Objective To investigate the effects of γ-secretase inhibitor (DAPT) on the secretory otitis media (SOM) of rats, and preliminarily explore the potential mechanisms of action.

Methods The SOM rat model was established by injecting type III pneumococcal suspension into the middle ear cavity. The SOM rats were randomly divided into the model group, positive drug group (1 mg/kg methylprednisolone solution, gavage), DAPT group (12 mg/kg DAPT solution, gavage) and NLRP3 activator group (12 mg/kg DAPT solution by gavage and 1 mg/kg nigericin by intraperitoneal injection), with 10 rats in each group. An additional 10 normal rats were served as the control group. After the treatment, the auditory brainstem response (ABR) threshold of rats was measured using evoked potential instrument. The tympanic membrane and middle ear effusion were observed using endoscopy. The pathological changes in rat middle ear mucosa tissue and middle ear mucosal thickness were observed using HE staining. The levels of TNF-α, IL-1β and IL-18 in middle ear mucosal tissue were detected using ELISA kit. RT-qPCR and Western blotting were used to detect the mRNA and protein expression levels of NLRP3, ASC, Caspase-1 and IL-1β.

Results Compared with the control group, the tympanic membrane in the model group was yellowing, the middle ear effusion was obvious, the middle ear mucosa was thickened, accompanied by a large infiltration of inflammatory cells. The hearing threshold, middle ear mucosa thickness, serum levels of TNF-α, IL-1β and IL-18 and mRNA and protein expression levels of NLRP3, ASC, Caspase-1 and IL-1β increased. The differences were statistically significant (P ＜ 0.05). Compared with the model group, the degree of tympanic membrane yellowing and middle ear effusion in the positive drug group, DAPT group and NLRP3 activator group were alleviated, and the pathological damage of middle ear mucosa also reduced. The hearing threshold, thickness of the middle ear mucosa, serum levels of TNF-α, IL-1β and IL-18 and mRNA and protein expression levels of NLRP3, ASC, Caspase-1 and IL-1β decreased, and the differences were statistically significant (P ＜ 0.05). Compared with the positive drug group, the tympanic membranes in the DAPT group almost returned to normal, with a small amount of effusion and mild thickening. The hearing threshold, thickness of the middle ear mucosa, serum levels of TNF-α, IL-1β and IL-18 and protein expression levels of NLRP3, ASC, Caspase-1 and IL-1β decreased, and the differences were statistically significant (P ＜ 0.05). Compared with the DAPT group, the NLRP3 activator group reversed the above results, and the differences were statistically significant (P ＜ 0.05).

Conclusions DAPT can improve the hearing damage in SOM rats, alleviate middle ear injury, and inhibit inflammatory responses, and its mechanism may be related to the inhibition of NLRP3 inflammasome activation.