Olfactory memory formation in Drosophila: from molecular to systems neuroscience

Insect olfactory memory in time and space

Recent studies using functional optical imaging have revealed that cellular memory traces form in different areas of the insect brain after olfactory classical conditioning. These traces are revealed as increased calcium signals or synaptic release from defined neurons, and include a short-lived trace that forms immediately after conditioning in antennal lobe projection neurons, an early trace in dopaminergic neurons, and a medium-term trace in dorsal paired medial neurons. New molecular genetic tools have revealed that for normal behavioral memory performance, synaptic transmission from the mushroom body neurons is required only during retrieval, whereas synaptic transmission from dopaminergic neurons is required at the time of acquisition and synaptic transmission from dorsal paired medial neurons is required during the consolidation period. Such experimental results are helping to identify the types of neurons that participate in olfactory learning and when their participation is required. Olfactory learning often occurs alongside crossmodal interactions of sensory information from other modalities. Recent studies have revealed complex interactions between the olfactory and the visual senses that can occur during olfactory learning, including the facilitation of learning about subthreshold olfactory stimuli due to training with concurrent visual stimuli.

The Drosophila larva as a model for studying chemosensation and chemosensory learning: a review

Understanding the relationship between brain and behavior is the fundamental challenge in neuroscience. We focus on chemosensation and chemosensory learning in larval Drosophila and review what is known about its molecular and cellular bases. Detailed analyses suggest that the larval olfactory system, albeit much reduced in cell number, shares the basic architecture, both in terms of receptor gene expression and neuronal circuitry, of its adult counterpart as well as of mammals. With respect to the gustatory system, less is known in particular with respect to processing of gustatory information in the central nervous system, leaving generalizations premature. On the behavioral level, a learning paradigm for the association of odors with food reinforcement has been introduced. Capitalizing on the knowledge of the chemosensory pathways, we review the first steps to reveal the genetic and cellular bases of olfactory learning in larval Drosophila. We argue that the simplicity of the larval chemosensory system, combined with the experimental accessibility of Drosophila on the genetic, electrophysiological, cellular, and behavioral level, makes this system suitable for an integrated understanding of chemosensation and chemosensory learning.

Getting a buzz out of the bee genome

The honey bee Apis mellifera displays the most complex behavior of any insect. This, and its utility to humans, makes it a fascinating object of study for biologists. Such studies are now further enabled by the release of the honey-bee genome sequence.

NovelDrosophilatwo-pore domain K+channels: rescue of channel function by heteromeric assembly

Ten genes with essential structural features of two-pore domain potassium channels were identified in the genome of Drosophila melanogaster. Two Drosophila two-pore domain potassium subunits displayed substantial amino acid similarity to human TWIK-related acid-sensitive K(+) (TASK) channels (38-43%), whereas all others were less than 26% similar to any human homolog. The cDNAs of Drosophila TASK (dTASK)-6 and dTASK-7 channels were isolated from adult fruit flies. In Northern blots dTASK transcripts were found predominantly in the head fraction of adult flies and whole-mount brain in situ hybridizations showed strongly overlapping expression patterns of both dTASK isoforms in the antennal lobes. When heterologously expressed in Drosophila Schneider 2 cells, dTASK-6 gave rise to rapidly activating K(+)-selective currents that steeply depended on external pH. Structural elements in the extracellular M1-P1 loop of dTASK-6 were found to be involved in proton sensation. In contrast to mammalian TASK channels, the pH sensitivity was independent of extracellular histidines adjacent to the GYG selectivity filter (His98). As revealed by mutational analysis, functional expression of dTASK-7 was prevented by two nonconserved amino acids (Ala92-Met93) in the pore domain. When these two residues were replaced by conserved Thr92-Thr93, typical K(+)-selective leak currents were generated that were insensitive to changes in external pH. Nonfunctional wildtype dTASK-7 channels appeared to form heteromeric assemblies with dTASK-6. Following cotransfection of dTASK-6 and wildtype dTASK-7 (or when engineered as concatemers), K(+) currents were observed that were smaller in amplitude, harbored slower activation kinetics and were considerably less inhibited by local anesthetics as compared with dTASK-6. Thus, pore-loop residues in dTASK-7 changed functional and pharmacological properties in heteromeric dTASK channels.

Roles for Drosophila mushroom body neurons in olfactory learning and memory

Olfactory learning assays in Drosophila have revealed that distinct brain structures known as mushroom bodies (MBs) are critical for the associative learning and memory of olfactory stimuli. However, the precise roles of the different neurons comprising the MBs are still under debate. The confusion surrounding the roles of the different neurons may be due, in part, to the use of different odors as conditioned stimuli in previous studies. We investigated the requirements for the different MB neurons, specifically the alpha/beta versus the gamma neurons, and whether olfactory learning is supported by different subsets of MB neurons irrespective of the odors used as conditioned stimuli. We expressed the rutabaga (rut)-encoded adenylyl cyclase in either the gamma or alpha/beta neurons and examined the effects on restoring olfactory associative learning and memory of rut mutant flies. We also expressed a temperature-sensitive shibire (shi) transgene in these neuron sets and examined the effects of disrupting synaptic vesicle recycling on Drosophila olfactory learning. Our results indicate that although we did not detect odor-pair-specific learning using GAL4 drivers that primarily express in gamma neurons, expression of the transgenes in a subset of alpha/beta neurons resulted in both odor-pair-specific rescue of the rut defect as well as odor-pair-specific disruption of learning using shi(ts1).

Differential microarray analysis of Drosophila mushroom body transcripts using chemical ablation

Mushroom bodies (MBs) are the centers for olfactory associative learning and elementary cognitive functions in the Drosophila brain. As a way to systematically elucidate genes preferentially expressed in MBs, we have analyzed genome-wide alterations in transcript profiles associated with MB ablation by hydroxyurea. We selected 100 genes based on microarray data and examined their expression patterns in the brain by in situ hybridization. Seventy genes were found to be expressed in the posterodorsal cortex, which harbors the MB cell bodies. These genes encode proteins of diverse functions, including transcription, signaling, cell adhesion, channels, and transporters. Moreover, we have examined developmental functions of 40 of the microarray-identified genes by transgenic RNA interference; 8 genes were found to cause mild-to-strong MB defects when suppressed with a MB-Gal4 driver. These results provide important information not only on the repertoire of genes that control MB development but also on the repertoire of neural factors that may have important physiological functions in MB plasticity.

Olfactory conditioning of proboscis activity in Drosophila melanogaster

Olfactory learning and memory processes in Drosophila have been well investigated with aversive conditioning, but appetitive conditioning has rarely been documented. Here, we report for the first time individual olfactory conditioning of proboscis activity in restrained Drosophila melanogaster. The protocol was adapted from those developed for proboscis extension conditioning in the honeybee Apis mellifera. After establishing a scale of small proboscis movements necessary to characterize responses to olfactory stimulation, we applied Pavlovian conditioning, with five trials consisting of paired presentation of a banana odour and a sucrose reward. Drosophila showed conditioned proboscis activity to the odour, with a twofold increase of percentage of responses after the first trial. No change occurred in flies experiencing unpaired presentations of the stimuli, confirming an associative basis for this form of olfactory learning. The adenylyl cyclase mutant rutabaga did not exhibit learning in this paradigm. This protocol generated at least a short-term memory of 15 min, but no significant associative memory was detected at 1 h. We also showed that learning performance was dependent on food motivation, by comparing flies subjected to different starvation regimes.

Modeling human mitochondrial diseases in flies

Human mitochondrial diseases are associated with a wide range of clinical symptoms, and those that result from mutations in mitochondrial DNA affect at least 1 in 8500 individuals. The development of animal models that reproduce the variety of symptoms associated with this group of complex human disorders is a major focus of current research. Drosophila represents an attractive model, in large part because of its short life cycle, the availability of a number of powerful techniques to alter gene structure and regulation, and the presence of orthologs of many human disease genes. We describe here Drosophila models of mitochondrial DNA depletion, deafness, encephalopathy, Freidreich's ataxia, and diseases due to mitochondrial DNA mutations. We also describe several genetic approaches for gene manipulation in flies, including the recently developed method of targeted mutagenesis by recombinational knock-in.

Drosophila Dorsal Paired Medial Neurons Provide a General Mechanism for Memory Consolidation

Memories are formed, stabilized in a time-dependent manner, and stored in neural networks. In Drosophila, retrieval of punitive and rewarded odor memories depends on output from mushroom body (MB) neurons, consistent with the idea that both types of memory are represented there. Dorsal Paired Medial (DPM) neurons innervate the mushroom bodies, and DPM neuron output is required for the stability of punished odor memory. Here we show that stable reward-odor memory is also DPM neuron dependent. DPM neuron expression of amnesiac (amn) in amn mutant flies restores wild-type memory. In addition, disrupting DPM neurotransmission between training and testing abolishes reward-odor memory, just as it does with punished memory. We further examined DPM-MB connectivity by overexpressing a DScam variant that reduces DPM neuron projections to the MB alpha, beta, and gamma lobes. DPM neurons that primarily project to MB alpha' and beta' lobes are capable of stabilizing punitive- and reward-odor memory, implying that both forms of memory have similar circuit requirements. Therefore, our results suggest that the fly employs the local DPM-MB circuit to stabilize punitive- and reward-odor memories and that stable aspects of both forms of memory may reside in mushroom body alpha' and beta' lobe neurons.

Cell type-specific processing of human Tau proteins in Drosophila

Accumulation of hyperphosphorylated Tau is associated with a number of neurodegenerative diseases collectively known as tauopathies. Differences in clinical and cognitive profiles among them suggest differential sensitivity of neuronal populations to Tau levels, phosphorylation and mutations. We used tissue specific expression of wild type and mutant human tau transgenes to demonstrate differential phosphorylation and stability in a cell type-specific manner, which includes different neuronal types and does not correlate with the level of accumulated protein. Rather, they likely reflect the spatial distribution or regulation of Tau-targeting kinases and phosphatases.

Invertebrate studies and their ongoing contributions to neuroscience

Invertebrates have been deployed very successfully in experimental studies of the nervous system and neuromuscular junctions. Many important discoveries on axonal conduction, synaptic transmission, integrative neurobiology and behaviour have been made by investigations of these remarkable animals. Their advantages as model organisms for investigations of nervous systems include (a) the large diameter of neurons, glia and muscle cells of some invertebrates, thereby facilitating microelectrode recordings; (b) simple nervous systems with few neurons, enhancing the tractability of neuronal circuitry; and (c) well-defined behaviours, which lend themselves to physiological and genetic dissection. Genetic model organisms such as Drosophila melanogaster and Caenorhabditis elegans have provided powerful genetic approaches to central questions concerning nervous system development, learning and memory and the cellular and molecular basis of behaviour. The process of attributing function to particular gene products has been greatly accelerated in recent years with access to entire genome sequences and the application of reverse genetic (e.g. RNA interference, RNAi) and other post-genome technologies (e.g. microarrays). Studies of many other invertebrates, notably the honeybee (Apis mellifera), a nudibranch mollusc (Aplysia californica), locusts, lobsters, crabs, annelids and jellyfish have all assisted in the development of major concepts in neuroscience. The future is equally bright with ease of access to genome-wide reverse genetic technologies, and the development of optical recordings using voltage and intracellular calcium sensors genetically targeted to selected individual and groups of neurons.

Focal and temporal release of glutamate in the mushroom bodies improves olfactory memory in Apis mellifera

In contrast to vertebrates, the role of the neurotransmitter glutamate in learning and memory in insects has hardly been investigated. The reason is that a pharmacological characterization of insect glutamate receptors is still missing; furthermore, it is difficult to locally restrict pharmacological interventions. In this study, we overcome these problems by using locally and temporally defined photo-uncaging of glutamate to study its role in olfactory learning and memory formation in the honeybee, Apis mellifera. Uncaging glutamate in the mushroom bodies immediately after a weak training protocol induced a higher memory rate 2 d after training, mimicking the effect of a strong training protocol. Glutamate release before training does not facilitate memory formation, suggesting that glutamate mediates processes triggered by training and required for memory formation. Uncaging glutamate in the antennal lobes shows no effect on memory formation. These results provide the first direct evidence for a temporally and locally restricted function of glutamate in memory formation in honeybees and insects.

Deconstructing memory in Drosophila

Unlike most organ systems, which have evolved to maintain homeostasis, the brain has been selected to sense and adapt to environmental stimuli by constantly altering interactions in a gene network that functions within a larger neural network. This unique feature of the central nervous system provides a remarkable plasticity of behavior, but also makes experimental investigations challenging. Each experimental intervention ramifies through both gene and neural networks, resulting in unpredicted and sometimes confusing phenotypic adaptations. Experimental dissection of mechanisms underlying behavioral plasticity ultimately must accomplish an integration across many levels of biological organization, including genetic pathways acting within individual neurons, neural network interactions which feed back to gene function, and phenotypic observations at the behavioral level. This dissection will be more easily accomplished for model systems such as Drosophila, which, compared with mammals, have relatively simple and manipulable nervous systems and genomes. The evolutionary conservation of behavioral phenotype and the underlying gene function ensures that much of what we learn in such model systems will be relevant to human cognition. In this essay, we have not attempted to review the entire Drosophila memory field. Instead, we have tried to discuss particular findings that provide some level of intellectual synthesis across three levels of biological organization: behavior, neural circuitry and biochemical pathways. We have attempted to use this integrative approach to evaluate distinct mechanistic hypotheses, and to propose critical experiments that will advance this field.

Drosophila DPM neurons form a delayed and branch-specific memory trace after olfactory classical conditioning

Formation of normal olfactory memory requires the expression of the wild-type amnesiac gene in the dorsal paired medial (DPM) neurons. Imaging the activity in the processes of DPM neurons revealed that the neurons respond when the fly is stimulated with electric shock or with any odor that was tested. Pairing odor and electric-shock stimulation increases odor-evoked calcium signals and synaptic release from DPM neurons. These memory traces form in only one of the two branches of the DPM neuron process. Moreover, trace formation requires the expression of the wild-type amnesiac gene in the DPM neurons. The cellular memory traces first appear at 30 min after conditioning and persist for at least 1 hr, a time window during which DPM neuron synaptic transmission is required for normal memory. DPM neurons are therefore "odor generalists" and form a delayed, branch-specific, and amnesiac-dependent memory trace that may guide behavior after acquisition.

Stimulation of muscarinic receptors mimics experience-dependent plasticity in the honey bee brain

Honey bees begin life working in the hive. At approximately 3 weeks of age, they shift to visiting flowers to forage for pollen and nectar. Foraging is a complex task associated with enlargement of the mushroom bodies, a brain region important in insects for certain forms of learning and memory. We report here that foraging bees had a larger volume of mushroom body neuropil than did age-matched bees confined to the hive. This result indicates that direct experience of the world outside the hive causes mushroom body neuropil growth in bees. We also show that oral treatment of caged bees with pilocarpine, a muscarinic agonist, induced an increase in the volume of the neuropil similar to that seen after a week of foraging experience. Effects of pilocarpine were blocked by scopolamine, a muscarinic antagonist. Our results suggest that signaling in cholinergic pathways couples experience to structural brain plasticity.

Punishment Prediction by Dopaminergic Neurons in Drosophila

The temporal pairing of a neutral stimulus with a reinforcer (reward or punishment) can lead to classical conditioning, a simple form of learning in which the animal assigns a value (positive or negative) to the formerly neutral stimulus. Olfactory classical conditioning in Drosophila is a prime model for the analysis of the molecular and neuronal substrate of this type of learning and memory. Neuronal correlates of associative plasticity have been identified in several regions of the insect brain. In particular, the mushroom bodies have been shown to be necessary for aversive olfactory memory formation. However, little is known about which neurons mediate the reinforcing stimulus. Using functional optical imaging, we now show that dopaminergic projections to the mushroom-body lobes are weakly activated by odor stimuli but respond strongly to electric shocks. However, after one of two odors is paired several times with an electric shock, odor-evoked activity is significantly prolonged only for the "punished" odor. Whereas dopaminergic neurons mediate rewarding reinforcement in mammals, our data suggest a role for aversive reinforcement in Drosophila. However, the dopaminergic neurons' capability of mediating and predicting a reinforcing stimulus appears to be conserved between Drosophila and mammals.

Remote control of fruit fly behavior

Electrical stimulation of neurons in the central nervous system of awake, behaving animals offers the ultimate test to determine whether the activation of specific neurons is sufficient to elicit perception, motor activity, or other behaviors. In this issue of Cell, Lima and Miesenböck (Lima and Miesenböck, 2005) dump the stimulating electrode in favor of a new remote control system to excite specific neurons--light activation of transgenically supplied ion channels.