CALCIUM-DEPENDENT SPIKE-FREQUENCY ACCOMMODATION IN HIPPOCAMPAL CA3 NONPYRAMIDAL NEURONS

Interneurons of the hippocampal formation are traditionally identified electrophysiologically as those cells that fire trains of weakly acco mmodating action potentials in response to depolarizing current inject ion. We studied the firing properties of nonpyramidal neurons in the f ive substrata of the CA3b region of hippocampus. With the use of whole cell recording methods we found that nonpyramidal neurons fired in a range from weak to strong spike-frequency accommodation (SFA) that was calcium dependent. Slow afterhyperpolarizations were not associated w ith strong SFA. In addition a subset of interneurons (similar to 20%) fired with an irregular firing pattern that was generally calcium inde pendent. These results suggest a calcium-dependent mechanism for SFA i n nonpyramidal neurons that is distinct from pyramidal cells and furth er demonstrates the heterogeneity of hippocampal interneurons.