PLATEAU POTENTIALS IN CAT NEOCORTICAL ASSOCIATION CELLS IN-VIVO - SYNAPTIC CONTROL OF DENDRITIC EXCITABILITY

The dendrites of neocortical pyramidal cells are bombarded by myriads of synaptic inputs and express active conductances generating prominen t plateau potentials. We have examined in vivo the possibility that sp ontaneous synaptic inputs trigger or terminate plateau potentials afte r blockage of K+ currents. Under barbiturate anaesthesia, pairs of cor tical cells were intracellularly recorded with sharp electrodes from t he cat's association cortex (areas 5-7). In one pyramidal cell, K+ cha nnels were blocked with intracellular Cs+, while in the simultaneously impaled pyramidal cell the K+ conductances were left intact to act as a control; this second cell allowed recognition of spontaneous spindl e-related synaptic activity. Depolarizing current pulses elicited sing le, all-or-none plateau potentials (60-70 mV, 0.1-0.4 a). Plateau pote ntials slowly repolarized towards a break point of fast repolarization around -20 mV. Thalamic-evoked inhibitory postsynaptic potentials con sistently shut off the plateaus. Synchronized spontaneous activity, as occurring during thalamic-generated spindle oscillations, either trig gered or blocked the plateaus. These results suggest that spontaneousl y occurring synaptic activation during synchronized oscillatory states , such as those that occur during sleep spindles in vivo, may exert a strong control over the dendritic excitability in neocortical pyramida l cells.