Sources of calcium at Connexin 36 gap junctions in the retina.

Synaptic plasticity is a fundamental feature of the central nervous system that controls the magnitude of signal transmission between communicating cells. Many electrical synapses exhibit substantial plasticity that modulates the degree of coupling within groups of neurons, alters the fidelity of signal transmission or even reconfigures functional circuits. In several known examples, such plasticity depends on calcium and is associated with neuronal activity. Calcium-driven signaling is known to promote potentiation of electrical synapses in fish Mauthner cells, mammalian retinal AII amacrine cells and inferior olive neurons, and to promote depression in thalamic reticular neurons. In order to measure local calcium dynamics in situ, we developed a transgenic mouse expressing a GCaMP calcium biosensor fused to Connexin 36 (Cx36) at electrical synapses. We examined the sources of calcium for activity-dependent plasticity in retina slices using confocal or SRRF imaging. More than half of Cx36-GCaMP gap junctions responded to puffs of glutamate with transient increases in fluorescence. The responses were strongly dependent on NMDA receptors, in keeping with known activity-dependent signaling in some amacrine cells. We also found that some responses depended on the activity of voltage-gated calcium channels, representing a previously unrecognized source of calcium to control retinal electrical synaptic plasticity. The high prevalence of calcium signals at electrical synapses in response to glutamate application indicates that a large fraction of electrical synapses has the potential to be regulated by neuronal activity. This provides a means to tune circuit connectivity dynamically based on local activity. Significance statement Electrical synapse plasticity is controlled at the level of the individual synapse and several mechanisms of plasticity depend on the presence of calcium. Tools available to researchers to study calcium dynamics generally provide a spatially diffuse view, without single-synapse resolution. We have developed a transgenic mouse that reports calcium dynamics at individual Cx36 electrical synapses, enabling investigators to study calcium microdomain dynamics in circuits of interest. Preliminary studies in retina reveal that calcium microdomains are dynamic at a large fraction of electrical synapses, demonstrating extensive potential for activity-dependent plasticity. Such plasticity is likely to be the norm for electrical synapses rather than an exception limited to a few studied circuits.