Vol. 275, Issue 3, E380-E385, September 1998
Blockage of gonadotropin-induced first ovulation
caused by thyroidectomy and its possible mechanisms in
rats
Kazuhiro
Tamura1,
Minoru
Hatsuta1,
Gen
Watanabe2,
Kazuyoshi
Taya2, and
Hiroshi
Kogo1
1 Department of Pharmacology, Tokyo University
of Pharmacy and Life Science, Hachioji, Tokyo 192-0392; and
2 Department of Veterinary Physiology, Tokyo
University of Agriculture and Technology, Fuchu, Tokyo 183, Japan
To determine the role of the thyroid gland on
the ovarian functions during the initiation process of puberty, we
examined the effects and its mechanisms of hypothyroidism on the first ovulation induced by equine chorionic gonadotropin (eCG) in immature female rats. Animals were thyroidectomized on day 22 and were injected with 5 IU of eCG on day 26 to induce the first
ovulation on day 29. The number of antral follicles that
secrete inhibin and the ovarian weight were significantly increased in
thyroidectomized rats (Tx rats) 48 h after eCG treatment compared with
those in non-Tx rats. However, thyroidectomy (Tx) significantly
suppressed the rates of ovulating animals on day 29. The
blockage of ovulation in Tx rats was recovered by administration of
human chorionic gonadotropin or luteinizing hormone (LH)-releasing
hormone (LHRH) on day 28. Inhibition of serum LH (not
follicle-stimulating hormone) levels induced by Tx was almost restored
to control levels by injection of LHRH. A significant increment in
prolactin levels in Tx rats was also observed on day 28. The
present data indicate that Tx before puberty in female rats causes the
blockage of the first ovulation and that the inhibitory effects on
ovulation are mainly due to the reduction in the preovulatory LH surge,
which is partially mediated through an inhibition of LHRH action on the
secretion of LH.
thyroid function; preovulatory luteinizing hormone; inhibin; prolactin
