Rescue of axotomized rubrospinal neurons by brain-derived neurotrophic factor (BDNF) in the developing opossum, Didelphis virginiana

Neurotrophins: potential therapeutic tools for the treatment of spinal cord injury

Spinal cord injury permanently disrupts neuroanatomical circuitry and can result in severe functional deficits. These functional deficits, however, are not immutable and spontaneous recovery occurs in some patients. It is highly likely that this recovery is dependent upon spared tissue and the endogenous plasticity of the central nervous system. Neurotrophic factors are mediators of neuronal plasticity throughout development and into adulthood, affecting proliferation of neuronal precursors, neuronal survival, axonal growth, dendritic arborization and synapse formation. Neurotrophic factors are therefore excellent candidates for enhancing axonal plasticity and regeneration after spinal cord injury. Understanding growth factor effects on axonal growth and utilizing them to alter the intrinsic limitations on regenerative growth will provide potent tools for the development of translational therapeutic interventions for spinal cord injury.

Cortical reorganization in NT3-treated experimental spinal cord injury: Functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) studies were performed for visualizing ongoing brain plasticity in Neurotrophin-3 (NT3)-treated experimental spinal cord injury (SCI). In response to the electrical stimulation of the forepaw, the NT3-treated animals showed extensive activation of brain structures that included contralateral cortex, thalamus, caudate putamen, hippocampus, and periaqueductal gray. Quantitative analysis of the fMRI data indicated significant changes both in the volume and center of activations in NT3-treated animals relative to saline-treated controls. A strong activation in both ipsi- and contralateral periaqueductal gray and thalamus was observed in NT3-treated animals. These studies indicate ongoing brain reorganization in the SCI animals. The fMRI results also suggest that NT3 may influence nociceptive pathways.

Inhibiting BDNF expression by antisense oligonucleotide infusion causes loss of nigral dopaminergic neurons

Brain derived neurotrophic factor (BDNF) expression is significantly reduced in the Parkinson's disease substantia nigra. This neurotrophin has potent affects on dopaminergic neuron survival protecting them from the neurotoxins MPTP and 6-hydroxydopamine (6-OHDA) commonly used to create animal models of Parkinson's disease and also promoting dopaminergic axonal sprouting. In this study, we demonstrate that an antisense oligonucleotide infusion (200 nM for 28 days) to prevent BDNF production in the substantia nigra of rats mimics many features of the classical animal models of Parkinson's disease. 62% of antisense treated rats rotate (P < or = 0.05) in response to dopaminergic receptor stimulation by apomorphine. 40% of substantia nigra pars compacta tyrosine hydroxylase immunoreactive neurons are lost (P < or = 0.00001) and dopamine uptake site density measured by (3)H-mazindol autoradiography is reduced by 34% (P < or = 0.005). Loss of haematoxylin and eosin stained nigral neurons is significant (P < or = 0.0001) but less extensive (34%). These observations indicate that loss of BDNF expression leads both to down regulation of the dopaminergic phenotype and to dopaminergic neuronal death. Therefore, reduced BDNF mRNA expression in Parkinson's disease substantia nigra may contribute directly to the death of nigral dopaminergic neurons and the development of Parkinson's disease.

Treatment of chronically injured spinal cord with neurotrophic factors stimulates betaII-tubulin and GAP-43 expression in rubrospinal tract neurons

Exogenous neurotrophic factors provided at a spinal cord injury site promote regeneration of chronically injured rubrospinal tract (RST) neurons into a peripheral nerve graft. The present study tested whether the response to neurotrophins is associated with changes in the expression of two regeneration-associated genes, betaII-tubulin and growth-associated protein (GAP)-43. Adult female rats were subjected to a right full hemisection lesion via aspiration of the C3 spinal cord. A second aspiration lesion was made 4 weeks later and gel foam saturated in brain-derived neurotrophic factor (BDNF), glial cell-line derived neurotrophic factor (GDNF), or phosphate-buffered saline (PBS) was applied to the lesion site for 60 min. Using in situ hybridization, RST neurons were examined for changes in mRNA levels of betaII-tubulin and GAP-43 at 1, 3, and 7 days after treatment. Based on analysis of gene expression in single cells, there was no effect of BDNF treatment on either betaII-tubulin or GAP-43 mRNA expression at any time point. betaII-Tubulin mRNA levels were enhanced significantly at 1 and 3 days in animals treated with GDNF relative to levels in animals treated with PBS. Treatment with GDNF did not affect GAP-43 mRNA levels at 1 and 3 days, but at 7 days there was a significant increase in mRNA expression. Interestingly, 7 days after GDNF treatment, the mean cell size of chronically injured RST neurons was increased significantly. Although GDNF and BDNF both promote axonal regeneration by chronically injured neurons, only GDNF treatment is associated with upregulation of betaII-tubulin or GAP-43 mRNA. It is not clear from the present study how exogenous BDNF stimulates regrowth of injured axons.

Regeneration of descending spinal axons after transection of the thoracic spinal cord during early development in the North American opossum, Didelphis virginiana

Opossums are born in an immature, fetal-like state, making it possible to lesion their spinal cord early in development without intrauterine surgery. When the thoracic spinal cord of the North American opossum, Didelphis virginiana, is transected on postnatal day 5, and injections of Fast Blue (FB) are made caudal to the lesion site 30-40 days or 6 months later, neurons are labeled in all of the spinal and supraspinal areas that are labeled after comparable injections in age-matched, unlesioned controls. Double-labeling studies document that regeneration of cut axons contributes to growth of axons through the lesion site and behavioral studies show that animals lesioned on postnatal day 5 use their hindlimbs in normal appearing locomotion as adults. The critical period for developmental plasticity of descending spinal axons extends to postnatal day 26, although axons which grow through the lesion site become fewer in number and more restricted as to origin with increasing age. Animals lesioned between postnatal day 12 and 26 use the hindlimbs better than animals lesioned as adults, but hindlimb function is markedly abnormal and uncoordinated with that of the forelimbs. We conclude that restoration of anatomical continuity occurs after transection of the spinal cord in developing opossums, that descending axons grow through the lesion site, that regeneration of cut axons contributes to such growth, and that animals lesioned early enough in development have relatively normal motor function as adults.