sb-334867-a and almorexant

This study tested the hypothesis that orexin plays a role in the elevated pressor response to acute stress in the spontaneously hypertensive rat (SHR). The pressor response to air jet stress (AJS) (n=11/group) was 2.5 times greater in vehicle treated SHR versus Wistar (WIS) rats. Systemic delivery of 30mg/kg of the dual orexin receptor antagonist almorexant did not significantly change resting mean arterial pressure (MAP) but did attenuate the pressor response elicited by AJS to a greater extent in the SHR compared to the Wistar rats (~65% versus ~33% reduction respectively; n=6/group). Alternatively 100mg/kg almorexant reduced resting MAP in the SHR (~25mm Hg drop) and attenuated both the heart rate (HR; ~50% reduction) and MAP (~62% reduction) response to AJS in both strains (n=6/group). Systemic application of SB-334867 (3mg/kg), an orexin receptor type 1 antagonist (n=5/group), selectively reduced resting MAP and attenuated the HR response to AJS in the SHR but had no effect on the pressor response in either strain. The potential role of endogenous orexin release in cardiovascular control in the SHR was linked to a significant increase in brain-derived neurotrophic factor mRNA expression in the hypothalamus and elevated orexin receptor expression (type 2 only) in the dorsal pons when compared to WIS (n=4/group). These results demonstrate that the exaggerated pressor response in the SHR to stress is linked to increased orexin receptor activation and possibly altered orexin receptor expression in the dorsal pons and BDNF expression in the hypothalamus.

Topics: Acetamides; Air; Animals; Benzoxazoles; Blood Pressure; Brain; Brain-Derived Neurotrophic Factor; Cardiovascular Agents; Dose-Response Relationship, Drug; Heart Rate; Isoquinolines; Male; Naphthyridines; Orexin Receptor Antagonists; Orexin Receptors; Physical Stimulation; Random Allocation; Rats, Inbred SHR; Rats, Wistar; RNA, Messenger; Species Specificity; Stress, Psychological; Urea

The hypocretin (hcrt) system has been implicated in addiction-relevant effects of several drugs, but its role in nicotine dependence has been little studied.. These experiments examined the role of the hcrt system in nicotine reinforcement.. Rats were trained for nicotine self-administration (NSA) on fixed-ratio schedules. The effects of acute, presession treatments with the hcrtR1 antagonist SB334867 and the hcrtR1/2 antagonist almorexant were examined on NSA maintained on a fixed-ratio (FR) 5 schedule. Gene expression for the hcrt system (mRNA for hcrt, hcrtR1, and hcrtR2) was measured in animals following NSA on a FR 1 schedule for a 19-day period.. The hcrtR1 antagonist SB334867 and the hcrtR1/2 antagonist almorexant both reduced NSA dose-dependently (significantly at doses of 30 and 300 mg/kg, respectively); SB334867 did not affect food-maintained responding whereas almorexant (at the 300 mg/kg) did. Tissue from animals collected 5 h after self-administration showed an increase in hcrtR1 mRNA in the arcuate nucleus compared to control subjects. In tissue collected immediately after a similar 19-day self-administration period, mRNA for hcrtR1 was decreased in the rostral lateral hypothalamus compared to controls.. These data confirm a previous report (Hollander et al., Proc Natl Acad Sci U S A 105:19480-19485, 2008) that the hypocretin receptor hcrtR1 is activated in nicotine reinforcement and in addition show that both the arcuate nucleus and lateral hypothalamus are sites at which hcrt receptor mechanisms may influence reinforcement. Different patterns of mRNA expression at different times after NSA suggest that changes in the hcrt system may be labile with time.

Topics: Acetamides; Animals; Arcuate Nucleus of Hypothalamus; Behavior, Animal; Benzoxazoles; Dose-Response Relationship, Drug; Hypothalamic Area, Lateral; Hypothalamus; Infusions, Intravenous; Intracellular Signaling Peptides and Proteins; Isoquinolines; Male; Naphthyridines; Neuropeptides; Nicotine; Nicotinic Agonists; Orexin Receptors; Orexins; Rats; Rats, Long-Evans; Receptors, G-Protein-Coupled; Receptors, Neuropeptide; Reinforcement, Psychology; RNA, Messenger; Self Administration; Urea

Recent preclinical and clinical research has shown that almorexant promotes sleep in animals and humans without disrupting the sleep architecture. Here, the pharmacology and kinetics of [(3)H]almorexant binding to human orexin 1 receptor (OX(1))- and human orexin 2 receptor (OX(2))-human embryonic kidney 293 membranes were characterized and compared with those of selective OX(1) and OX(2) antagonists, including 1-(5-(2-fluoro-phenyl)-2-methyl-thiazol-4-yl)-1-((S)-2-(5-phenyl-(1,3,4)oxadiazol-2-ylmethyl)-pyrrolidin-1-yl)-methanone (SB-674042), 1-(6,8-difluoro-2-methyl-quinolin-4-yl)-3-(4-dimethylamino-phenyl)-urea (SB-408124), and N-ethyl-2-[(6-methoxy-pyridin-3-yl)-(toluene-2-sulfonyl)-amino]-N-pyridin-3-ylmethyl-acetamide (EMPA). The effect of these antagonists was also examined in vitro on the spontaneous activity of rat ventral tegmental area (VTA) dopaminergic neurons. [(3)H]Almorexant bound to a single saturable site on hOX(1) and hOX(2) with high affinity (K(d) of 1.3 and 0.17 nM, respectively). In Schild analyses using the [(3)H]inositol phosphates assay, almorexant acted as a competitive antagonist at hOX(1) and as a noncompetitive-like antagonist at hOX(2). In binding kinetic analyses, [(3)H]almorexant had fast association and dissociation rates at hOX(1), whereas it had a fast association rate and a remarkably slow dissociation rate at hOX(2). In the VTA, orexin-A potentiated the basal firing frequency to 175 +/- 17% of control in approximately half of the neurons tested. In the presence of 1 microM SB-674042 or SB-408124, the effect of orexin-A was only partially antagonized. However, in the presence of 1 microM EMPA or 1 microM almorexant, the effect of orexin-A was completely antagonized. In conclusion, almorexant exhibited a noncompetitive and long-lasting pseudo-irreversible mode of antagonism as a result of its very slow rate of dissociation from OX(2). The electrophysiology data suggest that OX(2) might be more important than OX(1) in mediating the effect of orexin-A on slow-firing of VTA dopaminergic neurons.

Topics: Acetamides; Animals; Evoked Potentials; Humans; Isoquinolines; Kinetics; Molecular Structure; Neurons; Orexin Receptors; Phenylurea Compounds; Pyrrolidines; Rats; Receptors, G-Protein-Coupled; Receptors, Neuropeptide; Sleep; Thiazoles