Current Biology Vol 25 No 4 R144
End of session
Beginning of session 90 o
180 o
90 o
0o
180 o
0o
Eyes closed 270 o
270 o
90 o
90 o
180 o
0o
180 o
0o
Eyes open 270 o
270 o Current Biology
Figure 1. Development of the head direction cell system. The results of Bjerknes et al. [2] and Tan et al. [3] show that rat pups have head direction cells before the eyes are opened. Though directional, these cells appear to show within-recording session drift in firing directions. Once the eyes are opened, these cells are much more stable, and are anchored to visual landmarks in the environment.
does come online, the stability of the cells is much improved, and some head direction cells — particularly those in the anterodorsal thalamus — appear to develop directional firing. Finally, the capacity for head direction cells to form an associational anchor to visual landmarks appears within a day of access to these landmarks. Thus, a blank slate or compass is present in the mammalian brain prior to experience with the visual world. With vision, this directional slate is rapidly honed and an associational process allows this internal representation to
become linked to the contents of the outside world. References 1. Locke, J. (1690). An Essay Concerning Human Understanding (Adelaide: eBooks@Adelaide). 2. Bjerknes, T.L., Langston, R.F., Kruge, I.U., Moser, E.I., and Moser, M.-B. (2015). Coherence among head direction cells before eye opening in rat pups. Curr. Biol. 25, 103–108. 3. Tan, H.M., Bassett, J.P., O’Keefe, J., Cacucci, F., and Wills, T.J. (2015). The development of the head direction system before eye-opening in the rat. Curr. Biol. 25, 479–483. 4. Taube, J.S., Muller, R.U., and Ranck, J.B. (1990). Head-direction cells recorded from the postsubiculum in freely moving rats. I. Description and quantitative analysis. J. Neurosci. 10, 420–435.
Olfaction: Smells Like Fly Food Fruit flies love foods containing yeast. A new study now shows that they are attracted to and have dedicated olfactory neurons for detecting the scents produced by yeast metabolizing common phenolic compounds in fruit. Geraldine A. Wright A fruit is a tasty, nutritious parcel shaped by natural selection to entice animals to disperse seeds. It is a
reward provided by plants containing substances animals need. In fact, fruits have evolved to be nutritious to distract animals from eating seeds and instead move seeds to new habitats away from
5. Skaggs, W.E., Knierim, J.J., Kudrimoti, H.S., and McNaughton, B.L. (1995). A model of the neural basis of the rat’s sense of direction. Adv. Neural Inf. Process. Syst. 7, 173–180. 6. Redish, A.D., Elga, A.N., and Touretzky, D.S. (1996). A coupled attractor model of the rodent head direction system. Network. Comput. Neural Syst. 7, 671–685. 7. Zhang, K. (1996). Representation of spatial orientation by the intrinsic dynamics of the head-direction ensemble: a theory. J. Neurosci. 16, 2112–2126. 8. Langston, R.F., Ainge, J.A., Couey, J.J., Canto, C.B., Bjerknes, T.L., Witter, M.P., Moser, E.I., and Moser, M.-B. (2010). Development of the spatial representation system in the rat. Science 238, 1576–1580. 9. Wills, T.J., Cacucci, F., Burgess, N., and O’Keefe, J. (2010). Development of the hippocampal cognitive map in preweanling rats. Science 328, 1573–1576. 10. Bush, D., Barry, C., and Burgess, N. (2014). What do grid cells contribute to place cell firing? Trends Neurosci. 37, 136–145. 11. Taube, J.S. (2007). The head direction signal: origins and sensory-motor integration. Annu. Rev. Neurosci. 30, 181–207. 12. Boccara, C.N., Sargolini, F., Thoresen, V.H., Solstad, T., Witter, M.P., Moser, E.I., and Moser, M.-B. (2010). Grid cells in the pre- and parasubiculum. Nat. Neurosci. 13, 987–994. 13. Dudchenko, P.A., and Taube, J.S. (1997). Correlation between head direction cell activity and spatial behavior on a radial arm maze. Behav. Neurosci. 111, 3–19. 14. van der Meer, M., Richmond, Z., Braga, R.M., Wood, E.R., and Dudchenko, P.A. (2010). Evidence for the use of an internal sense of direction in homing. Behav. Neurosci. 124, 164–169. 15. Vann, S.D. (2010). Re-evaluating the role of the mammillary bodies in memory. Neuropsychologia 48, 2316–2327. 16. Vann, S.D., and Aggleton, J.P. (2004). The mammillary bodies: two memory systems in one? Nat. Rev. Neurosci. 5, 35–44. 17. Yoganarasimha, D., Yu, X., and Knierim, J.J. (2006). Head direction cell representations maintain internal coherence during conflicting proximal and distal cue rotations: comparison with hippocampal place cells. J. Neurosci. 26, 622–631. 18. Stackman, R.W., and Taube, J.S. (1997). Firing properties of head direction cells in the rat anterior thalamic nucleus: dependence on vestibular input. J. Neurosci. 17, 4349–4358.
University of Stirling, Psychology, School of Natural Sciences, Stirling FK9 4LA, UK. E-mail: [email protected]
http://dx.doi.org/10.1016/j.cub.2014.12.054
the parent plant. Fruits are mainly carbohydrates and water, but they also can be a source of fats, proteins, and micronutrients including vitamins, minerals and salts. Their colours and flavours arise mainly from ‘non-nutrient’ chemicals called phenolic compounds [1]. So far, over 8,000 phenolic compounds have been identified in fruit, vegetables, grains and other plant tissues [1,2]. They function as a form of UV protection
Dispatch R145
and as a defence against fungi, microorganisms, and herbivores [1,3]. These compounds, however, also convey potential health benefits to animals; they are antioxidants, scavenging damaging free radicals and protecting DNA, proteins and lipids from oxidative damage [1,3]. Unlike us primates, the fruit or vinegar fly Drosophila melanogaster is not a seed disperser. Instead, it takes advantage of fruits when they are not eaten by frugivores. When fruits get old or are damaged, they fall from the parent plant and begin to rot due to the action of yeasts and other microorganisms. The presence of some of these microbes, especially yeast, alters the chemical composition of the fruit in a way that makes it a perfect substrate for fruit fly larval development and maturation. So, while the fly may have no direct effect on dispersing a plant’s seeds, it can derive the same benefits. A new Current Biology paper by Dweck, Stensmyr and colleagues [4] is the first to test whether fruit flies are attracted to the phenolic content of the fruits they colonize. Rotting fruit emit a peculiar mixture of odour compounds that are produced by the metabolic activities of the microorganisms that colonize it (Figure 1). The compounds found in the headspace of rotting fruit attract fruit flies and give them information about the state of microbial colonization [5,6]. Several studies have now established that volatile compounds produced by fruit and yeasts play an important role as signals for the flies, attracting adults to congregate, mate and lay eggs on fruit [5,7–9]. In their new study, Dweck and colleagues [4] have found that fruit flies are highly attracted to food media containing hydroxycinnamic acids (HCAs): adult flies were more likely to run towards, feed and lay eggs on yeast-inoculated media containing the common HCAs p-coumaric acid and ferrulic acid. Fly larvae, too, were attracted to media containing these HCAs. The authors found that flies were neither repelled nor attracted to HCAs in food in the absence of yeast, suggesting that flies cannot detect these compounds by taste. Instead, the authors found convincing evidence that flies are attracted to the volatile signals produced by the metabolism of HCAs by yeasts. Yeasts themselves produce many volatile compounds that flies find
attractive [5,9]. When HCAs were added to media, the yeast species Brettanomyces bruxellensis produced two new compounds, the ethylphenols 4-ethylguiacol and 4-ethylphenol. Ethylphenols are volatile organic compounds that distinctly signal the presence of Brettanomyces sp. For example, they are found in the headspace of beer and wine when these yeasts actively convert p-coumaric acid and ferrulic acid from grapes or cereals into volatiles; a strong ethylphenol signal from beer or wine produces a smell we find unpleasant and musky [10]. Adult fruit flies found both of these compounds highly attractive and were more likely to feed and lay eggs on media containing them. Animals sense volatile compounds using olfactory receptor neurons (ORNs) in nasal epithelia (mammals) or in hair-like structures called sensilla (insects). ORNs detect volatile compounds by G protein-coupled receptors or by ligand-gated ion channels that bind volatiles and start a second messenger cascade that leads to neuronal spiking [11]. A curious truth about the olfactory system is that ORNs express only one functional receptor type (a heteromer of two OR gene products) [12]. Some olfactory receptors are narrowly-tuned to detect only a small suite of compounds; these receptors represent specializations in the olfactory system. Such olfactory specialization evolves when odor compounds are reliably associated with an important event that directly affects fitness (e.g. a sex pheromone emitted by a sexually receptive female). Previous studies have identified olfactory receptor types in Drosophila that are tuned to detect volatiles produced by yeasts and other microorganisms. In the present study [4], the authors performed a comprehensive series of experiments to establish that fruit flies use specific olfactory receptors to identify the ethylphenols produced by yeast metabolism of HCAs. Surprisingly, they found that adult flies use a different receptor (OR71a) than larval flies (OR94b) to detect these compounds. However, ethylphenols are not the only compounds produced by the metabolism of HCAs by yeast. To test whether all yeasts metabolize phenolics into the same compounds, Dweck and colleagues [4] examined the headspaces of five other yeast species on media containing HCAs. Unlike
CH3
HO CH3
HO
HO
+ HO
CH3
YUM!
O OH
Current Biology
Figure 1. Fruit flies are attracted to the by-products of HCA metabolism by yeasts colonizing fruit. Yeasts convert p-coumaric acid, a phenolic compound, into the volatile odor molecules, 4-ethylphenol and 4-ethylguiacol. These compounds are highly attractive to adult and larval fruit flies.
Brettanomyces, the other yeast species did not reliably produce 4-ethylguiacol. Instead, they produced other compounds that are precursors to the ethylphenols such as 4-vinylguaicol. These compounds also activated the ORNs housing OR71a in adult flies, but they also elicited responses from additional ORNs. Why should flies care about the presence of HCAs? HCAs are antioxidants — compounds that accept rogue electrons that might otherwise damage DNA, lipids or proteins. For this reason, HCAs also confer benefits to the animals that consume them. Indeed, one study observed that fruit-eating birds prefer fruits based on their anthocyanin (a phenolic compound) content, and hypothesized that this was because of these compounds’ antioxidant properties [13]. Antioxidants can be helpful in diet, and can extend lifespan, especially when animals are subject to oxidative stress. Indeed, to establish that flies also benefit from HCAs in diet, Dweck and colleagues [4] fed adult flies the HCA-laced media containing the redox-reactive compound paraquat and found that they lived longer. Alternatively, the ethylphenol volatile signals present in the headspace could represent a reliable signal of the presence of yeast on rotting fruit, indicating to the flies that good food is present. HCAs are found ubiquitously in plants and are commonly found in fruit [2]. In fact, the highest concentrations reported in fruit occur during early stages of fruit development. As fruits ripen, HCA concentrations decrease steadily [2]. In ripe fruit, HCAs are most concentrated in the epicarp (peel) and
Current Biology Vol 25 No 4 R146
not in the flesh [2], also suggesting that they serve as a form of fruit protection. Thus, when yeasts have first successfully started their colonization of fruit, they will undoubtedly encounter and metabolize the HCAs in the fruit peel, and adult flies that could detect this early colonization might be at an advantage. Fruit flies are not fruigivores, but they may still help plants disperse seeds. If fruit goes uneaten, it could prevent seed germination and even kill the young plant. The steady decline in HCAs as fruit ripen could make it easier for yeasts and other organisms to colonize fruit, exposing the seed and helping the young plant to emerge from the seed coat. Fruit flies may, therefore, unwittingly help plants by participating in the decomposition of uneaten fruit.
2.
3.
4.
5.
6.
7.
References 1. Soto-Vaca, A., Gutierrez, A., Losso, J.N., Xu, Z., and Finley, J.W. (2012). Evolution of phenolic compounds from color and flavor problems to
8.
health benefits. J. Agric. Food Chem. 60, 6658–6677. Hermann, K. (1989). Occurrence and content of hydroxycinnamic and hydroxybenzoic acid compounds in foods. CRC Crit. Rev. Food Sci. Nut. 28, 315–347. Dai, J., and Russell, J.N. (2010). Plant phenolics: Extraction, analysis and their antioxidant and anticancer properties. Molecules 15, 7313–7352. Dweck, H.K.M., Ebrahim, S.A.M., Farhan, A., Hansson, B.S., and Stensmyr, M.C. (2015). Olfactory proxy detection of dietary antioxidants in Drosophila. Curr. Biol. 25, 455–466. Becher, G., Flick, G., Rozpe˛dowska, E., Schmidt, A., Hagman, A., Lebreton, S., Larsson, kur, J., Witzgall, P., M.C., Hansson, B.S., Pis and Bengtsson, M. (2012). Yeast, not fruit volatiles mediate Drosophila melanogaster attraction, oviposition and development. Funct. Ecol. 26, 822–828. Stensmyr, M.C., Dweck, H.K., Farhan, A., Ibba, I., Strutz, A., Mukunda, L., Linz, J., Grabe, V., Steck, K., Lavista-Llanos, S., et al. (2012). A conserved dedicated olfactory circuit for detecting harmful microbes in Drosophila. Cell 151, 1345–1357. Zhu, J.W., Park, K.C., and Baker, T.C. (2003). Identification of odors from overripe mango that attract vinegar flies, Drosophila melanogaster. J. Chem. Ecol. 29, 899–909. Grosjean, Y., Rytz, R., Farine, J.P., Abuin, L., Cortot, J., Jefferis, G.S., and Benton, R. (2011). An olfactory receptor for food-derived odours
Neuromodulation: Letting Sources of Spinal Dopamine Speak for Themselves A recent study of dopaminergic neurons in the brain of larval zebrafish has important implications for interpreting the natural actions of neuromodulators in the spinal cord. Sandeep Kishore and David L. McLean* The surest way to get reliable information is to go to the source. In vertebrates, key sources of aminergic neuromodulators that help produce locomotion are located in the brain. However, much of our understanding of neuromodulation during locomotion has come from studies where the spinal cord is isolated, drugs are bath applied, and changes in locomotor output due to changes in spinal neuron excitability and connectivity are measured. Because of the difficulty of recording from sources of neuromodulators in the brain of intact, locomoting animals, the behavioral relevance of pharmacological manipulations in the spinal cord is still unclear. In this issue of Current Biology, Jay et al. [1] tackle this issue
head on, as it were, using the zebrafish model system. One of the earliest demonstrations that aminergic neuromodulators play a critical role in facilitating vertebrate locomotion was provided by experiments in the 1960s, where systemic application of a precursor to catecholamine synthesis, L-DOPA, rescued walking movements in spinalized cats [2]. This led to a focus on the actions of amines such as serotonin, noradrenaline and dopamine within the spinal cord, which contains the circuitry necessary for executing locomotion [3]. A body of work too large to adequately cover here has subsequently described myriad actions in numerous species [4]. Of particular relevance to the Jay et al. [1] study is the fact that dopamine can exert different effects via receptor subtypes with different affinities and second
9.
10.
11.
12.
13.
promotes male courtship in Drosophila. Nature 478, 236–240. Palanca, L., Gaskett, A.C., Gunther, C.S., Newcomb, R.D., and Goddard, M.R. (2013). Quantifying variation in the ability of yeasts to attract Drosophila melanogaster. PLoS ONE 8, e75332. Vanbeneden, N., Delvaux, F., and Delvaux, F.R. (2006). Determination of hydroxycinnamic acids and volatile phenols in wort and beer by isocratic high-performance liquid chromatography using electrochemical detection. J. Chrom. A 1136, 237–242. Kaupp, U.B. (2010). Olfactory signalling in vertebrates and insects: differences and commonalities. Nat. Rev. Neurosci. 11, 188–200. Vosshall, L.B., Wong, A.M., and Axel, R. (2000). An olfactory sensory map in the fly brain. Cell 102, 147–159. Schaefer, H.M., McGraw, K., and Catoni, C. (2008). Birds use fruit colour as honest signal of dietary antioxidant rewards. Funct. Ecol. 22, 303–310.
Centre for Behaviour and Evolution, Institute of Neuroscience, Newcastle University, Newcastle upon Tyne NE1 7RU, UK. E-mail: [email protected]
http://dx.doi.org/10.1016/j.cub.2014.12.052
messenger pathways [5,6]. In larval zebrafish, for example, dopamine can promote locomotion via activation of lower-affinity D1 or higher-affinity D4 receptors, and inhibit locomotion via higher-affinity D2 or D3 receptors [7,8]. To begin to place these observations in a behavioral context, Jay et al. [1] focused on an evolutionarily conserved group of dopaminergic diencephalospinal neurons (DDNs) in the forebrain, which provide the sole source of spinal dopamine not only in zebrafish [9,10], but also in mammals [11]. The authors took advantage of an enhancer trap transgenic line of zebrafish, Tg(ETvmat2:GFP), in which these neurons are labeled by green fluorescent protein (GFP) [12]. The relatively large size and location of the cells made it possible to monitor not only their activity patterns, but also their excitatory and inhibitory synaptic inputs, using patch-clamp recordings in intact, chemically-immobilized fish capable of generating ‘fictive’ swimming (Figure 1A). By including fluorescent dye in the patch pipette, or by using a post-hoc stain, the authors confirmed that their recordings were from spinal projecting neurons. Unexpectedly, they also found that DDN axons exited the central nervous system and targeted auditory and
End of session
Beginning of session 90 o
180 o
90 o
0o
180 o
0o
Eyes closed 270 o
270 o
90 o
90 o
180 o
0o
180 o
0o
Eyes open 270 o
270 o Current Biology
Figure 1. Development of the head direction cell system. The results of Bjerknes et al. [2] and Tan et al. [3] show that rat pups have head direction cells before the eyes are opened. Though directional, these cells appear to show within-recording session drift in firing directions. Once the eyes are opened, these cells are much more stable, and are anchored to visual landmarks in the environment.
does come online, the stability of the cells is much improved, and some head direction cells — particularly those in the anterodorsal thalamus — appear to develop directional firing. Finally, the capacity for head direction cells to form an associational anchor to visual landmarks appears within a day of access to these landmarks. Thus, a blank slate or compass is present in the mammalian brain prior to experience with the visual world. With vision, this directional slate is rapidly honed and an associational process allows this internal representation to
become linked to the contents of the outside world. References 1. Locke, J. (1690). An Essay Concerning Human Understanding (Adelaide: eBooks@Adelaide). 2. Bjerknes, T.L., Langston, R.F., Kruge, I.U., Moser, E.I., and Moser, M.-B. (2015). Coherence among head direction cells before eye opening in rat pups. Curr. Biol. 25, 103–108. 3. Tan, H.M., Bassett, J.P., O’Keefe, J., Cacucci, F., and Wills, T.J. (2015). The development of the head direction system before eye-opening in the rat. Curr. Biol. 25, 479–483. 4. Taube, J.S., Muller, R.U., and Ranck, J.B. (1990). Head-direction cells recorded from the postsubiculum in freely moving rats. I. Description and quantitative analysis. J. Neurosci. 10, 420–435.
Olfaction: Smells Like Fly Food Fruit flies love foods containing yeast. A new study now shows that they are attracted to and have dedicated olfactory neurons for detecting the scents produced by yeast metabolizing common phenolic compounds in fruit. Geraldine A. Wright A fruit is a tasty, nutritious parcel shaped by natural selection to entice animals to disperse seeds. It is a
reward provided by plants containing substances animals need. In fact, fruits have evolved to be nutritious to distract animals from eating seeds and instead move seeds to new habitats away from
5. Skaggs, W.E., Knierim, J.J., Kudrimoti, H.S., and McNaughton, B.L. (1995). A model of the neural basis of the rat’s sense of direction. Adv. Neural Inf. Process. Syst. 7, 173–180. 6. Redish, A.D., Elga, A.N., and Touretzky, D.S. (1996). A coupled attractor model of the rodent head direction system. Network. Comput. Neural Syst. 7, 671–685. 7. Zhang, K. (1996). Representation of spatial orientation by the intrinsic dynamics of the head-direction ensemble: a theory. J. Neurosci. 16, 2112–2126. 8. Langston, R.F., Ainge, J.A., Couey, J.J., Canto, C.B., Bjerknes, T.L., Witter, M.P., Moser, E.I., and Moser, M.-B. (2010). Development of the spatial representation system in the rat. Science 238, 1576–1580. 9. Wills, T.J., Cacucci, F., Burgess, N., and O’Keefe, J. (2010). Development of the hippocampal cognitive map in preweanling rats. Science 328, 1573–1576. 10. Bush, D., Barry, C., and Burgess, N. (2014). What do grid cells contribute to place cell firing? Trends Neurosci. 37, 136–145. 11. Taube, J.S. (2007). The head direction signal: origins and sensory-motor integration. Annu. Rev. Neurosci. 30, 181–207. 12. Boccara, C.N., Sargolini, F., Thoresen, V.H., Solstad, T., Witter, M.P., Moser, E.I., and Moser, M.-B. (2010). Grid cells in the pre- and parasubiculum. Nat. Neurosci. 13, 987–994. 13. Dudchenko, P.A., and Taube, J.S. (1997). Correlation between head direction cell activity and spatial behavior on a radial arm maze. Behav. Neurosci. 111, 3–19. 14. van der Meer, M., Richmond, Z., Braga, R.M., Wood, E.R., and Dudchenko, P.A. (2010). Evidence for the use of an internal sense of direction in homing. Behav. Neurosci. 124, 164–169. 15. Vann, S.D. (2010). Re-evaluating the role of the mammillary bodies in memory. Neuropsychologia 48, 2316–2327. 16. Vann, S.D., and Aggleton, J.P. (2004). The mammillary bodies: two memory systems in one? Nat. Rev. Neurosci. 5, 35–44. 17. Yoganarasimha, D., Yu, X., and Knierim, J.J. (2006). Head direction cell representations maintain internal coherence during conflicting proximal and distal cue rotations: comparison with hippocampal place cells. J. Neurosci. 26, 622–631. 18. Stackman, R.W., and Taube, J.S. (1997). Firing properties of head direction cells in the rat anterior thalamic nucleus: dependence on vestibular input. J. Neurosci. 17, 4349–4358.
University of Stirling, Psychology, School of Natural Sciences, Stirling FK9 4LA, UK. E-mail: [email protected]
http://dx.doi.org/10.1016/j.cub.2014.12.054
the parent plant. Fruits are mainly carbohydrates and water, but they also can be a source of fats, proteins, and micronutrients including vitamins, minerals and salts. Their colours and flavours arise mainly from ‘non-nutrient’ chemicals called phenolic compounds [1]. So far, over 8,000 phenolic compounds have been identified in fruit, vegetables, grains and other plant tissues [1,2]. They function as a form of UV protection
Dispatch R145
and as a defence against fungi, microorganisms, and herbivores [1,3]. These compounds, however, also convey potential health benefits to animals; they are antioxidants, scavenging damaging free radicals and protecting DNA, proteins and lipids from oxidative damage [1,3]. Unlike us primates, the fruit or vinegar fly Drosophila melanogaster is not a seed disperser. Instead, it takes advantage of fruits when they are not eaten by frugivores. When fruits get old or are damaged, they fall from the parent plant and begin to rot due to the action of yeasts and other microorganisms. The presence of some of these microbes, especially yeast, alters the chemical composition of the fruit in a way that makes it a perfect substrate for fruit fly larval development and maturation. So, while the fly may have no direct effect on dispersing a plant’s seeds, it can derive the same benefits. A new Current Biology paper by Dweck, Stensmyr and colleagues [4] is the first to test whether fruit flies are attracted to the phenolic content of the fruits they colonize. Rotting fruit emit a peculiar mixture of odour compounds that are produced by the metabolic activities of the microorganisms that colonize it (Figure 1). The compounds found in the headspace of rotting fruit attract fruit flies and give them information about the state of microbial colonization [5,6]. Several studies have now established that volatile compounds produced by fruit and yeasts play an important role as signals for the flies, attracting adults to congregate, mate and lay eggs on fruit [5,7–9]. In their new study, Dweck and colleagues [4] have found that fruit flies are highly attracted to food media containing hydroxycinnamic acids (HCAs): adult flies were more likely to run towards, feed and lay eggs on yeast-inoculated media containing the common HCAs p-coumaric acid and ferrulic acid. Fly larvae, too, were attracted to media containing these HCAs. The authors found that flies were neither repelled nor attracted to HCAs in food in the absence of yeast, suggesting that flies cannot detect these compounds by taste. Instead, the authors found convincing evidence that flies are attracted to the volatile signals produced by the metabolism of HCAs by yeasts. Yeasts themselves produce many volatile compounds that flies find
attractive [5,9]. When HCAs were added to media, the yeast species Brettanomyces bruxellensis produced two new compounds, the ethylphenols 4-ethylguiacol and 4-ethylphenol. Ethylphenols are volatile organic compounds that distinctly signal the presence of Brettanomyces sp. For example, they are found in the headspace of beer and wine when these yeasts actively convert p-coumaric acid and ferrulic acid from grapes or cereals into volatiles; a strong ethylphenol signal from beer or wine produces a smell we find unpleasant and musky [10]. Adult fruit flies found both of these compounds highly attractive and were more likely to feed and lay eggs on media containing them. Animals sense volatile compounds using olfactory receptor neurons (ORNs) in nasal epithelia (mammals) or in hair-like structures called sensilla (insects). ORNs detect volatile compounds by G protein-coupled receptors or by ligand-gated ion channels that bind volatiles and start a second messenger cascade that leads to neuronal spiking [11]. A curious truth about the olfactory system is that ORNs express only one functional receptor type (a heteromer of two OR gene products) [12]. Some olfactory receptors are narrowly-tuned to detect only a small suite of compounds; these receptors represent specializations in the olfactory system. Such olfactory specialization evolves when odor compounds are reliably associated with an important event that directly affects fitness (e.g. a sex pheromone emitted by a sexually receptive female). Previous studies have identified olfactory receptor types in Drosophila that are tuned to detect volatiles produced by yeasts and other microorganisms. In the present study [4], the authors performed a comprehensive series of experiments to establish that fruit flies use specific olfactory receptors to identify the ethylphenols produced by yeast metabolism of HCAs. Surprisingly, they found that adult flies use a different receptor (OR71a) than larval flies (OR94b) to detect these compounds. However, ethylphenols are not the only compounds produced by the metabolism of HCAs by yeast. To test whether all yeasts metabolize phenolics into the same compounds, Dweck and colleagues [4] examined the headspaces of five other yeast species on media containing HCAs. Unlike
CH3
HO CH3
HO
HO
+ HO
CH3
YUM!
O OH
Current Biology
Figure 1. Fruit flies are attracted to the by-products of HCA metabolism by yeasts colonizing fruit. Yeasts convert p-coumaric acid, a phenolic compound, into the volatile odor molecules, 4-ethylphenol and 4-ethylguiacol. These compounds are highly attractive to adult and larval fruit flies.
Brettanomyces, the other yeast species did not reliably produce 4-ethylguiacol. Instead, they produced other compounds that are precursors to the ethylphenols such as 4-vinylguaicol. These compounds also activated the ORNs housing OR71a in adult flies, but they also elicited responses from additional ORNs. Why should flies care about the presence of HCAs? HCAs are antioxidants — compounds that accept rogue electrons that might otherwise damage DNA, lipids or proteins. For this reason, HCAs also confer benefits to the animals that consume them. Indeed, one study observed that fruit-eating birds prefer fruits based on their anthocyanin (a phenolic compound) content, and hypothesized that this was because of these compounds’ antioxidant properties [13]. Antioxidants can be helpful in diet, and can extend lifespan, especially when animals are subject to oxidative stress. Indeed, to establish that flies also benefit from HCAs in diet, Dweck and colleagues [4] fed adult flies the HCA-laced media containing the redox-reactive compound paraquat and found that they lived longer. Alternatively, the ethylphenol volatile signals present in the headspace could represent a reliable signal of the presence of yeast on rotting fruit, indicating to the flies that good food is present. HCAs are found ubiquitously in plants and are commonly found in fruit [2]. In fact, the highest concentrations reported in fruit occur during early stages of fruit development. As fruits ripen, HCA concentrations decrease steadily [2]. In ripe fruit, HCAs are most concentrated in the epicarp (peel) and
Current Biology Vol 25 No 4 R146
not in the flesh [2], also suggesting that they serve as a form of fruit protection. Thus, when yeasts have first successfully started their colonization of fruit, they will undoubtedly encounter and metabolize the HCAs in the fruit peel, and adult flies that could detect this early colonization might be at an advantage. Fruit flies are not fruigivores, but they may still help plants disperse seeds. If fruit goes uneaten, it could prevent seed germination and even kill the young plant. The steady decline in HCAs as fruit ripen could make it easier for yeasts and other organisms to colonize fruit, exposing the seed and helping the young plant to emerge from the seed coat. Fruit flies may, therefore, unwittingly help plants by participating in the decomposition of uneaten fruit.
2.
3.
4.
5.
6.
7.
References 1. Soto-Vaca, A., Gutierrez, A., Losso, J.N., Xu, Z., and Finley, J.W. (2012). Evolution of phenolic compounds from color and flavor problems to
8.
health benefits. J. Agric. Food Chem. 60, 6658–6677. Hermann, K. (1989). Occurrence and content of hydroxycinnamic and hydroxybenzoic acid compounds in foods. CRC Crit. Rev. Food Sci. Nut. 28, 315–347. Dai, J., and Russell, J.N. (2010). Plant phenolics: Extraction, analysis and their antioxidant and anticancer properties. Molecules 15, 7313–7352. Dweck, H.K.M., Ebrahim, S.A.M., Farhan, A., Hansson, B.S., and Stensmyr, M.C. (2015). Olfactory proxy detection of dietary antioxidants in Drosophila. Curr. Biol. 25, 455–466. Becher, G., Flick, G., Rozpe˛dowska, E., Schmidt, A., Hagman, A., Lebreton, S., Larsson, kur, J., Witzgall, P., M.C., Hansson, B.S., Pis and Bengtsson, M. (2012). Yeast, not fruit volatiles mediate Drosophila melanogaster attraction, oviposition and development. Funct. Ecol. 26, 822–828. Stensmyr, M.C., Dweck, H.K., Farhan, A., Ibba, I., Strutz, A., Mukunda, L., Linz, J., Grabe, V., Steck, K., Lavista-Llanos, S., et al. (2012). A conserved dedicated olfactory circuit for detecting harmful microbes in Drosophila. Cell 151, 1345–1357. Zhu, J.W., Park, K.C., and Baker, T.C. (2003). Identification of odors from overripe mango that attract vinegar flies, Drosophila melanogaster. J. Chem. Ecol. 29, 899–909. Grosjean, Y., Rytz, R., Farine, J.P., Abuin, L., Cortot, J., Jefferis, G.S., and Benton, R. (2011). An olfactory receptor for food-derived odours
Neuromodulation: Letting Sources of Spinal Dopamine Speak for Themselves A recent study of dopaminergic neurons in the brain of larval zebrafish has important implications for interpreting the natural actions of neuromodulators in the spinal cord. Sandeep Kishore and David L. McLean* The surest way to get reliable information is to go to the source. In vertebrates, key sources of aminergic neuromodulators that help produce locomotion are located in the brain. However, much of our understanding of neuromodulation during locomotion has come from studies where the spinal cord is isolated, drugs are bath applied, and changes in locomotor output due to changes in spinal neuron excitability and connectivity are measured. Because of the difficulty of recording from sources of neuromodulators in the brain of intact, locomoting animals, the behavioral relevance of pharmacological manipulations in the spinal cord is still unclear. In this issue of Current Biology, Jay et al. [1] tackle this issue
head on, as it were, using the zebrafish model system. One of the earliest demonstrations that aminergic neuromodulators play a critical role in facilitating vertebrate locomotion was provided by experiments in the 1960s, where systemic application of a precursor to catecholamine synthesis, L-DOPA, rescued walking movements in spinalized cats [2]. This led to a focus on the actions of amines such as serotonin, noradrenaline and dopamine within the spinal cord, which contains the circuitry necessary for executing locomotion [3]. A body of work too large to adequately cover here has subsequently described myriad actions in numerous species [4]. Of particular relevance to the Jay et al. [1] study is the fact that dopamine can exert different effects via receptor subtypes with different affinities and second
9.
10.
11.
12.
13.
promotes male courtship in Drosophila. Nature 478, 236–240. Palanca, L., Gaskett, A.C., Gunther, C.S., Newcomb, R.D., and Goddard, M.R. (2013). Quantifying variation in the ability of yeasts to attract Drosophila melanogaster. PLoS ONE 8, e75332. Vanbeneden, N., Delvaux, F., and Delvaux, F.R. (2006). Determination of hydroxycinnamic acids and volatile phenols in wort and beer by isocratic high-performance liquid chromatography using electrochemical detection. J. Chrom. A 1136, 237–242. Kaupp, U.B. (2010). Olfactory signalling in vertebrates and insects: differences and commonalities. Nat. Rev. Neurosci. 11, 188–200. Vosshall, L.B., Wong, A.M., and Axel, R. (2000). An olfactory sensory map in the fly brain. Cell 102, 147–159. Schaefer, H.M., McGraw, K., and Catoni, C. (2008). Birds use fruit colour as honest signal of dietary antioxidant rewards. Funct. Ecol. 22, 303–310.
Centre for Behaviour and Evolution, Institute of Neuroscience, Newcastle University, Newcastle upon Tyne NE1 7RU, UK. E-mail: [email protected]
http://dx.doi.org/10.1016/j.cub.2014.12.052
messenger pathways [5,6]. In larval zebrafish, for example, dopamine can promote locomotion via activation of lower-affinity D1 or higher-affinity D4 receptors, and inhibit locomotion via higher-affinity D2 or D3 receptors [7,8]. To begin to place these observations in a behavioral context, Jay et al. [1] focused on an evolutionarily conserved group of dopaminergic diencephalospinal neurons (DDNs) in the forebrain, which provide the sole source of spinal dopamine not only in zebrafish [9,10], but also in mammals [11]. The authors took advantage of an enhancer trap transgenic line of zebrafish, Tg(ETvmat2:GFP), in which these neurons are labeled by green fluorescent protein (GFP) [12]. The relatively large size and location of the cells made it possible to monitor not only their activity patterns, but also their excitatory and inhibitory synaptic inputs, using patch-clamp recordings in intact, chemically-immobilized fish capable of generating ‘fictive’ swimming (Figure 1A). By including fluorescent dye in the patch pipette, or by using a post-hoc stain, the authors confirmed that their recordings were from spinal projecting neurons. Unexpectedly, they also found that DDN axons exited the central nervous system and targeted auditory and
