Association of medullary reticular formation ventral part with spasticity in mice suffering from photothrombotic stroke

•The mechanisms underlying spasticity following stroke are poorly understood.•Neuronal activity increased in the medullary reticular formation in mice after stroke.•The expression and ratio of synaptic receptor subunits in this region were altered.•Homeostatic plasticity is involved in the development of spasticity following stroke. Strokes cause spasticity via stretch reflex hyperexcitability in the spinal cord, and spastic paralysis due to involuntary muscle contraction in the hands and fingers can severely restrict skilled hand movements. However, the underlying neurological mechanisms remain unknown. Using a mouse model of spasticity after stroke, we demonstrate changes in neuronal activity with and without electrostimulation of the afferent nerve to induce the stretch reflex, measured using quantitative activation-induced manganese-enhanced magnetic resonance imaging. Neuronal activity increased within the ventral medullary reticular formation (MdV) in the contralesional brainstem during the acute post-stroke phase, and this increase was characterised by activation of circuits involved in spasticity. Interestingly, ascending electrostimulation inhibited the MdV activity on the stimulation side in normal conditions. Moreover, immunohistochemical staining showed that, in the acute phase, the density of GluA1, one of the α-amino-3 hydroxy‑5 methyl -4 isoxazolepropionic acid receptor (AMPAR) subunits, at the synapses of MdV neurons was significantly increased. In addition, the GluA1/GluA2 ratio in these receptors was altered at 2 weeks post-stroke, confirming homeostatic plasticity as the underlying mechanisms of spasticity. These results provide new insights into the relationship between impaired skilled movements and spasticity at the acute post-stroke phase.