Differential effects of intravitreal optic nerve and sciatic nerve grafts on the survival of retinal ganglion cells and the regeneration of their axons

We have investigated the effects of intravitreal sciatic nerve (SN) and/or optic nerve (ON) grafts on the survival and the axonal regeneration of retinal ganglion cells (RGCs). Following transection of the ON, approximately 40% RGCs survived at 7 days post-axotomy (dpa). Results showed that the intravitreal ON graft significantly promoted the survival of RGCs at 7 dpa (39,063 vs 28,246). Intravitreal SN graft, however, did not rescue axotomized RGCs at 5, 7 or 14 dpa. Axotomized RGCs could be induced to regenerate axons along a segment of SN graft attached to the proximal stump of ON. On average, 608 axotomized RGCs were induced to regenerate axons along the attached SN graft. The presence of intravitreal SN graft promoted about 100% increase in the number of regenerating RGCs (1,227) relative to the control groups. The intravitreal ON graft, surprisingly, also induced about 100% more regenerating RGCs (1220) than in the control group. When SN and ON grafts were co-transplanted into the vitreous, about 200% more regenerating RGCs (1916) were observed than in the control group. These findings illustrated that the intravitreal ON graft rescued axotomized RGCs and enhanced the regeneration of retinal axons. This is the first report to show that ON promotes RGC axonal regeneration. The intravitreal SN graft did not rescue RGCs but promoted axonal regeneration. The differential effects of intravitreal ON and SN grafts on the survival and the RGC regeneration suggest that these might be two independently operating events.     

Characterization of the sprouting response of axon-like processes from retinal ganglion cells after axotomy in adult hamsters: a model using intravitreal implantation of a peripheral nerve

Summary Peripheral nerves provide a favourable environment for damaged CNS axons to sprout and regenerate. It has also been demonstrated that retinal ganglion cells respond to a peripheral, nerve segment grafted to the retina by emitting axon-like processes from the somatodendritic compartment into the graft. The factors influencing the pattern of sprouting of axotomized retinal ganglion cells were explored in this study by implanting a short segment of peripheral nerve, which did not come into contact with the retina, into the vitreous body of an eye whose optic nerve was concurrently crushed. Silver staining was used to assess the morphology of the retinal ganglion cells which underwent sprouting.Some retinal ganglion cells were induced to sprout axon-like processes; these emerged primarily from dendrites and less frequently from the soma or intraretinal axon. Implantation of a nonviable graft (freezed-thawed) elicited only minimal sprouting. These results suggest that diffusible factors secreted by cells in the graft are a possible stimulus to sprouting in axotomized retinal ganglion cells.Examination of the pattern of dendritic sprouting indicates that sprouting was most intense (in terms of number of sprouts per cell) at early times post-axotomy. Moreover, a differential pattern of development of sprouts arising from individual primary dendrites of the same cell was observed; sprouts tend to arise from all primary dendrites initially but as the post-axotomy time increased, retraction of sprouts from some primary dendrites occurred. Concomitant with this retraction, however, there was an increase in the number of sprouts on those primary dendrites which were still in the active phase of sprouting. Selective stabilization of sprouts by extrinsic factors may account for this phenomenon.Changes in the area and outline (irregularity) of the somata of retinal ganglion cells with sprouts from two weeks to two months after optic nerve crush could be correlated temporally with the intensity of sprouting from the dendritic tree, suggesting that during sprouting, intrinsic mechanisms coordinate the responses of different cellular compartments.In contrast to extensive ectopic sprouting of axotomized retinal ganglion cells in the presence of an intravitreal graft, when a long peripheral nerve segment is grafted to the cut optic nerve, there is extensive axonal regeneration into the graft from retinal ganglion cells, most of which did not exhibit ectopic sprouting. Thus, a hierarchy of sprouting sites within a neuron seems to exist, with the damaged axonal tip being the most favoured site, followed by the dendrites, and then the intraretinal axon. The soma appears to be the least preferred compartment for sprout emission.     

Ciliary neurotrophic factor promotes the regrowth capacity but not the survival of intraorbitally axotomized retinal ganglion cells in adult hamsters

Ciliary neurotrophic factor has recently been shown to promote the axonal regrowth of retinal ganglion cells into a peripheral nerve graft following an intracranial transection of the optic nerve (approximately 7 mm from the optic disc). It is unclear whether the enhancement of axonal regrowth by ciliary neurotrophic factor application correlates with the enhancement of survival of retinal ganglion cells and/or the up-regulation of expression of growth-associated protein-43 messenger RNA in retinas. The present study evaluated the regenerative capacity of retinal ganglion cells following intraorbital transection of the optic nerve (approximately 1.5 mm from the optic disc) and the attachment of a peripheral nerve to the ocular stump of the optic nerve. In addition, we have determined the survival of retinal ganglion cells and the expression of growth-associated protein-43 messenger RNA in ciliary neurotrophic factor-treated retinas following optic nerve transection. The results showed that in the ciliary neurotrophic factor-treated retinas, the number of retinal ganglion cells which had regrown axons into a peripheral nerve is about four times more than the control. In the axotomized retinas, ciliary neurotrophic factor initiated sprouting of axon-like processes at 14 and 28 days post-axotomy and up-regulated the expression level of growth-associated protein-43 messenger RNA at 7, 14 and 28 days post-axotomy. However, ciliary neurotrophic factor did not prevent the death of axotomized retinal ganglion cells. We suggest that one possible mechanism for the axonal regeneration of axotomized retinal ganglion cells by ciliary neurotrophic factor could be mediated by the up-regulation of growth-associated protein-43 gene expression and not by increasing the pool of surviving retinal ganglion cells after axotomy.     

Peripheral nerve explants grafted into the vitreous body of the eye promote the regeneration of retinal ganglion cell axons severed in the optic nerve

Summary We have conducted experiments in the adult rat visual system to assess the relative importance of an absence of trophic factors versus the presence of putative growth inhibitory molecules for the failure of regeneration of CNS axons after injury. The experiments comprised three groups of animals in which all optic nerves were crushed intra-orbitally: an optic nerve crush group had a sham implant-operation on the eye; the other two groups had peripheral nerve tissue introduced into the vitreous body; in an acellular peripheral nerve group, a frozen/thawed teased sciatic nerve segment was grafted, and in a cellular peripheral nerve group, a predegenerate teased segment of sciatic nerve was implanted. The rats were left for 20 days and their optic nerves and retinae prepared for immunohistochemical examination of both the reaction to injury of axons and glia in the nerve and also the viability of Schwann cells in the grafts. Anterograde axon tracing with rhodamine-B provided unequivocal qualitative evidence of regeneration in each group, and retrograde HRP tracing gave a measure of the numbers of axons growing across the lesion by counting HRP filled retinal ganglion cells in retinal whole mounts after HRP injection into the optic nerve distal to the lesion. No fibres crossed the lesion in the optic nerve crush group and dense scar tissue was formed in the wound site. GAP-43-positive and rhodamine-B filled axons in the acellular peripheral nerve and cellular peripheral nerve groups traversed the lesion and grew distally. There were greater numbers of regenerating fibres in the cellular peripheral nerve compared to the acellular peripheral nerve group. In the former, 0.6–10% of the retinal ganglion cell population regenerated axons at least 3–4 mm into the distal segment. In both the acellular peripheral nerve and cellular peripheral nerve groups, no basal lamina was deposited in the wound. Thus, although astrocyte processes were stacked around the lesion edge, a glia limitans was not formed. These observations suggest that regenerating fibres may interfere with scarring. Viable Schwann cells were found in the vitreal grafts in the cellular peripheral nerve group only, supporting the proposition that Schwann cell derived trophic molecules secreted into the vitreous stimulated retinal ganglion cell axon growth in the severed optic nerve. The regenerative response of acellular peripheral nerve-transplanted animals was probably promoted by residual amounts of these molecules present in the transplants after freezing and thawing. In the optic nerves of all groups the astrocyte, microglia and macrophage reactions were similar. Moreover, oligodendrocytes and myelin debris were also uniformly distributed throughout all nerves. Our results suggest either that none of the above elements inhibit CNS regeneration after perineuronal neurotrophin delivery, or that the latter, in addition to mobilising and maintaining regeneration, also down regulates the expression of axonal growth cone-located receptors, which normally mediate growth arrest by engaging putative growth inhibitory molecules of the CNS neuropil.     

轴突损伤距离对成年金黄地鼠视网膜节细胞轴突在正常或预先溃变周围神经移植物中再生的影响

目的　系统研究轴突损伤的距离与轴突再生之间的关系 ,以及预先溃变周围神经移植物对视网膜节细胞 (简称节细胞 )轴突在不同距离损伤后再生的影响。　方法　在距成年金黄地鼠眼球后极 0 .5、1、1.5、2、3或7m m处切断左侧视神经 ,视神经眶侧断端同正常 (正常组 )或预先溃变 (溃变组 )的周围神经移植物相吻合。　结果　移植术后 2 8d,两组荧光金逆行标记的发出再生轴突的节细胞数量均随轴突损伤距离的增加而下降 ,并以 0 .5～3m m距离点之间下降最为明显。比较两组对应距离点 ,溃变组标记节细胞数量在 2、3mm距离点较正常组显著增多。　结论　成年金黄地鼠节细胞轴突损伤的距离 ,决定了发出再生轴突的节细胞数量。预先溃变周围神经移植物对节细胞轴突再生具有促进作用 ,并以轴突损伤距离为条件。     

成体金黄地鼠神经节细胞轴突在外周神经移植到视网膜后的再生

在成体金黄地鼠的视网膜上移植一段自体坐骨神经1～4个月后,视网膜神经节细胞纤维长入移植的坐骨神经中长达2 cm。具有再生纤维的神经节细胞分布于外周神经插入处与视网膜边缘之间的扇形或带状区域内。细胞数目与插入处的位置有关,接近视神经乳头处较远离视神经乳头处标记细胞数量多。移植后的视网膜标记神经元细胞体面积分布直方图表明:具有再生纤维的神经元胞体面积范围除包括正常大小的神经节细胞外,还包括相当多的胞体增大的神经元。用荧光染料核黄施于视束、真蓝施于移植的坐骨神经后的逆行荧光双标记法实验表明,再生的轴突起源于神经节细胞的损伤轴突,而不是完整神经节细胞轴突的侧芽。     

霍乱毒素及外周神经对视神经远端切断后视网膜谷氨酸能节细胞再生的作用

目的：探讨霍乱毒素(CTx)及外周神经对成年金黄地鼠远端视神经受损后视网膜谷氨酸((Glu)能节细胞(RGCs)再生的作用。方法：远端切断视神经并缝接自体坐骨神经(AG)，玻璃体内注射CTx及／或植入小段坐骨神经分支(SN)。动物分为AG CTx组；AG SN组；AG SN CTx组，分别存活4W、5W，荧光金和免疫荧光组织化学双标法标记再生的RGCs。结果：术后5W，AG CTx组；AG SN组；AG SN CTx组Glu免疫反应阳性RGCs再生数分别占再生总数的4.25％、2.50％及6.00％，AG SN组与AG SN CTx组间差异显著。结论：CTx与SN能协同促进视神经远端切断后Glu能节细胞的再生．     

Morphological plasticity of axotomized retinal ganglion cells following intravitreal transplantation of a peripheral nerve segment

Summary During normal development of retinal ganglion cells when the axons are growing, transient dendritic spines have been observed. Similar dendritic spine-like processes are also exhibited by retinal ganglion cells undergoing axonal regeneration into a peripheral nerve grafted to the damaged optic axons. Here we show, using the intracellular injection of Lucifer Yellow, that when a segment of peripheral nerve is transplanted to the vitreous body, a procedure which induces ectopic sprouting of axon-like processes from the cell bodies and dendrites of some retinal ganglion cells, similar spine-like processes appear on the dendrites of cells with ectopic sprouts. Quantitative analysis indicated that there were significant changes with post-transplantation survival time in the distributions of spine-like processes and axon-like processes on these sprouting retinal ganglion cells following the intravitreal transplantation of a piece of peripheral nerve. The remodelling of the spine-like processes and axon-like processes correlated with one another suggesting that plastic changes can occur in certain dendritic subcompartments independent of the growth activity of the other dendritic subcompartments.     

骨髓间充质干细胞对成年大鼠视网膜节细胞再生的影响

目的：探讨骨髓间充质干细胞(MSCs)对受损成年大鼠视网膜节细胞(RGCs)轴突再生的影响。方法：切断大鼠视神经,缝接自体坐骨神经(AG)。动物分对照组、AG MSCs组、AG MSCs tPNS(三七总皂苷)组。各组动物存活3、4周,用粒蓝逆行标记再生的RGCs,荧光镜下观察视网膜平铺片中再生RGCs的数量变化。免疫组化检测MSCs在玻璃体内的分化。结果：动物存活3、4周,再生的RGCs数目实验组与对照组有显著性差异。AG MSC组和AG MSCs tPNS组,存活的细胞表达Vimentin、NF和GFAP,Laminin无表达。结论：玻璃体植入MSCs可促进受损伤的视网膜节细胞轴突再生。     

H89和wortmannin对霍乱毒素促进远端视神经损伤后视网膜节细胞再生的影响

目的：探讨H89和wortmannin对霍乱毒素(CTx)促进成年金黄地鼠远端视神经受损后视网膜节细胞(RGCs)再生的影响。方法：远端切断视神经并对接一段自体坐骨神经(AG)，玻璃体内注射CTx及H89、wortmannin。动物随机分为AG；AG CTx组；AG CWx H89组；AG CTx wortmannin组，4w后粒蓝逆行标记再生的RGCs，荧光显微镜下观察计数。结果：AG CTx组再生的视网膜节细胞平均数(43．2)明显高出AG组(2.6)，AG CTx H89组平均再生数为3．2，与AG组相近，明显低于AG CTx组。AG CTx W组平均再生数为9.6，明显高于AG，低于AG CTx组。结论：H89和wortmannin可部分抑制CTx对视神经远端切断后视网膜节细胞再生的促进作用。     

霍乱毒素对远端视神经损伤后视网膜节细胞再生的影响

目的　探讨霍乱毒素 (CTx)对成年金黄地鼠远端视神经受损后视网膜节细胞 (RGCs)再生的作用。方法　远端切断视神经并对接一段自体坐骨神经 (AG) ,玻璃体内注射CTx及 或植入小段坐骨神经分支 (SN)。动物随机分为对照组Ⅰ (AG组 )与对照组Ⅱ (溶剂组 ) ;AG SN组 ;AG CTx组 ;AG SN CTx组和量效关系组 ,前五组分别存活 4周～ 6周 ,用粒蓝逆行标记再生的RGCs ,荧光显微镜下观察。　结果　AG CTx组各时间点再生的视网膜节细胞比对照组Ⅰ与对照组Ⅱ明显增加 ,具统计学意义 (P <0 0 5 ) ;AG SN组得出相似结果。AG SN CTx组与其它几组相比在各时间点上均存在极显著性差异 (P <0 0 1)。　结论　霍乱毒素可明显提高远端视神经受损后视网膜节细胞的再生。     

霍乱毒素对金黄地鼠视网膜节细胞再生的作用

目的：探讨霍成毒素（CTx)对金黄地鼠视网膜节细胞（RGCs)再生的促进作用，方法：成年金黄地鼠近端切断视神经（ON）并接一段自体坐骨神经（AG），玻璃体内注射CTx及／或插入小段坐骨神经分支（SN）。动物随机分为AG组和溶剂组；AG＋CTx组；AG＋SN组；AG＋CTx SN组；量效关系组。前4组动物存活2－6周，用粒蓝逆行标记再生的RGCs，在荧光镜下观察视网膜平铺片再生RGCs的数量变化。结果：AG＋CTx组各时间点视风膜再生RGCs平均数比AG组和溶剂组增加，具有统计学意义（P＜0．05）；AG＋SN组也得出相似结果。AG＋CTx＋SN组各时间点的视网膜再生RGCs平均数分别比AG组和AG＋SN组明显增加（P＜0．01）。结论提示霍乱霉素或／与坐骨神经具有显著促进视神经切断后视网膜节细胞再生的作用。     

Regrowth of retinal ganglion cell axons into a peripheral nerve graft in the adult hamster is enhanced by a concurrent optic nerve crush

Summary Transplantation of a segment of peripheral nerve to the retina of the adult hamster resulted in regrowth of damaged ganglion cell axons into the graft, with the fastest regenerating axons extending at 2 mm/day after an initial delay of 4.5 days (Cho and So 1987b). In this study, the effect of making 2 lesions on the same axon (the conditioning lesion effect) on the regrowth of ganglion cell axons into the peripheral nerve graft was examined. When a conditioning lesion (first lesion) was made by crushing the optic nerve 7 or 14 days before the peripheral nerve grafting (the second lesion) to the retina, the distance of regrowth achieved by the fastest regenerating axons in the graft, measured at the 7th post-grafting day, was lower than in animals with a peripheral nerve grafted to a normal eye. This indicated that in contrast to the situation in peripheral nerve axons (Forman et al. 1980) and goldfish optic axons (Edwards et al. 1981), the conditioning lesion was unable to enhance the regrowth of mammalian retinal ganglion cell axons. However, when crushing of the optic nerve was followed immediately by peripheral nerve grafting, an enhancement in axonal regrowth could be observed. The initial delay time before the axons extended into the peripheral nerve graft was reduced by 1 day while the rate of elongation of the fastest regrowing axons in the graft apparently remained unchanged. Moreover, the shortening of the initial delay could still be observed even when the sequence of performing the 2 lesions was reversed. From these data, it was concluded that the classical conditioning lesion effect was not responsible for the enhancement observed. Rather it was suggested that changes in the intra-retinal environment brought about by crushing of the optic nerve might account for it.     

MK-801和L-NA对成年仓鼠视网膜节细胞轴突损伤后存活与再生的影响

目的 研究NMDA受体阻断剂MK-801和一氧化氮合酶抑制剂L-NA对成年仓鼠视网膜节细胞(下称节细胞)轴突切断后存活和再生的影响。方法 切断动物左侧视神经后分两组：存活组存活2、7或14d；再生组视神经眶侧断端同一段坐骨神经吻合后存活28d。所有实验动物自视神经切断前1d开始，每日接受腹腔注射MK-801和／或LNA直至处死。结果存活节细胞均数在MK-801组与对照组间无显著性差异，但L-NA组在术后2和7d节细胞数较对照组显著增加，合用MK-801／LNA较单用MK-801或L-NA使更多节细胞存活。而MK-801或L-NA对节细胞轴突在周围神经移植物内的再生均无明显作用。结论 lmg／kg剂量的MK-801对节细胞的存活无明显作用，但同时阻断NMDA受体和抑制一氧化氮合酶比单纯抑制一氧化氮合酶对节细胞有更强的神经保护作用。1．0mg／kgMK-801或4．5mg／kgL-NA对节细胞轴突的再生无明显促进作用。     

GAP-43 in the axons of mammalian CNS neurons regenerating into peripheral nerve grafts

Summary Although mature mammalian CNS neurons do not normally regenerate axons after injury, it is well established that they will regrow axons over long distances into peripheral nerve implants. We have autografted segments of sciatic nerve into the brains of adult albino rats and have used light and electron microscopic immunocytochemistry to examine the distribution of the growth associated protein GAP-43 in and around the graft in the first two weeks following implantation. GAP-43 was present, 3–14 days after grafting, in small non-myelinated axonal sprouts in the brain parenchyma around the proximal tip of the graft. At 11–14 days after implantation similar sprouts within the graft itself were GAP-43 immunoreactive. The sprouts were either naked or associated with other cell processes (chiefly of Schwann cells; to a lesser extent of astrocytes). We also show that small numbers of neuronal perikarya around the tip of the graft become GAP-43 immunoreactive 11–14 days after implantation. Thus mature mammalian CNS neurons regenerating axons into a PNS graft display a marked increase in their content of GAP-43. In addition, we report that small plaques of GAP-43 reaction product are sometimes present on the plasma membranes of Schwann cells or astrocytes adjacent to immunoreactive axons, and that narrow sheet-like or filopodial processes of astrocytes, Schwann cells and possibly other non-neuronal cell types, may contain small amounts of GAP-43.     

Intravitreal injections of neurotrophic factors and forskolin enhance survival and axonal regeneration of axotomized beta ganglion cells in cat retina

Some retinal ganglion cells in adult cats survive axotomy for two months and regenerate their axons when a peripheral nerve is transplanted to the transected optic nerve. However, regenerated retinal ganglion cells were fewer than 4% of the total retinal ganglion cell population in the intact retina. The present study examined the effects of intravitreal injections of neurotrophic factors (brain-derived neurotrophic factor, ciliary neurotrophic factor, basic fibroblast growth factor, glial cell-derived neurotrophic factor, neurotrophin 4), first on the survival of axotomized cat retinal ganglion cells within 2 weeks, and then on axonal regeneration of the retinal ganglion cells for 2 months after peripheral nerve transplantation. We tested first enhancement of the survival by one of the factors, and then one or two of them supplemented with forskolin, which increases intracellular cAMP. Single injections of 0.5 microg or 1 microg brain-derived neurotrophic factor, 1 microg ciliary neurotrophic factor, or 1 microg glial cell-derived neurotrophic factor significantly increased total numbers of surviving retinal ganglion cells; 1.6-1.8 times those in control retinas. Identification of retinal ganglion cell types with Lucifer Yellow injections revealed that the increase of surviving beta cells was most conspicuous: 2.5-fold (brain-derived neurotrophic factor) to 3.6-fold (ciliary neurotrophic factor). A combined injection of 1 microg brain-derived neurotrophic factor, 1 microg ciliary neurotrophic factor, and 0.1 mg forskolin resulted in a 4.7-fold increase of surviving beta cells, i.e. 50% survival on day 14. On the axonal regeneration by peripheral nerve transplantation, a combined injection of brain-derived neurotrophic factor, ciliary neurotrophic factor, and forskolin resulted in a 3.4-fold increase of beta cells with regenerated axons. The increase of regenerated beta cells was mainly due to the enhancing effect of neurotrophic factors on their survival, and possibly to a change of retinal ganglion cell properties by cAMP to facilitate their axonal regeneration.     

Regrowth and connectivity of injured central nervous system axons in adult rodents

The capacity of injured nerve cells to regrow and form terminal connections in the CNS of adult mammals was investigated in axotomized retinal ganglion cells (RGCs) of rodents whose optic nerves were substituted by an autologous segment of peripheral nerve. While many RGCs died after axotomy approximately 20% of the surviving RGCs regenerated axons several cm in length. Some of the regenerated RGC axons entered the superior colliculus where they arborized and formed well differentiated synapses that transynaptically excited or inhibited tectal neurons.     

The growth-associated protein GAP-43 appears in dorsal root ganglion cells and in the dorsal horn of the rat spinal cord following peripheral nerve injury

When adult dorsal root ganglion cells are dissociated and maintained in vitro, both the small dark and the large light neurons show increases in the growth-associated protein GAP-43, a membrane phosphoprotein associated with neuronal development and plasticity. Immunoreactivity for GAP-43 appears in the cytoplasm of the cell bodies as early as 3.5 h post axotomy and is present in neurites and growth cones as soon as they develop. At early stages of culture (4 h to eight days) satellite/Schwann cells are also immunoreactive for GAP-43. Neurons in isolated whole dorsal root ganglion maintained in vitro become GAP-43-immunoreactive between 2 and 3 h after axotomy. It takes three days however, after cutting or crushing the sciatic nerve in adult rats in vivo, for GAP-43 immunoreactivity to appear in the axotomized dorsal root ganglion cells. GAP-43 immunoreactivity can be detected in the central terminals of primary afferent neurons in the superficial laminae of the dorsal horn of the lumbar enlargement four days after sciatic cut or crush. The intensity of the GAP-43 staining reaches a peak at 21 days and becomes undetectable nine weeks following crush injury and 36 weeks following sciatic nerve cut. The pattern of GAP-43 staining is identical to the distribution of sciatic small-calibre afferent terminals. Little or no staining is present in the deep dorsal horn, but GAP-43 does appear in the ipsilateral gracile nucleus 22 days after sciatic injury. In investigating the mechanism of GAP-43 regulation, blockade of axon transport in the sciatic nerve with vinblastine (10(-5) M-10(-4) M) or capsaicin (1.5%) was found to produce a pattern of GAP-43 immunoreactivity in the dorsal horn identical to that found with crush, while electrical stimulation of the sciatic nerve had no effect. Axotomy of primary sensory neurons or the interruption of axon transport in the periphery therefore acts to trigger GAP-43 production in the cell body. The GAP-43 is transported to both the peripheral and the central terminals of the afferents. In the CNS the elevated GAP-43 levels may contribute to an inappropriate synaptic reorganization of afferent terminals that could play a role in the sensory disorders that follow nerve injury.     

Effects of inosine on axonal regeneration of axotomized retinal ganglion cells in adult rats

Damage to the central nervous system (CNS) is always followed by an irreversible axon degeneration of injured neurons. The purine nucleoside inosine has been shown to induce neurons to regenerate axons in culture and in vivo. In the present study, we investigated the in vivo effects of inosine on the axon regeneration of axotomized retinal ganglion cells (RGCs) in adult rats, using the model of peripheral nerve (PN) grafting onto the ocular stump of the transected optic nerve. Animals were allowed to survive for 4 weeks after surgery with repeated intraperitoneal injections of inosine 1 day before PN grafting till they were killed. Treatment with inosine induced a significant increase (62%) in the number of FluroGold -labeled RGCs regrowing their axons into the PN graft, when compared with the control animals. The axon outgrowth-promoting effect of inosine in adult rodents may represent a potential clinical treatment for injured or degenerated CNS.