Melatonin, an essential pineal hormone, acts as a marker of the circadian clock that regulates biological rhythms in animals. The effects of exogenous melatonin on the circadian system of nocturnal rodents have been extensively studied; however, there is a paucity of studies on the phase-resetting characteristics of melatonin in diurnal rodents. We studied the phase shifting effects of exogenous melatonin as a single melatonin injection (1 mg/kg) at various phases of the circadian cycle on the circadian locomotor activity rhythm in the palm squirrel, Funambulus pennantii. A phase response curve (PRC) was constructed. Adult male squirrels (N = 10) were entrained to a 12:12 h light-dark cycle (LD) in a climate-controlled chronocubicle with food and water provided ad libitum. After stable entrainment, squirrels were transferred to constant dark condition (DD) for free-running. Following stable free run, animals were administered a single dose of melatonin (1 mg/kg in 2% ethanol-phosphate buffered saline (PBS) solution) or vehicle (2% ethanol-PBS solution) at circadian times (CTs) 3 h apart to evoke phase shifts. The phase shifts elicited at various CTs were plotted to generate the PRC. A dose response curve was generated using four doses (0.5, 1, 2 and 4 mg/kg) administered at the CT of maximum phase advance. Melatonin evoked maximum phase advances at CT0 (1.23 ± 0.28 h) and maximum phase delays at CT15 (0.31 ± 0.09 h). In the dose response experiment, maximal phase shifts were evoked with 1 mg/kg. In contrast, no significant shifts were observed in control groups. Our study demonstrates that the precise timing and appropriate dose of melatonin administration is essential to maximize the amelioration of circadian rhythm–related disorders in a diurnal model.

褪黑激素是一种必不可少的松果激素，是调节动物生物节律的生物钟的标志。已经广泛研究了外源褪黑素对夜间啮齿动物昼夜节律系统的影响。然而，关于日间啮齿动物中褪黑激素的相重置特性的研究很少。我们研究了昼夜周期各个阶段外源性褪黑素作为单次褪黑素注射液（1 mg / kg）的相移效应对棕鼠松鼠Funambulus pennantii的昼夜运动能力节律的影响。绘制了相位响应曲线（PRC）。成年雄性松鼠（N = 10）在气候受控的小夜蛾中被诱捕到12:12 h的明暗循环（LD），随意提供食物和水。稳定地带走后，将松鼠转移到恒定的黑暗条件下（DD）自由行驶。稳定的自由奔跑后，每隔3小时的昼夜节律（CT）向动物施用单剂量的褪黑激素（1 mg / kg的2％乙醇-磷酸盐缓冲盐水（PBS）溶液）或溶媒（2％乙醇-PBS溶液）引起相移。绘制了各种CT引起的相移以生成PRC。使用在最大相位超前的CT上给予的四个剂量（0.5、1、2和4 mg / kg）生成剂量响应曲线。褪黑素诱发的最大相位提前在CT0（1.23±0.28 h），最大相位延迟在CT15（0.31±0.09 h）。在剂量反应实验中，最大相移为1 mg / kg。相反，在对照组中没有观察到明显的变化。