The Identification And Seasonal Expression Of GnRH And GnIH In The Brain Of Eurasian Tree Sparrows(Passer Montanus)

Gonadotropin releasing hormone (GnRH) and gonadotropin-inhibiting hormone (GnIH)are key hormones to regulate the seasonal reproduction of vertebrate. In birds, GnRH andGnIH interact together in stimulating them to express periodic reproductive behavior byregulating the synthesis and release of gonadotropins. In this study, we determined GnRH andGnIH distribution among the whole brain of Eurasian tree sparrows (Passer montanus) fromlow altitude area, temperate zone (Shijiazhuang, Hebei Province) usingimmunochistochemical method, and compared the seasonal expression of GnRH and GnIHamong different life history periods (the wintering period, the prebreeding period, the firstegg-laying period, the first nestling period, the second egg-laying period, the second nestlingperiod, the pre-basic molting period), as well as the seasonal changes of gonadal volume ofmale sparrows. Moreover, we also compared the differences of GnRH and GnIH expressionbetween sexes at the same life history period. We concluded that:(1) Male tree sparrows showed remarkable differences in gonadal volume among thedifferent life history stages. The gonads volume in the prebreeding period is significantlyhigher than those from the wintering and the pre-basic molting periods. The gonads of malesparrows increased rapidly from the early breeding to the first egg-laying periods, and shrankquickly from the second brood to the pre-basic molting periods. Seasonal changes of thegonadal volume in tree sparrows showed that they expressed a typical seasonalbreeding-nonbreeding cycle regulated by the neuroendocrinolgical pathways.(2) GnRH and GnIH immune positive-fibers were widely distributed in thediencephalon and mesencephalon. GnRH immune positive cell bodies were mainlydistributed in the hypothalamus preoptic area, and GnIH immune positive cell bodieswere mainly distributed in the hypothalamic paraventricular nucleus. The terminals ofGnRH and GnIH nerve fibers extended to the median eminence. GnIH-related peptides,GnIH-RPs, were mainly distributed in the hippocampus, old striatum, neostriatum,cerebellum, optic lobe area, and vocal nuclei. Because of the widely distribution of GnRH, GnIH and GnIH-RPs in the brain, they may function many important roles inregulating seasonal breeding behavior and reproductive function, as well as vocal andforaging behaviors in birds.(3) The expression of GnRH increased gradually from wintering period to the firstegg-laying period, and the first egg-laying period is the maximum. The expression of GnRHdecreased gradually from the second egg-laying period to the second nestling period. Theexpression of GnIH reduce gradually from the wintering period to the first egg-layingperiod, and the first egg-laying period is the minimum. Tree sparrows became photorefractivefrom the second nestling period to the pre-basic periods, and closed HPG axis at this period,and we found the expression of GnIH cell bodies rapidly decreased. The seasonal patterns ofGnRH and GnIH indicate they may interact directly or indirectly involved in switching on-offHPG axis during different life history periods, which may play important functions inregulating the seasonal breeding in birds.