Synaptically-evoked field responses were elicited by stimulation of the lateral olfactory tract of rat olfactory cortex slices maintained in vitro. 2. Various concentrations of lignocaine (5-500 microM) were applied to the solution bathing the slices. These produced dose-dependent depressions of the synaptically-evoked potential over the concentration range 20-500 microM. The responses completely recovered on washing out the lignocaine. Similar depressions were also noted for procaine (100-1000 microM). 3. In the 47 slices tested, application of beta-phorbol 12,13-dibutyrate (1 microM) increased the amplitude of the synaptic response (from 0.99 +/- 0.05 to 1.36 +/- 0.06 mV). beta-Phorbol 13-monbutyrate (1 microM) had no effect. 4. In the presence of phorbol dibutyrate the depressant effect of lignocaine was increased: the EC50 changed from 91 +/- 10 to 24 +/- 2 microM (a mean potency increase of 3.47 +/- 0.14). A similar increase in potency for procaine was observed with phorbol dibutyrate (from 264 +/- 23 to 49 +/- 9 microM: a 5.49 +/- 0.82 increase in potency). If the tissue was pre-equilibrated in a concentration of lignocaine which produced a 60-80% depression, addition of phorbol ester caused a complete abolition of the evoked potential. 5. beta-Phorbol 13-monobutyrate (1 microM) had no effect on the potency of lignocaine. 6. The Na and K currents generating the action potential in the presynaptic nerve terminals were unaffected by phorbol dibutyrate. The depressant effect of lignocaine on these currents was not modified by phorbol dibutyrate. The depressant effect of lignocaine on these currents was not modified by phorbol dibutyrate. 7. The potentiation of lignocaine could not be accounted for by membrane depolarization or by nonspecific actions of phorbol dibutyrate, and was distinct from the action on transmitter release. Therefore, it seems likely that protein kinase C activation was responsible for the modified action of lignocaine, although the mechanism for this is unclear.
