The effect of neonatal capsaicin on the development of bronchial hyperresponsiveness in allergic rabbits

We have investigated the effects of neonatal capsaicin treatment of rabbits on the development of bronchial hyperresponsiveness following allergen exposure from birth. In vivo airways responsiveness was assessed as the concentrations of histamine to cause a 35% decrease in compliance (PC35) and a 50% increase in resistance (PC50). Rabbits treated with vehicle capsaicin (10% ethanol, 10% tween 80 and 80% saline) and then immunised with Altenaria tenius (40,000 protein nitrogen units (PNU)/ml + AL(OH)3 + saline in a ratio 2:1:1) were more responsive to aerosol histamine in vivo at three months of age when compared to sham-immunised (AL(OH)3 + saline 1:3) or saline-treated rabbits for compliance measurements (P < 0.05). However, immunised rabbits although not significantly different to sham-immunised (P > 0.05) were significantly more responsive than saline-treated rabbits for resistance measurements (P < 0.05). Neonatal capsaicin treatment significantly attenuated the increased responsiveness seen in immunised rabbits for both compliance and resistance measurements (P < 0.05). The bronchial hyperresponsiveness is unlikely to be due to cellular infiltration per se as cell numbers assessed by bronchoalveolar lavage were not significantly different between groups (P > 0.05). Exogenous in vitro functional bronchial responses to capsaicin were increased in vehicle-immunised rabbits, an effect attenuated by neonatal capsaicin treatment (P < 0.05) whereas responses to methacholine, histamine or electrical field stimulation remained similar between groups (P > 0.05). Bronchial levels of calcitonin gene-related peptide- and substance P-like immunoreactivity were unaffected by any treatment (P > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)