Gene expression and protein distribution of the orexin-1 receptor in the rat brain and spinal cord

Orexins-A and -B are neuropeptides derived from a single precursor prepro-o rexin. The mature peptides are mainly expressed in the lateral hypothalamic and perifornical areas. The orexins have been implicated in the control of arousal and appear to be important messengers in the regulation of food in take. Two receptors for orexins have been characterised so far: orexin-1 an d -2 receptors. To gain a further understanding of the biology of orexins, we studied the distribution of the orexin-1 receptor messenger RNA and prot ein in the rat nervous system. We first assessed the expression profile of the orexin-1 receptor gene (ox-r1) in different regions by using quantitati ve reverse transcription followed by polymerase chain reaction. Using immun ohistochemical techniques, we investigated the distribution of orexin-1 rec eptor protein in the rat brain using a rabbit affinity-purifred polyclonal antiserum raised against an N-terminal peptide. The orexin-1 receptor was w idely and strongly expressed in the brain. Thus, immunosignals were observe d in the cerebral cortex, basal ganglia, hippocampal formation, and various other subcortical nuclei in the hypothalamus, thalamus, midbrain and retic ular formation. In particular, robust immunosignals were present in many hy pothalamic and thalamic nuclei, as well as in the locus coeruleus. The dist ribution of the receptor protein was generally in agreement with the distri bution of the receptor messenger RNA in the brain as reported previously by others and confirmed in the present study. In addition, we present in situ hybridisation and immunohistochemical data showing the presence of orexin- 1 receptor messenger RNA and protein in the spinal cord and the dorsal root ganglia. Finally, due to the shared anatomical and functional similarities between orexins and melanin-concentrating hormone, we present a comparison between the neuroanatomical distribution of the orexin-1 receptor and mela nin-concentrating hormone receptor protein-like immunoreactivities in the r at central nervous system, and discuss some functional implications. In conclusion, our neuroanatomical data are consistent with the biological effects of orexins on food intake and regulation of arousal. In addition, t he data suggest other physiological roles for orexins mediated through the orexin-l receptor. (C) 2001 IBRO. Published by Elsevier Science Ltd. All ri ghts reserved.