Asha Chandola

Scoto/photo‐periodic responses of a sub‐tropical finch (Spotted munia) in relation to seasonal breeding cycle

Abstract Effects of continuous light and dark periods and long and ultrashort photoperiods on the gonadal function of a subtropical finch, Spotted munia, hitherto considered “non‐photosensitive”, were studied. This bird seems to be unique in that while gametogenesis and hormone production are stimulated by the continuous or long scoto periods the endogenous rhythm of reproduction continues in constant 24hr or 15hr long daily photoperiod.

Further studies on the regulation of the body weight of spotted Munia, Lonchura punctulota

Somatotropin (STH) and luteotropin (LTH) administration led to significant increases in the body weight and in the weight of spleen of juvenile Spotted Munia. However, except for a slight, though significant decrease in LTH-treated males, there was no effect on the weight of the liver. Maximal thyroid uptake of radioiodine occurred within 8 hours of injection and was maintained practically unchanged during the next 24 hours. Adult females showed higher 131I uptake than the males. Thyroid activity in juveniles was similar in both sexes but was higher than in the adult birds.

The effects of thyroidal hormones on the ovarian response to photoperiod in a tropical finch (Ploceus philippinus)

Abstract A constant 15-hr daily photoperiod led to a prolonged reproductive phase in the female tropical weaver finch Ploceus philippinus . Thyroxine treatment (1.4 and 2.8 μg on alternative days for 5 mo) completely suppressed follicular development, and ovarian and oviducal weight, but had no effect on the liver and body weight in birds maintained on 15-hr daily photoperiods. A “thyroid-sensitive” hypothalamo/hypophyseal/gonadal axis, as exhibited by this bird and many other birds of mid and low latitudes where the degree of insolation is large, may serve as a safety mechanism against unseasonal gonadal growth and hence might be of adaptive significance in tropical photoperiodic finches.

Restricted food intake and annual thyroid cycle of spotted munia

The spotted munia shows a seasonal cyclicity in total daily food intake as well as in thyroid function. The seasonal decline in thyroid activity is preceded by a 50% reduction in food intake. In January, artificially restricting the food consumption to late summer/fall level (by about 50%; 2.5 g/bird/day over 4 days) led to a significant decrease in conversion of monoiodotyrosine (MIT) to diiodotyrosine (DIT), relative proportions of thyroxine (%T4) and triiodothyronine (%T3) in the gland, and plasma protein-bound iodine after the injection of a tracer dose of I at almost all points studied. The plasma radioiodohormone fraction (%Tx), as measured by Sephadex gel filtration, was also significantly less than that in the adlibitum-fed controls at all the points studied. These results were confirmed later (May) by direct measurement (radioimmunoassay) of hormone levels when plasma concentration of total and free T4 and T3 were found significantly low in similarly treated birds as compared to controls. A 30% restriction in diet (simulating late spring food intake) did not alter percentage thyroidal 131I uptake, MIT/DIT ratio, glandular %T4 plasma PBI, and %Tx but decreased %T3 in the gland. A constant fixed restricted diet (2.5 g/bird!day, ambient conditions) over 8 months suppressed the seasonal variation in some thyroid activity parameters (% 131I uptake, % thyroidal T4 and T3). The declining ambient temperature failed to restore the thyroid of underfed birds to normal winter efficiency. It is concluded that the seasonal lowering of thyroid activity in spotted munia may be partly due to minimal levels of food intake reached in late summer and fall. The increase in thyroid activity in winter may partly result from an increased food intake in response to the low ambient temperature.

Iodine metabolism in spotted munia, Lonchura punctulata

Abstract Radioactive iodine metabolism was studied in both sexes of Spotted Munia, Lonchura punctulata , during different periods of the year. A characteristic “avian pattern” of thyroidal radioiodine retention was observed, which appeared to be due to a rapid turnover, as evident from a high uptake, rapid thyroxinogenesis, and release of hormone into blood. Monoiodotyrosine (MIT), diiodotyrosine (DIT), and thyroxine (T 4 ) were the three iodoamino acids identified. Triiodothyronine (T 2 ) was never detected. Thyroxinogenesis was of the typical vertebrate type. With time MIT:DIT ratio, which was always less than one, registered a decline, indicating a precursor-product relationship with the hormone. A pronounced seasonal variation was demonstrated in thyroidal 131 I uptake, thyroxine production, and serum PBI, which were all minimal during the peak phase of reproductive activity in this bird. Effect of starvation, exogenous iodide (KI), thyrotropin (TSH), and propylthiouracil (PTU) was also studied. Starvation increased 131 I uptake and favored high MIT formation, resulting in MIT:DIT ratios greater than one, indicating economic handling of iodine. KI drastically depressed 131 I uptake and favored rapid conversion of MIT to DIT. But T 4 production was not markedly altered by these two treatments. TSH stimulated 131 I uptake, organification of iodide, conversion of MIT to DIT and thyroxine production, pointing toward an active pituitarythyroid axis. PTU led to increase in DIT and a 5-fold decrease in T 4 , indicating that in wild birds, as in mammals, the DIT to T 4 -step may be the most sensitive to PTU treatment. The annual fluctuations in thyroid activity, which do not coincide with natural fluctuations in temperature and daylength, may be influenced by extra intake of iodide, by contributing to the extrathyroidal pool, and endogenous thyrotropin secretion.

Effect of photoperiod on thyroid function of spotted munia, Lonchura punctulata

Abstract Long day length ( 15 L 9D ) stimulated thyroid function in juvenile (spring) and adult (autumn) Spotted Munia but was without effect on the gonad and gonoduct weight.

Gonadal hormones and thyroid function in the spotted munia, Lonchura punctulata.

Abstract Female and male spotted munia, Lonchura punctulata , were gonadectomized in February, and their thyroid function was studied 4 (June) and 7 (September) months later. There was no effect on the thyrodial 131 I uptake, plasma protein-bound iodine (PBI), and proportions of radioiodoamino acids in the thyroid gland hydrolyzates of ovariectomized birds at any time. The maximal thyroidal 131 I uptake was increased and PBI decreased temporarily after radioisotope administration in orchidectomized birds in June, but the time curves of these two parameters were not affected in September. The maximal 131 I uptake in ovariectomized and orchidectomized birds were both lower in September as compared to June. Administration of estradiol benzoate (EB) and testosterone propionate (TP) into normal and gonadectomized adult birds failed to alter the proportions of radioiodoamino acids in the thyroid gland. Estradiol injection in intact juvenile females led to decrease in 131 I uptake and increase in PBI without any apparent effect on hormonogenesis. Administration of TP into intact juvenile males resulted in increased thyroidal 131 I uptake and decreased PBI, but the intrathyroidal conpounds were not affected.

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.

Role of gonadal feedback in annual reproduction of the weaver bird: interaction with photoperiod.

Abstract Involvement of gonadal steroids in the annual reproduction of the weaver bird has been demonstrated by castration experiments. Castration renders the pituitary acyclic, as observed by LH-dependent yellow nuptial plumage, indicating the role of gonads in seasonal decline of pituitary activity. It was also evident from the results of castration performed at different months that sensitivity of the hypothalamo-hypophyseal system to negative feed-back of testicular hormones changes with the phase of the reproductive cycle. The initiation of pituitary activity in this bird seems to be direct effect of photostimulation and not a consequence of altered sensitivity of the pituitary or the hypothalamus to gonadal steroids. Gonadal steroids appear to interfere with/alter the photoresponse of this bird. The annual reproduction in the weaver bird seems to be a result of interaction between steroidal feedback and the day length.

Premigratory increase in circulating triiodothyronine/thyroxine ratio in the red-headed bunting (Emberiza bruniceps)

Abstract The circulating triiodothyronine/thyroxine ( T 3 T 4 ) ratio in the red-headed bunting significantly increased prior to migration. Marked extrathyroidal conversion of T 4 to T 3 also occurred. The possibility that migratory disposition in this bird may be related to an increase in serum concentration of T 3 is, therefore, raised.