ACTIVATORS OF PROTEIN-KINASE-C MIMIC SEROTONIN-INDUCED MODULATION OF A VOLTAGE-DEPENDENT POTASSIUM CURRENT IN PLEURAL SENSORY NEURONS OF APLYSIA

1. In the pleural mechanoafferent sensory neurons of Aplysia, serotonin (5-HT)-induced spike broadening consists of at least two components: a cAMP and protein kinase A (PKA)-dependent, rapidly developing component and a protein kinase C (PKC)-dependent, slowly developing component. Voltage-clamp experiments were conducted to identify currents that are modulated by PKC and thus may contribute to the slowly developing component of 5-HT-induced spike broadening. 2. We compared the effects of phorbol esters, activators of PKC, on membrane currents with those of 5-HT. Bath application of 5-HT had complex modulatory effects on currents elicited by voltage-clamp pulses to potentials >0 mV. The kinetics of both activation and inactivation of the membrane currents were slowed by 5-HT. This led to a decrease in an outward current at the beginning of the voltage-clamp pulse and an increase at the end of the pulse. Previous work has shown that these effects represent, in part, the modulation of a large, voltage-dependent K+ current (I-K,I-V) by 5-HT. 3. Active phorbol esters mimicked some of the actions of 5-HT on membrane currents in that they slowed activation and inactivation kinetics of current responses to voltage-clamp pulses more positive than 0 mV. This led to a decrease in an outward current at the beginning of the pulse and an increase at the end of the pulse. Because inactive phorbols did not mimic the actions of 5-HT, the effects of active phorbol esters appeared to be PKC specific. In addition, preexposure of the sensory neurons to active phorbol esters appeared to occlude the modulatory actions of 5-HT on I-K,I-V. Thus it is likely that modulation of I-K,I-V, by 5-HT is mediated, at lease in part, by PKC. 4. To further characterize which currents were modulated by PKC, low concentrations of tetraethylammonium (TEA, 2 mM) were used to block Ca2+-activated K+ current (I-K,I-Ca). Low TEA partially blocked the phorbol ester-induced increase of the outward current at the end of voltage-clamp pulses. These results agreed with previous reports that activation of PKC enhanced a fast component of I-K,I-Ca in these sensory neurons. Such an enhance ment would lead to an increase in outward current that should be blocked by low TEA. Low TEA, however, did not affect phorbol ester-induced decrease of the outward current at the beginning of pulse, where the predominant current is I-K,I-V, which is less sensitive to TEA. Nor did low TEA block phorbol ester-induced spike broadening. Thus modulation of I-K,I-V appears to contribute to the PKC-mediated component of 5-HT-induced spike broadening. 5. On the basis of our results and those of others, a scheme is proposed in which the modulatory actions of 5-HT are mediated via two protein kinase systems (the PKC and PKA pathways) that converge and diverge to affect various ionic conductances.