MOLECULAR AND PHYSIOLOGICAL DIVERSITY OF CORTICAL NONPYRAMIDAL CELLS

The physiological and molecular features of nonpyramidal cells were in vestigated in acute slices of sensory-motor cortex using whole-cell re cordings 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 in testinal polypeptide, somatostatin, and cholecystokinin). In the 97 ne urons analyzed, all expressed mRNAs of at least one calcium binding pr otein, and the majority (n = 73) contained mRNAs of at least one neuro peptide. Three groups of nonpyramidal cells were defined according to their firing pattern. (1) Fast spiking cells (n = 34) displayed tonic discharges of fast action potentials with no accommodation. They expre ssed parvalbumin (n = 30) and/or calbindin (n = 19) mnNAs, and half of them also contained transcripts of at least one of the four neuropept ides. (2) Regular spiking nonpyramidal cells (n = 48) displayed a firi ng behavior characterized by a marked accommodation and presented a la rge diversity of expression patterns of the seven biochemical markers. (3) Finally, a small population of vertically oriented bipolar cells, termed irregular spiking cells (n = 15), fired bursts of action poten tials at an irregular frequency. They consistently cc-expressed calret inin and vasoactive intestinal polypeptide. Additional investigations of these cells showed that they also co-expressed glutamic acid decarb oxylase and choline acetyl transferase. Our results indicate that neoc ortical nonpyramidal neurons display a large diversity in their firing properties and biochemical patterns of co-expression and that both ch aracteristics could be correlated to define discrete subpopulations.