Role of microcircuit structure and input integration in hippocampal interneuron recruitment and plasticity

The proper operation of cortical neuronal networks depends on the temporally precise recruitment of GABAergic inhibitory interneurons. Inhibitory cells receive convergent excitatory inputs from afferent pathways, as well as local collaterals of principal cells, and provide feedforward or feedback inhibition within the circuitry. Accumulating evidence indicates that recruitment of GABAergic cells is highly diverse among interneuron types. Differences in the properties of input synapses, dendritic architecture and membrane properties, as well as the rich repertoire of plasticity mechanisms contribute to this diversity. Efficient and precise recruitment of interneurons is thought to depend on the coincident occurrence of rapid synaptic responses and their faithful propagation to the action potential initiation site. However, slow inputs can also play important roles by facilitating the activation of interneurons by rapid synaptic inputs and supporting associative synaptic plasticity. Here we review how the diversity in the synaptic and integrative properties as well as dendritic geometry of hippocampal inhibitory cells impact on their activation. We further discuss how the various modes of interneuron recruitment can support the versatile cell type- and input-specific computational functions which appear to be adapted to the structure and the function of the network they are embedded in.This article is part of a Special Issue entitled ‘Synaptic Plasticity & Interneurons’.

Research highlights
► Excitatory synapses onto GABAergic interneurons show cell type- and input-specific properties.
► Receptor composition and dendritic integration determine synaptic impact and plasticity.
► Precisely timed interneuron recruitment depends on coincidence of fast synaptic excitation.
► Convergent slow inputs can facilitate activation by the fast ones and promote Hebbian plasticity.
► Plasticity at feedback inputs to interneurons may underlie competitive assembly formation.