Changes in expiratory flow at 50% expired tidal volume (EF50), relaxation time (RT), and apneic pause (ApP). Panels A, C, and E: Actual EF50, RT, and ApP values during the various stages of the experiment, respectively. Panels B, D, and F: Sum of EF50, RT, and ApP responses recorded during the infusion of vehicle (VEH; 20 μL/min, IV) or L-CYSee (INF; 14.3 μmol/kg/min, IV), upon injection of morphine (MOR, 10 mg/kg, IV) and following injection of VEH (100 μL/100 g body weight, IV) or L-CYSee (250 μmol/kg, IV), respectively. Data are presented as mean ± SEM. There were 5 rats in the vehicle-infusion group and 6 rats in the L-CYSee-infusion group.*P < 0.05, significant change from Pre values. †P < 0.05, L-CYSee versus VEH.