Background: Functional neuroimaging studies have reported increased activation in the contralesional primary motor cortex (cM1) in both experimental and clinical settings of stroke. However, the cellular resolution of cM1 activity and the role of cM1 activation in post-stroke recovery is unclear. In this study we characterized the time course of neuronal activation in cM1 during post-stroke recovery using activity-dependent markers. Furthermore, we investigated the role of cM1 activation in post-stroke recovery using the chemogenetic approach to silence the neuronal activity in cM1.
Methods: Adult male C57Bl6 mice (6-7 weeks) were subjected to stereotaxic injection of AAV-CamKIIa-hM4D-mCherry into the cM1. Ischemic stroke was induced by transient MCAO. For time-course study, brains collected at post-stroke days (PD) 3, 5, 14 and 28 were processed for immunostaining using activity-dependent marker pCREB. For cM1 neuronal silencing, clozapine-N-oxide (CNO) was used to induce inhibitory effects via hM4D. Stroke mice received either CNO or saline intraperitoneally during early phase (PD5-14) or late phase (PD15-28). Functional recovery was evaluated using the rotating beam test at pre-stroke baseline and PD 4, 7, 10 and 14, 21 and 28.
Results: Timecourse analysis showed that pCREB expression was predominantly observed in cM1 when compared to iM1, as early as PD5 and gradually increased at PD14. While neuronal silencing of cM1 during early phase (PD5-14) did not have significant effect on recovery, neuronal silencing of cM1 during late phase (PD 15-28) showed a trend of beneficial effect on recovery (CNO group n=5, SAL group n=4), suggesting a time-dependent role of cM1 activation during post-stroke recovery.
Conclusions: Our results showed that basal cM1 neuronal activity increased after stroke and interhemispheric imbalance persisted until at least PD14. Chemogenetic neuronal silencing studies suggest that inhibiting cM1 activation during late phase of recovery may be beneficial. Ongoing studies are addressing the time dependent role of cM1 activation, and elucidating the neuronal subtypes (excitatory vs inhibitory) that are activated in cM1 after stroke.