Despite a hundred years of anatomical work and nearly fifty years of electrophysiology, a full understanding of the neuronal cell types that comprise the visual cortex is lacking. There is widespread disagreement as to how cell types defined on the basis of morphology, axonal projections, cellular physiology, and neurochemistry relate to one another or to functional categories of neurons defined by their response properties in vivo. Despite the difficulty of the problem, progress is crucial if cellular and molecular work done in rodents is to be related to in vivo physiology performed in carnivores and primates, and to clinical observations in humans. Here we propose to combine anatomy, physiology and gene expression analysis to discover sets of markers that can be used to identify cortical cell classes. Subsets of pyramidal neurons that share laminar and sublaminar position, dendritic morphology and axonal targets have been identified on the basis of expression of fluorescent proteins in transgenic mice. Fluorescence activated cell sorting will be used to isolate these neurons and Affymetrix gene chips will be used to measure their patterns of gene expression. Multiple strains of mice will be used in which variant fluorescent proteins are expressed in different subsets of neurons to allow comparisons of gene expression across different cell types. Whole cell recording in slices will be used to characterize the intrinsic electrophysiology of each candidate neuronal cell class. Their morphological properties will be measured from intracellular fills; their synaptic connectivity will be assessed from dual whole cell recording, and their suspected long range projections will be confirmed using retrograde tracers. This work may serve as a foundation for identifying homologous cortical cell types across regions, across species and across development. In addition, the assays of gene expression in identified neuronal types may provide a more selective and sensitive assay of changes in gene expression induced by altered sensory experience or by neurological diseases.
| Hempel, Chris M; Sugino, Ken; Nelson, Sacha B (2007) A manual method for the purification of fluorescently labeled neurons from the mammalian brain. Nat Protoc 2:2924-9 |
| Song, Sen; Sjostrom, Per Jesper; Reigl, Markus et al. (2005) Highly nonrandom features of synaptic connectivity in local cortical circuits. PLoS Biol 3:e68 |
