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Eurasian tree sparrow1/3/2024 Another study reported a significant rise in plasma LH in Japanese quail, Coturnix coturnix japonica exposed to single long day at 18 or 20 h after lights-on. A recent study, using this paradigm on white-throated sparrows, Zonotrichia albicollis, revealed an activation of GnRH-I neurons after exposure to just one long day thereby indicating the rapidness of photoperiodic induction. The “Single long-day induction paradigm” is quite useful in uncoding the photoperiodic molecular circuitry at the level of brain in birds. Thus, GnRH-I and GnIH are two significant components of the neuronal circuitry in the avian brain that play important role in the regulation of their reproductive responses. GnIH acting on the anterior pituitary and GnRH-I neurons inhibits the synthesis and release of gonadotropins in a direct and indirect manner, respectively, leading to gonadal regression and inhibition of reproductive behaviours. It acts as a neuroendocrine integrator of photoperiodic cue and regulates gonadotropins secretion to time avian seasonal reproduction. Another neurohormone, known as gonadotropin-inhibitory hormone (GnIH), has been reported to exert gonado-inhibitory effects. GnRH-I further stimulates gonadotropins (luteinizing hormone, LH and follicle-stimulating hormone, FSH) synthesis and release from the anterior pituitary causing seasonal variation in reproductive physiology, gonadal development and behaviour in birds. T 3 changes the structural arrangement in the nerve terminals of gonadotropin-releasing hormone-I (GnRH-I) expressing neurons at the median eminence causing retraction of glial end feet encasement around GnRH nerve terminals, thus, allowing access of nerve terminals to the basal lamina as well as portal blood supply to the pituitary. Long photoperiod stimulates synthesis of thyroid stimulating hormone beta (TSH-β) in the pars tuberalis leading to increase in production of type-2 iodothyronine deiodinase (DIO2) that converts tetraiodothyronine (T 4) to triiodothyronine (T 3). The mediobasal hypothalamus (MBH) has been suggested as the site of photoperiodic induction in birds. Some investigations directed to uncover the neuroendocrine mechanisms involved in photoperiodic regulation of reproductive responses have yielded significant results. The day length triggers neuroendocrine system of birds and initiates reproduction by subsequently activating the hypothalamus–pituitary–gonad (HPG) axis. The role of day length as a primary environmental factor regulating seasonal reproduction is well established in a variety of avian species belonging to mid as well as high latitudes. Majority of birds use day length as the most reliable environmental information to predict favourable season and to begin physiological preparations in advance of the conducive conditions in the environment for successful reproduction. To avoid physiological conflict, the above stages are temporally spaced and closely coupled. The day length measurement is extremely crucial for photoperiodic birds for timing of life history stages such as reproduction, molt and migration. Furthermore, they suggest that the GnRH-I and GnIH are expressed in the hypothalamus of tree sparrow in an anti-phasic manner and switching over of their expression occurs at late hours of exposure of birds to single long day. Our results clearly indicate that the photoperiodic response system of sparrow is highly sensitive to light and responds even to single long day. However, no significant change in GnRH-I or GnIH expression was observed at any time point under short day and birds maintained high and low expression levels of GnIH and GnRH-I, respectively. Observations revealed that birds exposed to a single long day (14L: 10D) showed an increase in hypothalamic expressions of GnRH-I mRNA and peptide and decrease in levels of GnIH mRNA only at ZT 16 and ZT 18 with no significant change in GnIH peptide. In addition, testicular size was measured to know testicular development. Birds of both the groups were sacrificed and perfused on fifth day at different time points, i.e., ZT 14, ZT 16 and ZT 18 and the expressions of GnRH-I and GnIH mRNAs and peptides were studied using real-time PCR and immunohistochemistry, respectively. On the fifth day, one group was exposed to long day (14L: 10D), while other was continued under short day for another 1 day. Sparrows were divided into two groups ( n = 24 each) and pre-treated under short days (9L: 15D) for 4 days. The present study aims to investigate the role of a single long day in triggering hypothalamic expressions of GnRH-I and GnIH in the Eurasian tree sparrow ( Passer montanus). Seasonal reproductive cycles of most birds are regulated by photoperiod via neuroendocrine control.
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