Activation of protein kinase C induces long-term changes of postsynaptic currents in neocortical neurons

Intracellularly injected phorbol 12,13-dibutyrate (PdiB), a phorbol ester that activates protein kinase C (PKC), altered the postsynaptic responses of neurons of the motor cortex of cats. PdiB increased the amplitudes and durations of EPSPs and decreased the amplitudes and durations of IPSPs elicited by stimulation of the ventrolateral (VL) thalamus or the pyramidal tract (PT). The changes lasted for 50 min or longer. Corresponding changes in peak excitatory and inhibitory postsynaptic currents (EPSCs, IPSCs) were measured directly with the single electrode voltage clamp technique. Quantitative analysis of EPSCs in response to VL thalamic stimulation and IPSCs in response to PT stimulation made in a subgroup of fast PT cells suggested that PdiB acted within the injected neuron rather than presynaptically to alter the synaptic currents. No consistent changes in resting membrane parameters that would account for these modifications were found. Control injections of a phorbol ester that did not activate PKC failed to induce changes in synaptic responses or resting membrane properties. These observations suggest that activation of PKC, in vivo, can induce long-lasting changes in synaptic responses of neocortical neurons by direct modification of postsynaptic ion channel conductivities.