Binding pattern of 125Iodine thyroxine and tri-iodothyronine in skin and liver tissues of spotted munia, Lonchura punctulata: Co-relation to seasonal cycles of breeding and molting.

Prevalent notion about thyroid hormones is that thyroxine (T4) is a mere precursor and physiological effects of thyroid hormones are elicited by tri-iodothyronine (T3) after mono-deiodination of T4. Earlier studies on feather regeneration and molt done on spotted munia L. punctulata suggest that T4 (mono-deiodination suppressed by iopanoic acid and thyroidectomized birds) is more effective than T3 in inducing feather regeneration. The binding pattern of 125I labeled T4 and T3 has been investigated in the nuclei prepared from skin and liver tissues (samples obtained during different months) of spotted munia using scatchard plot analysis. The results show that binding capacity (Bmax – pmole/80 µgm DNA) of 125I-T3 to nuclei of skin was significantly higher in November as compared to April and June, whereas the binding affinity (Kd-10-9M-1) was significantly lower in November as compared to April and June. During November, Bmax for binding of T3 and T4 did not vary in liver and skin nuclei but Kd varied significantly. Binding capacity of 125I- T3 to skin and liver did not vary but binding affinity of 125I- T4 to skin was approximately 7 times higher than that of liver. The results suggest that T4 does show a variation in binding pattern that co-relates to the molting pattern of spotted munia. These variations might play important role in different physiological phenomenon in this tropical bird. The experiments do point towards the possibility of independent role of T4 as a hormone, however, further experiments need to be done to ascertain the role of T4 in this model and work out the exact molecular mechanism of action.

Environmental and hormonal control of vernal migration in redheaded bunting (Emberiza bruniceps).

This paper reviews the factors and mechanisms which result in the development of the metabolic state characteristic of migration with special reference to a palaeotropic migrant the redheaded bunting, Emberiza bruniceps. Changes in climatic conditions and food supply act as proximate triggers of migratory behaviour in partial migrants. Typical migrants like buntings use daylength as a cue but the exact mechanism of how photoperiodic information is translated in terms of migratory events is still not known. Almost entirely the photoperiodic effects have been explained on the basis of the involvement of hypothalamo/hypophyseal system. We feel mechanism(s) other than those acting through neuroendocrine system may be equally important. Furthermore the role of temperature has not been adequately explored so far. Our observations indicate the possibility that redheaded buntings might integrate the information received from photoperiod with environmental temperature (and other factors?) resulting in the development of migratory state. The physiological control of avian migration is much less understood. Majority of papers have centered around the ‘gonadal hypothesis’ of Rowan supporting or contradicting it without providing conclusive evidence. Pituitary prolactin has also been shown to be implicated although the mechanism of action is only speculative. Conclusive evidence for the involvement of thyroid hormones (thyroxine, T4; triiodothyronine, T3) in the physiological timing of migration has been produced attributing independent roles to T4 and T3. It is suggested that seasonal variation in peripheral conversion of T4 to T3 could serve as an effective strategy to render available the required thyroid hormones T4 and/or T3 during different phases of the year thus accounting for the metabolic switch over from T4-dependent moult to T3-dependent migratory fat deposition and zugunruhe and also ensuring preclusion of simultaneous occurrence of these mutually incompatible events. Considering that the number of environmental and physiological factors influence this mechanism and considering that thyroid hormone molecule has been put to a wide range of usage during the course of evolution the mechanism(s) of peripheral conversion of T4-T3 may assume great flexibility and have selective value-especially in migration which is known to have evolved several times in diverse avian families. The attractiveness of this hypothesis lies in the fact that it has potential to explain the both physiological development of the metabolic state of migration and at the same time the physiological timing of migration not only with respect to the cycle of environment but also with respect to other conflicting seasonal events (moult and reproduction).

Short-day experience is not a prerequisite for the termination of photorefractoriness in the reproductive cycle of baya weaver, Ploceus philippinus.

In most photoperiodic avian forms (irrespective of temperate or tropical distribution) including the baya weaver, Ploceus philippinus, seasonal reproduction comes to an end due to the development of a photoperiodically controlled photorefractory phase when birds cease to respond to the stimulatory effect of long days. In the present paper photoperiodic control of the termination of photorefractory phase has been examined by studying the effect of short-day exposure lasting 4–6 months on long-day response of birds. Results indicate that unlike in other photoperiodic birds short-day exposure of winter is not a prerequisite for the termination of photorefractory phase in the reproductive cycle of baya weaver. Artificial long days on the other hand hasten the termination of this phase. Refractory phase in baya weaver, therefore, unlike that in temperate forms, is a temporary state resulting most likely from a sequel of physiological events triggered by long days of spring/summer which temporarily mask the photostimulatory response. Spontaneous termination of photorefractoriness in birds of tropical habitats may have a selective value imparting to the reproductive cycle the necessary elasticity for adaptation to diverse ecological conditions.