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Michael B. Manookin, Christian Puller, Joanna D. Crook, Dennis M. Dacey; Complementary Roles for Glutamate Excitation and Crossover Inhibition in Parasol Ganglion Cells of the Macaque Monkey Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3031.
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© ARVO (1962-2015); The Authors (2016-present)
Parasol ganglion cells of the primate retina comprise a major achromatic pathway to the lateral geniculate nucleus. Parasol cells show high contrast sensitivity and signal across a wide range of mean luminances. However, the synaptic mechanisms driving visual encoding in parasol cells remains uncertain. Here, we directly measure the contribution from ligand-gated neurotransmitter receptors to visual encoding in ON and OFF parasol cells across eight orders of luminance magnitude, from scotopic to high photopic mean luminance.
We recorded from parasol ganglion cells in whole-cell, voltage-clamp in an in vitro, whole-mount preparation of the macaque monkey retina. The stimulus was a spot projected on the receptive field center; spot contrast was sinusoidally modulated at 2 Hz (contrast, 100%). The stimulus was repeated at nine holding potentials between -85 and +50 mV. Recordings were performed at eight background luminances from scotopic to photopic mean luminance (10-1 to 106 R* cone-1 sec-1). Excitatory and inhibitory conductances were calculated from light-evoked currents using established methods.
The contribution from glutamate receptors and glycinergic crossover inhibition varied with mean luminance and cell type. ON cells were primarily driven by glutamate excitation under scotopic conditions and by crossover inhibition under photopic conditions. OFF cells showed the opposite pattern. Crossover inhibition dominated under scotopic lighting conditions and excitation dominated under photopic conditions.
These data suggest that the vertical glutamate pathways driving visual encoding in ON and OFF parasol cells are most sensitive under scotopic and photopic light conditions, respectively. Furthermore, when the excitatory glutamate circuitry is less sensitive, crossover inhibition drives visual encoding. Thus, glutamate excitation and crossover inhibition play complementary roles in visual encoding.
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