Repeated use of morphine induces anxiety via affecting a proinflammatory cytokine signaling pathway in the prefrontal cortex in rats

Abstract We examined the involvement 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 dependence in male Wistar rats was induced via twice-daily morphine injection (10 mg/kg) for eight days. On day 8, opioid dependence was confirmed by measuring morphine withdrawal signs precipitated with naloxone. On days 1 and 8, anxiety-like behaviors were evaluated using a light/dark box test. On day 8, TLR1 and 4, proinflammatory cytokines, and some of the downstream signaling molecules were evaluated at mRNA and protein levels in the PFC. The results revealed that morphine caused anxiolytic-like effects on day 1, which significantly decreased after eight days of the repeated injection. On day 8, a significant decrease in TLR1 expression in the PFC was detected in morphine-dependent rats but TLR4 remained unaffected. Repeated morphine injection significantly increased theIL1-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 of morphine-dependent rats. The NF-κB expression also increased significantly in the PFC after repeated injection of morphine. Significant increases in Let-7c , mir-133b , and mir-365 were also detected in the PFC in morphine-dependent rats. We conclude that TLR1 and proinflammatory cytokines in the PFC via a MAP kinase/NF-κB pathway may party underlie the decrease in the anxiolytic-like effect of morphine in dependent rats.