VOLTAGE-GATED CURRENTS IN IDENTIFIED PARASYMPATHETIC CARDIAC NEURONS IN THE NUCLEUS AMBIGUUS

Heart rate is normally dominated by the activity of the cardioinhibito ry parasympathetic nervous system, while abnormally low levels of para sympathetic cardiac activity have been implicated in many cardiovascul ar diseases including hypertension, heart failure and sudden cardiac d eath. In this study we have examined the voltage-gated currents in par asympathetic cardiac neurons that were identified with a retrograde fl uorescent tracer in visualized sections (250 mu m) of nucleus ambiguus . Depolarization of parasympathetic cardiac neurons to potentials more positive than -50 mV evoked a rapidly activating and inactivating inw ard current which could be blocked by tetrodotoxin (TTX), although in some neurons up to 10 mu M was required for complete block, The voltag e-dependent inactivation properties of this Na current showed relative ly broad inactivation characteristics, a characteristic of TTS-resista nt Na channels, Depolarization also elicited biphasic outward currents , which were separated into a transient I-A type K current using the s pecific channel antagonist 4-aminopyridine and a long-lasting delayed rectified K current. These voltage-gated Na and K currents define the action potential firing patterns of parasympathetic cardiac neurons, s uch as frequency adaptation and spike delay, and also determine the ac tivity of these neurons in response to depolarizing and hyperpolarizin g synaptic innervation.