A CARDIAC-LIKE SODIUM CURRENT IN MOTOR-NEURONS OF A JELLYFISH

1. Whole cell voltage-clamp recordings from isolated swimming motor ne urons (SMNs) reveal a rapidly activating and inactivating sodium curre nt. 2. Permeability ratios of P-Li/P-Na = 0.941 and P-guanidinium/P-Na = 0.124 were measured for the mediating channel, which was impermeabl e to rubidium. 3. The conductance/voltage and steady state inactivatio n curves are shifted in a depolarizing direction by similar to 45 mV r elative to most neuronal sodium currents in higher animals. 4. Activat ion could be fitted with two exponents and maximal current peaked at 0 .74 +/- 0.06 ms (mean +/- SD). 5. Inactivation could be fitted with fa st (tau(1) = 1.91 +/- 0.07 ms at +10 mV) and slow (tau(2) = 11.65 +/- 0.55 ms at +10 mV) exponents. 6. Half-recovery from inactivation occur red slowly (52.6 +/- 2.9 ms). 7. A second class of identifiable neuron s, ''B'' neurons, possesses a distinctly different population of sodiu m channels. They showed different inactivation kinetics and far more r apid recovery from inactivation (half-recovery <5 ms).8. We conclude t hat there was physiological diversification of sodium channels early i n metazoan evolution and that there has been considerable cell-specifi c selection of channel properties.