We have previously shown that some features of the defense response, such as increases in arterial blood pressure (AP), heart rate (HR), and ventilation were attenuated in prepro-orexin knockout (ORX-KO) mice. Here, we examined whether the same was true in orexin neuron-ablated [orexin/ataxin-3 transgenic mice (ORX/ATX-Tg)] mice. In addition, we examined other features of the defense response: skeletal muscular vasodilation and shift of baroreceptor reflex. In both anesthetized and conscious conditions, basal AP in ORX/ATX-Tg mice was significantly lower by approximately 20 mmHg than in wild-type (WT) controls, as was the case in ORX-KO mice. The difference in AP disappeared after treatment with an alpha-blocker but not with a beta-blocker, indicating lower sympathetic vasoconstrictor outflow. Stimulation of the perifornical area (PFA) in urethane-anesthetized ORX/ATX-Tg mice elicited smaller and shorter-lasting increases in AP, HR, and ventilation, and skeletal muscle vasodilation than in WT controls. In addition, air jet stress-induced elevations of AP and HR were attenuated in conscious ORX/ATX-Tg mice. After pretreatment with a beta-blocker, atenolol, stimulation of PFA suppressed phenylephrine (50 microg/kg iv)-induced bradycardia (DeltaHR=-360+/-29 beats/min without PFA stimulation vs. -166+/-26 during stimulation) in WT. This demonstrated the resetting of the baroreflex. In ORX/ATX-Tg mice, however, no significant suppression was observed (-355+/-16 without stimulation vs. -300+/-30 during stimulation). The present study provided further support for our hypothesis that orexin-containing neurons in PFA play a role as a master switch to activate multiple efferent pathways of the defense response and also operate as a regulator of basal AP.