Inhibitory microcircuit modules in hippocampal learning.

It has recently become possible to investigate connectivities and roles of identified hippocampal GABAergic interneurons (INs) in behaving rodents. INs targeting distinct pyramidal neuron subcompartments are recruited dynamically at defined phases of behavior and learning. They include Parvalbumin Axo-axonic and perisomatic Basket cells, and Somatostatin radiatum-oriens and oriens-lacunosum moleculare cells. Each IN is in turn either activated or inhibited upon specific behavioral and network state requirements through specific inputs and neuromodulators. Subpopulations of these principal neurons and INs interconnect selectively, suggesting selective processing and routing of alternate information streams. First canonical functional modules have emerged, which will have to be further defined and linked to identified afferents and efferents towards a circuit understanding of how hippocampal networks support behavior.