Effect of a Conditioning Lesion on Axonal Regeneration and Recovery of Function

Conditioning electrical stimulation fails to enhance sympathetic axon regeneration

Peripheral nerve injuries are common, and there is a critical need for the development of novel therapeutics to complement surgical repair. Conditioning electrical stimulation (CES) is a novel variation to the well-studied perioperative electrical stimulation, both of which have displayed success in enhancing the regeneration of motor and sensory axons in an injured peripheral nerve. CES is a clinically attractive alternative not only because of its ability to be performed at the bedside prior to a scheduled nerve repair surgery, but it has also been shown to be superior to perioperative electrical stimulation in the enhancement of motor and sensory regeneration. However, the effects of CES on sympathetic regeneration are unknown. Therefore, we tested the effects of two clinically relevant CES paradigms on sympathetic axon regeneration and distal target reinnervation. Because of the long history of evidence for the enhancement of motor and sensory axons in response to electrical stimulation, we hypothesize that CES will also enhance sympathetic axon regeneration. Our results indicate that the growth of sympathetic axons is acutely inhibited by CES; however, at a longer survival time point post-injury, there is no difference between sham CES and the CES groups. There has been evidence to suggest that the growth of sympathetic axons is inhibited by a conditioning lesion, and that sympathetic axons may respond to electrical stimulation by sprouting rather than elongation. Our data indicate that sympathetic axons may retain some regenerative ability after CES, but no enhancement is exhibited, which may be accounted for by the inability of the current clinically relevant electrical stimulation paradigm to recruit the small-caliber sympathetic axons into activity. Further studies will be needed to optimize electrical stimulation parameters in order to enhance the regeneration of all neuron types.

Axonal outgrowth and neuronal apoptosis in cultured adult mouse dorsal root ganglion preparations: effects of neurotrophins, of inhibition of neurotrophin actions and of prior axotomy

Dorsal root ganglia (L4 and L5) with attached spinal roots and nerve stumps were isolated from young adult mice and cultured in a layer of extracellular matrix material (matrigel). Within one day, a large number of axons grew out from the cut ends of the nerve and the dorsal root. The average outgrowth length was more than doubled by nerve growth factor, which also strongly increased the number of fibres, showing extensive branching. There was also a significant outgrowth stimulation by neurotrophin-3, but no observable effect by brain-derived neurotrophic factor. In preparations isolated and cultured six days after peripheral nerve transection in vivo, there was an increase in both the outgrowth length (about 1.5- to 2-fold) and in the number of axons. Stimulation of axonal outgrowth, which concerned outgrowth from both the peripheral nerve and the dorsal root, could be further enhanced by the addition of nerve growth factor to the culture. K-252a, a selective inhibitor of neurotrophin receptor-associated tyrosine kinase activity, did not affect either the normal outgrowth or the increased outgrowth in pre-axotomized preparations, at a concentration which abolished the stimulating effects by exogenous nerve growth factor and neurotrophin-3. Under the culturing conditions used, spontaneous apoptosis occurred, but none of the neurotrophins tested, nor K-252a, affected the number of apoptotic neuronal cells analysed by nick-labelling DNA breaks at the end of a 48-h culturing period. Altogether, the present data suggest that for most dorsal root ganglia neurons, signalling through the trk receptors does not influence the apoptosis in vitro and is not required for either the spontaneous axonal outgrowth in matrigel or the increased outgrowth which occurs after prior axotomy in vivo.

Acrylamide neuropathy: changes in the composition of proteins of fast axonal transport resemble those observed in regenerating axons

Proteins conveyed by fast axonal transport along sensory and motor axons of rat sciatic nerve were labelled with L-[35S]methionine and characterized by one- and two-dimensional electrophoresis on polyacrylamide gels, followed by fluorography. Nerves from normal or bis-acrylamide-treated animals were compared with nerves from acrylamide-treated animals and nerves regenerating after a crush axotomy. In both sensory and motor axons significant changes in the pattern of labelled bands on one-dimensional gels occurred after 10 days of acrylamide treatment (50 mg/kg daily, i.p.). These changes resembled those seen in regenerating axons, but were less pronounced. No changes were detectable after shorter periods of treatment, even though the onset of the neuropathy, assessed by a behavioral test, occurred on days 4-6 of treatment. Two-dimensional separations of the labelled proteins revealed increased labelling of growth-associated protein 43 in acrylamide-treated animals, but again this was less pronounced than in regenerating nerves. Acrylamide treatment induces changes in composition of fast-transported protein that are qualitatively similar to those seen after axotomy. Since these changes are not detectable until the neuropathy is advanced, it is unlikely that they are causative factors. Instead, they are most likely a result of the cell body reaction previously observed in acrylamide intoxication, a reaction that resembles that produced by axotomy.

Cytofluorometric quantification of somatopetal axonal transport: effects of a conditioning lesion and 2,5-hexanedione

Cytofluorometric quantification of axonally transported fluorescein isothiocyanate (FITC)-labelled wheat germ agglutinin (WGA) from the injection site in the snout area was performed in the facial nucleus at various times during regeneration of one of the facial nerves. Measurements were made on single neurons on both operated and non-operated sides in three different groups of mice 8, 12 and 16 days after a nerve crush, Group 1: (control group) animals with a nerve crush, Group 2: animals with a conditioning lesion (nerve injury) made 3 days before the nerve crush, and Group 3: animals exposed daily to 2,5-hexanedione from 2 weeks before nerve crush until killing. A conditioning lesion caused a more rapid return of transport in regenerating nerves but there was no evidence for an increase in the total amount of transported FITC-WGA. For mice exposed to 2,5-hexanedione a transient increase of tracer transport in regenerating nerves could be demonstrated on day 12 after nerve crush. On day 16, however, a reduction of transport was seen in both operated and non-operated nerves. This study shows that it is possible experimentally to manipulate the influx of macromolecules to the nerve cell body from the periphery during nerve regeneration, and the present method offers the opportunity to study quantitatively the effects of various treatments on reinnervation of a muscle.

Effect of conditioning lesion on axonal sprout formation at nodes of Ranvier

The effect of a conditioning lesion on the time-course of axonal sprout formation after a subsequent testing lesion was evaluated in myelinated axons of the rat sciatic nerve. Transmission electron microscopy of longitudinal nerve sections was used to examine nodes of Ranvier located 200-500 micron proximal to the testing lesion. The conditioning lesion was a cut of the tibial nerve at the ankle; the testing lesion, made 2 weeks later, was a crush of the sciatic nerve at the hip. Sprouts were defined as unmyelinated evaginations of the nodal axolemma that (1) had reached the basement membrane of the Schwann cell, and (2) were located between the myelin sheath of the distal paranode and the basement membrane. Photomicrographs of the nodes at 9, 18, and 27 hours after the testing lesion were assigned to one of seven categories: normal, retracted, myelin degeneration, axonal degeneration, type A sprout formation (cytoskeleton absent), type B sprout formation (cytoskeleton present), and type B sprout degeneration. By 9 hours after the testing lesion, type B sprout formation was found in 9% of the nodes in control nerves (testing lesion alone) and 33% of those in conditioned nerves (P less than .01). A 33% incidence of type B sprout formation was not reached in control nerves until 27 hours after the testing lesion. Since the conditioning lesion was located 50 mm distal to the testing lesion and did not induce neuronal death, earlier sprout formation can be attributed to a neuronal response to the conditioning lesion rather than to a putative factor that arises from pre-degenerated fibers and acts on newly formed sprouts.

Axonal regeneration in wobbler motor neuron disease: quantitative histologic and axonal transport studies

The regenerative capacity of the cervical anterior horn cells was studied at 4 and 7 days following forelimb nerve crush in 19 wobbler mice and 18 normal littermates. Quantitative histologic and radiolabeled axonal transport techniques showed that the axotomized neurons of the wobbler mouse supported active axonal elongation. However, the average axon outgrowth rate determined by histologic technique was diminished by 25% and the fastest axon outgrowth rate determined by axonal transport technique was also decreased by 30% in wobbler mice as compared to controls. The distal labeled peak was absent in the wobbler mouse at 7 days, indicating that the regeneration rate of individual axons was widely dispersed. Histologic studies also showed that the wobbler axons grew slowly. This study suggests that axonal regeneration does occur in motor neurons undergoing a primary neuronopathy. However, the regenerative capacity was reduced and this appears to reflect an impairment of functional integrity in the anterior horn cells of the wobbler mouse.

Effect of a conditioning lesion on optic nerve regeneration in goldfish

Following a 'test lesion' (crush) of the optic nerve in goldfish, histological study of axons in silver-stained sections showed that outgrowth of the leading axons began after an initial delay of 4.3 days and proceeded at 0.34 +/- 0.03 mm/day. When a 'conditioning lesion' (crush at the same site) preceded the testing lesion by 2 weeks, the initial delay was 2.5 days and the outgrowth rate was 0.74 +/- 0.13 mm/day (P less than 0.01). Two additional methods, utilizing intraocular injections of tritiated proline or fucose to label axonally transported proteins, were used to examine the outgrowth of leading optic axons. (a) Measurement of the distances reached by labeled axons in the nerve at 6 and 10 days after a testing lesion alone yielded an initial delay of 4.6 days and an outgrowth rate of 0.41 +/- 0.04 mm/day. However, when a conditioning lesion preceded the testing lesion, labeled optic axons were already found to have reached the optic tectum by 10 days after the testing lesion, indicating an outgrowth rate in excess of 0.64 mm/day. (b) Determination of the times at which labeled axons arrived at the optic tectum showed that the outgrowth rate after a testing lesion along was 0.40 mm/day whereas when the testing lesion was preceded by a conditioning lesion it was 0.74 mm/day. Thus, as a result of a conditioning lesion the initial delay was reduced by nearly half and the outgrowth rate was nearly doubled.