INTERNEURONS OF THE DENTATE-HILUS BORDER OF THE RAT DENTATE GYRUS - MORPHOLOGICAL AND ELECTROPHYSIOLOGICAL HETEROGENEITY

Interneurons located near the border of the dentate granule cell layer and the hilus were studied in hippocampal slices using whole-cell cur rent clamp and biocytin staining. Because these interneurons exhibit b oth morphological and electrophysiological diversity, we asked whether passive electrotonic parameters or repetitive firing behavior correla ted with axonal distribution. Each interneuron was distinguished by a preferred axonal distribution in the molecular layer or granule cell l ayer, and four groups could be discerned, the axons of which arborized in (1) the granule cell layer, (2) the inner molecular layer, (3) the outer molecular layer, and (4) diffusely in the molecular layer. In o ur sample, interneurons with axons arborizing diffusely in the molecul ar layer were most frequent, and those with axons restricted to the gr anule cell layer were least frequent. Resting potential, input resista nce, time constant, electrotonic length, and spike frequency adaptatio n (SFA) were not significantly different among the four groups, and th e variability in SFA between cells with similar axonal distributions w as striking. Clear differences in action potential morphology and afte rhyperpolarizations, however, emerged when nonadapting interneurons we re compared with those exhibiting SFA. Interneurons exhibiting SFA had characteristically broader spikes, progressive slowing of action pote ntial repolarization during repetitive firing, and slow afterhyperpola rizations that distinguished them from nonadapting interneurons. We pr opose that the variability in repetitive firing behavior and morpholog y exhibited by each of these interneurons makes each interneuron uniqu e and may provide a high level of fine tuning of inhibitory control cr itical to information processing in the dentate.