Firing properties of identified parasympathetic cardiac neurons in nucleus ambiguus

This study tests the hypothesis that identified parasympathetic cardiac neurons in the nucleus ambiguus possess pacemaker-like activity or, alternatively, that these neurons are inherently silent. To test this hypothesis and to examine the firing properties of these neurons, parasympathetic cardiac neurons were identified by the presence of a fluorescent tracer previously applied to their terminals surrounding the heart. Perforated patch-clamp electrophysiological techniques were used to study the spontaneous and depolarization-evoked firing patterns of these identified parasympathetic cardiac neurons in an in vitro brain stem slice. Parasympathetic cardiac neurons were silent. On injection of depolarizing current, however, these neurons fired with both little delay and spike frequency adaptation. Hyperpolarizing prepulses elicited a significant delay before depolarization-evoked firing. The Ca2+-activated K+ channel blocker apamin, but not charybdotoxin, increased the depolarization-activated firing frequency of these neurons and inhibited the afterhyperpolarization. In summary, parasympathetic cardiac neurons do not have pacemaker-like properties, but they do possess discharge characteristics that would enable them to closely follow excitatory synaptic activation for prolonged periods.