Increase in ganglion cell size after optic nerve regeneration in the frog, Rana pipiens

Axonal sprouting in the optic nerve is not a prerequisite for successful regeneration

Axonal sprouting, the production of axons additional to the parent one, occurs during optic nerve regeneration in goldfish and the frog Rana pipiens, with numbers of regenerate axons exceeding normal values four- to sixfold (Murray [1982] J. Comp. Neurol. 209:352-362; Stelzner and Strauss [1986] J. Comp. Neurol. 245:83-103). To determine whether axonal sprouting is a prerequisite for regeneration, the frog Litoria moorei was examined, a species that undergoes successful optic nerve regeneration but with a different time course compared with R. pipiens. Sprouting was assessed, as in goldfish and R. pipiens, from electron microscopic counts between the lesion and chiasm. However, disconnected axons that persist after axotomy would have falsely elevated the counts. The suspected overlap of these two axon populations was confirmed by labeling regenerate axons anterogradely with DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) and disconnected ones retrogradely with DiA (4-4-dihexadecylaminostyrl 1-N methylpyridinium iodide). Numbers of disconnected axons were estimated after preventing regeneration and subtracted from numbers in regenerate nerves. Throughout, the total number of regenerate axons was approximately one third lower than normal (P < 0.05) supporting a previous finding of minimal axonal sprouting in L. moorei (Dunlop et al. [2002] J. Comp. Neurol. 446:276-287). The validity of the subtractive electron microscopic method was confirmed by retrograde labeling to estimate numbers of retinal ganglion cells whose axons had crossed the lesion; values were approximately one third lower than normal. The data suggest that sprouting is not essential for either axon outgrowth or topographic map refinement.

Intravitreal injections of neurotrophic factors support the survival of axotomized retinal ganglion cells in adult rats in vivo

After transection of the optic nerve (ON) in adult rats, retinal ganglion cells (RGC) progressively degenerate until, after two months, a residual population of only about 5% of these cells survives. In this study, we investigated the effect of regeneration-associated factors from sciatic nerve (ScN), BDNF, and CNTF on the survival of adult rat RGC after intraorbital ON transection. Neurotrophic factors were injected into the vitreous body. Rats were allowed to survive 3, 5, or 7 weeks, and the remaining viable RGC were then labelled by retrograde staining with the carbocyanine dye, 4Di-10Asp, which was applied onto the proximal nerve stump in vivo. The animals were sacrificed 3 days later and RGC counted in retinal whole mounts. Due to progressive degeneration following nerve transection the number of surviving RGC decreased to about 10% of the initially labelled population after 3 weeks, to about 8% after 5 weeks, and to about 5% after 7 weeks. Survival of axotomized cells could be prolonged using either of the neurotrophic factors: after 3 weeks a 2-3-fold increase in the number of viable RGC could be obtained compared to uninjected controls and to those which received injection of buffer. The prolonged survival effect vanished after 5 and 7 weeks, and no additive effect could be seen when combining brain-derived neurotrophic factor (BDNF) and ciliary neuronotrophic factor (CNTF) treatment. Morphometric analysis of labelled cells revealed that all neurotrophic factors supported predominantly large RGC with somal areas > 250 micron 2. In retinae from rats that survived the ON transection for several months, a characteristic population of axotomy-resistant RGC remained alive. Their few, very large, and often curled dendrites showed signs of placticity in the depleted inner nuclear layer of the adult rat retina. We conclude that the intraocular injection of CNTF, BDNF, and ScN-derived medium, which retard the process of lesion-induced RGC degeneration, may be successfully used as a subsidiary strategy in transplatation protocols. This would result in larger populations of RGC which can be recruited to regenerate their axons and provide a basis for functional recovery.

Frog tectal efferent axons fail to regenerate within the CNS but grow within peripheral nerve implants

Tectal efferent axons, located adjacent to the optic tract, fail to regenerate past diencephalic lesions in Rana pipiens even though optic axons regenerate after the same injury (M. J. Lyon and D. J. Stelzner, J. Comp. Neurol. 255: 511-525). We tested the possibility that tectal efferent axons can regenerate within peripheral nerve implants. A 6- to 8-mm segment of autologous sciatic nerve was implanted into the anterolateral (N = 23) or centrolateral (N = 22) portion of the dorsal surface of the tectum. Frogs survived for 6 (N = 16) or 12 weeks (N = 29) before the free end of the nerve was recut and HRP applied. A control group had the nerve crushed prior to the HRP application. Neurons within the tectum, near and medial to the implant site, were retrogradely labeled from the nerve graft in most experimental operates but no neurons were labeled in controls. In addition, neurons were also labeled in nuclei which projected to the tectum in a number of cases. Three times as many neurons were labeled in 12-week operates (42 +/- 46) as in 6-week operates (15 +/- 12). The morphology and location of labeled neurons in the tectum was similar to tectal efferent neurons except that the somal area of neurons labeled from the graft was significantly larger (41%) than normal tectal efferent neurons. The basic finding is similar to experiments using the same paradigm in the mammalian central nervous system (CNS). One difference is the minimal glial reaction at the graft insertion site.(ABSTRACT TRUNCATED AT 250 WORDS)

Further study of the outward displacement of retinal ganglion cells during optic nerve regeneration, with a note on the normal cells of Dogiel in the adult frog

In a previous study we observed massive retinal ganglion cell death in adult Rana pipiens after periods of optic nerve regeneration, and reported that large numbers of the surviving cells had become displaced bodily into the inner plexiform layer of the affected eye (Scalia et al.: Brain Research 344:267-280, 1985). The outwardly displaced cells could be identified as retinal ganglion cells because they could be back-filled with horseradish peroxidase (HRP) injected into the regenerated optic nerve. Quantitative observations on the abnormal outward displacement of ganglion cells are reported here. Parallel observations on normally displaced ganglion cells (cells of Dogiel) are also reported to clarify the distinctions between these two classes of cells. For the present work, injections of HRP of varying size were placed in the optic tectum bilaterally in 3 normal frogs and 9 frogs sustaining unilateral optic nerve regeneration. Most injections were centered at loci mapping the middle region of the nasal retina. The retinas were examined as flat-mounts and in-section. In 8 other frogs sustaining optic nerve regeneration, the HRP was administered bilaterally directly to the optic nerves in the orbit. Ganglion cells were labeled by retrograde transport of the HRP in the retinal ganglion cell layer in both the normal and affected eyes in areas topographically isomorphic with the tectal areas subtended by the injections. In the normal eyes, the orthotopic ganglion cells formed a strict monolayer, and virtually no cells existed in the inner plexiform layer. In the retinas sustaining optic nerve regeneration, the retinal ganglion cells abnormally displaced into the inner plexiform layer were also labeled topographically in correspondence with the injection sites. The abnormally displaced cells comprised 5.5% of the total population of surviving neurons in the retinal ganglion cell and inner plexiform layers. The mean outward dislocation of the displaced cells, as measured in one frog surviving optic nerve crush for 8 weeks, was 69.9 +/- 2.4% of the distance across the inner plexiform layer, which itself was uniformly 14.3 +/- 0.39 microns thick. Cells of Dogiel, which were embedded within the inner nuclear layer, were also labeled when the injections of HRP spread to include the area of representation of the optic disc. The labeled cells were restricted to a dorsal, peripapillary locus capping the optic disc. Therefore, some cells of Dogiel project to the tectum normally, but only from the central retina.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in alpha-tubulin and actin gene expression during optic nerve regeneration in frog retina

The optic nerve of the bullfrog was transected and the regeneration process was investigated. We previously reported that alpha-tubulin mRNA in the retina increased to a maximum 1-2 h after optic nerve transection with no specific change in actin mRNA. In the present investigation, we examined the long-term effect of optic nerve transection. Northern blot analysis revealed that alpha-tubulin mRNA increased again gradually after the rapid and transient increase and actin mRNA increased to a maximum at 7 days (more than twofold compared to the control retinas). The period during which actin mRNA reaches a maximal increase almost corresponds to the time lag between the axotomy and the initiation of axonal outgrowth. The main cytoskeletons of neuronal growth cones have been shown to consist of actin-containing microfilaments. Therefore, the transient increase of actin mRNA may have a relationship to the initial outgrowth of axons. On the other hand, the rapid and transient increase of alpha-tubulin mRNA observed in our previous studies is probably one of the initial responses of retinal ganglion cells to the axotomy, and the gradual increase in alpha-tubulin mRNA observed in this study can probably be interpreted as provision of the structural materials necessary for axonal elongation.

Morphological and physiological properties of neurons after long-term axonal regeneration: observations on chronic and delayed sequelae of peripheral nerve injury

Axonal regeneration has been the focus of extensive investigation of mechanisms which mediate structural and functional recovery after injury to mammalian peripheral nerves and has proven to be a valuable model for development and plasticity in the nervous system. Although details of the acute morphological and physiological responses to nerve injury are well-described, less information is available to nerve injury are well-described, less information is available about long-term alterations which persist or develop after regenerated axons have established connections with their targets. The present paper briefly discusses the mammalian neuron's initial response to peripheral nerve injury and subsequent events which occur during regeneration. Morphological and physiological alterations observed in neurons after long-term axonal regeneration are described and are considered in the context of their potential implications for clinical recovery after nerve injury, as well as their potential contribution to the appearance of delayed neurological dysfunction. Selective responses to neuronal injury during development and in different fiber populations are discussed.

Morphological changes in spinal motor neurons giving rise to long-term regenerated sciatic nerve axons

Morphological properties of rat spinal motor neurons were examined 14-16 months following unilateral sciatic nerve crush and compared to the properties observed in neurons contralateral to injury and in cord segments from age-matched control rats. Regenerated and control motor neurons were identified by retrograde labelling with HRP applied to sciatic nerves distal to the site of crush or at a comparable location in control nerves. Many of the experimental motor neurons were enlarged and had thickened dendritic processess compared to the finer dendrites seen in control cells. Mean cell area ipsilateral to the crush lesions was larger than mean control cell area (P-value less than 0.001) despite representation of all control cell areas in the regenerated population. These data suggest that persistent or continued morphological abnormalities occur in mammalian motor neurons following simple sciatic crush injury when examined at extended times beyond the period of axonal regeneration and clinical recovery.