METHODS	Twelve patients with PD received biperiden hydrochloride ( a muscarinic antagonist that crosses the blood-brain barrier ) , pirenzepine hydrochloride ( a muscarinic antagonist that does not cross the blood-brain barrier ) , or placebo 2 hours before a 35 % carbon dioxide-65 % oxygen respiratory challenge ( vs air as a placebo ) on 3 separate days , in a double-blind , random crossover design .\