Calcium- and sodium-dependent action potentials of mouse spinal cord and dorsal root ganglion neurons in cell culture.

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Abstract
Resting membrane properties and action potentials were recorded from somata of mouse spinal cord (SC) and dorsal root ganglion (DRG) neurons in primary dissociated cell culture. To determine the role of Na+, Ca2+ and K+ conductances, recordings were made following addition of blockers of specific ion channels to or removal of single ionic species from the bathing medium. Input resistance (Rin) of SC neurons was unaltered in Ca2+-free medium but was increased in Na+-free medium. Addition of tetraethylammonium (TEA) to Na+-free medium increased Rin. In DRG neurons, Rin was unaltered in Na+-free medium but was reduced in Ca2+-free medium. Addition of TEA to Na+-free medium increased Rin. SC and DRG neurons had significant resting Na+ conductance and TEA-sensitive K+ conductance. In DRG, but not SC neurons, Ca2+ reduced a resting conductance (probably Na+). Action potentials recorded from the somata of SC neurons could not be evoked in the majority of neurons following addition of the Na+-channel blocker tetrodotoxin (TTX) or in a Na+-free bathing medium with or without TTX. Removal of Ca2+ from the bathing medium increased action-potential duration but did not alter .ovrhdot.Vmax. Action potentials of SC neurons in cell culture were Na+-dependent and TTX-sensitive but not Ca2+-dependent. Action potentials were recorded from the somata of DRG neurons. In control and Na+-free media action potentials were TTX-insensitive, but in Ca2+-free medium TTX reduced .ovrhdot.Vmax. Action potentials in DRG neurons in cell culture have 3 types of voltage-dependent cation channels; fast TTX-sensitive Na+ channels, fast TTX-insensitive Na+ channels and slow Ca2+ channels. Conductance of TTX-sensitive but not TTX-insensitive Na+ channels was reduced by intracellular Ca2+. After addition of 100 mM TEA to Na+-free medium, action potentials in SC and DRG neurons had the following properties: amplitude and duration increased following Ca2+ addition; amplitude and duration decreased following addition of the Ca2+-channel blocker Mn; overshoot was proportional to the logarithm of extracellular Ca2+ concentration; and .ovrhdot.Vmax varied with the extracellular Ca2+ concentration. Action potentials in Na+-free medium containing TEA were Ca2+-dependent, while TEA blocked a K+ conductance in both cell types. The potential significance of Ca2+-dependent action potentials in SC and DRG neurons is discussed.