Impaired ovulation by 2,3,7,8 tetrachlorodibenzo- p-dioxin (TCDD) in immature rats treated with equine chorionic gonadotropin

Abstract

Several studies have established that 2,3,7,8 tetrachloro- p-dioxin (TCDD) blocks ovulation. The main purpose of this study was to determine if induced ovulation was delayed temporarily by TCDD. The ovulation model used was that of the gonadotropin-primed intact or hypophysectomized rat. Immature intact female Sprague-Dawley rats (IIR) were given 32 μg TCDD/kg by gavage on day 24 of age. The next day equine chorionic gonadotropin (eCG) (5 IU) was injected sc to stimulate follicular development. The number of ova in the oviducts, the ovulation rate, and steroid concentrations were determined at 72, 96, 120, and 144 h after eCG. Immature female Sprague-Dawley rats (IHR) were hypophysectomized on day 23 of age. On day 26, the IHR were given 20 μg TCDD/kg by gavage. The next day eCG (10 IU) was injected sc to stimulate follicle development and at 52 h after eCG, 10 IU human chorionic gonadotropin (hCG) was given to induce ovulation. The same parameters as in IIR were determined in IHR at 72, 96, and 120 h after eCG. TCDD decreased body and ovarian weight gains in both IIR and IHR. In IIR, TCDD delayed ovulation by 24 to 48 h reducing the number of ova shed as well as the number of animals ovulating at 72 and 96 h after eCG. In IHR, however, TCDD reduced only the number of ova shed but caused no delay in ovulation. The IIR treated with TCDD had low levels of progesterone (P4) at 72 and 96 h after eCG but high levels of estradiol (E2) at the same time points. This sustained high level of E2 production coincided with a transient decrease in serum concentrations of androstenedione (A4). The alteration of steroid hormones by TCDD was restored to normal by 48 h after ovulation in IIR. Serum P4 concentration was not altered by TCDD in IHR at 72 h after eCG but was decreased thereafter. The delay in ovulation induced by TCDD in IIR indicates the disruption of the hypothalamus-pituitary-ovary axis during proestrus. The decrease in number of ova shed in IHR induced by exogenous gonadotropins indicates an additional direct ovarian effect of TCDD in blocking ovulation.