Orexin microinjection in the medullary raphe increases heart rate and arterial pressure but does not reduce tail skin blood flow in the awake rat.

The rostral medullary raphe region is an important target of hypothalamic orexin neurons; however, little is known of the effect of orexin in this key autonomic and somatic premotor region. Here we tested the effect of orexin-A (3 and 30 pmol) microinjected in the medullary raphe, on heart rate (HR), mean arterial pressure (MAP), tail skin blood flow, body temperature, and behavior in freely moving, awake rats. HR, MAP, and body activity were recorded by radio-telemetry. Changes in tail skin blood flow and body temperature, as well as potential interscapular brown adipose tissue thermogenesis were recorded indirectly by infrared thermography of the skin of the tail, lumbosacral back, and interscapular back areas, respectively. Compared with saline, orexin-A (30 pmol) evoked significant and long lasting increases in HR (+99 bpm), MAP (+11 mmHg), and body activity (grooming, not locomotor activity). However, it did not reduce tail skin blood flow more than saline, and there was no significant increase in body temperature. A small, though significant, thermogenic effect was observed in the interscapular region, but this effect is more likely to have originated from activity in neck and shoulder muscles than brown adipose tissue. Thus, orexin projections to the rostral medullary raphe can mediate significant cardiovascular changes, but does not seem to affect tail skin vasomotor tone or brown adipose tissue in the awake rat. This important brainstem relay may contribute to the cardiovascular changes evoked by arousal and various forms of stress that are associated with activation of orexin neurons.