INHIBITORY CONTROL OF EXCITABLE DENDRITES IN NEOCORTEX

1. Many dendrites of pyramidal cells in mature neocortex express active Na+ and Ca2+ conductances. Dendrites are also the target of numerous inhibitory synapses. We examined the interactions between the intrinsic excitability of dendrites and synaptic inhibition using whole cell recordings from the apical dendrites of layer 5 pyramidal cells. Experiments were performed on slices of somatosensory cortex from mature rats. Slices were bathed in the glutamate receptor antagonists 2-amino-5-phosphonopentanoic acid and 6,7-dinitroquinoxaline-2,3-dione, and maintained at 32-36 degrees C. 2. In agreement with previous findings, intradendritic current injection evoked two distinct types of dendritic firing. Type I dendrites generated monophasic fast spikes, whereas type II dendrites showed more complex firing patterns, consisting of fast and slow spike components. 3. Stimulation of cortical layers 2/3 evoked fast inhibitory postsynaptic potentials (IPSPs) in all dendrites tested. IPSP reversal potentials were bimodally distributed, with means of about -53 and -85 mV when recorded with high-Cl- -concentration-filled electrodes. Interestingly, IPSP reversal potentials were correlated with the type of dendritic spiking pattern. 4. IPSPs were able to delay, completely block, or partially block spiking in dendrites, depending on the relative timing between inhibition and dendritic spiking. Slow, Ca2+-dependent spike components could be blocked selectively by IPSPs. Furthermore, inhibition could either phase advance or phase delay repetitive patterns of dendritic spiking, depending on the timing of the IPSP.