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Dennis M Dacey, Joanna D Crook, Lauren Anderson, Beth B Peterson, Orin S Packer; Distinct contribution of kainate and NMDA receptors to OFF-pathway circuitry for midget, parasol and small bistratified ganglion cells in the macaque monkey retina. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2386.
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© ARVO (1962-2015); The Authors (2016-present)
In primate the midget, parasol and small bistratified ganglion cell populations together provide the major contribution to the primary visual pathway and thus to the perception of form, color and motion. Receptive field properties of these pathways have been well characterized but less is known about the underlying circuits and synaptic mechanisms utilized by each pathway. The purpose of this study was to characterize the contributions of AMPA, kainate and NMDA type glutamate receptors to the midget and parasol OFF-center types and the "yellow-OFF" pathway of the small bistratified cell.
Ganglion cell types were identified in the macaque monkey retina in vitro and were voltage clamped using standard methods. Light-evoked postsynaptic currents were measured with spots restricted to the receptive field center that selectively modulated the long and middle wavelength-sensitive cones (L+M, 50% contrast). Conductance analysis was used to resolve excitatory NMDA, non-NMDA and inhibitory conductances (Crook et al., Vis Neurosci, 2013; first view, 1-28).
NMDA receptors contributed about 20% to the total excitatory conductance in OFF-parasol cells at physiological membrane potentials (-55 mV). By contrast, for both the OFF-midget and the L+M-OFF response of the small bistratified cells, NMDA receptors mediated a surprisingly large fraction, from 50-70%, of the excitatory conductance. Unexpectedly, after application of the AMPA/kainate receptor antagonist NBQX (10-50 µM), the NMDA-mediated conductance was preserved with little or no attenuation in all three ganglion cell types. Further addition of UBP 310 (1-5 µM), a selective kainate receptor antagonist, abolished all light evoked synaptic currents, suggesting that in midget, parasol and small bistratified cell circuits the transmission from cones to OFF-bipolar cells is mediated largely by an NBQX-resistant kainate receptor.
The NMDA receptor-mediated component of OFF-pathway excitation in midget and small bistratified cells is surprisingly large, comprising over 50% of the excitatory conductance at physiological membrane potentials suggesting a role for NMDA receptors in color-opponent circuitry. Unexpectedly, in all three major pathways, kainate receptors appear to mediate the major fraction of synaptic transmission from cones to OFF-bipolar cells.
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