Airway hyper- or hyporeactivity to inhaled spasmogens 24 h after ovalbumin challenge of sensitized guinea-pigs

Asthma: The Use of Animal Models and Their Translational Utility

Asthma is characterized by chronic lower airway inflammation that results in airway remodeling, which can lead to a permanent decrease in lung function. The pathophysiology driving the development of asthma is complex and heterogenous. Animal models have been and continue to be essential for the discovery of molecular pathways driving the pathophysiology of asthma and novel therapeutic approaches. Animal models of asthma may be induced or naturally occurring. Species used to study asthma include mouse, rat, guinea pig, cat, dog, sheep, horse, and nonhuman primate. Some of the aspects to consider when evaluating any of these asthma models are cost, labor, reagent availability, regulatory burden, relevance to natural disease in humans, type of lower airway inflammation, biological samples available for testing, and ultimately whether the model can answer the research question(s). This review aims to discuss the animal models most available for asthma investigation, with an emphasis on describing the inciting antigen/allergen, inflammatory response induced, and its translation to human asthma.

Inhibitory effects of inhaled complex traditional Chinese medicine on early and late asthmatic responses induced by ovalbumin in sensitized guinea pigs

Background

Many formulae of traditional Chinese medicines (TCMs) have been used for antiasthma treatment dating back many centuries. There is evidence to suggest that TCMs are effective as a cure for this allergenic disease administered via gastric tubes in animal studies; however, their efficacy, safety and side effects as an asthmatic therapy are still unclear.

Methods

In this study, guinea pigs sensitized with ovalbumin (OVA) were used as an animal model for asthma challenge, and the sensitization of animals by bronchial reactivity to methacholine (Mch) and the IgE concentration in the serum after OVA challenge were estimated. Complex traditional Chinese herbs (CTCM) were administered to the animals by nebulization, and the leukocytes were evaluated from bronchoalveolar lavage fluid (BALF).

Results

The results showed that inhalation of CTCM could abolish the increased lung resistance (13-fold increase) induced by challenge with OVA in the early asthmatic response (EAR), reducing to as low as baseline (1-fold). Moreover, our results indicated higher IgE levels (range, 78-83 ng/ml) in the serum of sensitized guinea pigs than in the unsensitized controls (0.9 ± 0.256 ng/ml). In addition, increased total leukocytes and higher levels of eosinophils and neutrophils were seen 6 hours after challenge, and the increased inflammatory cells were reduced by treatment with CTCM inhalation. The interleukin-5 (IL-5) level in BALF was also reduced by CTCM.

Conclusion

Our findings indicate a novel method of administering traditional Chinese medicines for asthma treatment in an animal model that may be more effective than traditional methods.

Role of 5-HT2A, 5-HT4 and 5-HT7 receptors in the antigen-induced airway hyperresponsiveness in guinea-pigs

BACKGROUND

A possible role of 5-hydroxytryptamine (5-HT) in the origin of antigen-induced airway hyperresponsiveness (AI-AHR) has been scarcely investigated.

OBJECTIVE

To explore the participation of different 5-HT receptors in the development of AI-AHR in guinea-pigs.

METHODS

Lung resistance was measured in anaesthetized guinea-pigs sensitized to ovalbumin (OVA). Dose-response curves to intravenous (i.v.) acetylcholine (ACh) were performed before and 1 h after antigenic challenge and expressed as the 200% provocative dose (PD(200)). Organ bath experiments, confocal microscopy and RT-PCR were additionally used. The 5-HT content in lung homogenates was measured by HPLC.

RESULTS

Antigenic challenge significantly decreased PD(200), indicating the development of AI-AHR. This hyperresponsiveness was abolished by a combination of methiothepin (5-HT(1)/5-HT(2)/5-HT(5)/5-HT(6)/5-HT(7) receptors antagonist) and tropisetron (5-HT(3)/5-HT(4) antagonist). Other 5-HT receptor antagonists showed three different patterns of response. Firstly, WAY100135 (5-HT(1A) antagonist) and ondansetron (5-HT(3) antagonist) did not modify the AI-AHR. Secondly, SB269970 (5-HT(7) antagonist), GR113808 (5-HT(4) antagonist), tropisetron or methiothepin abolished the AI-AHR. Thirdly, ketanserin (5-HT(2A) antagonist) produced airway hyporresponsiveness. Animals with bilateral vagotomy did not develop AI-AHR. Experiments in tracheal rings showed that pre-incubation with LP44 or cisapride (agonists of 5-HT(7) and 5-HT(4) receptors, respectively) induced a significant increase of the cholinergic contractile response to the electrical field stimulation. In sensitized lung parenchyma strips, ketanserin diminished the contractile responses to ACh. Sensitization was associated with a ninefold increase in the 5-HT content of lung homogenates. Confocal microscopy showed that sensitization enhanced the immunolabelling and co-localization of nicotinic receptor and 5-HT in airway epithelium, probably located in pulmonary neuroendocrine cells (PNECs). RT-PCR demonstrated that neither sensitization nor antigen challenge modified the 5-HT(2A) receptor mRNA levels.

CONCLUSIONS

Our results suggested that 5-HT was involved in the development of AI-AHR to ACh in guinea-pigs. Specifically, 5-HT(2A), 5-HT(4) and 5-HT(7) receptors seem to be particularly involved in this phenomenon. Participation of 5-HT might probably be favoured by the enhancement of the PNECs 5-HT content observed after sensitization.

Ozone differentially modulates airway responsiveness in atopic versus nonatopic guinea pigs

While acute exposures to ozone (O(3)) can alter airway responsiveness, effects from long-term exposures at low concentrations are less clear. This study assessed whether such exposures could induce nonspecific hyperresponsiveness in nonatopic (nonsensitized) guinea pigs and/or could exacerbate the pre-existing hyperresponsive state in atopic (sensitized) animals, and whether gender was a factor modulating any effect of O(3). Responsiveness was measured during and following exposures to 0.1 and 0.3 ppm O(3) for 4 h/day, 4 days/wk for 24 wk in male and female nonsensitized animals, those sensitized to allergen (ovalbumin) prior to initiation of O(3) exposures, and those sensitized concurrently with exposures. Ozone did not produce hyperresponsiveness in nonsensitized animals, but did exacerbate hyperresponsiveness to both specific and nonspecific bronchoprovocation challenges in sensitized animals, an effect that persisted through at least 4 wk after exposures ended. Gender was not a factor modulating response to O(3). Induced effects on responsiveness were not associated with numbers of eosinophils in the lungs nor with any chronic pulmonary inflammatory response, but were correlated with antigen-specific antibodies in blood. This study supports a role for chronic O(3) exposure in the exacerbation of airways dysfunction in a certain segment of the general population, namely, those demonstrating atopy.

Animal models of asthma: potential usefulness for studying health effects of inhaled particles

Asthma is now recognized to be a chronic inflammatory disease that affects the whole lung. Incidence appears to be increasing despite improved treatment regimens. There is substantial epidemiological evidence suggesting a relationship between the incidence and severity of asthma (e.g., hospitalizations) and exposure to increased levels of air pollution, especially fine and ultrafine particulate material, in susceptible individuals. There have been a few studies in animal models that support this concept, but additional animal studies to test this hypothesis are needed. However, such studies must be performed with awareness of the strengths and weaknesses of the currently available animal models. For studies in mice, the most commonly used animal, a broad spectrum of molecular and immunological tools is available, particularly to study the balance between Th1 and Th2 responses, and inbred strains may be useful for genetic dissection of susceptibility to the disease. However, the mouse is a poor model for bronchoconstriction or localized immune responses that characterize the human disease. In contrast, allergic lung diseases in dogs and cats may more accurately model the human condition, but fewer tools are available for characterization of the mechanisms. Finally, economic issues as well as reagent availability limit the utility of horses, sheep, and primates.

Effect of vagotomy on airway hyperreactivity to endogenously released neurotransmitters at 18-24 h after inhaled antigen

Airway reactivity was examined in anaesthetized guinea-pigs 18-24 h after inhalation challenge of ovalbumin-sensitized animals with ovalbumin. Bronchoconstrictor responses were measured from the increases in pulmonary inflation pressure. The study was undertaken to examine whether ovalbumin challenge induced airway hyperreactivity to neurotransmitters released endogenously by vagal nerve stimulation. Stimulation parameters were selected to cause release of either acetylcholine (0.3 ms pulse width for 3 s, 20 V, 2-40 Hz), both acetylcholine and neuropeptide (5 ms pulse width for 15 s, 20 V, 0.5-8 Hz) or neuropeptide only, using the latter parameters in the presence of atropine. The vagi were paired for stimulation and in some experiments were cut central to the stimulation point. Frequency-response curves for acetylcholine- and neuropeptide-mediated bronchoconstrictor responses to vagal stimulation when the nerves were intact revealed no airway hyperreactivity after ovalbumin challenge. The presence of atropine failed to reveal airway hyperreactivity. However, when the vagi were cut, the frequency-response curves were displaced to the left after ovalbumin challenge compared with saline challenged animals, indicating airway hyperreactivity. This airway hyperreactivity was significant after atropine and suggests an increase in sensitivity to endogenously released neuropeptides rather than acetylcholine. It also indicates that the airway hyperreactivity is dependent on removal of the afferent vagal pathways. Frequency-response curves for cholinergic stimulation (0.3 ms) with intact vagi revealed no airway hyperreactivity after antigen challenge. Comparisons of exogenously administered 5-hydroxytryptamine (5-HT, 300 ng/100 g i.v.) and a single vagal stimulation of 0.3 ms pulse width (cholinergic) revealed no airway hyperreactivity to either stimulus after ovalbumin challenge. However if the vagi were cut, airway hyperreactivity was observed, again suggesting that removal of afferent pathways is important for revealing airway hyperreactivity in the anaesthetized guinea-pig. Ovalbumin challenge caused significant increases in the bronchoconstrictor responses to a single dose of capsaicin (50 microg/100 g i.v.) or dose-response curves to bradykinin. Since these agents release neuropeptides from sensory C-fibres, this is further support for a raised sensitivity to endogenously released neuropeptides.

PDE4 inhibition and a corticosteroid in chronically antigen exposed conscious guinea-pigs

BACKGROUND

The physiological and pharmacological consequences of repeated aero-allergen challenge have not been previously characterized in conscious, sensitized guinea-pigs.

OBJECTIVES

This study was undertaken to compare the effects of two anti-inflammatory compounds, dexamethasone and Ro 20- 1724, on an acute and chronic airway inflammation, in terms of airway function, reactivity and leucocyte infiltration.

METHODS

Sensitized guinea-pigs received eight saline or ovalbumin (OvA) inhalation exposures over 4 weeks and either vehicle, the type 4 PDE inhibitor, Ro 20-1724 (3 mgkg(-1)), or dexamethasone (1.5 mg/kg(-1)), 30 min before and 6 h after each challenge. Airway function of the conscious animal (sGaw) was monitored over the duration of the first and final OvA challenge. Airway reactivity to the thromboxane mimetic, U46619, was also determined following the final OvA exposure as was the leucocyte infiltration.

RESULTS

The first antigen challenge induced a large early (0-3h) and smaller late (17-24h) bronchoconstrictor response. Neither phase was affected by the drug treatments. The final OvA challenge induced early and late phase bronchoconstrictor responses but of similar magnitude. The late phase was also significantly prolonged. Ro 20-1724 and dexamethasone significantly attenuated both phases. Airway reactivity to the inhaled thromboxane mimetic, U46619, was also significantly enhanced at 120h after the final OvA exposure in contrast to the saline challenged group. This hyperreactivity was attenuated by Ro 20-1724 and dexamethasone. Bronchoalveolar lavage after repeated OvA exposures revealed eosinophilia which was attenuated by Ro 20-1724 and dexamethasone.

CONCLUSIONS

This model demonstrates differential airway responses to acute and chronic antigen challenge. Repeated administration of dexamethasone and Ro 20-1724 with each OvA exposure attenuated all of the chronic inflammatory responses: early and late phase responses, hyperreactivity and eosinophilia.

Effects of inhibitors of phosphodiesterase, on antigen-induced bronchial hyperreactivity in conscious sensitized guinea-pigs and airway leukocyte infiltration

1. The aim of this study was to determine the effects of inhibitors of phosphodiesterase (PDE) on the early and late phase bronchoconstriction in sensitized, conscious guinea-pigs and the subsequent development of acute airway hyperreactivity to the inhaled thromboxane mimetic, U46619, and leukocyte infiltration following ovalbumin (OvA) challenge. 2. Following an inhalation challenge with OvA, there was an early bronchoconstriction which peaked at 15 min with recovery after 3-4 h. A late phase bronchoconstriction occurred between 17 and 24 h after challenge. The PDE 4 inhibitors, Ro 20-1724 (3 mg kg-1, i.p.) and rolipram (1 mg kg-1, i.p.) administered 30 min before and 6 h after antigen challenge (double dosing regimen), did not affect the development of the early or late phase responses. 3. Seventeen to twenty four hours following an acute OvA or saline challenge, a consistently greater bronchoconstrictor response to inhaled U46619 was observed in the OvA challenged group. This increase in responsiveness was significantly attenuated by the administration of Ro 20-1724 and rolipram 30 min before and 6 h after antigen challenge (P < 0.05); this was not attributable to a residual bronchodilator effect of these compounds. There was a trend towards inhibition of the hyperreactivity to U46619 by aminophylline but not by the PDE3 inhibitors, siguazodan or SKF 95654. 4. Aminophylline, rolipram and Ro 20-1724 when administered as the double dose regimen attenuated the rise in macrophages, eosinophils and neutrophils recovered in bronchial lavage fluid 17 to 24 h after antigen challenge. 5. The dose of Ro 20-1724 given at 6 h post challenge was essential for attenuation of airway hyperreactivity and to protect against leukocyte influx. 6. In summary, aminophylline, rolipram and Ro 20-1724 have anti-inflammatory effects against antigen-induced airway leukocyte infiltration. Rolipram and Ro 20-1724 additionally attenuated the development of acute airway hyperreactivity, effects which are probably mediated through inhibition of PDE type 4. A dose of PDE inhibitor 6 h after the antigen challenge appears to be essential to achieve this protection. Inhibitors of PDE type 3 were generally without effect. However, there was no effect of rolipram or Ro 20-1724 on the development of either the early or late phase type responses.

Influence of spasmogen inhalation by guinea pigs upon subsequent demonstration of ovalbumin-induced hyperreactivity in isolated airways tissues

Guinea pigs were sensitized with ovalbumin (i.p.) 14 days before use. In vivo airway hyperreactivity induced by ovalbumin inhalation was determined by challenging with aerosolized spasmogen (5-hydroxytryptamine [5-HT] methacholine, the thromboxane-mimetic, U-46619, or adenosine) 24 hr before (7 days with adenosine) and again 18-24 hr after the ovalbumin inhalation. One hour later they were killed and isolated airways perfused lung halves and tracheal spirals were set up for determination of tissue sensitivity to carbachol, histamine, and adenosine. This study examines whether the spasmogen interferes with the ovalbumin-induced in vitro hyperreactivity and the combined effects of ovalbumin followed by spasmogen challenge upon tissue sensitivity. The influence of the spasmogen upon the in vitro measurement of ovalbumin-induced hyperreactivity was variable, depending upon which spasmogen was used and whether the lung or trachea was examined. The inhalation of the spasmogen in ovalbumin-challenged guinea pigs had clear effects upon the subsequent measurement of tissue sensitivity. This depended upon the spasmogen used, but 5-HT, methacholine, and U-46619 usually depressed responsiveness, while adenosine was without significant effect. As a consequence, the appearances of in vitro hyperreactivity due to the ovalbumin challenge could be masked (e.g., bolus doses of agonist in the trachea when 5-HT was the spasmogen) or the degree of hyperreactivity could be enhanced (e.g., in the perfused lung when adenosine was the spasmogen). Thus, isolated airways tissues should not be used for evaluating tissue sensitivity when the animals have been previously exposed to inhalations of spasmogens.