Principles of visual cortex excitatory microcircuit organization

Public summary

*Optomapping is a fast two-photon optogenetic technique that charts brain microcircuits at ～100 times the speed of traditional patching methods.

*In mouse visual cortex, optomapping verified canonical pyramidal circuits but found surprising excitation patterns in basket and Martinotti cells, concentrated in layers 5 and 2/3.

*Excitatory inputs distribute log normally, with a handful of strong synapses among mostly weaker ones, extending this principle from excitatory to inhibitory neurons.

*Short-term synaptic changes that influence information transfer surprisingly depend on cortical layer in addition to target cell.

*This work sheds light on cortical circuit structure and synaptic dynamics, offering a faster approach to mapping microcircuits at synaptic resolution.

Abstract

Synapse-specific connectivity and dynamics determine microcircuit function but are challenging to explore with classic paired recordings due to their low throughput. We therefore implemented optomapping, a ～100-fold faster two-photon optogenetic method. In mouse primary visual cortex (V1), we optomapped 30,454 candidate inputs to reveal 1,790 excitatory inputs to pyramidal, basket, and Martinotti cells. Across these cell types, log-normal distribution of synaptic efficacies emerged as a principle. For pyramidal cells, optomapping reproduced the canonical circuit but unexpectedly uncovered that the excitation of basket cells concentrated to layer 5 and that of Martinotti cells dominated in layer 2/3. The excitation of basket cells was stronger and reached farther than the excitation of pyramidal cells, which may promote stability. Short-term plasticity surprisingly depended on cortical layer in addition to target cell. Finally, optomapping revealed an overrepresentation of shared inputs for interconnected layer-6 pyramidal cells. Thus, by resolving the throughput problem, optomapping uncovered hitherto unappreciated principles of V1 structure.