Rod/Cone Gap Junctions: Plastic Electrical Synapses at the First Visual Synapse

Electrical coupling mediated by gap junctions is a defining feature of almost every neuron in the retina, as it is throughout the central nervous system. By allowing direct current flow between neurons, electrical coupling helps establish functional circuits, allows signals to be shared, and improves the signal-to-noise ratio, thereby regulating neuronal activity at the dendritic, single-neuron, and network levels. Remarkably, electrical coupling is not static or constitutive. It shows a high degree of plasticity, changing in strength on time scales ranging from milliseconds to days, thereby playing a crucial role in signal processing by dynamically regulating signal transmission between neurons. To understand visual processing in the retina, we need to first elucidate the mechanisms underlying this plasticity, its dynamic range, and its impact on circuit function. This review focuses on electrical coupling between rods and cones, which has been investigated using state-of-the-art structural, functional, and computational methods. Knowledge of rod/cone coupling function and plasticity may also reveal fundamental features of electrical synapse structure, function, and plasticity relevant to other brain areas.