Differential expression of rat hippocampal microRNAs in two rat models of chronic pain.

The two most common forms of chronic pain are inflammatory pain and neuropathic pain. Nevertheless, the underlying mechanisms of these pain conditions and their therapeutic responses are poorly understood. MicroRNAs (miRNAs) negatively regulate cell genes, and thus control cell proliferation, inflammation and metabolism. In the present study, we examined gene expression in the hippocampus of rats in two models of chronic pain. In addition, we used the left hindpaw procedure to identify differences in the bilateral hippocampus. We divided the rats into the 4 following groups: the group with chronic constriction injury (CCI), the sham-operated group, the group injected with complete Freund's adjuvant (CFA) and the group injected with normal saline. miRNA expression profiles were analyzed using TaqMan low-density array (TLDA). We observed 54 miRNAs (22.7%) in the rats with CCI rats that were differentially expressed, including 7 miRNAs that were downregulated compared with the sham-operated rats. In the CFA-injected rats, 40 miRNAs (16.8%) were differentially expressed, including 8 miRNAs that were downregulated compared with the normal saline-injected rats. Pearson's correlation co-efficient for all detected miRNAs in the rat hippocampus failed to identify differences between the hippocampi bilaterally. An unsupervised cluster analysis produced separate clusters between the control and experimental groups. In this study, we demonstrate the differential expression of hippocampal miRNAs in two rat models of chronic pain; however, no significant differences were observed bilaterally in hippocampal miRNA expression. Further research is required to determine the correlation among miRNAs, messenger RNAs (mRNAs) and proteins.