POTASSIUM CURRENTS CONTRIBUTING TO ACTION-POTENTIAL REPOLARIZATION AND THE AFTERHYPERPOLARIZATION IN RAT VAGAL MOTONEURONS

1. Intracellular recordings were made from neurons in the dorsal motor nucleus of the vagus (DMV) in transverse slices of rat medulla maintained in vitro at 30-degrees-C. Neurons had a resting potential of -59.8 +/- 1.4 (SE) mV (n = 39) and input resistance of 293 +/- 23 MOMEGA (n = 44). 2. Depolarization elicited overshooting action potentials that were blocked by tetrodotoxin (TTX; 1 muM). In the presence of TTX, two types of action potentials having low and high thresholds could be elicited. The action potentials were blocked by cobalt (2 mM) indicating they were mediated by calcium currents. 3. Under voltage clamp, depolarization of the cell from membrane potentials negative of the resting potential activated a transient potassium current. This current was selectively blocked by 4-aminopyridine (4-AP) (5 mM) and catechol (5 mM) indicating that it is an A-type current. This current inactivated with a time constant of 420 ms and recovered from inactivation with a time constant of 26 ms. 4. When calcium currents were blocked by cadmium or cobalt, the rate of action potential repolarization was slower. In the presence of tetraethylammonium (TEA; 200-400 muM) or charybdotoxin (CTX; 30 nM) a small "hump" appeared on the repolarizing phase of the action potential that was abolished by addition of cadmium. These results indicate that a calcium-activated potassium current (I(C)) contributes to action potential repolarization. 5. Actions potentials elicited from hyperpolarized membrane potentials repolarized faster than those elicited from resting membrane potential. This effect could be blocked by catechol, indicating that voltage-dependent potassium currents (I(A)) can also contribute to action-potential repolarization. In the presence of catechol and calcium channel blockers, action potentials still had a significant early after hyperpolarization suggesting that another calcium independent outward current is also active during repolarization. This fast after hyperpolarizations (AHP) was not blocked by TEA. 6. Action potentials were followed by prolonged AHPs, which had two phases. The initial part of the AHP was blocked by apamin (100 nM) indicating that it results from activation of SK type calcium-activated potassium channels. The slow phase was selectively blocked by catechol suggesting that it is due to activation of I(A). 7. It is concluded that a TTX-sensitive sodium current and two calcium currents contribute to the action potential in rat DMV neurons. At least three different currents contribute to action-potential repolarization: I(C), I(A), and a third unidentified calcium-insensitive outward current. The long AHP results from the sequential activation of apamin sensitive calcium-activated potassium channels and a voltage-activated A-current.