Properties of bipolar VIPergic interneurons and their excitation by pyramidal neurons in the rat neocortex

In the rat neocortex, a subset of GABAergic interneurons express the neurop eptide vasoactive intestinal peptide (VIP). Previously, we demonstrated tha t a population of VIPergic interneurons could be accurately identified by t heir irregular spiking (IS) pattern and their bipolar morphology. IS intern eurons were studied in neocortical slices from 16-22-day-old rats using who le-cell recordings, intracellular labelling and single-cell RT-PCR. In resp onse to a depolarizing pulse, IS interneurons typically discharged a burst of action potentials followed by spikes emitted at an irregular frequency. Several seconds of depolarization, micromolar concentrations of 4-aminopyri dine, and nanomolar concentrations of either dendrotoxin I or K converted t his irregular pattern to a sustained discharge, suggesting the involvement of an I-D-like K+ current. The main glutamate receptor subunits detected in IS cells were GluR1 flop and GluR2 flop, GluR5 and GluR6, and NR2B and NR2 D for the alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA), kainate and N-methyl-D-aspartic acid (NMDA) subtypes, respectively. Paired whole-cell patch-clamp recordings indicated that pyramidal neurons provide intracortical glutamatergic inputs onto IS interneurons, Most connections had high probabilities of response and exhibited frequency-dependent paired pulse depression. Comparison of the amplitude distribution of paired respo nses suggested that most of these connections consisted of multiple functio nal release sites. Finally, two discrete subpopulations of IS cells could b e identified based on the duration of the initial burst of action potential s and the differential expression of calretinin and choline acetyltransfera se.