Elsevier

Neuroscience

Volume 173, 26 January 2011, Pages 19-29
Neuroscience

Cellular and Molecular Neuroscience
Research Paper
Melatonin inhibits tetraethylammonium-sensitive potassium channels of rod ON type bipolar cells via MT2 receptors in rat retina

https://doi.org/10.1016/j.neuroscience.2010.11.028Get rights and content

Abstract

By challenging specific receptors, melatonin synthesized and released by photoreceptors regulates various physiological functions in the vertebrate retina. Here, we studied modulatory effects of melatonin on K+ currents of rod-dominant ON type bipolar cells (Rod-ON-BCs) in rat retinal slices by patch-clamp techniques. Double immunofluorescence experiments conducted in isolated cell and retinal section preparations showed that the melatonin MT2 receptor was expressed in somata, dendrites and axon terminals of rat Rod-ON-BCs. Electrophysiologically, application of melatonin selectively inhibited the tetraethylammonium (TEA)-sensitive K+ current component, but did not show any effect on the 4-aminopyridine (4-AP)-sensitive component. Consistent with the immunocytochemical result, the melatonin effect was blocked by co-application of 4-phenyl-2-propionamidotetralin (4-P-PDOT), a specific MT2 receptor antagonist. Neither protein kinase A (PKA) nor protein kinase G (PKG) seemed to be involved because both the PKA inhibitor Rp-cAMP and the PKG inhibitor KT5823 did not block the melatonin-induced suppression of the K+ currents. In contrast, application of the phospholipase C (PLC) inhibitor U73122 or the protein kinase C (PKC) inhibitor bisindolylmaleimide IV (Bis IV) eliminated the melatonin effect, and when the Ca2+ chelator BAPTA-containing pipette was used, melatonin failed to inhibit the K+ currents. These results suggest that suppression of the TEA-sensitive K+ current component via activation of MT2 receptors expressed on rat Rod-ON-BCs may be mediated by a Ca2+-dependent PLC/inositol 1,4,5-trisphosphate (IP3)/PKC signaling pathway.

Research Highlights

▶MT2 melatonin receptor is expressed on rat retinal Rod-ON-BCs. ▶Melatonin selectively suppresses TEA-sensitive K+ current component in Rod-ON-BCs. ▶Ca2+-dependent PLC/IP3/PKC signaling pathway mediates the melatonin effect.

Section snippets

Experimental procedures

All experimental procedures described here were in accordance with the National Institute of Health (NIH) guidelines for the Care and Use of Laboratory Animals (NIH Publications No. 80-23) revised 1996 and the guidelines of Fudan University on the ethical use of animals. During this study all efforts were made to minimize the number of animals used and their suffering. Male Sprague–Dawley rats weighing 100∼150 g were obtained from SLAC Laboratory Animal Co. Ltd (Shanghai, China) and maintained

Expression of MT2 receptors on Rod-ON-BCs

Expression of MT2 receptors on Rod-ON-BCs was first examined by double immunofluorescence labeling with the antibodies against MT2 receptors and PKC in both acutely isolated cells and retinal sections. The specificity of the MT2 antibody was tested in our previous study by Western blot analysis and immunocytochemistry in isolated RGCs (Zhao et al., 2010). Fig. 1A1–4 show a typical isolated Rod-ON-BC, which was characterized by short dendrites emerging at one end of the soma and a long axon with

Expression of MT2 receptors on rat Rod-ON-BCs

MT2 receptors are widely expressed in both mammalian and non-mammalian retinas (Reppert et al., 1995, Wiechmann et al., 2004, Alarma-Estrany and Pintor, 2007, Savaskan et al., 2007, Huang et al., 2005, Ping et al., 2008, Zhao et al., 2010). Consistent with the observation that MT2 receptors are functionally expressed in Rod-ON-BCs of carp (Ping et al., 2008), the present work, by double immunofluorescence labeling experiments in isolated cell and retinal section preparations, demonstrated for

Acknowledgments

This work was supported by grants from the National Program of Basic Research sponsored by the Ministry of Science and Technology of China (2006CB500805; 2007CB512205; 2011CB504602), the Natural Science Foundation of China (31070966; 30900427; 30870803; 30930034), Pujiang Talent Project of the Shanghai Science and Technology Committee (08PJ1401600), and the 211 Project sponsored by the Ministry of Education of China.

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