Changes in the synaptology of spinal motoneurons in zebrafish following spinal cord transection

Optimizing spinal cord injury in zebrafish larvae: effects of age on the injury response

Zebrafish are an increasingly popular model to study spinal cord injury (SCI) regeneration. The transparency of larval zebrafish makes them ideal to study cellular processes in real time. Standardized approaches, including age of injury, are not readily available making comparisons of the results with other models challenging. In this study, we systematically examined the response to spinal cord transection of larval zebrafish at three different ages (3-7 days post fertilization or dpf) to determine whether the developmental complexity of the central nervous system affects the overall response to SCI. We then used imaging and behavioral analysis to evaluate whether differences existed based on the age of injury. All ages of larval zebrafish upregulated the required genes for glial bridge formation, ctgfa and gfap, at the site of injury, consistent with studies from adult zebrafish. Though all larval ages upregulated factors required to promote glial bridging, young larval zebrafish (3 dpf) were better able to regenerate axons independent of the glial bridge, unlike older zebrafish (7 dpf). Consistent with this data, locomotor experiments demonstrated that some swimming behavior occurs independent of glial bridge formation, further highlighting the need for standardization of this model and recovery assays. Overall, we found subtle cellular differences based on the age of transection in zebrafish, underlining the importance of considering age when designing experiments aimed at understanding regeneration.

Electric fish: new insights into conserved processes of adult tissue regeneration

Biology is replete with examples of regeneration, the process that allows animals to replace or repair cells, tissues and organs. As on land, vertebrates in aquatic environments experience the occurrence of injury with varying frequency and to different degrees. Studies demonstrate that ray-finned fishes possess a very high capacity to regenerate different tissues and organs when they are adults. Among fishes that exhibit robust regenerative capacities are the neotropical electric fishes of South America (Teleostei: Gymnotiformes). Specifically, adult gymnotiform electric fishes can regenerate injured brain and spinal cord tissues and restore amputated body parts repeatedly. We have begun to identify some aspects of the cellular and molecular mechanisms of tail regeneration in the weakly electric fish Sternopygus macrurus (long-tailed knifefish) with a focus on regeneration of skeletal muscle and the muscle-derived electric organ. Application of in vivo microinjection techniques and generation of myogenic stem cell markers are beginning to overcome some of the challenges owing to the limitations of working with non-genetic animal models with extensive regenerative capacity. This review highlights some aspects of tail regeneration in S. macrurus and discusses the advantages of using gymnotiform electric fishes to investigate the cellular and molecular mechanisms that produce new cells during regeneration in adult vertebrates.

A surgery protocol for adult zebrafish spinal cord injury

Adult zebrafish has a remarkable capability to recover from spinal cord injury, providing an excellent model for studying neuroregeneration. Here we list equipment and reagents, and give a detailed protocol for complete transection of the adult zebrafish spinal cord. In this protocol, potential problems and their solutions are described so that the zebrafish spinal cord injury model can be more easily and reproducibly performed. In addition, two assessments are introduced to monitor the success of the surgery and functional recovery: one test to assess free swimming capability and the other test to assess extent of neuroregeneration by in vivo anterograde axonal tracing. In the swimming behavior test, successful complete spinal cord transection is monitored by the inability of zebrafish to swim freely for 1 week after spinal cord injury, followed by the gradual reacquisition of full locomotor ability within 6 weeks after injury. As a morphometric correlate, anterograde axonal tracing allows the investigator to monitor the ability of regenerated axons to cross the lesion site and increasingly extend into the gray and white matter with time after injury, confirming functional recovery. This zebrafish model provides a paradigm for recovery from spinal cord injury, enabling the identification of pathways and components of neuroregeneration.

Differences in the regenerative response of neuronal cell populations and indications for plasticity in intraspinal neurons after spinal cord transection in adult zebrafish

In zebrafish, the capacity to regenerate long axons varies among different populations of axotomized neurons after spinal cord transection. In specific brain nuclei, 84-92% of axotomized neurons upregulate expression of the growth-related genes GAP-43 and L1.1 and 32-51% of these neurons regrow their descending axons. In contrast, 16-31% of spinal neurons with axons ascending to the brainstem upregulate these genes and only 2-4% regrow their axons. Dorsal root ganglion (DRG) neurons were not observed to regrow their ascending axons or to increase expression of GAP-43 mRNA. Expression of L1.1 mRNA is high in unlesioned and axotomized DRG neurons. In the lesioned spinal cord, expression of growth-related molecules is increased in a substantial population of non-axotomized neurons, suggesting morphological plasticity in the spinal-intrinsic circuitry. We propose that locomotor recovery in spinal-transected adult zebrafish is influenced less by recovery of ascending pathways, but more by regrowth of descending tracts and rearrangement of intraspinal circuitry.

Recovery of locomotion correlated with axonal regeneration after a complete spinal transection in the eel

This research has examined the relationship between axonal regeneration and the return of normal movement following complete transection of the spinal cord. We made measurements of tail beat frequency and amplitude of the caudal body wave from video recordings of eels (Anguilla anguilla) swimming in a water tunnel at several speeds. Each eel was then anaesthetised and the spinal cord cut caudal to the anus; in some animals the resulting gap was filled with a rubber block. All animals were kept at 25 degrees C for recovery periods ranging from 7 to 128 days, during which their swimming performance was monitored regularly. Each fish was then re-anaesthetised and perfused with fixative and the regrowing descending axons labelled with 1,1'-diotadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate. For all animals and at all speeds after surgery, tail beat frequency increased, while amplitude decreased. In non-blocked animals, an improvement in performance was first seen from 8 days following transection and thereafter tail beat frequency decreased progressively until it had returned to normal after 35 to 45 days, while amplitude remained below baseline until at least 45 days. In these animals, few axonal growth cones had penetrated the caudal stump by 7 days, but some had extended as much as 3 mm by 15 days. Many had reached as far as 6 mm between 25 and 36 days, while by 128 days they had progressed up to 10.5 mm. Contralateral crossing was never observed. Functional recovery was never witnessed in animals in which the cord had been blocked and these eels swam at all times with elevated tail beat frequency and reduced caudal amplitude. No labelled axons could be traced into the caudal spinal cord at any recovery stage in such animals. We conclude that re-innervation of only 1-2 segments caudal to the injury is necessary for functional recovery, although continued axonal growth may be important for the refinement of some aspects of movement.

Long term effects of spinal cord transection in zebrafish: swimming performances, and metabolic properties of the neuromuscular system

This study concerns functional recovery of zebrafish following spinal cord transection. Spinal cords were transected at the level of the 14th vertebra, just rostral to the dorsal fin. Recovery was tested at one month after transection when descending fibers start to regrow across the transection site and at three months after transection when fish perform kick and glide swimming. To estimate the rate of regrowth across the lesion site we analysed the tyrosine hydroxylase (TH) and dorsal 5-hydroxytryptamine (5-HT) systems in distal parts of lesioned cords. Both systems have cell bodies in the brainstem and in control fish TH- and dorsal 5-HT-containing fibers descend to all spinal segments. Swimming performance was studied by subjecting lesioned fish to endurance tests in a swimming tunnel with water flowing at a constant rate of 2 or 4.5 body lengths per second (BL/s). At 2 BL/s slow myotomal muscles are active whereas at 4.5 BL/s fast myotomal muscles are recruited. Control fish endured sustained swimming at both speeds for at least 3 hours. As a measure for the condition of the neuromuscular system in trunk and tail, we analysed aerobic metabolic capacities, assessed by NADH-tetrazolium reductase (NADH-TR) histochemistry of myotomal muscle fibers and spinal lateral neuropil. We found that TH- and dorsal 5-HT-immunoreactive fibers were absent in the entire distal part of lesioned cords at one month but at two months after transection they were present at approximately 6000 microns caudally to the site of the lesion. Thus the rate of outgrowth of these fibers is at least 200 microns per day. Sustained swimming at the slow speed (2 BL/s) could be endured for about 14.4 min at one month and for 23.5 min at two months after transection; there was no further improvement in the period that followed. In contrast, in the 10 weeks following transection, fast swimming (4.5 BL/s) could be endured for about 5 to 6 minutes. A significant improvement was gained in the period of 10 to 12 weeks after transection when fish could endure the high speed for almost 15 min. The aerobic capacity of muscle fibers in distal parts of the body was not strongly affected by the lesion. The only important change in aerobic capacity was observed in the neuropil of distal parts of the cords where, at three months after transection, NADH-TR activity was increased to approximately 150% of control values. On the basis of our findings, we assume that it is not the condition of the neuromuscular system, but rather a deficient co-ordination between proximal and distal body parts of lesioned fish that accounts for the relatively poor performances in endurance tests. Furthermore, differences in timing of improvements in swimming at 2 and 4.5 BL/s indicate that the spinal circuitries serving the slow parts of the neuromuscular system recover at an earlier stage than those serving the fast parts.