Molecular and Neuroendocrine Mechanisms of Avian Seasonal Reproduction

Animals living outside tropical zones experience seasonal changes in the environment and accordingly, adapt their physiology and behavior in reproduction, molting, and migration. Subtropical birds are excellent models for the study of seasonal reproduction because of their rapid and dramatic response to changes in photoperiod. For example, testicular weight typically changes by more than a 100-fold. In birds, the eyes are not necessary for seasonal reproduction, and light is instead perceived by deep brain photoreceptors. Functional genomic analysis has revealed that long day (LD)-induced thyrotropin from the pars tuberalis of the pituitary gland causes local thyroid hormone (TH) activation within the mediobasal hypothalamus. This local bioactive TH, triiodothyronine (T₃), appears to regulate seasonal gonadotropin-releasing hormone (GnRH) secretion through morphological changes in neuro-glial interactions. GnRH, in turn, stimulates gonadotropin secretion and hence, gonadal development under LD conditions. In marked contrast, low temperatures accelerate short day (SD)-induced testicular regression in winter. Interestingly, low temperatures increase circulating levels of T₃ to support adaptive thermogenesis, but this induction of T₃ also triggers the apoptosis of germ cells by activating genes involved in metamorphosis. This apparent contradiction in the role of TH has recently been clarified. Central activation of TH during spring results in testicular growth, while peripheral activation of TH during winter regulates adaptive thermogenesis and testicular regression.