Genomic organization of mouse orexin receptors: Characterization of two novel tissue-specific splice variants

In humans and rat, orexins orchestrate divergent actions through their G protein-coupled receptors, orexin-1 (OX1R) and orexin-2 (OX2R). Orexins also play an important physiological role in mouse, but the receptors through which they function are not characterized. To characterize the physiological role(s) of orexins in the mouse, we cloned and characterized the mouse orexin receptor(s), mOX1R and mOX2R, using rapid amplification of cDNA (mouse brain) ends, RT-PCR, and gene structure analysis. The mOX1R cDNA encodes a 416-amino acid (aa) receptor. We have identified two alternative C terminus splice variants of the mOX2R; mOX2alphaR (443 aa) and mOX2betaR (460 aa). Binding studies in human embryonic kidney 293 cells transfected with mOX1R, mOX2alphaR, and the mOX2betaR revealed specific, saturable sites for both orexin-A and -B. Activation of these receptors by orexins induced inositol triphosphate (IP3) turnover. However, human embryonic kidney 293 cells transfected with mOXRs demonstrated no cAMP response to either orexin- A or orexin- B challenge, although forskolin and GTPgammaS revealed a dose-dependent increase in cAMP. Although, orexin- A and -B showed no difference in binding characteristics between the splice variants; interestingly, orexin- B led to an increase in IP3 production at all concentrations in the mOX2alphaR variant. Orexin-A, however, showed no difference in IP3 production between the two variants. Additionally, in the mouse, we demonstrate that these splice variants are distributed in a tissue-specific manner, where OX2alphaR mRNA was undetectable in skeletal muscle and kidney. Moreover, food deprivation led to a greater increase in hypothalamic mOX2betaR gene expression, compared with both mOX1R and mOX2alphaR. This potentially implicates a fundamental physiological role for these splice variants.