CALCIUM-DEPENDENT PLATEAU POTENTIALS IN ROSTRAL AMBIGUUS NEURONS IN THE NEWBORN MOUSE-BRAIN STEM IN-VITRO

The nucleus ambiguus contains vagal and glossopharyngeal motoneurons a nd preganglionic neurons involved in respiration, swallowing, vocaliza tion, and control of heart beat. Here we show that the rostral compact formation's ambiguus neurons, which control the esophageal phase of s wallowing, display calcium-dependent plateau potentials in response to tetanic orthodromic stimulation or current injection. Whole cell reco rdings were made from visualized neurons in the rostral nucleus ambigu us using a slice preparation from the newborn mouse. Biocytin-labeling revealed dendritic trees with pronounced rostrocaudal orientations co nfined to the nucleus ambiguus, a morphological profile matching that of vagal motoneurons projecting to the esophagus. Single-stimulus orth odromic activation, using an electrode placed in the dorsomedial slice near the nucleus tractus solitarius, evoked single excitatory postsyn aptic potentials (EPSPs) or short trains of EPSPs (500 ms to 1 s). How ever, 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 pul ses elicited voltage-dependent afterdepolarizations or plateau potenti als lasting a few seconds to several minutes. Constant spike activity accompanied the long-lasting plateau potentials, which ended spontaneo usly or could be terminated by weak hyperpolarizing current pulses. Cu rrent-induced afterdepolarizations and plateau potentials were depende nt on extracellular and intracellular Ca2+, as they were blocked compl etely by extracellular Co2+, Cd2+, or intracellular bis-(o-aminophenox y)N, N, N', N'-tetraacetic acid (BAPTA). Orthodromically induced after depolarizations and plateau potentials were blocked by intracellular B APTA. Afterdepolarizations and plateau potentials were completely bloc ked by substitution of extracellular Na+ with choline. Afterdepolariza tions persisted in tetrodotoxin. We conclude that rostral ambiguus neu rons have a Ca2+-activated inward current carried by Na+. Synaptic act ivation of this conductance may generate prolonged spike activity in t hese neurons during the esophageal phase of swallowing.