Neuroscience produces a vast amount of data from an enormous diversity of neurons. A neuronal classification system is essential to organize such data and the knowledge that is derived from them. Classification depends on the unequivocal identification of the features that distinguish one type of neuron from another. The problems inherent in this are particularly acute when studying cortical interneurons. To tackle this, we convened a representative group of researchers to agree on a set of terms to describe the anatomical, physiological and molecular features of GABAergic interneurons of the cerebral cortex. The resulting terminology might provide a stepping stone towards a future classification of these complex and heterogeneous cells. Consistent adoption will be important for the success of such an initiative, and we also encourage the active involvement of the broader scientific community in the dynamic evolution of this project.

The physiological and molecular features of nonpyramidal cells were investigated in acute slices of sensory-motor cortex using whole-cell recordings combined with single-cell RT-PCR to detect simultaneously the mRNAs of three calcium binding proteins (calbindin D28k, parvalbumin, and calretinin) and four neuropeptides (neuropeptide Y, vasoactive intestinal polypeptide, somatostatin, and cholecystokinin). In the 97 neurons analyzed, all expressed mRNAs of at least one calcium binding protein, and the majority (n ϭ 73) contained mRNAs of at least one neuropeptide. Three groups of nonpyramidal cells were defined according to their firing pattern. (1) Fast spiking cells (n ϭ 34) displayed tonic discharges of fast action potentials with no accommodation. They expressed parvalbumin (n ϭ 30) and/or calbindin (n ϭ 19) mRNAs, and half of them also contained transcripts of at least one of the four neuropeptides. (2) Regular spiking nonpyramidal cells (n ϭ 48) displayed a firing behavior characterized by a marked accommodation and presented a large diversity of expression patterns of the seven biochemical markers. (3) Finally, a small population of vertically oriented bipolar cells, termed irregular spiking cells (n ϭ 15), fired bursts of action potentials at an irregular frequency. They consistently co-expressed calretinin and vasoactive intestinal polypeptide. Additional investigations of these cells showed that they also co-expressed glutamic acid decarboxylase and choline acetyl transferase. Our results indicate that neocortical nonpyramidal neurons display a large diversity in their firing properties and biochemical patterns of co-expression and that both characteristics could be correlated to define discrete subpopulations.

The role of interneurons in neurovascular coupling was investigated by patch-clamp recordings in acute rat cortical slices, followed by single-cell reverse transcriptase-multiplex PCR (RT-mPCR) and confocal observation of biocytin-filled neurons, laminin-stained microvessels, and immunodetection of their afferents by vasoactive subcortical cholinergic (

We studied neurogliaform neurons in the stratum lacunosum moleculare of the CA1 hippocampal area. These interneurons have short stellate dendrites and an extensive axonal arbor mainly located in the stratum lacunosum moleculare. Single-cell reverse transcription-PCR showed that these neurons were GABAergic and that the majority expressed mRNA for neuropeptide Y. Most neurogliaform neurons tested were immunoreactive for ␣-actinin-2, and many stratum lacunosum moleculare interneurons coexpressed ␣-actinin-2 and neuropeptide Y. Neurogliaform neurons received monosynaptic, DNQX-sensitive excitatory input from the perforant path, and 40 Hz stimulation of this input evoked EPSCs displaying either depression or initial facilitation, followed by depression. Paired recordings performed between neurogliaform neurons showed that 85% of pairs were electrically connected and 70% were also connected via

A classification of fusiform neocortical interneurons (n ‫؍‬ 60) was performed with an unsupervised cluster analysis based on the comparison of multiple electrophysiological and molecular parameters studied by patch-clamp and single-cell multiplex reverse transcription-PCR in rat neocortical acute slices. The multiplex reverse transcription-PCR protocol was designed to detect simultaneously the expression of GAD65, GAD67, calbindin, parvalbumin, calretinin, neuropeptide Y, vasoactive intestinal peptide (VIP), somatostatin (SS), cholecystokinin, ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, kainate, N-methyl-D-aspartate, and metabotropic glutamate receptor subtypes. Three groups of fusiform interneurons with distinctive features were disclosed by the cluster analysis. The first type of fusiform neuron (n ‫؍‬ 12), termed regular spiking nonpyramidal (RSNP)-SS cluster, was characterized 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, predominantly expressed VIP and also showed firing properties of RSNP neurons with accommodation profiles different from those of RSNP-SS cells. Finally, the last type of fusiform neuron (n ‫؍‬ 16) contained a majority of irregular spiking-VIPergic neurons. In addition, the analysis of glutamate receptors revealed cell-type-specific expression profiles. This study shows that combinations of multiple independent criteria define distinct neocortical populations of interneurons potentially involved in specific functions. B ecause, in part, of their diversity, the function of neuron subtypes in the physiology of the neocortex is still poorly understood. A better knowledge of the different neuronal populations that compose this heterogeneous brain structure may therefore contribute to elucidating their specific role.Attempts to classify neurons rely on several independent criteria (morphological, physiological, and molecular). In the neocortex, neurons are classified as pyramidal cells or nonpyramidal cells according to their morphology. Pyramidal cells accumulate glutamate and constitute the main class of excitatory projecting neurons (1). In contrast, nonpyramidal cells, also termed interneurons, are mainly inhibitory ␥-aminobutyric acid-ergic neurons with a short axon involved in local circuits (2) and have a large diversity of morphology (3). The expression of biochemical markers has been used to define different classes of nonpyramidal cells. The distribution of three calcium binding proteins, calbindin (CB), parvalbumin (PV), and calretinin (CR), and four neuropeptides, neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), somatostatin (SS), and cholecystokinin (CCK), defines partially overlapping groups of interneurons (4 -12). In addition to this morphological and molecular diversity, nonpyramidal cells also have a large repertoire of firing behaviors (13-18), such as fast spiking (FS), regular spiking nonpyramidal (RSNP), or irregular spiking (IS).Some types of interneurons have b...