Pharmacological Mechanisms of Low-intensity Focused Ultrasound for Motor Cortex Neuroplasticity

Overview

Low-intensity focused ultrasound (LIFUS) has been shown to be an effective and safe non-invasive brain stimulation technique, capable of reaching greater brain depth and a greater spatial resolution than other brain stimulation tools. Its use as a potential clinical tool for treatment of neurological disorders is reliant on an understanding of its mechanisms of action. Although it has been shown to induce immediate (online) and prolonged (offline) changes in plasticity in the motor cortex, researchers have not studied its effects on neurotransmitter receptors and ion channels responsible for neuronal signaling in humans. The purpose of this study is to explore the effects of online and offline LIFUS stimulation in tandem with administration of various brain-active drugs, to elucidate the effects of this technique on specific cortical receptors and channels. 20 healthy, screened subjects will be recruited to participate in 5 sessions in-lab. Each session will represent the double-blinded administration of four known and studied pharmacological agents known to safely induce changes in the motor cortex, as well as a placebo. Investigators will use carbamazepine (sodium channel blocker), lorazepam (GABAA positive allosteric modulator), nimodipine (calcium channel blocker), and dextromethorphan (glutamate N-Methyl-D-aspartate receptor antagonist). Single- and paired-pulse transcranial magnetic stimulation (TMS) measures will be recorded for online LIFUS before and after drug intervention, and induction of offline LIFUS during placebo will be compared with its induction following the various drug interventions. Investigators predict that due to the differential effects of online and offline LIFUS on motor parameters, the mechanisms in which it alters the receptors and channels of interest will also be differentially modulated.

In the current study investigators aim to describe how common neurotransmitter receptors and ion channels in the brain, particularly the motor cortex, are dynamically involved in the mechanisms generated by LIFUS. By selectively modulating the activity of these receptors and channels with previously studied and known drugs, it can be empirically determined how common components of signal propagation and prolonged neuroplasticity are being affected by the sonication waves induced by LIFUS. This will be done through baseline recording of FUS-induced neuroplasticity, and then compared within-subject to FUS-induced neuroplasticity after administration of pharmacological agents of interest. Random double-blinded administration of four drugs and a placebo will ensure that results are accurate and reduce any effects of expectation. Its predicted that online FUS will lead to a reduction in motor excitation and an increase in intracortical inhibition, and offline FUS will lead to an increase in motor excitation, an increase in intracortical facilitation, and a reduction in intracortical inhibition. Its predicted that drugs of interest to similarly replicate previous work; that is, carbamazepine, lorazepam, dextromethorphan, and nimodipine will all reduce cortical excitation and increase intracortical inhibition compared to placebo. Its predicted that carbamazepine, lorazepam, nimodipine and dextromethorphan will all further reduce motor excitation in conjunction with online FUS in an additive manner, compared to placebo. Its predicted that carbamazepine will have no effect on the prolonged effects of offline FUS on the motor cortex, but that lorazepam, nimodipine, and dextromethorphan will all interfere with the offline effects of FUS on motor cortex, compared to placebo.