Spontaneous Regeneration of the Serotonergic Descending Innervation in the Sea Lamprey after Spinal Cord Injury

Abstract

In contrast to mammals, lampreys are capable of recovering apparently normal locomotion after complete spinal cord transection, and the spinal axons regenerate selectively in their correct paths. Descending serotonergic projections to the spinal cord play a role in the modulation of locomotion at spinal levels in both mammals and lampreys. In this study, we used combined immunofluorescence and tract-tracing techniques to show that in the sea lamprey, serotonergic descending neurons of the caudal rhombencephalon (vagal nucleus) regenerate their axons across the lesion site after complete spinal cord transection. The spinal cord of mature larval sea lampreys was transected at the level of the fifth gill, then after a recovery period of 5 months, the spinal cord was exposed again, 1 mm caudal to the injury site, and the tracer Neurobiotin(™) was applied. Double-labeled cells were observed in the caudal portion of the serotonin-immunoreactive vagal nucleus of the caudal rhombencephalon. In order to investigate whether the reinnervation was due to sprouting from axons above the injury site or to regeneration of axotomized axons, the experiments were performed again, but the tracer Fluoro-Gold(™) was applied at the time of transection. Triple-labeled cells were observed in the vagal nucleus, indicating that at least part of the reinnervation corresponds to true regeneration. This study provides a new and interesting model for investigating the intrinsic molecular mechanisms involved in regeneration of the serotonergic descending axons in vertebrates. Use of this model may provide valuable information for proposing new therapies for patients with spinal cord injury.

Anatomical recovery of the spinal glutamatergic system following a complete spinal cord injury in lampreys

“…Complete transection of the spinal cord at the level of the 5 th gill was performed as previously described25272834. Sham-operated controls were processed in the same way but only performing the surgical incision of the dorsal body wall without injuring the spinal cord.…”

“…Excessive glutamate release in response to a lesion causes glutamate excitotoxicity, which can lead to neuronal and oligodendrocyte death212223, thus worsening the outcome of the lesion. While the short term changes in glutamate levels after SCI are well documented2124, even in lampreys25, as well as the changes of other neurotransmitter systems during regeneration in lampreys or zebrafish (for example, the serotonergic system)26272829; surprisingly, much less is known about the long term changes in the glutamatergic system.…”

Use of Fluorochrome-Labeled Inhibitors of Caspases to Detect Neuronal Apoptosis in the Whole-Mounted Lamprey Brain after Spinal Cord Injury

“…The unique evolutionary position of lampreys as early-evolved vertebrates, the sequencing of the sea lamprey ( Petromyzon marinus L.) genome, and the adaptation and optimization of many established molecular biology and histochemistry techniques for use in this species have made it an emerging nonmammalian model organism of choice for investigations into the evolution of hallmark vertebrate characteristics. In addition, the sea lamprey has proven to be a valuable animal model in spinal cord (SC) injury and regeneration studies [2, 3]. …”

Plasticity of tyrosine hydroxylase and serotonergic systems in the regenerating spinal cord of adult zebrafish

“…Also, in the spinal cord of the sea lamprey, regeneration of serotonergic descending axons has been reported (Cornide-Petronio et al, 2011). In addition, there is lesion-induced generation of serotonergic cells in the spinal cord of the closely related goldfish (Takeda et al, 2008).…”