FUNCTIONAL AND MOLECULAR DIFFERENCES BETWEEN VOLTAGE-GATED K-SPIKING INTERNEURONS AND PYRAMIDAL NEURONS OF RAT HIPPOCAMPUS( CHANNELS OF FAST)

We have examined gating and pharmacological characteristics of somatic K+ channels in fast-spiking interneurons and regularly spiking princi pal neurons of hippocampal slices. In nucleated patches isolated from basket cells of the dentate gyrus, a fast delayed rectifier K+ current component that was highly sensitive to tetraethylammonium (TEA) and 4 -aminopyridine (4-AP) (half-maximal inhibitory concentrations <0.1 mM) pre-dominated, contributing an average of 58% to the total K+ current in these cells. By contrast, in pyramidal neurons of the CA1 region a rapidly inactivating A-type K+ current component that was TEA-resista nt prevailed, contributing 61% to the total K+ current. Both types of neurons also showed small amounts of the K+ current component mainly f ound in the other type of neuron and, in addition, a slow delayed rect ifier K+ current component with intermediate properties (slow inactiva tion, intermediate sensitivity to TEA). Single-cell RT-PCR analysis of mRNA revealed that Kv3 (Kv3.1, Kv3.2) subunit transcripts were expres sed in almost all (89%) of the interneurons but only in 17% of the pyr amidal neurons. In contrast, Kv4 (Kv4.2, Kv4.3) subunit mRNAs were pre sent in 87% of pyramidal neurons but only in 55% of interneurons. Sele ctive block of fast delayed rectifier K+ channels, presumably assemble d from Kv3 subunits, by 4-AP reduced substantially the action potentia l frequency in interneurons. These results indicate that the different ial expression of Kv3 and Kv4 subunits shapes the action potential phe notypes of principal neurons and interneurons in the cortex.