Calcium-dependent plateau potentials in rostral ambiguus neurons in the newborn mouse brain stem in vitro

The nucleus ambiguus contains vagal and glossopharyngeal motoneurons and preganglionic neurons involved in respiration, swallowing, vocalization, and control of heart beat. Here we show that the rostral compact formation's ambiguus neurons, which control the esophageal phase of swallowing, display calcium-dependent plateau potentials in response to tetanic orthodromic stimulation or current injection. Whole cell recordings were made from visualized neurons in the rostral nucleus ambiguus using a slice preparation from the newborn mouse. Biocytin-labeling revealed dendritic trees with pronounced rostrocaudal orientations confined to the nucleus ambiguus, a morphological profile matching that of vagal motoneurons projecting to the esophagus. Single-stimulus orthodromic activation, using an electrode placed in the dorsomedial slice near the nucleus tractus solitarius, evoked single excitatory postsynaptic potentials (EPSPs) or short trains of EPSPs (500 ms to 1 s). However, tetanic stimulation (5 pulses, 10 Hz) induced Voltage dependent afterdepolarizations or long-lasting plateau potentials (>1 min) with a constant firing pattern. Depolarizing or hyperpolarizing current pulses elicited voltage-dependent afterdepolarizations or plateau potentials lasting a few seconds to several minutes. Constant spike activity accompanied the long-lasting plateau potentials, which ended spontaneously or could be terminated by weak hyperpolarizing current pulses. Current-induced afterdepolarizations and plateau potentials were dependent on extracellular and intracellular Ca2+, as they were blocked completely by extracellular Co2+, Cd2+, or intracellular bis-(o-aminophenoxy)N, N, N', N'-tetraacetic acid (BAPTA). Orthodromically induced afterdepolarizations and plateau potentials were blocked by intracellular BAPTA. Afterdepolarizations and plateau potentials were completely blocked by substitution of extracellular Na+ with choline. Afterdepolarizations persisted in tetrodotoxin. We conclude that rostral ambiguus neurons have a Ca2+-activated inward current carried by Na+. Synaptic activation of this conductance may generate prolonged spike activity in these neurons during the esophageal phase of swallowing.