Distinct expressions of contrast gain control in parallel synaptic pathways converging on a retinal ganglion cell.
Visual neurons adapt to increases in stimulus contrast by reducing their response sensitivity and decreasing their integration time, a collective process known as 'contrast gain control.' In retinal ganglion cells, gain control arises at two stages: an intrinsic mechanism related to spike generation, and a synaptic mechanism in retinal pathways. Here, we tested whether gain control is expressed similarly by three synaptic pathways that converge on an OFF alpha/Y-type ganglion cell: excitatory inputs driven by OFF cone bipolar cells; inhibitory inputs driven by ON cone bipolar cells; and inhibitory inputs driven by rod bipolar cells. We made whole-cell recordings of membrane current in guinea pig ganglion cells in vitro. At high contrast, OFF bipolar cell-mediated excitatory input reduced gain and shortened integration time. Inhibitory input was measured by clamping voltage near 0 mV or by recording in the presence of ionotropic glutamate receptor (iGluR) antagonists to isolate the following circuit: cone --> ON cone bipolar cell --> AII amacrine cell --> OFF ganglion cell. At high contrast, this input reduced gain with no effect on integration time. Mean luminance was reduced 1000-fold to recruit the rod bipolar pathway: rod --> rod bipolar cell --> AII cell --> OFF ganglion cell. The spiking response, measured with loose-patch recording, adapted despite essentially no gain control in synaptic currents. Thus, cone bipolar-driven pathways adapt differently, with kinetic effects confined to the excitatory OFF pathway. The ON bipolar-mediated inhibition reduced gain at high contrast by a mechanism that did not require an iGluR. Under rod bipolar-driven conditions, ganglion cell firing showed gain control that was explained primarily by an intrinsic property.
Beaudoin
Deborah Langrill
DL
Department of Ophthalmology & Visual Sciences, University of Michigan, Ann Arbor, MI 48105, USA.
Manookin
Michael B
MB
Demb
Jonathan B
JB
D000818:Animals; D015350:Contrast Sensitivity / Q000502:physiology*; D004594:Electrophysiology; D006168:Guinea Pigs; D008959:Models, Neurological; D017711:Nonlinear Dynamics; D010775:Photic Stimulation; D012165:Retinal Ganglion Cells / Q000502:physiology*; D013569:Synapses / Q000502:physiology; D014795:Visual Pathways / Q000502:physiology