Vagal nerve stimulation in whole-animals. A 2 ms electric stimulation was delivered to the vagal nerve − at increasing frequency but fixed voltage (2 V) or increasing voltage at a fixed frequency (1.5 Hz) − in the absence (A and C) or presence of 0.3 mg/Kg physostigmine (B and D). In absence of physostigmine chagasic rats (open circles) the heart rate decreased proportionally to stimulus frequency (panel A) or intensity (panel C). Physostigmine increased the vagal response in both groups; however, when higher stimulation frequencies were applied(i.e., ≥ 3 Hz), the vagal response of chagasic animals became similar as compared with healthy rats. p < 0.05 indicated statistical significant differences between groups (*) or to the basal rate (·).