Increases in Pro-inflammatory Cytokines in the Ventral Striatum Following Frequent Morphine Exposure Impose New Setpoints in the Expression of Biologically Relevant Genes to Addiction and Neuroinflammation in Rats

Background: The ventral striatum is an integrated drug reward and addiction center. Objectives: We aimed to examine changes in pro-inflammatory cytokines and subsequent changes in the expression of biologically relevant genes to addiction and neuroinflammation in the ventral striatum in response to frequent morphine exposure. Methods: Sixteen male Wistar rats were divided into two experimental groups, a control, and a morphine-treated group, receiving eight days of twice-daily injections of either saline or morphine sulfate (10 mg/kg). Results: The results revealed that frequent morphine treatment increased both gene and protein levels of pro-inflammatory cytokines, including tumor necrosis factor α, interleukin 1-β, and interleukin 6 in the ventral striatum. Frequent morphine treatment also induced significant upregulations in the mRNA levels of mu-opioid receptor, dopamine D1 receptor (Drd1), Fos, nuclear factor- kappa B, and pre-miRNAs expression including, mir-124, mir-133b, mir-339, mir-365, and Let-7c1 in the ventral striatum. On the contrary, frequent morphine injection significantly downregulated mRNA levels of toll-like receptor 4, cannabinoid CB1 and CB2 receptors, Drd2, Il1r, Il6r, tnfr, protein kinase Cγ, calcium/calmodulin-dependent protein kinase IIα, nitric oxide synthase, cAMP-response element-binding protein as well as p38 and Jnk3 MAP kinases in the ventral striatum. However, no group differences were detected in the expression of Erk1 and mir-219 in the ventral striatum. Conclusions: It can be concluded that dysregulations in pro-inflammatory cytokines and, subsequently, in the downstream signaling pathways impair physiological functions of the ventral striatum following chronic morphine exposure, affecting reward pathways and the expression of morphine tolerance and dependence.