Cardiovascular pressor effects of orexins in the dorsomedial hypothalamus
Orexins are important regulators of cardiovascular functions in various physiological and pathological conditions. The dorsomedial hypothalamus (DMH), an essential mediator of cardiovascular responses to stress, contains dense orexinergic innervations and receptors. We examined whether orexins can regulate cardiovascular functions through their actions in the DMH in anesthetized rats. An intra-DMH injection of orexin A (30 pmol) produced elevation of arterial pressure and heart rate. Orexin A-sensitive sites were located within or immediately adjacent to the DMH and larger responses were induced at the compact part of the dorsomedial hypothalamic nucleus. Orexin A-induced responses were attenuated by intra-DMH pretreatment with an orexin receptor 1 (OX1R) antagonist, SB-334867 (15 nmol) (17.7 ± 2.8 vs. 5.2 ± 1.0 mmHg; 54.6 ± 10.0 vs. 22.8 ± 7.4 beats/min). Intra-DMH applied [Ala11,D-Leu15]-orexin B (300 pmol), an orexin receptor 2 (OX2R) agonist, elicited cardiovascular responses mimicking the responses of orexin A, except for a smaller pressor response (7.4 ± 1.7 vs. 16.4 ± 1.8 mmHg). In a series of experiment, effects of orexin B (100 pmol) and then orexin A (30 pmol), were examined at a same site. Two patterns of responses were observed in 12 intra-DMH sites: (1) both orexin A and B (9 sites), and (2) only orexin A (3 sites) induced cardiovascular responses, respectively suggesting OX1 R/OX2R-mediated and OX1R-predominant mechanisms. In conclusion, orexins regulated cardiovascular functions through OX1R/OX2R- or OX1R-mediated mechanisms at different locations in the DMH.