Circannual and circadian variations in plasma levels of steroids (cortisol, estradiol-17 beta, estrone, and testosterone) correlated with the annual gonadal cycle in the catfish, Heteropneustes fossilis (Bloch).

Circannual and circadian variations in plasma levels of steroids were estimated by radioimmunoassay in the female and male catfish, Heteropneustes fossilis, over two consecutive annual reproductive cycles. In the female catfish, testosterone (T), estradiol-17 beta (E2), and estrone (E1) were detectable in the plasma only during the reproductively active (preparatory through spawning) period and their levels increased during vitellogenesis. In the fully gravid catfish, when vitellogenesis was nearly complete, levels of E2 declined but those of T continued to increase suggesting a product-precursor relationship between the two steroids. Plasma cortisol (F) was detectable throughout the year and exhibited three peaks coinciding with summer, monsoon, and winter; the first and second peaks coincided with vitellogenesis and spawning, respectively. In the male catfish, changes in plasma T and F levels closely paralleled the seasonal recrudescence and activity of testes and seminal vesicles. After spawning, gonads regressed and levels of sex steroids declined sharply. In the absence of natural spawning due to scanty monsoon rains, as during the second year of this study, gonadal regression was delayed and the sex steroids persisted in the plasma well beyond the normal spawning season. In addition, the first two peaks of F levels merged to form a plateau extending from the preparatory period until the late spawning period. The three sex steroids (T, E2, and E1) exhibited identical circadian rhythms; a major peak occurred at the onset of the dark phase (20:00 hr) and a minor peak was generally observed 4 hr after the onset of the light phase (12:00 hr). The amplitude of rhythms was greatest during the prespawning and the spawning periods. Cortisol peak levels generally alternated with those of sex steroids. Steroid rhythms show rather precise correlations with environmental factors such as photoperiod, temperature, and rainfall as well as with seasonal reproductive activity in both sexes of catfish.