Causal interhemispheric neuromodulation sharpens synaptic and neurobehavioral inhibition in stroke.

Hemispheric functional asymmetries are a key aspect of human brain organization. Revealing how causal manipulations of hemispheric (im)balances dynamically change pre- and post-synaptic GABA regulation, in terms of mean and variance changes, is essential to understand the plasticity of brain asymmetries in health and disease. This has direct implications for treatment approaches in stroke relying on interhemispheric inhibition using transcranial magnetic stimulation. We investigated neurobehavioral and neurochemical effects of cortical stimulation using a unique multimodal brain imaging and stimulation set-up, in both healthy and unilaterally lesioned hemispheres, in stroke. We performed two molecular imaging sessions measuring GABA receptor levels, with sham and real neurostimulation, functional magnetic resonance imaging (MRI) and GABA neurospectroscopy, all in the same day. We found that noninvasive inhibitory neuromodulation of interhemispheric interactions causes pre- and post-synaptic sharpening of GABA neurotransmitter and receptor levels, providing a mechanism of action for transcranial magnetic stimulation contralateral inhibitory protocols in stroke. Moreover, we found a positive association between GABA neurotransmitter and receptor levels, suggesting increased synaptic coupling. This discovered plasticity mechanism, based on correlated reduction of pre- and post-synaptic variability, represents a basic principle underlying long-range inhibitory hemispheric interactions and allows for the stabilization of synapses and rebalancing of brain activity in disorders of hemispheric imbalance, such as stroke.