rats the estrous cycle ceases and the animals develop phases of constant estrous (CE) or constant diestrous (CD) prior to the irreversible transition into anestrous. In young rats, gamma-aminobutyric acid (GABA) is of pivotal importance for the release of GnRH. In the medial-preoptic area (MPO) where the majority of the GnRH perikarya are located in the rat, GABA release decreases at the time of the preovulatory LH surge. The suprachiasmatic nucleus (SCN) contains numerous GABA neurons. Neurochemical signals from this hypothalamic nucleus provide temporal information to GnRH neurons and thereby influence the preovulatory LH surge and the length of estrous cycles. To investigate aging-related changes of the activity of hypothalamic GABAergic neurons, we determined GABA turnover rates in various hypothalamic nuclei of CE and CD rats and compared them to those determined in young estrous (E) or diestrous rats (D1). In old female rats, GABA activity in the MPO was significantly decreased compared to E and D1 rats. A selective increase of GABA turnover rates was observed in the SCN of CE animals. No age-related changes were observed in the other examined brain areas. These data provide the first evidence for alterations in GABAergic activity in specific hypothalamic areas that depend on age and reproductive status. These may cause changes in ability to induce preovulatory LH surges and to maintain regular estrous cyclicity.