Patch-clamp study of the properties of the sodium current in single isolated adult aminergic neurons

The properties of the fast Na+ inward current of the aminergic neurosecretory dorsal unpaired median (DUM) cells isolated from the sixth abdominal ganglion of the cockroach Periplaneta americana were studied with the whole-cell clamp technique in the presence of Ca2+ and K+ channel blockers. In about 80 % of the cells, the current activated at-35 mV, was maximal at- lOmV and reversed at +48mV (Vrev), very close to VNa, the equilibrium potential for Na+ (+47.9mV). Vrev followed VNa when the external Na+ concentration was varied and the current was entirely suppressed by 1.0xlO~7moll~1 saxitoxin (STX), indicating that it was carried by Na+. In the remaining cells, an STX-sensitive maintained current was observed, the peak current-voltage relationship having almost the same characteristics except that an additional small shoulder was present between —90 and —35 mV, suggesting the existence of two types of Na+ channels. Na+ channels were half-inactivated at —41.1 mV and half-activated at — 25.8mV. The overlap of inactivation (hoo) and activation (mx) parameters was important (—70 to + 10mV). hx was not well fitted by a single Boltzmann distribution for potentials more negative than — 70 mV, indicating more than one process of inactivation. The half-times of activation and of inactivation of the Na+ current were voltage-dependent, strongly decreasing with increasing pulse amplitude. The time courses of both inactivation and recovery from inactivation were best described by the sum of two exponentials. The two components of inactivation did not contribute equally to the decay of the current; the fast component accounting for more than 90 % of the inactivation at positive potentials. Taken together, the results suggest that the Na+ channels of DUM neurones have more than one open state and/or more than one inactivated state