Lewis Jo E, Ebling Francis J P
School of Life Sciences, University of Nottingham Medical School, Queen's Medical Centre , Nottingham , UK.
Front Neurol. 2017 Mar 10;8:79. doi: 10.3389/fneur.2017.00079. eCollection 2017.
Annual cycles of physiology and behavior are highly prevalent in organisms inhabiting temperate and polar regions. Examples in mammals include changes in appetite and body fat composition, hibernation and torpor, growth of antlers, pelage and horns, and seasonal reproduction. The timing of these seasonal cycles reflects an interaction of changing environmental signals, such as daylength, and intrinsic rhythmic processes: circannual clocks. As neuroendocrine signals underlie these rhythmic processes, the focus of most mechanistic studies has been on neuronal systems in the hypothalamus. Recent studies also implicate the pituitary stalk () and hypothalamic tanycytes as key pathways in seasonal timing. The expresses a high density of melatonin receptors, so is highly responsive to changes in the nocturnal secretion of melatonin from the pineal gland as photoperiod changes across the year. The in turn regulates tanycyte function in the adjacent hypothalamus via paracrine signals. Tanycytes are radial glial cells that persist into adulthood and function as a stem cell niche. Their cell soma are embedded in the ependymal lining of the third ventricle, and they also send elaborate projections through the arcuate nucleus, many of which terminate on capillaries in the median eminence. This anatomy underlies their function as sensors of nutrients in the circulation, and as regulators of transport of hormones and metabolites into the hypothalamus. hybridization studies reveal robust seasonal changes in gene expression in tanycytes, for example, those controlling transport and metabolism of thyroid hormone and retinoic acid. These hormonal signals play a key role in the initial development of the brain, and experimental manipulation of thyroid hormone availability in the adult hypothalamus can accelerate or block seasonal cyclicity in sheep and Siberian hamsters. We hypothesize that seasonal rhythms depends upon reuse of developmental mechanisms in the adult hypothalamus and that tanycytes are key orchestrators of these processes.
在栖息于温带和极地地区的生物中，生理和行为的年度周期非常普遍。哺乳动物中的例子包括食欲和身体脂肪组成的变化、冬眠和蛰伏、鹿角、皮毛和角的生长以及季节性繁殖。这些季节性周期的时间反映了不断变化的环境信号（如日照长度）与内在节律过程——年生物钟之间的相互作用。由于神经内分泌信号是这些节律过程的基础，大多数机制研究的重点都放在了下丘脑的神经元系统上。最近的研究还表明，垂体柄和下丘脑室管膜细胞是季节性定时的关键途径。[此处原文缺失具体结构名称]表达高密度的褪黑素受体，因此随着一年中光周期的变化，它对松果体夜间褪黑素分泌的变化高度敏感。[此处原文缺失具体结构名称]进而通过旁分泌信号调节相邻下丘脑中室管膜细胞的功能。室管膜细胞是一种放射状胶质细胞，持续到成年并作为干细胞龛发挥作用。它们的细胞体嵌入第三脑室的室管膜内衬中，并且它们还通过弓状核发出精细的投射，其中许多终止于正中隆起的毛细血管上。这种解剖结构是它们作为循环中营养物质传感器以及激素和代谢物进入下丘脑运输调节剂功能的基础。[此处原文缺失具体结构名称]杂交研究揭示了室管膜细胞中基因表达的强烈季节性变化，例如那些控制甲状腺激素和视黄酸运输和代谢的基因。这些激素信号在大脑的初始发育中起关键作用，并且在成年下丘脑实验性地操纵甲状腺激素的可用性可以加速或阻断绵羊和西伯利亚仓鼠的季节性周期。我们假设季节性节律取决于成年下丘脑中发育机制的重新利用，并且室管膜细胞是这些过程的关键协调者。
