Phorbol esters, protein phosphorylation and the regulation of neuronal ion channels

Protein kinase C is an enzyme whose activity is modulated by its lipid environment and which is fully activated by diacylglycerol in the presence of phosphatidyl serine and calcium ions. This kinase is highly enriched in the nervous systems of both vertebrates and invertebrates. The activity of protein kinase C can be stimulated in intact cells by certain synthetic diacylglycerols as well as by phorbol esters which substitute for endogenous diacylglycerol. The effects of such activators on the endogenous electrical properties of neurones, as well as on synaptic transmission, have recently been investigated in several vertebrate and invertebrate preparations of neurones. One example is that of the bag cell neurones of Aplysia which, in response to brief stimulation, generate a prolonged discharge during which the height of their action potentials is increased. Exposure of isolated bag cell neurones to activators of protein kinase C results in the enhancement of their action potentials through an increase in the amplitude of their voltage-dependent calcium current. This is caused by the unmasking of a previously inactive species of calcium channel in the plasma membrane.