Predator-prey relationships provide a classic paradigm for the study of innate animal behavior. Odors from carnivores elicit stereotyped fear and avoidance responses in rodents, although sensory mechanisms involved are largely unknown. Here, we identified a chemical produced by predators that activates a mouse olfactory receptor and produces an innate behavioral response. We purified this predator cue from bobcat urine and identified it to be a biogenic amine, 2-phenylethylamine. Quantitative HPLC analysis across 38 mammalian species indicates enriched 2-phenylethylamine production by numerous carnivores, with some producing >3,000-fold more than herbivores examined. Calcium imaging of neuronal responses in mouse olfactory tissue slices identified dispersed carnivore odor-selective sensory neurons that also responded to 2-phenylethylamine. Two prey species, rat and mouse, avoid a 2-phenylethylamine odor source, and loss-of-function studies involving enzymatic depletion of 2-phenylethylamine from a carnivore odor indicate it to be required for full avoidance behavior. Thus, rodent olfactory sensory neurons and chemosensory receptors have the capacity for recognizing interspecies odors. One such cue, carnivore-derived 2-phenylethylamine, is a key component of a predator odor blend that triggers hard-wired aversion circuits in the rodent brain. These data show how a single, volatile chemical detected in the environment can drive an elaborate danger-associated behavioral response in mammals.kairomone | olfaction | pheromone | trace amine-associated receptors | G protein-coupled receptor P redator-prey relationships provide a classic paradigm for understanding the molecular basis of complex behavior (1). Predator-derived visual, auditory, and olfactory cues induce hardwired defensive responses in prey that are sculpted by strong evolutionary pressure and are critical for survival. For example, odors from felines, canines, and other predators elicit innate reactions in rodents, including stereotyped avoidance behaviors and stimulation of the hypothalamic-pituitary-adrenal axis that coordinates sympathetic stress responses (1). Aversive reactions to odors can function in reverse as well, as skunk thiols facilitate prey escape by repelling predator species (2).Predator odors contain a class of ecological chemosignals termed kairomones, cues transmitted between species that benefit the detecting organism. Predator odor-derived kairomones that elicit defensive responses in rodents are largely unknown and can be found in fur, dander, saliva, urine, or feces of divergent predator species. One volatile chemical produced by foxes, 2,5-dihydro-2,4,5-trimethylthiazole (TMT), and two nonvolatile lipocalins produced by cats and rats elicit fear-like or aversive behavior in mice, enabling remote or contact-based detection of predator cues (3-5). Each of these chemicals is not broadly produced by predators, raising the possibility that rodents detect a multitude of species-specific predator signals, each of which triggers a hardwir...

Carrion smell is strongly repugnant to humans and triggers distinct innate behaviors in many other species. This smell is mainly carried by two small aliphatic diamines, putrescine and cadaverine, which are generated by bacterial decarboxylation of the basic amino acids ornithine and lysine. Depending on the species, these diamines may also serve as feeding attractants, oviposition attractants, or social cues. Behavioral responses to diamines have not been investigated in zebrafish, a powerful model system for studying vertebrate olfaction. Furthermore, olfactory receptors that detect cadaverine and putrescine have not been identified in any species so far. Here, we show robust olfactory-mediated avoidance behavior of zebrafish to cadaverine and related diamines, and concomitant activation of sparse olfactory sensory neurons by these diamines. The large majority of neurons activated by low concentrations of cadaverine expresses a particular olfactory receptor, trace amineassociated receptor 13c (TAAR13c). Structure-activity analysis indicates TAAR13c to be a general diamine sensor, with pronounced selectivity for odd chains of medium length. This receptor can also be activated by decaying fish extracts, a physiologically relevant source of diamines. The identification of a sensitive zebrafish olfactory receptor for these diamines provides a molecular basis for studying neural circuits connecting sensation, perception, and innate behavior.Danio rerio | aversion | heterologous expression | polyamines C adaverine, putrescine, and other biogenic diamines are strongly repulsive odors to humans, for whom these odors presumably signal bacterial contamination. It may be expected that animal species feeding on carcasses attribute a more positive valence to diamines, and indeed both putrescine and cadaverine have been reported to be feeding attractants for rats (1) as well as goldfish (2). Similarly, insects depositing their eggs in carcasses or other proteineacous materials are attracted by these diamines (3). Beyond signaling danger or food, putrescine and cadaverine also serve as social cues in several vertebrate species, both for marking of territories-for example, in feline species (4)-and for burial of conspecifics (5).Very little is known about the molecular and cellular basis of cadaverine-driven behaviors. Cadaverine and putrescine evoke electrophysiological responses in the olfactory epithelium of two fish species (2, 6) and cadaverine-responsive olfactory sensory neurons and glomeruli have been identified in the mouse (7,8). However, chemosensory receptors that detect cadaverine or related diamines are unknown in any species, and could provide valuable tools to study how the olfactory system mediates innate aversion or attraction.Here, we show that cadaverine is a major product of zebrafish tissue decay, activates a zebrafish olfactory receptor (trace amineassociated receptor 13c, TAAR13c) with high affinity, and elicits a strong, low-threshold, and olfactory-mediated avoidance response in zebrafish. In vivo me...

SUMMARY Background Rodents use olfactory cues for species-specific behaviors. For example, mice emit odors to attract mates of the same species but not competitors of closely related species. This implies rapid evolution of olfactory signaling, although odors and chemosensory receptors involved are unknown. Results Here, we identify a mouse chemosignal, trimethylamine, and its olfactory receptor, trace amine-associated receptor 5 (TAAR5), to be involved in species-specific social communication. Abundant (>1,000-fold increased) and sex-dependent trimethylamine production arose de novo along the Mus lineage after divergence from Mus caroli. The two-step trimethylamine biosynthesis pathway involves synergy between commensal microflora and a sex-dependent liver enzyme, flavin-containing monooxygenase 3 (FMO3), which oxidizes trimethylamine. One key evolutionary alteration in this pathway is the recent acquisition in Mus of male-specific Fmo3 gene repression. Coincident with its evolving biosynthesis, trimethylamine evokes species-specific behaviors, attracting mice but repelling rats. Attraction to trimethylamine is abolished in TAAR5 knockout mice, and furthermore, attraction to mouse scent is impaired by enzymatic depletion of trimethylamine or TAAR5 knockout. Conclusions TAAR5 is an evolutionarily conserved olfactory receptor required for a species-specific behavior. Synchronized changes in odor biosynthesis pathways and odor-evoked behaviors could ensure species-appropriate social interactions.

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BackgroundPreterm birth is the most common single cause of perinatal and infant mortality, affecting 15 million infants worldwide each year with global rates increasing. Understanding of risk factors remains poor, and preventive interventions have only limited benefit. Large differences exist in preterm birth rates across high income countries. We hypothesized that understanding the basis for these wide variations could lead to interventions that reduce preterm birth incidence in countries with high rates. We thus sought to assess the contributions of known risk factors for both spontaneous and provider-initiated preterm birth in selected high income countries, estimating also the potential impact of successful interventions due to advances in research, policy and public health, or clinical practice.MethodsWe analyzed individual patient-level data on 4.1 million singleton pregnancies from four countries with very high human development index (Czech Republic, New Zealand, Slovenia, Sweden) and one comparator U.S. state (California) to determine the specific contribution (adjusting for confounding effects) of 21 factors. Both individual and population-attributable preterm birth risks were determined, as were contributors to cross-country differences. We also assessed the ability to predict preterm birth given various sets of known risk factors.FindingsPrevious preterm birth and preeclampsia were the strongest individual risk factors of preterm birth in all datasets, with odds ratios of 4.6–6.0 and 2.8–5.7, respectively, for individual women having those characteristics. In contrast, on a population basis, nulliparity and male sex were the two risk factors with the highest impact on preterm birth rates, accounting for 25–50% and 11–16% of excess population attributable risk, respectively (p<0.001). The importance of nulliparity and male sex on population attributable risk was driven by high prevalence despite low odds ratios for individual women. More than 65% of the total aggregated risk of preterm birth within each country lacks a plausible biologic explanation, and 63% of difference between countries cannot be explained with known factors; thus, research is necessary to elucidate the underlying mechanisms of preterm birth and, hence, therapeutic intervention. Surprisingly, variation in prevalence of known risk factors accounted for less than 35% of the difference in preterm birth rates between countries. Known risk factors had an area under the curve of less than 0.7 in ROC analysis of preterm birth prediction within countries. These data suggest that other influences, as yet unidentified, are involved in preterm birth. Further research into biological mechanisms is warranted.ConclusionsWe have quantified the causes of variation in preterm birth rates among countries with very high human development index. The paucity of explicit and currently identified factors amenable to intervention illustrates the limited impact of changes possible through current clinical practice and policy interventions. Our research high...

Trace amine-associated receptors (TAARs) are vertebrate olfactory receptors. However, ligand recognition properties of TAARs remain poorly understood, as most are ‘orphan receptors’ without known agonists. Here, we identify the first ligands for many rodent TAARs, and classify these receptors into two subfamilies based on phylogeny and binding preference for primary or tertiary amines. Some mouse and rat orthologs have similar response profiles, although independent Taar7 gene expansions led to highly related receptors with altered ligand specificities. Using chimeric TAAR7 receptors, we identified an odor contact site in transmembrane helix III that functions as a selectivity filter. Homology models based on the β2 adrenergic receptor structure indicate close proximity of this site to the ligand. Gain-of-function mutations at this site created olfactory receptors with radically altered odor recognition properties. These studies provide new TAAR ligands, valuable tools to study receptor function, and general insights into the molecular pharmacology of GPCRs.

Mating drive is balanced by a need to safeguard resources for offspring, yet the neural basis for negative regulation of mating remains poorly understood. In rodents, pheromones critically regulate sexual behavior. Here, we observe suppression of adult female sexual behavior in mice by exocrine gland-secreting peptide 22 (ESP22), a lacrimal protein from juvenile mice. ESP22 activates a dedicated vomeronasal receptor, V2Rp4, and V2Rp4 knockout eliminates ESP22 effects on sexual behavior. Genetic tracing of ESP22-responsive neural circuits reveals a critical limbic system connection that inhibits reproductive behavior. Furthermore, V2Rp4 counteracts a highly related vomeronasal receptor, V2Rp5, that detects the male sex pheromone ESP1. Interestingly, V2Rp4 and V2Rp5 are encoded by adjacent genes, yet couple to distinct circuits and mediate opposing effects on female sexual behavior. Collectively, our study reveals molecular and neural mechanisms underlying pheromone-mediated sexual rejection, and more generally, how inputs are routed through olfactory circuits to evoke specific behaviors.

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