Hochuekkito attenuates anxiety-like behavior associated with pulmonary inflammation induced by intratracheal lipopolysaccharides in mice

Introduction We have previously demonstrated that intratracheal lipopolysaccharide (LPS) injection induces significant pulmonary inflammation accompanied by hippocampal microglial activation, indicative of neuroinflammation. Hochuekkito (HET) is a traditional Japanese herbal medicine used to treat various conditions, including mental disorders and physical weakness. We have previously reported that HET ameliorates anxiety-like behaviors induced by intraperitoneal LPS injections in mice. However, its anxiolytic effects on anxiety-like behaviors due to pulmonary inflammation remain poorly understood. Therefore, in the present study, we aimed to investigate the effects of HET on anxiety-like behaviors induced by intratracheal LPS injection in mice. Methods Mice received HET (1.0 g/kg) once daily for 2 weeks through oral gavage prior to LPS treatment. The light-dark box test was conducted 24 h following LPS injection to assess anxiety-like behaviors. Diazepam, a clinically used anxiolytic, served as a positive control. The lung wet-to-dry weight ratio was determined, and the concentrations of interleukin-6 (IL-6) in the lungs and serum were assessed. Results Repeated administration of HET prevented the development of anxiety-like behaviors and reduced serum IL-6 concentrations and hippocampal Il6 mRNA expression levels in LPS-treated mice. Diazepam failed to exert significant effects in LPS-treated mice, whereas HET remained effective under inflammatory conditions. Moreover, LPS injections significantly increased the number of Iba-1-immunoreactive microglial cells in the CA1 region of the hippocampus, whereas this effect was suppressed by treatment with HET. In the bronchoalveolar lavage fluid (BALF), the LPS-induced increase in white blood cell count was significantly reduced by treatment with HET. Furthermore, HET alleviated LPS-induced pulmonary inflammation, as evidenced by decreased lung wet-to-dry weight ratios. Conclusion This study suggests that inflammation induced by intratracheal LPS injection contributes to anxiety-like behaviors in mice and that IL-6 may play a key role in linking peripheral inflammation to neuroinflammatory responses. The anxiolytic effects of HET appear to be associated, at least in part, with the suppression of IL-6 elevation in both the periphery and the hippocampus, along with attenuation of microglial activation. Our findings suggest that HET may serve as a potential therapeutic agent for anxiety-like behaviors associated with pulmonary inflammation.