The second and third optic ganglia of the worker bee: Golgi studies of the neuronal elements in the medulla and lobula

Illusional Perspective across Humans and Bees

For two centuries, visual illusions have attracted the attention of neurobiologists and comparative psychologists, given the possibility of investigating the complexity of perceptual mechanisms by using relatively simple patterns. Animal models, such as primates, birds, and fish, have played a crucial role in understanding the physiological circuits involved in the susceptibility of visual illusions. However, the comprehension of such mechanisms is still a matter of debate. Despite their different neural architectures, recent studies have shown that some arthropods, primarily Hymenoptera and Diptera, experience illusions similar to those humans do, suggesting that perceptual mechanisms are evolutionarily conserved among species. Here, we review the current state of illusory perception in bees. First, we introduce bees' visual system and speculate which areas might make them susceptible to illusory scenes. Second, we review the current state of knowledge on misperception in bees (Apidae), focusing on the visual stimuli used in the literature. Finally, we discuss important aspects to be considered before claiming that a species shows higher cognitive ability while equally supporting alternative hypotheses. This growing evidence provides insights into the evolutionary origin of visual mechanisms across species.

Brain size and behavioral specialization in the jataí stingless bee (Tetragonisca angustula)

Social insects are instructive models for understanding the association between investment in brain size and behavioral variability because they show a relatively simple nervous system associated with a large set of complex behaviors. In the jataí stingless bee (Tetragonisca angustula), division of labor relies both on age and body size differences among workers. When young, both minors and soldiers engage in intranidal tasks and move to extranidal tasks as they age. Minors switch to foraging activities, while soldiers take over defensive roles. Nest defense performed by soldiers includes two different tasks: (1) hovering around the nest entrance for the detection and interception of heterospecific bees (a task relying mostly on vision) and (2) standing at the nest entrance tube for inspection of returning foragers and discrimination against conspecific non-nestmates based on olfactory cues. Here, using different-sized individuals (minors and soldiers) as well as same-sized individuals (hovering and standing soldiers) performing distinct tasks, we investigated the effects of both morphological and behavioral variability on brain size. We found a negative allometric growth between brain size and body size across jataí workers, meaning that minors had relatively larger brains than soldiers. Between soldier types, we found that hovering soldiers had larger brain compartments related to visual processing (the optic lobes) and learning (the mushroom bodies). Brain size differences between jataí soldiers thus correspond to behavioral specialization in defense (i.e., vision for hovering soldiers) and illustrate a functional neuroplasticity underpinning division of labor.

Phosphoproteomic analysis of duck egg yolk provides novel insights into its characteristics and biofunctions

BACKGROUND

Although the importance of phosphorylation in the function of proteins is known, investigation of the protein phosphorylation of duck egg yolk (DEY) is still very limited. This study aimed to conduct a detailed phosphoproteomic study of DEY using immobilized metal affinity chromatography and ultra-high liquid chromatography tandem mass spectrometry.

RESULTS

A total of 253 phosphorylation sites assigned to 66 phosphoproteins were identified in DEY, of which VTG-1, VTG-2, and fibrinogen alpha chain were found to be the highly phosphorylated proteins in DEY. The biological functions of the identified phosphoproteins were illuminated through gene ontology analysis, which showed that they were mainly involved in binding, catalytic, immune response, and metabolic activity. S-X-E and S-X-S were found to be the most conserved serine motifs of phosphorylation in DEY. The comparison of DEY phosphoproteins with those of chicken egg yolk (CEY) revealed that differences mostly involved molecular functions and biological processes. The comparison also revealed a higher phosphorylation level in DEY proteins.

CONCLUSION

The higher phosphorylation level in DEY proteins than that in CEY proteins are supposed to help enhance duck growth performance and biological activities (e.g. antibacterial and antioxidant ability) for better adapting the humid environment the duck lived. © 2021 Society of Chemical Industry.

The complex synaptic pathways onto a looming-detector neuron revealed using serial block-face scanning electron microscopy

The ability of locusts to detect looming stimuli and avoid collisions or predators depends on a neuronal circuit in the locust's optic lobe. Although comprehensively studied for over three decades, there are still major questions about the computational steps of this circuit. We used fourth instar larvae of Locusta migratoria to describe the connection between the lobula giant movement detector 1 (LGMD1) neuron in the lobula complex and the upstream neuropil, the medulla. Serial block-face scanning electron microscopy (SBEM) was used to characterize the morphology of the connecting neurons termed trans-medullary afferent (TmA) neurons and their synaptic connectivity. This enabled us to trace neurons over several hundred micrometers between the medulla and the lobula complex while identifying their synapses. We traced two different TmA neurons, each from a different individual, from their synapses with the LGMD in the lobula complex up into the medulla and describe their synaptic relationships. There is not a simple downstream transmission of the signal from a lamina neuron onto these TmA neurons; there is also a feedback loop in place with TmA neurons making outputs as well as receiving inputs. More than one type of neuron shapes the signal of the TmA neurons in the medulla. We found both columnar and trans-columnar neurons connected with the traced TmA neurons in the medulla. These findings indicate that there are computational steps in the medulla that have not been included in models of the neuronal pathway for looming detection.

Phosphoproteomic analysis of duck egg white and insight into the biological functions of identified phosphoproteins

Phosphorylation of proteins is one of the most important and pleiotropic modifications. It plays a vital role in controlling protein functions. However, the phosphoproteome of duck egg white (DEW) has not been studied yet. To investigate the role of phosphorylation on DEW proteins, a detailed phosphoproteome analysis of DEW was performed using an immobilized metal affinity chromatography enrichment strategy. A total of 92 phosphosites representing 36 phosphoproteins were identified. [S-x-E] and [T-x-E] were found to be the most overrepresented motifs in DEW. The identified phosphoproteins in DEW were mainly involved in the binding, transport activity, biological regulation, and metabolic processes. Gene ontology analysis was used to elucidate the biological functions of DEW phosphoproteins and compare them with those of chicken egg white (CEW), which showed the differences mostly involved molecular functions and biological processes. PRACTICAL APPLICATIONS: These findings provide fundamental insight into the biological functions of identified phosphoproteins of DEW to better understand the roles of phosphorylated DEW proteins for food industries and human health. Phosphoproteomic study of DEW would be valuable for the food and nutrition industry to exploit the potential of this avian proteins in the processing of health benefit products.

Honeybees solve a multi-comparison ranking task by probability matching

Honeybees forage on diverse flowers which vary in the amount and type of rewards they offer, and bees are challenged with maximizing the resources they gather for their colony. That bees are effective foragers is clear, but how bees solve this type of complex multi-choice task is unknown. Here, we set bees a five-comparison choice task in which five colours differed in their probability of offering reward and punishment. The colours were ranked such that high ranked colours were more likely to offer reward, and the ranking was unambiguous. Bees' choices in unrewarded tests matched their individual experiences of reward and punishment of each colour, indicating bees solved this test not by comparing or ranking colours but by basing their colour choices on their history of reinforcement for each colour. Computational modelling suggests a structure like the honeybee mushroom body with reinforcement-related plasticity at both input and output can be sufficient for this cognitive strategy. We discuss how probability matching enables effective choices to be made without a need to compare any stimuli directly, and the use and limitations of this simple cognitive strategy for foraging animals.

Randomly weighted receptor inputs can explain the large diversity of colour-coding neurons in the bee visual system

True colour vision requires comparing the responses of different spectral classes of photoreceptors. In insects, there is a wealth of data available on the physiology of photoreceptors and on colour-dependent behaviour, but less is known about the neural mechanisms that link the two. The available information in bees indicates a diversity of colour opponent neurons in the visual optic ganglia that significantly exceeds that known in humans and other primates. Here, we present a simple mathematical model for colour processing in the optic lobes of bees to explore how this diversity might arise. We found that the model can reproduce the physiological spectral tuning curves of the 22 neurons that have been described so far. Moreover, the distribution of the presynaptic weights in the model suggests that colour-coding neurons are likely to be wired up to the receptor inputs randomly. The perceptual distances in our random synaptic weight model are in agreement with behavioural observations. Our results support the idea that the insect nervous system might adopt partially random wiring of neurons for colour processing.

Afferent neural pathways from the photoperiodic receptor in the bean bug, Riptortus pedestris

Adult diapause in the bean bug, Riptortus pedestris, is controlled by the photoperiod, which is received by retinal cells in the central region of the compound eyes. To resolve the afferent neural pathways involved in the photoperiodic response, we examine fibre projections from the photoperiodic receptors to the brain and investigate the roles of the posterior optic tract (POT) in the photoperiodic response. Reduced-silver impregnation and synapsin immunolabelling revealed that the medulla was divided into nine strata: the outer layer comprises 4 strata, the inner layer comprises 4 strata and a serpentine layer separates the inner and outer layers. Biotin injection revealed that retinal fibres from the central region of the compound eye terminated in either the central part of the lamina or the central part of the medulla 3rd or 4th layer. Biotin injection into the central part of the medulla labelled 5 distinct afferent pathways: two terminated in a region of ipsilateral anterior protocerebrum, while the other three had contralateral projections. One pathway ran through the POT and connected to the bilateral medulla serpentine layers. When the POT was surgically severed, diapause incidence under short-day conditions was significantly reduced compared to that observed following a sham operation. However, an incision at a posterior part of the medulla and lobula boundary, as a control experiment, did not affect the photoperiodic response. These results suggest that photoperiodic signals from the central region of the compound eye are transferred to neurons with fibres running in the POT for photoperiodic response in R. pedestris.

Synaptic Organization of Microglomerular Clusters in the Lateral and Medial Bulbs of the Honeybee Brain

The honeybee Apis mellifera is an established model for the study of visual orientation. Yet, research on this topic has focused on behavioral aspects and has neglected the investigation of the underlying neural architectures in the bee brain. In other insects, the anterior optic tubercle (AOTU), the lateral (LX) and the central complex (CX) are important brain regions for visuospatial performances. In the central brain of the honeybee, a prominent group of neurons connecting the AOTU with conspicuous microglomerular synaptic structures in the LX has been recently identified, but their neural organization and ultrastructure have not been investigated. Here we characterized these microglomerular structures by means of immunohistochemical and ultrastructural analyses, in order to evaluate neurotransmission and synaptic organization. Three-dimensional reconstructions of the pre-synaptic and post-synaptic microglomerular regions were performed based on confocal microscopy. Each pre-synaptic region appears as a large cup-shaped profile that embraces numerous post-synaptic profiles of GABAergic tangential neurons connecting the LX to the CX. We also identified serotonergic broad field neurons that probably provide modulatory input from the CX to the synaptic microglomeruli in the LX. Two distinct clusters of microglomerular structures were identified in the lateral bulb (LBU) and medial bulb (MBU) of the LX. Although the ultrastructure of both clusters is very similar, we found differences in the number of microglomeruli and in the volume of the pre-synaptic profiles of each cluster. We discuss the possible role of these microglomerular clusters in the visuospatial behavior of honeybees and propose research avenues for studying their neural plasticity and synaptic function.

Gene expression profiles and neural activities of Kenyon cell subtypes in the honeybee brain: identification of novel 'middle-type' Kenyon cells

In the honeybee (Apis mellifera L.), it has long been thought that the mushroom bodies, a higher-order center in the insect brain, comprise three distinct subtypes of intrinsic neurons called Kenyon cells. In class-I large-type Kenyon cells and class-I small-type Kenyon cells, the somata are localized at the edges and in the inner core of the mushroom body calyces, respectively. In class-II Kenyon cells, the somata are localized at the outer surface of the mushroom body calyces. The gene expression profiles of the large- and small-type Kenyon cells are distinct, suggesting that each exhibits distinct cellular characteristics. We recently identified a novel gene, mKast (middle-type Kenyon cell-preferential arrestin-related gene-1), which has a distinctive expression pattern in the Kenyon cells. Detailed expression analyses of mKast led to the discovery of novel 'middle-type' Kenyon cells characterized by their preferential mKast-expression in the mushroom bodies. The somata of the middle-type Kenyon cells are localized between the large- and small-type Kenyon cells, and the size of the middle-type Kenyon cell somata is intermediate between that of large- and small-type Kenyon cells. Middle-type Kenyon cells appear to differentiate from the large- and/or small-type Kenyon cell lineage(s). Neural activity mapping using an immediate early gene, kakusei, suggests that the small-type and some middle-type Kenyon cells are prominently active in the forager brain, suggesting a potential role in processing information during foraging flight. Our findings indicate that honeybee mushroom bodies in fact comprise four types of Kenyon cells with different molecular and cellular characteristics: the previously known class-I large- and small-type Kenyon cells, class-II Kenyon cells, and the newly identified middle-type Kenyon cells described in this review. As the cellular characteristics of the middle-type Kenyon cells are distinct from those of the large- and small-type Kenyon cells, their careful discrimination will be required in future studies of honeybee Kenyon cell subtypes. In this review, we summarize recent progress in analyzing the gene expression profiles and neural activities of the honeybee Kenyon cell subtypes, and discuss possible roles of each Kenyon cell subtype in the honeybee brain.

A Model for an Angular Velocity-Tuned Motion Detector Accounting for Deviations in the Corridor-Centering Response of the Bee

We present a novel neurally based model for estimating angular velocity (AV) in the bee brain, capable of quantitatively reproducing experimental observations of visual odometry and corridor-centering in free-flying honeybees, including previously unaccounted for manipulations of behaviour. The model is fitted using electrophysiological data, and tested using behavioural data. Based on our model we suggest that the AV response can be considered as an evolutionary extension to the optomotor response. The detector is tested behaviourally in silico with the corridor-centering paradigm, where bees navigate down a corridor with gratings (square wave or sinusoidal) on the walls. When combined with an existing flight control algorithm the detector reproduces the invariance of the average flight path to the spatial frequency and contrast of the gratings, including deviations from perfect centering behaviour as found in the real bee's behaviour. In addition, the summed response of the detector to a unit distance movement along the corridor is constant for a large range of grating spatial frequencies, demonstrating that the detector can be used as a visual odometer.

Transmedulla Neurons in the Sky Compass Network of the Honeybee (Apis mellifera) Are a Possible Site of Circadian Input

Honeybees are known for their ability to use the sun's azimuth and the sky's polarization pattern for spatial orientation. Sky compass orientation in bees has been extensively studied at the behavioral level but our knowledge about the underlying neuronal systems and mechanisms is very limited. Electrophysiological studies in other insect species suggest that neurons of the sky compass system integrate information about the polarization pattern of the sky, its chromatic gradient, and the azimuth of the sun. In order to obtain a stable directional signal throughout the day, circadian changes between the sky polarization pattern and the solar azimuth must be compensated. Likewise, the system must be modulated in a context specific way to compensate for changes in intensity, polarization and chromatic properties of light caused by clouds, vegetation and landscape. The goal of this study was to identify neurons of the sky compass pathway in the honeybee brain and to find potential sites of circadian and neuromodulatory input into this pathway. To this end we first traced the sky compass pathway from the polarization-sensitive dorsal rim area of the compound eye via the medulla and the anterior optic tubercle to the lateral complex using dye injections. Neurons forming this pathway strongly resembled neurons of the sky compass pathway in other insect species. Next we combined tracer injections with immunocytochemistry against the circadian neuropeptide pigment dispersing factor and the neuromodulators serotonin, and γ-aminobutyric acid. We identified neurons, connecting the dorsal rim area of the medulla to the anterior optic tubercle, as a possible site of neuromodulation and interaction with the circadian system. These neurons have conspicuous spines in close proximity to pigment dispersing factor-, serotonin-, and GABA-immunoreactive neurons. Our data therefore show for the first time a potential interaction site between the sky compass pathway and the circadian clock.

It takes two-coincidence coding within the dual olfactory pathway of the honeybee

To rapidly process biologically relevant stimuli, sensory systems have developed a broad variety of coding mechanisms like parallel processing and coincidence detection. Parallel processing (e.g., in the visual system), increases both computational capacity and processing speed by simultaneously coding different aspects of the same stimulus. Coincidence detection is an efficient way to integrate information from different sources. Coincidence has been shown to promote associative learning and memory or stimulus feature detection (e.g., in auditory delay lines). Within the dual olfactory pathway of the honeybee both of these mechanisms might be implemented by uniglomerular projection neurons (PNs) that transfer information from the primary olfactory centers, the antennal lobe (AL), to a multimodal integration center, the mushroom body (MB). PNs from anatomically distinct tracts respond to the same stimulus space, but have different physiological properties, characteristics that are prerequisites for parallel processing of different stimulus aspects. However, the PN pathways also display mirror-imaged like anatomical trajectories that resemble neuronal coincidence detectors as known from auditory delay lines. To investigate temporal processing of olfactory information, we recorded PN odor responses simultaneously from both tracts and measured coincident activity of PNs within and between tracts. Our results show that coincidence levels are different within each of the two tracts. Coincidence also occurs between tracts, but to a minor extent compared to coincidence within tracts. Taken together our findings support the relevance of spike timing in coding of olfactory information (temporal code).

Photoreceptor projections and receptive fields in the dorsal rim area and main retina of the locust eye

In many insect species, photoreceptors of a small dorsal rim area of the eye are specialized for sensitivity to the oscillation plane of polarized skylight and, thus, serve a role in sky compass orientation. To further understand peripheral mechanisms of polarized-light processing in the optic lobe, we have studied the projections of photoreceptors and their receptive fields in the main eye and dorsal rim area of the desert locust, a model system for polarization vision analysis. In both eye regions, one photoreceptor per ommatidium, R7, has a long visual fiber projecting through the lamina to the medulla. Axonal fibers from R7 receptors of the dorsal rim area have short side branches throughout the depth of the dorsal lamina and maintain retinotopic projections to the dorsal medulla following the first optic chiasma. Receptive fields of dorsal rim photoreceptors are considerably larger (average acceptance angle 33°) than those of the main eye (average acceptance angle 2.04°) and, taken together, cover almost the entire sky. The data challenge previous reports of two long visual fibers per ommatidium in the main eye of the locust and provide data for future analysis of peripheral networks underlying polarization opponency in the locust brain.

In-depth phosphoproteomic analysis of royal jelly derived from western and eastern honeybee species

The proteins in royal jelly (RJ) play a pivotal role in the nutrition, immune defense, and cast determination of honeybee larvae and have a wide range of pharmacological and health-promoting functions for humans as well. Although the importance of post-translational modifications (PTMs) in protein function is known, investigation of protein phosphorylation of RJ proteins is still very limited. To this end, two complementary phosphopeptide enrichment materials (Ti(4+)-IMAC and TiO2) and high-sensitivity mass spectrometry were applied to establish a detailed phosphoproteome map and to qualitatively and quantitatively compare the phosphoproteomes of RJ produced by Apis mellifera ligustica (Aml) and Apis cerana cerana (Acc). In total, 16 phosphoproteins carrying 67 phosphorylation sites were identified in RJ derived from western bees, and nine proteins phosphorylated on 71 sites were found in RJ produced by eastern honeybees. Of which, eight phosphorylated proteins were common to both RJ samples, and the same motif ([S-x-E]) was extracted, suggesting that the function of major RJ proteins as nutrients and immune agents is evolutionary preserved in both of these honeybee species. All eight overlapping phosphoproteins showed significantly higher abundance in Acc-RJ than in Aml-RJ, and the phosphorylation of Jelleine-II (an antimicrobial peptide, TPFKLSLHL) at S(6) in Acc-RJ had stronger antimicrobial properties than that at T(1) in Aml-RJ even though the overall antimicrobial activity of Jelleine-II was found to decrease after phosphorylation. The differences in phosphosites, peptide abundance, and antimicrobial activity of the phosphorylated RJ proteins indicate that the two major honeybee species employ distinct phosphorylation strategies that align with their different biological characteristics shaped by evolution. The phosphorylation of RJ proteins are potentially driven by the activity of extracellular serine/threonine protein kinase FAM20C-like protein (FAM20C-like) through the [S-x-E] motif, which is supported by evidence that mRNA and protein expression of FAM20C-like protein kinase are both found in the highest level in the hypopharyngeal gland of nurse bees. Our data represent the first comprehensive RJ phosphorylation atlas, recording patterns of phosphorylated RJ protein abundance and antibacterial activity of some RJ proteins in two major managed honeybee species. These data constitute a firm basis for future research to better understand the biological roles of each RJ protein for honeybee biology and human health care.

Visual motion-sensitive neurons in the bumblebee brain convey information about landmarks during a navigational task

Bees use visual memories to find the spatial location of previously learnt food sites. Characteristic learning flights help acquiring these memories at newly discovered foraging locations where landmarks—salient objects in the vicinity of the goal location—can play an important role in guiding the animal's homing behavior. Although behavioral experiments have shown that bees can use a variety of visual cues to distinguish objects as landmarks, the question of how landmark features are encoded by the visual system is still open. Recently, it could be shown that motion cues are sufficient to allow bees localizing their goal using landmarks that can hardly be discriminated from the background texture. Here, we tested the hypothesis that motion sensitive neurons in the bee's visual pathway provide information about such landmarks during a learning flight and might, thus, play a role for goal localization. We tracked learning flights of free-flying bumblebees (Bombus terrestris) in an arena with distinct visual landmarks, reconstructed the visual input during these flights, and replayed ego-perspective movies to tethered bumblebees while recording the activity of direction-selective wide-field neurons in their optic lobe. By comparing neuronal responses during a typical learning flight and targeted modifications of landmark properties in this movie we demonstrate that these objects are indeed represented in the bee's visual motion pathway. We find that object-induced responses vary little with object texture, which is in agreement with behavioral evidence. These neurons thus convey information about landmark properties that are useful for view-based homing.

Mechanisms, functions and ecology of colour vision in the honeybee

Research in the honeybee has laid the foundations for our understanding of insect colour vision. The trichromatic colour vision of honeybees shares fundamental properties with primate and human colour perception, such as colour constancy, colour opponency, segregation of colour and brightness coding. Laborious efforts to reconstruct the colour vision pathway in the honeybee have provided detailed descriptions of neural connectivity and the properties of photoreceptors and interneurons in the optic lobes of the bee brain. The modelling of colour perception advanced with the establishment of colour discrimination models that were based on experimental data, the Colour-Opponent Coding and Receptor Noise-Limited models, which are important tools for the quantitative assessment of bee colour vision and colour-guided behaviours. Major insights into the visual ecology of bees have been gained combining behavioural experiments and quantitative modelling, and asking how bee vision has influenced the evolution of flower colours and patterns. Recently research has focussed on the discrimination and categorisation of coloured patterns, colourful scenes and various other groupings of coloured stimuli, highlighting the bees' behavioural flexibility. The identification of perceptual mechanisms remains of fundamental importance for the interpretation of their learning strategies and performance in diverse experimental tasks.

Selective attention in the honeybee optic lobes precedes behavioral choices

Attention allows animals to respond selectively to competing stimuli, enabling some stimuli to evoke a behavioral response while others are ignored. How the brain does this remains mysterious, although it is increasingly evident that even animals with the smallest brains display this capacity. For example, insects respond selectively to salient visual stimuli, but it is unknown where such selectivity occurs in the insect brain, or whether neural correlates of attention might predict the visual choices made by an insect. Here, we investigate neural correlates of visual attention in behaving honeybees (Apis mellifera). Using a closed-loop paradigm that allows tethered, walking bees to actively control visual objects in a virtual reality arena, we show that behavioral fixation increases neuronal responses to flickering, frequency-tagged stimuli. Attention-like effects were reduced in the optic lobes during replay of the same visual sequences, when bees were not able to control the visual displays. When bees were presented with competing frequency-tagged visual stimuli, selectivity in the medulla (an optic ganglion) preceded behavioral selection of a stimulus, suggesting that modulation of early visual processing centers precedes eventual behavioral choices made by these insects.

Parallel processing in the honeybee olfactory pathway: structure, function, and evolution

Animals face highly complex and dynamic olfactory stimuli in their natural environments, which require fast and reliable olfactory processing. Parallel processing is a common principle of sensory systems supporting this task, for example in visual and auditory systems, but its role in olfaction remained unclear. Studies in the honeybee focused on a dual olfactory pathway. Two sets of projection neurons connect glomeruli in two antennal-lobe hemilobes via lateral and medial tracts in opposite sequence with the mushroom bodies and lateral horn. Comparative studies suggest that this dual-tract circuit represents a unique adaptation in Hymenoptera. Imaging studies indicate that glomeruli in both hemilobes receive redundant sensory input. Recent simultaneous multi-unit recordings from projection neurons of both tracts revealed widely overlapping response profiles strongly indicating parallel olfactory processing. Whereas lateral-tract neurons respond fast with broad (generalistic) profiles, medial-tract neurons are odorant specific and respond slower. In analogy to “what-” and “where” subsystems in visual pathways, this suggests two parallel olfactory subsystems providing “what-” (quality) and “when” (temporal) information. Temporal response properties may support across-tract coincidence coding in higher centers. Parallel olfactory processing likely enhances perception of complex odorant mixtures to decode the diverse and dynamic olfactory world of a social insect.

Chromatic processing in the anterior optic tubercle of the honey bee brain

Color vision in honey bees (Apis mellifera) has been extensively studied at the behavioral level and, to a lesser degree, at the physiological level by means of electrophysiological intracellular recordings of single neurons. Few visual neurons have been so far characterized in the lateral protocerebrum of bees. Therefore, the possible implication of this region in chromatic processing remains unknown. We performed in vivo calcium imaging of interneurons in the anterior optic tubercle (AOTu) of honey bees upon visual stimulation of the compound eye to analyze chromatic response properties. Stimulation with distinct monochromatic lights (ultraviolet [UV], blue, and green) matching the sensitivity of the three photoreceptor types of the bee retina induced different signal amplitudes, temporal dynamics, and spatial activity patterns in the AOTu intertubercle network, thus revealing intricate chromatic processing properties. Green light strongly activated both the dorsal and ventral lobes of the AOTu's major unit; blue light activated the dorsal lobe more while UV light activated the ventral lobe more. Eye stimulation with mixtures of blue and green light induced suppression phenomena in which responses to the mixture were lower than those to the color components, thus concurring with color-opponent processing. These data provide evidence for a spatial segregation of color processing in the AOTu, which may serve for navigation purposes.

Honeybees (Apis mellifera) learn color discriminations via differential conditioning independent of long wavelength (green) photoreceptor modulation

BACKGROUND

Recent studies on colour discrimination suggest that experience is an important factor in how a visual system processes spectral signals. In insects it has been shown that differential conditioning is important for processing fine colour discriminations. However, the visual system of many insects, including the honeybee, has a complex set of neural pathways, in which input from the long wavelength sensitive ('green') photoreceptor may be processed either as an independent achromatic signal or as part of a trichromatic opponent-colour system. Thus, a potential confound of colour learning in insects is the possibility that modulation of the 'green' photoreceptor could underlie observations.

METHODOLOGY/PRINCIPAL FINDINGS

We tested honeybee vision using light emitting diodes centered on 414 and 424 nm wavelengths, which limit activation to the short-wavelength-sensitive ('UV') and medium-wavelength-sensitive ('blue') photoreceptors. The absolute irradiance spectra of stimuli was measured and modelled at both receptor and colour processing levels, and stimuli were then presented to the bees in a Y-maze at a large visual angle (26°), to ensure chromatic processing. Sixteen bees were trained over 50 trials, using either appetitive differential conditioning (N = 8), or aversive-appetitive differential conditioning (N = 8). In both cases the bees slowly learned to discriminate between the target and distractor with significantly better accuracy than would be expected by chance. Control experiments confirmed that changing stimulus intensity in transfers tests does not significantly affect bee performance, and it was possible to replicate previous findings that bees do not learn similar colour stimuli with absolute conditioning.

CONCLUSION

Our data indicate that honeybee colour vision can be tuned to relatively small spectral differences, independent of 'green' photoreceptor contrast and brightness cues. We thus show that colour vision is at least partly experience dependent, and behavioural plasticity plays an important role in how bees exploit colour information.

Segregation of visual inputs from different regions of the compound eye in two parallel pathways through the anterior optic tubercle of the bumblebee (Bombus ignitus)

Visually guided behaviors require the brain to extract features of the visual world and to integrate them in a context-specific manner. Hymenopteran insects have been prime models for ethological research into visual behaviors for decades but knowledge about the underlying central processing is very limited. This is particularly the case for sky-compass navigation. To learn more about central processing of visual information in general and specifically to reveal a possible polarization vision pathway in the bee brain, we used tracer injections to investigate the pathways through the anterior optic tubercle, a prominent output target of the insect optic lobe, in the bumblebee Bombus ignitus. The anterior optic tubercle of the bumblebee is a small neuropil of 200 μm width and is located dorsolateral to the antennal lobe at the anterior surface of the brain. It is divided into a larger upper and a smaller lower subunit, both of which receive input from the optic lobe and connect to the lateral accessory lobe, and the contralateral tubercle, via two parallel pathways. The lower subunit receives input from the dorsal rim area (DRA) of the compound eye. The bumblebee DRA shares structural similarities with polarization-sensitive DRAs of other insects and looks similar to that of honeybees. We identified several neurons within this pathway that could be homologous to identified polarization-sensitive neurons in the locust brain. We therefore conclude that the pathway through the lower subunit of the anterior optic tubercle could carry polarization information from the periphery to the central brain.

Neural organization and visual processing in the anterior optic tubercle of the honeybee brain

The honeybee Apis mellifera represents a valuable model for studying the neural segregation and integration of visual information. Vision in honeybees has been extensively studied at the behavioral level and, to a lesser degree, at the physiological level using intracellular electrophysiological recordings of single neurons. However, our knowledge of visual processing in honeybees is still limited by the lack of functional studies of visual processing at the circuit level. Here we contribute to filling this gap by providing a neuroanatomical and neurophysiological characterization at the circuit level of a practically unstudied visual area of the bee brain, the anterior optic tubercle (AOTu). First, we analyzed the internal organization and neuronal connections of the AOTu. Second, we established a novel protocol for performing optophysiological recordings of visual circuit activity in the honeybee brain and studied the responses of AOTu interneurons during stimulation of distinct eye regions. Our neuroanatomical data show an intricate compartmentalization and connectivity of the AOTu, revealing a dorsoventral segregation of the visual input to the AOTu. Light stimuli presented in different parts of the visual field (dorsal, lateral, or ventral) induce distinct patterns of activation in AOTu output interneurons, retaining to some extent the dorsoventral input segregation revealed by our neuroanatomical data. In particular, activity patterns evoked by dorsal and ventral eye stimulation are clearly segregated into distinct AOTu subunits. Our results therefore suggest an involvement of the AOTu in the processing of dorsoventrally segregated visual information in the honeybee brain.

Colour processing in complex environments: insights from the visual system of bees

Colour vision enables animals to detect and discriminate differences in chromatic cues independent of brightness. How the bee visual system manages this task is of interest for understanding information processing in miniaturized systems, as well as the relationship between bee pollinators and flowering plants. Bees can quickly discriminate dissimilar colours, but can also slowly learn to discriminate very similar colours, raising the question as to how the visual system can support this, or whether it is simply a learning and memory operation. We discuss the detailed neuroanatomical layout of the brain, identify probable brain areas for colour processing, and suggest that there may be multiple systems in the bee brain that mediate either coarse or fine colour discrimination ability in a manner dependent upon individual experience. These multiple colour pathways have been identified along both functional and anatomical lines in the bee brain, providing us with some insights into how the brain may operate to support complex colour discrimination behaviours.

Allatostatin immunoreactivity in the honeybee brain

Information transmission and processing in the brain is achieved through a small family of chemical neurotransmitters and neuromodulators and a very large family of neuropeptides. In order to understand neural networks in the brain it will be necessary, therefore, to understand the connectivity, morphology, and distribution of peptidergic neurons, and to elucidate their function in the brain. In this study we characterize the distribution of substances related to Dip-allatostatin I in the honeybee brain, which belongs to the allatostatin-A (AST) peptide family sharing the conserved c-terminal sequence -YXFGL-NH(2). We found about 500 AST-immunoreactive (ASTir) neurons in the brain, scattered in 18 groups that varied in their precise location across individuals. Almost all areas of the brain were innervated by ASTir fibers. Most ASTir neurites formed networks within functionally distinct areas, e.g., the antennal lobes, the mushroom bodies, or the optic lobes, indicating local functions of the peptide. A small number of very large neurons had widespread arborizations and neurites were found in the corpora cardiaca and in the cervical connectives, suggesting that AST also has global functions. We double-stained AST and GABA and found that a subset of ASTir neurons were GABA-immunoreactive (GABAir). Double staining AST with backfills of olfactory receptor neurons or mass fills of neurons in the antennal lobes and in the mushroom bodies allowed a more fine-grained description of ASTir networks. Together, this first comprehensive description of AST in the bee brain suggests a diverse functional role of AST, including local and global computational tasks.

In situ hybridization analysis of the expression of futsch, tau, and MESK2 homologues in the brain of the European honeybee (Apis mellifera L.)

BACKGROUND

The importance of visual sense in Hymenopteran social behavior is suggested by the existence of a Hymenopteran insect-specific neural circuit related to visual processing and the fact that worker honeybee brain changes morphologically according to its foraging experience. To analyze molecular and neural bases that underlie the visual abilities of the honeybees, we used a cDNA microarray to search for gene(s) expressed in a neural cell-type preferential manner in a visual center of the honeybee brain, the optic lobes (OLs).

METHODOLOGY/PRINCIPAL FINDINGS

Expression analysis of candidate genes using in situ hybridization revealed two genes expressed in a neural cell-type preferential manner in the OLs. One is a homologue of Drosophila futsch, which encodes a microtubule-associated protein and is preferentially expressed in the monopolar cells in the lamina of the OLs. The gene for another microtubule-associated protein, tau, which functionally overlaps with futsch, was also preferentially expressed in the monopolar cells, strongly suggesting the functional importance of these two microtubule-associated proteins in monopolar cells. The other gene encoded a homologue of Misexpression Suppressor of Dominant-negative Kinase Suppressor of Ras 2 (MESK2), which might activate Ras/MAPK-signaling in Drosophila. MESK2 was expressed preferentially in a subclass of neurons located in the ventral region between the lamina and medulla neuropil in the OLs, suggesting that this subclass is a novel OL neuron type characterized by MESK2-expression. These three genes exhibited similar expression patterns in the worker, drone, and queen brains, suggesting that they function similarly irrespective of the honeybee sex or caste.

CONCLUSIONS

Here we identified genes that are expressed in a monopolar cell (Amfutsch and Amtau) or ventral medulla-preferential manner (AmMESK2) in insect OLs. These genes may aid in visualizing neurites of monopolar cells and ventral medulla cells, as well as in analyzing the function of these neurons.

Analysis of GABAergic and non-GABAergic neuron activity in the optic lobes of the forager and re-orienting worker honeybee (Apis mellifera L.)

Background

European honeybee (Apis mellifera L.) foragers have a highly developed visual system that is used for navigation. To clarify the neural basis underlying the highly sophisticated visual ability of foragers, we investigated the neural activity pattern of the optic lobes (OLs) in pollen-foragers and re-orienting bees, using the immediate early gene kakusei as a neural activity marker.

Methodology/Principal Findings

We performed double-in situ hybridization of kakusei and Amgad, the honeybee homolog of the GABA synthesizing enzyme GAD, to assess inhibitory neural activity. kakusei-related activity in GABAergic and non-GABAergic neurons was strongly upregulated in the OLs of the foragers and re-orienting bees, suggesting that both types of neurons are involved in visual information processing. GABAergic neuron activity was significantly higher than non-GABAergic neuron activity in a part of the OLs of only the forager, suggesting that unique information processing occurs in the OLs of foragers. In contrast, GABAergic neuron activity in the antennal lobe was significantly lower than that of GABAergic neurons in the OLs in the forager and re-orienting bees, suggesting that kakusei-related visual activity is dominant in the brains of these bees.

Conclusions/Significance

The present study provides the first evidence that GABAergic neurons are highly active in the OL neurons of free-moving honeybees and essential clue to reveal neural basis of the sophisticated visual ability that is equipped in the small and simple brain.

Visual processing in the central bee brain

Visual scenes comprise enormous amounts of information from which nervous systems extract behaviorally relevant cues. In most model systems, little is known about the transformation of visual information as it occurs along visual pathways. We examined how visual information is transformed physiologically as it is communicated from the eye to higher-order brain centers using bumblebees, which are known for their visual capabilities. We recorded intracellularly in vivo from 30 neurons in the central bumblebee brain (the lateral protocerebrum) and compared these neurons to 132 neurons from more distal areas along the visual pathway, namely the medulla and the lobula. In these three brain regions (medulla, lobula, and central brain), we examined correlations between the neurons' branching patterns and their responses primarily to color, but also to motion stimuli. Visual neurons projecting to the anterior central brain were generally color sensitive, while neurons projecting to the posterior central brain were predominantly motion sensitive. The temporal response properties differed significantly between these areas, with an increase in spike time precision across trials and a decrease in average reliable spiking as visual information processing progressed from the periphery to the central brain. These data suggest that neurons along the visual pathway to the central brain not only are segregated with regard to the physical features of the stimuli (e.g., color and motion), but also differ in the way they encode stimuli, possibly to allow for efficient parallel processing to occur.

Color processing in the medulla of the bumblebee (Apidae: Bombus impatiens)

The mechanisms of processing a visual scene involve segregating features (such as color) into separate information channels at different stages within the brain, processing these features, and then integrating this information at higher levels in the brain. To examine how this process takes place in the insect brain, we focused on the medulla, an area within the optic lobe through which all of the visual information from the retina must pass before it proceeds to central brain areas. We used histological and immunocytochemical techniques to examine the bumblebee medulla and found that the medulla is divided into eight layers. We then recorded and morphologically identified 27 neurons with processes in the medulla. During our recordings we presented color cues to determine whether response types correlated with locations of the neural branching patterns of the filled neurons among the medulla layers. Neurons in the outer medulla layers had less complex color responses compared to neurons in the inner medulla layers and there were differences in the temporal dynamics of the responses among the layers. Progressing from the outer to the inner medulla, neurons in the different layers appear to process increasingly complex aspects of the natural visual scene.

Higher order visual input to the mushroom bodies in the bee, Bombus impatiens

To produce appropriate behaviors based on biologically relevant associations, sensory pathways conveying different modalities are integrated by higher-order central brain structures, such as insect mushroom bodies. To address this function of sensory integration, we characterized the structure and response of optic lobe (OL) neurons projecting to the calyces of the mushroom bodies in bees. Bees are well known for their visual learning and memory capabilities and their brains possess major direct visual input from the optic lobes to the mushroom bodies. To functionally characterize these visual inputs to the mushroom bodies, we recorded intracellularly from neurons in bumblebees (Apidae: Bombus impatiens) and a single neuron in a honeybee (Apidae: Apis mellifera) while presenting color and motion stimuli. All of the mushroom body input neurons were color sensitive while a subset was motion sensitive. Additionally, most of the mushroom body input neurons would respond to the first, but not to subsequent, presentations of repeated stimuli. In general, the medulla or lobula neurons projecting to the calyx signaled specific chromatic, temporal, and motion features of the visual world to the mushroom bodies, which included sensory information required for the biologically relevant associations bees form during foraging tasks.

The processing of color, motion, and stimulus timing are anatomically segregated in the bumblebee brain

Animals use vision to perform such diverse behaviors as finding food, interacting socially with other animals, choosing a mate, and avoiding predators. These behaviors are complex and the visual system must process color, motion, and pattern cues efficiently so that animals can respond to relevant stimuli. The visual system achieves this by dividing visual information into separate pathways, but to what extent are these parallel streams separated in the brain? To answer this question, we recorded intracellularly in vivo from 105 morphologically identified neurons in the lobula, a major visual processing structure of bumblebees (Bombus impatiens). We found that these cells have anatomically segregated dendritic inputs confined to one or two of six lobula layers. Lobula neurons exhibit physiological characteristics common to their respective input layer. Cells with arborizations in layers 1-4 are generally indifferent to color but sensitive to motion, whereas layer 5-6 neurons often respond to both color and motion cues. Furthermore, the temporal characteristics of these responses differ systematically with dendritic branching pattern. Some layers are more temporally precise, whereas others are less precise but more reliable across trials. Because different layers send projections to different regions of the central brain, we hypothesize that the anatomical layers of the lobula are the structural basis for the segregation of visual information into color, motion, and stimulus timing.

Eph receptor and ephrin signaling in developing and adult brain of the honeybee (Apis mellifera)

Roles for Eph receptor tyrosine kinase and ephrin signaling in vertebrate brain development are well established. Their involvement in the modulation of mammalian synaptic structure and physiology is also emerging. However, less is known of their effects on brain development and their function in adult invertebrate nervous systems. Here, we report on the characterization of Eph receptor and ephrin orthologs in the honeybee, Apis mellifera (Am), and their role in learning and memory. In situ hybridization for mRNA expression showed a uniform distribution of expression of both genes across the developing pupal and adult brain. However, in situ labeling with Fc fusion proteins indicated that the AmEphR and Amephrin proteins were differentially localized to cell body regions in the mushroom bodies and the developing neuropiles of the antennal and optic lobes. In adults, AmEphR protein was localized to regions of synaptic contacts in optic lobes, in the glomeruli of antennal lobes, and in the medial lobe of the mushroom body. The latter two regions are involved in olfactory learning and memory in the honeybee. Injections of EphR-Fc and ephrin-Fc proteins into the brains of adult bees, 1 h before olfactory conditioning of the proboscis extension reflex, significantly reduced memory 24 h later. Experimental amnesia in the group injected with ephrin-Fc was apparent 1 h post-training. Experimental amnesia was also induced by post-training injections with ephrin-Fc suggesting a role in recall. This is the first demonstration that Eph molecules function to regulate the formation of memory in insects.

Patterns of PERIOD and pigment-dispersing hormone immunoreactivity in the brain of the European honeybee (Apis mellifera): age- and time-related plasticity

We explored the neural basis of age- and task-related plasticity in circadian patterns of activity in the honeybee. To identify putative circadian pacemakers in the bee brain, we used antibodies against Drosophila melanogaster and Antheraea pernyi PERIOD and an antiserum to crustacean pigment-dispersing hormone (PDH) known to cross-react with insect pigment-dispersing factors (PDFs). In contrast to previous results from Drosophila, PDH and PER immunoreactivity (-ir) were not colocalized in bee neurons. The most intense PER-ir was cytoplasmic, in two groups of large neurons in the protocerebrum. The number of protocerebral PER-ir neurons and PER-ir intensity within individual cells were highest in brains collected during subjective night and higher in old bees than in young bees. These results are consistent with previous analyses of brain per mRNA in honeybees. Nuclear PER-ir was found throughout the brain, including the optic and antennal lobes. A single group of PDH-ir neurons (approximately 20/optic lobe) was consistently and intensely labeled at the medial margin of the medulla, independent of age or time of day. The processes of these neurons extended to specific neuropils in the protocerebrum and the optic lobes but not to the deutocerebrum. The patterns displayed by PER- and PDH-ir do not completely match any patterns previously described. This suggests that, although clock proteins are conserved across insect groups, there is no universal pattern of coexpression that allows ready identification of pacemaker neurons within the insect brain.

Lung development and function in preterm infants in the surfactant treatment era

Mortality of infants of < 1-kg birth weight has decreased because of surfactant treatments, antenatal glucocorticoid treatments, and new ventilation strategies. However, many of these infants develop a chronic lung disease characterized by an arrest of lung development and interference with alveolarization. Antenatal glucocorticoids can induce early lung maturation clinically, but new information from transgenic and other experimental models indicates that traditional explanations for glucocorticoid effects on the developing lung are inadequate. These very preterm infants have lungs with small lung gas volumes and delicate lung tissue that are susceptible to injury with the initiation of ventilation and subsequent ventilation. Antenatal proinflammatory exposures are frequent in very preterm infants, and postnatal injury is associated with elevations of proinflammatory cytokines in the lungs. One hypothesis is that proinflammatory cytokines can promote or interfere with lung development as well as promote lung injury. Mechanisms of lung injury being characterized in the adult lung may have unique characteristics in the developing lung.

Role of transcription factors in fetal lung development and surfactant protein gene expression

Branching morphogenesis of the lung and differentiation of specialized cell populations is dependent upon reciprocal interactions between epithelial cells derived from endoderm of embryonic foregut and surrounding mesenchymal cells. These interactions are mediated by elaboration and concerted actions of a variety of growth and differentiation factors binding to specific receptors. Such factors include members of the fibroblast growth factor family, sonic hedgehog, members of the transforming growth factor-beta family, epidermal growth factor, and members of the platelet-derived growth factor family. Hormones that increase cyclic AMP formation, glucocorticoids, and retinoids also play important roles in branching morphogenesis, alveolar development, and cellular differentiation. Expression of the genes encoding these morphogens and their receptors is controlled by a variety of transcription factors that also are highly regulated. Several of these transcription factors serve dual roles as regulators of genes involved in early lung development and in specialized functions of differentiated cells. Targeted null mutations of genes encoding many of these morphogens and transcription factors have provided important insight into their function during lung development. In this chapter, the cellular and molecular mechanisms that control lung development are considered, as well as those that regulate expression of the genes encoding the surfactant proteins.

Monoclonal antibody labels olfactory and visual pathways in Drosophila and Apis brains

We employed a monoclonal antibody raised against Drosophila brain homogenate for a comparative immunocytochemical analysis of visual and olfactory pathways in brains of two insect species. On Western blots of Drosophila and Apis nervous tissue, antibody fb45 recognized an antigen with an apparent molecular weight higher than 180 kD. Application of the antibody to sections of Drosophila and Apis brain stained certain interneurons which conspicuously fasciculate in common tracts or neuropilar compartments. Both in Drosophila and in Apis, the antigen was also expressed on the perineural sheath and granular cell compartments in the majority of neuronal cell bodies. The antibody stained monopolar cells in the visual system of both species, and in Apis those fibers of the anterior superior optic tract which link the medulla with the mushroom bodies. In Drosophila, bundles of Kenyon cells of the mushroom bodies were stained. In worker bees and drones, the relay neurons of the median and lateral antennoglomerular tracts were labelled. Since the recognition of the antigen does not require fixation, the antibody can be employed to label selectively living neurons in dissociated cell culture. This opens up the possibility for future functional studies on the role of the antigen in vitro.

Common projection areas of antennal and visual pathways in the honeybee brain, Apis mellifera

The convergence of primary sensory neurons of the antennae, higher order visual interneurons, and antennal motoneurons was analysed with neuroanatomical techniques in the honeybee, Apis mellifera. The different modalities evoke specific antennal responses in this insect. Three different fluorescent dyes were applied successively in the same preparation in order to visualise the various fiber projections from the antennae and the lobula in the brain of the honeybee. Three neuropile areas where sensory fibers of the antennae overlap with visual projection neurons from the lobula were found. Within the posterior-median protocerebrum the antennal tract T6-1 comes in close vicinity to the lobula tract LoT-9 and to some other lobula fibers that cannot be assigned to a special tract. Antennal T6-3 fibers overlap with lobula LoT-7 neurons within the posterior protocerebrum more laterally. Antennal T5 fibers arborise in the dorsal lobe and show common projection sites with lobula LoT-3 neurons. The multimodal convergence in the three common neuropiles demonstrates that these areas are important centers for multimodal information processing between sensory, motor, and descending neurons in insects.

FMRFamide-like immunoreactivity in the brain of the honeybee (Apis mellifera). A light-and electron microscopical study

Peptide-FMRFamide-like immunoreactivity in the brain and suboesophageal ganglion of the honeybee Apis mellifera L. is demonstrated with the peroxidase-antiperoxidase technique. Immunoreactivity is found in about 120 perikarya of the brain and in about 30 of the suboesophageal ganglion. These cells are distributed in 13 paired clusters representing neurons of different types including neurosecretory neurons projecting to neurohemal organs. Immunoreactivity of different intensity is found in the non-glomerular neuropil around the mushroom bodies, in the lateral protocerebrum, the central body, the optic tubercles, the lobula and medulla of optic lobe, the ocellar neuropil, in multiglomerular elements of the antennal lobes and in the dorsal deuterocerebrum. In the mushroom bodies, immunoreactivity is located in layers of the lobes and stalks, corresponding to intrinsic fibre bundles of some Kenyon cell types. The somata of these intrinsic cells did not show FMRFamide-like immunoreactivity. Electron microscopy of immunostained somata and nerve fibres was performed employing a pre-embedding peroxidase-antiperoxidase technique. Fibres of optic lobes and the non-glomerular neuropil contain immunoreactive dense core vesicles (diameter 50-165 nm) accumulated in boutons besides small synaptic vesicles and synaptic membrane specializations. Immunoreactive layers of the mushroom body neuropil were analysed at the ultrastructural level. Axon profiles with dense-core vesicles of a small type (diameter 35-75 nm) show only faint immunoreactive products. Immunoreactivity of intrinsic mushroom body neurons does not appear to be specifically correlated with synaptic organelles. Our results indicate that FMRFamide or related peptides peptides may be neuroactive compounds in different classes of nerve cells in the bee brain.

Histochemistry of acetylcholinesterase and immunocytochemistry of an acetylcholine receptor-like antigen in the brain of the honeybee

A histochemical staining method for acetylcholinesterase (AChE) and an antiserum raised against nicotinic acetylcholine receptors (AChR) of locust nervous tissue were applied in order to reveal certain candidates of cholinergic pathways in the brain of the honeybee. The AChE staining marked layers in the optic lobes, fibers connecting the two brain hemispheres, and fiber tracts as well as soma clusters within the protocerebrum. The calycal input regions of the mushroom bodies were labelled, whereas the intrinsic Kenyon cells showed no staining. Although the antennal afferents projecting into the dorsal lobe showed strong AChE activity, projections into the antennal lobe showed rather weak staining. Application of the antiserum against the AChR showed immunoreactivity in neuropiles, tracts, somata, and the antennal nerve. The immunoreactivity of the optic lobes coincided with the banding pattern of the AChE staining. A particularly striking overlap of AChR immunoreactivity and AChE staining was found in the lip neuropile of the mushroom bodies, which would suggest a cholinergic input into this neuropile via fibers of the median antennoglomerular tract. Because the antiserum against locust AChR binds in neuropiles displaying AChE activity, we conclude that this antiserum also cross-reacts with the bee's receptor. This interpretation is supported by experiments showing alpha-bungarotoxin (alpha-BTX) binding sites in some areas of strong immunoreactivity.

Distribution of GABA-like immunoreactivity in the brain of the honeybee

The distribution of GABA-like immunoreactivity in the brain of the honeybee was investigated with antisera generated against GABA protein conjugates. The binding of the antisera in paraffin serial sections was studied with the peroxidase-antiperoxidase method. GABA-like immunoreactive fibers appeared in all main neuropile areas. The staining of the optic lobes showed pronounced stratification. The receptor cells of compound eyes, ocelli, and antennae were not labelled. Several prominent fiber tracts showed GABA-like immunoreactivity, whereas other tracts were devoid of staining. There are no major immunoreactive commissures linking the two brain hemispheres with the exception of small commissures that bridge short distances between the beta-lobes and the antennal lobes. Several fibers in the cervical connective were also labelled; some of those may descend from the suboesophageal ganglion to the thoracic ganglia. The dense reactivity seen in the optic and antennal neuropiles implies that GABA is more important in mediating local rather than more distant neural interactions.

Insect optic lobe neurons identifiable with monoclonal antibodies to GABA

Five monoclonal antibodies against GABA were tested on glutaraldehyde fixed sections of optic lobes of three insect species, blowflies, houseflies and worker bees. The specificity of these antibodies was analyzed in several tests and compared with commercially available anti-GABA antiserum. A very large number of GABA-like immunoreactive neurons innervate all the neuropil regions of these optic lobes. Immunoreactive processes are found in different layers of the neuropils. The immunoreactive neurons are amacrines and columnar or noncolumnar neurons connecting the optic lobe neuropils. In addition some large immunoreactive neurons connect the optic lobes with centers of the brain. Some neuron types could be matched with neurons previously identified with other methods. The connections of a few of these neuron types are partly known from electron microscopy or electrophysiology and a possible role of GABA in certain neural circuits can be discussed.