Institutes of Brain Science and Institute of Neurobiology, State Key Laboratory of Medical Neurobiology, Fudan University, 138 Yixueyuan Road, Shanghai 200032, China.
J Physiol. 2010 Jul 15;588(Pt 14):2605-19. doi: 10.1113/jphysiol.2010.187641. Epub 2010 Jun 2.
In vertebrate retina, melatonin regulates various physiological functions. In this work we investigated the mechanisms underlying melatonin-induced potentiation of glycine currents in rat retinal ganglion cells (RGCs). Immunofluorescence double labelling showed that rat RGCs were solely immunoreactive to melatonin MT(2) receptors. Melatonin potentiated glycine currents of RGCs, which was reversed by the MT(2) receptor antagonist 4-P-PDOT. The melatonin effect was blocked by intracellular dialysis of GDP-beta-S. Either preincubation with pertussis toxin or application of the phosphatidylcholine (PC)-specific phospholipase C (PLC) inhibitor D609, but not the phosphatidylinositol (PI)-PLC inhibitor U73122, blocked the melatonin effect. The protein kinase C (PKC) activator PMA potentiated the glycine currents and in the presence of PMA melatonin failed to cause further potentiation of the currents, whereas application of the PKC inhibitor bisindolylmaleimide IV abolished the melatonin-induced potentiation. The melatonin effect persisted when Ca(2+) was chelated by BAPTA, and melatonin induced no increase in Ca(2+). Neither cAMP-PKA nor cGMP-PKG signalling pathways seemed to be involved because 8-Br-cAMP or 8-Br-cGMP failed to cause potentiation of the glycine currents and both the PKA inhibitor H-89 and the PKG inhibitor KT5823 did not block the melatonin-induced potentiation. In consequence, a distinct PC-PLC/PKC signalling pathway, following the activation of G(i/o)-coupled MT(2) receptors, is most likely responsible for the melatonin-induced potentiation of glycine currents of rat RGCs. Furthermore, in rat retinal slices melatonin potentiated light-evoked glycine receptor-mediated inhibitory postsynaptic currents in RGCs. These results suggest that melatonin, being at higher levels at night, may help animals to detect positive or negative contrast in night vision by modulating inhibitory signals largely mediated by glycinergic amacrine cells in the inner retina.
在脊椎动物视网膜中，褪黑素调节各种生理功能。在这项工作中，我们研究了褪黑素诱导大鼠视网膜神经节细胞（RGC）甘氨酸电流增强的机制。免疫荧光双重标记显示，大鼠 RGC 仅对褪黑素 MT（2）受体呈免疫反应性。褪黑素增强了 RGC 的甘氨酸电流，该作用可被 MT（2）受体拮抗剂 4-P-PDOT 逆转。褪黑素的作用被 GDP-β-S 的细胞内透析所阻断。预先孵育百日咳毒素或应用磷脂酰胆碱（PC）特异性磷脂酶 C（PLC）抑制剂 D609，但不是磷脂酰肌醇（PI）PLC 抑制剂 U73122，均可阻断褪黑素的作用。蛋白激酶 C（PKC）激活剂 PMA 增强甘氨酸电流，并且在 PMA 存在下，褪黑素未能进一步增强电流，而 PKC 抑制剂双吲哚马来酰亚胺 IV 的应用则消除了褪黑素诱导的增强作用。当 [Ca（2+）]（i）被 BAPTA 螯合时，褪黑素的作用仍然存在，并且褪黑素不会引起 [Ca（2+）]（i）的增加。cAMP-PKA 或 cGMP-PKG 信号通路似乎都不参与，因为 8-Br-cAMP 或 8-Br-cGMP 不能引起甘氨酸电流的增强，PKA 抑制剂 H-89 和 PKG 抑制剂 KT5823 也不能阻断褪黑素诱导的增强作用。因此，与 G(i/o)-偶联 MT（2）受体的激活后，一个独特的 PC-PLC/PKC 信号通路可能是大鼠 RGC 中褪黑素诱导甘氨酸电流增强的原因。此外，在大鼠视网膜切片中，褪黑素增强了光诱导的甘氨酸受体介导的 RGC 中的抑制性突触后电流。这些结果表明，褪黑素在夜间水平较高，可能通过调制主要由内视网膜中的甘氨酸能无长突细胞介导的抑制性信号，帮助动物在夜间视觉中检测正或负对比。
