Insect Mushroom Bodies Research Articles

What Do the Mushroom Bodies Do for the Insect Brain? An Introduction

Despite public claims to the contrary, our functional understanding of the brain is still rudimentary. Starting with F.J. Gall’s Phrenology of the late eighteenth century, one of the major drives has been to assign functions to discrete regions of the brain. In humans and other primates, lesions and noninvasive imaging techniques have provided fascinating insights into the underlying functional topology of the brain. However, lesions in humans relate brain areas comprising millions of neurons to clinical tests, interrogation, and life histories. Similarly, functional imaging usually ties regions of increased blood flow to mental activity conveyed by verbal or visual instructions. The striking specificity of behavioral and mental deficits after local brain damage is difficult to reconcile with the high degree of integration in the neural circuitry. The situation is as perplexing as that met in brain stimulation experiments in which gross perturbations of sophisticated neural activity patterns in the brain lead, often with long delays, to highly organized behaviors or mental experiences. These approaches, therefore, reveal structure–function relationships that are difficult to interpret in terms of network-based brain models. After all, assigning behavioral and mental functions to locations in the brain does not explain how the brain works. Brain regions need first to be interpreted as parts of a neural circuitry. One then can hope to obtain cues about the functioning of this system by interfering with a certain part of it and observing how the rest still controls behavior. From this perspective, the interventionist approach is a difficult and largely unexplored task. It is not even obvious (to me) that the brain sciences are ready for it. Given the motivational nature of behavior and the enormous diversity of neuronal interactions, can a basic general model of the brain be established at all at the network level? Is the conceptual and methodological inventory of the behavioral sciences sufficiently sophisticated? A basic model of how the brain controls behavior, would, in principle, allow the building of a robot with the same behavioral properties as the respective animal. Such explicit models have been designed for partial aspects of behavior such as color vision in the honeybee (Backhaus 1992), directional hearing in owls (Konishi 1993), or for walking in insects (Graham 1985). However, no such model exists for behavioral control in general. This is where smaller brains can contribute to our understanding of brain function. Animals with small brains, in particular insects, provide the opportunity to analyze the behavioral consequences of interventions (e.g., lesions) in much greater detail and have already, in cases like the ones cited above made it possible to match behavioral functions with the properties of the corresponding neural networks in the brain. The insect mushroom body (MB) may be a uniquely suitable structure for intervention. Its internal cytoarchitecture seems less complex than that of most other neuropil regions and its bizarre shape suggests a highly specialized function. Being closely associated with the chemosensory system, its role in olfaction can serve as a point of entry for the analysis, and a comparison of the MBs from species with different life styles and ecological constraints may provide useful cues for functional studies. Moreover, a variety of new approaches for studying the MBs have recently been developed. They take advantage of the rapid advances in areas such as developmental genetics, transgenics, cell and tissue culture, microchemistry, pharmacology, functional imaging, and patch clamping, to name a few. They all recognize the fact that without knowledge of the cellular properties of the tissues involved, their functional involvement cannot be properly assessed. Several of these new approaches are highlighted in this volume. When Dujardin (1850) first described the MBs in bees and ants, he compared them to the vertebrate cerebral cortex and considered them the seat of intelligence. It took nearly 150 years to establish beyond doubt that they are, indeed, involved in something related to intelligence: They are essential for short term memory in odor discrimination tasks. This is a promising start, but it may still be a long way until their function is understood at the circuit level. I appreciate the editorial decision of Learning & Memory to devote a special issue of the journal to this topic. The papers that have been contributed provide a good overview of the forefront of this field. They are not narrowly focused on learning and memory but address a variety of problems broadly related to MB function. Even if the MBs eventually turn out not to be the high-level integration center of the insect brain envisaged by Dujardin, understanding their contribution to a network model of brain function will be of great importance for learning and memory research.

Adult insect mushroom body neurons in primary culture: cell morphology and characterization of potassium channels.

The dissociation and maintenance in culture of cells derived from the mushroom bodies of adult crickets (Acheta domesticus) are described. This primary culture was developed in order to investigate maturation and differentiation of mushroom-body cells including Kenyon cells, the major intrinsic interneurons of mushroom bodies, which have been shown to be involved in learning and memory in insects. Three distinct cell types were observed, all identified as neural cells on the basis of their size, morphology and immunocytochemical staining with horseradish peroxidase. These cells appear to correspond to the three cell types observed in vivo: Kenyon cells, ganglion mother cells and neuroblasts. Some cells showed neurite growth, usually with long unipolar processes, occasionally with either bipolar or, more rarely, multipolar processes. Neuronal cell bodies readily formed seals with patch pipettes, allowing stable, whole-cell, patch-clamp electrophysiological recordings. Depolarization of the cell under voltage-clamp resulted in at least two types of outwardly directed potassium currents: a delayed rectifier-type of current that was sensitive to tetraethylammonium, and a cadmium-sensitive current with rapid inactivation. Neither type of current was affected by quinidine, a blocker of potassium currents recorded from pupal honeybee Kenyon cells. Other ionic currents, which have yet to be characterized, were also observed.

Gamma-Aminobutyric acid receptor distribution in the mushroom bodies of a fly (Calliphora erythrocephala): a functional subdivision of Kenyon cells?

Antibodies against the Drosophila gamma-aminobutyric acid (GABA) receptor subunit RDL were used to investigate the significance of inhibitory inputs to the mushroom bodies in the blowfly (Calliphora erythrocephala) brain. The pedunculus and the lobes of the mushroom body, which mainly consist of Kenyon cell fibers, revealed strong immunoreactivity against RDL. Pedunculi, alpha- and beta-lobe show characteristic unstained core structures with concentric labeling along the neuropile axis. The gamma-lobes in contrast exhibit a compartmentalized RDL-immunoreactive pattern. These data suggest an important role of GABAergic inhibition in the pedunculus and the lobes of insect mushroom bodies. It is most likely that the RDL-immunoreactivity in the mushroom bodies is closely related to Kenyon cell fibers suggesting that Kenyon cells are an inhomogeneous class of neurons, only part of which receive inhibitory GABAergic input from extrinsic elements. GABAergic inhibition, therefore, may play a substantial role in the process of learning and memory formation in the insect mushroom bodies.

Neurogenesis in adult insect mushroom bodies.

The occurrence of neurogenesis in mushroom bodies of adult insects belonging to several orthopteroid and coleopteran families is described. Using injections of 5-bromo, T2'-deoxyuridine, we showed that neuroblasts, which are progenitors of Kenyon cells during preimaginal instars, continue to divide in adult Acheta domesticus. Their progeny constitute a central column in mushroom body cortices of 3-week-old females. Other Gryllidae, Gryllus bimaculatus and Gryllomorpha dalmatina, show the same pattern of neuroblast activity and migration of their progeny. Immunocytochemical staining of glial cells failed to reveal any immunoreactivity, either in proliferating regions or in the resulting cells. In another orthopteran, Locusta migratoria, discrete clusters of cells, located dorsolateral to the Kenyon cells, incorporated 5-bromo, 2'-deoxyuridine, but we could not detect any neuronal progeny migrating to the mushroom body cortices. These cells were strongly labeled with an antiglial antibody, indicating that the replicating cells are glioblasts rather than neuroblasts. In Periplaneta americana (Dictyoptera), cells replicating their DNA were similarly shown to immunoreact with glial antibodies. In contrast, three coleopterans (Tenebrio molitor, Zophobas species, Harmonia axyridis) have two large neuroblasts located in the middle of the mushroom body cortices. These produce cells which migrate within the group of Kenyon cells, their nuclei having the same shape and size as those of surrounding Kenyon cells. In adult insects, neurogenesis in mushroom bodies occurs in Gryllidae and several coleopteran families, but could not be demonstrated in Dictyoptera and Acrididae. Its occurrence and distribution raise the issue of unexpected plasticity in the adult insect brain.

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.

Substance P antibody reveals homologous neurons with axon terminals among somata in the crayfish and crab brain

In the search for particular neurons that stain selectively and can be identified, the cerebral ganglia (brains) of the crayfish Cherax destructor and the crab Leptograpsus variegatus were immunocytochemically treated with a monoclonal antibody raised against substance P. Four large neurons in the cerebral ganglion of the crayfish and crab label selectively with a monoclonal antibody raised against substance P. Two of the large neurons have their cell bodies in the protocerebrum and two in the deutocerebrum in both animals. Each protocerebral cell in both animals projects through the ipsilateral and contralateral olfactory lobes to end among the lateral cell somata of the olfactory lobe and not in the neuropile. Electron micrographs show the presence of synapses within the cell somata area and on the cell somata themselves. Each deutocerebral cell in both animals projects only ipsilaterally and ends within the neuropile of the olfactory lobes. The immunoreactivity to substance P antibody and the shapes and the unique projections of the four cells suggest that they are homologous in the two species. Synaptic connections between axons and cell somata are rare in the arthropods but have been found on the Kenyon cells of the mushroom bodies of Limulus. This raises questions about homologies between the crustacean olfactory lobe and the mushroom bodies of Limulus and insects.