Estradiol increases multiunit electrical activity in the A15 area of ewes exposed to inhibitory photoperiods

Seasonal anestrus in ewes results from an increase in response to the negat ive feedback action of estradiol (E-2). This increase in the inhibitory eff ects of E-2 is controlled by photoperiod and appears to be mediated, in par t, by dopaminergic neurons in the retrochiasmatic area of the hypothalamus (A15 group). This study was designed to test the hypothesis that E-2 increa ses multiunit electrical activity (MUA) in the A15 during inhibitory long d ays, MUA was monitored in the retrochiasmatic area of 14 ovariectomized ewe s from 4 h before to 24 h after insertion of an E-2-containing implant subc utaneously, In six of these ewes, MUA activity was also monitored before an d after insertion of blank implants, Three of the 14 ewes were excluded fro m analysis because E-2 failed to inhibit LH. When MUA was recorded within t he A15, E-2 produced a gradual increase in MUA that was sustained for 24 h, Blank implants failed to increase MUA in the A15 area, and E-2 did not alt er MUA if recording electrodes were outside the A15, These data demonstrate that E-2 increases MUA in the A15 region of ewes and are consistent with t he hypothesis that these neurons mediate E-2 negative feedback during long photoperiods.