DIFFERENTIAL IMPACT OF MINIATURE SYNAPTIC POTENTIALS ON THE SOMA AND DENDRITES OF PYRAMIDAL NEURONS IN-VIVO

We studied the impact of transmitter release resistant to tetrodotoxin (TTX) in morphologically identified neocortical pyramidal neurons rec orded intracellularly in barbiturate-anesthetized cats. It was observe d that TTX-resistant release occurs in pyramidal neurons in vivo and a t much higher frequencies than was previously reported in vitro. Furth er, in agreement with previous findings indicating that GABAergic and glutamatergic synapses are differentially distributed in the somata an d dendrites of pyramidal cells, we found that most miniature synaptic potentials were sensitive to gamma-aminobutyric acid-A (GABA(A)) or lp ha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid(AMPA)antagonists in presumed somatic and dendritic impalements, respectively. Pharmaco logical blockage of spontaneous synaptic events produced large increas es in input resistance that were more important in dendritic (approxim ate to 50%) than somatic (approximate to 10%) impalements. These findi ngs imply that in the intact brain, pyramidal neurons are submitted to an intense spike-independent synaptic bombardment that decreases the space constant of the cells. These results should be taken into accoun t when extrapolating in vitro findings to intact brains.