Transcriptional expression of inflammatory mediators in various somatosensory relay centers in the brain of rat models of peripheral mononeuropathy and local inflammation

Abstract

Contradictory results have been reported regarding the role of inflammatory mediators in the central nervous system in mediating neuropathic pain and inflammatory hyperalgesia following peripheral nerve injury or localized inflammation. The present study aims to correlate between the mRNA expression and protein secretion of proinflammatory cytokines and nerve growth factor (NGF), in the dorsal root ganglia (DRGs), spinal cord, brainstem and thalamus, and pain-related behavior in animal models of peripheral mononeuropathy and localized inflammation. Different groups of rats (n=8, each) were subjected to either lesion of the nerves of their hindpaws to induce mononeuropathy or intraplantar injection of endotoxin (ET) and were sacrificed at various time intervals. TNF-alpha, IL-1beta and NGF mRNA expression and protein levels in the various centers involved in processing nociceptive information were determined, by RT-PCR and ELISA. Control groups were either subjected to sham surgery or to saline injection. Mononeuropathy and ET injection produced significant and sustained increases in the mRNA expression and protein levels of TNF-alpha, IL-1beta and NGF in the ipsilateral and contralateral DRGs, spinal cord, and brainstem. No significant and consistent changes in the mRNA expression of cytokines were noticed in the thalamus, while a downregulation of the NGF-mRNA level was observed. The temporal and spatial patterns of the observed changes in mRNA expression of cytokines and NGF are not closely in phase with the observed allodynia and hyperalgesia in the different models, suggesting that the role of these mediators may not be reduced exclusively to the production and maintenance of pain.