Activation of the reward system ameliorates passive cutaneous anaphylactic reaction in mice
Authors: Shotaro Nakajima, Satoshi Manita, Guannan Yu, Kayoko Ishimaru, Koji Kono, Kazuo Kitamura, Atsuhito Nakao
Journal: Allergy (2020)

The activation of the brain's reward system by chemogenetics, natural reward, or pharmacological approaches ameliorates IgE/mast cell-mediated allergic reaction in mice. These findings suggest a functional connection between a mood-regulating neuronal circuit and allergic reaction. Positive emotional states are often associated with better physical health outputs, including cancer survival.1 Some allergic diseases may also benefit from positive emotional states, as a high placebo effect has frequently been observed in clinical trials of allergic diseases such as allergic rhinitis.2 However, the causal link between positive emotional states and allergic reaction has not yet been investigated. The brain's reward system, consisting of the dopaminergic neurons (DNs) in the ventral tegmental area (VTA) and the dopaminergic projections from the VTA to components of the limbic system, mediates positive emotions, expectations, and motivation,3 which may contribute to the clinical benefit of placebo effects.4 Given the close relationships between positive emotional states and the reward system, we hypothesized that activation of the reward system may affect allergic reactions. To test this hypothesis, we examined the effects of chemogenetic, spontaneous, and pharmacological activation of the brain reward system on the passive cutaneous anaphylactic (PCA) reaction, a classical model of IgE/mast cell-mediated allergic skin reaction, in mice. Six-week-old male C57BL/6 mice and tyrosine hydroxylase (Th)-Cre mice housed under 12-hour light/12-hour dark conditions with ad libitum access to food and water were used. All animal experiments were approved by the Institutional Review Board of the University of Yamanashi. For more information, see the Supplementary Methods section. First, we used the designer receptor exclusively activated by designer drugs (DREADD) system to specifically control reward system activity as previously described.5 We injected the VTA of Th-Cre mice with a viral vector encoding DREADDs, mutated muscarinic receptors that respond to the synthetic ligand clozapine-N-oxide (CNO) but not their endogenous ligand, and the fluorescent reporter mCherry (AAV-hM3Dq); control mice were injected with a viral vector encoding mCherry but lacking DREADDs (AAV-control) (Figure 1A). Five weeks after the viral injections, the mice were treated intraperitoneally with CNO and subjected to the PCA reaction (Figure 1B). After AAV injection, we assessed the efficacy of virus expression in the VTA by immunohistochemistry for Th, a marker for DNs, as well as mCherry, an indicator of virus expression. DREADDs were specifically expressed in the VTA, but not in the substantia nigra pars compacta (SNc), which also contains DNs, as judged by the expression levels of mCherry and Th (Figure 1C). DREADD (AAV-hM3Dq) expression was observed in >60% of Th-positive area in the VTA, while DREADD expression was less than 10% of Th-positive area in the SNc (Figure S1). The expression levels of mCherry in the VTA were comparable between AAV-control injected mice and AAV-hM3Dq-injected mice (Figure 1C). DNs in the VTA were activated in response to CNO in AAV-hM3Dq-injected but not AAV-control-injected mice, as judged by c-fos expression (an indicator of neuron activation) (Figure 1D). Interestingly, the extent of PCA reaction was significantly smaller in VTA-activated mice than in control mice (Figure 1E). These results suggest a causal link between activation of the brain's reward system and amelioration of the PCA reaction. Next, we aimed to determine whether natural reward would affect allergic reactions. Rodents exhibit preferential saccharin intake when they are given an ad libitum two-bottle choice between saccharin solution and water, in association with activation of the VTA and the nucleus accumbens (NAcc), a dopamine projection area from the VTA.6 We used this experimental system for the present study (Figure 2A). Voluntary saccharin drinking markedly activated DNs in the VTA, as judged by c-fos expression (Figure 2B). The extent of PCA reaction was significantly smaller in mice that engaged in voluntary saccharin drinking than in control mice (Figure 2C). Importantly, administration of similar amounts of saccharin by gastric tube did not affect the extent of PCA reaction (Figure 2D). These results suggest that VTA activation triggered by voluntary saccharin drinking ameliorates the PCA reaction. Finally, we examined the effects of the dopamine precursor levodopa (L-DOPA) on PCA reaction (Figure S2, A). L-DOPA is converted to dopamine in the brain and is to replenish depleted levels of dopamine in patients with Parkinson's disease. Normally, L-DOPA is administered with a dopamine decarboxylase inhibitor such as benserazide to prevent conversion of L-DOPA to dopamine in the bloodstream,7 as dopamine cannot cross the blood-brain barrier. We found that administration of L-DOPA with benserazide decreased the extent of the PCA reaction relative to control mice (Figure S2B). Administration of L-DOPA or benserazide alone did not affect the extent of PCA reaction (Figure S2C,D), suggesting that dopamine levels in the brain, but not in the periphery, are important for suppression of the PCA reaction. Several studies have reported relationships between dopamine and IgE/mast cell-mediated allergic reactions, including the PCA reaction. However, most of these studies focus on the peripheral, but not brain (central), activity of dopamine/dopamine receptors, and some of their results are controversial. For instance, Mori et al reported that bone marrow-derived mast cells (BMMCs) express D1-like dopamine receptors and that the antagonist SCH23390 inhibited IgE-mediated degranulation in BMMCs and the PCA reaction in mice,8 suggesting that dopamine promotes allergic reactions. By contrast, Casale et al reported that intradermal or intravenous dopamine suppressed histamine-mediated cutaneous responses in humans,9 suggesting that dopamine antagonizes allergic reaction. We wish to emphasize that in this study, we used several independent approaches—DREADDs, spontaneous saccharine drinking, and L-DOPA with benserazide—that selectively or preferentially target the reward system in the mouse brain. This allows us to exclude peripheral or net (ie, both peripheral and central) effects of dopamine/dopamine receptors on the PCA reaction. The mechanisms linking the reward system and allergic reaction remain to be determined. We observed that plasma histamine levels upon PCA reaction were comparable between VTA-activated and control mice, as well as between voluntarily saccharin-drinking mice and control mice (Figure S3A,B), suggesting that the reward system might affect PCA reaction downstream of mast cell activation. Consistently, activation of the brain's reward system by voluntary saccharin drinking slightly but significantly suppressed plasma extravasation by intradermal injection of histamine in the skin (Figure S4). In addition, we did not find any significant effects of 6-hydroxydopamines (6-OHDA) that chemically ablate peripheral catecholaminergic neurons of the sympathetic nervous system (SNS) on the inhibition of PCA reaction in VTA-activated mice by voluntary saccharin-drinking mice (Figure S5). 6-OHDA did not affect basal PCA reaction in mice (Figure S5). These data suggest that the brain's reward system might affect PCA reaction in part through inhibition of histamine-induced vascular permeability, independent on the SNS. The extent of control PCA reaction appears to be smaller in virus-injected mice than mice without virus-injected mice (Figure 1E, Figure 2C,D, Figure S6). Thus, viral injection might affect PCA responses perhaps because viral injection affects innate immune response. In summary, this study shows that activation of the reward system in the brain by chemogenetic, natural reward, or pharmacological approaches can ameliorate PCA reaction in mice. These findings suggest a functional connection between a mood-regulating neuronal circuit, specifically the reward system, and type I allergic reactions. We thank Prof. Keisuke Masuyama, Dr Naohito Miyamoto, and Suguru Fukumoto for financial support. We are also grateful to Naomi Yaguchi, Maiko Aihara, and Chika Numano for technical assistance, and Tomoko Tohno and Yukino Fukasawa for helpful secretarial assistance. The authors declare that they have no conflicts of interest. This work was supported by the grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan (18H02848) (to Atsuhito Nakao) and by the academic crowdfunding platform “Academist” (to Shotaro Nakajima). Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.