, 1998). It became clear that the numbers of GPCRs outnumbered the number of known neuromodulators selleck inhibitor (later the completion of the human genome revealed the full extent of the GPCR diversity). The conclusion of this recognition was (and still is) that many orphan GPCRs must be activated by undiscovered neuromodulators, since inactive GPCRs would have been evolutionarily discarded. The corollary of this conclusion was that orphan receptors may be used as baits to identify novel neuromodulators (Civelli et al., 2001). The approach used to isolate novel neuromodulators consists of expressing an orphan GPCR in heterologous cells and test these against brain tissue extracts. Receptor
reactivity is monitored by quantifying
changes in second messenger levels. The extract displaying reactivity is fractionated and its purification is pursued to homogeneity. The first orphan GPCR used in this approach was ORL-1, an orphan GPCR sequentially related to the opioid receptors (Henderson and McKnight, 1997). Its activation was monitored by monitoring intracellular decreases in cAMP levels. Its neuromodulator was extracted from brain tissues PD0332991 purchase and shown to be a neuropeptide, named orphanin FQ or nociceptin (OFQ/N) (Meunier et al., 1995; Reinscheid et al., 1995). This neuropeptide shares sequence similarities to the opioid peptides but also precise differences that render it inactive at opioid receptors (Reinscheid et al., 1998). This strategy has since been used to discover the following neuropeptides (Figure 5): the two orexins (Oxs) (Sakurai et al., 1998), also identified STK38 through an RNA subtraction approach as hypocretins (Hcrts) (de Lecea et al., 1998); prolactin-releasing peptide (PrRP) (Hinuma et al., 1998); apelin (Tatemoto et al., 1998); ghrelin (Kojima et al., 1999); kisspeptin/metastin (Ohtaki
et al., 2001); the two prokineticins (Lin et al., 2002; Masuda et al., 2002); neuropeptide B and neuropeptide W (NPB/W) (Brezillon et al., 2003; Fujii et al., 2002; Shimomura et al., 2002; Tanaka et al., 2003); neuropeptide S (NPS) (Sato et al., 2002; Xu et al., 2004); neuromedin S (Mori et al., 2005); and finally relaxin-3 (Liu et al., 2003). Each of these neuropeptides has been the subject of intense research, which has helped better understand a number of physiological processes. I will not cover all the advances that they have brought to neuroscience here, but in the next section, I review, as examples, three of the brain-directed responses for which our understanding has been drastically impacted by orphan GPCR research. Studies on one orphan GPCR system, the orexin/hypocretin (Oxs/Hcrts) system, has had a great impact on our understanding sleep/wakefulness states. Soon after the discovery of the Oxs/Hcrts it was shown that mice devoid of Oxs/Hcrts exhibit a pronounced narcoleptic behavior (Chemelli et al., 1999).