Innate immune signaling in the brain especially via toll-like receptors (TLRs) has emerged as a contributor to many central nervous system (CNS) pathologies, including addiction. In particular, the interaction between morphine and TLR4 is likely to be an important factor in morphine dependence and withdrawal. Αlpha-pinene is a member of the monoterpene family that has exhibited several pharmacological properties, including anti-inflammatory effects. However, there is no report evaluating effects of alpha-pinene on TLRs signaling in induction of morphine dependence and withdrawal. This study aimed to examine the effects of alpha-pinene treatment on TLRs signaling pathway in the rat hippocampus after morphine dependence and withdrawal. Six groups of male Wistar rats were used in two categories, including dependent and withdrawal. In the first category, three experimental groups received 10 days of either saline + DMSO, morphine (10 mg/kg) + DMSO, or morphine + alpha-pinene (20 mg/kg). In the second category, three experimental groups received 10 days of either saline (group 1) or morphine treatments (groups 2 and 3) followed by a 30-day withdrawal period in which the saline-treated group and one of the groups treated with morphine received daily injections of DMSO, while the second group of morphine-treated animals received daily injections of alpha-pinene. The results revealed that expression of TLR2, 4, and 10 as well as MyD88 altered in the hippocampus both after morphine dependence and withdrawal period. However, hippocampal levels of PI3K, p-AKT1B, and Il-1Ra decreased in the rat hippocampus after both morphine dependence and withdrawal. The results also revealed that treatment with alpha-pinene during either 10 days of the induction of dependence or 30 days of withdrawal period significantly restored the alterations observed either after dependence or withdrawal in TLRs signaling pathway in the hippocampus. It can be concluded that alpha-pinene has a clear effect on the central immune system by controlling the changes in the expression of the TLR signaling pathway, thereby modulating the onset and progression of morphine addiction and withdrawal complications. These results can serve as a valuable insight to guide future therapeutic approaches in the treatment of complications caused by repeated use of morphine such as tolerance, dependence and addiction.
