A pivotal role for enhanced brainstem Orexin receptor 1 signaling in the central cannabinoid receptor 1-mediated pressor response in conscious rats.

Orexin receptor 1 (OX1R) signaling is implicated in cannabinoid receptor 1 (CB1R) modulation of feeding. Further, our studies established the dependence of the central CB1R-mediated pressor response on neuronal nitric oxide synthase (nNOS) and extracellular signal-regulated kinase1/2 (ERK1/2) phosphorylation in the RVLM. Here, we tested the novel hypothesis that brainstem orexin-A/OX1R signaling plays a pivotal role in the central CB1R-mediated pressor response. Our multiple labeling immunofluorescence findings revealed co-localization of CB1R, OX1R and the peptide orexin-A within the C1 area of the rostral ventrolateral medulla (RVLM). Activation of central CB1R following intracisternal (i.c.) WIN55,212-2 (15μg/rat) in conscious rats caused significant increases in BP and orexin-A level in RVLM neuronal tissue. Additional studies established a causal role for orexin-A in the central CB1R-mediated pressor response because (i) selective blockade of central CB1R (AM251, 30μg/rat; i.c.) abrogated WIN55,212-2-evoked increases in RVLM orexin-A level, (ii) the selective OX1R antagonist SB-408124 (10nmol/rat; i.c.) attenuated orexin-A (3nmol/rat; i.c.) or WIN55,212-2 (15μg/rat; i.c.)-evoked pressor response while selective CB1R blockade (AM251) had no effect on orexin-A (3nmol/rat; i.c.)-evoked pressor response, (iii) direct CB1R activation in the RVLM (WIN55,212-2; 0.1μg/rat) increased RVLM orexin-A and BP. Finally, SB-408124 attenuated WIN55,212-2-evoked increases in RVLM nNOS and ERK1/2 phosphorylation and BP. Our findings suggest that orexin-A/OX1R dependent activation of the RVLM nNOS/ERK1/2 cascade is essential neurochemical mechanism for the central CB1R-mediated pressor response in conscious rats.