A rapid suppressive effect of estrogen in the paraventricular nucleus on pulsatile LH release in fasting-ovariectomized rats.

The paraventricular nucleus (PVN) and A2 are novel estrogen feedback sites where estrogen may modulate the neural response to adrenergic inputs during fasting. In the present study, the effects of local estradiol (E(2)) perfusion through a microdialysis probe placed in the PVN or A2 on pulsatile luteinizing hormone (LH) secretion and on norepinephrine (NE) release in the PVN were examined in 48-h fasting ovariectomized (OVX) rats to determine whether local estrogen administered in the PVN or A2 rapidly inhibits LH secretion during fasting and whether this inhibition is mediated by an increase of NE release in the PVN. Five days after ovariectomy, animals (n=5 per group) stereotaxically implanted with a guide cannula for microdialysis in the PVN (experiment 1) or both PVN and A2 (experiment 2) were deprived of food for 48 h. Blood samples and dialysates were then collected every 6 min for 3 h and every 12 min (experiment 1) or 20 min (experiment 2) for 3 h, respectively. The PVN or A2 was perfused with E(2) (5 ng/ml in artificial cerebrospinal fluid) through a microdialysis probe after the first hour of sampling. E(2) perfusion in the PVN caused a rapid and significant suppression of mean plasma LH levels and LH pulse frequency in fasting rats but no changes in unfasting animals. NE release in the PVN was not affected by the local E(2) perfusion of the PVN in either fasting or unfasting groups. This perfusion in A2, however, did not cause any apparent changes in plasma LH and perfusate NE levels in the PVN and A2. The present results indicate that estrogen feedback action at the PVN suppresses LH secretion rapidly during fasting and does not involve an increase of NE release in the PVN.