Seasonal study of the interrenal function of the European green frog, in vivo and in vitro.

The existence of seasonal changes in the secretory activity of the interrenal glands in European Green frogs has been investigated in vivo, in a natural environment, and in vitro, using a perifusion system technique. In vivo, plasma corticosterone levels have been systematically measured at six different moments during a 24-hr period (03, 07, 11, 15, 19, and 23 hr), for almost all the months, during 2 consecutive years. In this part of the study, corticosterone concentrations were assayed in duplicate, in an average of seven animals at each of the six definite moments of the 24-hr cycle. Significant fluctuations in circulating corticosterone have been recorded: a single peak in late spring (May and June) coinciding with the spawning period; a decrease during summer and autumn; the nadir in the depth of winter (February); and a sharp increase at the emergence from hibernation (end of March). The amplitude of the fluctuations ranged from 0.28 ± 0.07 μg/100 ml in late February to 2.68 ± 0.36 μg/100 ml in late May. No significant difference in plasma corticosterone concentrations was observed between males and females. The in vitro studies consisted of comparing the responses of perifused frog interrenal tissue to increasing doses of homologous pituitary extracts, at various times of the year. According to the date of the experiment, important differences in maximum corticosterone snd aldosterone secretions were recorded. As an example, under submaximum ACTH stimulations, 188 and 193% increases in corticosterone and aldosterone outputs, respectively, were recorded on June 4th, whereas the corresponding figures on October 17th were 38 and 54%. The maximum capacity of interrenal tissue to secrete corticosterone and aldosterone after stimulation by homologous pituitary extract was observed in May and June, whereas the minimum capacity was recorded in winter. These results are in complete agreement with our in vivo data. From the present results, we conclude that corticosterone production undergoes seasonal fluctuations in European Green frog. Since the fluctuations cannot simply originate from variations of ambient temperature (F. Leboulenger, C. Delarue, M. C. Tonon, S. Jegou, and H. Vaudry, 1978, Gen. Comp. Endocrinol.36, 327–338), our in vitro results support the view that plasma corticosterone rhythms are due, at least in part, to seasonal variations of interrenal sensitivity to ACTH.