Intrasegmental and propriospinal pre-phrenic interneuron circuitry in intact CNS and following cervical spinal cord injury
Cervical spinal cord injury (SCI) disrupts descending respiratory circuitry, resulting in debilitating and often persistent ventilatory deficits. Respiratory drive emerges within medulla from the rostral ventral respiratory group (rVRG), whose neurons project to C3-C6 phrenic motor neurons (PhMNs), which then innervate diaphragm, the primary muscle of inspiration. Though rVRG neurons make extensive monosynaptic connection with PhMNs, rVRG input to PhMNs can also be relayed through pre-phrenic interneurons (PP-INs) via polysynaptic pathways. However, the neuroanatomical connectivity between PP-INs and PhMNs remains incompletely understood. In both uninjured rats and the C2 hemisection (C2HS) model of cervical SCI, we performed tracing of PP-INs that were synaptically connected to PhMNs located in rostral (C3-C4) or caudal (C5-C6) portions of the phrenic nucleus by unilaterally injecting retrograde trans-synaptic tracer, pseudorabies (PRV), selectively into ventral or dorsal regions of hemi-diaphragm. We quantified numbers of PRV-labeled PP-INs individually at segments across cervical spinal cord, including separately in dorsal horn, intermediate gray, and ventral horn. We found that PP-INs are widespread throughout C1-C7 spinal cord in the uninjured condition. These PP-INs have a predominant intrasegmental connectivity pattern with PhMNs, though significant numbers of longer distance projecting propriospinal PP-INs also exist both rostral and caudal to the PhMN pool. Furthermore, while PP-INs connect both ipsilaterally and contralaterally with PhMNs, there is a strong bias to ipsilateral projection. C2HS induced major disconnection between PhMNs and ipsilesional propriospinal PP-INs located rostral to the SCI, but conversely induced limited plasticity in connectivity of intrasegmental PP-INs located within C3-C6 spinal cord. These findings greatly improve our knowledge about PP-IN circuitry and also provide important information to aid in developing approaches to target PP-IN plasticity for promoting spinal cord repair.