Blocking glutamate receptors eliminated these events, and bipolar cells provide the only known glutamatergic input to RGCs. Hence, we conclude that inputs from amacrine cells, bipolar cells, Selleckchem Nutlin 3a and to a lesser extent, the intrinsic K+ conductances of RGCs, all combine to shape and amplify the AAQ-mediated RGC light response. Visual acuity is determined by the size of receptive fields of neurons in the visual system. In the healthy retina, the receptive field
of an RGC is defined by the spatial extent of all of the photoreceptors that influence its activity. By definition, the receptive fields of RGCs in rd1 mice are eliminated after the photoreceptors have degenerated. However because AAQ makes presynaptic neurons light-sensitive, it is possible to measure the spatial extent of their light-driven influence on RGC firing. While this is not a conventional measurement of the RGC receptive field, it does indicate the spatial precision of the AAQ-mediated RGC light response. We illuminated AAQ-treated retinas with small spots (60 μm diameter) of 380 nm light centered on one of the 60 electrodes in an MEA (Figure 3A). In the example shown in Figure 3A, upon switching BMS-354825 mouse from 500 to 380 nm light, the average RGC activity increased in the targeted electrode by ∼81% but not in the surrounding electrodes. In each
of a total of eight targeted spots from three different retinas, only neurons near the targeted electrode exhibited Tryptophan synthase a significant increase in firing (median PI = 0.517; Figure 3B). Since RGCs are detected by only one electrode and they are spaced 200 μm apart, this puts an upper limit on the radius of the AAQ-mediated RGC collecting area of 100 μm. Analysis of electrodes outside the illuminated spot showed that
380 nm light significant decreased RGC firing. Decreased firing was detected in electrodes centered at 300, 500, and 700 μm from the mid-point of the targeted electrode (Figure 3C; Table 1). Hence, RGCs in the center of an illuminated spot are stimulated, whereas those in a surrounding annulus (from 200 to 800 μm) are inhibited. Inhibition in the surrounding RGCs implies that a sign-inverting synapse from a laterally-projecting neuron is involved in transmitting information from the center illuminated area to the surround. Amacrine cells are known to form a mutually inhibitory network, making them the likely source of the inhibitory signal. We determined the optimal wavelength for turning off RGC firing when the AAQ photoswitch is driven from the cis to the trans configuration. First, a conditioning 380 nm stimulus was used to turn on firing and then we measured suppression of firing in response to test flashes of different wavelengths. We found that 500 nm light is best at suppressing activity ( Figure 4A), as expected from previous results ( Fortin et al., 2008).