To address Specific Aim 1, a prospective, parallel-group, repeated-measures design will be used to compare behavioral, physiological, and biochemical (plasma ORX) responses in 2 independent, unmedicated groups of PD outpatients (n=6 in each group) at baseline/resting state and after panic provocation due to brief (1 minute) inhalation of a 35% CO2 / 65% O2 gas mixture. PD patients will be randomized, in a double-blind manner, to receive either a single, oral dose of the mixed ORX1/2 receptor antagonist, suvorexant (10 mg dose), or identical placebo, 120 minutes before CO2 challenge.