Astman, Nadav, Michael J. Gutnick, and Ilya A. Fleidervish. Activation of protein kinase C increases neuronal excitability by regulating persistent Na+ current in mouse neocortical slices. J. Neurophysiol. 80: 1547–1551, 1998. Effects of the protein kinase C activating phorbol ester, phorbol 12-myristate 13-acetate (PMA), were studied in whole cell recordings from layer V neurons in slices of mouse somatosensory neocortex. PMA was applied intracellularly (100 nM to 1 μM) to restrict its action to the cell under study. In current-clamp recordings, it enhanced neuronal excitability by inducing a 10- to 20-mV decrease in voltage threshold for action-potential generation. Because spike threshold in neocortical neurons critically depends on the properties of persistent Na+ current ( I NaP), effects of PMA on this current were studied in voltage clamp. After blocking K+ and Ca2+ currents, I NaP was revealed by applying slow depolarizing voltage ramps from −70 to 0 mV. Intracellular PMA induced a decrease in I NaP at very depolarized membrane potentials. It also shifted activation of I NaP in the hyperpolarizing direction, however, such that there was a significant increase in persistent inward current at potentials more negative than −45 mV. When tetrodotoxin (TTX) was added to the bath, blocking I NaP and leaving only an outward nonspecific cationic current ( I cat), PMA had no apparent effect on responses to voltage ramps. Thus PMA did not affect I cat, and it did not induce any additional current. Intracellular application of the inactive PMA analogue, 4α-PMA, did not affect I NaP. The specific protein kinase C inhibitors, chelerythrine (20 μM) and calphostin C (10 μM), blocked the effect of PMA on I NaP. The data suggest that PMA enhances neuronal excitability via a protein kinase C–mediated increase in I NaP at functionally critical subthreshold voltages. This novel effect would modulate all neuronal functions that are influenced by I NaP, including synaptic integration and active backpropagation of action potential from the soma into the dendrites.