Harbeck, A. | Search bioRxiv

bioRxiv2026

Recruitment of intrasegmental and propriospinal interneuron networks of forelimb motor circuitry in chronic cervical contusion spinal cord injury

Cervical spinal cord injury (SCI) disrupts descending supraspinal input and spinal networks controlling upper extremity motor function, making recovery of arm and hand function a major priority for affected individuals. The vast majority of regenerative strategies have been studied during the acute and sub-acute phases following SCI, while therapeutic testing remains limited for chronic injury, when persistent barriers significantly limit circuit remodeling. We asked whether pharmacologic enhancement of neuronal-intrinsic axon growth capacity through PTEN inhibition could promote forelimb motor recovery and circuit plasticity in a rodent model of chronic cervical SCI. Following C5 hemi-contusion, female rats underwent behavioral testing of forelimb motor function and received PTEN antagonist peptide (PAP) beginning at a chronic time point, and the trans-synaptic tracer, pseudorabies virus (PRV), was injected into ipsilesional triceps to trace forelimb-associated motor circuitry. PAP selectively improved forelimb usage during cylinder paw-preference testing, but not strength, sensorimotor, or gait-related forelimb measures. PAP increased recruitment of PRV-traced local-projecting and propriospinal interneuron networks. In addition, PAP enhanced synaptic input to these recruited propriospinal interneurons, raising the possibility that spared descending pathways may contribute to recovery through spinal relay circuits. Lastly, we mapped the supraspinal connectome providing descending input from brainstem to the lower motor neurons innervating forelimb muscle, and we determined how this connectome was impacted both by SCI and PAP. Together, these findings show that, even in chronic SCI, pharmacological PTEN inhibition can promote forelimb functional recovery and significant circuit remodeling, with spinal interneuron networks emerging as a key site of PAP-induced plasticity.

neuroscience↗

bioRxiv2026

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.

neuroscience↗