Mind the Gap Junctions: The Importance of Electrical Synapses to Visual Processing

Abstract

In this issue of Neuron, Kuo et al. (2016) show that coordinated interaction between electrical and chemical synapses in a defined retinal circuit enhances sensitivity to moving objects. Their work demonstrates how electrical and chemical synapses combine to improve information processing in a specific area of the CNS.

Figure 1. Signal Propagation through the Retinal Network

(A) Parallel, excitatory retinal pathways: a uniform photoreceptor output (p) is distributed to various cone bipolar (CB) cell types, each of which makes synapses onto a unique population of GCs. CBs and postsynaptic GCs are characterized broadly as either ON or OFF, excited by increments or decrements in luminance, respectively, and then more specifically by response properties. The dendrites of different GC types, and the axon terminals of their specific presynaptic CB partners, costratify in narrow laminae within the inner plexiform layer (IPL). (B1) Electrical coupling within the AII-ON CB network (top) allows AII V_M to alter ON CB V_M (bottom). Coupling between AIIs and ON CBs in the mouse retina is well established; direct coupling between CBs has not been demonstrated (question marks). (B2) Electrical coupling permits synaptic input to one ON CB to influence V_M in a different ON CB. As illustrated schematically, a postsynaptic potential (PSP) evoked by a stimulus in the receptive field of CB1 (e.g., an ∼18 μm bar, as in Figures 1C and 1D; Kuo et al., 2016) changes VM in CB2. If CB2 receives a synaptic input soon after a PSP is generated in CB1, the resulting PSP in CB2 is amplified. And the PSP in CB2 then serves to lengthen the duration of depolarization in CB1. (C) Depolarization or hyperpolarization of the AII-ON CB network alters the gain of signaling at ON CB→ON GC synapses by shifting mean V_M relative to the conductance (G)-voltage relationship of the presynaptic Ca channels (left). Changes in tonic Ca conductance alter the tonic release rate and thereby affect the availability of vesicles in the RRP, as illustrated schematically at right.