Glutamate-mediated synaptic excitation of cortical interneurons

Principal neurons and interneurons are the two main classes of cells in cortical neuronal networks. Principal neurons (granule cells or pyramidal neurons) have transregional axonal projections and release glutamate onto their postsynaptic target cells. In contrast, interneurons have local, but often extensive, axonal arborizations and use γ-aminobutyric acid (GABA) as a transmitter. Although interneurons represent only approximately 10% of the neuronal population, they control the electrical activity of the entire network (FREUND and BUZSÁKI 1996). Interneurons forming inhibitory synapses on the somata or axon initial segments of their postsynaptic target cells are thought to set the threshold of action potential initiation (MILES et al. 1996) and can synchronize the collective activities of large principal neuron ensembles (COBB et al. 1995). In contrast, interneurons establishing inhibitory synapses mainly on dendrites could suppress dendritic Na+ or Ca2+ spikes (BUZSÁKI et al. 1996; MILES et al. 1996) and, thus, regulate plasticity at glutamatergic synapses in the cortex (DAVIES et al.1991).