Functional connectomics analysis of synaptic transmission using optogenetics

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The aim was to develop a method applicable to slice-based analysis of synaptic transmission using optogenetics that can determine the origin of fibers being stimulated. Five to eight weeks after transfecting adeno-associated virus vectors for ChR2 expression into the lateral parabrachial nucleus (LPB) in the dorsal pons of rats, blue light pulses robustly triggered CNQX-sensitive EPSCs in the neurons in the capsular part of the central amygdala recorded in acute slices of distinct angles (coronal and horizontal). These light-evoked EPSCs (leEPSCs) were blocked by TTX but re-appeared by addition of 4-AP, suggesting monosynaptic innervation, and was followed by a large and long-lasting IPSC, indicative of the presence of polysynaptic feed-forward inhibition. This "functional connectomics" technique has advantages in identifying the synaptic properties of long-distance connections, which has been otherwise almost impossible in the conventional electrophysiology setups.