Bentley G E
Department of Psychology, Behavioral Neuroendocrinology Group, The Johns Hopkins University, Baltimore, Maryland, USA.
Microsc Res Tech. 2001 Apr 1;53(1):63-71. doi: 10.1002/jemt.1069.
Birds, unlike mammals, do not use the annual profile of pineal melatonin secretion to coordinate their reproductive efforts with a favorable time of year. Melatonin in birds mediates the entrainment of circadian activity rhythms, and thus helps to time hatching of eggs and facilitate migration. However, the role of melatonin as a reliable indicator of day length for seasonal processes has remained equivocal for many years. Recently, the influence of melatonin on two physiological processes involved in aspects of seasonal reproduction has been identified in European starlings: 1) the regulation of seasonal changes in immune function, and 2) neuroplasticity in the song control system. Melatonin can enhance cell-mediated immune function and acts as an inhibitory hormone on the song control system. Melatonin receptor (MelR) density in a forebrain song control nucleus, Area X, is regulated as a function of reproductive state; there is marked downregulation of MelR in Area X during the breeding season in starlings. Seasonal regulation of immune function and neural plasticity within the song control system, and the efficacy of the action of melatonin on these two processes, appears to be modified by the same central, thyroid-dependent mechanism that controls the reproductive state of birds. These data indicate that the interaction of day length and hormones of different classes affects the ability of melatonin to affect seasonal processes in birds. The downstream consequences of MelR regulation within the song control system are discussed with regard to the cellular action of melatonin and its possible interaction with immediate-early genes and transcription factors.
与哺乳动物不同，鸟类并不利用松果体褪黑素分泌的年度变化来使它们的繁殖活动与一年中的适宜时间相协调。鸟类体内的褪黑素介导昼夜活动节律的同步，从而有助于确定孵卵时间并促进迁徙。然而，多年来褪黑素作为季节变化可靠的日长指标的作用一直不明确。最近，在欧洲椋鸟中发现了褪黑素对季节性繁殖相关的两个生理过程的影响：1）免疫功能季节性变化的调节，以及2）鸣唱控制系统中的神经可塑性。褪黑素可以增强细胞介导的免疫功能，并对鸣唱控制系统起抑制激素的作用。前脑鸣唱控制核X区中的褪黑素受体（MelR）密度作为繁殖状态的函数受到调节；在椋鸟的繁殖季节，X区的MelR会显著下调。鸣唱控制系统内免疫功能和神经可塑性的季节性调节，以及褪黑素对这两个过程作用的效力，似乎受到控制鸟类繁殖状态的相同的、依赖甲状腺的中枢机制的调节。这些数据表明，日长与不同类别激素之间的相互作用会影响褪黑素影响鸟类季节性过程的能力。文中讨论了鸣唱控制系统内MelR调节的下游后果，涉及褪黑素的细胞作用及其与即早基因和转录因子可能的相互作用。
