Persistence of Serotonergic Enhancement of Airway Response in a Model of Childhood Asthma

The persistence of airway hyperresponsiveness (AHR) and serotonergic enhancement of airway smooth muscle (ASM) contraction induced by ozone (O-3) plus allergen has not been evaluated. If this mechanism persists after a prolonged recovery, it would indicate that early-life exposure to O-3 plus allergen induces functional changes predisposing allergic individuals to asthma-related symptoms throughout life, even in the absence of environmental insult. A persistent serotonergic mechanism in asthma exacerbations may offer a novel therapeutic target, widening treatment options for patients with asthma. The objective of this study was to determine if previously documented AHR and serotonin-enhanced ASM contraction in allergic monkeys exposed to O-3 plus house dust mite allergen (HDMA) persist after prolonged recovery. Infant rhesus monkeys sensitized to HDMA were exposed to filtered air (FA) (n = 6) or HDMA plus O-3 (n = 6) for 5 months. Monkeys were then housed in a FA environment for 30 months. At 3 years, airway responsiveness was assessed. Airway rings were then harvested, and ASM contraction was evaluated using electrical field stimulation with and without exogenous serotonin and serotonin-subtype receptor antagonists. Animals exposed to O-3 plus HDMA exhibited persistent AHR. Serotonin exacerbated the ASM contraction in the exposure group but not in the FA group. Serotonin subtype receptors 2, 3, and 4 appear to drive the response. Our study shows that AHR and serotonin-dependent exacerbation of cholinergic-mediated ASM contraction induced by early-life exposure to O-3 plus allergen persist for at least 2.5 years and may contribute to a persistent asthma phenotype.