Department of Biology, University of Memphis, 313 Life Sciences Building, Memphis, TN 38152, USA.
J Biol Rhythms. 2013 Jun;28(3):193-200. doi: 10.1177/0748730413487974.
Siberian hamsters (Phodopus sungorus) exhibit robust seasonal rhythms of reproduction driven by changes in day length. Day length is encoded endogenously by the duration of nocturnal melatonin (Mel) secretion from the pineal gland. Short duration Mel signals stimulate whereas long duration Mel signals inhibit reproduction. The mechanism by which Mel regulates the reproductive axis has not been fully characterized. In Siberian hamsters, the thyroid hormone triiodothyronine (T₃) is thought to be part of the photoperiodic mechanism. The availability of T₃ is decreased in hamsters housed in short day lengths, and injections of exogenous T₃ stimulate testicular growth in short-day (SD) Siberian hamsters. Thus, T₃ acts as a neuroendocrine intermediate between the Mel rhythm and the reproductive axis. The RFamides kisspeptin (Kiss1) and gonadotropin-inhibitory hormone (GnIH) also act as a link between the Mel rhythm and the reproductive axis. Expression of both of these neuropeptides is regulated by photoperiod and Mel. Kiss1 stimulates, and GnIH inhibits, the reproductive axis in long-day housed hamsters. It remains unknown whether T₃ acts through changes in RFamide expression in the regulation of reproduction or whether these molecules act independently of one another. We tested the hypothesis that exogenous T₃ administered to SD hamsters, a treatment that stimulates testicular growth, would also result in alterations in the patterns of Kiss1- and GnIH-immunoreactivity. Administration of T₃ to SD hamsters resulted in significant testicular growth as well as a long day-like pattern of RFamide peptide expression. Thus, exogenous T₃ elicited increased numbers of Kiss1-positive cells in the hypothalamic anteroventral periventricular nucleus, decreased numbers of Kiss1-positive cells in the arcuate nucleus, and a greater number of GnIH-positive cells in the dorsomedial hypothalamus compared with SD controls. The results are consistent with the hypothesis that T₃ elicits alterations in the reproductive axis through alterations in RFamide peptide expression.
黑线仓鼠（Phodopus sungorus）表现出强烈的季节性繁殖节律，这种节律由日长变化驱动。日长由松果体夜间褪黑素（Mel）分泌的持续时间内在编码。短持续时间的 Mel 信号刺激繁殖，而长持续时间的 Mel 信号抑制繁殖。Mel 调节生殖轴的机制尚未完全阐明。在黑线仓鼠中，甲状腺激素三碘甲状腺原氨酸（T₃）被认为是光周期机制的一部分。在短日长度下饲养的仓鼠中，T₃ 的可用性降低，外源性 T₃ 注射可刺激短日（SD）黑线仓鼠的睾丸生长。因此，T₃ 作为 Mel 节律和生殖轴之间的神经内分泌中间物发挥作用。RFamides 促性腺激素抑制激素（GnIH）和 kisspeptin（Kiss1）也作为 Mel 节律和生殖轴之间的联系。这两种神经肽的表达都受光周期和 Mel 的调节。在长日饲养的仓鼠中，Kiss1 刺激，而 GnIH 抑制生殖轴。目前尚不清楚 T₃ 是否通过改变 RFamide 表达来调节生殖，或者这些分子是否相互独立发挥作用。我们检验了这样一个假设，即给予 SD 仓鼠外源性 T₃，这种治疗方法可刺激睾丸生长，也会导致 Kiss1 和 GnIH 免疫反应性模式的改变。给予 SD 仓鼠 T₃ 治疗导致睾丸显著生长，以及类似长日的 RFamide 肽表达模式。因此，与 SD 对照组相比，外源性 T₃ 处理增加了下丘脑腹前脑室核 Kiss1 阳性细胞的数量，减少了弓状核 Kiss1 阳性细胞的数量，并且增加了背内侧下丘脑 GnIH 阳性细胞的数量。这些结果与 T₃ 通过改变 RFamide 肽表达来引发生殖轴改变的假设一致。
