[Regrowth of central respiratory pathways in neural graft. From research tool on the axonal regeneration to a strategy of post-traumatic reparation]

Abstract

This review focuses on the regrowth of respiratory pathways after nerve grafting within the central nervous system of the adult rat. After a general presentation of the background and of the grafting procedure, we summarize our nerve grafting results of while it is now well established that severed axons of adult central neurons can regenerate within segments of peripheral nerve partially implanted within the brain or spinal cord, the functional properties of the regenerating neurons remain generally unknown. With a view to assessing the extent to which the functional capacities of central neurons can be maintained after axonal regeneration, we have carried out experiments on central respiratory neurons which are a good example of a highly organized neuronal network with characteristic patterns of spontaneous discharge. We have shown that axonal regrowth of central respiratory neurons was successfully induced in blind-ended medullary and spinal autografts implanted respectively within the respiratory centers of the medulla oblongata and within the cervical spinal cord at the level of descending respiratory pathways. The grafts consisted of true "supplementary nerve" in which normal afferent and efferent respiratory pathways were confirmed by recording respiratory unitary discharges from teased fibers within the grafts. The efferent discharges reflected the activity of central respiratory neurons that had regenerated axons within the grafts: these neurons manifested spontaneous activity and normal responsiveness to respiratory stimuli that resemble those of normal respiratory cells. In order to evaluate the possibility of experimental nerve banking, the feasibility of using short-term and long-term stored nerves as potential spinal nerve grafts was established using in vitro pre-degenerated nerve and cryopreserved nerve grafts after assessment of Schwann cell viability. The extent of respiratory reinnervation of the different grafts (medullary, spinal and stored nerve grafts) was compared. The discussion focuses on the main data and the strategy for future nerve grafting is evoked: functional characteristics of regenerating respiratory axons, extent of graft reinnervation, functional schwann cell survey within stored/grafted nerve and post-traumatic grafting.