GABA(A) RECEPTOR-MEDIATED FEEDBACK INHIBITION IN PYRAMIDAL NEURONS OF CAT MOTOR CORTEX

Feedback or recurrent inhibition is generally proposed as a basic component of circuit organization in the brain. However, there is little direct evidence for its existence in the cortex, nor for the nature of the neurotransmitters and receptors involved. Here we address this issue by analyzing the potentials following a single action potential in cat neocortical pyramidal neurons. Using 3 M KCl-filled electrodes, we observed a distinct depolarizing potential that was superimposed on the spike after-potentials in seven out of 81 recorded neurons. This depolarizing potential had a latency of 4.4 +/- 1.6 ms and an amplitude of 4.8 +/- 2.1 mV. Its short risetime (1.39 +/- 0.27 ms) and the resemblance between its falling phase and the passive decay of membrane potential suggested that the depolarizing potential originated from the cell body or proximal dendrites. This potential was blocked by bicuculline methiodide (10 mu M), but not by phaclofen (0.2mM), suggesting that it was a reversed recurrent IPSP mediated by GABA(A) receptors.