Accelerated axonal sprouting after nerve transection

Effects of repeated transection and coaptation of peripheral nerves on axonal regeneration and motoneuron survival

BACKGROUND AND PURPOSE

Salvage procedures for facial reanimation can involve a second neurorrhaphy operation. It remains unclear whether reuse of the original donor nerve in the salvage procedure remains likely to produce successful outcome. This study aimed to investigate the effect of repeated transection and coaptation of a nerve on axonal regrowth and motoneuron survival.

MATERIALS AND METHODS

The sciatic nerves of Sprague Dawley rats were transected and microsutured once (the one-time group) or repeatedly at eight-week intervals (the repeated group), and the animals remained alive for eight weeks before sacrifice. The gastrocnemius muscle was weighed, and muscle fiber diameter was measured with hematoxylin-eosin staining. Axonal count of the distal nerve stump was calculated by toluidine blue staining. Myelin thickness and axonal diameter were analyzed by transmission electronic microscopy. Finally, motoneurons were retrogradely traced to the spinal cord using Fluoro-Gold.

RESULTS

Repeated coaptation of nerves resulted in significant decreases of the wet weight ratio of gastrocnemius and muscle fiber diameter. The axonal counts and myelin thicknesses of the distal stumps were comparable between the groups, whereas axonal diameter was significantly smaller after repeated injury. Additionally, retrograde tracing demonstrated significantly less motoneurons in the L4-L6 spinal segments of the repeatedly injured animals than that of the one-time group.

CONCLUSIONS

Compared with one-time nerve injury, repetitive transection and coaptation of nerves resulted in compromised axonal regeneration, motoneuron survival, and target muscle recovery. It is possible that the final functional outcome could also be compromised, and the patients should be counseled accordingly.

Influence of peripheral inflammation on growth-associated phosphoprotein (GAP-43) expression in dorsal root ganglia and on nerve recovery after crush injury

An experimental study was performed to investigate the influence of the inflammation in peripheral target tissue on growth-associated phosphoprotein (GAP-43) expression in dorsal root ganglia (DRG) and on recovery of crushed nerve. Fifty-four male Wistar rats were used for this study. The sciatic nerve was operatively crushed unilaterally with an aneurysm clip. Inflammation in peripheral target tissue was induced by injection of complete Freund's adjuvant (CFA) at 1 week before crush. In crushed or crushed with arthritis rats DRGs were examined in immunohistochemistry for GAP-43 and the sciatic nerves were observed in Epon embedded sections with toluidine blue stain. In addition, electrophysiological studies of the nerves were performed to evaluate the recovery of function. Immunohistochemical studies showed the ratio of GAP-43 immunopositive cells in crushed with arthritis rats was significantly lower than that in crushed rats at 1 week after crush (P<0.01). Electrophysiological studies at 4 weeks after crush showed functional nerve recovery in crushed with arthritis rats was significantly suppressed compared with that in crushed rats (P<0.01). Histological studies showed the mean diameter of the axons in crushed with arthritis rats was significantly smaller than that in crushed rats (P<0.01). All these findings indicate that inflammation in peripheral target tissue suppresses GAP-43 expression in DRG and eventually suppresses functional and morphological recovery of the crushed nerve.

Peripheral but not central axotomy promotes axonal outgrowth and induces alterations in neuropeptide synthesis in the nodose ganglion of the rat

We investigated the effects of central and peripheral axotomy of the sensory neurons in the nodose ganglion on neurite outgrowth and neuropeptide expression. Axonal outgrowth was studied in ganglia subjected to a conditioning lesion of the vagus nerve 6 days prior to in vitro explantation. In such cultures, a conditioning effect, i. e. a shorter initial delay and faster axonal outgrowth, was observed after peripheral axotomy, while central axotomy had no effect. Neuropeptide expression was measured by immunocytochemistry 3 days after axotomy. Peripheral axotomy induced an increase in the number of neurons expressing the C-terminal flanking peptide of neuropeptide Y (C-PON), galanin (GAL) and vasoactive intestinal peptide (VIP). In contrast, central axotomy did not affect neuropeptide expression. These results suggest that both axonal outgrowth and expression of neuropeptides in the sensory neurons of the nodose ganglion could be regulated by the contact of the cells with their peripheral, but not their central targets.

Regenerative axonal sprouting in the cat trochlear nerve

Following peripheral trochlear nerve axotomy in the cat, the normal number of myelinated axons is restored despite significant motor neuron death, suggesting regulation of the number of myelinated axons in the regenerated nerve. In this study we used light and electron microscopy to examine the production and maintenance of axonal sprouts at different locations in the nerve and at different postoperative intervals. Despite proliferative sprouting and an overproduction of nonmyelinated axons in the regenerating trochlear nerve, the number of myelinated axons was strictly regulated. Only approximately 1,000 regenerated axons were eventually remyelinated, but many nonmyelinated axons were still present 6-8 months postaxotomy. Regenerated axons were remyelinated in a proximal-to-distal direction between 3 and 4 weeks postaxotomy. We also examined the maturation of regenerated myelinated axons by measuring axon diameter and myelin index (an expression of myelin thickness). Mean myelinated axon diameter remained significantly below normal in long-term regenerated nerves. Mean myelin index was not different from normal at 4 weeks postaxotomy but was significantly decreased at long postoperative intervals, reflecting a slightly thicker myelin sheath relative to the axon diameter. This relative increase in mean myelin thickness could serve to restore normal conduction velocity despite the decrease in mean axon diameter. We suggest that the regulation of the number of myelinated axons at the normal number despite cell death and the increase in mean myelin thickness may both be compensatory mechanisms that function to restore preoperative conditions and maximize functional recovery.

Tenotomy prevents the functional improvement of a muscle reinnervated with a chronically severed nerve

During the early stages of nerve implantation, we followed the dynamic properties of the lateral gastrocnemius muscle of the rat, reinnervated with an acutely or chronically severed peroneal nerve. The aim of this study was to ascertain whether 1) the better functional recovery of a muscle reinnervated by a chronically severed foreign nerve is present from the onset of reinnervation, and 2) whether such functional improvement is due to the conditioning lesion effect. Our results indicate that better functional recovery is already apparent one week after nerve implantation, and it is due to the conditioning lesion effect, since tenotomy prevents such improvement. The tenotomy effect underlines the fact that some environmental factors concerning the target tissue, and not only the predegenerated nerve, are involved in the conditioning effect.

Progressive degeneration of motor nerve terminals in GAD mutant mouse with hereditary sensory axonopathy

The evolution of motor nerve degeneration was examined in gracile axonal dystrophy (GAD) mutant mice, which develop initial sensory ataxia and subsequent motor paresis. Using the anterior gracilis (AG) muscle, which is innervated at two discrete and well-separated endplate zones, we demonstrated that axonal degeneration occurred first at motor nerve terminals in the distal endplate zone, and then extended gradually from the distal to the more proximal parts of affected axons in the intra-muscular nerve trunk. In contrast to the degeneration in the distal zone, active degeneration was less marked in the proximal endplate zone and, furthermore, most terminal axons had begun to produce regenerating sprouts. Ventral horn cells were histologically normal, even at advanced stages. These results indicate that, as previously observed in sensory nerves, dying back degeneration progresses later in the lower motor neuron system, even within one muscle. The mechanism(s) influencing the activation of axonal regeneration are discussed. This mutant mouse will be a useful model for the study of regenerating phenomena in dying back degeneration of genetically compromised motor neurons, as well as for the study of the pathogenesis of hereditary sensory and motor neuropathies in man.

Myelinated fiber regeneration after sciatic nerve crush: morphometric observations in young adult and aging mice and the effects of macrophage suppression and conditioning lesions

To study myelinated nerve fiber regeneration during aging, the right sciatic nerves of 6- and 24-month-old mice were crushed at the sciatic notch. Two, 4, and 8 weeks later, both groups of mice were perfused. The sciatic nerves were processed so that the transverse sections of each nerve subsequently studied by light and electron microscopy included the entire posterior tibial fascicle 5 mm distal to the crush site. Two weeks after axotomy, fascicles of aging mice contained significantly fewer regenerated myelinated fibers than those of young adults. After 4 weeks, the difference in the number of myelinated fibers was less. However, measurements of myelinated fibers in fascicles of aging mice showed that areas of Schwann cell cytoplasm and myelin were significantly reduced at all intervals. In contrast, although axon diameters in aging mice were somewhat less 2 weeks after crushing, the difference decreased with time, suggesting that in nerves of aging mice, regenerative responses of Schwann cells were more affected than those of axons. Other experiments in young mice showed that myelinated fiber regeneration could be retarded by suppressing macrophage responses and was not significantly changed by conditioning lesions before crush injury.

Neuronal intermediate filament expression during neurite outgrowth from explanted goldfish retina: effect of retinoic acid

Regulation of the goldfish neuronal intermediate filament proteins ON1 and ON2 was investigated in a retinal explant system. The synthesis of these proteins in explanted retina decreased with increasing time in culture, despite continuing neurite outgrowth. Thus, ON1/ON2 neurofilament expression is regulated independently from neurite outgrowth. During regeneration of the goldfish optic nerve in vivo, the expression of these proteins increased during the later phase of the process, when growing axons make contact with the optic tectum. The declining synthesis of ON1 and ON2 during neurite outgrowth in culture suggests that factors extrinsic to the retina are necessary to support synthesis of these proteins. Treating retinal explants with retinoic acid stimulated the synthesis of the ON1/ON2 proteins in a dose-dependent manner. This stimulation was effective during a period of declining synthesis of the ON1/ON2 proteins, restoring their synthesis towards initial levels of expression. These results show that retinoic acid serves as a modulator of neurofilament expression in this in vitro model of nerve regeneration.

The initial period of peripheral nerve regeneration and the importance of the local environment for the conditioning lesion effect

The aim of this study was to investigate the early period of neurite outgrowth in the regenerating rat sciatic nerve and to determine if the non-neuronal cells were important for the conditioning lesion effect. Regeneration distance was evaluated with the pinch-reflex test 6 h to 5 days after a test crush lesion. The regeneration velocity accelerated during approximately 3 days, whereupon outgrowth continued with a constant velocity. In unconditioned nerves the initial delay was 2.8 h and the constant rate of regeneration was 3.2 mm/day. In nerves with a distal conditioning lesion the initial delay was 2.4 h and the rate of regeneration increased by 52%. When the test crush was applied at the same place as the conditioning crush the initial delay was 1.9 h and the rate of regeneration increased by 61%. The conditioning lesion effect was not influenced by the distance between the cell body and the conditioning crush lesion. Furthermore, the conditioning lesion effect could not be expressed if conditioned axons grew into a freeze injured nerve section. Incorporation of [3H]thymidine increased in the regenerating nerve segment. The increase occurred earlier if this segment had been subjected to a conditioning crush lesion. The results of these experiments showed that peripheral neurites start to regenerate within a few hours after an injury, suggesting that growth cone formation is independent of the cell body reaction. A conditioning crush lesion increases the regeneration velocity and its acceleration, and the conditioning lesion effect cannot be expressed in the absence of living Schwann and other non-neuronal cells.

The effect of a conditioning lesion on sudomotor axon regeneration

The effects of a conditioning lesion on the rate of sudomotor axon regeneration were judged by the recovery of sweat gland (SG) secretion after cholinergic stimulation. Three groups of mice were given a conditioning lesion by crushing the sciatic nerve at mid-thigh 4, 7, and 14 days before a test lesion. A 4th group received a conditioning crush of the tibial nerve at the ankle 7 days before the test lesion. Control mice had a single test lesion. SG reinnervation in control mice began 19 days after the test lesion, and was functionally complete by 41 days. In groups with the conditioning lesion 4, 7, and 14 days before the test operation, the first reactive SGs reappeared at 16, 15, and 16 days respectively after the test lesion, and maximal recovery occurred by 33, 32, and 39 days. In mice with the distal conditioning lesion, reinnervation began at 19 days and was maximal by 36 days. In summary, a nerve conditioning lesion placed from 4 to 14 days prior to and at the same site as a test lesion significantly accelerated the growth rate of the fastest regenerating unmyelinated sudomotor axons and reduced the time until most SGs were reinnervated. A more distally placed test lesion reduced the interval for recovery.

Conditioning lesions of peripheral nerves change regenerated axon numbers

The present study investigates the effects of conditioning lesions on regenerated axon numbers in tributary nerves after a test lesion. If a rat sciatic nerve is crushed 7 and 14 days prior to a test crush, the numbers of regenerated myelinated axons 8 weeks later in the sural nerve (SN) and nerve to the medial gastrocnemius (NMG) are increased, both over normal and over numbers after a single crush. If the lesions are only separated by 2 days, however, the numbers are similar to the numbers after a single crush. Thus conditioning occurs, but a minimum time between crushes is necessary for the effects of conditioning to be manifest. If the intervals between lesions are 14 days, the numbers are similar to those after the 7-day intervals. Moving each successive crush proximally or distally does not change regenerated myelinated axon numbers. Thus increasing the time between lesions after conditioning occurs, at least within the constraints of our paradigm, does not change regenerated axon numbers and the location of the lesion has relatively little bearing on the numbers of axons that regenerate. These findings allow us to change axonal numbers in these tributary nerves in a predictable way, and they are also compatible with the hypothesis that conditioning results from priming of the cell body rather than changes in the environment of the regenerating axons.

Effects of a conditioning lesion on bullfrog sciatic nerve regeneration: analysis of fast axonally transported proteins

We have shown that bullfrog sciatic nerves respond to a conditioning lesion similarly to goldfish optic nerve and rat or mouse sciatic nerve; that is, following a crush the rate of regeneration is faster in nerves that have received a conditioning lesion compared to nerves that have not. Also, damaged nerve fibres show initial growth or sprouting earlier in a previously conditioned nerve compared to nerves that have not received a prior conditioning lesion. We have not detected changes in the transport of fast axonally transported proteins with the conditioning lesion paradigm, other than those changes seen in regenerating nerves after receiving a single lesion. However, more label was present in a few fast axonally transported proteins at the lesion site in conditioned nerves compared to non-conditioned nerves, and this difference is not apparently due to increased transport. It seems that changes in fast axonally transported proteins probably do not contribute directly to the mechanism underlying the conditioning lesion effect of higher out growth rates, although some of the fast transported proteins may be involved in functions, possibly at the growing tip of damaged fibres, which promote or result from the conditioning effect.

Single vs multiple somatic nerve crushes in the rat

Nerve lesions modify regenerative responses to subsequent lesions. Some of the modifications might be useful. To increase our understanding of these modifications, the present study determines myelinated and unmyelinated axon numbers in the distal part of rat sciatic nerve and in 2 smaller branches, the nerve to the medial gastrocnemius muscle and the sural nerve, 8 weeks and 9 months following either single or the last of 3 crushes to the rat sciatic nerve. For myelinated axons, there is a significant and proportional increase distal to the crush in the sciatic nerve and in its smaller tributaries following both single and triple crushes. These increased axons persist. We interpret these data to indicate that some of the regenerating myelinated axons branch at the site of lesion, pass without branching into the tributary nerves, and then presumably find attachments at the periphery. If true, single or multiple crushes might be useful in conditions where it would be desirable to increase numbers of processes from surviving neurons. The major differences between single and triple crushes are that myelinated axons are increased more after triple crush and increase significantly between 8 weeks and 9 months after triple crush but not after single crush. Thus not only myelinated axon numbers, but the timing of the myelination process seems to change if regeneration following single crush is compared to similar regeneration following multiple crushes. Unmyelinated axons do not regenerate in the same way as the myelinated axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular and cellular aspects of nerve regeneration

Injury of an axon leads to at least four independent events, summarized in Figure 1: first, deprivation of the nerve cell body from target-derived or mediated substances, which leads to a derepressed or a permissive state; second, disruption of anterograde transport, with a resultant accumulation of anterogradely transported molecules; third, environmental response with possible consequent changes in constituents of the extracellular matrix and substances secreted from the surrounding cells; and fourth, appearance of growth inhibitors and modified protease activity. It seems that the first three of these events are obligatory, but not sufficient, i.e., they lead to a growth state only if the cell body is able to respond to the injury-induced signals from the environment (a and b). The regenerative state is characterized by alterations in protein synthesis and axonal transport and by sprouting activity. The subsequent elongation of the growing fibers depends on a continuous supply of appropriate growth factors. These factors are presumably anchored to the appropriate extracellular matrix that serves as a substratum for elongating fibers. It should be mentioned that the proliferating nonneuronal cells have a conducive effect on regeneration by forming a scaffold for the growing fibers. Accordingly, the lack of regeneration may stem from a deficiency in the ability of glial cells to provide the appropriate soluble components or from insufficient formation of extracellular matrix. In this respect, one may consider regeneration of an injured axon as a process which involves regeneration of both the nonneuronal cells and the supported axons. The regeneration of glial cells may fulfill the rules which are applied to regeneration of any other proliferating tissue. Furthermore, the processes of regeneration in the axon and the glial cells are mutually dependent. Perhaps the triggering factors provided by the nonneuronal cells affect the nonneuronal cells themselves by modulating their postlesion gliosis and thereby inducing their appropriate activation. In such a case, regeneration of nonneuronal cells may resemble an autocrine type of regulation that exists also during ontogeny. The growth regulation is shifted back to the paracrine type upon neuronal maturation or cessation of axonal growth. When the elongating fibers reach the vicinity of the target organ, they are under the influence of the target-derived factors, which guide the fibers and eventually cease their elongation.

Effect of conditioning lesion on axonal sprout formation at nodes of Ranvier

The effect of a conditioning lesion on the time-course of axonal sprout formation after a subsequent testing lesion was evaluated in myelinated axons of the rat sciatic nerve. Transmission electron microscopy of longitudinal nerve sections was used to examine nodes of Ranvier located 200-500 micron proximal to the testing lesion. The conditioning lesion was a cut of the tibial nerve at the ankle; the testing lesion, made 2 weeks later, was a crush of the sciatic nerve at the hip. Sprouts were defined as unmyelinated evaginations of the nodal axolemma that (1) had reached the basement membrane of the Schwann cell, and (2) were located between the myelin sheath of the distal paranode and the basement membrane. Photomicrographs of the nodes at 9, 18, and 27 hours after the testing lesion were assigned to one of seven categories: normal, retracted, myelin degeneration, axonal degeneration, type A sprout formation (cytoskeleton absent), type B sprout formation (cytoskeleton present), and type B sprout degeneration. By 9 hours after the testing lesion, type B sprout formation was found in 9% of the nodes in control nerves (testing lesion alone) and 33% of those in conditioned nerves (P less than .01). A 33% incidence of type B sprout formation was not reached in control nerves until 27 hours after the testing lesion. Since the conditioning lesion was located 50 mm distal to the testing lesion and did not induce neuronal death, earlier sprout formation can be attributed to a neuronal response to the conditioning lesion rather than to a putative factor that arises from pre-degenerated fibers and acts on newly formed sprouts.

Axonal regeneration in wobbler motor neuron disease: quantitative histologic and axonal transport studies

The regenerative capacity of the cervical anterior horn cells was studied at 4 and 7 days following forelimb nerve crush in 19 wobbler mice and 18 normal littermates. Quantitative histologic and radiolabeled axonal transport techniques showed that the axotomized neurons of the wobbler mouse supported active axonal elongation. However, the average axon outgrowth rate determined by histologic technique was diminished by 25% and the fastest axon outgrowth rate determined by axonal transport technique was also decreased by 30% in wobbler mice as compared to controls. The distal labeled peak was absent in the wobbler mouse at 7 days, indicating that the regeneration rate of individual axons was widely dispersed. Histologic studies also showed that the wobbler axons grew slowly. This study suggests that axonal regeneration does occur in motor neurons undergoing a primary neuronopathy. However, the regenerative capacity was reduced and this appears to reflect an impairment of functional integrity in the anterior horn cells of the wobbler mouse.

A nerve-conditioning lesion accelerates limb regeneration in the newt

A nerve-conditioning lesion induced sustained acceleration of limb regeneration. Newt limb nerves were subjected to a conditioning lesion by unilateral axotomy at the elbow 2 weeks prior to amputating both limbs above the elbows. Limbs on the side that had received a conditioning lesion began the regeneration process 3-4 days earlier than contralateral controls and this difference was observed up to recognizable digit formation. Limb buds on the conditioned sides had a twofold greater axonal density than contralateral counterparts at 2 weeks after amputation. Since limb bud formation is dependent on a sufficient quantity of axonal regrowth, accelerated limb regeneration is apparently due to accelerated reinnervation.

Animal model for peripheral nerve grafting

We propose an animal model from which it is possible to follow nerve-muscle unit recovery after a nerve graft easily, consistently, and relatively inexpensively. The model is also compatible with subsequent histologic or histochemical analysis. We document the recovery of a group of animals after nerve grafting to demonstrate the flexibility of the model.

Enzyme changes in the rat facial nucleus following a conditioning lesion

The enzymatic changes in the facial nucleus of the rat occurring after single nerve transection were compared with those after double lesion. In a first operation the left facial nerve was transected and 2 weeks later, both the left and the right facial nerves were axotomized. The double or "conditioning" lesion produced a complex pattern of changes that differed from those after a single lesion. Three enzymes were investigated both biochemically and histochemically. Acetylcholinesterase is representative of the group of transmitter-related enzymes which in general showed a decrease after a single lesion. The hexose monophosphate shunt enzymes, represented here by glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, are known to increase in the perikaryon. 5'-Nucleotidase is a marker enzyme for the perineuronal satellite glia which also increase in number during chromatolysis. The following results were obtained: (i) In comparison with the single-lesion side the conditioning-lesion side exhibited less activity of the pentose phosphate shunt enzymes on days 7 and 12 after the second operation. On the conditioning-lesion side the amount of enzyme per perikaryon was higher on days 1 and 3, approximately the same on day 7, and less on day 12 compared with the single-lesion side. (ii) The conditioning-lesion side displayed a more pronounced decrease of acetylcholinesterase. (iii) 5'-Nucleotidase increased again after a second axotomy and reached the same level of activity as after a single lesion. These data suggest that a conditioning lesion does not simply amplify the ongoing axonal reaction of the cells in a linear fashion, but that it leads to a complex response. The data are in favor of a shorter initial delay prior to the axonal outgrowth which occurs after a conditioning lesion. However, our data could not explain an enhancement of axonal outgrowth velocity after the second operation.

Induced homotypic collateral sprouting of serotonergic fibers in the hippocampus of rat

The organization of the raphe cells efferent to the dorsal hippocampus (DHipp) has been shown to consist of two homologous groups of cells in the median raphe nucleus (MRN). Their axons project by way of the cingulum bundle (CB) and the fornix-fimbria (FF)58. A group of neurons located in the interfascicular part of the dorsal raphe nucleus (IFN) was found to use the CB route exclusively. Quantitation of the HRP-labeled cells in the MRN neurons projecting to the DHipp decreased by 55% and 92% 3 days following CB and CB-FF 5,7-DHT injections respectively. There was a significant increase in HRP-labeled neurons in the MRN at 21 and 42 days compared to 3 days after CB injection (a 2% decrease at 21 days and 38% increase at 42 days compared to sham injected). No change in the number of labeled MRN cells was seen at these long-term time points after combined CB-FF 5,7-DHT microinjections. The labeled cells in the MRN increased in size in the long survival group of CB lesioned animals. The labeled cells localized in the IFN which were lost after CB 5,7-DHT injections did not reappear after long-term survival. These results indicate that CB 5-HT fibers do not regenerate into the DHipp, but FF 5-HT fibers increase their terminal territory by collateral sprouting in response to homotypic denervation.

Parent and daughter axon density in the limb stump and regenerating limb bud of the newt

Axonal density in the amputated limb stump and regenerating limb bud of the newt was determined by axon counts and planimetry of silver-stained sections at various distances from the amputation level. Traumatic degeneration occurred for at least 250 micron proximal to the amputation level. Axonal density at 250 micron distal to the amputation was less than half that at 100 micron. Attempts to increase in situ axonal density in limbs of animals naturally incapable of limb regeneration can be based on specific knowledge of natural traumatic degeneration and sprouting prior to axonal manipulation.

Protein synthesis and axonal transport in goldfish retinal ganglion cells during regeneration accelerated by a conditioning lesion

Axonal outgrowth in goldfish retinal ganglion cells following a testing lesion of the optic axons is accelerated by a prior conditioning lesion. Changes in protein synthesis and axonal transport were examined during the accelerated regeneration. The conditioning lesion was an optic tract cut made 2 weeks prior to the testing lesion, which consisted of a tract cut at the chiasma, so that nerves subjected to either a conditioning lesion ('conditioned nerves') or a sham operation ('sham-conditioned nerves') could be examined in the same animal. In the retinal ganglion cells of conditioned nerves, the incorporation of [3H]proline into protein began to increase between 1 and 8 days after the testing lesion. The amount of fast-transported labeled protein was elevated to about 8 X normal by 1 day after the testing lesion but had decreased to about 3-5X normal at 8 and 22 days. The 8 and 22 day values were not significantly different from those in sham-conditioned nerves or nerves that had received a testing lesion alone. For slow protein transport, the instantaneous amount transported was 15-16 X normal in the conditioned nerves at 1 and 8 days after the testing lesion, and the velocity of slow transport, which was already elevated above normal by 1 day after the testing lesion, was elevated still further by 8 days--to a value in excess of 1.5 mm/day (compared to 0.2-0.4 mm/day in normal animals). We believe that the enhanced outgrowth resulting from the conditioning lesion is due to a transient increase in the amount of fast transport (possibly responsible for a decreased delay in the initiation of sprouting), and a sustained increase in the amount and velocity of slow transport (which may account for an increased rate of elongation).

Effect of a conditioning lesion on optic nerve regeneration in goldfish

Following a 'test lesion' (crush) of the optic nerve in goldfish, histological study of axons in silver-stained sections showed that outgrowth of the leading axons began after an initial delay of 4.3 days and proceeded at 0.34 +/- 0.03 mm/day. When a 'conditioning lesion' (crush at the same site) preceded the testing lesion by 2 weeks, the initial delay was 2.5 days and the outgrowth rate was 0.74 +/- 0.13 mm/day (P less than 0.01). Two additional methods, utilizing intraocular injections of tritiated proline or fucose to label axonally transported proteins, were used to examine the outgrowth of leading optic axons. (a) Measurement of the distances reached by labeled axons in the nerve at 6 and 10 days after a testing lesion alone yielded an initial delay of 4.6 days and an outgrowth rate of 0.41 +/- 0.04 mm/day. However, when a conditioning lesion preceded the testing lesion, labeled optic axons were already found to have reached the optic tectum by 10 days after the testing lesion, indicating an outgrowth rate in excess of 0.64 mm/day. (b) Determination of the times at which labeled axons arrived at the optic tectum showed that the outgrowth rate after a testing lesion along was 0.40 mm/day whereas when the testing lesion was preceded by a conditioning lesion it was 0.74 mm/day. Thus, as a result of a conditioning lesion the initial delay was reduced by nearly half and the outgrowth rate was nearly doubled.

The aberrant retino-retinal projection during optic nerve regeneration in the frog. III. Effects of crushing both nerves

Previous reports from this laboratory have shown that a substantial number of optic axons are misrouted after optic nerve regeneration in the adult frog, Rana pipiens. Regenerating axons from a crushed optic nerve are distributed throughout the opposite nerve. In this study, we report the effect of crushing both optic nerves (double crush) on the pattern and degree of axonal misrouting. In 28 frogs both optic nerves were crushed at the same time (simultaneous double crush) and animals survived for varying periods before the right eye was injected with 3H-proline and the brain processed for autoradiography 24 hours later. In every frog with postoperative survivals longer than 2 weeks, labeled axons from the right eye were found in the left optic nerve. However, when compared to the amount of label seen in frogs in which only the right optic nerve was crushed (single crush) there was substantially less label within the left nerve of frogs after crushing both nerves. Label was also found only at the edge of the left nerve in material from double crush frogs, unlike that found after single crush. In four of six frogs where the left nerve was crushed 1 week after the right nerve (delayed double crush), the proximal end of the left nerve was completely filled with label, but more distally, label was found only along the edge of this nerve. Although fewer optic axons were labeled in the opposite optic nerve of double crush frogs, label did extend to the optic disc of that eye. However, label was not apparent in the ganglion cell fiber layer of the opposite eye. Instead, it was confined to the edge of the optic disc in a space apparently associated with papilledema resulting from crushing the optic nerve of that eye. In six frogs the retina of the left eye was removed at the same time the right optic nerve was crushed. Labeled axons of the right eye filled the left optic nerve to the retina-less shell of the left eye. Thus, these data show that the amount and distribution of axonal misrouting into the opposite optic nerve during optic nerve regeneration is affected by intact or regenerating optic axons from the other eye.

The aberrant retino-retinal projection during optic nerve regeneration in the frog. II. Anterograde labeling with horseradish peroxidase

Previous experiments have shown that a substantial number of regenerating optic axons in adult frogs (Rana pipiens) are misrouted into the opposite optic nerve and retina during early stages of regeneration. This projection is maximal at 5 and 6 weeks after optic nerve crush. To further characterize this anomalous projection, small quantities of horseradish peroxidase (HRP) were injected into the right eye or right optic nerve 5 or 6 weeks after right optic nerve crush. Twenty-four hours later the animals were killed and regenerating axons anterogradely filled with HRP were reacted with the tetramethyl-benzidine method or a diaminobenzidine-CoCl2 method. Serial reconstruction tracing the course of individual axons through the optic chiasm showed that few of the axons projecting into the opposite optic nerve were collaterals of axons projecting centrally. Instead, the majority of labeled axons misdirected into the opposite nerve or contributing to an expanded projection into the ipsilateral optic tract turned out of the chiasm without branching. Many of the labeled regenerating axons had unusual trajectories within the chiasm, making abrupt turns or changing their direction of growth. Most of the axons misrouted into the opposite nerve came from portions of the chiasm nearest to the nerve of other eye. In three of eight frogs with an intact optic nerve, a small number of HRP-labeled axons were found in the left nerve after right nerve injection, but there was no indication that these axons reached the left eye. The results from this investigation suggest that the most parsimonious explanation for the chiasmal misrouting of regenerating frog optic axons is that axons are mechanically deflected into inappropriate pathways.

Formation of ectopic neuromuscular junctions in adult rats

The formation of ectopic neuromuscular junctions between a transplanted foreign nerve (the superficial fibular nerve) and the denervated soleus muscle was examined in adult rats. Formation of new junctions was induced by denervating the soleus muscle by cutting the tibial nerve. Junctional transmission began 3 days after denervation when denervation was done 3 weeks after transplantation of the foreign nerve, and progressively later when denervation was done 8, 16 and 24 weeks after transplantation. The rate at which the transmission, once started, acquired mature characteristics was approximately the same in each case. Initially, spontaneous m.e.p.p.s were infrequent, long lasting with a skewed amplitude distribution. The e.p.p.s evoked by stimulation of the foreign nerve were then commonly below threshold for eliciting action potentials and were occasionally no larger than the size of the spontaneous m.e.p.p.s. M.e.p.p. characteristics became normal in the 2nd week after transmission had started. Fully effective evoked transmission with every innervated fibre responding with overshooting action potential occurred 1-3 months after onset of transmission.