DEPOLARIZATION-INDUCED FACILITATION OF A PLATEAU-GENERATING CURRENT IN VENTRAL HORN NEURONS IN THE TURTLE SPINAL-CORD

Plasticity at the neuronal level commonly involves use-dependent chang es in strength of particular synaptic pathways or regulation of postsy naptic properties by modulatory transmitters. Here we analyze a novel form of short-term plasticity mediated by use-dependent facilitation o f postsynaptic responsiveness. Using current-and voltage-clamp recordi ngs, we found that all spinal ventral horn neurons able to generate pl ateau potentials showed depolarization-induced facilitation of the und erlying inward current. Facilitation was noticeable when the neurons w ere depolarized to more than -50 mV at intervals <4 s. When stimulatio n with fast triangular voltage ramps was used, the inward current acti vated at a less depolarized potential during the second ramp. The inwa rd current and facilitation was eliminated by nifedipine, a selective antagonist of L-type calcium channels. Depolarization-induced facilita tion of low-voltage-activated L-type calcium channels is suggested to be the underlying mechanism. It is noted that facilitation occurs on a time scale compatible with a role in phasic motor activity.