The timing and location of action potential initiation in layer V pyramidal cells of the rat somatosensory cortex depends on different ligand-gated and intrinsic conductances. In order to assess the shunting effect of GABAA receptor activation and Ih on the summative properties of these neurons, we recorded simultaneously from their somata and dendrites and induced combined excitatory and inhibitory postsynaptic potentials using extracellular stimulation. Three major consequences of GABAA receptor activation on the integration properties were found: (i) the action potential could be initiated in the distal apical dendrite, if GABAA receptors were simultaneously activated in the soma; (ii) GABAA receptor activation narrowed the time window for suprathreshold summation of two coincident excitatory synaptic inputs by a factor of 3.4; and (iii) uneven weights of the GABAergic components between two inputs could lead to unbalanced shunting and consequently to a time shift in the peak of the coincidence detection window. Blockade of Ih practically abolished coincidence detection within a time window of 30 ms. Location of spike initiation and summation of coincident inputs in pyramidal cells is determined by at least two shunting mechanisms: activation of the presynaptic inhibitory network and modulation of Ih as a function of the postsynaptic membrane potentia
