Neuroimmune Imbalance: The Key for the Treatment of Anxiety?

Bai Li, Tangxin Gao, Jing Du *

School of Medicine, Yunnan University, 2 Cuihu North Road, Kunming, 650091, Yunnan, China

Anxiety disorder, a common mood disorder, seems associated with neuro-immune aberration on the pathophysiology, revealed by many clinical and preclinical evidences^1-5. Furthermore, previous research suggested that the prefrontal cortex (PFC) is a critical brain region involved in anxiety disorders^6-9. Now, we show that to keep the balance of pro-inflammatory immune-status and the anti-inflammatory immune-status in the PFC is the key for the treatment of anxiety¹⁰.

3’-deoxyadenosine (3’-dA), one of the major bioactive metabolites in the Cordyceps Militaris, possesses multiple beneficial effects of anti-inflammatory, anti-cancer, anti-viral, and anti-fungal activities^{11, 12}. In recent study, we found that 3'-dA treatments significantly increased IL-4 expression in neurons, and IL-4 expression strongly correlated with the parameters representing reduced anxiety behaviours, suggesting that expression of the key anti-inflammatory molecule IL-4 is critical for the anxiolytic effect¹⁰. In addition, both 3'-dA and traditional anxiolytic imipramine enhanced the expression levels of IL-4 and IL-10, and decreased the expression levels of IL-1β and TNF-α, synchronous with the anxiolytic effects produced after chronic treatment for 5 days. The blockade of IL-4 action using the specific inhibitor RIL-4Rα prevented the anxiolytic effect of both 3'-dA and imipramine, indicating that correction of imbalance of proinflammatory and anti-inflammatory pathways is the common mechanism for the treatment of anxiety disorders¹⁰. Other anxiolytics, including diazepam, oryzanol, and chlordiazepoxide were shown enhancing anti-inflammatory cytokines and inhibiting pro-inflammatory cytokines in support of the findings^13-16.

It is well established that pro-inflammatory cytokines, such as interleukin (IL)-6, TNF-α and IL-1β, are increased in patients with anxiety disorders¹. Conversely, the anti-inflammatory cytokine IL-4 is negatively associated with trait anxiety in adolescent girls² and with anxiety symptoms in women in mid-pregnancy^{2, 3}. In an animal model, IL-4 was reduced in anxious mice⁴, and an IL-4 injection attenuated anxiety-like behaviour⁵ and a previous study demonstrated that IL-4-knockout mice exhibited anxiety-like behaviour¹⁸. Immobilization stress leads to decreased release of IL-4 from the neurons of the locus coeruleus⁴. Growing evidence indicates the balance between pro-inflammatory and anti-inflammatory factors is associated with the pathophysiology of anxiety disorder¹⁷. Increased inflammatory reactions in the CNS are commonly seen in patients with anxiety disorders.

3'-dA was reported to be able to modulate peripheral immune functions, Downregulating the gene expression of the pro-inflammatory factors IL-1β, TNF-α, and IL-2 and upregulating the anti-inflammatory factors IL-4, IL-10, IL-1Rα, and TGF-β in macrophages and mononuclear cells in the peripheral immune system^18-20, similar to what happened in CNS. The lack of IL-4, a key molecule to modulate neuroimmune status, leads to leads to increased neuroinflammation in response to lipopolysaccharide in mice²¹. The action of IL-4 results from its ability to inhibit the release of pro-inflammatory cytokines, such as IL-1β and TNF-α, by innate immune cells and to upregulate the synthesis of anti-inflammatory factors, such as IL-1β receptor antagonist^{19, 22}. In addition, neuronal IL-4 plays a vital role in brain clean-up and repair after ischemic damage²³. Endogenous IL-4 is expressed mainly in neurons, although small amounts co-localize with GFAP-positive cells and Iba1-immuno-reactive cells⁴. Our study shows that 3'-dA exhibited a rapid and stable anxiolytic effect in the mice behavioural tests via activating the IL-4 pathway, regulating the expression levels of the IL-10 IL-1β and TNF-α in PFC,¹⁰, suggesting 3'-dA functions to correct the imbalance between pro-inflammatory factors and anti-inflammatory factors, which is helpful for restraining inflammatory reactions, activating anti-inflammatory factor pathways, and promoting neural-functional repair in the CNS.

Furthermore, cumulative studies show that transition of pro-inflammatory to anti-inflammatory status helps to modulate neuronal and synaptic functions. It was previously reported that restoration of IL-4 concentrations in the hippocampus to the levels of those observed in young rats is associated with the successful maintenance of long-term potentiation (LTP), and the downregulation of IL-4-stimulated signalling significantly contributes to the deficit in LTP^{19, 24, 25}. LTP is an important indicator of neural synaptic plasticity, which plays a key role in behavioural manifestations^{26, 27}, and may be a relevant mechanism in IL-4-mediated anxiolytic effects of 3'-dA and imipramine.

In summary, both 3'-dA and the traditional antianxietic imipramine produced an anxiolytic effect via increasing the IL-4 expression levels to correct the pro-inflammatory/anti-inflammatory balance, subsequently to change the neuronal functions and behaviours, and the IL-4 specific inhibitor RIL-4Rα could prevented the anxiolytic effect of both 3'-dA and imipramine. These results indicate that IL-4 pathway is the key to the treatment of anxiety disorder and provide an innovative drug with a novel neuro-immune mechanism for the treatment of anxiety disorder.

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