Both Ox1r and Ox2r orexin receptors contribute to the cardiovascular and locomotor components of the novelty stress response in the rat.

Orexin contributes to the expression of the cardiovascular and behavioural response of some forms of stress, including novelty stress. Thus, Almorexant, a dual receptor antagonist that blocks the two known orexin receptors, Ox1R and Ox2R, reduces these responses. However, it is not known if the reduction results from blockade of one receptor only or both. To answer this question, the selective Ox1R antagonist ACT335827 and the selective Ox2R antagonist EMPA were injected intragastrically (300 mg/kg) or intraperitoneally (30 and 100 mg/kg) either alone or as a cocktail and compared to Almorexant in rats exposed to novelty stress. Cardiovascular and locomotor responses were recorded by radio-telemetry. Triple immunolabelling was also conducted to establish the distribution of Ox1R and Ox2R in sympathetic preganglionic neurons and orexin neurons. Intraperitoneal injections of ACT335827 (100 mg/kg) reduced the pressor and tachycardic but not the locomotor response of novelty (by 32% and 48%, respectively). Intraperitoneal injections of EMPA (100 mg/kg) only reduced the pressor response (42%). However when given together, ACT335827 and EMPA reduced all 3 components (65%, 60% and 57% of the tachycardic, pressor and locomotor responses, respectively) as Almorexant (100 mg/kg) did (69%, 87% and 72%, respectively). Triple immunolabelling revealed that sympathetic preganglionic neurons were mainly Ox1R positive only while orexin neurons were both Ox1R and Ox2R positive. This study shows that orexin's contribution to the cardiovascular and locomotor components of the novelty stress response is not mediated by one receptor alone, but by both receptors and at different levels of the neuraxis.