Delayed viral vector mediated delivery of neurotrophin-3 improves skilled hindlimb function and stability after thoracic contusion in rats

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Abstract It has been reported that intramuscular injection of an Adeno-associated viral vector serotype 1 (AAV1) encoding Neurotrophin-3 (NT3) into hindlimb muscles 24 hours after a severe T9 contusion in rats induced lumbar spinal neuroplasticity, partially restored locomotive function and reduced spasms during swimming. Here we investigated whether a targeted delivery of NT3 to lumbar and thoracic motor neurons 48 hours following a severe contusive injury aids locomotive recovery in rats. AAV1-NT3 was injected into the tibialis anterior, gastrocnemius and rectus abdominus muscles 48-hours following trauma, persistently elevating serum levels of the neurotrophin. NT3 improved trunk stability, accuracy of stepping during skilled locomotive tasks, and alternation of the hindlimbs during swimming, but it had no effect on gross locomotion function in the open field. The number of vGlut1 + (likely proprioceptive afferent) boutons on gastrocnemius α-motor neurons was increased after injury but normalised following NT3 treatment suggestive of a mechanism in which the functional effects may be mediated through proprioceptive feedback. Ex vivo MRI revealed substantial loss of grey and white matter at the lesion epicentre but no effect of delayed NT3 treatment to induce neuroprotection or prevent secondary damage. Spasms and hyperreflexia were not reliably induced in this severe injury model suggesting a more complex anatomical or physiological cause to their induction. We have shown that delayed intramuscular AAV-NT3 treatment can promote recovery in skilled stepping and coordinated swimming supporting a role for NT3 as a therapeutic strategy for spinal injuries potentially through modulation of somatosensory feedback. Key Points Targeted delivery of NT3 to hindlimb and trunk muscles at a clinically relevant 48h following a severe thoracic contusion aids fine locomotor control and synchronised movement. NT3 mediated improvements in trunk stability, accuracy of stepping during skilled locomotive tasks, and alternation of the hindlimbs during swimming through the normalisation of vGlut1+ boutons on presumptive proprioceptive afferents innervating these muscles. 250kDyn thoracic contusion does not reliably result in measurable signs of spasticity.

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