Differential regulation by exercise of BDNF and NT-3 in rat spinal cord and skeletal muscle

We have investigated the impact of neuromuscular activity on the expression of neurotrophins in the lumbar spinal cord region and innervating skeletal muscle of adult rats. Rats were exercised on a treadmill for 1 day or 5 consecutive days and euthanized at 0, 2 or 6 h after the last bout of exercise. By Day 1, there was no clear evidence of an increase in brain-derived neurotrophic factor (BDNF) mRNA in the spinal cord or the soleus muscle. By Day 5, there was a significant increase in BDNF mRNA in the spinal cord at 2 h post-training, and the soleus muscle showed a robust increase between 0 and 6 h post-training. Immunoassays showed significant increases in BDNF protein in the soleus muscle by training Day 5. Immunohistochemical analyses showed elevated BDNF levels in motoneuron cell bodies and axons in the ventral horn. Neurotrophin-3 (NT-3) mRNA was measured to determine whether selected neurotrophins respond with a selective pattern of induction to neuromuscular activity. In the spinal cord, there was a progressive post-training decrease in NT-3 mRNA following a single bout of training, while there was a significant increase in NT-3 mRNA at 2 h post-training by Day 5. The soleus muscle showed a progressive increase in NT-3 mRNA by Days 1 and 5 following training. These results show that neuromuscular activity has specific effects on the BDNF and NT-3 systems, and that repetitive exercise affects the magnitude and stability of these responses.     

Voluntary exercise increases neurotrophin-3 and its receptor TrkC in the spinal cord

We have evaluated changes in the expression of neurotrophin-3 (NT-3) and its tyrosine kinase C (TrkC) receptor in the neuromuscular system as a result of voluntary physical activity. We assessed changes in the mRNAs and proteins for NT-3 and TrkC in the lumbar spinal cord and associated soleus muscle following 3 and 7 days of voluntary wheel running. We used quantitative Taqman RT-PCR to measure mRNA and ELISA to assess protein levels. NT-3 mRNA and protein levels increased in the spinal cord to reach statistical significance after 7 days of exercise compared to sedentary control rats. Immunohistochemical analyses localized the elevated NT-3 to the substantia gelatinosa (SG) and nucleus of the dorsal horn. TrkC mRNA levels were significantly elevated in the spinal cord after 3 and 7 days of running. In the soleus muscle, NT-3 mRNA levels and its receptor TrkC were elevated after 3 days, while NT-3 protein levels remained unaffected. The results demonstrate that voluntary exercise has a differential effect on NT-3 as well as its receptor TrkC in the neural and muscular components of the neuromuscular system, and emphasize the role of voluntary activity on the spinal cord and muscle.     

神经营养因子3基因修饰神经干细胞移植脊髓损伤的相关蛋白表达☆

背景：研究表明神经干细胞和神经营养因子3基因修饰的神经细胞联合移植能够在移植后存活并有效促进脊髓横断后脊髓的功能恢复,但神经营养因子3基因修饰的神经干细胞能否在脊髓受损部位发挥功能并促进脊髓损伤大鼠的功能恢复? 目的：观察神经营养因子3基因修饰胚胎脊髓神经干细胞移植后脊髓损伤大鼠的功能恢复情况及损伤局部的基因表达。 方法：将30只SD大鼠在T9水平进行脊髓半切后,随机分为3组,分别在受损脊髓内植入细胞培养液、神经干细胞及神经营养因子3基因修饰神经干细胞。另取10只仅行椎板切除设置为空白对照。移植后通过行为学测试评价脊髓功能的恢复,RT-PCR和Western blot检测脊髓损伤部位神经营养因子3和髓鞘碱性蛋白的表达。 结果与结论：移植神经营养因子3基因修饰神经干细胞组行为学测试结果最好,移植细胞培养液组行为学测试最差。与移植细胞培养液组相比,移植神经干细胞及神经营养因子3基因修饰神经干细胞组大鼠脊髓组织中神经营养因子3基因和髓鞘碱性蛋白基因的mRNA水平明显上调,在蛋白水平也有类似的结果,且神经营养因子3基因修饰神经干细胞组效果更明显。提示移植神经营养因子3基因修饰神经干细胞能促进脊髓受损部位出现更多向少突胶质细胞分化的细胞,并能更强的表达神经营养因子3。     

Neurotrophin expression by spinal motoneurons in adult and developing rats

Expression of the neurotrophins NT-4, brain-derived neurotrophic factor (BDNF), and NT-3 in adult rat lumbosacral spinal cord motoneurons is reported. A sensitive in situ hybridization procedure demonstrates localization of the mRNA for each of these neurotrophins within spinal motoneurons of the adult and in early postnatal development. A majority of adult rat spinal cord lumbar motoneurons (approximately 63%) express NT-4 mRNA as assessed by counting motoneurons in the L4 and L5 segments of two adult rat spinal cords on adjacent cresyl violet-stained and in situ hybridization sections. Similarly, a majority of lumbar motoneurons (approximately 73%) express BDNF mRNA. Further analyses of adjacent lumbar spinal cord sections revealed that many, although not all motoneurons coexpress both NT-4 and BDNF mRNAs. At birth, the mRNA encoding NT-3 is expressed in motoneurons, but BDNF mRNA is not apparent until postnatal day 5 (P5) and NT-4 mRNA first appears at P9. The potential biological significance of neurotrophin mRNA expression in spinal motoneurons is supported by immunohistochemical localization of each neurotrophin protein in adult motoneurons. We discuss the potential role of spinal cord neurotrophins as autocrine or paracrine factors involved in modulating motoneuron synaptic function.     

Changes in urinary bladder neurotrophic factor mRNA and NGF protein following urinary bladder dysfunction

Spinal cord injury and cyclophosphamide-induced cystitis dramatically alter lower urinary tract function and produce neurochemical, electrophysiological, and anatomical changes that may contribute to reorganization of the micturition reflex. Mechanisms underlying this neural plasticity may involve alterations in neurotrophic factors in the urinary bladder. These studies have determined neurotrophic factors in the urinary bladder that may contribute to reorganization of the micturition reflex following cystitis or spinal cord injury. A ribonuclease protection assay was used to measure changes in urinary bladder neurotrophic factor mRNA (betaNGF, BDNF, GDNF, CNTF, NT-3, and NT-4) following spinal cord injury (acute/chronic) or cyclophosphamide-induced cystitis (acute/chronic). The correlation between urinary bladder nerve growth factor mRNA and nerve growth factor protein expression was also determined. Each experimental paradigm resulted in significant (P 相似文献   

Different expressions of BDNF, NT3, and NT4 in muscle and nerve after various types of peripheral nerve injuries

The changes in the expression of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) in the rat neuromuscular system as a result of three different types of sciatic nerve injuries have been evaluated. The changes in mRNA and protein levels for BDNF, NT-3, and NT-4 in the soleus muscle and sciatic nerve were assessed 4-28 days after sciatic nerve transection (neurotmesis), sciatic nerve crush (axonotmesis), and mild acute compression (neurapraxia). BDNF mRNA levels increased dramatically with nerve transection in the soleus muscle and the sciatic nerve 7-14 days after injury, whereas the changes were low in other types of injury. The changes of protein levels for BDNF were also similar. The mRNA and the protein levels of NT-3 in the soleus muscle did not show any significant difference. The mRNA for NT-4 in the soleus muscle decreased from 4 to 14 days after sciatic nerve transection, and the protein level was also minimum 14 days after sciatic nerve transection. Our results indicate that the neurotrophic factors in the neuromuscular system could play a role in differentiating peripheral nerve injury.     

Differences in neurotrophic factor gene expression profiles between neonate and adult rat spinal cord after injury

The capacity of the central nervous system for axonal growth decreases as the age of the animal at the time of injury increases. Changes in the expression of neurotrophic factors within embryonic and early postnatal spinal cord suggest that a lack of trophic support contributes to this restrictive growth environment. We examined neurotrophic factor gene profiles by ribonuclease protection assay in normal neonate and normal adult spinal cord and in neonate and adult spinal cord after injury. Our results show that in the normal developing spinal cord between postnatal days 3 (P3) and P10, compared to the normal adult spinal cord, there are higher levels of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and glial-derived neurotrophic factor (GDNF) mRNA expression and a lower level of ciliary neurotrophic factor (CNTF) mRNA expression. Between P10 and P17, there is a significant decrease in the expression of NGF, BDNF, NT-3, and GDNF mRNA and a contrasting steady and significant increase in the level of CNTF mRNA expression. These findings show that there is a critical shift in neurotrophic factor expression in normal developing spinal cord between P10 and P17. In neonate spinal cord after injury, there is a significantly higher level of BDNF mRNA expression and a significantly lower level of CNTF mRNA expression compared to those observed in the adult spinal cord after injury. These findings suggest that high levels of BDNF mRNA expression and low levels of CNTF mRNA expression play important roles in axonal regrowth in early postnatal spinal cord after injury.     

Temporal changes in the expression of some neurotrophins in spinal cord transected adult rats

Functional recovery of neurons in the spinal cord after physical injury is essentially abortive in clinical cases. As neurotrophins had been reported to be responsible, at least partially, for the lesion-induced recovery of spinal cord, it is not surprising that they have become the focus of numerous studies. Studies on endogenous neurotrophins, especially the three more important ones, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in injured spinal cord might provide some important clues in clinical treatment. Here we investigate the immunohistological expression of the above three factors at lower thoracic levels of the spinal cord as well as changes in the motor functions of the adult rat hindlimbs after cord transection. The injured rats were allowed to survive 3, 7, 14 and 21 days post operation (dpo). Flaccid paralysis was seen at 3 dpo following cord transection, however, hindlimb function showed partial recovery from 7 dpo to 21 dpo. The numbers of NGF, BDNF and NT-3 immunopositive neurons and their optical densities all increased in the lesion-induced cord. The immuno-expression of NGF and BDNF peaked at 7 dpo, while that of NT-3 peaked at 7 dpo and remained so at least up to 14 dpo. These results suggested that neurotrophins might play essential roles in functional recovery of after spinal cord injury, but the time points for the expression of the three factors differed somewhat.     

Exercise-induced gene expression in soleus muscle is dependent on time after spinal cord injury in rats

Cycling exercise attenuates atrophy in hindlimb muscles and causes changes in spinal cord properties after spinal cord injury in rats. We hypothesized that exercising soleus muscle expresses genes that are potentially beneficial to the injured spinal cord. Rats underwent spinal cord injury at T10 and were exercised on a motor-driven bicycle. Soleus muscle and lumbar spinal cord tissue were used for messenger RNA (mRNA) analysis. Gene expression of brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) was elevated 11- and 14-fold, respectively, in soleus muscle after one bout of exercise performed 5 days after spinal cord transection. Also, c-fos and heat shock protein-27 (HSP27) mRNA abundance were increased 11- and 7-fold, respectively. When exercise was started 2 days after the injury, the changes in gene expression were not observed. By contrast, at 2 but not at 5 days after transection, expression of the HSP27 gene was elevated sixfold in the lumbar spinal cord, independent of exercise. Electromyographic activity in soleus muscles was also decreased at 2 days, indicating that the spinal cord was less permissive to exercise at this early time. Long-term exercise for 4 weeks attenuated muscle atrophy equally well in rats started at 2 days or 5 days after injury. We conclude that BDNF and GDNF released from exercising muscle may be involved in exercise-induced plasticity of the spinal cord. Furthermore, the data suggest that the lumbar spinal cord undergoes time-dependent changes that temporarily impede the ability of the muscle to respond to exercise.     

BDN F Down-regulates Neurotrophin Responsiveness, TrkB Protein and TrkB mRNA Levels in Cultured Rat Hippocampal Neurons

Regulation of Trk receptors by their ligands, the neurotrophins, was investigated in dissociated cultures of embryonic day 18 rat hippocampal neurons. Cultures were exposed to brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) or NT-4/5 for 24 h upon plating followed by factor washout. As determined by immunohistochemical staining and phosphotyrosine blotting, the functional responses to acute stimulation with BDNF, NT-3 and NT-4/5, including c-Fos induction and phosphorylation of Trk and extracellular signal-regulated kinase (ERK) proteins, were significantly decreased after 6 days in culture by prior exposure to BDNF. As determined by Western and Northern blot analysis respectively, there was a parallel down-regulation of TrkB protein as well as of trkB and trkC mRNA levels in BDNF-pretreated cultures. Exposure to NT-3 or NT-4/5 at the same concentrations as BDNF did not down-regulate any of the measured cellular responses or TrkB protein and/or trkB and trkC mRNA levels. Regulation of hippocampal neuronal TrkB protein does not appear to be just a developmental phenomenon, as infusion of BDNF into the hippocampus of adult rats for 6 days produced an 80% decrease in levels of full-length TrkB protein. We thus show that exposure of hippocampal neurons to BDNF, both in culture and in the adult brain, results in down-regulation of TrkB. At least in vitro , this leads to long-term functional desensitization to BDNF. NT-3 and NT-4/5. as well as down-regulation of trkB and trkC mRNA.     

Developmentally Regulated Expression of HDNF/NT-3 mRNA in Rat Spinal Cord Motoneurons and Expression of BDNF mRNA in Embryonic Dorsal Root Ganglion

Northern blot analysis was used to demonstrate high levels of hippocampus-derived neurotrophic factor/neurotrophin-3 (HDNF/NT-3) mRNA in the embryonic day (E) 13 - 14 and 15 - 16 spinal cord. The level decreased at E18 - 19 and remained the same until postnatal day (P) 1, after which it decreased further to a level below the detection limit in the adult. In situ hybridization revealed that the NT-3 mRNA detected in the developing spinal cord was derived from motoneurons and the decrease seen at E18 - 19 was caused by a reduction in the number of motoneurons expressing NT-3 mRNA. The distribution of NT-3 mRNA-expressing cells in the E15 spinal cord was very similar to the distribution of cells expressing choline acetyltransferase or nerve growth factor receptor (NGFR) mRNA. Moreover, a striking similarity between the developmentally regulated expression of NT-3 and NGFR mRNA was noted in spinal cord motoneurons. A subpopulation of all neurons in the dorsal root ganglia expressed brain-derived neurotrophic factor (BDNF) mRNA from E13, the earliest time examined, to adulthood. These results are consistent with a trophic role of NT-3 for proprioceptive sensory neurons innervating the ventral horn, and imply a local action of BDNF for developing sensory neurons within the dorsal root ganglia.     

Neurotrophic factors expressed in both cortex and spinal cord induce axonal plasticity after spinal cord injury

We reported recently that overexpression of neurotrophin-3 (NT-3) by motoneurons in the spinal cord of rats will induce sprouting of corticospinal tract (CST) axons (Zhou et al. [2003] J. Neurosci. 23:1424-1431). We now report that overexpression of brain-derived neurotrophic factor (BDNF) or glial cell-derived neurotrophic factor (GDNF) in the rat sensorimotor cortex near the CST neuronal cell bodies together with overexpression of NT-3 in the lumbar spinal cord significantly increases axonal sprouting compared to that induced by NT-3 alone. Two weeks after unilaterally lesioning the CST at the level of the pyramids, we injected rats with saline or adenoviral vectors (Adv) carrying genes coding for BDNF (Adv.BDNF), GDNF (Adv.GDNF) or enhanced green fluorescent protein (Adv.EGFP) at six sites in the sensorimotor cortex, while delivering Adv.NT3 to motoneurons in each of these four groups on the lesioned side of the spinal cord by retrograde transport from the sciatic nerve. Four days later, biotinylated dextran amine (BDA) was injected into the sensorimotor cortex on the unlesioned side to mark CST axons in the spinal cord. Morphometric analysis of axonal sprouting 3 weeks after BDA injection showed that the number of CST axons crossing the midline in rats treated with Adv.BDNF or Adv.GDNF were 46% and 52% greater, respectively, than in rats treated with Adv.EGFP or PBS (P < 0.05). These data demonstrate that sustained local expression of neurotrophic factors in the sensorimotor cortex and spinal cord will promote increased axonal sprouting after spinal cord injury, providing a basis for continued development of neurotrophic factor therapy for central nervous system damage.     

Combined delivery of neurotrophin-3 and NMDA receptors 2D subunit strengthens synaptic transmission in contused and staggered double hemisected spinal cord of neonatal rat

We investigated whether administration of neurotrophin-3 (NT-3) and NMDA-2D-expressing units, found previously to enhance transmission in neonatal rat spinal cord, strengthens synaptic connections in the injured neonatal cord. We employed electrophysiological methods to evaluate the strength of synaptic transmission to individual motoneurons in the contusion and staggered double hemisection spinal cord injury (SCI) models. SCI at caudal thoracic levels (T11-T12) was carried out at postnatal day 2 (P2). Plugs containing NT-3- secreting fibroblasts and NR2D-expressing HSV-1 amplicons (HSVnr2d) were implanted above the lesion. Control animals were treated with an amplicon-expressing beta-galactosidase (HSVlac). After 8-10 days of treatment, the rats were sacrificed and spinal cords were removed for intracellular recording. Untreated contused cords preserved a fraction of white matter and weak monosynaptic responses were observed through the injury region. However, no synaptic connections were observed in control cords receiving double hemisection injury. Combined treatment with NT-3 and HSVnr2d strengthened monosynaptic connections in contused cords and induced the appearance of weak but functional multisynaptic connections in double hemisected cords. In contrast, treatment with either NT-3 or HSVnr2d alone failed to induce appearance of synaptic responses through the hemisected region. These results suggest that chronic treatment with NT-3 secreting fibroblasts combined with facilitated function of NMDA receptors by HSVnr2d treatment strengthens connections that survive incomplete SCI and therefore that such combined treatment might facilitate recovery of function following SCI.     

Increase in neurotrophin-3 expression followed by Purkinje cell degeneration in the adult rat cerebellum after spinal cord transection

Changes in brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) contents following thoracic spinal cord transection were investigated in the cerebral cortex, hippocampus, and cerebellum of rats. The NT-3 content became significantly elevated at 3 days after transection only in the cerebellum and gradually declined to the control level by 6 days after the injury, remaining unchanged in the cerebral cortex and hippocampus. No significant change in the BDNF content was observed in any of the regions tested. Immunohistochemical analysis showed that the labeling indicating NT-3-like immunoreactivity was intensified in both cerebellar granule and Purkinje cells 3 days after the injury. The number of Purkinje cells with aggregation of chromatin around the nuclear membrane and swelling of the cytoplasm and/or organelles gradually increased with time starting 4 days after the injury, demonstrating morphological changes indicative of necrosis. However, no abnormal morphology was found in cerebellar granule cells at any time examined. We suggest that it is reasonable that increased NT-3 stimulated the death of Purkinje cells, because 1) the degeneration was necrosis, which is known to be accelerated by neurotrophins under certain pathological conditions, and 2) the increase in NT-3 occurred prior to Purkinje cell degeneration. Therefore, our present results may imply that spinal cord injury-induced NT-3 accelerates injury rather than alleviates degeneration of Purkinje cells.     

Neurotrophic and migratory properties of an olfactory ensheathing cell line

Olfactory ensheathing cells (OECs) are a unique type of macroglia required for normal olfactory axonal regeneration throughout the lifetime of an individual. Recent evidence in the literature suggests that OECs transplanted into injured spinal cords may facilitate axonal regeneration. In this study, we evaluated the neurotrophic properties of OECs using a homogeneous clonal cell line (nOEC), which does not contain contaminating cell types found in all primary OEC cultures. The results indicate that nOECs express mRNA for NGF, BDNF, NT-4/5, and neuregulins, but not for NT-3 or CNTF. In addition, nOECs secrete NGF, BDNF, and neuregulin, but retain NT-4/5 intracellularly. Finally, prelabeled nOECs derived from rat survived transplantation into a dorsal hemisected region of the hamster spinal cord and migrated only in the injured, dorsal portion of the spinal cord. This migratory pattern suggests that the nOECs are viable in vivo and respond to signals originating from the injured neuronal cells and their processes.     

BDNF abolishes the survival effect of NT-3 in axotomized Clarke neurons of adult rats

Neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) have previously been shown to support survival and axonal regeneration in various types of neurons. Also, synergistic neuroprotective effects of these neurotrophins have been reported in descending rubrospinal neurons after cervical spinal cord injury (Novikova et al., [2000] Eur. J. Neurosci. 12:776-780). The present study investigates the effects of intrathecally delivered NT-3 and BDNF on the survival and atrophy of ascending spinocerebellar neurons of Clarke nucleus (CN) after cervical spinal cord injury in adult rats. At 8 weeks after cervical spinal cord hemisection, 40% of the axotomized CN neurons had been lost, and the remaining cells exhibited marked atrophy. Microglial activity was significantly increased in CN of the operated side. Intrathecal infusion of NT-3 for 8 weeks postoperatively resulted in 91% cell survival and a reduction in cell atrophy, but did not reduce microglial activity. In spite of the fact that the CN neurons expressed both TrkC and TrkB receptors, only NT-3 had a neuroprotective effect, whereas BDNF was ineffective. Furthermore, when a combination of BDNF and NT-3 was administered, the neuroprotective effect of NT-3 was lost. The present results indicate a therapeutic potential for NT-3 in the treatment of spinal cord injury, but also demonstrate that in certain neuronal populations the neuroprotection obtained by a combination of neurotrophic factors may be less than that of a single neurotrophin.     

Preparation of brain-derived neurotrophic factor-and neurotrophin-3-secreting schwann cells by infection with a retroviral vector

One reason that the central nervous system of adult mammals does not regenerate after injury is that neurotrophic factors are present only in low concentrations in these tissues. Recent studies have shown that the application of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) acts to encourage the regrowth of motor and sensory fibers after spinal cord injury. Other studies have reported that the regrowth of axons after injury was enhanced by the implantation of Schwann cells, which normally secrete BDNF and NT-3. The purpose of the present study was to genetically modify Schwann cells to secrete increased amounts of BDNF or NT-3 by infection with a retroviral vector. Retroviral vectors were constructed by the ligation of BDNF or NT-3 cDNA to the LXSN vector. Viruses were generated from the plasmid forms of the vectors by transient transfection of PA317 amphotrophic retroviral packaging cells. Viruses were harvested and used to infect the human Schwann cell line designated NF-1T. Northern blot analysis of poly (A⁺) RNA prepared from Schwann cells that were infected with BDNF- or NT-3-containing virus showed the presence of BDNF or NT-3 mRNA. An enzyme-linked immunosorbent assay (ELISA) for BDNF and NT-3 was performed on media the cells were grown in, and on cellular extracts prepared from the BDNF- and NT-3-infected Schwann cells. The ELISA results demonstrated that the Schwann cells were secreting increased levels of immunologically active BDNF or NT-3. Immunocytochemical staining of these cells revealed the presence of these two neurotrophic factors located in perinuclear granules. These neurotrophic factor-secreting Schwann cells are currently being evaluated for their efficacy in the treatment of spinal cord injury.     

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.