Early functional and morphological changes in locomotor networks induced by focal lesion in an in vitro spinal cord injury model

Using an isolated neonatal rat spinal cord, the early consequences of a lesion on the operation of locomotor circuits were studied by mimicking the hypoxia, aglycemia, low pH, ROS and excitotoxicity through 1 h application of a toxic solution to lower thoracic segments and monitoring the consequences on lumbar networks with 24 h recording from ventral roots. Preparations were then processed for histological analysis. Sham experiments indicated that setting up transverse barriers to circumscribe the lesion did not damage spinal cord activity. Applying the toxic solution to few segments only determined a large depolarization recorded not only from the directly affected segments but also from those above and below. Maintenance of leakproof barriers was checked with phenol red solution.Within the toxic-exposed segments reflex responses elicited by electrical stimulating the homosegmental dorsal root were fully and irreversibly abolished in conjunction with extensive cell loss throughout the various laminae. Conversely, the reflex responses from motor pools outside the damage site were depressed by 40 % in amplitude without area change. Signal propagation between segments above and below the lesion was usually preserved, indicating conserved impulse conduction. Lumbar segments containing locomotor networks did not show extensive morphological damage, although they showed transient loss of locomotor-like oscillations induced by neurochemicals (NMDA and serotonin), which recovered after overnight washout despite the limited impairment in reflex activity. In contrast, locomotor discharges elicited by trains of electrical stimuli applied to one dorsal root were completely suppressed during the whole observation period without recovery. Our results suggest that during the first minutes from the start of an experimental injury a lesioning mechanism spread beyond the affected spinal areas with functional impairment of locomotor-like activity. This was a temporary deficit as spinal networks located below the damaged areas recuperated their ability to generate a locomotor rhythm in response to neurochemicals. Nevertheless, the irreversible suppression of electrically induced alternating oscillations suggests different vulnerability of this pathway not attributable to white matter damage. Such observations raise interesting implications for strategies to reactivate locomotor networks with functional stimulations. In particular, to exploit the residual capacity of the lumbosacral central pattern generator to produce neurorehabilitative benefits to spinal cord injured persons might require focal stimuli targeted to surviving networks.