Abstract
Methamphetamine (METH) is a globally abused, highly addictive stimulant. While investigations of the rewarding and motivational effects of METH have focused on neuronal actions, increasing evidence suggests that METH can also target microglia, the innate immune cells of the central nervous system, causing release of proinflammatory mediators and therefore amplifying the reward changes in the neuronal activity induced by METH. However, how METH induces neuroinflammatory responses within the central nervous system (CNS) is unknown. Herein, we provide direct evidence that METH creates neuroinflammation, at least in part, via the activation of the innate immune Toll-like receptor 4 (TLR4). Biophysical studies revealed that METH bound to MD-2, the key coreceptor of TLR4. Molecular dynamics simulations showed METH binding stabilized the active heterotetramer (TLR4/MD-2)2 conformation. Classic TLR4 antagonists LPS-RS and TAK-242 attenuated METH induced NF-κB activation of microglia, whereas added MD-2 protein boosted METH-induced NF-κB activation. Systemically administered METH (1 mg/kg) was found to specifically up-regulate expression of both CD11b (microglial activation marker) and the proinflammatory cytokine interleukin 6 (IL-6) mRNAs in the ventral tegmental area (VTA), but not in either the nucleus accumbens shell (NAc) or prefrontal cortex (PFC). Systemic administration of a nonopioid, blood-brain barrier permeable TLR4 antagonist (+)-naloxone inhibited METH-induced activation of microglia and IL-6 mRNA overexpression in VTA. METH was found to increase conditioned place preference (CPP) as well as extracellular dopamine concentrations in the NAc, with both effects suppressed by the nonopioid TLR4 antagonist (+)-naloxone. Furthermore, intra-VTA injection of LPS-RS or IL-6 neutralizing antibody suppressed METH-induced elevation of extracellular NAc dopamine. Taken together, this series of studies demonstrate that METH-induced neuroinflammation is, at least in part, mediated by TLR4-IL6 signaling within the VTA, which has the downstream effect of elevating dopamine in the NAc shell. These results provide a novel understanding of the neurobiological mechanisms underlying acute METH reward that includes a critical role for central immune signaling and offers a new target for medication development for treating drug abuse.