Responses of rod bipolar cells isolated from dogfish retinal slices to concentration-jumps of glutamate.
Rod on-bipolar cell light responses are mediated by a class of metabotropic glutamate receptor which is coupled via a G-protein to the control of a cGMP cascade, with cGMP acting to open cation channels, whilst off-bipolar cells possess ionotropic glutamate receptors. Whole-cell voltage-clamp recordings were obtained from on- and off-bipolar cells of dark-adapted dogfish retinal slices, identified by their light responses. Isolated cells were exposed to concentration-jumps of glutamate. At negative voltage-clamp potentials, on-bipolar cells responded to glutamate with outward currents with a mean delay of 10.8 ms, whilst off-bipolar cells responded with inward currents without any delay. Neither cell type showed desensitization to applied steps of glutamate. The dose-response relation for on-bipolar cells showed no gradual saturation, but increased linearly with a sharp cutoff above 200 microM glutamate. This dose-response relation could be fitted with a theoretical expression assuming Michaelis-Menten kinetics for the action of glutamate on receptors and a linear relation between the concentration of receptors bound to glutamate and the fall in cGMP this induces. The dose-response relation of off-bipolar cells showed saturation with a limiting slope of 2 at low glutamate concentrations, suggesting that two molecules of glutamate are required to open each channel by a cooperative mechanism. The glutamate receptor coupled cGMP cascade of rod on-bipolar cells can account for high synaptic voltage gain.
Department of Physiology, University College, London, UK.
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Shiells
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Falk
D000818:Animals; D006152:Cyclic GMP / Q000378:metabolism; D003623:Dark Adaptation; D004284:Dogfish / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D018698:Glutamic Acid / Q000494:pharmacology*; D007395:Interneurons / Q000502:physiology*; D008027:Light; D008564:Membrane Potentials / Q000502:physiology; D017470:Receptors, Glutamate / Q000378:metabolism; D012160:Retina / Q000187:drug effects* / Q000502:physiology; D017948:Retinal Rod Photoreceptor Cells / Q000502:physiology*; D013997:Time Factors
D017470:Receptors, Glutamate; D018698:Glutamic Acid; D006152:Cyclic GMP