Responses of rod bipolar cells isolated from dogfish retinal slices to concentration-jumps of glutamate

Abstract

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 nM 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.

Regulation of ON bipolar cell activity

“…Several groups (Shiells and Falk, 1994;van Rossum and Smith, 1998;Sampath and Rieke, 2004) have proposed variations on a threshold model to account for the nonlinear relationship between the concentration of mGluR6 agonist and the fraction of open transduction channels. An essential feature of the model is that there is an excess number of molecules within the dendrites of the ON bipolar cell that serve to keep the synaptic channels closed in the dark.…”

A Retinal-Specific Regulator of G-Protein Signaling Interacts with Gαoand Accelerates an Expressed Metabotropic Glutamate Receptor 6 Cascade

“…It is therefore tempting to speculate that in rod bipolar cells Ret-RGS1 should increase glutamate response. The rod bipolar cell carries the tiny single photon signal from rods to the AII amacrine cell via a high gain system (Ashmore and Falk, 1979;Falk, 1988;Shiells and Falk, 1994). During dark periods, glutamate is released continuously by the rod and hyperpolarizes the rod bipolar cell.…”

A rise in intracellular Ca2+ underlies light adaptation in dogfish retinal ‘on’ bipolar cells

“…High sensitivity in the dark-adapted state is achieved by high gain in phototransduction in rods (Pugh & Lamb, 1993) and in synaptic transduction in 'on' bipolar cells (Shiells & Falk, 1994), each involving a cGMP cascade linked to rhodopsin or to a metabotropic glutamate receptor (mGluR), respectively. This mGluR has been cloned and identified as mGluR6 (Nakajima et al 1993).…”

“…This system inverts the rod photoresponse giving rise to the 'on' pathway of the visual system. The hyperpolarizing rod response is conserved on synaptic transmission to 'off' bipolar cells since these cells possess non-desensitizing ionotropic glutamate receptors (Shiells & Falk, 1994). Voltage gain in synaptic transmission from rods to 'on' bipolar cells is in the order of 100-fold when fully dark adapted (Ashmore & Falk, 1980).…”