Classification of fusiform neocortical interneurons based on unsupervised clustering

A classification of fusiform neocortical interneurons (n = 60) was performe d with an unsupervised cluster analysis based on the comparison of multiple electrophysiological and molecular parameters studied by patch-clamp and s ingle-cell multiplex reverse transcription-PCR in rat neocortical acute sli ces. The multiplex reverse transcription-PCR protocol was designed to detec t simultaneously the expression of GAD65, GAD67, calbindin, parvalbumin, ca lretinin, neuropeptide Y, vasoactive intestinal peptide (VIP), somatostatin (SS), cholecystokinin, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, kainate, N-methyl-D-aspartate, and metabotropic glutamate receptor s ubtypes. Three groups of fusiform interneurons with distinctive features we re disclosed by the cluster analysis. The first type of fusiform neuron (n = 12), termed regular spiking nonpyramidal (RSNP)-SS cluster, was character ized by a firing pattern of RSNP cells and by a high occurrence of SS. The second type of fusiform neuron (n = 32), termed RSNP-VIP cluster, predomina ntly expressed VIP and also showed firing properties of RSNP neurons with a ccommodation profiles different from those of RSNP-SS cells. Finally, the l ast type of fusiform neuron (n = 16) contained a majority of irregular spik ing-VIPergic neurons. In addition, the analysis of glutamate receptors reve aled cell-type-specific expression profiles. This study shows that combinat ions of multiple independent criteria define distinct neocortical populatio ns of interneurons potentially involved in specific functions.