Photomapping electrically coupled networks of the mature thalamus and cortex

Electrical synapses are expressed ubiquitously across the brain and are crucial components of active neural circuitry and connectomes. Identification of coupled networks in living tissue is limited by technical demands of multiplexed recordings, and no dyes, fluorescent reporters, or genetic labels can now fill the gap. We introduce a novel method of identifying and quantifying electrical synapses, opto-δL, that combines focal photostimulation of soma-targeted opsins with a spike timing-based computation for the strength of electrical synapses to rapidly measure and map electrically coupled networks in vitro. We leverage opto-δL to show that coupled networks of the mature thalamic reticular nucleus extend as far as 100 micrometers, synapse promiscuously across genetic subtypes of neurons, and couple one to four neighboring neurons to each recorded hub cell. We applied opto-δL to cortical networks, where we found that somatostatin neurons were more strongly and prevalently coupled. These results highlight the broad potential of opto-δL to interrogate the identity and roles of electrical synapses in circuitry, behavior, and cognition.