PubMed1983
Studies of seasonal gonadal cycles combined with direct measurements of plasma levels of the two gonadotropins (FSH and LH), several gonadal steroids (estrogen, E; progesterone, P; testosterone, T; and 5 alpha-dihydrotestosterone, DHT), and the interrenal steroid corticosterone (B) were made in male and female bullfrogs in central California over a 5-year period (between 1976 and 1981). During the course of these studies, we discovered that levels of plasma gonadotropins and steroids are highly labile and particularly sensitive to the effects of captivity, especially in males. In animals captured and sampled repeatedly in the field over a 3-day period, hormone levels remained constant, but if held in collecting sacks, gonadotropin and gonadal steroids began to drop within 2--4 hr and usually reached "baseline" levels within 20 hr. This effect was apparent in all seasons, except occasionally in early spring when hormone levels dropped by only about half. Hormone levels were also generally depressed by the time commercially collected frogs reached local supply houses. Plasma B increased within 30 min of capture and remained high for days in captive animals. Blood samples taken from several hundred animals at the time of capture reveal pronounced seasonal cycles in all hormones measured. These changes are discussed in connection with other gross changes in gonadal condition and with regard to possible interactions among gonadotropins and steroids. Both sexes show a general elevation of hormone levels in spring and early summer, but the sexes differ somewhat both in timing and in magnitude of the changes, as well as in the nature of the dominant steroids. Pronounced "surges" in gonadotropins are evident around the time of gamate release in bot sexes, but the temporal pattern of these surges is not the same for ovulation and spermiation; an elevation in plasma P is associated with the periovulatory surge in gonadotropins. Results were not entirely consistent with expectation of pituitary--gonadal relationships. Levels of plasma gonadotropins and steroids did not show the reciprocal relationship expected from a simple negative feed-back between gonadal and pituitary secretion, nor did changes in gonadotropins and gonadal activities show the consistent positive correlation expected from a direct dependence of gonadal function on circulating gonadotropins. In females, plasma T, but not E, correlated with ovarian growth. Plasma T in females reached much higher levels than in males, but DHT was higher in males. Androgens were generally elevated during the period of sexual activity in males, but absolute levels did not correlate well with individual differences in sexual behavior. Thus, seasonal changes in testicular and ovarian activities cannot be accounted for solely by seasonal cycles in circulating gonadotropin levels.