New Insights Into Dendritic Inhibition

Inhibitory interneurons control fundamental aspects of cortical neuronal network function by means of their GABAergic output synapses. The precise location as well as the functional and dynamic characteristics of inhibitory synapses are the key determinants of their functional influence on target cells. Interneurons are highly diverse and can be subdivided into various types on the basis of their physiological characteristics and neurochemical profile, most importantly, however, on the location of their output synapses. In the past, high attention was concentrated on perisoma-inhibiting interneurons with synapses located at the soma, proximal dendrites and the axon initial segment of target cells provided by parvalbumin (PV)-, cholecystokinin (CCK)-expressing basket cells and PV-positive axo-axonic cells, respectively. Perisomatic inhibition has been shown to control action potential discharges. Moreover, it efficiently synchronizes principal cell assemblies and contributes to the generation of gamma oscillations. However, the vast majority of inhibitory synapses are located in the dendritic tree. In the CA1 hippocampal area, for example, at least 15 different types of interneurons innervate different dendritic compartments of pyramidal cells and interneurons. The layer-specific distribution of their axons suggests that dendrite-targeting interneurons provide spatially defined inhibition to control local dendritic electrogenesis (Miles et al., 1996). Over the last two years, several laboratories have invested efforts into defining the functional significance of different inhibitory inputs converging onto neuronal dendrites. In addition to a critical role of dendritic inhibition in synaptic input discrimination and plasticity (Muller et al., 2012; Bar-Ilan et al., 2013), we learnt about its ability to control multiple aspects of neuronal output, from dendritic and somatic spiking to discharge timing, rate and pattern (Muller et al., 2012; Lovett-Baron et al., 2012; Royer et al., 2012). Thus, in this Research Topic we would like to reunite recent fascinating findings on the multiple roles of dendritic inhibition originating from different sources by addressing the following questions: How many different subtypes of dendrite-targeting interneurons operate in different cortical regions? Under which conditions are dendrite-targeting interneurons recruited, especially in behaving animals? What advances have been made towards the identification of the cell-type-specific markers necessary for selective targeting of distinct subtypes of dendrite-targeting interneurons? What do we currently know about their connectivity? What are the functional and dynamic properties of dendritic inhibitory synapses? How may the activation of distinct dendritic inhibitory inputs interact with local intrinsic and synaptic conductances? With this Research Topic, we intend to build a comprehensive view on the structural and functional organization of dendritic inhibition in complex cortical networks making a bridge to its relevance for behavioural states and cognitive functions.