Annual cycle of pituitary and plasma gonadotropins and plasma sex steroids in a wild population of the toad, Bufo japonicus.

Pituitary and plasma gonadotropins and plasma sex steroids of free-living toads, Bufo japonicus, were measured monthly from March 1981 to February 1982 and examined in relation to gonadal cycles. Toads were captured at Mizuno, Saitama Prefecture. Individual blood samples were collected by cardiac puncture within 3 min of capture in the field. In males, testicular weight was maximal in August. Plasma follicle-stimulating hormone (FSH) levels changed in association with testicular weight. Plasma androgen levels showed a small peak in November and a large peak in March just prior to breeding. Plasma luteinizing hormone (LH) levels changed in parallel with plasma androgen levels. In females, neither plasma FSH nor LH alone were correlated significantly with ovarian weight. However, ovarian and oviductal weights both correlated significantly with plasma steroid levels. Plasma estradiol levels showed a sharp peak in March, followed by a rapid decrease to the minimum in April. A gradual increase of estradiol occurred from July to November in parallel with an increase in ovarian weight. Changes in plasma progesterone and androgen levels in females resembled those for estradiol. However, the changes in progesterone were not so marked as in estradiol. Plasma androgen levels in females were especially high between January and March. In both sexes, the pituitary gonadotropin contents changed in parallel with plasma levels of both FSH and LH. The pituitary almost always contained more LH than FSH, while the reverse was true in the plasma in both sexes. In addition, plasma FSH levels increased markedly in early summer when plasma LH remained unchanged (males) or increased only slightly (females). These results indicate that the toad may serve as excellent material for the study of differential control of FSH and LH secretion.