Extensive Sprouting of Sensory Afferents and Hyperalgesia Induced by Conditional Expression of Nerve Growth Factor in the Adult Spinal Cord

Abstract

Genetic transfer of growth-promoting molecules was proposed as a potential strategy to modify the nonpermissive nature of the adult CNS to induce axonal regeneration. To evaluate whether overexpression of neurotrophins or cellular adhesion molecules would effect axonal plasticity, adenoviruses encoding fibroblast growth factor-2 (FGF-2/Adts), nerve growth factor (NGF/Adts), neurotrophin-3, and the cell adhesion molecules N-cadherin and L1 were injected into the dorsal horn of the adult spinal cord. Transgene expression was primarily localized to astrocytes in the dorsal horn and motor neurons within the ventral horn. Overexpression of these factors, with the exception of NGF/Adts, failed to increase axonal sprouting. Eight days after NGF/Adts injections, axonal sprouting within the dorsal horn was apparent, and after 4 weeks, extensive spouting was observed throughout the entire dorsal horn, extending into the ventral horn and the white matter of the lateral funiculus. These axons were identified primarily as a subpopulation of nociceptive fibers expressing calcitonin gene-related peptide and substance-P. Behavioral analysis revealed thermal hyperalgesia and perturbation of accurate paw placement on gridwalking tasks for both FGF-2-and NGF-treated animals. These results indicate that the administration of growth-promoting molecules can induce robust axonal plasticity of normal adult primary sensory neurons into areas of transgene expression, causing significant alterations in behavioral responses. This observation also indicates that gene transfer protocols that aim to reconstruct diseased or injured pathways should also be designed to prevent the sprouting of the normal circuitry from adjacent unaffected neurons.

Setting the stage for functional repair of spinal cord injuries: a cast of thousands

“…The most obvious example here is NGF, which while promoting the robust regeneration of small-diameter peptidergic afferents into the spinal cord following rhizotomy also produces severe hyperalgesia. 163 One of the apparent issues is that NGF-treated axons far overshoot their targets 164,165 to terminate deep in the dorsal horn. Tang et al 166 have taken advantage of a developmental mechanism of axon repulsion in the spinal grey matter by transfecting ventral cells with semaphorin 3A.…”

“…While NGF has been shown to promote intraspinal sprouting of nociceptive primary afferents, 164,165 the usefulness of NGF in SCI is severely limited by its painful side effects (discussed above). 163 Exogenous NT-3 promotes sprouting of intact CST axons developmentally 197 and following rubrospinal tract ablation in adults. 184 Like neurotrophic factors, neutralizing inhibitory myelin and ECM molecules can promote sprouting of some intact systems: blocking Nogo-A promotes sprouting of adult CST axons, 198 and digestion of glial scarassociated CSPGs promotes behavioral recovery following dorsal column injury in the absence of robust regeneration of injured axons.…”

Repair of spinal cord injuries: where are we, where are we going?

“…A concern has always been that small pain ®bres may regenerate better than larger sensory axons, bringing with it the danger of worsening the pain that many people with spinal injuries su er: however, this has not been seen in experiments except where nerve growth factor, which acts on pain ®bres, has been increased in the cord. 5 In addition to the speci®c innervation from sensory and motor axons there is evidence that di use innvervation from serotonergic axons may be bene®cial. In rats with complete spinal transections a transplant of embryonic serotonergic neurones below the injury led to extensive innvervation by small serotonergic ®bres, and a considerable return of stepping and weightbearing function.…”

“…30 Nerve growth factor also promotes regeneration, but receptors for it are prominent on small pain axons, so hyperalgesia can be caused by its presence in excess. 5 An alternative to the application of neurotrophins is to apply treatments that have direct e ects on axonal signalling pathways. There is evidence that the positive e ects of neurotrophins are mediated via cyclic nucleotides, and that inhibitory molecules in the environment act via the small GTPase Rho.…”

“…Treatment of the cord with neurotrophins produces local sprouting of axons with functional e ects. 5,52,53 One of these e ects is not helpful, which is NGF-induced sprouting of pain ®bres, leading to hyperalgesia. Overall, despite the fact that there are now treatments that can in¯uence plasticity in the cord, there remains a need to establish whether these treatments could be helpful in animal models of partial cord damage, and therefore to patients with partial lesions of the cord.…”