e human genome that are known or anticipated to respond to endogenously generated regulators of homeostatic function. The advent of `reverse pharmacology’, designed to either pair `orphan’ GPCRs with endogenous modulators or to identify surrogate, low MW chemical ligands useful to interrogate the function of these receptors, promised to expand significantly the proportion of GPCRs that could be considered as validated therapeutic targets. At least in part, this effort has begun to deliver. This has included work on receptors that are activated by free fatty acids, including the two GPCRs that provide the focus of the current review. Four GPCRs, free fatty acid receptors 14 are currently defined as receptors for free fatty acids while a further receptor, GPR84, although clearly activated by medium-chain fatty acids, officially remains an PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19819037 `orphan’. FFA2 and FFA3 receptors are activated by short-chain fatty acids that are produced in high concentrations by bacterial fermentation of dietary fibre, whereas FFA1 and FFA4 receptors, although displaying only limited relatedness to each other, are both activated by medium- and long-chain saturated and unsaturated fatty acids derived from dietary triglycerides. FFA1 receptor agonists: from identification to clinical studies The `orphan’ receptor GPR40 was ini- tially shown to be expressed selectively by beta cells of rat islets. In parallel with these studies, ligand fishing experiments using FFA1 receptors demonstrated this receptor to be activated by a broad range of both medium- and longer-chain saturated and unsaturated fatty acids. Interestingly, within this group of ligands, only modest variation in potency was MedChemExpress AVE8062A observed, and therefore, in an in vivo context, it might be anticipated that FFA1 receptor-mediated effects of fatty acids at the level of the pancreas would largely reflect their relative circulating concentrations. There is a substantial literature on the health benefits of various fatty acids, including -3 fatty acids derived from fish oils and other sources. However, the relatively high overall concentration of circulating fatty acids might, therefore, be anticipated to limit the effectiveness of fatty acids provided as dietary supplements, unless key effects are produced largely within the gut, for example, or at targets other than the GPCRs that are activated by the broader group of fatty acids. FFA1 receptors are also expressed by a range of gut enteroendocrine cells that generate, store and release hormones such as glucagon-like peptide-1 and cholecystokinin. Initial de-orphanization studies also demonstrated the high-level expression of FFA1 receptors in a broad range of regions of the human brain. Expression of this receptor in rodent brain and its potential function in the CNS has subsequently been a matter of debate. Recently, however, a number of studies have used combinations of in situ hybridization and receptor-selective pairs of agonist and antagonist to provide substantial support for regional expression and function, although the exact role of FFA1 receptors here remains uncertain. Equally, FFA1 receptors are expressed in osteoclastic cells and regulation of apoptosis of such cells and inhibition of osteoclast differBritish Journal of Pharmacology 172 32543265 3255 BJP G Milligan et al. entiation by fatty acids and synthetic FFA1 receptor agonists has hinted at other applications of FFA1 receptor ligands, although these ideas have yet been explored in any detail. B