Photoperiodic Control of Seasonal Breeding: A Review
Photoperiod is a powerful synchronizer of seasonal changes in endocrine and metabolic physiology in vertebrates living in tropical to polar zone. The purpose of this review is to study the factors which are involved in the process of photoperiodism in control of seasonal breeding in birds and mammals. The gonadotropin-releasing hormone (GnRH) is crucial in seasonal breeding mammals and birds, whereby under the influence of thyroid hormone, triiodothyronine (T3), it stimulates the anterior pituitary gland to release gonadotropins responsible for maturation of reproductive organs. The process that leads to stimulation of GnRH production differs between birds and mammals. In birds, light is perceived by deep brain photoreceptors and long day induced thyroid-stimulating hormone (TSH) causes local T3 activation. In mammals, light is perceived by the eyes and melatonin which is secreted during the night from the pineal gland regulates TSH. Therefore eyes, pineal gland and melatonin are not essential for regulation of seasonal reproduction in birds. Despite the differences, birds and mammals are subjected to common features i.e. light exposure and TSH during photostimulation. Long photoperiod causes photostimulation followed by photorefractoriness. The latter, has been thought to occur due to the influence of prolactin. The initial gonadal regression which is encountered during photorefractoriness occurs while there is still releasable GnRH. The timing of photorefractoriness is influenced by food availability, temperature and social factors like songs. The dissipation of photorefractoriness occurs during short photoperiods and is characterized by an increase in hypothalamic GnRH.