Elsevier

Neuroscience

Volume 85, Issue 2, 8 April 1998, Pages 443-458
Neuroscience

Sprouting of primary afferent fibers after spinal cord transection in the rat

https://doi.org/10.1016/S0306-4522(97)00622-2Get rights and content

Abstract

After spinal cord injury, hyper-reflexia can lead to episodic hypertension, muscle spasticity and urinary bladder dyssynergia. This condition may be caused by primary afferent fiber sprouting providing new input to partially denervated spinal interneurons, autonomic neurons and motor neurons. However, conflicting reports concerning afferent neurite sprouting after cord injury do not provide adequate information to associate sprouting with hyper-reflexia. Therefore, we studied the effect of mid-thoracic spinal cord transection on central projections of sensory neurons, quantified by area measurements. The area of myelinated afferent arbors, immunolabeled by cholera toxin B, was greater in laminae I–V in lumbar, but not thoracic cord, by one week after cord transection. Changes in small sensory neurons and their unmyelinated fibers, immunolabeled for calcitonin gene-related peptide, were assessed in the cord and in dorsal root ganglia. The area of calcitonin gene-related peptide-immunoreactive fibers in laminae III–V increased in all cord segments at two weeks after cord transection, but not at one week. Numbers of sensory neurons immunoreactive for calcitonin gene-related peptide were unchanged, suggesting that the increased area of immunoreactivity reflected sprouting rather than peptide up-regulation. Immunoreactive fibers in the lateral horn increased only above the lesion and in lumbar segments at two weeks after cord transection. They were not continuous with dorsal horn fibers, suggesting that they were not primary afferent fibers. Using the fluorescent tracer DiI to label afferent fibers, an increase in area could be seen in Clarke's nucleus caudal to the injury two weeks after transection.

In conclusion, site- and time-dependent sprouting of myelinated and unmyelinated primary afferent fibers, and possibly interneurons, occurred after spinal cord transection. Afferent fiber sprouting did not reach autonomic or motor neurons directly, but may cause hyper-reflexia by increasing inputs to interneurons.

Section snippets

Spinal cord transection

All protocols for these experiments were approved by the University of Western Ontario Animal Care Committee in accordance with the policies established in the Guide to Care and Use of Experimental Animals prepared by the Canadian Council on Animal Care. Twenty-six male Wistar rats (Charles River), weighing 300–400 g, were premedicated with atropine (1 mg/kg, i.p.) and diazepam (2.5 mg/kg, i.p.). After 10 min, these rats were anesthetized with 35 mg/kg sodium pentobarbital (i.p.), and supplemental

Cholera toxin subunit B immunoreactivity

In control rats, CTB-IR in myelinated fibers was found in individual fibers and in a punctate form associated with terminals. This CTB-IR was found in segments T6–T9, corresponding to the intercostal muscle injection, and in segments L1–L7, transported from the quadriceps muscle injection site. CTB-IR was found throughout laminae I–V in T6–T9 and L1–L7, and in Clarke's nucleus in the lumbar segments (Fig. 2A, D). No CTB-IR fibers could be seen extending into the ventral or lateral horn anywhere

Discussion

Both myelinated and unmyelinated primary afferent neurons sprouted, but the changes in each group occurred in different spinal cord segments and followed a different time-course after the injury. The area of myelinated afferent fibers, labeled with CTB, increased in the dorsal horn of lumbar spinal cord segments by one week after SCT and this change persisted for two weeks, while no changes were seen in the thoracic cord. In contrast, the unmyelinated afferent fibers, identified by their

Conclusions

These experiments have demonstrated sprouting of both myelinated and unmyelinated primary afferent fibers after spinal cord injury. As each afferent population sprouts at a different time after transection, each may be responding to a different cue within the injured cord. Although no direct projections were observed into the lateral or ventral horns, the putative new neurites may provide excitatory inputs to interneurons antecedent to SPNs or motor neurons. Thus, primary afferent sprouting may

Acknowledgements

This research was supported by grant no. T2679 from the Heart and Stroke Foundation of Ontario and a grant from the Medical Research Council of Canada. L. C. Weaver is the recipient of a Career Investigator award from the Heart and Stroke Foundation of Ontario, and N. R. Krenz holds a studentship from the Medical Research Council of Canada. The authors are grateful to M. Korkola for carrying out pilot studies, S. Hota, C. Nutt and M. Columbus for technical assistance, and B. Atkinson for

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