Abstract
Progesterone has a facilitating effect on luteinizing hormone (LH) surges induced by estrogen in ovariectomized (OVX) rats. The aim of the present study was to elucidate the involvement of excitatory amino acids such as glutamate and aspartate, and noradrenaline (NA) in the mediobasal hypothalamus/median eminence (MBH) region in a progesterone-enhanced LH surge. Ovariectomized animals were implanted with a Silastic tubing containing crystalline estradiol (E2) for 2 days, and some of them received sc injection of progesterone (4 mg) at 0900 h on the day of sampling. The MBH was perfused using the push-pull perfusion method to determine the levels of glutamate, aspartate, glycine, glutamine, and NA during the LH surge. Blood samples were collected through an atrial cannula to determine plasma LH levels. In animals treated with both E2 and progesterone, levels of glutamate and glutamine significantly (P<0.01) increased simultaneously 1 to 2 h before the peak of LH surge, whereas those levels did not significantly change by E2 treatment alone. Levels of aspartate, glycine and NA showed no significant changes during steroid(s)-induced LH surges. These results suggest that the facilitating effect of progesterone on estrogen-induced LH surges might be mediated, at least in part, by glutamate released at the MBH where the majority of LHRH nerve terminals are distributed.
