Kriegsfeld L J, Demas G E, Lee S E, Dawson T M, Dawson V L, Nelson R J
Department of Psychology, Behavioral Neuroendocrinology Group, Johns Hopkins University, Baltimore, MD 21218-2686, USA.
J Biol Rhythms. 1999 Feb;14(1):20-7. doi: 10.1177/074873099129000407.
Nitric oxide (NO) is an endogenous gas that functions as a neurotransmitter. Because NO is very labile with a half-life of less than 5 sec, most functional studies of NO have manipulated its synthetic enzyme, NO synthase (NOS). Three isoforms of NOS have been identified: (1) in the endothelial lining of blood vessels (eNOS), (2) an inducible form found in macrophages (iNOS), and (3) in neurons (nNOS). Most pharmacological studies to date have blocked all three isoforms of NOS. Previous studies using such agents have revealed that NO might be necessary for photic entrainment of circadian rhythms; general NOS inhibitors attenuate phase shifts of free-running behavior, light-induced c-fos expression in the suprachiasmatic nucleus (SCN), and phase shifts of neural firing activity in SCN maintained in vitro. To assess the specific role of nNOS in mediating entrainment of circadian rhythms, mice with targeted deletion of the gene encoding the neuronal isoform of NOS (nNOS-/-) were used. Wild-type (WT) and nNOS-/- mice initially were entrained to a 14:10 light:dark (LD) cycle. After 3 weeks, the LD cycle was either phase advanced or phase delayed. After an additional 3 weeks, animals were held in either constant dim light or constant dark. WT and nNOS-/- animals did not differ in their ability to entrain to the LD cycle, phase shift locomotor activity, or free run in constant conditions. Animals held in constant dark were killed after light exposure during either the subjective day or subjective night to assess c-fos induction in the SCN. Light exposure during the subjective night increased c-fos expression in the SCN of both WT and nNOS-/- mice relative to animals killed after light exposure during the subjective day. Taken together, these findings suggest that NO from neurons might not be necessary for photic entrainment.
一氧化氮（NO）是一种内源性气体，起神经递质的作用。由于NO非常不稳定，半衰期不到5秒，因此大多数关于NO的功能研究都通过操纵其合成酶——一氧化氮合酶（NOS）来进行。已鉴定出NOS的三种同工型：（1）血管内皮中的（eNOS），（2）巨噬细胞中发现的诱导型（iNOS），以及（3）神经元中的（nNOS）。迄今为止，大多数药理学研究都阻断了NOS的所有三种同工型。此前使用此类药物的研究表明，NO可能是昼夜节律光诱导同步所必需的；一般的NOS抑制剂会减弱自由活动行为的相位偏移、视交叉上核（SCN）中光诱导的c-fos表达，以及体外维持的SCN中神经放电活动的相位偏移。为了评估nNOS在介导昼夜节律同步中的具体作用，使用了靶向缺失编码神经元型NOS（nNOS-/-）基因的小鼠。野生型（WT）和nNOS-/-小鼠最初被同步到14:10的明:暗（LD）循环。3周后，LD循环要么提前相位，要么延迟相位。再过3周后，将动物置于持续弱光或持续黑暗中。WT和nNOS-/-动物在同步到LD循环、相位偏移运动活动或在恒定条件下自由活动的能力方面没有差异。在主观白天或主观夜间光照后，将处于持续黑暗中的动物处死，以评估SCN中c-fos的诱导情况。与在主观白天光照后处死的动物相比，主观夜间光照增加了WT和nNOS-/-小鼠SCN中c-fos的表达。综上所述，这些发现表明神经元产生的NO可能不是光诱导同步所必需的。
