Changes in brain-derived neurotrophic factor and trkB receptor in the adult Rana pipiens retina and optic tectum after optic nerve injury

In this study we used immunocytochemistry to investigate the distribution of brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase (trkB) in retina and optic tectum of the frog Rana pipiens during regeneration after axotomy. We also measured changes in BDNF mRNA in retina and tectum. Retrograde labeling was used to identify retinal ganglion cells (RGCs) prior to quantification of the BDNF immunoreactivity. In control animals, BDNF was found in the majority of RGCs and displaced amacrine cells and in some cells in the inner nuclear layer (INL). After axotomy, BDNF immunoreactivity was reduced in RGCs but increased in the INL. BDNF mRNA levels in the retina remained high before and after axotomy. Three months after axotomy, after reconnection to the target, the staining intensity of many of the surviving RGCs had partially recovered. In the control tectum, BDNF staining was present in ependymoglial cells and in neurons throughout layers 4, 6, 8, and 9. After axotomy, BDNF staining in tectal neurons became more intense, even though mRNA synthesis was transiently down-regulated. In control retinas, trkB receptor immunostaining was present in most RGCs; no significant changes were observed after axotomy. In control tectum, trkB was detected only in ependymoglial cells. After axotomy, many neuronal cell bodies were transiently labeled. Our data are consistent with the hypothesis that a considerable fraction of the BDNF normally present in RGCs is acquired from their targets in the tectum. However, there are also intraretinal sources of BDNF that could contribute to the survival of RGCs.     

Brain-derived neurotrophic factor modulates GAP-43 but not tα1 expression in injured retinal ganglion cells of adult rats

The administration of neurotrophins affects neuronal survival and growth, but less is known about their ability to modify the expression of growth associated genes following injury to CNS neurons. Here we characterize the effect of brain-derived neurotrophic factor (BDNF) on mRNA levels for Tα1 α-tubulin, and for GAP-43, two genes whose expression levels in retinal ganglion cells (RGC) tend to correlate with growth. We first determined that most adult rat RGCs can retrogradely transport BDNF by injecting ¹²⁵I-BDNF into RGC target sites in vivo. We then used quantitative in situ hybridization to characterize the effect of axotomy, or axotomy and BDNF administration on mRNA levels for GAP-43 and Tα1. Axotomy alone resulted in a general decrease in Tα1 α-tubulin mRNA levels by 2 weeks, and elicited an increase in GAP-43 mRNA levels in an average of 30% of surviving RGCs. The intravitreal administration of a single dose of BDNF (5 μg) to axotomized RGCs on the day of injury did not affect Tα1 α-tubulin mRNA levels, but was followed by a moderate (approximately 80%), and short-lasting enhancement of GAP-43 mRNA levels in most RGCs during the first week after axotomy. No significant increase in GAP-43 mRNA levels was observed when BDNF was injected into the uninjured eye. We conclude that BDNF specifically enhances GAP-43 but not Tα1 mRNA levels in injured RGCs. Because BDNF is known to stimulate branch length of injured RGCs, we suggest that changes in the expression of GAP-43, but not Tα1 tubulin, correlate with branching of injured neurons as opposed to long distance regrowth. J. Neurosci. Res. 47:561–572, 1997. © 1997 Wiley-Liss, Inc.     

Effect of different optic nerve lesions on retinal ganglion cell death in the frog Rana pipiens

Following optic nerve crush in various species of frog, a proportion of the retinal ganglion cells re-establishes functional contact with the optic tectum. However, as much as 50% of the retinal ganglion cells die during this process. The determinants of an individual ganglion cell's fate have not been established. In this study of Rana pipiens, cell survival after optic nerve crush was compared with that after nerve cut followed by stump separation, a procedure that considerably delayed entry of optic axons to the brain. It was also ascertained, in the case of delayed ingrowth, whether application of nerve growth factor immediately after lesion influenced the cell death process. This study confirmed that retinal ganglion cell death is a relatively late event in regeneration, because in several animals where anterograde HRP labeling demonstrated regenerating axons within the tectum, no cell death had occurred. There was no statistically significant difference in cell death at 75 days after lesion between animals receiving nerve crush and those receiving nerve cut with stump separation, even though most crush animals had regenerated a complete visual projection, whereas most nerve cut animals had not. The application of NGF did not influence the level of cell death at 75 days after lesion. These results suggest that contact of optic axons with the optic tract or tectum is not necessary for retinal ganglion cell death to occur. However, this does not necessarily mean that contact with the brain is not involved with cell death during regeneration following nerve crush because it is possible that the mechanisms of cell death are different when axons are prevented from regenerating. Further investigations are therefore required to establish the reasons for this cell death.     

Erythropoietin upregulates growth associated protein-43 expression and promotes retinal ganglion cell axonal regeneration in vivo after optic nerve crush

In this study, we established a rat model of optic nerve crush to explore the effects of erythropoietin on retinal ganglion cell axonal regeneration. At 15 days after injury in erythropoietin treated rats, retinal ganglion cell densities in regions corresponding to the 1/6, 3/6 and 5/6 ratios of the retinal radius were significantly increased. In addition, the number of growth associated protein-43 positive axons was significantly increased at different distances (50, 250 and 500 ?m) from the crush site after erythropoietin treatment. Erythropoietin significantly increased growth associated protein-43 protein levels in the retina after crush injury, as determined by western blot and immunofluorescence analysis. These results demonstrate that erythropoietin protects injured retinal ganglion cells and promotes axonal regeneration.     

Up-regulation of brain-derived neurotrophic factor by application of fibroblast growth factor-2 to the cut optic nerve is important for long-term survival of retinal ganglion cells

Application of basic fibroblast growth factor (FGF-2) to the optic nerve after axotomy promotes the survival of retinal ganglion cells (RGCs) in the frog Rana pipiens and results in a rapid up-regulation of brain-derived neurotrophic factor (BDNF) and TrkB synthesis by the RGCs. Here we investigate whether this up-regulation is maintained over the long term and whether it is required for FGF-2's survival effect. At 6 weeks after axotomy and FGF-2 treatment, we found more RGCs immunopositive for BDNF protein and higher intensity of BDNF and TrkB immunostaining, accompanied by increases in BDNF and TrkB mRNA in RGCs. Application of fluorescently labeled siRNA targeted against BDNF to the cut RGC axons showed that it was transported to the cell bodies. Axonal siRNA treatment eliminated the increases in BDNF immunostaining and mRNA that were induced by FGF-2 and had no effect on TrkB mRNA. This reduction in BDNF synthesis by siRNA greatly reduced the long-term survival effect of FGF-2 on RGCs. This, taken together with previous results, suggests that, although FGF-2 may initially activate survival pathways via ERK signaling, its main long-term survival effects are mediated via its up-regulation of BDNF synthesis by the RGCs.     

依托咪酯促进大鼠视神经损伤后再生的研究

目的探讨依托咪酯对成年大鼠视神经损伤后再生的影响。方法选取25只成年SD大鼠,按随机数字表法随机分为依托咪酯治疗组（腹腔注射依托咪酯脂肪乳注射液,15只）、治疗对照组f腹腔注射脂肪乳,5只）和空白对照组（5只）;治疗组又分为低（2mg／kg）、中（4mg／kg）和高（6mg／kg）剂量3个亚组,每亚组5只。采用自体坐骨神经移植模型和荧光金逆行标记再生视网膜神经节细胞（RGCs）。自体坐骨神经移植术后4周,采用视网膜平铺技术计数再生RGCs。结果空白对照组每张视网膜中再生RGCs数量平均为（1032±147）个,治疗对照组为（1114±179）个,两者之间无明显差异fP〉0．05）。低、中和高剂量依托咪酯治疗组每张视网膜中再生RGCs数量分别为（2054±349）个、（2853±498）个和（4118±615）个,与空白对照组和治疗对照组相比均显著增高（P〈0．01）,而且不同剂量之间均差异显著（P〈0．01）。结论依托咪酯能显著促进大鼠视神经损伤后轴突再生,且具有剂量依赖性。     

Quantitative study of the tectally projecting retinal ganglion cells in the adult frog. II. Cell survival and functional recovery after optic nerve transection.

It is known from previous work that ganglion cells disappear from the retina in significant numbers during optic nerve regeneration in the adult frog. In the present study, the population size of surviving ganglion cells that have returned axon terminals to the correct tectal loci was estimated by counts of retrogradely labeled cells in retina-flat-mounts after tectal injections of HRP. Bilaterally symmetric injections were delivered to allow comparison of the normal and affected retinas. The frogs studied had regenerated the left optic nerve and had visually guided behaviors initiated by the recovered eye (see below). The proportion of tectally projecting ganglion cells in the normal retinas and in retinas of normal frogs studied in parallel ranged from 83-86% (Singman and Scalia: J. Comp. Neurol. 302:792-809, 1991). In the affected retinas, the subpopulation of tectally projecting cells was reduced by 40-90% after regeneration, and the relative size of this subpopulation ranged from 67-86%. The optic tectum was injected unilaterally in one specimen, on the side ipsilateral to the regenerated (left) optic nerve. The HRP-labeled ganglion cells in the ipsilateral (left) retina accounted for only 0.8% of the surviving ganglion cells in this animal, whereas it was previously found that the ipsilateral tectally projecting ganglion cells normally amount to 0.9-2.3% (Singman and Scalia, op. cit.) In frogs recovering from transection of the left optic nerve, the frequency, latency, and accuracy of the prey-acquisition responses initiated by the recovering eye were compared with those initiated by the normal eye. Mealworms or lure dummies were used to stimulate prey acquisition. In one experiment, the stimuli were presented unilaterally in the monocular fields of frogs permitted to use both eyes. Prior to the fourteenth postoperative week, the affected eye initiated responses of abnormally long latency and low frequency. In contrast, responses initiated by the affected eye after 14 weeks appeared to be normal in all respects. In another experiment, the normal eye was sutured shut in some frogs recovering for at least 24 weeks and then the affected eye was retested. The affected eye was capable of consistently initiating brisk and accurate prey acquisition. In a final experiment, two stimuli were presented simultaneously in bilaterally symmetric regions of the monocular fields of frogs surviving at least 42 weeks. These fully recovered frogs showed no preference for using either the normal or the recovered eye. Despite severe loss of tectally projecting ganglion cells during optic nerve regeneration, frogs are capable of apparently normal visual responses in prey acquisition tests.     

Molecular plasticity of retinal ganglion cells after partial optic nerve injury

In the past few years we established the partial crush of the optic nerve as an in vivo model system for the study of signaling pathways involved in molecular plasticity after axonal injury. The simplicity of this model at the cellular level allows decisive questions to be anwsered whilst functional aspects of visual information processing can be studied in parallel. A major advantage of a partial optic nerve crush model is the opportunity to directly compare different cell populations: (i) the rapidly degenerating retinal ganglion cells (RGC), (ii) the axotomized RGC population that eventually dies over the period of the next few weeks, (iii) the axotomized RGC population surviving for a long time in the retina without an axon and (iv) the surviving RGC population that maintains axonal connections to their brain targets. Thus, differential aspects of post-lesion plasticity can be analyzed. Using this axonal injury model we investigated the expression of immediate early genes, glutamate receptors, and other differentially expressed genes that we identified with a combined subtractive hybridization and suppression polymerase chain reaction (PCR) screen. Moreover, we characterized time course of cell death, the astroglia response of the retina and optic nerve as well as the topography of anterograde and retrograde axonal transport.     

Molecular plasticity of retinal ganglion cells after partial optic nerve injury

In the past few years we established the partial crush of the optic nerve as an in vivo model system for the study of signaling pathways involved in molecular plasticity after axonal injury. The simplicity of this model at the cellular level allows decisive questions to be anwsered whilst functional aspects of visual information processing can be studied in parallel. A major advantage of a partial optic nerve crush model is the opportunity to directly compare different cell populations: (i) the rapidly degenerating retinal ganglion cells (RGC), (ii) the axotomized RGC population that eventually dies over the period of the next few weeks, (iii) the axotomized RGC population surviving for a long time in the retina without an axon and (iv) the surviving RGC population that maintains axonal connections to their brain targets. Thus, differential aspects of post-lesion plasticity can be analyzed. Using this axonal injury model we investigated the expression of immediate early genes, glutamate receptors, and other differentially expressed genes that we identified with a combined subtractive hybridization and suppression polymerase chain reaction (PCR) screen. Moreover, we characterized time course of cell death, the astroglia response of the retina and optic nerve as well as the topography of anterograde and retrograde axonal transport.     

Artemin augments survival and axon regeneration in axotomized retinal ganglion cells

Artemin, a recently discovered member of the glial cell line‐derived neurotrophic factor (GDNF) family, has neurotrophic effects on damaged neurons, including sympathetic neurons, dopamine neurons, and spiral ganglion neurons both in vivo and in vitro. However, its effects on retinal cells and its intracellular signaling remain relatively unexplored. During development, expression of GFRα3, a specific receptor for artemin, is strong in the immature retina and gradually decreases during maturation, suggesting a possible role in the formation of retinal connections. Optic nerve damage in mature rats causes levels of GFRα3 mRNA to increase tenfold in the retina within 3 days. GFRα3 mRNA levels continue to rise within the first week and then decline. Artemin, a specific ligand for GFRα3, has a neuroprotective effect on axotomized retinal ganglion cells (RGCs) in vivo and in vitro via activation of the extracellular signal‐related kinase? and phosphoinositide 3‐kinase?Akt signaling pathways. Artemin also has a substantial effect on axon regeneration in RGCs both in vivo and in vitro, whereas other GDNF family members do not. Therefore, artemin/GFRα3, but not other GDNF family members, may be of value for optic nerve regeneration in mature mammals. © 2014 Wiley Periodicals, Inc.     

Substance P,bombesin, and leucine-enkephalin immunoreactivities are restored in the frog tectum after optic nerve regeneration

Extensive regeneration of the optic nerve takes place in adult Amphibia. In this study, we have determined whether one aspect of retinotectal organisation, namely immunoreactive laminae in the retinorecipient layers of the optic tectum, is restored after optic nerve regeneration. To do so, the distributions of substance-P, bombesin, and leucine-enkephalin immunoreactivities were examined in the optic tectum of the frog Litoria (Hyla) moorei. Results of a normal series were compared with those at intervals up to 84 days and at 196 days after either unilateral deafferentation or optic nerve crush. In the normal series, distinct neuropeptide immunoreactive laminae were located within the retinorecipient tectal layers. There were two major laminae with substance-P, two with bombesin, and one with leucine-enkephalin immunoreactivities. Additional faint laminae of both substance-P and bombesin immunoreactivity were present in the tectal region that receives input from the visual streak. In addition, labelling of cell bodies and dendrites was seen elsewhere in the tectum. All except one immunoreactive lamina changed after deafferentation. The deeper of those with substance-P immunoreactivity, along with both bombesin laminae, were eventually lost the lamina with leucine-enkephalin immunoreactivity was halved in intensity. We assume that these laminae are wholely or, in the case of the leucine-enkephalin lamina, partially associated with primary optic input. By contrast, the more superficial lamina with substance-P immunoreactivity remained unchanged and is presumably not directly related to visual input. During nerve regeneration, the intensity of all laminae associated with optic input initially fell as in the deafferentation series but, in the long term, recovered to approximately 80% of normal intensities. We conclude that ganglion cells associated with each of the immunoreactivities tested had successfully regenerated. The reduced intensity of immunoreactivities after regeneration is due presumably in part to the cell loss from the ganglion cell population. Furthermore, we discuss the findings of similar studies for Rana pipiens (Kuljis and Karten [1983] J. Comp. Neurol. 217:239–251 and [1985] 240:1–15) in light of the present findings. We argue that some of the previous observations can be reinterpreted to indicate that regeneration was not limited to ganglion cells associated with substance-P immunoreactivity as first thought. © 1995 Wiley-Liss, Inc.     

RNA content of normal and axotomized retinal ganglion cells of rat and goldfish

The responses of rat and goldfish retinal ganglion cells to axotomy were examined by a quantitative cytochemical method for RNA and by morphometric measurement 1-60 (rat) and 3-90 (goldfish) days after interruption of one optic nerve or tract intracranially. Unoperated control animals were studied also. The RNA content of axotomized neurons of rat fell 7-60 days postoperatively. Additionally, atrophy of the axotomized somas occurred. Over time, neuronal atrophy approximately paralleled the loss of RNA, and mean cell area and RNA content were reduced by about 25% 60 days after axotomy. Incorporation of 3H-uridine by axotomized neurons declined also. Axotomized retinal ganglion cells of goldfish behaved differently from those of the rat and showed increases in RNA content, most conspicuously 14-60 days postoperatively. Enlargement of axotomized fish neurons occurred but was less proportionately than concomitant increases in RNA content. The nonaxotomized ganglion cells of goldfish displayed statistically significant increases in size and RNA content 14-49 days after unilateral optic nerve or tract lesions. In contrast, alterations in rat retinal ganglion cells contralateral to interruption of one optic nerve were of limited and questionable significance. The contrasting reactions to axotomy by the retinal ganglion cells of these two vertebrates, one of which regenerates optic axons and one of which does not, may support the proposition that the somal response to axon injury has an important bearing upon the success or failure of CNS regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Retinal ganglion cell death during optic nerve regeneration in the frog Hyla moorei

In the frog Hyla moorei we have estimated there to be between approximately 450,000 and 750,000 cells in the retinal ganglion cell layer. Optic axon counts and retrograde transport of horseradish peroxidase (HRP) indicated that 72-76% of these were ganglion cells. Cells of this type were distributed as a temporally situated area centralis within a horizontal visual streak. Cell and optic axon counts showed that there was an approximately 40% loss of ganglion cells during optic nerve regeneration. Ganglion cells appeared chromatolysed by 6-8 days after an extracranial nerve crush but there was no indication of cell death until 15 days. By this stage anterograde transport of HRP indicated that axons had reached the chiasma. Death was first seen in the area centralis, extended along the streak, and finally was observed in the periphery by 65 days; cell counts demonstrated that at this time the wave of death was almost complete. We have previously shown by electrophysiological visual mapping (Humphrey and Beazley, '82) and confirmed in this study that visuotectal projections were retinotopically organized during regeneration. Multiunit receptive fields were initially large but progressively refined starting in nasal field (temporal retina) to restore a normal projection. The similar sequences whereby the visuotectal projection became refined and death took place in the retinal ganglion cell layer suggested that death may be related to a process of organization within the regenerating projection. In normal animals primary visual pathways revealed by anterograde transport of HRP were essentially similar to those of Rana pipiens and R. esculenta. Regenerating axons generally remained within optic pathways. Exceptions were a retinoretinal projection which was not completely withdrawn even after 1,028 days and a direct projection to the ipsilateral tectum via an inappropriate part of the optic tract.     

Peripheral nerve injury down-regulates CNTF expression in adult rat sciatic nerves.

Ciliary neurotrophic factor (CNTF) is a 200-amino acid protein expressed in high concentrations by peripheral nerves and is thought to be important for the survival and regeneration of injured motoneurons (Lin et al., J Biol Chem 265:8942-8947, 1990). To better understand CNTF's role in nerve injury we have characterized the effects of crush injury on the expression of CNTF in adult rat sciatic nerves using specific antibody and RNA probes. Following a crush injury, both the protein and mRNA levels undergo pronounced decreases distal to the crush. These changes in CNTF expression were qualitatively distinct from changes in the expression of the low-affinity NGF receptor (p75NGFR), which increases following crush. Thus, the changes in CNTF levels do not reflect an overall down-regulation of mRNA during degeneration, and are inconsistent with the proposed role of CNTF in neuronal injury, since its levels are decreasing at the same time as the requirement for neurotrophic support is increasing.     

Different spatial and temporal protein expressions of repulsive guidance molecule a and neogenin in the rat optic nerve after optic nerve crush with and without lens injury

The failure of lesioned mammalian CNS neurons to regenerate their axons remains a challenge. Evidence is emerging that repulsive proteins contribute to this failure. The repulsive guidance molecule A (RGMA) induces growth cone collapse in vitro, accumulates in the scar after spinal cord injury, and is up-regulated in glaucoma. In this study, we evaluated the spatial and temporal localization pattern of RGMA and its receptor neogenin in the optic nerve after optic nerve crush (ONC) without or with lens injury (LI) at up to nine time points (6 hr to 20 days) postsurgery by performing immunohistochemistry and Western blots. We found RGMA at the crush site (CS) and in the developing scar of ONC rats at every time point investigated, whereas it was absent in the CS of ONC + LI rats. Independent of the model, many cells were RGMA(+) in the ON: nerve fibers, blood vessels, astrocytes, oligodendrocytes, some microglia, some macrophages, and the sheath of the ON. Western blots showed a significantly lowered amount of RGMA in ONC + LI animals at 2, 4, and 6 days after crush compared with ONC animals. Furthermore, LI in sham-operated animals showed an increase of RGMA in six of eight time points compared with the sham-operated animals. Moreover, the effects of LI on the morphology of the ON were characterized at a level of detail never reported before. Our results show that RGMA is present and might contribute to the inhibitory environment in the ON, especially in and around the CS after ONC.     

Regulatory effects of trophic factors on expression and distribution of CGRP and GAP-43 in rat motoneurons

Trophic factors play important roles for the regulation of several motoneuron characteristics. In this report, we have studied the effects of different trophic factors on the regulation of two substances that have been linked with axon sprouting and regeneration in rat spinal motoneurons, i.e., calcitonin gene-related peptide (CGRP) and growth-associated protein-43 (GAP-43). The expression of both GAP-43 and CGRP is regulated developmentally and after different lesions. In the adult animal a sciatic nerve transection normally leads to an increased expression of both alpha-CGRP and GAP-43. Local administration of insulin-like growth factor-1 (IGF-1), IGF-2, or ciliary neurotrophic factor (CNTF) did not significantly change this response, whereas basic fibroblast growth factor (bFGF) attenuated the lesion-induced upregulation of alpha-CGRP mRNA. None of the studied factors had any influence on the upregulation of GAP-43 after lesion. In vitro, GAP-43 and alpha-CGRP mRNAs could be detected in enriched motoneuron cultures prepared from E14 rat embryos. The GAP-43 expression was upregulated by bFGF, IGF-1, IGF-2, and CNTF. CNTF also upregulated alpha-CGRP mRNA in cultured motoneurons, whereas bFGF had the opposite effect. IGF-1 and -2 did not significantly affect the alpha-CGRP mRNA levels. The decrease in GAP-43-immunopositive neuromuscular junctions (NMJs), seen during normal postnatal development, was attenuated after intramuscular injections of bFGF, CNTF, IGF-2, or CGRP 8–37, a CGRP antagonist. At the same time bFGF decreased and CNTF increased the number of CGRP-immunoreactive NMJs, whereas no difference from control was seen in IGF-2-treated muscles. These results show important regulatory effects of trophic factors on the expression of both GAP-43 and alpha-CGRP in motoneurons. However, it also is shown that the trophic factors used here influence the expression of these two substances in distinctively different patterns. This may have important implications for the understanding of trophic interactions in the spinal motor system and also should be considered in the evaluation of possible effects of CGRP and GAP-43 in motoneurons. The effect of a CGRP antagonist on GAP-43 in muscle indicates a role for CGRP in sprouting-related processes.     

Cavernous nerve injury elicits GAP-43 mRNA expression but not regeneration of injured pelvic ganglion neurons

Recovery of erectile dysfunction after cavernous nerve injury takes a long period. To elucidate this mechanism, unilateral cavernous nerve of male rat was cut, and the expression level of a nerve regeneration marker, the growth associated protein-43 (GAP-43) mRNA was evaluated by in situ hybridization and RT-PCR. While GAP-43 mRNA expression was transiently increased in the injured neurons of the major pelvic ganglion (MPG) at 7 days after nerve injury, continuous increase of GAP-43 mRNA was observed in the contralateral MPG from 7 days to 6 months after the nerve injury. Histochemical double-labeling studies for either neuronal NOS (nNOS) or tyrosine hydroxylase (TH) and the GAP-43 mRNA expression demonstrated that in injured MPG the transient up-regulation of GAP-43 mRNA was mainly seen in nNOS negative and/or TH positive neurons, suggesting non-parasympathetic post-ganglionic neurons, and also demonstrated that in contralateral MPG GAP-43 mRNA positive neurons were gradually increased in nNOS positive but TH negative neurons, suggesting parasympathetic post-ganglionic neurons. When a retrograde tracer Fluorogold (FG) was injected into the penile crus 7 days before histological experiments, FG-positive neurons were, if any, hardly seen in nNOS-positive neurons of the injured MPG for at least 6 months, whereas numerous FG-positive cells were seen in nNOS-positive neurons of the contralateral MPG. These results suggest that post-ganglionic projecting neurons of the intact side, which express increased GAP-43 mRNA, would be most likely to contribute to the recovery of the erectile function after unilateral cavernous nerve injury possibly by a plastic change such as nerve sprouting.