The dopamine D3 receptor regulates the effects of methamphetamine on LPS-induced cytokine production in murine mast cells

Immunity on ice: The impact of methamphetamine on peripheral immunity

Methamphetamine (METH) regulation of the dopamine transporter (DAT) and central nervous system (CNS) dopamine transmission have been extensively studied. However, our understanding of how METH influences neuroimmune communication and innate and adaptive immunity is still developing. Recent studies have shed light on the bidirectional communication between the CNS and the peripheral immune system. They have established a link between CNS dopamine levels, dopamine neuronal activity, and peripheral immunity. Akin to dopamine neurons in the CNS, a majority of peripheral immune cells also express DAT, implying that in addition to their effect in the CNS, DAT ligands such as methamphetamine may have a role in modulating peripheral immunity. For example, by directly influencing DAT-expressing peripheral immune cells and thus peripheral immunity, METH can trigger a feed-forward cascade that impacts the bidirectional communication between the CNS and peripheral immune system. In this review, we aim to discuss the current understanding of how METH modulates both innate and adaptive immunity and identify areas where knowledge gaps exist. These gaps will then be considered in guiding future research directions.

Dopamine D3 receptor signaling alleviates mouse rheumatoid arthritis by promoting Toll-like receptor 4 degradation in mast cells

Dopamine receptors are involved in several immunological diseases. We previously found that dopamine D3 receptor (D3R) on mast cells showed a high correlation with disease activity in patients with rheumatoid arthritis, but the mechanism remains largely elusive. In this study, a murine collagen-induced arthritis (CIA) model was employed in both DBA/1 mice and D3R knockout mice. Here, we revealed that D3R-deficient mice developed more severe arthritis than wild-type mice. D3R suppressed mast cell activation in vivo and in vitro via a Toll-like receptor 4 (TLR4)-dependent pathway. Importantly, D3R promoted LC3 conversion to accelerate ubiquitin-labeled TLR4 degradation. Mechanistically, D3R inhibited mTOR and AKT phosphorylation while enhancing AMPK phosphorylation in activated mast cells, which was followed by autophagy-dependent protein degradation of TLR4. In total, we found that D3R on mast cells alleviated inflammation in mouse rheumatoid arthritis through the mTOR/AKT/AMPK-LC3-ubiquitin-TLR4 signaling axis. These findings identify a protective function of D3R against excessive inflammation in mast cells, expanding significant insight into the pathogenesis of rheumatoid arthritis and providing a possible target for future treatment.

Associations between D3R expression in synovial mast cells and disease activity and oxidant status in patients with rheumatoid arthritis

Dopamine D3 receptor (D3R) on immune cells is involved in the pathogenesis of rheumatoid arthritis (RA). Mast cells (MCs) are currently identified as important effector cells in synovial inflammation of RA, but little is known about the role of D3R on synovial MCs in the pathogenesis of RA. Several inflammatory cells in the synovium induce reactive oxygen species (ROS) formation which are involved in the progression of RA. However, it is unclear whether D3R on synovial MCs is related to the levels of ROS in RA patients. In this study, a total of 73 patients with RA were divided into three groups according to disease activity DAS28 scores. The number of cases in group 1, group 2, and group 3 was 19, 26, and 28, respectively. We examined D3R-positive MC numbers in the synovial fluid and ROS levels in each group of RA patients, and we also analyzed the association of D3R-positive MC numbers with RA disease activity and ROS levels. MDA and protein carbonylation in the serum and synovial fluid were measured to reflect the level of lipid peroxidation and protein oxidation, respectively. Additionally, superoxide dismutase (SOD) and catalase (CAT) in the serum and synovial fluid were used to be markers of antioxidant levels. Our results showed that D3R-positive MCs in the synovial fluid showed a declining trend with the increased disease activity DAS28 score in RA patients. There was negative correlation between D3R-positive MC numbers in the synovial fluid and disease severity DAS28 score of RA patients. Moreover, D3R-positive MC numbers in the synovial fluid were negatively correlated with the level of MDA and protein carbonylation while were positively correlated with antioxidant levels such as SOD and CAT in RA patients. Our results suggested that D3R on MCs may be involved in ROS-mediated pathogenesis of RA.

Inhibitory effects of methamphetamine on mast cell activation and cytokine/chemokine production stimulated by lipopolysaccharide in C57BL/6J mice

Previous studies have demonstrated that methamphetamine (MA) influences host immunity; however, the effect of MA on lipopolysaccharide (LPS)-induced immune responses remains unknown. Mast cells (MCs) are considered to serve an important role in the innate and acquired immune response, but it remains unknown whether MA modulates MC activation and LPS-stimulated cytokine production. The present study aimed to investigate the effect of MA on LPS-induced MC activation and the production of MC-derived cytokines in mice. Markers for MC activation, including cluster of differentiation 117 and the type I high affinity immunoglobulin E receptor, were assessed in mouse intestines. Levels of MC-derived cytokines in the lungs and thymus were also examined. The results demonstrated that cytokines were produced in the bone marrow-derived mast cells (BMMCs) of mice. The present study demonstrated that MA suppressed the LPS-mediated MC activation in mouse intestines. MA also altered the release of MC cytokines in the lung and thymus following LPS stimulation. In addition, LPS-stimulated cytokines were decreased in the BMMCs of mice following treatment with MA. The present study demonstrated that MA may regulate LPS-stimulated MC activation and cytokine production.

Methamphetamine modulates the production of interleukin-6 and tumor necrosis factor-alpha via the cAMP/PKA/CREB signaling pathway in lipopolysaccharide-activated microglia

Methamphetamine (METH) elicits neuroinflammatory effects that may implicate its regulatory role on the microglial immune response. However, the mechanism underlying this remains unclear. In the present study, the effects of METH on lipopolysaccharide (LPS)-induced interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) productions were tested in BV-2 cells and primary microglial cells. Additionally, western blot analysis was used to examine the phosphorylation of mitogenactivated protein kinases (MAPKs). Next, we detected the alterations in cAMP content and the phosphorylation levels of CREB in microglial cells to determine the involvement of the cAMP/CREB signaling pathway. We also used an adenylyl cyclase (AC) agonist (forskolin) and antagonist (MDL-12330A) and a PKA antagonist (H89) to confirm their participation. We observed that METH alone did not affect the production of IL-6 or TNF-α. In contrast, METH augmented the IL-6 production and inhibited the TNF-α production induced by LPS. A similar effect of forskolin was also observed in BV-2 cells. While MAPK activation was not influenced by METH alone, the LPS-induced phosphorylation of p38, JNK and ERK1/2 were all reduced by METH. Both the concentration of cAMP and the phosphorylation of CREB were increased by METH in LPS-activated microglial cells. The effects of METH were altered by MDL-12330A and H89. Moreover, the inhibition of the phosphorylation of ERK1/2 by METH was also reversed. These results suggest that the differential regulation of IL-6 and TNF-α by METH in LPS-activated microglial cells may be attributable to the cAMP/PKA/CREB signaling pathway.

Thalamic mast cell activity is associated with sign-tracking behavior in rats

Mast cells are resident immune cells in the thalamus that can degranulate and release hundreds of signaling molecules (i.e., monoamines, growth factors, and cytokines) both basally and in response to environmental stimuli. Interestingly, mast cell numbers in the brain show immense individual variation in both rodents and humans. We used a Pavlovian conditioned approach (PCA) procedure to examine whether mast cells are associated with individual variation in the attribution of incentive-motivational value to reward-related cues. During the PCA procedure, a lever response-independently predicts the delivery of a food pellet into a magazine, and over training sessions three conditioned responses (CRs) develop: sign-tracking (lever-directed CRs), goal-tracking (magazine-directed CRs), and an intermediate response (both CRs). In Experiment 1, we measured thalamic mast cell number/activation using toluidine blue and demonstrated that sign-trackers have increased degranulated (activated) but not granulated (inactive) mast cells. In Experiment 2, we infused the mast cell inhibitor, cromolyn (200µg/rat; i.c.v.), immediately before five daily PCA training sessions and demonstrated that mast cell inhibition selectively impairs the acquisition of sign-tracking behavior. Taken together, these results demonstrate that thalamic mast cells contribute to the attribution of incentive-motivational value to reward-related cues and suggest that mast cell inhibition may be a novel target for addiction treatment.

The effects of D3R on TLR4 signaling involved in the regulation of METH-mediated mast cells activation

Accumulating studies have revealed that the dopamine D3 receptor (D3R) plays an important role in methamphetamine (METH) addiction. However, the action of D3R on METH-mediated immune response and the underlying mechanism remain unclear. Mast cells (MCs) are currently identified as effector cells in many processes of immune responses, and MC activation is induced by various stimuli such as lipopolysaccharide (LPS). Moreover, CD117 and FcεRI are known as MC markers due to their specific expression in MCs. To investigate the effects of D3R on METH-mediated alteration of LPS-induced MCs activation and the underlying mechanism, in this study, we examined the expression of CD117 and FcεRI in the intestines of wild-type (D3R(+/+)) and D3R-deficient (D3R(-/-)) mice. We also measured the production of MC-derived cytokines, including TNF-α, IL-6, IL-4, IL-13 and CCL-5, in the bone marrow-derived mast cells (BMMCs) of WT and D3R(-/-) mice. Furthermore, we explored the effects of D3R on METH-mediated TLR4 and downstream MAPK and NF-κB signaling induced by LPS in mouse BMMCs. We found that METH suppressed MC activation induced by LPS in the intestines of D3R(+/)mice. In contrast, LPS-induced MC activation was less affected by METH in D3R(-/-) mice. Furthermore, METH altered LPS-induced cytokine production in BMMCs of D3R(+/+) mice but not D3R(-/-) mice. D3R was also involved in METH-mediated modulation of LPS-induced expression of TLR4 and downstream MAPK and NF-κB signaling molecules in mouse BMMCs. Taken together, our findings demonstrate that the effect of D3R on TLR4 signaling may be implicated in the regulation of METH-mediated MCs activation induced by LPS.