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J.-J. Pang, F. Gao, S. M. Wu; Ionotropic Glutamate Receptors Mediate OFF Responses in Light-Adapted Depolarizing Bipolar Cells. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5806.
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© ARVO (1962-2015); The Authors (2016-present)
The objective is to understand functional roles of metabotropic and ionotropic glutamate receptors in mediating light-evoked synaptic inputs to depolarizing bipolar cells (DBCs).
Rod/DBC pairs were recorded from salamander retinal slices with dual-electrode whole-cell voltage clamp techniques, and the cell morphology was revealed by Lucifer yellow fluorescence with a confocal microscope. Glutamate receptors in the outer retina were localized with immunocytochemical methods.
Postsynaptic current responses to light steps or to positive/negative voltage steps in rods were recorded from DBCs held near ECl. Light-evoked current responses at ECl (leΔIC) in all DBCs were inward and they could be suppressed by L-AP4. Rod-elicited current responses at ECl (reΔIC) were transient and those elicited at the positive voltage transitions (offset of a negative voltage step and onset of a positive step) were always larger than those elicited at the negative voltage transitions (onset of a negative voltage step and offset of a positive step). In about 30% of DBCs, reΔICs were sign-inverting and L-AP4-sensitive (type I response). In another 30% of DBCs, large transient inward reΔICs were observed at the positive voltage transitions, and they were L-AP4-insensitive but DNQX-sensitive (type II response). In the remaining 40% of DBCs, reΔICs were mixtures of type I and type II responses. Under dark-adapted conditions, rods responded to light steps with abrupt hyperpolarization at onset and slow depolarizing recovery (no offset response), and thus the DNQX-sensitive ionotropic glutamate receptors play a negligible role in the DBC light responses. Under light-adapted conditions, rods responded to light steps with abrupt onset hyperpolarization and offset depolarization (due to fast offset cone response via rod-cone coupling), and DBCs exhibited transient off responses that are mediated by ionotropic glutamate receptors. Immunocytochemical results showed that GluR4 receptors are localized in DBC dendrites postsynaptic to rod output synapses.
Ionotropic glutamate receptors co-exist with metabotropic glutamate receptors in most DBCs at the rod-DBC synapses. They are silenced under dark-adapted conditions and mediate an off transient response in DBCs under light-adapted conditions.
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