Elsevier

Neuroscience

Volume 99, Issue 1, 26 July 2000, Pages 189-197
Neuroscience

Opioid peptide messenger RNA expression is increased at spinal and supraspinal levels following excitotoxic spinal cord injury

https://doi.org/10.1016/S0306-4522(00)00150-0Get rights and content

Abstract

Spinal cord injury in rats is known to cause anatomical, physiological and molecular changes within the spinal cord. These changes may account for behavioral syndromes that appear following spinal cord injury, syndromes believed to be related to the clinical condition of chronic pain. Intraspinal injection of quisqualic acid produces an excitotoxic injury with pathological characteristics similar to those associated with ischemic and traumatic spinal cord injury. In addition, recent studies have demonstrated changes in blood flow, neuronal excitability and gene expression in the brain following excitotoxic injury, indicating that behavioral changes may result from modification of neuronal substrates at supraspinal levels of the neuraxis. Because changes in spinal opioid peptide expression have been demonstrated in models of traumatic spinal cord injury and chronic pain, the present study investigated messenger RNA expression of the opioid peptides, preproenkephalin and preprodynorphin, at spinal and supraspinal levels following excitotoxic spinal cord injury. Male, Long–Evans rats were given three intraspinal injections of quisqualic acid (total 1.2 μl, 125 mM). After one, three, five, seven or 10 days, animals were killed and quantitative in situ hybridization performed on regions of the spinal cord surrounding the lesion site, as well as whole-brain sections through various levels of the thalamus. Preproenkephalin and preprodynorphin expression was increased in spinal cord areas adjacent to the site of quisqualic injection and in cortical regions associated with nociceptive function, preproenkephalin in the cingulate cortex and preprodynorphin in the parietal cortex, both ipsilaterally and contralaterally at various time-points following injury.

These results further our knowledge of the secondary events that occur following spinal cord injury, specifically implicating supraspinal opioid systems in the CNS response to spinal cord injury.

Section snippets

Intraspinal microinjection

All experimental procedures were approved by the Institutional Animal Care and Use Committee of East Carolina University. The technique of intraspinal injection was similar to that described in previous reports.22., 53. In brief, male, Long–Evans rats (250–300 g) were anesthetized with a mixture of Ketamine, Acepromazine and Xylazine (0.80 ml/kg, i.p.). After the vertebral column was exposed, the spinous process and vertebral lamina were removed from one spinal level and the dura incised

Histology

The general characteristics of spinal cords following QUIS injections were similar to those described in previous reports.51., 53. Consistently, the greatest area of neuronal loss was seen in the intermediate gray matter of the cord (Fig. 1). The area of neuronal loss and extent of spinal cavitation was generally larger in animals with longer survival times. Although not specifically examined, Thionin staining also suggested the presence of inflammatory cells, reactive astrocytes and

Discussion

This study demonstrates that intraspinal injection of QUIS results in an increase in the expression of PPE mRNA and PPD mRNA in the spinal cord adjacent to the site of injection, as well as in cortical areas associated with pain processing, i.e. the cingulate cortex and the parietal cortex. This is the first evidence of changes in opioid peptide gene expression in cortical areas in response to SCI, and suggests that both spinal and supraspinal regulation of endogenous opioid peptide expression

Conclusions

The present study reveals an increase in both PPD and PPE mRNA expression in the spinal cord rostral and caudal to and ipsilateral and contralateral to the site of intraspinal QUIS injection at various time-points following injury. In addition, there is an increase in PPD expression in the parietal cortex and an increase in PPD expression in the cingulate cortex following intraspinal QUIS injection. These results further our understanding of the secondary events that occur following SCI,

Acknowledgements

This research was supported by DA03982. We thank Denise Mayer and G. W. Lanford for technical assistance.

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