ABNORMAL ESTROUS CYCLICITY AND BEHAVIORAL HYPORESPONSIVENESS TO OVARIAN HORMONES IN GENETICALLY-OBESE ZUCKER FEMALE RATS

Obese Zucker female rats are infertile. The present study was designed to assess estrous cyclicity in adult, ovary-intact, lean ana. obese Z ucker rats and to compare reproductive behaviors induced by exogenous steroid hormones in ovariectomized (ovx) lean and obese Zucker rats. T he majority (90%) of obese rats had incomplete cycles in comparison wi th the normal, l-day cycles displayed by lean Zucker rats. After ovari ectomy, all rats were treated with estradiol benzoate (EB, 15-100 mu g /kg) or EB plus progesterone (P, 2-20 mg/kg), and tested for sexual re ceptivity and proceptivity (PRO). At the highest EB dose, obese Zucker females displayed lordosis less frequently than lean rats (lordosis q uotient, LQ, 8 +/- 68 vs. 32 +/- 138, respectively). At the lowest dos es of EB plus P, lean females were extremely receptive and preceptive (LQ = 93 +/- 4%, PRO = 6.2 +/- 2 bouts/min). Zucker obese females, in contrast, were only slightly receptive (LQ = 26 +/- 11%) and showed le ss PRO than lean rats (PRO = 2.4 +/- 0.6 bouts/min). Increasing the do se of either EB or P, administered in combination with the lowest dose of the other hormone, produced receptivity and PRO in obese Zucker fe males that were comparable with those observed in lean rats. Serum est radiol and P levels in ovx obese rats were either equivalent to or hig her than those in the ovx lean rats when both were given the same dose s of hormones. These data suggest that considerably higher doses and s erum concentrations of EB and/or P are required to elicit robust lordo sis and PRO in ovx obese Zucker than in lean rats. This behavioral hyp oresponsiveness to sex steroid hormones may contribute to infertility in the obese Zucker female rat.