Retinal inhibitory inputs shape the kinetics of graded, sustained photoreceptor signals as they pass through the retina. At the network level, retinal inhibition mediates the antagonistic center-surround organization of the bipolar cell's receptive field, and in turn, that of the ganglion cell. This retinal circuit underlies contrast detection, a fundamental feature of signal processing by the retina. The goal of the proposed research is to study the distribution, functional characteristics and regulation of inhibitory inputs targeting an identified retinal bipolar cell. I will take advantage of a unique preparation made from the goldfish retina, in which patch-clamp recordings of light evoked membrane currents can be made either from the soma or the axon terminal of an ON-type, Mb bipolar cell in a slice preparation. Stable recordings from these compartments is possible even after axon severing, thus outer or inner retinal inhibitory inputs can be separately studied. Using this preparation I will revisit fundamental questions of visual information processing in the vertebrate retina. My preliminary data show that light evokes different responses at the bipolar cell soma compared to the axon terminal, due to differences in their respective synaptic inputs and voltage-gated channels. The first specific aim will characterize the light-evoked responses of the axon terminal in intact Mb bipolar cells and will study with high time-resolution capacitance measurements how these physiological responses trigger glutamate release. The second specific aim will focus on separating outer and inner retinal inhibition targeting Mb bipolar cells. I will determine how each influence the glutamate output, and separate their contributions to the antagonistic surround response. I recorded light-evoked inhibitory synaptic currents from bipolar axon terminals with severed axons, which represents pure lateral inhibition. I will characterize electrophysiologically and pharmacologically these synaptic inputs at various light adaptation levels, and determine their underlying synaptic circuitry. I have preliminary evidence that NMDA receptors provide enough calcium to trigger GABA release from amacrine cells directly and locally, bypassing voltage-gated calcium channels. The last specific aim will test the hypothesis that NMDA receptor regulation in amacrine cells influences synaptically triggered GABA release and in turn, bipolar cell output. Overall, these experiments will use a combination of capacitance, membrane potential/ current measurements and pharmacological manipulations to determine how the excitatory visual information reaching the ganglion cells is regulated by progressive levels of inhibition as the signal passes through the retinal network.
Images of the environment are translated into neural codes by all vertebrate retinas, including that of humans, in a very similar manner: the retina codes for contours, colors and motion. The proposed research investigates the natural mechanisms underlying contour detection. The results can be used to guide the design of prosthetic vision devices to restore some sight to blind people.
