Differential Regulation of Human Paired Associative Stimulation-Induced and Theta-Burst Stimulation-Induced Plasticity by L-type and T-type Ca2+ Channels.

Activity-dependent changes of postsynaptic Ca2+-concentration are influenced by a variety of different Ca2+-channels and play an important role in synaptic plasticity. Paired associative stimulation (PAS) and theta-burst stimulation (TBS) are noninvasive magnetic stimulation protocols used in human subjects to induce lasting corticospinal excitability changes that have been likened to synaptic long-term potentiation and long-term depression. To better characterize the Ca2+-related physiological mechanisms underlying PAS- and TBS-induced plasticity, we examined the impact of different Ca2+-sources. PAS-induced facilitation of corticospinal excitability was blocked by NMDA-receptor blocker dextromethorphan (DXM) and L-type voltage gated Ca2+ channels (VGCC) blocker nimodipine (NDP), but turned into depression by T-type VGCC blocker ethosuximide (ESM). Although, surprisingly, static corticospinal excitability was increased by the combination of DXM and NDP, PAS-induced facilitation was blocked. TBS-induced facilitation of corticospinal excitability, which has previously been shown to be turned into depression by L-type VGCC blocker NDP (Wankerl K, Weise D, Gentner R, Rumpf J, Classen J. 2010. L-type voltage-gated Ca2+ channels: a single molecular switch for long-term potentiation/long-term depression-like plasticity and activity-dependent metaplasticity in humans. J Neurosci. 30(18):6197-6204.), was blocked, but not reverted, by T-type VGCC blocker ESM. The different patterns of Ca2+-channel modulation of PAS- and TBS-induced plasticity may point to an important role of backpropagating action potentials in PAS-induced plasticity, similar as in spike-timing dependent synaptic plasticity, and to a requirement of dendritic Ca2+-dependent spikes in TBS-induced plasticity.