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 (30pmol) 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 (15nmol) (17.7 ± 2.8 vs. 5.2 ± 1.0mmHg; 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 ± 1.8mmHg). In a series of experiment, effects of orexin B (100pmol) and then orexin A (30pmol), 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 OX1R/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.

Expression of the M3 Muscarinic Receptor on Orexin Neurons that Project to the Rostral Ventrolateral Medulla.

Activation of central cholinergic receptors causes a pressor response in rats, and the hypothalamus is important for this response. Projections from hypothalamic orexin neurons to the rostral ventrolateral medulla (RVLM) are involved in sympatho-excitation of the cardiovascular system. A small population of orexin neurons is regulated by cholinergic inputs through M3 muscarinic acetylcholine receptor (M3 R). To elucidate whether the M3 R on orexin neurons is involved in cardiosympathetic regulation through the RVLM, we examined the presence of the M3 R on retrograde-labeled RVLM-projecting orexin neurons. The retrograde tracer was unilaterally injected into the RVLM. Within the hypothalamus, retrograde-labeled neurons were located predominantly ipsilateral to the injection side. In the anterior hypothalamus (-1.5 to -2.3 mm to the bregma), retrograde-labeled neurons were densely distributed in the paraventricular nuclei and scattered in the retrochiasmatic area. At -2.3 to -3.5 mm from the bregma, labeled neurons were located in the regions where orexin neurons were situated, that is, the tuberal lateral hypothalamic area, perifornical area, and dorsomedial nuclei. Very few retrograde-labeled neurons were observed in the hypothalamus at -3.5 to -4.5 mm from the bregma. About 19.5% ± 1.6% of RVLM-projecting neurons in the tuberal hypothalamus were orexinergic. The M3 R was present on 18.7% ± 3.0% of RVLM-projecting orexin neurons. Injection of a muscarinic agonist, oxotremorine, in the perifornical area resulted in a pressor response, which was attenuated by a pretreatment of atropine. We conclude that cholinergic inputs to orexin neurons may be involved in cardiosympathetic regulation through the M3 R on the orexin neurons that directly project to the RVLM.