Temporal overlap of excitatory and inhibitory afferent input in guinea-pigCA1 pyramidal cells

1. The temporal interaction of evoked synaptic excitation and GABA(A)-media ted inhibition was examined in CA1 pyramidal cells. Single and paired intra cellular recordings were carried out in pyramidal cell dendrites and somata , and interneurons of the guinea-pig hippocampal slice. Current-clamp, shar p electrode and whole-cell voltage-clamp recordings were made. 2. Kinetics of dendritic and somatic inhibitory responses were similar. Not ably, kinetics of dendritic unitary IPSPs were as fast as kinetics of somat ic unitary IPSPs. 3. GABA(A)-mediated influences were present throughout the orthodromic pyra midal cell EPSP/EPSC. Comparison of the kinetics of pharmacologically isola ted monosynaptic IPSPs, IPSCs and inhibitory conductances (S-GABAA), showed fastest kinetics for g(GABAA). Close temporal overlap was observed between monosynaptic g(GABBA) and the rising phase of the evoked EPSP/EPSC. The on set of g(GABAA) coincided with or preceded onset of the EPBP/EPSC. 4. Onsets of feedforward IPSPs coincided with the rising phase of the pyram idal cell EPSP in > 80% of paired recordings. Fastest feedforward inhibitor y responses exerted near complete overlap with evoked excitation. 5. Onsets of recurrent IPSPs did not occur during the rising phase of the e voked EPSP, but > 3.0 ms after the peak of the pyramidal cell EPSP. 6. Orthodromically evoked interneuron spikes were observed at stimulation i ntensities that were below the threshold for eliciting EPSPs in concomitant ly recorded pyramidal cells. The activation of feedforward inhibitory respo nses by weakest excitatory input, and the large temporal overlap between fe edforward inhibition and evoked excitation, suggest that in situ any excita tory input in CA1. is effectively controlled by fast synaptic inhibition.