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The Journal of Neuroscience, April 15, 1999, 19(8):2938-2944

The Metabotropic Receptor mGluR6 May Signal Through G_o, But Not Phosphodiesterase, in Retinal Bipolar Cells

Scott Nawy

Departments of Ophthalmology and Visual Science, and Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461

Bipolar cells are retinal interneurons that receive synaptic input from photoreceptors. Glutamate, the photoreceptor transmitter, hyperpolarizes On bipolar cells by closing nonselective cation channels, an effect mediated by the metabotropic receptor mGluR6. Previous studies of mGluR6 transduction have suggested that the receptor couples to a phosphodiesterase (PDE) that preferentially hydrolyzes cGMP, and that cGMP directly gates the nonselective cation channel. This hypothesis was tested by dialyzing On bipolar cells with nonhydrolyzable analogs of cGMP. Whole-cell recordings were obtained from On bipolar cells in slices of larval tiger salamander retina. Surprisingly, On bipolar cells dialyzed with 8-(4-chlorophenylthio)-cyclic GMP (8-pCPT-cGMP), or 8-bromo-cyclic GMP (8-Br-cGMP) responded normally to glutamate or L-2-amino-4-phosphonobutyrate (L-APB). Response amplitudes and kinetics were not significantly altered compared with cells dialyzed with cGMP alone. Comparable results were obtained with the PDE inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) or with 8-pCPT-cGMP and IBMX together, indicating that PDE is not required for mGluR6 signal transduction. Addition of the G-protein subunit G_o to the pipette solution suppressed the cation current and occluded the glutamate response, whereas dialysis with G_i or with transducin G had no significant effect on either the cation current or the response. Dialysis of an antibody directed against G_o also reduced the glutamate response, indicating a functional role for endogenous G_o. These results indicate that mGluR6 may signal through G_o, rather than a transducin-like G-protein.

Key words: cGMP; 8-pCPT-cGMP; mGluR6; phosphodiesterase; retinal bipolar cell; G_o; phosphodiesterase

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Copyright © 1999 Society for Neuroscience  0270-6474/99/1982938-07$05.00/0

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