Voltage gated currents in vagal cardiac neurons in the nucleus ambiguus
A fundamental question concerning the origin of parasympathetic cardiac activity is whether vagal cardiac neurons in the nucleus ambiguus (NA) possess pacemaker-like activity and can fire spontaneously, or alternatively, whether these neurons are normally silent and depend on excitatory synaptic activity to initiate and maintain their activity. To test this hypothesis cell attached, whole cell and perforated patch clamp electrophysiological techniques were used to study the spontaneous firing properties and \oltage-gated currents in identified preganglionic parasympathetic cardiac neurons. The soma of these neurons were visualized in an in-vitro brainstem slice and identified by the presence of a fluorescent tracer that had been previously applied to their peripheral terminals surrounding the heart. DIC optics, infrared illumination and detection cameras were used to guide and position the patch pipette onto the surface of the identified neuron. All parasympathetic cardiac neurons tested for activity in the cell-attached configuration (n>35) were silent and did not demonstrate any spontaneous firing. Furthermore when intracellular access was obtained, no spontaneous firing was ever observed. However, when the neuron was depolarized to voltages more positive than -50 mV, inward and outward voltage gated currents were elicited. The inward currents in these neurons were comprised of both a tetrodotoxin-sensitive sodium (Na) current and a voltage gated calcium (Ca) current. The outward currents in these neurons are due to activation of both a transient potassium (K) current that is blocked by 4-aminopyridine (4-AP) and a long lasting component that could be inhibited by tetraethylammonium (TEA). In conclusion, these results demonstrate that parasympathetic cardiac neurons in the NA do not possess spontaneous discharge characteristics. However these neurons do possess voltage-gated inward Na and Ca currents, as well as two types of voltage gated K channels.
Mendelowitz, D., Neff, R., & Mihalevich, M. (1996). Voltage gated currents in vagal cardiac neurons in the nucleus ambiguus. FASEB Journal, 10 (3). Retrieved from https://hsrc.himmelfarb.gwu.edu/smhs_pharm_facpubs/861