Current source-density analysis in the mushroom bodies of the honeybee (Apis mellifera carnica)

Alpha oscillations govern interhemispheric spike timing coordination in the honey bee brain

In 1929 Hans Berger discovered the alpha oscillations: prominent, ongoing oscillations around 10 Hz in the electroencephalogram of the human brain. These alpha oscillations are among the most widely studied brain signals, related to cognitive phenomena such as attention, memory and consciousness. However, the mechanisms by which alpha oscillations affect human cognition await demonstration. Here, we suggest the honey bee brain as an experimentally more accessible model system for investigating the functional role of alpha oscillations. We found a prominent spontaneous oscillation around 18 Hz that is reduced in amplitude upon olfactory stimulation. Similar to alpha oscillations in primates, the phase of this oscillation biased both timing of neuronal spikes and amplitude of high-frequency gamma activity (40-450 Hz). These results suggest a common role of alpha oscillations across phyla and provide an unprecedented new venue for causal studies on the relationship between neuronal spikes, brain oscillations and cognition.

Current Source Density Analysis of Electroantennogram Recordings: A Tool for Mapping the Olfactory Response in an Insect Antenna

The set of chemosensory receptors expressed by the olfactory receptor neurons lying in an insect's antennae and maxillary palps define the ability of this insect to perceive the volatile chemicals of its environment. The main two electrophysiological methods of antennal recordings for studying the range of chemicals that activate chemosensory receptors have limitations. Single-sensillum recording (SSR) samples a subset of olfactory receptor neurons and therefore does not reveal the full capacity of an insect to perceive an odor. Electroantennography (EAG), even if less resolutive than SSRs, is sometimes preferred since it samples the activity of a large number of the olfactory receptor neurons. But, at least in flies, the amplitude of the EAG signal is not directly correlated with the degree of sensitivity of the insect to the olfactory compound. Such dual methodology was also used to study mammalian brains, and the current source density (CSD) analysis was developed to bridge the gap between the cellular and the population recordings. This paper details the use of a similar approach adapted to the study of olfactory responses within insects with bulbous antennae. The EAG was recorded at multiple antennal positions and the CSD that generates the EAG potentials were estimated. The method measures the activation of olfactory receptor neurons (ORNs) across the antennae and thus it quantifies the olfactory sensitivity of the insect. It allows a rapid mapping of olfactory responses and thus can be used to guide further SSRs or to determine that two chemicals are detected by independent ORNs. This study further explored biases resulting from a limited number of recording positions or from an approximation of the antennal geometry that should be considered for interpreting the CSD maps. It also shows that the CSD analysis of EAGs is compatible with a gas chromatograph stimulator for analyzing the response to complex odors. Finally, I discuss the origin of the EAG signal in light of the CSD theory.

Current source density mapping of antennal sensory selectivity reveals conserved olfactory systems between tephritids and Drosophila

Ecological specialization of insects involves the functional and morphological reshaping of olfactory systems. Little is known about the degree to which insect sensitivity to odorant compounds is conserved between genera, tribes, or families. Here we compared the olfactory systems of six tephritid fruit fly species spanning two tribes and the distantly related Drosophila melanogaster at molecular, functional, and morphological levels. Olfaction in these flies is mediated by a set of olfactory receptors (ORs) expressed in different functional classes of neurons located in distinct antennal regions. We performed a phylogenetic analysis that revealed both family-specific OR genes and putative orthologous OR genes between tephritids and Drosophila. With respect to function, we then used a current source density (CSD) analysis to map activity across antennae. Functional maps mirrored the intrinsic structure of antennae observed with scanning electron microscopy. Together, the results revealed partial conservation of the olfactory systems between tephritids and Drosophila. We also demonstrate that the mapping of olfactory responses is necessary to decipher antennal sensory selectivity to olfactory compounds. CSD analysis can be easily applied to map antennae of other species and therefore enables the rapid deriving of olfactory maps and the reconstructing of the target organisms' history of evolution.

Learning performance and brain structure of artificially-reared honey bees fed with different quantities of food

Background

Artificial rearing of honey bee larvae is an established method which enables to fully standardize the rearing environment and to manipulate the supplied diet to the brood. However, there are no studies which compare learning performance or neuroanatomic differences of artificially-reared (in-lab) bees in comparison with their in-hive reared counterparts.

Methods

Here we tested how different quantities of food during larval development affect body size, brain morphology and learning ability of adult honey bees. We used in-lab rearing to be able to manipulate the total quantity of food consumed during larval development. After hatching, a subset of the bees was taken for which we made 3D reconstructions of the brains using confocal laser-scanning microscopy. Learning ability and memory formation of the remaining bees was tested in a differential olfactory conditioning experiment. Finally, we evaluated how bees reared with different quantities of artificial diet compared to in-hive reared bees.

Results

Thorax and head size of in-lab reared honey bees, when fed the standard diet of 160 µl or less, were slightly smaller than hive bees. The brain structure analyses showed that artificially reared bees had smaller mushroom body (MB) lateral calyces than their in-hive counterparts, independently of the quantity of food they received. However, they showed the same total brain size and the same associative learning ability as in-hive reared bees. In terms of mid-term memory, but not early long-term memory, they performed even better than the in-hive control.

Discussion

We have demonstrated that bees that are reared artificially (according to the Aupinel protocol) and kept in lab-conditions perform the same or even better than their in-hive sisters in an olfactory conditioning experiment even though their lateral calyces were consistently smaller at emergence. The applied combination of experimental manipulation during the larval phase plus subsequent behavioral and neuro-anatomic analyses is a powerful tool for basic and applied honey bee research.

A High-Bandwidth Dual-Channel Olfactory Stimulator for Studying Temporal Sensitivity of Olfactory Processing

Animals encounter fine-scale temporal patterns of odorant mixtures that contain information about the distance and number of odorant sources. To study the role of such temporal cues for odorant detection and source localization, one needs odorant delivery devices that are capable of mimicking the temporal stimulus statistics of natural odor plumes. However, current odorant delivery devices either lack temporal resolution or are limited to a single odorant channel. Here, we present an olfactory stimulator that features precise control of high-bandwidth stimulus dynamics, which allows generating arbitrary fluctuating binary odorant mixtures. We provide a comprehensive characterization of the stimulator's performance and use it to demonstrate that odor background affects the temporal resolution of insect olfactory receptor neurons, and we present a hitherto unknown odor pulse-tracking capability of up to 60 Hz in Kenyon cells, which are higher order olfactory neurons of the insect brain. This stimulator might help investigating whether and how animals use temporal stimulus cues for odor detection and source localization. Because the stimulator is easy to replicate it can facilitate generating the same odor stimulus dynamics at different experimental setups and across different labs.

Mushroom body volume is related to social aggression and ovary development in the paperwasp Polistes instabilis

The mushroom bodies (MB) are a complex neuropil in insect brains that have been implicated in higher-order information processing such as sensory integration and various types of learning and memory. Eusocial insects are excellent models to test functional neural plasticity in the MB because genetically related nest mates differ in task performance, environmental experience and social interactions. Previous research on eusocial insects shows that experience-dependent changes in brain anatomy (i.e., enlarged MB calyces) are positively correlated with task performance and social interactions. In this study, we quantified relationships of task performance and social and reproductive dominance with MB volume in Polistes instabilis, a primitively eusocial paper wasp. We used experimental removals of dominant workers to induce changes in aggressive behavior and foraging by workers. Ovary development and social dominance were positively associated with the volume of the MB calyces relative to the region containing the Kenyon cell bodies. In contrast to highly eusocial insect workers, foraging behavior was not positively correlated with MB calycal volume. We conclude that mushroom body volume is more strongly associated with dominance rank than with foraging behavior in Polistes instabilis.

Electrical potentials indicate stimulus expectancy in the brains of ants and bees

In vertebrates, and in humans in particular, so-called 'omitted stimulus potentials' can be electrically recorded from the brain or scalp upon repeated stimulation with simple stimuli such as light flashes. While standard evoked potentials follow each stimulus in a series, 'omitted stimulus potentials' occur when an additional stimulus is expected after the end of a stimulus series. These potentials represent neuronal plasticity and are assumed to be involved in basic cognitive processes. We recorded electroretinograms from the eyes and visually evoked potentials from central brain areas of honey bees and ants, social insects to which cognitive abilities have been ascribed and whose rich-behavioral repertoires include navigation, learning and memory. We demonstrate that omitted stimulus potentials occur in these insects. Omitted stimulus potentials in bees and ants show similar temporal characteristics to those found in crayfish and vertebrates, suggesting that common mechanisms may underlie this form of short-term neuronal plasticity.

Brain Allometry in Bumblebee and Honey Bee Workers

Within a particular animal taxon, larger bodied species generally have larger brains. Increased brain size usually correlates with increased behavioral repertoires and often with superior cognitive abilities. Bumblebees are eusocial insects that show pronounced size polymorphism among workers, whereas in honey bees size variation is much less pronounced. Recent studies suggest that within a given colony, large bumblebee workers are more efficient foragers and are better learners than their smaller sisters. Here we examine the allometric relationship between brain and body size of worker bumblebees and honey bees. We find that larger bees have larger brains and that most brain components show a similar size increase as the overall brain. One particular brain structure, the central body, is relatively smaller in large bumblebees compared to small bees. The same is true for the mushroom body lobes, whereas the mushroom body calyces, which receive sensory input, are not reduced in larger bumblebees or honey bees. Honey bees have relatively smaller brains, as well as smaller mushroom bodies, than bumblebee workers. We discuss why brain or mushroom body size does not necessarily correlate with the degree of a species' social organization.

Subdivisions of hymenopteran mushroom body calyces by their afferent supply

The mushroom bodies are regions in the insect brain involved in processing complex multimodal information. They are composed of many parallel sets of intrinsic neurons that receive input from and transfer output to extrinsic neurons that connect the mushroom bodies with the surrounding neuropils. Mushroom bodies are particularly large in social Hymenoptera and are thought to be involved in the control of conspicuous orientation, learning, and memory capabilities of these insects. The present account compares the organization of sensory input to the mushroom body's calyx in different Hymenoptera. Tracer and conventional neuronal staining procedures reveal the following anatomic characteristics: The calyx comprises three subdivisions, the lip, collar, and basal ring. The lip receives antennal lobe afferents, and these olfactory input neurons can terminate in two or more segregated zones within the lip. The collar receives visual afferents that are bilateral with equal representation of both eyes in each calyx. Visual inputs provide two to three layers of processes in the collar subdivision. The basal ring is subdivided into two modality-specific zones, one receiving visual, the other antennal lobe input. Some overlap of modality exists between calycal subdivisions and within the basal ring, and the degree of segregation of sensory input within the calyx is species-specific. The data suggest that the many parallel channels of intrinsic neurons may each process different aspects of sensory input information.

Cholinergic synaptic transmission in insect mushroom bodies in vitro

The mushroom body of the bee brain is an important site for learning and memory. Here we investigate synaptic transmission in the mushroom body using extracellular recording techniques in a whole bee brain in vitro preparation. The postsynaptic response showed attenuation by cadmium and paired-pulse facilitation, similar to in vivo findings. This confirms the viability of the in vitro preparation and supports the isolated whole bee brain as a useful model of the in vivo preparation. Bath application of the acetylcholine receptor antagonists, D-tubocurarine and alpha-bungarotoxin attenuated the postsynaptic response by 61 and 62% of control, respectively. The glutamate receptor antagonists, (+)-2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitroquinoxaline-2,3-dione, had no effect. The invertebrate monoamine and neuromodulator, octopamine, transiently increased the postsynaptic response by 130% of control. These results suggest that synaptic transmission of the olfactory input pathway in the mushroom body is 1) mediated primarily by acetylcholine and 2) modulated by octopamine.

Representation of the calyces in the medial and vertical lobes of cockroach mushroom bodies

Previous studies of honey bee and cockroach mushroom bodies have proposed that afferent terminals and intrinsic neurons (Kenyon cells) in the calyces are arranged according to polar coordinates. It has been suggested that there is a transformation by Kenyon cell axons of the polar arrangements of their dendrites in the calyces to laminar arrangements of their terminals in the lobes. Findings presented here show that cellular organization in the calyx of an evolutionarily basal neopteran, Periplaneta americana, is instead rectilinear, as it is in the lobes. It is shown that each calyx is divided into two halves (hemicalyces), each supplied by its own set of Kenyon cells. Each calyx is separately represented in the medial lobe where the dendritic trees of some efferent neurons receive inputs from one calyx only. Kenyon cell dendrites are arranged as narrow elongated fields, organized as rows in each hemicalyx. Dendritic fields arise from 14 to 16 sheets of Kenyon cell axons stacked on top of each other lining the inner surface of the calyx cup. A sheet consists of approximately 60 small bundles, each containing 5-15 axons that converge from the rim of the calyx to its neck. Each sheet contributes to a pair oflaminae, one dark one pale, called a doublet, that extends through the mushroom body. Dark laminae contain Kenyon cell axons packed with synaptic vesicles. Axons in pale laminae are sparsely equipped with vesicles. By analogy with photoreceptors, and with reference to field potential recordings, it is speculated that dark laminae are continuously active, being modulated by odor stimuli, whereas pale laminae are intermittently activated. Timm's silver staining and immunocytology reveal a second type of longitudinal division of the lobes. Five layers extend through the pedunculus and lobes, each composed of subsets of doublets. Four layers represent zones of afferent endings in the calyces. A fifth (the y layer) represents a specific type of Kenyon cell. It is concluded that the mushroom bodies comprise two independent modular systems, doublets and layers. Developmental studies show that new doublets are added at each instar to layers that are already present early in second instar nymphs. There are profound similarities between the mushroom bodies of Periplaneta, an evolutionarily basal taxon, and those of Drosophila melanogaster and the honey bee.

Evolution, Discovery, and Interpretations of Arthropod Mushroom Bodies

Mushroom bodies are prominent neuropils found in annelids and in all arthropod groups except crustaceans. First explicitly identified in 1850, the mushroom bodies differ in size and complexity between taxa, as well as between different castes of a single species of social insect. These differences led some early biologists to suggest that the mushroom bodies endow an arthropod with intelligence or the ability to execute voluntary actions, as opposed to innate behaviors. Recent physiological studies and mutant analyses have led to divergent interpretations. One interpretation is that the mushroom bodies conditionally relay to higher protocerebral centers information about sensory stimuli and the context in which they occur. Another interpretation is that they play a central role in learning and memory. Anatomical studies suggest that arthropod mushroom bodies are predominately associated with olfactory pathways except in phylogenetically basal insects. The prominent olfactory input to the mushroom body calyces in more recent insect orders is an acquired character. An overview of the history of research on the mushroom bodies, as well as comparative and evolutionary considerations, provides a conceptual framework for discussing the roles of these neuropils.

Long-term synaptic plasticity in the honeybee

A monosynaptic response was recorded in vivo in the mushroom body of the bee brain, an important site for memory consolidation. Focal electrical stimulation of a major afferent input evoked an extracellular field potential that consisted of a presynaptic fiber volley and a postsynaptic response. We report a long-lasting potentiation of the synaptic response (2.6-fold increase; < or = 3.5 h). Potentiation of the response was induced by low-frequency stimulation (0.02-1.0 Hz), was input specific, and was maintained in the absence of stimulation. Paired-pulse facilitation of the response was converted to paired-pulse depression after potentiation, suggesting a presynaptic mechanism. This is the first demonstration of long-term synaptic plasticity in the insect brain.

Neuroanatomy: mushrooming mushroom bodies

A revolution is spreading in the study of mushroom bodies, structures within the insect brain that mediate learning and memory processes and pheromonal discrimination of the opposite sex.

Development and experience lead to increased volume of subcompartments of the honeybee mushroom body

The mushroom bodies of insects are believed to be involved in higher order sensory integration and learning. In the honeybee, the mushroom body can be separated into three different, modality-specific input compartments and several morphologically inseparable output regions. By means of morphometric analysis we show that the volumes of these subcompartments depend on both the age of the adult bee and its experience. For the most part a significant, age-dependent increase in neuropile volume is observed. Additionally, the olfactory and visual input regions show experience-related differences. Unlike other subcompartments, the visual input region does not change in volume with age, but only with experience. We thus suggest that experience is an important factor in the structural development of higher order brain regions of an insect, the honeybee.

Two visual systems in one brain: neuropils serving the secondary eyes of the spider Cupiennius salei

Like other araneans, the wandering spider Cupiennius salei is equipped with one pair of principal eyes and three pairs of secondary eyes. Primary and secondary eyes serve two distinct sets of visual neuropils in the brain. This paper describes cellular organization in neuropils supplied by the secondary eyes, which individually send axons into three laminas resembling their namesakes serving insect superposition eyes. Secondary eye photoreceptors send axons to small-field projection neurons (L-cells) which extend from each lamina to supply three separate medullas. Each medulla is a vault of neuropil comprising only a few morphological types of neurons. These can be compared to a subset of retinotopic neurons in the medullas of calliphorid Diptera supplying giant motion-sensitive neurons in the lobula plate. In Cupiennius, neurons from secondary eye medullas converge at a single target neuropil called the "mushroom body." This region contains giant output neurons which, like their counterparts in the calliphorid lobula plate, lead to descending pathways that supply thoracic motor circuits. It is suggested that the cellular arrangements serving Cupiennius's secondary eyes are color independent pathways specialized for detecting horizontal motion. The present results do not support the classical view that the spider "mushroom body" is phylogenetically homologous or functionally analogous to its namesake in insects.