At the periphery of the olfactory system, the binding of odorants on olfactory receptors (ORs) is usually thought to be the first level of the perception of smell. However, at this stage, there is evidence that other molecular mechanisms also interfere with this chemoreception by ORs. These perireceptor events are mainly supported by two groups of proteins present in the olfactory nasal mucus or in the nasal epithelium. Odorantbinding proteins (OBPs), the first group of proteins have been investigated for many years. OBPs are small carrier proteins capable of binding odorants with affinities in the micromolar range. Although there is no absolute evidence to support their functional roles in vertebrates, OBPs are good candidates for the transport of inhaled odorants towards the ORs via the nasal mucus. The second group of proteins involves xenobiotic metabolizing enzymes, which are strongly expressed in the olfactory epithelium and supposed to be involved in odorant transformation, degradation, and/or olfactory signal termination. Following an overview of these proteins, this review explores their roles, which are still a matter of debate. Anat Rec, 296:1333Rec, 296: -1345

Caffeine (1, 3, 7-trimethylxanthine), an alkaloid produced by plants, has antioxidant and insecticide properties that can affect metabolism and cognition. In vertebrates, the metabolites derived from caffeine have been identified, and their functions have been characterized. However, the metabolites of caffeine in insects remain unknown. Thus, using radiolabelled caffeine, we have identified some of the primary caffeine metabolites produced in the body of Drosophila melanogaster males, including theobromine, paraxanthine and theophylline. In contrast to mammals, theobromine was the predominant metabolite (paraxanthine in humans; theophylline in monkeys; 1, 3, 7-trimethyluric acid in rodents). A transcriptomic screen of Drosophila flies exposed to caffeine revealed the coordinated variation of a large set of genes that encode xenobiotic-metabolizing proteins, including several cytochromes P450s (CYPs) that were highly overexpressed. Flies treated with metyrapone—an inhibitor of CYP enzymes—showed dramatically decreased caffeine metabolism, indicating that CYPs are involved in this process. Using interference RNA genetic silencing, we measured the metabolic and transcriptomic effect of three candidate CYPs. Silencing of CYP6d5 completely abolished theobromine synthesis, whereas CYP6a8 and CYP12d1 silencing induced different consequences on metabolism and gene expression. Therefore, we characterized several metabolic products and some enzymes potentially involved in the degradation of caffeine. In conclusion, this pioneer approach to caffeine metabolism in insects opens novel perspectives for the investigation of the physiological effects of caffeine metabolites. It also indicates that caffeine could be used as a biomarker to evaluate CYP phenotypes in Drosophila and other insects.