We performed whole-cell recordings from cones, bipolar, and amacrine cells in ground squirrel retina slices. We designed an S-cone isolating stimulus using the silence substitution method. Two Hz sine waves of green (578 nm) and blue (435 nm) stimuli that are 180o out of phase were presented to green cones for live calibration until the modulation of the membrane potential of green cones was abolished. Consequently, any inner retinal neurons that respond to this stimulus are the ones that receive prominent S-cone inputs. Fluorescent tracer was included in the recording pipette to reconstruct the recorded cells. We identified a type of amacrine cell that responded strongly to the S-cone isolating stimulus. They displayed a sustained depolarizing response to a one second blue light pulse with a hyperpolarizing overshoot at light Off appropriate for relaxing tonic inhibition and providing an Off excitatory drive to the postsynaptic cell. The reconstruction of the cells revealed long descending dendrites reaching the bottom of the IPL suitable for receiving inputs from SCBs. Pharmacological dissection of this putative s-cone pathway is ongoing. In the past year, we continued our work by collecting more amacrine cell samples and generated a plot to separate different types of amacrine cells. For each amacrine cell, we plotted the S-cone response index (S-cone response/(S-cone response + M-cone response) against the dendritic ramification. In so doing, we were able to unequivocally identify the S-cone amacrine cell population. Pharmacological experiments demonstrated that the S-cone isolating response can be blocked by L-AP4 and NBQX, consistent with the proposed circuit. In addition, by applying gap junction blocker, we verified that the S-cone responses were not mediated by gap junctions between the S-cone bipolar cell and the S-cone amacrine cell. In sum, we have evidence to suggest that the S-Off signal in the mammalian retina may originate from a type of amacrine cell that receives SCB input and inverts the S-On signal through its inhibitory output synapse. Mammalian amacrine cells are canonically considered as achromatic. We present first evidence of a color-coding amacrine cell in the mammalian retina.

National Institute of Health (NIH)
National Eye Institute (NEI)
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U.S. National Eye Institute
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