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

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.