Zawilska J B, Derbiszewska T, Nowak J Z
Institute of Biogenic Amines, Polish Academy of Sciences, Lódź, Poland.
J Pineal Res. 1997 Mar;22(2):59-64. doi: 10.1111/j.1600-079x.1997.tb00304.x.
Dopamine plays an important role in regulation of melatonin biosynthesis in retinas of several vertebrate species. In the avian retina, the dopamine receptor that controls melatonin production represents a D4-like subtype. Stimulation of this receptor by quinpirole (QNP) results in a dose-dependent decline of the nighttime activity of serotonin N-acetyltransferase (NAT, a key regulatory enzyme in melatonin biosynthesis) and melatonin level of the retina. The present study was undertaken to determine whether the ability of QNP to suppress nocturnal NAT activity of chick retina was affected by prolonged adaptation of animals to light and darkness. In the retina of chicks kept under a light:dark (LD) illumination cycle, dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) levels measured at the end of the light phase were significantly higher than those found in the middle of the dark phase. In animals maintained under continuous light (LL) or darkness (DD) dopamine and DOPAC contents of the retina measured at these two time points were similar and resembled levels found during, respectively, the light and dark phase in the retina of chicks kept under LD illumination cycle. Adaptation of chicks to LL and DD resulted in an attenuated and enhanced, respectively, response of the retinal NAT activity to the suppressive action of QNP. When compared to the LD group, a parallel shift to the right (LL group) or left (DD group) of the dose-response curve for QNP was observed, and the ED50 values for this dopamine receptor agonist were 3.4-times higher (LL) or 2.8-lower (DD) than those calculated for the control LD animals. It is suggested that prolonged exposure to light or darkness, by altering the level of the retinal dopaminergic neurotransmission, may modify the reactivity of the D4-like dopamine receptors regulating NAT activity of the chick retina.
多巴胺在几种脊椎动物视网膜中褪黑素生物合成的调节中起着重要作用。在鸟类视网膜中，控制褪黑素产生的多巴胺受体属于D4样亚型。喹吡罗（QNP）刺激该受体导致视网膜中血清素N-乙酰转移酶（NAT，褪黑素生物合成中的关键调节酶）夜间活性和褪黑素水平呈剂量依赖性下降。本研究旨在确定动物长期适应光照和黑暗是否会影响QNP抑制雏鸡视网膜夜间NAT活性的能力。在明暗（LD）光照周期下饲养的雏鸡视网膜中，在光照阶段结束时测得的多巴胺和3,4-二羟基苯乙酸（DOPAC）水平显著高于黑暗阶段中期测得的水平。在持续光照（LL）或黑暗（DD）条件下饲养的动物中，在这两个时间点测得的视网膜多巴胺和DOPAC含量相似，分别类似于在LD光照周期下饲养的雏鸡视网膜光照和黑暗阶段测得的水平。雏鸡对LL和DD的适应分别导致视网膜NAT活性对QNP抑制作用的反应减弱和增强。与LD组相比，观察到QNP剂量反应曲线向右（LL组）或向左（DD组）平行移动，该多巴胺受体激动剂的ED50值比对照LD动物计算值高3.4倍（LL）或低2.8倍（DD）。提示长期暴露于光照或黑暗中，通过改变视网膜多巴胺能神经传递水平，可能改变调节雏鸡视网膜NAT活性的D4样多巴胺受体的反应性。
