Neuropathic pain promotes gene expression adaptations in brain networks involved in stress and depression

Neuropathic pain is a complex chronic condition, characterized by a wide range of sensory, cognitive, and affective symptoms. Indeed, a large percentage of neuropathic pain patients are also afflicted with depression and anxiety disorders -- a pattern that is reliably replicated in animal models. Mounting evidence from clinical and preclinical studies indicates that chronic pain corresponds with adaptations in several brain networks involved in mood, motivation, and reward. Chronic stress is also a major determinant for depression. However, whether chronic pain and chronic stress affect similar mechanisms, and whether chronic pain can affect gene expression patterns known to be involved in depression, remains poorly understood. We employed the spared nerve injury model (SNI) of neuropathic pain in adult C57BL\6 mice and performed next-generation RNA-sequencing in order to monitor changes in gene expression in three brain regions known to be implicated in the pathophysiology of depression and in the modulation of pain: the nucleus accumbens (NAc), the medial prefrontal cortex (mPFC), and the periaqueductal grey (PAG). We observed mostly unique transcriptome profiles across the three brain regions but found common intracellular signal transduction pathways and biological functions were affected. A large amount of genes showing SNI-induced altered expression have been implicated in depression, anxiety, or chronic pain. In addition, we identified genes that are similarly regulated in a murine model of depression: chronic unpredictable stress. Our study provides the first unbiased characterization of neuropathic pain-induced long-term gene expression changes in three distinct brain regions, and presents evidence that neuropathic pain affects the expression of several genes that are also regulated by chronic stress. RNA-seq samples were generated from 3 brain regions (nucleus accumbens, medial prefrontal cortex, and periaqueductal grey) of adult male mice, 2.5 months after sham or spared nerve injury to the sciatic nerve.