Spinal plasticity mediated by postsynaptic L-type Ca2+ channels.

In the spinal cord, motoneurons and specific subgroups of interneurons express L-type Ca(2+) channels. As elsewhere, these dihydropyridine-sensitive channels mediate a slowly activating inward current in response to depolarisation and show little or no inactivation. The slow kinetics for activation and deactivation provide voltage-sensitive properties in a time range from hundreds of milliseconds to tens of seconds and lead to plateau potentials, bistability and wind-up in neurons in both sensory and motor networks. This slow dynamics is in part due to facilitation of L-type Ca(2+) channels by depolarisation. The voltage sensitivity of L-type Ca(2+) channels is also regulated by a range of metabotropic transmitter receptors. Up-regulation is mediated by receptors for glutamate, acetylcholine, noradrenaline and serotonin in motoneurons and by receptors for glutamate and substance P in plateau-generating dorsal horn interneurons. In both cell types, L-type Ca(2+) channels are down-regulated by activation of GABA(B) receptors. In this way, metabotropic regulation in cells expressing L-type Ca(2+) channels provides mechanisms for flexible adjustment of excitability and of the contribution of plateau currents to the intrinsic properties. This type of regulation also steers the magnitude and compartmental distribution of Ca(2+) influx during depolarisation, thus providing a signal for local synaptic plasticity.