ESTRADIOL ACTS LOCALLY WITHIN THE RETROCHIASMATIC AREA TO INHIBIT PULSATILE LUTEINIZING-HORMONE RELEASE IN THE FEMALE SHEEP DURING ANESTRUS

In the present study we have identified a site of action of estradiol in the inhibition of LH secretion during anestrus in the ewe. In the f irst experiment, we studied six sites: the medial preoptic area, the l ateral preoptic area, the ventromedial hypothalamus, the ventrolateral hypothalamus, the retrochiasmatic area (RCh), and the periventricular posterior hypothalamus. We compared the changes in parameters of puls atile LH secretion (interpulse interval, mean nadir, mean amplitude, a nd mean area under curve) during three 6-h sampling periods: before an d 30-36 h and 9 days after intracerebral implantation of crystalline e stradiol. Animals that received estradiol in the RCh (n = 5) showed a significantly greater increase in both the intervals between pulses of LH (up 116%, p < 0.03) and the area under the curve (up 180%, p < 0.0 1) than any of the other groups of 7 animals. In the second experiment , implantation of estradiol in the RCh (n = 6) induced an increase in the intervals between pulses of LH (p < 0.03), whereas receiving an em pty implant (n = 6) had no effect, showing that estradiol specifically induced increases in the intervals between pulses. Thus, estradiol ap pears to act in the RCh where the dopaminergic A15 nucleus, known to i nhibit pulsatile LH release, is located.