The present study investigates the involvement of circadian rhythm in photoperiodic expressions of GnRH-I and GnIH in the hypothalamus controlling seasonal reproduction in the Eurasian tree sparrow (Passer montanus). Groups of photosensitive birds were exposed for four weeks to resonance light dark cycles comprising of a light phase of 6 h (L) combined with dark phase of different durations (D) such that the period of LD cycles varied by 12 h increments viz. 12- (6 L/6D), 24- (6 L/18D), 36- (6 L/30D), 48- (6 L/42D), 60- (6 L/54D) and 72- (6 L/66D)h. In addition, a control group (C) was maintained under long day length (14 L/10D). Observations, recorded at the beginning and end of experiment, revealed significant testicular growth with corresponding increase in the hypothalamic expression of GnRH-I peptide but low levels of GnIH mRNA and peptide in the birds exposed to resonance cycles of 12, 36 and 60 h which were read as long days. On the other hand, birds experiencing resonance cycles of 24, 48 and 72 h read them as short days wherein they maintained their quiescent gonads and low levels of GnRH-I peptide but exhibited significant increase in GnIH mRNA and peptide expressions. Thus, sparrows responded to resonance light dark cycles differently despite the fact that each of them contained only 6 h of light. These findings suggest that an endogenous circadian rhythm is involved in photoperiodic expressions of above molecules and indicate a shift in their expressions depending upon whether the light falls in the photoinducible or non-photoinducible phase of an endogenous circadian rhythm.

本研究调查了昼夜节律参与控制欧亚树麻雀（Passer montanus）下丘脑下丘脑中GnRH-1和GnIH光周期表达的过程。）。使成组的光敏禽类暴露于包括6h的亮相（L）和不同持续时间（D）的暗相的共振浅暗周期四周，使得LD周期的周期以12h的增量变化。12-（6 L / 6D），24-（6 L / 18D），36-（6 L / 30D），48-（6 L / 42D），60-（6 L / 54D）和72-（6 L / 66D）h。另外，将对照组（C）维持在一天的白天（14L / 10D）下。在实验开始和结束时记录的观察结果显示，睾丸明显生长，GnRH-1肽的下丘脑表达相应增加，但GnIH的水平较低暴露于12、36和60 h共振周期的家禽中的mRNA和肽被视为很长的几天。另一方面，经历24、48和72 h共振周期的鸟类将它们读为短日，其中它们保持其静止的性腺和低水平的GnRH-1肽，但显示出GnIH mRNA和肽表达显着增加。因此，尽管它们中的每只仅包含6小时的光，但它们对共振光暗周期的响应却有所不同。这些发现表明内源性昼夜节律与上述分子的光周期表达有关，并且根据光是属于内源性昼夜节律的光诱导相还是非光诱导相，表明它们的表达发生了变化。