MOLECULAR MECHANISM OF HYPERPOLARIZATION OF ON-BIPOLAR CELLS IN RETINA

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Light reduce the release of glutamate from photoreceptor cell, resulting in opening of cation channels on on-bipolar cell and subsequent depolarization of the cell. In this respect, retinal onbipolar cell is a novel neuron which hyperpolarizes on contact with neurotransmitter molecules. Electrophysiological studies showed that cGMP act as the second messenger mediating the action of glutamate through the G protein that striking similarities to transduction in photoreceptors. In order to elucidate molecular mechanism of hyperpolarization of on-bipolar cell, we studied signal coupling proteins of on-bipolar cell in bovine retina.GTP-binding protein (G protein) linking to metabotropic glutamate receptor of bovine retinal on-bipolar cell was studied by use of pharmacologically selective ligands, 2-amino-4-phosphonobutyric acid (APB) on bacteria toxin-catalyzed ADP-ribosylation and GTP S-binding. In contrast to the electrophysiological finding reported, G-protein coupling to APB-sensitive glutamate receptor served as a substrate for pertussis toxin but did not for cholera toxin. Several glutamate analogues effective on on-bipolar cell, as well as APB,increased GTP S binding to retinal membranes devoid of rod outer segments. The enhancement of GTP S binding by APB was completely abolished when the membranes were pretreated with pertussis toxin and NAD.These results suggest that, in retinal on-bipolar cell, the G-protein which couples metabotropic glutamate receptor to hyperpolarizing response of the cell is sensitive to pertussis toxin.