Multisite imaging of neural activity using a genetically encoded calcium sensor in the honey bee.

Understanding of the neural bases for complex behaviors in Hymenoptera insect species has been limited by a lack of tools that allow measuring neuronal activity simultaneously in different brain regions. Here, we developed the first pan-neuronal genetic driver in a Hymenopteran model organism, the honey bee, and expressed the calcium indicator GCaMP6f under the control of the honey bee synapsin promoter. We show that GCaMP6f is widely expressed in the honey bee brain, allowing to record neural activity from multiple brain regions. To assess the power of this tool, we focused on the olfactory system, recording simultaneous responses from the antennal lobe, and from the more poorly investigated lateral horn (LH) and mushroom body (MB) calyces. Neural responses to 16 distinct odorants demonstrate that odorant quality (chemical structure) and quantity are faithfully encoded in the honey bee antennal lobe. In contrast, odor coding in the LH departs from this simple physico-chemical coding, supporting the role of this structure in coding the biological value of odorants. We further demonstrate robust neural responses to several bee pheromone odorants, key drivers of social behavior, in the LH. Combined, these brain recordings represent the first use of a neurogenetic tool for recording large-scale neural activity in a eusocial insect and will be of utility in assessing the neural underpinnings of olfactory and other sensory modalities and of social behaviors and cognitive abilities.

Neuronal Response Latencies Encode First Odor Identity Information across Subjects.

Odorants are coded in the primary olfactory processing centers by spatially and temporally distributed patterns of glomerular activity. Whereas the spatial distribution of odorant-induced responses is known to be conserved across individuals, the universality of its temporal structure is still debated. Via fast two-photon calcium imaging, we analyzed the early phase of neuronal responses in the form of the activity onset latencies in the antennal lobe projection neurons of honeybee foragers. We show that each odorant evokes a stimulus-specific response latency pattern across the glomerular coding space. Moreover, we investigate these early response features for the first time across animals, revealing that the order of glomerular firing onsets is conserved across individuals and allows them to reliably predict odorant identity, but not concentration. These results suggest that the neuronal response latencies provide the first available code for fast odor identification.SIGNIFICANCE STATEMENT Here, we studied early temporal coding in the primary olfactory processing centers of the honeybee brain by fast imaging of glomerular responses to different odorants across glomeruli and across individuals. Regarding the elusive role of rapid response dynamics in olfactory coding, we were able to clarify the following aspects: (1) the rank of glomerular activation is conserved across individuals, (2) its stimulus prediction accuracy is equal to that of the response amplitude code, and (3) it contains complementary information. Our findings suggest a substantial role of response latencies in odor identification, anticipating the static response amplitude code.

Morphofunctional experience-dependent plasticity in the honeybee brain.

Repeated or prolonged exposure to an odorant without any positive or negative reinforcement produces experience-dependent plasticity, which results in habituation and latent inhibition. In the honeybee (Apis mellifera), it has been demonstrated that, even if the absolute neural representation of an odor in the primary olfactory center, the antennal lobe (AL), is not changed by repeated presentations, its relative representation with respect to unfamiliar stimuli is modified. In particular, the representation of a stimulus composed of a 50:50 mixture of a familiar and a novel odorant becomes more similar to that of the novel stimulus after repeated stimulus preexposure. In a calcium-imaging study, we found that the same functional effect develops following prolonged odor exposure. By analyzing the brains of the animals subjected to this procedure, we found that such functional changes are accompanied by morphological changes in the AL (i.e., a decrease in volume in specific glomeruli). The AL glomeruli that exhibited structural plasticity also modified their functional responses to the three stimuli (familiar odor, novel odor, binary mixture). We suggest a model in which rebalancing inhibition within the AL glomeruli may be sufficient to elicit structural and functional correlates of experience-dependent plasticity.

Spatially resolved time-frequency analysis of odour coding in the insect antennal lobe.

Antennal lobes constitute the first neurophils in the insect brain involved in coding and processing of olfactory information. With their stereotyped functional and anatomical organization, they provide an accessible model with which to investigate information processing of an external stimulus in a neural network in vivo. Here, by combining functional calcium imaging with time-frequency analysis, we have been able to monitor the oscillatory components of neural activity upon olfactory stimulation. The aim of this study is to investigate the presence of stimulus-induced oscillatory patterns in the honeybee antennal lobe, and to analyse the distribution of those patterns across the antennal lobe glomeruli. Fast two-photon calcium imaging reveals the presence of low-frequency oscillations, the intensity of which is perturbed by an incoming stimulus. Moreover, analysis of the spatial arrangement of this activity indicates that it is not homogeneous throughout the antennal lobe. On the contrary, each glomerulus displays an odorant-specific time-frequency profile, and acts as a functional unit of the oscillatory activity. The presented approach allows simultaneous recording of complex activity patterns across several nodes of the antennal lobe, providing the means to better understand the network dynamics regulating olfactory coding and leading to perception.

Asymmetric neural coding revealed by in vivo calcium imaging in the honey bee brain.

Left-right asymmetries are common properties of nervous systems. Although lateralized sensory processing has been well studied, information is lacking about how asymmetries are represented at the level of neural coding. Using in vivo functional imaging, we identified a population-level left-right asymmetry in the honey bee's primary olfactory centre, the antennal lobe (AL). When both antennae were stimulated via a frontal odour source, the inter-odour distances between neural response patterns were higher in the right than in the left AL. Behavioural data correlated with the brain imaging results: bees with only their right antenna were better in discriminating a target odour in a cross-adaptation paradigm. We hypothesize that the differences in neural odour representations in the two brain sides serve to increase coding capacity by parallel processing.

Associative Learning of Quantitative Mechanosensory Stimuli in Honeybees.

The proboscis extension response (PER) has been widely used to evaluate honeybees' (Apis mellifera) learning and memory abilities, typically by using odors and visual cues for the conditioned stimuli. Here we asked whether honeybees could learn to distinguish between different magnitudes of the same type of stimulus, given as two speeds of air flux. By taking advantage of a novel automated system for administering PER experiments, we determined that the bees were highly successful when the lower air flux was rewarded and less successful when the higher flux was rewarded. Importantly, since our method includes AI-assisted analysis, we were able to consider subthreshold responses at a high temporal resolution; this analysis revealed patterns of rapid generalization and slowly acquired discrimination between the rewarded and unrewarded stimuli, as well as indications that the high air flux may have been mildly aversive. The learning curve for these mechanosensory stimuli, at least when the lower flux is rewarded, more closely mimics prior data from olfactory PER studies rather than visual ones, possibly in agreement with recent findings that the insect olfactory system is also sensitive to mechanosensory information. This work demonstrates a new modality to be used in PER experiments and lays the foundation for deeper exploration of honeybee cognitive processes when posed with complex learning challenges.

Trail pheromone identification in the ant Crematogaster scutellaris.

In this work, we identified the trail pheromone of the ant Crematogaster scutellaris. We combined gas chromatography-mass spectrometry analysis of extracts from the hind tibia, the location of the respective glands, with automated trail following assays. The study found tridecan-2-ol to be the strongest discriminator between hind tibia and other body part extracts. Tridecan-2-ol elicited trail-following behaviour at concentrations of 1 ng/µL. A separation of the enantiomers showed responses to (R)-tridecan-2-ol already at 0.001 ng/µL and only at a 1000-fold higher concentration for (S)-tridecan-2-ol, suggesting that only the R enantiomer is used by C. scutellaris in its natural environment. We also found strong behavioural responses to 2-dodecanol, a substance that was not detectable in the hind tibia extract of C. scutellaris, but which has been reported to be the trail pheromone of the related species C. castanea. We discuss the contribution of these results to the 'dissection and reconstruction' of strategies and mechanisms underlying the social organization of ants.

Intracellular trafficking of RNASET2, a novel component of P-bodies.

BACKGROUND INFORMATION: The ribonucleases (RNases) constitute a heterogeneous group of enzymes, which exert diverse and specific biological functions. Several RNases have been shown to control gene expression and cell differentiation. RNASET2, a novel member of the Rh/T2/S family of RNases, exerts micro-environmental control of malignancy in different experimental models with a general onco-suppressor activity, since it prevents cancer proliferation. Indeed, RNASET2 was found to be downregulated at the transcript level in several primary ovarian tumours or cell lines and in melanoma cell lines. Although recent works shed light on the biological role of RNASET2 in delaying tumour growth, its trafficking within the cell is still poorly understood. RNASET2 seems to play diverse biological roles including turnover of tRNA in yeast as well as rRNA degradation in zebrafish. RESULTS: Here, we have studied the intracellular trafficking of RNASET2 in mammalian cells. RNASET2 co-localizes with markers for the trans-Golgi network (TGN), which is the central sorting and processing station of the secretory pathway. Moreover, using the temperature-sensitive vesicular stomatitis glycoprotein, we demonstrate that RNASET2 undergoes delivery to the plasma membrane. In contrast to other RNA-interacting proteins, RNASET2 does not accumulate in stress granules upon metabolic stress in mammalian cells. Surprisingly, RNASET2 shows co-localization with processing bodies (P-bodies), which increases upon metabolic stress. Finally, cells lacking RNASET2 show a reduced numbers of P-bodies. CONCLUSIONS: In this study, we have identified two distinct cellular pools of RNASET2-containing granules. One pool undergoes membrane delivery using the TGN, and it is released to the extracellular environment. The second pool is recruited into P-bodies, suggesting a possible involvement of RNASET2 in P-body formation in mammalian cells.

Granger Causality Analysis of Transient Calcium Dynamics in the Honey Bee Antennal Lobe Network.

Odorant processing presents multiple parallels across animal species, and insects became relevant models for the study of olfactory coding because of the tractability of the underlying neural circuits. Within the insect brain, odorants are received by olfactory sensory neurons and processed by the antennal lobe network. Such a network comprises multiple nodes, named glomeruli, that receive sensory information and are interconnected by local interneurons participating in shaping the neural representation of an odorant. The study of functional connectivity between the nodes of a sensory network in vivo is a challenging task that requires simultaneous recording from multiple nodes at high temporal resolutions. Here, we followed the calcium dynamics of antennal lobe glomeruli and applied Granger causality analysis to assess the functional connectivity among network nodes in the presence and absence of an odorous stimulus. This approach revealed the existence of causal connectivity links between antennal lobe glomeruli in the absence of olfactory stimulation, while at odor arrival, the connectivity network's density increased and became stimulus-specific. Thus, such an analytical approach may provide a new tool for the investigation of neural network plasticity in vivo.

Geosmin suppresses defensive behaviour and elicits unusual neural responses in honey bees.

Geosmin is an odorant produced by bacteria in moist soil. It has been found to be extraordinarily relevant to some insects, but the reasons for this are not yet fully understood. Here we report the first tests of the effect of geosmin on honey bees. A stinging assay showed that the defensive behaviour elicited by the bee's alarm pheromone component isoamyl acetate (IAA) is strongly suppressed by geosmin. Surprisingly, the suppression is, however, only present at very low geosmin concentrations, and disappears at higher concentrations. We investigated the underlying mechanisms at the level of the olfactory receptor neurons by means of electroantennography, finding the responses to mixtures of geosmin and IAA to be lower than to pure IAA, suggesting an interaction of both compounds at the olfactory receptor level. Calcium imaging of the antennal lobe (AL) revealed that neuronal responses to geosmin decreased with increasing concentration, correlating well with the observed behaviour. Computational modelling of odour transduction and coding in the AL suggests that a broader activation of olfactory receptor types by geosmin in combination with lateral inhibition could lead to the observed non-monotonic increasing-decreasing responses to geosmin and thus underlie the specificity of the behavioural response to low geosmin concentrations.

All-optical manipulation of the Drosophila olfactory system.

Thanks to its well-known neuroanatomy, limited brain size, complex behaviour, and the extensive genetic methods, Drosophila has become an indispensable model in neuroscience. A vast number of studies have focused on its olfactory system and the processing of odour information. Optogenetics is one of the recently developed genetic tools that significantly advance this field of research, allowing to replace odour stimuli by direct neuronal activation with light. This becomes a universal all-optical toolkit when spatially selective optogenetic activation is combined with calcium imaging to read out neuronal responses. Initial experiments showed a successful implementation to study the olfactory system in fish and mice, but the olfactory system of Drosophila has been so far precluded from an application. To fill this gap, we present here optogenetic tools to selectively stimulate functional units in the Drosophila olfactory system, combined with two-photon calcium imaging to read out the activity patterns elicited by these stimuli at different levels of the brain. This method allows to study the spatial and temporal features of the information flow and reveals the functional connectivity in the olfactory network.

Impact of Chronic Exposure to Two Neonicotinoids on Honey Bee Antennal Responses to Flower Volatiles and Pheromonal Compounds.

Volatile compounds provide important olfactory cues for honey bees (Apis mellifera L.), which are essential for their ecology, behavior, and social communication. In the external environment bees locate food sources by the use of floral scents, while inside the hive, pheromones such as the queen mandibular pheromone (QMP) and alarm pheromones serve important functions in regulating colony life and inducing aggressive responses against intruders and parasites. Widely reported alterations of various behaviors in- and outside the hive following exposure to pesticides could therefore be associated with a disturbance of odor sensitivity. In the present study, we tested the effects of neonicotinoid pesticides at field concentrations on the ability of honey bees to perceive volatiles at the very periphery of the olfactory system. Bee colonies were subjected to treatments during the summer with either Imidacloprid or Thiacloprid at sublethal concentrations. Antennal responses to apple (Malus domestica L.) flower volatiles were studied by GC-coupled electro-antennographic detection (GC-EAD), and a range of volatiles, a substitute of the QMP, and the alarm pheromone 2-heptanone were tested by electroantennography (EAG). Short-term and long-term effects of the neonicotinoid treatments were investigated on bees collected in the autumn and again in the following spring. Treatment with Thiacloprid induced changes in antennal responses to specific flower VOCs, with differing short- and long-term effects. In the short term, increased antennal responses were observed for benzyl-alcohol and 1-hexanol, which are common flower volatiles but also constituents of the honey bee sting gland secretions. The treatment with Thiacloprid also affected antennal responses to the QMP and the mandibular alarm pheromone 2-heptanone. In the short term, a faster signal degeneration of the response signal to the positive control citral was recorded in the antennae of bees exposed to Thiacloprid or Imidacloprid. Finally, we observed season-related differences in the antennal responses to multiple VOCs. Altogether, our results suggest that volatile-specific alterations of antennal responses may contribute to explaining several behavioral changes previously observed in neonicotinoid-exposed bees. Treatment effects were generally more prominent in the short term, suggesting that adverse effects of neonicotinoid exposure may not persist across generations.

A multimodal approach for tracing lateralisation along the olfactory pathway in the honeybee through electrophysiological recordings, morpho-functional imaging, and behavioural studies.

Recent studies have revealed asymmetries between the left and right sides of the brain in invertebrate species. Here we present a review of a series of recent studies from our laboratories, aimed at tracing asymmetries at different stages along the honeybee's (Apis mellifera) olfactory pathway. These include estimates of the number of sensilla present on the two antennae, obtained by scanning electron microscopy, as well as electroantennography recordings of the left and right antennal responses to odorants. We describe investigative studies of the antennal lobes, where multi-photon microscopy was used to search for possible morphological asymmetries between the two brain sides. Moreover, we report on recently published results obtained by two-photon calcium imaging for functional mapping of the antennal lobe aimed at comparing patterns of activity evoked by different odours. Finally, possible links to the results of behavioural tests, measuring asymmetries in single-sided olfactory memory recall, are discussed.

Parallel Processing of Olfactory and Mechanosensory Information in the Honey Bee Antennal Lobe.

In insects, neuronal responses to clean air have so far been reported only episodically in moths. Here we present results obtained by fast two-photon calcium imaging in the honey bee Apis mellifera, indicating a substantial involvement of the antennal lobe, the first olfactory neuropil, in the processing of mechanical stimuli. Clean air pulses generate a complex pattern of glomerular activation that provides a code for stimulus intensity and dynamics with a similar level of stereotypy as observed for the olfactory code. Overlapping the air pulses with odor stimuli reveals a superposition of mechanosensory and odor response codes with high contrast. On the mechanosensitive signal, modulations were observed in the same frequency regime as the oscillatory motion of the antennae, suggesting a possible way to detect odorless airflow directions. The transduction of mechanosensory information via the insect antennae has so far been attributed primarily to Johnston's organ in the pedicel of the antenna. The possibility that the antennal lobe activation by clean air originates from Johnston's organ could be ruled out, as the signal is suppressed by covering the surfaces of the otherwise freely moving and bending antennae, which should leave Johnston's organ unaffected. The tuning curves of individual glomeruli indicate increased sensitivity at low-frequency mechanical oscillations as produced by the abdominal motion in waggle dance communication, suggesting a further potential function of this mechanosensory code. The discovery that the olfactory system can sense both odors and mechanical stimuli has recently been made also in mammals. The results presented here give hope that studies on insects can make a fundamental contribution to the cross-taxa understanding of this dual function, as only a few thousand neurons are involved in their brains, all of which are accessible by in vivo optical imaging.

Pheromone components affect motivation and induce persistent modulation of associative learning and memory in honey bees.

Since their discovery in insects, pheromones are considered as ubiquitous and stereotyped chemical messengers acting in intraspecific animal communication. Here we studied the effect of pheromones in a different context as we investigated their capacity to induce persistent modulations of associative learning and memory. We used honey bees, Apis mellifera, and combined olfactory conditioning and pheromone preexposure with disruption of neural activity and two-photon imaging of olfactory brain circuits, to characterize the effect of pheromones on olfactory learning and memory. Geraniol, an attractive pheromone component, and 2-heptanone, an aversive pheromone, improved and impaired, respectively, olfactory learning and memory via a durable modulation of appetitive motivation, which left odor processing unaffected. Consistently, interfering with aminergic circuits mediating appetitive motivation rescued or diminished the cognitive effects induced by pheromone components. We thus show that these chemical messengers act as important modulators of motivational processes and influence thereby animal cognition.

Structural and transcriptional evidence of mechanotransduction in the Drosophila suzukii ovipositor.

Drosophila suzukii is an invasive pest that prefers to lay eggs in ripening fruits, whereas most closely related Drosophila species exclusively use rotten fruit as oviposition site. This behaviour is allowed by an enlarged and serrated ovipositor that can pierce intact fruit skin, and by multiple contact sensory systems (mechanosensation and taste) that detect the optimal egg-laying substrates. Here, we tested the hypothesis that bristles present in the D. suzukii ovipositor tip contribute to these sensory modalities. Analysis of the bristle ultrastructure revealed that four different types of cuticular elements (conical pegs type 1 and 2, chaetic and trichoid sensilla) are present on the tip of each ovipositor plate. All of them have a poreless shaft and are innervated at their base by a single neuron that ends in a distal tubular body, thus resembling mechanosensitive structures. Fluorescent labelling in D. suzukii and D. melanogaster revealed that pegs located on the ventral side of the ovipositor tip are innervated by a single neuron in both species. RNA-sequencing profiled gene expression, notably sensory receptor genes of the terminalia of D. suzukii and of three other Drosophila species with changes in their ovipositor structure (from serrated to blunt ovipositor: Drosophila subpulchrella, Drosophila biarmipes and D. melanogaster). Our results revealed few species-specific transcripts and an overlapping expression of candidate mechanosensitive genes as well as the presence of some chemoreceptor transcripts. These experimental evidences suggest a mechanosensitive function for the D. suzukii ovipositor, which might be crucial across Drosophila species independently from ovipositor shape.

Automated quantification of synaptic boutons reveals their 3D distribution in the honey bee mushroom body.

Synaptic boutons are highly plastic structures undergoing experience-dependent changes in their number, volume, and shape. Their plasticity has been intensively studied in the insect mushroom bodies by manually counting the number of boutons in small regions of interest and extrapolating this number to the volume of the mushroom body neuropil. Here we extend this analysis to the synaptic bouton distribution within a larger subregion of the mushroom body olfactory neuropil of honey bees (Apis mellifera). This required the development of an automated method combining two-photon imaging with advanced image post-processing and multiple threshold segmentation. The method was first validated in subregions of the mushroom body olfactory and visual neuropils. Further analyses in the olfactory neuropil suggested that previous studies overestimated the number of synaptic boutons. As a reason for that, we identified boundaries effects in the small volume samples. The application of the automated analysis to larger volumes of the mushroom body olfactory neuropil revealed a corrected average density of synaptic boutons and, for the first time, their 3D spatial distribution. This distribution exhibited a considerable heterogeneity. This additional information on the synaptic bouton distribution provides the basis for future studies on brain development, symmetry, and plasticity.

The Neurophysiological Bases of the Impact of Neonicotinoid Pesticides on the Behaviour of Honeybees.

Acetylcholine is the main excitatory neurotransmitter in the honeybee brain and controls a wide range of behaviours that ensure the survival of the individuals and of the entire colony. Neonicotinoid pesticides target this neurotransmission pathway and can thereby affect the behaviours under its control, even at doses far below the toxicity limit. These sublethal effects of neonicotinoids on honeybee behaviours were suggested to be partly responsible for the decline in honeybee populations. However, the neural mechanisms by which neonicotinoids influence single behaviours are still unclear. This is mainly due to the heterogeneity of the exposure pathways, doses and durations between studies. Here, we provide a review of the state of the science in this field and highlight knowledge gaps that need to be closed. We describe the agonistic effects of neonicotinoids on neurons expressing the different nicotinic acetylcholine receptors and the resulting brain structural and functional changes, which are likely responsible for the behavioural alterations reported in bees exposed to neonicotinoids.

In Vivo Two-Photon Imaging of the Olfactory System in Insects.

This chapter describes how to apply two-photon neuroimaging to study the insect olfactory system in vivo. It provides a complete protocol for insect brain functional imaging, with some additional remarks on the acquisition of morphological information from the living brain. We discuss the most important choices to make when buying or building a two-photon laser-scanning microscope. We illustrate different possibilities of animal preparation and brain tissue labeling for in vivo imaging. Finally, we give an overview of the main methods of image data processing and analysis, followed by a short description of pioneering applications of this imaging modality.

Evolutionary compromises to metabolic toxins: Ammonia and urea tolerance in Drosophila suzukii and Drosophila melanogaster.

The invasive pest Drosophila suzukii has evolved morphological and behavioural adaptations to lay eggs under the skin of fresh fruits. This results in severe damage to a wide range of small fruits. Drosophila suzukii females typically lay few eggs per fruit, preferring healthy fruits. Hence, larvae are exposed to a reduced amount of nitrogenous waste. Differently, the innocuous Drosophila melanogaster lays eggs on fermented fruits already infested by conspecifics, with larvae developing in a crowded environment with the accumulation of nitrogenous waste such as ammonia and urea. These compounds derive from nitrogen metabolism, protein degradation, and amino acids catabolism and are relatively toxic at high concentrations in an organism. The observed differences in oviposition site and larval ecological niche suggest that these species might differ in behavioural and physiological mechanisms used to cope with nitrogenous waste. We investigated how different concentrations of ammonia and urea affect oviposition and larval development in both species. Females and larvae of D. suzukii showed greater susceptibility to high concentrations of both compounds, with a dramatic decrease in the number of eggs laid and egg viability. Moreover, we tested the chemotactic response of third instar larvae to high concentrations of the compounds. Interestingly, ammonia resulted in a repulsive behaviour in respect of the control and urea groups. To better understand the pathways underlying these differences, we evaluated the effect on ornithine aminotransferase and glutathione-S-transferase, two enzymes involved in nitrogen metabolism and stress response that are expressed during larval development. Both ammonia and urea significantly reduced the expression of these enzymes in D. suzukii compared to D. melanogaster. This shows how the ecological shift of D. suzukii to fresh fruit is accompanied by less efficient detoxifying and excretory mechanisms, with important implications for evolutionary biology and applied research. Our data suggest that the ecological shift of D. suzukii to fresh fruit as oviposition substrate is accompanied by a reduced tolerance to metabolic toxins during larval development.