Reciprocal interaction between seasonal testis and thyroid activity in Zembra Island wild @entity95 ( XXXX ): effects of castration, thyroidectomy, temperature, and photoperiod.

@entity2493 @entity251 @entity95 @entity1955
The reproductive cycle of wild @entity95 ( @entity95 ) living in Zembra Island (North Tunisia) is dependent on an external factor, the photoperiod: the gonads are inhibited by long days and stimulated by short days or @entity2493 implants. Here we studied the role of an internal factor, thyroid hormones and the possible thyroid-gonadal interrelationships, in animals captured on Zembra Island and maintained in natural conditions of photoperiod and temperature. We determined the seasonal profile of the thyroid and testis cycles and investigated the effects of castration and thyroidectomy on the seasonal @entity251 and @entity1955 cycles. Plasma @entity1955 and @entity251 levels followed similar, parallel seasonal patterns, with a peak in autumn (October) and low values from January to August. In thyroidectomized animals, plasma @entity251 levels, although significantly higher than those in controls (P < 0.001), remained low throughout the 13 mo of the experiment, and no testicular reactivation was observed in the fall. In castrated animals, despite the increase in @entity1955 concentration in the 3 mo following castration (P < 0.01), plasma @entity1955 levels remained low during the 2 yr of the study. We then investigated the combined effects of long days (16L:8D) and moderately high temperature (25 degrees C) on these two endocrine axes. In constant gonado-inhibiting conditions (16L:8D), whether the temperature was kept constantly high or allowed to fluctuate naturally, no reactivation of the thyroid and testicular axes was observed in the fall. In control animals, the peaks of @entity251 and @entity1955 concentrations observed in September were larger (P < 0.001) than those in animals subjected to the same natural photoperiod conditions but with constantly high temperature. The lower level of autumnal testis stimulation (P < 0.001) in animals maintained in conditions of constant high temperature (25 degrees C) may be attributed to the low @entity1955 levels induced by high temperature. These results clearly confirm that the thyroid and testicular cycles display similar seasonal variations and show that the thyroid and gonadal axes are strictly interdependent. This study provides the first demonstration, for a given species, that the seasonal reactivation of gonad activity is controlled by the thyroid, and thyroid activity is controlled by the gonads.
