Effects of GABA-B agonist baclofen on bronchial hyperreactivity to inhaled histamine in subjects with cervical spinal cord injury

Bronchial provocation studies performed in our research center have consistently demonstrated airway hyperresponsiveness to both inhaled methacholine and histamine in subjects with chronic cervical spinal cord injury (SCI). More recently, we reported that the airways of such subjects maintained on chronic baclofen (γ-aminobutyric acid) therapy were not hyperreactive to inhaled methacholine. In this study we determined whether baclofen also blocks the effects of the bronchoprovocative agent histamine in subjects with cervical SCI. Twenty-four male subjects with cervical SCI participated in this study; 14 were maintained on oral baclofen, and 10 served as age-matched controls. The subjects were challenged with increasing concentrations of aerosolized histamine until either a 20% fall in forced expiratory volume in 1 s (FEV₁) from baseline (defined as PC₂₀) was observed, or a maximum of 25 mg/ml histamine was administered. We found that 11 of the 14 baclofen subjects (78.5%) and 8 of the 10 control subjects (80%) responded (PC₂₀ < 8 mg/ml) to the histamine challenge. Mean PC₂₀ values among responders in the baclofen (PC₂₀ = 2.91 ± 2.3) and control (PC₂₀ = 2.18 ± 1.9) groups did not differ significantly. Because histamine acts directly on histamine receptors and indirectly on cholinergic pathways, our findings that baclofen blocks bronchoconstriction clue to inhaled methacholine, but not that due to histamine, suggests that hyperresponsiveness in subjects with cervical SCI may be secondary to nonspecific airway hyperreactivity.