Moderately selective excitatory receptor for orexin-A and, with a lower affinity, for orexin-B neuropeptide. Triggers an increase in cytoplasmic Ca(2+) levels in response to orexin-A binding. Highly expressed in the brain in the prefrontal cortex, hippocampus, paraventricular thalamus, ventromedial hypothalamus, arcuate nucleus, dorsal raphe nucleus, and locus coeruleus. Not detected in the spleen, lung, liver, skeletal muscle, kidney and testis. Orexin receptor mRNA expression has also been reported in the adrenal gland, enteric nervous system, and pancreas. By nutritional state, up-regulated by fasting. The N-terminal region is required for orexin signaling. Belongs to the G-protein coupled receptor 1 family.