Acute intermittent hypoxia - a novel non-invasive therapy that promotes regeneration akin to brief electrical stimulation in peripheral nerve repair

Abstract

Peripheral nerve regeneration often results in poor functional outcomes, a reality we aim to change. Injured peripheral neurons mount an intrinsic repair response as they undergo regenerative neuronal reprogramming, which can be enhanced by brief electrical stimulation (ES) of the injured nerve at the time of surgical repair, resulting in improved regeneration in rodents and humans. However, this approach is invasive. Acute intermittent hypoxia (AIH) - breathing alternate cycles of regular air and air with ~50% normal oxygen levels (11% O2) is emerging as a promising non-invasive intervention that promotes respiratory and non-respiratory motor function in spinal cord injured rats and humans. However, this therapy has the potential to globally impact the nervous system beyond the motor system. Of note, hypoxic conditions can increase neural activity in injured sensory neurons and peripheral axons and promote repair. Thus, I hypothesized that an AIH paradigm similar to that used for spinal cord repair, will improve peripheral nerve repair in a manner akin to ES, including its impact on regeneration-associated gene (RAG) expression – a predictor of growth states. To this end, alterations in early RAG expression (growth-associated protein 43, brain-derived neurotrophic factor, hypoxia-inducible factor alpha and a neural specific growth- associated protein, superior cervical ganglion 10) were examined for rats that had undergone tibial nerve transection and repair with either 2 days of normoxia treatment or AIH treatment begun two days post-repair, or 1 hour continuous ES treatment (20 Hz) at the time of repair. Three days post-repair, AIH or ES treatments effected significant and parallel elevated RAG expression relative to normoxia control treatment, most evident at the growing axon front. Behavioural, thermal and mechanical sensitivity assessments revealed that neither ES nor AIH elevated regeneration-associated pain states. Finally, ES showed significant impacts on functional recovery relative to normoxia controls in mid (25 day)- and late (70 day)-regeneration stages and AIH in mid (25 day)-regeneration stages. These indicators of an enhanced regenerative state for AIH and ES were supported by significantly increased numbers of newly myelinated fibers detected 20 mm distal to the tibial nerve repair site at 25 days post nerve repair. Collectively, these results support a role for brief AIH treatment, as a promising noninvasive adjunct therapy for improved peripheral nerve repair in a manner consistent with that observed with the more invasive direct nerve stimulation.