LAMINAR ORIGINS OF INHIBITORY SYNAPTIC INPUTS TO PYRAMIDAL NEURONS OFTHE RAT NEOCORTEX

1. Inhibitory neuron-pyramidal cell interactions were investigated in slices of rat somatosensory cortex in which excitatory synaptic transm ission was blocked with bath-applied glutamate receptor antagonists. L ocal inhibitory neurons were excited by focal pressure ejections of sm all (similar to 40 pl) volumes of 1-10 mM acetylcholine. 2. The freque ncy of inhibitory postsynaptic potentials (IPSPs) ('responses per tria l' or R/T) declined as the stimulation distance was increased. Inhibit ory inputs were most prevalent in layer II/III regular spiking (RS) py ramidal 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 low er for burst-firing (IB) neurons (17 cells), where median R/T was 0.00 7 (P = 0.039). 3. IPSPs in individual layer V pyramidal cells were rec orded with CsCl electrodes. In eight neurons, spontaneous picrotoxin-s ensitive IPSPs were recorded and found to display a wide range of 10-9 0% rise times (1-34 ms), not correlated with amplitude (0.2-18 mV). Fo r a further ten pyramidal neurons, extracellular stimulating electrode s were placed simultaneously in layers II/III and V/VI in order to evo ke 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 electrod es (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 inte rconnections 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.