Proximally targeted GABAergic synapses and gap junctions synchronize cortical interneurons

Networks of GABAergic interneurons are implicated in synchronizing cortical activity at gamma frequencies (30-70 Hz). Here we demonstrate that the com bined electrical and GABAergic synaptic coupling of basket cells instantane ously entrained gamma-frequency postsynaptic firing in layers 2/3 of rat so matosensory cortex. This entrainment was mediated by rapid curtailment of g ap junctional coupling potentials by GABA(A) receptor-mediated IPSPs. Elect ron microscopy revealed spatial proximity of gap junctions and GABAergic sy napses on somata and dendrites. Electrical coupling alone entrained postsyn aptic firing with a phase lag, whereas unitary GABAergic connections were i neffective in gamma-frequency phasing. These observations demonstrate preci se spatiotemporal mechanisms underlying action potential timing in oscillat ing interneuronal networks.