A guinea pig model of acute and chronic asthma using permanently instrumented and unrestrained animals Herman Meurs 1 , Ruud E Santing 1 , Rene ´ Remie 1 , Thomas W van der Mark 2 , Fiona J Westerhof 1 , Annet B Zuidhof 1 , I Sophie T Bos 1 & Johan Zaagsma 1 1 Department of Molecular Pharmacology, University Centre for Pharmacy, Antonius Deusinglaan 1, 9713 AVGroningen, The Netherlands. 2 Department of Pulmonology, University Medical Centre Groningen, Hanzeplein 1, 9713 GZ Groningen, The Netherlands. Correspondence should be addressed to H.M. (h.meurs@rug.nl). Published online 27 July 2006; doi:10.1038/nprot.2006.144 To investigate mechanisms underlying allergen-induced asthmatic reactions, airway hyperresponsiveness and remodeling, we have developed a guinea pig model of acute and chronic asthma using unanesthetized, unrestrained animals. To measure airway function, ovalbumin (IgE)-sensitized animals are permanently instrumented with a balloon-catheter, which is implanted inside the pleural cavity and exposed at the neck of the animal. Via an external cannula, the balloon-catheter is connected to a pressure transducer, an amplifier, an A/D converter and a computer system, enabling on-line measurement of pleural pressure (P pl )—closely correlating with airway resistance—for prolonged periods of time. Using aerosol inhalations, the method has been successfully applied to measure ovalbumin-induced early and late asthmatic reactions and airway hyperresponsiveness. Because airway function can be monitored repeatedly, intra-individual comparisons of airway responses (e.g., to study drug effects) are feasible. Moreover, this model is suitable to investigate chronic asthma and airway remodeling, which occurs after repeated allergen challenges. The protocol for establishing this model takes about 4 weeks. INTRODUCTION Allergic asthma Allergic asthma is a chronic inflammatory disease of the airways. Characteristic features of this disease are allergen-induced early and late bronchial obstructive reactions, associated with infiltration and activation of inflammatory cells in the airways—particularly Th2 lymphocytes and eosinophils—and the development of airway hyperresponsiveness to a variety of stimuli, including allergens, chemical irritants, cold air and pharmacological agents like hista- mine and methacholine 1 . Alterations in the neurogenic and non- neurogenic control of airway smooth muscle function, as well as physical changes in the airways, including epithelial damage, thickening of the mucosa due to edema and local vasodilation, and mucous secretions in the airway lumen, may be involved in the development of airway hyperresponsiveness after the early and late asthmatic reactions in acute asthma. In chronic asthma, airway remodeling due to irreversible structural changes of the airway wall, including thickening of the basement membrane, mucous gland hypertrophy, subepithelial fibrosis and increased airway smooth muscle mass, may be importantly involved in persistent airway hyperresponsiveness and decline of lung function. All of these changes are induced by a complex cascade of inflammatory reac- tions involving various allergic mediators, neurotransmitters, cyto- kines, growth factors and reactive oxygen and nitrogen species 1,2 . Animal models of asthma Although clinically well characterized, the exact pathological mechanisms underlying the allergen-induced early and late asth- matic reactions, airway hyperresponsiveness and remodeling are still incompletely understood. These mechanisms cannot be inves- tigated exhaustively in human subjects because of trivial experi- mental limitations; hence, there is considerable interest in animal models of human asthma, each with its strengths and weaknesses 3 . Of these, the sensitized guinea pig has received considerable attention as a small laboratory animal that develops allergen- induced asthmatic reactions; guinea pigs have been used to study pathophysiological mechanisms and pharmacological interven- tions in asthma since the 1930s 4 . From a physiological point of view, the sensitized guinea pig, being a non-rodent 3,5 , may be preferred as an animal model to investigate mechanisms of airway hyperresponsiveness in asthma as compared to commonly used rodents such as mice and rats. Although rodents have been invaluable for elucidating putative mechanisms of allergic airway inflammation, the mediators that govern airway smooth muscle tone and ultimately determine airway responsiveness in rodents differ appreciably from those found in humans and guinea pigs 3 . From a technical point of view, present disadvantages of the guinea pig as an experimental animal may be the unavailability of genetically modified animals and the relative scarcity of immuno- logical tools for this species, although the latter circumstance is rapidly improving. Various studies, including our own, indicate that sensitized guinea pigs develop early and late asthmatic reactions, airway hyperresponsiveness and airway inflammation upon a single expo- sure to an aerosolized allergen solution; it has also been shown that airway remodeling occurs after repeated inhalation challenges with aerosolized allergen solutions 6–12 . Techniques of airway function measurement In order to study the complex relationship between these aspects within the same animal, it is necessary to perform airway func- tion measurements continuously and repeatedly over prolonged periods of time. Most of the available methods for the assessment of airway function in guinea pigs are not very well suited for this purpose. Airway function can be measured in anesthetized, tra- cheotomized, mechanically ventilated animals by changes in venti- lation circuit pressure or air overflow as the lungs are inflated 13 . Because these measurements require surgical intervention and anesthesia, these methods are not suited to long-term or repeat 840 | VOL.1 NO.2 | 2006 | NATURE PROTOCOLS PROTOCOL © 2006 Nature Publishing Group http://www.nature.com/natureprotocols