Experimental studies on the annual cycles of thyroid and adrenocortical functions in relation to the reproductive cycle of drakes.

The annual variations in the basal plasma contents of testosterone, thyroxine and corticosterone have been measured in Peking drakes living outdoors, in Southern France. 10 The plasma testosterone titer underwent a more than 20-fold increase during the vernal reproductive period (March-April). In early June the circulating testosterone fell to near autumnal values, and the testosterone MCR was augmented. These were the first manifestations of the cessation of the vernal reproductive period. 20 The plasma thyroxine levels were minimal in autumn, moderately augmented (40%) in winter (January-March), but exhibited a 3-fold increase in early June. The resulting steep (13-fold) increase of the plasma thyroxine/testosterone ratio preceded the onset of the post-nuptial moult. 30 Modifications of testosterone secretion and clearance rate similar to those occurring in June were initiated in spring by i.m. injections of thyroxine at a dosage (1 mg/d) that induced June-July thyroxine plasma levels. On the other hand, an experimentally induced steep decrease of testosterone (castration) induced enhanced plasma thyroxine concentrations similar to the June values, while an induced (10 mg/d testosterone i.m.) hypertestosteronemia corresponding to the reproductive period depressed the plasma thyroxine levels. Strong reciprocal negative interactions between testis and thyroid might therefore afford a partial explanation of the peculiar thyroxine/testosterone imbalance that occurred in June immediately prior to the moult. 40 Cold-exposed "short-day" (December) ducks exhibited a marked increase in plasma thyroxine levels, while exposure of December ducks to "long days" (18L-6D) at 25 degrees C depressed the thyroxine titers. The inhibitory effect of "long days" on the blood level of thyroxine was further evident in castrated ducks. Exposure of "short day" ducks (December) to a combined treatment by "long days" (18L-6D) and cold (4 degrees C) produced an endocrine picture similar to the January-March pattern, i.e. highly increased testosterone plasma levels, but unaffected testosterone MCR, together with a moderate increase in plasma thyroxine concentration. 50 Corticosterone plasma concentrations increased during the reproductive season, as a result of a seasonally augmented binding-capacity of the CBG. Exogenous testosterone (10 mg/d) which induced spring-like circulating levels of male hormones, caused a similar increase in CBG-bound and total corticosterone levels. 60 In June the MCR of both corticosterone and aldosterone were elevated (as was the testosterone MCR). Similarly enhanced MCR of both corticosteroids were brought on in spring by an exogenous thyroxine treatment, leading to a June-like state of hyperthyroxinemia.