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The majority of γ-aminobutyric acid (GABA)ergic interneurons have smooth dendrites with no or only few dendritic spines, but certain types of spiny GABAergic interneurons do actually contain substantial numbers of spines. The explanation for such spines has so far been purely structural: They increase the dendritic surface area and thus provide the opportunity to accommodate larger numbers of synapses. We reasoned that there may be specific functional properties for these spines and therefore, undertook to characterize interneuron spines functionally. We find a remarkable similarity to pyramidal cell spines: They receive excitatory synapses with calcium impermeable α-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors, compartmentalize biochemical signals, and display activity-dependent morphological plasticity. Nevertheless, notable differences in spine density, neck length, and spine–dendrite coupling exist. Thus, dendritic spines on inhibitory interneurons have a number of important functional properties that go substantially beyond simply expanding the dendritic surface area. It therefore seems likely that spiny and aspiny interneurons may have very different roles in neural circuit function and plasticity.