PROPRIOSPINAL PACAPERGIC NEURONS MEDIATE ELECTRICAL STIMULATION-ASSISTED RECOVERY OF RESPIRATION AFTER SPINAL CORD INJURY IN MICE

Although mammalian breathing is predominantly controlled by central respiratory circuits in the medulla, spinal regulation of respiration could play an important role in the recovery of respiration after spinal cord injury (SCI). The precise neuronal types and neural circuits underlying the spinal regulation remains unclear. Here we report that epidural electrical stimulation (EES) interleaved with mechanical ventilation could induce post-treatment spontaneous respiration in mice with severe C2 dorsal over-hemisection, and this EES effect is mediated by spinal PACAPergic neurons. We identified the two PACAPergic cell types in cervical spinal cord that mediate both hypercapnia-induced respiratory regulation and EES facilitated post-SCI respiratory recovery: Type 1 PACAPergic neurons modulated the respiratory frequency and type 2 PACAPergic neurons modulated the amplitude of respiration. Type 1 PACAPergic neurons were innervated by projections from the respiratory-related reticulospinal neurons, while type 2 PACAPergic neurons were local interneurons. In an acute incomplete cervical SCI mice, optogenetic and pharmacogenetic activation of type 1 PACAPergic neurons or PACAP peptide-activation of their downstream neurons facilitated restoration of respiration. The EES also promoted extensive axon sprouting of type 2 PACAPergic neurons in chronic SCI mice. Thus, spinal PACAPergic neurons represent a novel substrate for neuromodulatory interventions targeting post-SCI respiratory dysfunction.