Cell Cycle Activation Contributes to Increased Neuronal Activity in the Posterior Thalamic Nucleus and Associated Chronic Hyperesthesia after Rat Spinal Cord Contusion

Abstract

Spinal cord injury (SCI) causes not only sensorimotor and cognitive deficits, but frequently also severe chronic pain that is difficult to treat (SCI pain). We previously showed that hyperesthesia, as well as spontaneous pain induced by electrolytic lesions in the rat spinothalamic tract, is associated with increased spontaneous and sensory-evoked activity in the posterior thalamic nucleus (PO). We have also demonstrated that rodent impact SCI increases cell cycle activation (CCA) in the injury region and that post-traumatic treatment with cyclin dependent kinase inhibitors reduces lesion volume and motor dysfunction. Here we examined whether CCA contributes to neuronal hyperexcitability of PO and hyperpathia after rat contusion SCI, as well as to microglial and astroglial activation (gliopathy) that has been implicated in delayed SCI pain. Trauma caused enhanced pain sensitivity, which developed weeks after injury and was correlated with increased PO neuronal activity. Increased CCA was found at the thoracic spinal lesion site, the lumbar dorsal horn, and the PO. Increased microglial activation and cysteine-cysteine chemokine ligand 21 expression was also observed in the PO after SCI. In vitro, neurons co-cultured with activated microglia showed up-regulation of cyclin D1 and cysteine-cysteine chemokine ligand 21 expression. In vivo, post-injury treatment with a selective cyclin dependent kinase inhibitor (CR8) significantly reduced cell cycle protein induction, microglial activation, and neuronal activity in the PO nucleus, as well as limiting chronic SCI-induced hyperpathia. These results suggest a mechanistic role for CCA in the development of SCI pain, through effects mediated in part by the PO nucleus. Moreover, cell cycle modulation may provide an effective therapeutic strategy to improve reduce both hyperpathia and motor dysfunction after SCI.

Spinal Cord Injury Causes Brain Inflammation Associated with Cognitive and Affective Changes: Role of Cell Cycle Pathways

“…However, supratentorial changes have been reported in localized brain regions associated with pain modulation after SCI-induced hyperesthesia, including chronic inflammatory changes associated with plasticity or electrophysiological alterations (Hains et al, 2005;Hubscher and Johnson, 2006;Zhao et al, 2007;KnerlichLukoschus et al, 2011;Yoon et al, 2013). In our own recent studies examining mechanisms of SCI-induced hyperpathia in rodent models, we observed sustained neuroinflammation not only in brain regions regulating pain sensation (Wu et al, 2013b), but also more diffusely in cortex, thalamus, and hippocampus. Because the degree and pattern of changes was similar to those associated with the chronic neurodegeneration observed after traumatic brain injury (TBI) in rodents (Kumar and Loane, 2012), we initiated an independent study to determine whether brain neurodegeneration and functional neurological correlates occur after isolated thoracic SCI.…”

Endoplasmic Reticulum Stress and Disrupted Neurogenesis in the Brain Are Associated with Cognitive Impairment and Depressive-Like Behavior after Spinal Cord Injury

“…49,53 More recently, CCL21 signaling was found increased in neuronal cell bodies and parenchyma within both the hippocampus and cortex at 7 days after SCI. 19 Here, we examined CCL21 immunoreactivity in key brain regions from sham, mild, moderate, and severe SCI mice at 16 weeks post-injury.…”

“…For each experiment, data from all images from one region in each mouse were summed up and used for final statistical analysis. 49,50 Unbiased stereological quantification of microglial phenotypes and neuronal survival Brain coronal sections were stained with cresyl-violet and Stereo Investigator software (MBF Biosciences, VT) was used to count the total number of surviving neurons in the cortex, thalamus, as well as Cornu Ammonis (CA) 1, CA2/3, and DG sub-regions of the hippocampus using the optical fractionator method of unbiased stereology as described previously. 18 Every fourth 60-lm section was analyzed beginning from a random start point.…”

“…Neurolucida software (MBF Biosciences) was used to create reconstructions of microglia at different stages of activation after injury by tracing the cell bodies and dendrites. 49 …”

“…54 Distal release of CCL21 produced following SCI by injured neurons at the trauma site has been proposed as a molecular mechanism that triggers microglial activation at spinal segments far from the site of injury, and in the thalamus in association with pain phenomena. 49,53 However, CCL21 changes appear to extend beyond these centers and impact other regions. We have shown that increased CCL21 signal in various brain regions are associated with microglial activation in these areas related to chronic neuronal cell loss after SCI.…”