Direct gating by retinoic acid of retinal electrical synapses.
Retinoic acid (RA), a signaling molecule derived from vitamin A, controls growth and differentiation of a variety of cell types through regulation of gene transcription. In the vertebrate retina, RA also regulates gap junction-mediated physiological coupling of retinal neurons through a nontranscriptional mechanism. Here we report that RA rapidly and specifically modulates synaptic transmission at electrical synapses of cultured retinal horizontal cells through an external RAR(beta)(/gamma)-like binding site, the action of which is independent of second messenger cascades. External application of all-trans retinoic acid (at-RA) reversibly reduced the amplitude of gap junctional conductance in a dose-dependent manner, but failed to affect non-gap-junctional channels, including glutamate receptors. In contrast, internal dialysis with at-RA was ineffective, indicating an external site of action. Selective RAR(beta)(/gamma) ligands, but not an RAR(alpha)-selective agonist, mimicked the action of at-RA, suggesting that gating of gap junctional channels is mediated through an RAR(beta)(/gamma)-like binding site. At-RA did not act on gap junctional conductance by lowering [pH](i) or by increasing [Ca(2+)](i). A G protein inhibitor and protein kinase inhibitors did not block at-RA uncoupling effects indicating no second messenger systems were involved. Direct action of at-RA on gap junction channels was further supported by its equivalent action on whole-cell hemi-gap-junctional currents and on cell-free excised patch hemichannel currents. At-RA significantly reduced single-channel open probability but did not change unitary conductance. Overall, the results indicate that RA modulates horizontal cell electrical synapses by activation of novel nonnuclear RAR(beta)(/gamma)-like sites either directly on, or intimately associated with, gap junction channels.
Zhang
D Q
DQ
Department of Physiology, University of Kentucky, 800 Rose Street, Lexington, KY 40536, USA.
McMahon
D G
DG
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001492:Bass; D002478:Cells, Cultured; D004594:Electrophysiology; D017629:Gap Junctions / Q000187:drug effects / Q000502:physiology*; D020719:Glucuronides / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D018168:Receptors, Retinoic Acid / Q000378:metabolism; D012160:Retina / Q000166:cytology / Q000502:physiology*; D000072482:Retinoic Acid Receptor alpha; D013569:Synapses / Q000502:physiology*; D014212:Tretinoin / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000494:pharmacology
D020719:Glucuronides; D018168:Receptors, Retinoic Acid; D000072482:Retinoic Acid Receptor alpha; C082925:retinoic acid receptor beta; C018058:5,6-epoxyretinoic acid; D014212:Tretinoin; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate