Abstract: 88

PHOTOPERIODIC VERSUS METABOLIC SIGNALS AS DETERMINANTS OF SEASONAL ANESTRUS IN THE MARE.

Barry P. Fitzgerald^{1 *}, Christina J. McManus^{1 *}
Dept. Veterinary Sci. Univ. KY, Lexington, KY ¹

Continuation of estrous cycles in mares during anestrus might reflect reduced responsiveness or incorrect translation of photoperiodic cues. Alternatively, metabolic signals might play a role, perhaps gating the response to photoperiod. To examine this hypothesis the timing of anestrus was compared in young, lean mares (n=11) and mature, fat mares (n=12). Responsiveness to constant treatment with melatonin (MEL) was examined in 6 mares from each group which received 4 melatonin implants (18 mg MEL/implant Sanofi, France), beginning June 21 (MEL 27±1.7 pg/ml; n=12), and reimplanted every 3 months. Ovulation was identified by RIA of progesterone in samples collected x3 per week. Every 3 weeks bodyweights (BW) were determined and percent total body fat estimated (EBF) by ultrasound. Leptin and melatonin levels were determined by RIA at selected timepoints between June and Feb. Anestrus occurred earlier in young, lean mares compared to mature mares (7/11 vs 0/12 anestrus in Dec; P<0.001). Treatment with melatonin failed to modify this response. BW was similar between groups (P>0.05) and decreased during winter (P<0.001). However, EBF was consistently greater (P<0.05) in mature versus young mares and in both groups changed with time (P<0.001), similar to BW. In mature mares, leptin was consistently greater than in young, lean mares (eg. Nov: 6.4±1.02 vs 2.38±.55, P<0.001) and declined from 5.92 ± 0.86 to 2.41 ± 0.25 ng/ml between Dec and Jan; in Jan 4/12 mature mares entered anestrus. Overall, the results suggest that low circulating concentrations of leptin are a prerequisite for a response to non-stimulatory photoperiodic cues. This proposal supports the hypothesis that metabolic signals (eg. leptin) play a permissive role in reproductive function.

    This abstract is being presented on Sunday, August 1 at 8:00 AM to 10:15 AM at CUB 2nd Floor Ballroom.