Abstract
Allodynia, hyperalgesia and spontaneous pain are cardinal sensory signs of neuropathic pain. Clinically, many neuropathic pain patients experience affective-motivational state changes, including reduced familial and social interactions, decreased motivation, anhedonia and depression which are severely debilitating. In earlier studies we have shown that sciatic nerve chronic constriction injury (CCI) disrupts social interactions, sleep-wake-cycle and endocrine function in one third of rats, a subgroup reliably identified six days after injury. CCI consistently produces allodynia and hyperalgesia, the intensity of which was unrelated either to the altered social interactions, sleep-wake-cycle or endocrine changes. This decoupling of the sensory consequences of nerve injury from the affective-motivational changes is reported in both animal experiments and human clinical data. The sensory changes triggered by CCI are mediated primarily by functional changes in the lumbar dorsal horn, however, whether lumbar spinal changes may drive different affective-motivational states has never been considered. In these studies, we used microarrays to identify the unique transcriptomes of rats with altered social behaviours following sciatic CCI to determine whether specific patterns of lumbar spinal adaptations characterised this subgroup. Rats underwent CCI and on the basis of reductions in dominance behaviour in resident-intruder social interactions were categorised as having Pain & Disability, Pain & Transient Disability or Pain alone. We examined the lumbar spinal transcriptomes two and six days after CCI. Fifty-four ‘disability-specific’ genes were identified. Sixty-five percent were unique to Pain & Disability rats, two-thirds of which were associated with neurotransmission, inflammation and/or cellular stress. In contrast, 40% of genes differentially regulated in rats without disabilities were involved with more general homeostatic processes (cellular structure, transcription or translation). We suggest that these patterns of gene expression lead to either the expression of disability, or to resilience and recovery, by modifying local spinal circuitry at the origin of ascending supraspinal pathways.
Highlights
The affective-motivational consequences of nerve injury have become an emerging focus of rat models of neuropathic pain
In earlier studies we have shown that sciatic nerve chronic constriction injury (CCI) disrupts social interactions, sleep-wake-cycle and endocrine function in one third of rats, a subgroup reliably identified six days after injury
Mechanical and thermal threshold testing alters gene expression [52, 53] and because we have previously reported that changes in allodynia, hyperalgesia and spontaneous pain behaviours occur in all sciatic nerve CCI animals [25, 27], it was not conducted on these rats
Summary
The affective-motivational consequences of nerve injury have become an emerging focus of rat models of neuropathic pain. The negative affective consequences of the spared nerve injury (SNI) [1]; spinal nerve ligation (SNL) [2]; and chronic constriction injury of sciatic nerve (CCI) [3] models of neuropathy, have been assessed using conditioned place aversion / place escape avoidance paradigms, and inferred from analgesic conditioned place preference [4–8]. These studies provide evidence for the immediate (day-3 post injury) and persisting (4 weeks post-injury) perception of unpleasantness of neuropathic injury [4, 5, 8], and for its reversal by: (i) analgesics acting at spinal alpha-2 adrenoreceptors or, N-type calcium channels and; (ii) inhibition of supra-spinal facilitatory pathways [6, 7]. Anxiety and depression-like behaviours have been first reported at 7 days following injury and persist for up to 6 months after injury
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