We examined the role of toll-like receptors (TLRs) and proinflammatory cytokine signaling pathways in the prefrontal cortex (PFC) in anxiety-like behaviors after repeated use of morphine. Morphine (10 mg/kg) was used twice daily for 8 days to induce morphine dependence in male Wistar rats. On day 8, opioid dependence was confirmed by measuring naloxone-precipitated withdrawal signs. On days 1 and 8, anxiety-like behaviors were evaluated using a light/dark box test. Expression of TLR1 and 4, proinflammatory cytokines, and some of the downstream signaling molecules was also evaluated in the bilateral PFC at mRNA and protein levels following morphine dependence. The results revealed that morphine caused anxiolytic-like effects on day 1 while induced anxiety following 8 days of repeated injection. On day 8, a significant decrease in TLR1 expression was detected in the PFC in morphine-dependent rats, but TLR4 remained unaffected. Repeated morphine injection significantly increased IL1-β, TNFα, and IL6 expression, but decreased IL1R and TNFR at mRNA and protein levels except for IL6R at the protein level in the PFC. The p38α mitogen-activated protein (MAP) kinase expression significantly increased but the JNK3 expression decreased in the PFC in morphine-dependent rats. Repeated injection of morphine also significantly increased the NF-κB expression in the PFC. Further, significant increases in Let-7c, mir-133b, and mir-365 were detected in the PFC in morphine-dependent rats. We conclude that TLR1 and proinflammatory cytokines signaling pathways in the PFC are associated with the anxiogenic-like effects of morphine following its chronic use in rats via a MAP kinase/NF-κB pathway.
