PARENT SESSION
SLIDE SESSION 4: NEUROENDOCRINOLOGY
Chairs: Donal Skinner, Fred Karsch, Heather Billings (Trainee)
Univ Ottawa-Arts Hall 257
2:30 PM-4:30 PM

SEASONAL VARIATION IN THE RESPONSE TO LOCAL ADMINISTRATION OF ESTRADIOL TO THE OVINE RETROCHIASMATIC AREA.

Hardy, Steven¹, Connors, John¹, Valent, Miro¹, Goodman, Robert¹, ¹

ABSTRACT- In the ewe, a reduction in LH pulse frequency during anestrus results from a change in the hypothalamic responsiveness to estradiol (E) negative feedback. Considerable evidence has implicated a group of dopaminergic neurons (A15) in the retrochiasmatic area (RCh) in mediating this seasonal variation in response to E. Moreover, E administered locally to the RCh of ovariectomized (OVX) anestrus ewes suppresses LH pulse frequency. This study was designed to determine if the response of the RCh to E varies with season. This was done by comparing the effects of local microimplants of E into the RCh of OVX breeding season ewes to that observed in OVX anestrus ewes (Hardy et al., Abtsr Soc Neurosci 26: 1447, 2000). Eight OVX breeding season ewes were stereotaxically implanted with chronic bilateral guide tubes targeting the RCh and allowed to recover for at least 2 weeks. Crystalline E was tamped into the lumen of 22 gauge tubing, which were inserted through the guide tubes into the RCh. Blood samples (every 12 min for 4 hr) taken prior to, and 4 days after implant insertion, were analyzed for LH concentrations. LH pulse frequency was compared before and during treatment. Seven of the eight breeding season ewes had correct placements. In these ewes E microimplants had no effect on LH pulse frequency (4.1±0.6 vs. 3.1±0.5), amplitude (9.3±1.3 vs. 7.8±1.2) or mean LH (10.2±2.4 vs. 7.9±1.6). This contrasts markedly with OVX anestrous ewes, in which E microimplants in the RCh inhibited LH pulse frequency and mean LH levels. These data lead to the conclusion that the response of neurons in the RCh to E varies with season. Interestingly, we have observed a similar seasonal variation in the response to E microimplants in the preoptic area (POA). Since POA neurons project to the RCh, we propose that these two areas are key elements in a neural circuit that is responsible for the seasonal variation in E negative feedback in the ewe. Supported by NIH-HD17864

KEY WORDS: NEUROENDOCRINE, HYPOTHALAMUS, ESTRADIOL, DOPAMINERGIC