Seasonal variations in extrathyroidal conversion of thyroxine to tri-iodothyronine and migratory disposition in redheaded bunting

Abstract

Thyroid gland has been shown to be involved in the migratory disposition in redheaded bunting, Emberiza bruniceps, ( Chandola & Pathak, 1980 a,b; Pathak & Chandola, 1982). Extrathyroidal conversion of thyroxine (T4) to triiodothyronine (T3) was now studied in thyroidectomised buntings by measuring the appearance of plasma T3 (RIA) 4h after an injection of 2.5ug I-T4 during non-migratory (January), spring pre-migratory (March) and post-migratory (June) periods of the year. A significantly high peripheral conversion of T4-T3 was recorded during pre-migratory period compared to non-migratory and postmigratory periods. This suggests that the increase in circulating T3 T4 ratio observed in buntings prior to spring migration could be to a certain extent, the result of an enhanced peripheral monodeiodination of T4. The inhibition of extrathyroidal conversion of exogenously administered I-T4 into T3 in thyroidectomised buntings (by propylthiouracil) prevented the T4-induced fattening significantly during non-migratory period (January). Same dose of PTU failed to inhibit T4-induced fattening as well as peripheral monodeiodination of T4 in thyroidectomised birds in spring premigratory period. This indicates that the T4-effects on fattening may be brought about by its conversion to T3, and also that the peripheral deiodinating enzyme systems in this bird may be comparatively more active during spring premigratory period than that during nonmigratory period. The results argue for a role of T3 in premigratory fattening, a characteristic of migratory disposition in birds. Also the seasonal variations in peripheral conversion of T4-T3 may be of great adaptive significance in seasonal breeders where energy requiring events like migratory fattening (and reproduction and molting-all influenced by thyroid status) must be temporally spaced across the year.