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Department of Neuroscience, Brown University, Providence, RI 02912, USA.

1. Inhibitory neuron-pyramidal cell interactions were investigated in slices of rat somatosensory cortex in which excitatory synaptic transmission was blocked with bath-applied glutamate receptor antagonists. Local inhibitory neurons were excited by focal pressure ejections of small (approximately 40 pl) volumes of 1-10 mM acetylcholine. 2. The frequency of inhibitory postsynaptic potentials (IPSPs) ("responses per trial' or R/T) declined as the stimulation distance was increased. Inhibitory inputs were most prevalent in layer II/III regular spiking (RS) pyramidal neurons (30 cells) where median R/T was 0.020. In layer V, the median R/T was 0.024 for RS neurons (25 cells), but significantly lower for burst-firing (IB) neurons (17 cells), where median R/T was 0.007 (P = 0.039). 3. IPSPs in individual layer V pyramidal cells were recorded with CsCl electrodes. In eight neurons, spontaneous picrotoxin-sensitive IPSPs were recorded and found to display a wide range of 10-90% rise times (1-34 ms), not correlated with amplitude (0.2-18 mV). For a further ten pyramidal neurons, extracellular stimulating electrodes were placed simultaneously in layers II/III and V/VI in order to evoke pairs of IPSPs whose waveforms were averaged and compared. In seven cells, IPSPs evoked from layer II/III (distal location) had longer 10-90% rise times than IPSPs evoked from layer V/VI stimulating electrodes (proximal location). In addition, "proximal' IPSPs could always be reversed by membrane depolarization whereas "distal' ones could not (n = 4/4). 4. This study showed that pyramid cell-inhibitory neuron interconnections are extensive but their spatial organization varies with cell class and with cortical layer. In addition, pyramidal neurons can receive inhibitory inputs from locations on their apical dendrites.