Morphological identification of neuron types in the rat hippocampus

The cerebral cortex ensures an optimal interaction of mammals, including humans, with their environment, by encoding, storing and combining information about the surrounding world and the internal milieu. Probably the simplest and the most popular region for studying the cortical network is the hippocampal CA1 area, because it has the least heterogeneous neuronal population, the somata and dendrites of principal neurons (pyramidal cells) are arranged into well defined layers and the extrinsic and intrinsic inputs are segregated. The relatively homogeneous pyramidal cell population is supported by a very heterogeneous GABAergic interneuron population, which provides not only general inhibition, but also regulates the precise timing of pyramidal cell activity. Interneurons usually innervate distinct domains of the surface of their target cell. The strategic placement of inhibitory synapses, indicate that GABAergic interneurons belonging to different classes serve distinct functions in the hippocampal network. Neuron types are usually defined according to various morphological, molecular and physiological features. Under typical experimental conditions only some of these parameters are available, therefore an important scientific question is: which partial measures are sufficient for correct recognition of a class of cell. By immunohistochemistry it is possible to stain all neurochemically identical neurons in a given brain region, therefore it is the most widely used method for identifying neuron classes. This review presents the neuron types identified so far in the area CA1 of the rat hippocampus with special emphasis on the immunocytochemical characterization of these cells.