Sodium-Mediated Plateau Potentials in Lumbar Motoneurons of Neonatal Rats

The development and the ionic nature of bistable behavior in lumbar motoneurons were investigated in rats. One week after birth, almost all (∼80%) ankle extensor motoneurons recorded in whole-cell configuration displayed self-sustained spiking in response to a brief depolarization that emerged when the temperature was raised >30°C. The effect of L-type Ca2+ channel blockers on self-sustained spiking was variable, whereas blockade of the persistent sodium current (INaP) abolished them. When hyperpolarized, bistable motoneurons displayed a characteristic slow afterdepolarization (sADP). The sADPs generated by repeated depolarizing pulses summed to promote a plateau potential. The sADP was tightly associated with the emergence of Ca2+ spikes. Substitution of extracellular Na+ or chelation of intracellular Ca2+ abolished both sADP and the plateau potential without affecting Ca2+ spikes. These data suggest a key role of a Ca2+-activated nonselective cation conductance (ICaN) in generating the plateau potential. In line with this, the blockade of ICaN by flufenamate abolished both sADP and plateau potentials. Furthermore, 2-aminoethoxydiphenyl borate (2-APB), a common activator of thermo-sensitive vanilloid transient receptor potential (TRPV) cation channels, promoted the sADP. Among TRPV channels, only the selective activation of TRPV2 channels by probenecid promoted the sADP to generate a plateau potential. To conclude, bistable behaviors are, to a large extent, determined by the interplay between three currents: L-type ICa, INaP, and a Na+-mediated ICaN flowing through putative TRPV2 channels.