INHIBITORY CONTROL OF EXCITABLE DENDRITES IN NEOCORTEX

1. Many dendrites of pyramidal cells in mature neocortex express activ e Na+ and Ca2+ conductances. Dendrites are also the target of numerous inhibitory synapses. We examined the interactions between the intrins ic excitability of dendrites and synaptic inhibition using whole cell recordings from the apical dendrites of layer 5 pyramidal cells. Exper iments were performed on slices of somatosensory cortex from mature ra ts. Slices were bathed in the glutamate receptor antagonists 2-amino-5 -phosphonopentanoic acid and 6,7-dinitroquinoxaline-2,3-dione, and mai ntained at 32-36 degrees C. 2. In agreement with previous findings, in tradendritic current injection evoked two distinct types of dendritic firing. Type I dendrites generated monophasic fast spikes, whereas typ e II dendrites showed more complex firing patterns, consisting of fast and slow spike components. 3. Stimulation of cortical layers 2/3 evok ed fast inhibitory postsynaptic potentials (IPSPs) in all dendrites te sted. IPSP reversal potentials were bimodally distributed, with means of about -53 and -85 mV when recorded with high-Cl- -concentration-fil led electrodes. Interestingly, IPSP reversal potentials were correlate d with the type of dendritic spiking pattern. 4. IPSPs were able to de lay, completely block, or partially block spiking in dendrites, depend ing on the relative timing between inhibition and dendritic spiking. S low, Ca2+-dependent spike components could be blocked selectively by I PSPs. Furthermore, inhibition could either phase advance or phase dela y repetitive patterns of dendritic spiking, depending on the timing of the IPSP.