The capacity of the brain to make structural, physiological, and genetic adaptations following stroke, otherwise known as plasticity, is likely to be critical for improving sensorimotor impairments and functional activities. Promotion of adaptive plasticity in the central nervous system (CNS) leading to sustained functional improvement is of paramount importance, given the personal suffering and cost associated with post-stroke disability (Ma et al., 2014). In addition to rehabilitation therapies to retrain degraded motor skills, animal and human studies have tried to augment recovery with neuropharmacologic interventions. Unfortunately, few if any have had a notable effect in patients or have come into routine use (Martinsson et al., 2007; Chollet et al., 2011; Cramer, 2015; Simpson et al., 2015). Methods to screen drugs based on their presumed mechanism of action on plasticity in human motor systems could speed translation to patients. However, there is currently no accepted method in stroke patients for evaluating the potential effectiveness or individual responsiveness to putative “plasticity enhancing” drugs in an efficient, low-cost, cross-sectional manner, in order to establish target engagement in humans and to avoid the extensive time and cost of protracted clinical trials.
Paired associative stimulation (PAS) is a safe, painless, and non-invasive technique known to result in short-term modulation of corticospinal excitability in the adult human motor system, lasting ∼90 min (Stefan et al., 2000; Wolters et al., 2003). Post-PAS excitability enhancement has been considered an LTP-like response thought to relate to transient changes in synaptic efficacy in the glutamatergic system at the N-methyl-D-aspartate (NMDA) receptor, since both human NMDA receptor deficiency (Volz et al., 2016) and pharmacological manipulation with dextromethorphan (Stefan et al., 2002) can block the effect. While PAS has been explored as a potential therapeutic intervention in patients with residual motor deficits after stroke (Jayaram and Stinear, 2008; Castel-Lacanal et al., 2009), it has not previously been investigated for its potential use as an assay of motor system plasticity in this context. Prior studies have suggested that motor practice and PAS share the same neuronal substrates, modulating LTP and LTD-like plasticity in the human motor system (Ziemann et al., 2004; Jung and Ziemann, 2009); therefore, as an established non-invasive human neuromodulation method (Suppa et al., 2017), we reasoned that PAS would be a suitable assay in the present study to examine the effect of a drug on motor system plasticity.
Here, we examine the effect of memantine, a drug used for treatment of Alzheimer’s disease, on the PAS response in patients with chronic stroke. Memantine is described pharmacologically as a low affinity, voltage dependent, non-competitive, NMDA antagonist (Rogawski and Wenk, 2003). At high concentrations, like other NMDA-R antagonists, it can inhibit synaptic plasticity. At lower, clinically relevant concentrations, memantine can, under some circumstances, promote synaptic plasticity by selectively inhibiting extra-synaptic glutamate receptor activity while sparing normal synaptic transmission, and hence may have clinical utility for rehabilitation (Xia et al., 2010). Interest in specifically using the drug for its interaction with stroke pathophysiology stems from animal models of both prevention (Trotman et al., 2015), in which pre-conditioning reduced infarct size, as well as for functional recovery, in which chronic oral administration starting >2 h post-stroke resulted in improved function through a non-neuroprotective mechanism (López-Valdés et al., 2014). In humans, memantine taken over multiple days has been used to demonstrate that the NMDA receptor is implicated in specific transcranial magnetic paired-pulse measures (Schwenkreis et al., 1999), and short-term training-induced motor map reorganization (Schwenkreis et al., 2005). In studies of neuromodulation, memantine blocked the facilitatory effect of intermittent theta-burst stimulation (iTBS) (Huang et al., 2007). Similarly, LTP-like plasticity induced by associative pairing of painful laser stimuli and TMS over primary motor cortex (M1) can also be blocked by memantine (Suppa et al., 2013). The effects of memantine on the PAS response have not yet been demonstrated, including examination of potential dose-response effects, which would be important for the potential clinical application of memantine for stroke recovery.
In our study, we set out to determine whether PAS might be a useful tool to probe the potential for plasticity after stroke in persons with chronic hemiparesis and apply PAS as an assay to look at drug effects on motor system plasticity using memantine. We hypothesized that (a) PAS would enhance corticospinal excitability in the contralesional hemisphere of stroke patients, and that (b) since PAS-induced plasticity is thought to involve a short-term change in glutamatergic synaptic efficacy, memantine would have a dose-dependent effect on PAS response. We predicted that at low doses, memantine would enhance PAS-induced plasticity through selective blockade of extrasynaptic NMDA receptors, whereas higher doses would inhibit PAS-induced plasticity.[…]