Formation of functional endplates by spinal axons regenerating through a peripheral nerve graft. A study in the adult rat.

Peripheral nerve (PN) autografts were used in the adult rat to join the midcervical spinal cord to a nearby denervated skeletal muscle. Retrograde tracing, morphological and electrophysiological studies indicated the following: 1) a great number of neurons, located bilaterally, between C3 and C7 in most laminae of the grey matter, extended axons into the PN grafts, 2) a lesser number of neurons regenerated up to the reconnected muscle, but most of them were typical motoneurons, 3) neuromuscular junctions were formed in ectopic locations, around the tip of the grafted nerve, and at the sites of original endplates, 4) these junctions were functional and formed by axons that had regenerated into the PN bridges, as muscle contraction was obtained by electrical stimulation of the grafted nerves, 5) they were proved to be cholinergic since endplate potentials, evoked by stimulating the PN graft, were suppressed by curare. These results strongly suggest that spinal neurons, and especially motoneurons, are involved in the formation, through PN bridges, of new functional cholinergic connections with denervated skeletal muscles.

[Post-traumatic reconnection of the cervical spinal cord with skeletal striated muscles. Study in adult rats and marmosets]. / Reconnexion post-traumatique de la moelle épinière cervicale avec la musculature striée squelettique. Etude chez le rat et le marmouset adultes.

In an attempt at repairing the injured spinal cord of adult mammals (rat, dog and marmoset) and its damaged muscular connections, we are currently using: 1) peripheral nerve autografts (PNG), containing Schwann cells, to trigger and direct axonal regrowth from host and/or transplanted motoneurons towards denervated muscular targets; 2) foetal spinal cord transplants to replace lost neurons. In adult rats and marmosets, a PNG bridge was used to joint the injured cervical spinal cord to a denervated skeletal muscle (longissimus atlantis [rat] or biceps brachii [rat and marmoset]). The spinal lesion was obtained by the implantation procedure of the PNG. After a post-operative delay ranging from 2 to 22 months, the animals were checked electrophysiologically for functional muscular reconnection and processed for a morphological study including retrograde axonal tracing (HRP, Fast Blue, True Blue), histochemistry (AChE, ATPase), immunocytochemistry (ChAT) and EM. It was thus demonstrated that host motoneurons of the cervical enlargement could extend axons all the way through the PNG bridge as: a) in anaesthetized animals, contraction of the reconnected muscle could be obtained by electrical stimulation of the grafted nerve; b) the retrograde axonal tracing studies indicated that a great number of host cervical neurons extended axons into the PNG bridge up to the muscle; c) many of them were assumed to be motoneurons (double labelling with True Blue and an antibody against ChAT); and even alpha-motoneurons (type C axosomatic synapses in HRP labelled neurons seen in EM in the rat); d) numerous ectopic endplates were seen around the intramuscular tip of the PNG. In larger (cavitation) spinal lesions (rat), foetal motoneurons contained in E14 spinal cord transplants could similarly grow axons through PNG bridges up to the reconnected muscle. Taking all these data into account, it can be concluded that neural transplants are interesting tools for evaluating both the plasticity and the repair capacities of the mammalian spinal cord and of its muscular connections.