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Dietary magnesium (MgThe proverbial saying ‘you are what you eat’ perfectly summarizes the concept that our diet can influence both our mental and physical health. We know that foods that are good for the heart, such as nuts, oily fish and berries, are also good for the brain. We know too that vitamins and minerals are essential for overall good health. But is there any evidence that increasing your intake of specific vitamins or minerals could help boost your brain power? While it might sound almost too good to be true, there is some evidence that this is the case for at least one mineral, magnesium. Studies in rodents have shown that adding magnesium supplements to food improves how well the animals perform on memory tasks. Both young and old animals benefit from additional magnesium. Even elderly rodents with a condition similar to Alzheimer’s disease show less memory loss when given magnesium supplements. But what about other species? Wu et al. now show that magnesium supplements also boost memory performance in fruit flies. One group of flies was fed with standard cornmeal for several days, while the other group received cornmeal supplemented with magnesium. Both groups were then trained to associate an odor with a food reward. Flies that had received the extra magnesium showed better memory for the odor when tested 24 hours after training. Wu et al. show that magnesium improves memory in the flies via a different mechanism to that reported previously for rodents. In rodents, magnesium increased levels of a receptor protein for a brain chemical called glutamate. In fruit flies, by contrast, the memory boost depended on a protein that transports magnesium out of neurons. Mutant flies that lacked this transporter showed memory impairments. Unlike normal flies, those without the transporter showed no memory improvement after eating magnesium-enriched food. The results suggest that the transporter may help adjust magnesium levels inside brain cells in response to neural activity. Humans produce four variants of this magnesium transporter, each encoded by a different gene. One of these transporters has already been implicated in brain development. The findings of Wu et al. suggest that the transporters may also act in the adult brain to influence cognition. Further studies are needed to test whether targeting the magnesium transporter could ultimately hold promise for treating memory impairments.

Summary Animals exhibit innate and learned preferences for temperature and humidity—conditions critical for their survival and reproduction. Leveraging a whole-brain electron microscopy volume, we studied the adult Drosophila melanogaster circuitry associated with antennal thermo- and hygrosensory neurons. We have identified two new target glomeruli in the antennal lobe, in addition to the five known ones, and the ventroposterior projection neurons (VP PNs) that relay thermo- and hygrosensory information to higher brain centers, including the mushroom body and lateral horn, seats of learned and innate behavior. We present the first connectome of a thermo- and hygrosensory neuropil, the lateral accessory calyx (lACA), by reconstructing neurons downstream of heating- and cooling-responsive VP PNs. A few mushroom body-intrinsic neurons solely receive thermosensory input from the lACA, while most receive additional olfactory and thermo- and/or hygrosensory PN inputs. Furthermore, several classes of lACA-associated neurons form a local network with outputs to other brain neuropils, suggesting that the lACA serves as a hub for thermo- and hygrosensory circuitry. For example, DN1a neurons link thermosensory PNs in the lACA to the circadian clock via the accessory medulla. Finally, we survey strongly connected downstream partners of VP PNs across the protocerebrum; these include a descending neuron targeted by dry-responsive VP PNs, meaning that just two synapses might separate hygrosensory inputs from motor circuits. These data provide a comprehensive first- and second-order layer analysis of Drosophila thermo- and hygrosensory systems and an initial survey of third-order neurons that could directly modulate behavior., Graphical Abstract, Highlights • Two novel thermo- and/or hygrosensory glomeruli in the fly antennal lobe • First complete set of thermosensory and hygrosensory projection neurons • First connectome for a thermo- and hygrosensory neuropil • Third-order thermo- and hygrosensory neurons, including link to circadian clock, Marin et al. use connectomics and genetics for comprehensive identification of temperature and humidity sensory neurons in the Drosophila brain. They reconstruct all projections to higher brain areas and select higher-order targets, including the mushroom body lateral accessory calyx, linking thermosensation to memory and the circadian clock.

After previously discovering that the ion channel TRPA1 is used as an internal temperature sensor in Drosophila to control the slow response of flies to shallow thermal gradients, the authors show here that the rapid response of flies to steep warming gradients relies on a different protein, GR28B, providing the first example of a thermosensory role for a gustatory receptor. Flies use the TRPA1 ion channel as an internal temperature sensor to slowly adjust their response to shallow thermal gradients. Now Paul Garrity and colleagues show that the rapid response of flies exposed to steep warmth gradients does not require TRPA1, but instead relies on the gustatory receptor GR28B(D), acting in peripheral thermosensing cells. Gustatory receptors have been implicated in taste, olfaction and host-seeking by disease-vector insects, but have not previously been linked with thermosensation. Behavioural responses to temperature are critical for survival, and animals from insects to humans show strong preferences for specific temperatures1,2. Preferred temperature selection promotes avoidance of adverse thermal environments in the short term and maintenance of optimal body temperatures over the long term1,2, but its molecular and cellular basis is largely unknown. Recent studies have generated conflicting views of thermal preference in Drosophila, attributing importance to either internal3 or peripheral4 warmth sensors. Here we reconcile these views by showing that thermal preference is not a singular response, but involves multiple systems relevant in different contexts. We found previously that the transient receptor potential channel TRPA1 acts internally to control the slowly developing preference response of flies exposed to a shallow thermal gradient3. We now find that the rapid response of flies exposed to a steep warmth gradient does not require TRPA1; rather, the gustatory receptor GR28B(D) drives this behaviour through peripheral thermosensors. Gustatory receptors are a large gene family, widely studied in insect gustation and olfaction, and are implicated in host-seeking by insect disease vectors5,6,7, but have not previously been implicated in thermosensation. At the molecular level, GR28B(D) misexpression confers thermosensitivity upon diverse cell types, suggesting that it is a warmth sensor. These data reveal a new type of thermosensory molecule and uncover a functional distinction between peripheral and internal warmth sensors in this tiny ectotherm reminiscent of thermoregulatory systems in larger, endothermic animals2. The use of multiple, distinct molecules to respond to a given temperature, as observed here, may facilitate independent tuning of an animal’s distinct thermosensory responses.

Ionotropic Receptors (IRs) are a large subfamily of variant ionotropic glutamate receptors present across Protostomia. While these receptors are most extensively studied for their roles in chemosensory detection, recent work has implicated two family members, IR21a and IR25a, in thermosensation in Drosophila. Here we characterize one of the most evolutionarily deeply conserved receptors, IR93a, and show that it is co-expressed and functions with IR21a and IR25a to mediate physiological and behavioral responses to cool temperatures. IR93a is also co-expressed with IR25a and a distinct receptor, IR40a, in a discrete population of sensory neurons in the sacculus, a multi-chambered pocket within the antenna. We demonstrate that this combination of receptors is required for neuronal responses to dry air and behavioral discrimination of humidity differences. Our results identify IR93a as a common component of molecularly and cellularly distinct IR pathways important for thermosensation and hygrosensation in insects. DOI: http://dx.doi.org/10.7554/eLife.17879.001

Our bodies are constantly bombarded by a diversity of environmental stimuli, such as touch, taste, sound, smell, light, etc. To detect and process this broad array of signals, nature has evolved a variety of cellular sensory mechanisms and pathways that interface with the environment and transmit neural signals back to the CNS where they are translated into behavioral decisions. Transient Response Potential (TRP) cation channels were first identified in invertebrates (i.e., Drosophila) and represent a sizeable receptor/channel family in mammals, consisting of 28 individual members grouped into subclasses denoted TRPC, TRPV, TRPM, TRPML, TRPP and TRPA (for a recent review, see ref. 1). Although originally described as ion channels, we now know that many members of the TRP family also function as receptors for a range of stimuli, including temperature, pH, chemical compounds and membrane voltage. In fact, several TRP isoforms display multimodal sensitivity, meaning that they can respond to more than one stimulus. For example, TRPV1, or the capcaisin receptor, displays both thermal and chemical sensitivity, and the two stimuli may act synergistically to increase channel activity. Physiologically, TRP family members are expressed in a variety of sensory afferent nerves that feed environmental information to the CNS, and also in smaller C-type afferent fibers responsible for peripheral pain sensation and transmission. Therapeutically, manipulation of TRP channel activity may represent an effective strategy to treat peripheral pain associated with inflammation and chronic tissue injury.

The ability to sense and respond to subtle variations in environmental temperature is critical for animal survival. Animals avoid temperatures that are too cold or too warm and seek out temperatures favorable for their survival. At the molecular level, members of the transient receptor potential (TRP) family of cation channels contribute to thermosensory behaviors in animals from flies to humans. In Drosophila melanogaster larvae, avoidance of excessively warm temperatures is known to require the TRP protein dTRPA1. Whether larval avoidance of excessively cool temperatures also requires TRP channel function, and whether warm and cool avoidance use the same or distinct TRP channels has been unknown. Here we identify two TRP channels required for cool avoidance, TRPL and TRP. Although TRPL and TRP have previously characterized roles in phototransduction, their function in cool avoidance appears to be distinct, as neither photoreceptor neurons nor the phototransduction regulators NORPA and INAF are required for cool avoidance. TRPL and TRP are required for cool avoidance; however they are dispensable for warm avoidance. Furthermore, cold-activated neurons in the larvae are required for cool but not warm avoidance. Conversely, dTRPA1 is essential for warm avoidance, but not cool avoidance. Taken together, these data demonstrate that warm and cool avoidance in the Drosophila larva involves distinct TRP channels and circuits.

Brain morphogenesis depends on the maintenance of boundaries between populations of non-intermingling cells. We used molecular markers to characterize a boundary within the optic lobe of the Drosophila brain and found that Slit and the Robo family of receptors, well-known regulators of axon guidance and neuronal migration, inhibit the mixing of adjacent cell populations in the developing optic lobe. Our data suggest that Slit is needed in the lamina to prevent inappropriate invasion of Robo-expressing neurons from the lobula cortex. We show that Slit protein surrounds lamina glia, while the distal cell neurons in the lobula cortex express all three Drosophila Robos. We examine the function of these proteins in the visual system by isolating a novel allele of slit that preferentially disrupts visual system expression of Slit and by creating transgenic RNA interference flies to inhibit the function of each Drosophila Robo in a tissue-specific fashion. We find that loss of Slit or simultaneous knockdown of Robo, Robo2 and Robo3 causes distal cell neurons to invade the lamina,resulting in cell mixing across the lamina/lobula cortex boundary. This boundary disruption appears to lead to alterations in patterns of axon navigation in the visual system. We propose that Slit and Robo-family proteins act to maintain the distinct cellular composition of the lamina and the lobula cortex.

Cell death plays an essential role in development, and the removal of cell corpses presents an important challenge for the developing organism. Macrophages are largely responsible for the clearance of cell corpses in Drosophila melanogaster and mammalian systems. We have examined the developmental requirement for macrophages in Drosophila and find that macrophage function is essential for central nervous system (CNS)morphogenesis. We generate and analyze mutations in the Pvr locus,which encodes a receptor tyrosine kinase of the PDGF/VEGF family that is required for hemocyte migration. We find that loss of Pvr function causes the mispositioning of glia within the CNS and the disruption of the CNS axon scaffold. We further find that inhibition of hemocyte development or of Croquemort, a receptor required for macrophage-mediated corpse engulfment,causes similar CNS defects. These data indicate that macrophage-mediated clearance of cell corpses is required for proper morphogenesis of the Drosophila CNS.

The neuronal wiring of the Drosophila melanogaster visual system is constructed through an intricate series of cell–cell interactions. Recent studies have identified some of the gene regulatory and cytoskeletal signaling pathways responsible for the layer-specific targeting of Drosophila photoreceptor axons. Target selection decisions of the R1–R6 subset of photoreceptor axons have been found to be influenced by the nuclear factors Brakeless and Runt, and target selection decisions of the R7 subset of axons have been found to require the cell-surface proteins Ptp69d, Lar and N-cadherin. A role for the visual system glia in orienting photoreceptor axon outgrowth and target selection has also been uncovered.

Complex animal behaviors are built from dynamical relationships between sensory inputs, neuronal activity, and motor outputs in patterns with strategic value. Connecting these patterns illuminates how nervous systems compute behavior. Here, we study Drosophila larva navigation up temperature gradients toward preferred temperatures (positive thermotaxis). By tracking the movements of animals responding to fixed spatial temperature gradients or random temperature fluctuations, we calculate the sensitivity and dynamics of the conversion of thermosensory inputs into motor responses. We discover three thermosensory neurons in each dorsal organ ganglion (DOG) that are required for positive thermotaxis. Random optogenetic stimulation of the DOG thermosensory neurons evokes behavioral patterns that mimic the response to temperature variations. In vivo calcium and voltage imaging reveals that the DOG thermosensory neurons exhibit activity patterns with sensitivity and dynamics matched to the behavioral response. Temporal processing of temperature variations carried out by the DOG thermosensory neurons emerges in distinct motor responses during thermotaxis.

Animals use thermosensory systems to achieve optimal temperatures for growth and reproduction and to avoid damaging extremes. Thermoregulation is particularly challenging for small animals like the fruit fly Drosophila melanogaster, whose body temperature rapidly changes in response to environmental temperature fluctuation. Recent work has uncovered some of the key molecules mediating fly thermosensation, including the Transient Receptor Potential (TRP) channels TRPA1 and Painless, and the Gustatory Receptor Gr28b, an unanticipated thermosensory regulator normally associated with a different sensory modality. There is also evidence the Drosophila phototransduction cascade may have some role in thermosensory responses. Together, the fly's diverse thermosensory molecules act in an array of functionally distinct thermosensory neurons to drive a suite of complex, and often exceptionally thermosensitive, behaviors.

Imaging a fluorophore in a living tissue presents several unique problems. The fluorescence from the labeled cell(s) may be weak, the labeled cells may be buried deep within tissue and the presence of a fluorophore may render the cells photo-sensitive. Two-photon laser-scanning microscopy (TPLSM) offers several advantages in meeting these challenges. We show that TPLSM provides greater sensitivity, better resolution and less photo-bleaching, as compared to confocal laser-scanning microscopy. The dramatically reduced photo-bleaching makes it possible to image cells continuously for long periods of time. Therefore, TPLSM allows a safer and higher-resolution means of imaging living cells labeled with a variety of fluorophores, including green fluorescent protein.

Pain is a significant medical concern and represents a major unmet clinical need. The ability to perceive and react to tissue-damaging stimuli is essential in order to maintain bodily integrity in the face of environmental danger. To prevent damage the systems that detect noxious stimuli are therefore under strict evolutionary pressure. We developed a high-throughput behavioral method to identify genes contributing to thermal nociception in the fruit fly and have reported a large-scale screen that identified the Ca²⁺ channel straightjacket (stj) as a conserved regulator of thermal nociception. Here we present the minimal anatomical and neuronal requirements for Drosophila to avoid noxious heat in our novel behavioral paradigm. Bioinformatics analysis of our whole genome data set revealed 23 genes implicated in Ca²⁺ signaling that are required for noxious heat avoidance. One of these genes, the conserved thermoreceptor TrpA1, was confirmed as a bona fide "pain" gene in both adult and larval fly nociception paradigms. The nociceptive function of TrpA1 required expression within the Drosophila nervous system, specifically within nociceptive multi-dendritic (MD) sensory neurons. Therefore, our analysis identifies the channel TRPA1 as a conserved regulator of nociception.

When placed on a temperature gradient, a Drosophila larva navigates away from excessive cold or heat by regulating the size, frequency, and direction of reorientation maneuvers between successive periods of forward movement. Forward movement is driven by peristalsis waves that travel from tail to head. During each reorientation maneuver, the larva pauses and sweeps its head from side to side until it picks a new direction for forward movement. Here, we characterized the motor programs that underlie the initiation, execution, and completion of reorientation maneuvers by measuring body segment dynamics of freely moving larvae with fluorescent muscle fibers as they were exposed to temporal changes in temperature. We find that reorientation maneuvers are characterized by highly stereotyped spatiotemporal patterns of segment dynamics. Reorientation maneuvers are initiated with head sweeping movement driven by asymmetric contraction of a portion of anterior body segments. The larva attains a new direction for forward movement after head sweeping movement by using peristalsis waves that gradually push posterior body segments out of alignment with the tail (i.e., the previous direction of forward movement) into alignment with the head. Thus, reorientation maneuvers during thermotaxis are carried out by two alternating motor programs: (1) peristalsis for driving forward movement and (2) asymmetric contraction of anterior body segments for driving head sweeping movement.

Like other ectotherms, the roundworm Caenorhabditis elegans and the fruit fly Drosophila melanogaster rely on behavioral strategies to stabilize their body temperature. Both animals use specialized sensory neurons to detect small changes in temperature, and the activity of these thermosensors governs the neural circuits that control migration and accumulation at preferred temperatures. Despite these similarities, the underlying molecular, neuronal, and computational mechanisms responsible for thermotaxis are distinct in these organisms. Here, we discuss the role of thermosensation in the development and survival of C. elegans and Drosophila, and review the behavioral strategies, neuronal circuits, and molecular networks responsible for thermotaxis behavior.

A mechanistic understanding of animal navigation requires quantitative assessment of the sensorimotor strategies used during navigation and quantitative assessment of how these strategies are regulated by cellular sensors. Here, we examine thermotactic behavior of theDrosophila melanogasterlarva using a tracking microscope to study individual larval movements on defined temperature gradients. We discover that larval thermotaxis involves a larger repertoire of strategies than navigation in smaller organisms such as motile bacteria andCaenorhabditis elegans. Beyond regulating run length (i.e., biasing a random walk), theDrosophila melanogasterlarva also regulates the size and direction of turns to achieve and maintain favorable orientations. Thus, the sharp turns in a larva's trajectory represent decision points for selecting new directions of forward movement. The larva uses the same strategies to move up temperature gradients during positive thermotaxis and to move down temperature gradients during negative thermotaxis. Disrupting positive thermotaxis by inactivating cold-sensitive neurons in the larva's terminal organ weakens all regulation of turning decisions, suggesting that information from one set of temperature sensors is used to regulate all aspects of turning decisions. TheDrosophila melanogasterlarva performs thermotaxis by biasing stochastic turning decisions on the basis of temporal variations in thermosensory input, thereby augmenting the likelihood of heading toward favorable temperatures at all times.

Animals from flies to humans are able to distinguish subtle gradations in temperature and show strong temperature preferences. Animals move to environments of optimal temperature and some manipulate the temperature of their surroundings, as humans do using clothing and shelter. Despite the ubiquitous influence of environmental temperature on animal behaviour, the neural circuits and strategies through which animals select a preferred temperature remain largely unknown. Here we identify a small set of warmth-activated anterior cell (AC) neurons located in the Drosophila brain, the function of which is critical for preferred temperature selection. AC neuron activation occurs just above the fly's preferred temperature and depends on dTrpA1, an ion channel that functions as a molecular sensor of warmth. Flies that selectively express dTrpA1 in the AC neurons select normal temperatures, whereas flies in which dTrpA1 function is reduced or eliminated choose warmer temperatures. This internal warmth-sensing pathway promotes avoidance of slightly elevated temperatures and acts together with a distinct pathway for cold avoidance to set the fly's preferred temperature. Thus, flies select a preferred temperature by using a thermal sensing pathway tuned to trigger avoidance of temperatures that deviate even slightly from the preferred temperature. This provides a potentially general strategy for robustly selecting a narrow temperature range optimal for survival.

BEACH proteins, an evolutionarily conserved family characterized by the presence of a BEACH (Beige and Chediak-Higashi) domain, have been implicated in membrane trafficking, but how they interact with the membrane trafficking machinery is unknown. Here we show that the Drosophila BEACH protein Bchs (Blue cheese) acts during development as an antagonist of Rab11, a small GTPase involved in vesicle trafficking. We find that reduction in, or loss of, bchs function restores viability and normal bristle development in animals with reduced rab11 function, while reductions in rab11 function exacerbate defects caused by bchs overexpression in the eye. Consistent with a role for Bchs in modulating Rab11-dependent trafficking, Bchs protein is associated with vesicles and extensively colocalized with Rab11 at the neuromuscular junction (NMJ). At the NMJ, we find that rab11 is important for synaptic morphogenesis, as reductions in rab11 function cause increases in bouton density and branching. These defects are also suppressed by loss of bchs . Taken together, these data identify Bchs as an antagonist of Rab11 during development and uncover a role for these regulators of vesicle trafficking in synaptic morphogenesis. This raises the interesting possibility that Bchs and other BEACH proteins may regulate vesicle traffic via interactions with Rab GTPases.

How a nucleus is positioned within a highly polarized postmitotic animal cell is not well understood. In this work, we demonstrate that the Dynactin complex (a regulator of the microtubule motor protein Dynein) is required to maintain the position of the nucleus within post-mitotic Drosophila melanogaster photoreceptor neurons. We show that multiple independent disruptions of Dynactin function cause a relocation of the photoreceptor nucleus toward the brain, and that inhibiting Dynactin causes the photoreceptor to acquire a bipolar appearance with long leading and trailing processes. We find that while the minus-end directed motor Dynein cooperates with Dynactin in positioning the photoreceptor nucleus, the plus-end directed microtubule motor Kinesin acts antagonistically to Dynactin. These data suggest that the maintenance of photoreceptor nuclear position depends on a balance of plus-end and minus-end directed microtubule motor function.

The daily sleep cycle in humans and other mammals is driven by a complex circuit within which GABAergic sleep-promoting neurons oppose arousal systems. The latter includes the circadian system, aminergic/cholinergic systems as well as neurons secreting the peptide orexin/hypocretin, which contribute to sharp behavioral transitions (Lu and Greco, 2006). Drosophila sleep has recently been shown also to be controlled by GABAergic inputs, which act on unknown cells expressing the Rdl GABAA receptor (Agosto et al., 2008). We identify here the relevant Rdl-containing cells as a subset of the well-studied Drosophila circadian clock neurons, the PDF-expressing small and large ventral lateral neurons (LNvs). LNv activity regulates the total amount of sleep as well as the rate of sleep onset, and both large and small LNvs are part of the sleep circuit. Flies mutant for either the pdf gene or its receptor are hypersomnolent, and PDF acts on the LNvs themselves to control sleep. These features of the Drosophila sleep circuit, GABAergic control of sleep onset and maintenance as well as peptidergic control of arousal, support the idea that features of sleep circuit architecture as well as the mechanisms governing the behavioral transitions between sleep and wake are conserved between mammals and insects.