In contrast, inhibitory interneurons, which by definition release the neurotransmitter GABA, have their cell bodies scattered throughout all major subfields, and the positioning of their somatodendritic arbors allows them to integrate from a more restricted intrinsic and extrinsic afferent input repertoire than their pyramidal cell counterparts. This organization permits pyramidal neurons to receive afferent input from a variety of both intrinsic and extrinsic sources across well-defined dendritic domains.
Across all subfields of the hippocampus, the cell bodies of glutamatergic pyramidal neurons are organized in a three- to five-cell-deep laminar arrangement in stratum pyramidale (s.p.) and have orthogonal dendrites that span from the deep stratum oriens (s.o.) to the superficial layers of the stratum lacunosum moleculare (s.l.m.).
Despite being in the minority, this diverse neuronal population serves as a major determinant of virtually all aspects of cortical circuit function and regulation. In a 30-day-old Wistar rat it has been estimated that the total CA1 hippocampal neuronal population is ~350,000, which contains a conservative estimate of ~38,500 inhibitory interneurons ( 102). In hippocampus, GABAergic local circuit inhibitory interneurons account for ~10–15% of the total neuronal cell population. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10–15% of the total neuronal population however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function.
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