Introduction
Animal olfactory receptors (ORs) make up a G proteincoupled chemosensory receptor (GPCR) family with 7 transmembrane alpha-helices located in the cytoplasmic membrane of cells. They are known to work by binding heterotrimeric guanine nucleotide-binding proteins (G proteins), composed of the olfactory Gα (Gαolf) subunit and the Gβγ dimer. GPCRs are classified by their sequence homology [1] or based on their phylogenetic origin [2], and there are more than 16 types of G protein α subunits [3, 4]. ORs are also known to recognize a wide spectrum of organic compounds in accordance with its cognate ligand. When it was originally identified from rat olfactory epithelium [5], the expression of the protein was thought to be confined to that tissue. After Gαolf [6] and rat ORs [5], OR expression in mammalian germ cells [7, 8] and heart [9] was reported in the early 1990s. In the late 20th century, OR expression in an insulin-secreting cell line and the spleen of rats [10] and human erythroid cells [11] was reported. ORs are largely distributed in olfactory sensory neurons of the nasal epithelium but are also expressed in other non-olfactory tissues [12].
We discuss the physiological functions of ORs and suggest future perspectives for their research.