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The Journal of Neuroscience, July 15, 1998, 18(14):5253-5263

Membrane Current Induced by Protein Kinase C Activators in Rhabdomeric Photoreceptors: Implications for Visual Excitation

Maria del Pilar Gomez and Enrico Nasi

Department of Physiology, Boston University School of Medicine, Boston, Massachusetts 02118, and Marine Biological Laboratory, Woods Hole, Massachusetts 02543

Visual excitation in rhabdomeric photoreceptors is thought to be mediated by activation of a light-regulated phospholipase C (PLC) and the consequent hydrolysis of phosphatidylinositol bisphosphate. Whereas much attention has been devoted to inositol trisphosphate (IP₃) production and intracellular Ca²⁺ release, little is known about the possible role of the DAG branch in the generation of the light response. We have tested the effect of chemically distinct surrogates of DAG on isolated Lima photoreceptors. Application of the phorbol ester PMA (0.5-10 µM) or the alkaloid ()-indolactam (20-100 µM) from a holding potential of 50 mV elicited an inward current, several hundred picoamperes in amplitude, accompanied by a pronounced increase in membrane conductance. The stereoisomers 4-PMA and (+)-indolactam were both inactive, arguing for the specificity of the effects. Elevation of cytosolic Ca²⁺ by intracellular dialysis accelerated this current, whereas chelerythrine antagonized it, suggesting the involvement of PKC. The reversal potential of the membrane current induced by PKC activators was approximately +10 mV; replacement of extracellular Na with impermeant N-methyl-D-glucamine decreased its amplitude and shifted the reversal potential in the negative direction. Stimulation by PMA and ()-indolactam was accompanied by a pronounced depression of light responsiveness; conversely, steady illumination reduced the size of the current elicited by these PKC activators. Taken together, these results support the notion that the DAG branch of the PLC cascade, in addition to its suggested participation in visual adaptation, may play a role in the activation of the photoresponse or a component thereof, probably in synergy with IP₃-mediated Ca²⁺ release.

Key words: visual excitation; PKC; diacylglycerol; rhabdomeric photoreceptors; light-dependent conductance; calcium

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Copyright © 1998 Society for Neuroscience  0270-6474/98/18145253-11$05.00/0

This article has been cited by other articles:

				M. del Pilar Gomez and E. Nasi Prolonged calcium influx after termination of light-induced calcium release in invertebrate photoreceptors J. Gen. Physiol., September 1, 2009; 134(3): 177 - 189. [Abstract] [Full Text] [PDF]

				M. del Pilar Gomez and E. Nasi A Direct Signaling Role for Phosphatidylinositol 4,5-Bisphosphate (PIP2) in the Visual Excitation Process of Microvillar Receptors J. Biol. Chem., April 29, 2005; 280(17): 16784 - 16789. [Abstract] [Full Text] [PDF]

				G. Piccoli, M. del Pilar Gomez, and E. Nasi Role of protein kinase C in light adaptation of molluscan microvillar photoreceptors J. Physiol., September 1, 2002; 543(2): 481 - 494. [Abstract] [Full Text] [PDF]

				R. Payne and J. Demas Timing of Ca2+ Release from Intracellular Stores and the Electrical Response of Limulus Ventral Photoreceptors to Dim Flashes J. Gen. Physiol., June 1, 2000; 115(6): 735 - 748. [Abstract] [Full Text] [PDF]

				A. Dabdoub and R. Payne Protein Kinase C Activators Inhibit the Visual Cascade in Limulus Ventral Photoreceptors at an Early Stage J. Neurosci., December 1, 1999; 19(23): 10262 - 10269. [Abstract] [Full Text] [PDF]

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