## 4 Discussion
These results provide evidence that the oral administration of IAP extract significantly and dose-dependently suppresses PM\({}_{2.5}\)-induced pulmonary edema. Regarding inflammatory lung damage caused by intranasal injection of PM\({}_{2.5}\), there were significant increases in lymphocytes, neutrophils, amount of total leukocytes, eosinophil, monocytes, and total cell number in BALF. In contrast, in all three IAP-extract-administered groups, the number of cells, white blood cells, lymphocytes, monocytes, neutrophils, and eosinophils in BALF significantly decreased in a dose-dependent manner. In particular, the amount of leukocytes and cells in BALF of the IAP400 group were comparable to those of the DEXA group, indicating that the oral administration of IAP extract significantly suppresses lung damage in PM\({}_{2.5}\)-induced inflammation. Mucus, present on the surface of the respiratory system, serves as the primary defense against various foreign substances and infectious agents. Mucin, a glycoprotein, is a crucial component of mucus [56; 57]. The cleansing action of the microglia in the respiratory system is directly influenced by the quantity and viscosity of mucin. In other words, impaired cleansing action of the microglia is indicated by the presence of a large amount of mucin with high viscosity [58]. MUC5 AC and MUC5B are the predominant polymeric substances secreted by the respiratory system [56; 57], and alterations in the expression of MUC5 AC and MUC5B are induced in various respiratory diseases [59; 60; 61]. Particularly, PM\({}_{2.5}\) is known to promote mucin production by increasing the expression of MUC5 AC and MUC5B mRNA, resulting in the formation of high-viscosity sputum [16; 56; 57]. In the respiratory system, secretion is primarily regulated through the nervous system [62]. Stimulation or excitation of the vagus nerve leads to the local secretion of ACh and significant glandular secretion [63]. Indeed, Substance P is a well-known representative factor that promotes serous secretion in the respiratory system [64; 65; 66]. Hence, serous secretion in the respiratory system is primarily regulated by ACh and Substance P [62; 63]. Furthermore, Substance P also functions as a neurogenic inflammatory factor in the respiratory system [67]. The inhalation of PM\({}_{2.5}\) leads to a noteworthy increase in Substance P levels [68,69]. Moreover, PM\({}_{2.5}\) exposure escalates vascular permeability, contributing to the inflammatory response [2,3], as a result causing an increase in ACh levels, which acts as a vasorelasant factor [16,70]. However, in PM\({}_{2.5}\)-induced lung injury mice, the vascular reactivity to ACh notably diminishes [71]. In the current experiment, the PM\({}_{2.5}\) control group exhibited heightened production of high-viscosity mucus, driven by increased levels of ACh and substance P [16]. Additionally, there was an increase in mRNA expression of MUC5 AC and MUC5B [16,64,65], contributing to a pronounced neurogenic inflammatory response within lung tissue. On the contrary, as part of the mucolytic expectorant activity, significant increases in the secretion-promoting factors Substance P and ACh [16,62,63] were observed. Additionally, a reduction in mRNA expression of mucus-production-related genes MUC5 AC and MUC5B [16,56,57] in lung tissue was observed in a dose-dependent manner in all three IAP-extract-administered groups. These results verify that the oral administration of IAP extract (400, 200, or 100 mg/kg) results in mucolytic expectorant activity through mucin-reducing effects by decreasing MUC5 AC and MUC5B mRNA expression, as well as promoting serous secretion by increasing substance P and ACh production. In contrast, a significant reduction in MUC5 AC and MUC5B mRNA expression was observed in the DEXA group. These expression levels were comparable to those observed in the IAP400 group. However, the levels of substance P and ACh were significantly decreased in the DEXA group when compared to the PM\({}_{2.5}\) control group.