Heterosynaptic Plasticity Underlies Aversive Olfactory Learning in Drosophila

Inhibitory muscarinic acetylcholine receptors enhance aversive olfactory learning in adult Drosophila

Olfactory associative learning in Drosophila is mediated by synaptic plasticity between the Kenyon cells of the mushroom body and their output neurons. Both Kenyon cells and their inputs from projection neurons are cholinergic, yet little is known about the physiological function of muscarinic acetylcholine receptors in learning in adult flies. Here, we show that aversive olfactory learning in adult flies requires type A muscarinic acetylcholine receptors (mAChR-A), particularly in the gamma subtype of Kenyon cells. mAChR-A inhibits odor responses and is localized in Kenyon cell dendrites. Moreover, mAChR-A knockdown impairs the learning-associated depression of odor responses in a mushroom body output neuron. Our results suggest that mAChR-A function in Kenyon cell dendrites is required for synaptic plasticity between Kenyon cells and their output neurons.

Two Parallel Pathways Assign Opposing Odor Valences during Drosophila Memory Formation

During olfactory associative learning in Drosophila, odors activate specific subsets of intrinsic mushroom body (MB) neurons. Coincident exposure to either rewards or punishments is thought to activate extrinsic dopaminergic neurons, which modulate synaptic connections between odor-encoding MB neurons and MB output neurons to alter behaviors. However, here we identify two classes of intrinsic MB γ neurons based on cAMP response element (CRE)-dependent expression, γCRE-p and γCRE-n, which encode aversive and appetitive valences. γCRE-p and γCRE-n neurons act antagonistically to maintain neutral valences for neutral odors. Activation or inhibition of either cell type upsets this balance, toggling odor preferences to either positive or negative values. The mushroom body output neurons, MBON-γ5β'2a/β'2mp and MBON-γ2α'1, mediate the actions of γCRE-p and γCRE-n neurons. Our data indicate that MB neurons encode valence information, as well as odor information, and this information is integrated through a process involving MBONs to regulate learning and memory.

Neurogenetic dissection of the Drosophila lateral horn reveals major outputs, diverse behavioural functions, and interactions with the mushroom body

Animals exhibit innate behaviours to a variety of sensory stimuli including olfactory cues. In Drosophila, one higher olfactory centre, the lateral horn (LH), is implicated in innate behaviour. However, our structural and functional understanding of the LH is scant, in large part due to a lack of sparse neurogenetic tools for this region. We generate a collection of split-GAL4 driver lines providing genetic access to 82 LH cell types. We use these to create an anatomical and neurotransmitter map of the LH and link this to EM connectomics data. We find ~30% of LH projections converge with outputs from the mushroom body, site of olfactory learning and memory. Using optogenetic activation, we identify LH cell types that drive changes in valence behavior or specific locomotor programs. In summary, we have generated a resource for manipulating and mapping LH neurons, providing new insights into the circuit basis of innate and learned olfactory behavior.

Courtship behavior induced by appetitive olfactory memory

Reinforcement signals such as food reward and noxious punishment can change diverse behaviors. This holds true in fruit flies, Drosophila melanogaster, which can be conditioned by an odor and sugar reward or electric shock punishment. Despite a wide variety of behavior modulated by learning, conditioned responses have been traditionally measured by altered odor preference in a choice, and other memory-guided behaviors have been only scarcely investigated. Here, we analyzed detailed conditioned odor responses of flies after sugar associative learning by employing a video recording and semi-automated processing pipeline. Trajectory analyses revealed that multiple behavioral components were altered along with conditioned approach to the rewarded odor. Notably, we found that lateral wing extension, a hallmark of courtship behavior of D. melanogaster, was robustly increased specifically in the presence of the rewarded odor. Strikingly, genetic disruption of the mushroom body output did not impair conditioned courtship increase, while markedly weakening conditioned odor approach. Our results highlight the complexity of conditioned responses and their distinct regulatory mechanisms that may underlie coordinated yet complex memory-guided behaviors in flies.

Recurrent loops: Incorporating prediction error and semantic/episodic theories into Drosophila associative memory models

In 2003, Martin Heisenberg et al. presented a model of how associative memories could be encoded and stored in the insect brain. This model was extremely influential in the Drosophila memory field, but did not incorporate several important mammalian concepts, including ideas of separate episodic and semantic types of memory and prediction error hypotheses. In addition, at that time, the concept of memory traces recurrently entering and exiting the mushroom bodies, brain areas where associative memories are formed and stored, was unknown. In this review, I present a simple updated model incorporating these ideas, which may be useful for future studies.

Suppression of GABAergic neurons through D2-like receptor secures efficient conditioning in Drosophila aversive olfactory learning

The GABAergic system serves as a vital negative modulator in cognitive functions, such as learning and memory, while the mechanisms governing this inhibitory system remain to be elucidated. In Drosophila, the GABAergic anterior paired lateral (APL) neurons mediate a negative feedback essential for odor discrimination; however, their activity is suppressed by learning via unknown mechanisms. In aversive olfactory learning, a group of dopaminergic (DA) neurons is activated on electric shock (ES) and modulates the Kenyon cells (KCs) in the mushroom body, the center of olfactory learning. Here we find that the same group of DA neurons also form functional synaptic connections with the APL neurons, thereby emitting a suppressive signal to the latter through Drosophila dopamine 2-like receptor (DD2R). Knockdown of either DD2R or its downstream molecules in the APL neurons results in impaired olfactory learning at the behavioral level. Results obtained from in vivo functional imaging experiments indicate that this DD2R-dependent DA-to-APL suppression occurs during odor-ES conditioning and discharges the GABAergic inhibition on the KCs specific to the conditioned odor. Moreover, the decrease in odor response of the APL neurons persists to the postconditioning phase, and this change is also absent in DD2R knockdown flies. Taken together, our findings show that DA-to-GABA suppression is essential for restraining the GABAergic inhibition during conditioning, as well as for inducing synaptic modification in this learning circuit. Such circuit mechanisms may play conserved roles in associative learning across species.

Mechanisms of Sensory Discrimination: Insights from Drosophila Olfaction

All an animal can do to infer the state of its environment is to observe the sensory-evoked activity of its own neurons. These inferences about the presence, quality, or similarity of objects are probabilistic and inform behavioral decisions that are often made in close to real time. Neural systems employ several strategies to facilitate sensory discrimination: Biophysical mechanisms separate the neuronal response distributions in coding space, compress their variances, and combine information from sequential observations. We review how these strategies are implemented in the olfactory system of the fruit fly. The emerging principles of odor discrimination likely apply to other neural circuits of similar architecture.

Olfactory Object Recognition Based on Fine-Scale Stimulus Timing in Drosophila

Odorants of behaviorally relevant objects (e.g., food sources) intermingle with those from other sources. Therefore to determine whether an odor source is good or bad—without actually visiting it—animals first need to segregate the odorants from different sources. To do so, animals could use temporal stimulus cues, because odorants from one source exhibit correlated fluctuations, whereas odorants from different sources are less correlated. However, the behaviorally relevant timescales of temporal stimulus cues for odor source segregation remain unclear. Using behavioral experiments with free-flying flies, we show that (1) odorant onset asynchrony increases flies' attraction to a mixture of two odorants with opposing innate or learned valence and (2) attraction does not increase when the attractive odorant arrives first. These data suggest that flies can use stimulus onset asynchrony for odor source segregation and imply temporally precise neural mechanisms for encoding odors and for segregating them into distinct objects.

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Flies can detect whether two mixed odorants arrive synchronously or asynchronously

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This temporal sensitivity occurs for odorants with innate and learned valences

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Flies' behavior suggests use of odor onset asynchrony for odor source segregation

A neural data structure for novelty detection

Novelty detection is a fundamental biological problem that organisms must solve to determine whether a given stimulus departs from those previously experienced. In computer science, this problem is solved efficiently using a data structure called a Bloom filter. We found that the fruit fly olfactory circuit evolved a variant of a Bloom filter to assess the novelty of odors. Compared with a traditional Bloom filter, the fly adjusts novelty responses based on two additional features: the similarity of an odor to previously experienced odors and the time elapsed since the odor was last experienced. We elaborate and validate a framework to predict novelty responses of fruit flies to given pairs of odors. We also translate insights from the fly circuit to develop a class of distance- and time-sensitive Bloom filters that outperform prior filters when evaluated on several biological and computational datasets. Overall, our work illuminates the algorithmic basis of an important neurobiological problem and offers strategies for novelty detection in computational systems.

Communication from Learned to Innate Olfactory Processing Centers Is Required for Memory Retrieval in Drosophila

The behavioral response to a sensory stimulus may depend on both learned and innate neuronal representations. How these circuits interact to produce appropriate behavior is unknown. In Drosophila, the lateral horn (LH) and mushroom body (MB) are thought to mediate innate and learned olfactory behavior, respectively, although LH function has not been tested directly. Here we identify two LH cell types (PD2a1 and PD2b1) that receive input from an MB output neuron required for recall of aversive olfactory memories. These neurons are required for aversive memory retrieval and modulated by training. Connectomics data demonstrate that PD2a1 and PD2b1 neurons also receive direct input from food odor-encoding neurons. Consistent with this, PD2a1 and PD2b1 are also necessary for unlearned attraction to some odors, indicating that these neurons have a dual behavioral role. This provides a circuit mechanism by which learned and innate olfactory information can interact in identified neurons to produce appropriate behavior. VIDEO ABSTRACT.

Roles for sleep in memory: insights from the fly

Sleep has been universally conserved across animal species. The basic functions of sleep remain unclear, but insufficient sleep impairs memory acquisition and retention in both vertebrates and invertebrates. Sleep is also a homeostatic process that is influenced not only by the amount of time awake, but also by neural activity and plasticity. Because of the breadth and precision of available genetic tools, the fruit fly has become a powerful model system to understand sleep regulation and function. Importantly, these tools enable the dissection of memory-encoding circuits at the level of individual neurons, and have allowed the development of genetic tools to induce sleep on-demand. This review describes recent investigations of the role for sleep in memory using Drosophila and current hypotheses of sleep's functions for supporting plasticity, learning, and memory.

Multiple neurons encode CrebB dependent appetitive long-term memory in the mushroom body circuit

Lasting changes in gene expression are critical for the formation of long-term memories (LTMs), depending on the conserved CrebB transcriptional activator. While requirement of distinct neurons in defined circuits for different learning and memory phases have been studied in detail, only little is known regarding the gene regulatory changes that occur within these neurons. We here use the fruit fly as powerful model system to study the neural circuits of CrebB-dependent appetitive olfactory LTM. We edited the CrebB locus to create a GFP-tagged CrebB conditional knockout allele, allowing us to generate mutant, post-mitotic neurons with high spatial and temporal precision. Investigating CrebB-dependence within the mushroom body (MB) circuit we show that MB α/β and α’/β’ neurons as well as MBON α3, but not in dopaminergic neurons require CrebB for LTM. Thus, transcriptional memory traces occur in different neurons within the same neural circuit.

Our brains can store different types of memories. You may have forgotten what you had for lunch yesterday, but still be able to remember a song from your childhood. Short-term memories and long-term memories form via different mechanisms. To establish long-term memories, the brain must produce new proteins, many of which are common to all members of the animal kingdom. By studying these proteins in organisms such as fruit flies, we can learn more about their role in our own memories.

Widmer et al. used this approach to explore how a protein called CrebB helps fruit flies to remember for several days that a specific odor is associated with a sugary reward. These odor-reward memories form in a brain region called the mushroom body, which has three lobes. Input neurons supply information about the odor and the reward to the region, while output neurons pass on information to other parts of the fly brain. CrebB regulates the production of new proteins required to form these long-term odor-reward memories: but where exactly does CrebB act during this process?

Using a gene editing technique called CRISPR, Widmer et al. generated mutant flies. In these insects CrebB could be easily deactivated ‘at will’ in either the entire brain, the whole mushroom body, each of the three lobes or in specific output neurons. The flies were then trained on the odor-reward task, and their memory tested 24 hours later. The results revealed that for the memories to form, CrebB is only required in two of the three lobes of the mushroom body, and in certain output neurons. Future studies can now focus on the cells shown to need CrebB to create long-term memories, and identify the other proteins involved in this process.

In humans, defects in CrebB are associated with intellectual disability, addiction and depression. The mutant fly created by Widmer et al. could be a useful model in which to investigate how the protein may play a role in these conditions.

Integration of Parallel Opposing Memories Underlies Memory Extinction

Accurately predicting an outcome requires that animals learn supporting and conflicting evidence from sequential experience. In mammals and invertebrates, learned fear responses can be suppressed by experiencing predictive cues without punishment, a process called memory extinction. Here, we show that extinction of aversive memories in Drosophila requires specific dopaminergic neurons, which indicate that omission of punishment is remembered as a positive experience. Functional imaging revealed co-existence of intracellular calcium traces in different places in the mushroom body output neuron network for both the original aversive memory and a new appetitive extinction memory. Light and ultrastructural anatomy are consistent with parallel competing memories being combined within mushroom body output neurons that direct avoidance. Indeed, extinction-evoked plasticity in a pair of these neurons neutralizes the potentiated odor response imposed in the network by aversive learning. Therefore, flies track the accuracy of learned expectations by accumulating and integrating memories of conflicting events.

Dopamine Neurons Mediate Learning and Forgetting through Bidirectional Modulation of a Memory Trace

It remains unclear how memory engrams are altered by experience, such as new learning, to cause forgetting. Here, we report that short-term aversive memory in Drosophila is encoded by and retrieved from the mushroom body output neuron MBOn-γ2α'1. Pairing an odor with aversive electric shock creates a robust depression in the calcium response of MBOn-γ2α'1 and increases avoidance to the paired odor. Electric shock after learning, which activates the cognate dopamine neuron DAn-γ2α'1, restores the response properties of MBOn-γ2α'1 and causes behavioral forgetting. Conditioning with a second odor restores the responses of MBOn-γ2α'1 to a previously learned odor while depressing responses to the newly learned odor, showing that learning and forgetting can occur simultaneously. Moreover, optogenetic activation of DAn-γ2α'1 is sufficient for the bidirectional modulation of MBOn-γ2α'1 response properties. Thus, a single DAn can drive both learning and forgetting by bidirectionally modulating a cellular memory trace.

Learning Distinct Chemical Labels of Nestmates in Ants

Colony coherence is essential for eusocial insects because it supports the inclusive fitness of colony members. Ants quickly and reliably recognize who belongs to the colony (nestmates) and who is an outsider (non-nestmates) based on chemical recognition cues (cuticular hydrocarbons: CHCs) which as a whole constitute a chemical label. The process of nestmate recognition often is described as matching a neural template with the label. In this study, we tested the prevailing view that ants use commonalities in the colony odor that are present in the CHC profile of all individuals of a colony or whether different CHC profiles are learned independently. We created and manipulated sub-colonies by adding one or two different hydrocarbons that were not present in the original colony odor of our Camponotus floridanus colony and later tested workers of the sub-colonies in one-on-one encounters for aggressive responses. We found that workers adjust their nestmate recognition by learning novel, manipulated CHC profiles, but still accept workers with the previous CHC profile. Workers from a sub-colony with two additional components showed aggression against workers with only one of the two components added to their CHC profile. Thus, additional components as well as the lack of a component can alter a label as “non-nestmate.” Our results suggest that ants have multiple-templates to recognize nestmates carrying distinct labels. This finding is in contrast to what previously has been proposed, i.e., a widening of the acceptance range of one template. We conclude that nestmate recognition in ants is a partitioned (multiple-template) process of the olfactory system that allows discrimination and categorization of nestmates by differences in their CHC profiles. Our findings have strong implications for our understanding of the underlying mechanisms of colony coherence and task allocation because they illustrate the importance of individual experience and task associated differences in the CHC profiles that can be instructive for the organization of insect societies.

Deep(er) Learning

Animals successfully thrive in noisy environments with finite resources. The necessity to function with resource constraints has led evolution to design animal brains (and bodies) to be optimal in their use of computational power while being adaptable to their environmental niche. A key process undergirding this ability to adapt is the process of learning. Although a complete characterization of the neural basis of learning remains ongoing, scientists for nearly a century have used the brain as inspiration to design artificial neural networks capable of learning, a case in point being deep learning. In this viewpoint, we advocate that deep learning can be further enhanced by incorporating and tightly integrating five fundamental principles of neural circuit design and function: optimizing the system to environmental need and making it robust to environmental noise, customizing learning to context, modularizing the system, learning without supervision, and learning using reinforcement strategies. We illustrate how animals integrate these learning principles using the fruit fly olfactory learning circuit, one of nature's best-characterized and highly optimized schemes for learning. Incorporating these principles may not just improve deep learning but also expose common computational constraints. With judicious use, deep learning can become yet another effective tool to understand how and why brains are designed the way they are.

Beyond STDP-towards diverse and functionally relevant plasticity rules

Synaptic plasticity, induced by the close temporal association of two neural signals, supports associative forms of learning. However, the millisecond timescales for association often do not match the much longer delays for behaviorally relevant signals that supervise learning. In particular, information about the behavioral outcome of neural activity can be delayed, leading to a problem of temporal credit assignment. Recent studies suggest that synaptic plasticity can have temporal rules that not only accommodate the delays relevant to the circuit, but also be precisely tuned to the behavior the circuit supports. These discoveries highlight the diversity of plasticity rules, whose temporal requirements may depend on circuit delays and the contingencies of behavior.

The Organization of Projections from Olfactory Glomeruli onto Higher-Order Neurons

Each odorant receptor corresponds to a unique glomerulus in the brain. Projections from different glomeruli then converge in higher brain regions, but we do not understand the logic governing which glomeruli converge and which do not. Here, we use two-photon optogenetics to map glomerular connections onto neurons in the lateral horn, the region of the Drosophila brain that receives the majority of olfactory projections. We identify 39 morphological types of lateral horn neurons (LHNs) and show that different types receive input from different combinations of glomeruli. We find that different LHN types do not have independent inputs; rather, certain combinations of glomeruli converge onto many of the same LHNs and so are over-represented. Notably, many over-represented combinations are composed of glomeruli that prefer chemically dissimilar ligands whose co-occurrence indicates a behaviorally relevant "odor scene." The pattern of glomerulus-LHN connections thus represents a prediction of what ligand combinations will be most salient.

Neural Correlates of Odor Learning in the Presynaptic Microglomerular Circuitry in the Honeybee Mushroom Body Calyx

The mushroom body (MB) in insects is known as a major center for associative learning and memory, although exact locations for the correlating memory traces remain to be elucidated. Here, we asked whether presynaptic boutons of olfactory projection neurons (PNs) in the main input site of the MB undergo neuronal plasticity during classical odor-reward conditioning and correlate with the conditioned behavior. We simultaneously measured Ca²⁺ responses in the boutons and conditioned behavioral responses to learned odors in honeybees. We found that the absolute amount of the neural change for the rewarded but not for the unrewarded odor was correlated with the behavioral learning rate across individuals. The temporal profile of the induced changes matched with odor response dynamics of the MB-associated inhibitory neurons, suggestive of activity modulation of boutons by this neural class. We hypothesize the circuit-specific neural plasticity relates to the learned value of the stimulus and underlies the conditioned behavior of the bees.

A GABAergic Feedback Shapes Dopaminergic Input on the Drosophila Mushroom Body to Promote Appetitive Long-Term Memory

Memory consolidation is a crucial step for long-term memory (LTM) storage. However, we still lack a clear picture of how memory consolidation is regulated at the neuronal circuit level. Here, we took advantage of the well-described anatomy of the Drosophila olfactory memory center, the mushroom body (MB), to address this question in the context of appetitive LTM. The MB lobes, which are made by the fascicled axons of the MB intrinsic neurons, are organized into discrete anatomical modules, each covered by the terminals of a defined type of dopaminergic neuron (DAN) and the dendrites of a corresponding type of MB output neuron (MBON). We previously revealed the essential role of one DAN, the MP1 neuron, in the formation of appetitive LTM. The MP1 neuron is anatomically matched to the GABAergic MBON MVP2, which has been attributed feedforward inhibitory functions recently. Here, we used behavior experiments and in vivo imaging to challenge the existence of MP1-MVP2 synapses and investigate their role in appetitive LTM consolidation. We show that MP1 and MVP2 neurons form an anatomically and functionally recurrent circuit, which features a feedback inhibition that regulates consolidation of appetitive memory. This circuit involves two opposite type 1 and type 2 dopamine receptors in MVP2 neurons and the metabotropic GABAB-R1 receptor in MP1 neurons. We propose that this dual-receptor feedback supports a bidirectional self-regulation of MP1 input to the MB. This mechanism displays striking similarities with the mammalian reward system, in which modulation of the dopaminergic signal is primarily assigned to inhibitory neurons.

Synaptic depression induced by postsynaptic cAMP production in the Drosophila mushroom body calyx

KEY POINTS

Synaptic potentiation in Drosophila is observed at cholinergic synapses between antennal lobe (AL) and mushroom body (MB) neurons in the adult brain; however, depression at the AL-MB synapses has not yet been identified. By ex vivo Ca²⁺ imaging in an isolated cultured Drosophila brain, we found novel activity-dependent depression at the AL-MB synapses. The degree of Ca²⁺ responses after repetitive AL stimulation is significantly reduced in the dendritic region of MB neurons (calyx) compared with those before AL stimulation, and this reduction of Ca²⁺ responses remains for at least 30 min. The expression of rutabaga, which encodes Ca²⁺ /calmodulin-dependent adenylyl cyclase, is essential in the MB neurons for the reduction of Ca²⁺ responses in the calyx. Our study reveals that elevation of cAMP production in the calyx during repetitive AL stimulation induces the depression at the AL-MB synapses.

ABSTRACT

Synaptic plasticity has been studied to reveal the molecular and cellular mechanisms of associative and non-associative learning. The fruit fly Drosophila melanogaster can be used to identify the molecular mechanisms of synaptic plasticity because vast genetic information or tools are available. Here, by ex vivo Ca²⁺ imaging of an isolated cultured Drosophila brain, we examined the novel activity-dependent synaptic depression between the projection neurons of the antennal lobe (AL) and mushroom body (MB). Ex vivo Ca²⁺ imaging analysis revealed that electrical stimulation of AL elicits Ca²⁺ responses in the dendritic (calyx) and axonal (α lobe) regions of MB neurons, and the responses are reduced after repetitive AL stimulation. Since the cAMP signalling pathway plays an important role in synaptic plasticity in invertebrates and vertebrates, we examined whether the reduction of Ca²⁺ responses is also regulated by the cAMP signalling pathway. The expression of rutabaga (rut), which encodes Ca²⁺ /calmodulin-dependent adenylyl cyclase, was essential for the reduction of Ca²⁺ responses in the calyx and α lobe. Furthermore, imaging analysis using a fluorescence resonance energy transfer-based cAMP indicator revealed that the cAMP level increased in the wild-type calyx during repetitive AL stimulation, whereas it decreased in rut¹ mutant flies with a loss-of-function mutation of rut. Thus, our study suggests that an increase in postsynaptic cAMP level during repetitive AL stimulation contributes to the attenuation of inputs at AL-MB synapses.

Odor Perception on the Two Sides of the Brain: Consistency Despite Randomness

Neurons in piriform cortex receive input from a random collection of glomeruli, resulting in odor representations that lack the stereotypic organization of the olfactory bulb. We have performed in vivo optical imaging and mathematical modeling to demonstrate that correlations are retained in the transformation from bulb to piriform cortex, a feature essential for generalization across odors. Random connectivity also implies that the piriform representation of a given odor will differ among different individuals and across brain hemispheres in a single individual. We show that these different representations can nevertheless support consistent agreement about odor quality across a range of odors. Our model also demonstrates that, whereas odor discrimination and categorization require far fewer neurons than reside in piriform cortex, consistent generalization may require the full complement of piriform neurons.

Reinforcement signaling of punishment versus relief in fruit flies

Painful events establish opponent memories: cues that precede pain are remembered negatively, whereas cues that follow pain, thus coinciding with relief are recalled positively. How do individual reinforcement-signaling neurons contribute to this “timing-dependent valence-reversal?” We addressed this question using an optogenetic approach in the fruit fly. Two types of fly dopaminergic neuron, each comprising just one paired cell, indeed established learned avoidance of odors that preceded their photostimulation during training, and learned approach to odors that followed the photostimulation. This is in striking parallel to punishment versus relief memories reinforced by a real noxious event. For only one of these neuron types, both effects were strong enough for further analyses. Notably, interfering with dopamine biosynthesis in these neurons partially impaired the punishing effect, but not the relieving after-effect of their photostimulation. We discuss how this finding constraints existing computational models of punishment versus relief memories and introduce a new model, which also incorporates findings from mammals. Furthermore, whether using dopaminergic neuron photostimulation or a real noxious event, more prolonged punishment led to stronger relief. This parametric feature of relief may also apply to other animals and may explain particular aspects of related behavioral dysfunction in humans.

Calcium in Kenyon Cell Somata as a Substrate for an Olfactory Sensory Memory in Drosophila

Animals can form associations between temporally separated stimuli. To do so, the nervous system has to retain a neural representation of the first stimulus until the second stimulus appears. The neural substrate of such sensory stimulus memories is unknown. Here, we search for a sensory odor memory in the insect olfactory system and characterize odorant-evoked Ca2+ activity at three consecutive layers of the olfactory system in Drosophila: in olfactory receptor neurons (ORNs) and projection neurons (PNs) in the antennal lobe, and in Kenyon cells (KCs) in the mushroom body. We show that the post-stimulus responses in ORN axons, PN dendrites, PN somata, and KC dendrites are odor-specific, but they are not predictive of the chemical identity of past olfactory stimuli. However, the post-stimulus responses in KC somata carry information about the identity of previous olfactory stimuli. These findings show that the Ca2+ dynamics in KC somata could encode a sensory memory of odorant identity and thus might serve as a basis for associations between temporally separated stimuli.

Dendritic Integration of Sensory Evidence in Perceptual Decision-Making

Perceptual decisions require the accumulation of sensory information to a response criterion. Most accounts of how the brain performs this process of temporal integration have focused on evolving patterns of spiking activity. We report that subthreshold changes in membrane voltage can represent accumulating evidence before a choice. αβ core Kenyon cells (αβ_c KCs) in the mushroom bodies of fruit flies integrate odor-evoked synaptic inputs to action potential threshold at timescales matching the speed of olfactory discrimination. The forkhead box P transcription factor (FoxP) sets neuronal integration and behavioral decision times by controlling the abundance of the voltage-gated potassium channel Shal (K_V4) in αβ_c KC dendrites. αβ_c KCs thus tailor, through a particular constellation of biophysical properties, the generic process of synaptic integration to the demands of sequential sampling.

Behavioral readout of spatio-temporal codes in olfaction

Neural recordings performed at an increasing scale and resolution have revealed complex, spatio-temporally precise patterns of activity in the olfactory system. Multiple models may explain the functional consequences of the spatio-temporal olfactory code, but the link to behavior remains unclear. Recent evidence in the field suggests a behavioral sensitivity to both fine spatial and temporal features in the code. How these features and combinations of features give rise to olfactory behavior is the subject of active research in the field. Modern genetic and optogenetic methods show great promise in testing the link between olfactory codes and behavior.

Do the right thing: neural network mechanisms of memory formation, expression and update in Drosophila

When animals learn, plasticity in brain networks that respond to specific cues results in a change in the behavior that these cues elicit. Individual network components in the mushroom bodies of the fruit fly Drosophila melanogaster represent cues, learning signals and behavioral outcomes of learned experience. Recent findings have highlighted the importance of dopamine-driven plasticity and activity in feedback and feedforward connections, between various elements of the mushroom body neural network. These computational motifs have been shown to be crucial for long term olfactory memory consolidation, integration of internal states, re-evaluation and updating of learned information. The often recurrent circuit anatomy and a prolonged requirement for activity in parts of these underlying networks, suggest that self-sustained and precisely timed activity is a fundamental feature of network computations in the insect brain. Together these processes allow flies to continuously adjust the content of their learned knowledge and direct their behavior in a way that best represents learned expectations and serves their most pressing current needs.

Drosophila mushroom bodies integrate hunger and satiety signals to control innate food-seeking behavior

The fruit fly can evaluate its energy state and decide whether to pursue food-related cues. Here, we reveal that the mushroom body (MB) integrates hunger and satiety signals to control food-seeking behavior. We have discovered five pathways in the MB essential for hungry flies to locate and approach food. Blocking the MB-intrinsic Kenyon cells (KCs) and the MB output neurons (MBONs) in these pathways impairs food-seeking behavior. Starvation bi-directionally modulates MBON responses to a food odor, suggesting that hunger and satiety controls occur at the KC-to-MBON synapses. These controls are mediated by six types of dopaminergic neurons (DANs). By manipulating these DANs, we could inhibit food-seeking behavior in hungry flies or promote food seeking in fed flies. Finally, we show that the DANs potentially receive multiple inputs of hunger and satiety signals. This work demonstrates an information-rich central circuit in the fly brain that controls hunger-driven food-seeking behavior.

Functional architecture of reward learning in mushroom body extrinsic neurons of larval Drosophila

The brain adaptively integrates present sensory input, past experience, and options for future action. The insect mushroom body exemplifies how a central brain structure brings about such integration. Here we use a combination of systematic single-cell labeling, connectomics, transgenic silencing, and activation experiments to study the mushroom body at single-cell resolution, focusing on the behavioral architecture of its input and output neurons (MBINs and MBONs), and of the mushroom body intrinsic APL neuron. Our results reveal the identity and morphology of almost all of these 44 neurons in stage 3 Drosophila larvae. Upon an initial screen, functional analyses focusing on the mushroom body medial lobe uncover sparse and specific functions of its dopaminergic MBINs, its MBONs, and of the GABAergic APL neuron across three behavioral tasks, namely odor preference, taste preference, and associative learning between odor and taste. Our results thus provide a cellular-resolution study case of how brains organize behavior.

Internal State Dependent Odor Processing and Perception-The Role of Neuromodulation in the Fly Olfactory System

Animals rely heavily on their sense of olfaction to perform various vital interactions with an ever-in-flux environment. The turbulent and combinatorial nature of air-borne odorant cues demands the employment of various coding strategies, which allow the animal to attune to its internal needs and past or present experiences. Furthermore, these internal needs can be dependent on internal states such as hunger, reproductive state and sickness. Neuromodulation is a key component providing flexibility under such conditions. Understanding the contributions of neuromodulation, such as sensory neuron sensitization and choice bias requires manipulation of neuronal activity on a local and global scale. With Drosophila's genetic toolset, these manipulations are feasible and even allow a detailed look on the functional role of classical neuromodulators such as dopamine, octopamine and neuropeptides. The past years unraveled various mechanisms adapting chemosensory processing and perception to internal states such as hunger and reproductive state. However, future research should also investigate the mechanisms underlying other internal states including the modulatory influence of endogenous microbiota on Drosophila behavior. Furthermore, sickness induced by pathogenic infection could lead to novel insights as to the neuromodulators of circuits that integrate such a negative postingestive signal within the circuits governing olfactory behavior and learning. The enriched emporium of tools Drosophila provides will help to build a concrete picture of the influence of neuromodulation on olfaction and metabolism, adaptive behavior and our overall understanding of how a brain works.

Persistent activity in a recurrent circuit underlies courtship memory in Drosophila

Recurrent connections are thought to be a common feature of the neural circuits that encode memories, but how memories are laid down in such circuits is not fully understood. Here we present evidence that courtship memory in Drosophila relies on the recurrent circuit between mushroom body gamma (MBγ), M6 output, and aSP13 dopaminergic neurons. We demonstrate persistent neuronal activity of aSP13 neurons and show that it transiently potentiates synaptic transmission from MBγ>M6 neurons. M6 neurons in turn provide input to aSP13 neurons, prolonging potentiation of MBγ>M6 synapses over time periods that match short-term memory. These data support a model in which persistent aSP13 activity within a recurrent circuit lays the foundation for a short-term memory.

Memories help to shape behavior, and can last from a few seconds to an entire lifetime. Working memory, in which information is temporarily held available for use in an ongoing task, is the most fleeting form of memory. It relies on persistent activation of a network of nerve cells or neurons that represent the information in question. Strengthening the connections between those neurons may result in a longer-lasting memory. But the mechanisms that support the formation of memories of different durations are not fully understood.

Zhao et al. have now explored these mechanisms in the fruit fly by studying memory for courtship behavior. Inexperienced male fruit flies will attempt to court both virgin females and females who have recently mated. But the latter reject courtship attempts, and male fruit flies therefore learn to avoid them. This is known as courtship memory, and it relies on a network of neurons within a region of the fruit fly brain called the mushroom body.

Within the mushroom body, dopamine neuron sends signals to a neuron called the Kenyon cell, which in turn sends signals to a mushroom body output neuron. The latter activates circuits responsible for decision-making and movement. But it also activates the dopamine neuron, thereby forming a recurrent circuit or loop. When the courtship is rejected, the dopamine neuron becomes persistently active, which generates a working memory of the experience. If the circuit is activated again during this period of persistent firing, the working memory may be converted into a longer-lasting memory.

The results of Zhao et al. provide insights into the mechanisms by which memories form and undergo strengthening. They suggest that distinct processes within a single neural circuit give rise to memories of different durations. Recurrent loops are also present within the brains of mammals. Similar processes may thus support the formation and persistence of our own memories.

Ex vivo Whole-cell Recordings in Adult Drosophila Brain

Cost-effective and efficient, the fruit fly (Drosophila melanogaster) has been used to make many key discoveries in the field of neuroscience and to model a number of neurological disorders. Great strides in understanding have been made using sophisticated molecular genetic tools and behavioral assays. Functional analysis of neural activity was initially limited to the neuromuscular junction (NMJ) and in the central nervous system (CNS) of embryos and larvae. Elucidating the cellular mechanisms underlying neurological processes and disorders in the mature nervous system have been more challenging due to difficulty in recording from neurons in adult brains. To this aim we developed an ex vivo preparation in which a whole brain is isolated from the head capsule of an adult fly and placed in a recording chamber. With this preparation, whole cell recording of identified neurons in the adult brain can be combined with genetic, pharmacological and environmental manipulations to explore cellular mechanisms of neuronal function and dysfunction. It also serves as an important platform for evaluating the mechanism of action of new therapies identified through behavioral assays for treating neurological diseases. Here we present our protocol for ex vivo preparations and whole-cell recordings in the adult Drosophila brain.

Cyclic AMP-dependent plasticity underlies rapid changes in odor coding associated with reward learning

Learning and memory rely on dopamine and downstream cAMP-dependent plasticity across diverse organisms. Despite the central role of cAMP signaling, it is not known how cAMP-dependent plasticity drives coherent changes in neuronal physiology that encode the memory trace, or engram. In Drosophila, the mushroom body (MB) is critically involved in olfactory classical conditioning, and cAMP signaling molecules are necessary and sufficient for normal memory in intrinsic MB neurons. To evaluate the role of cAMP-dependent plasticity in learning, we examined how cAMP manipulations and olfactory classical conditioning modulate olfactory responses in the MB with in vivo imaging. Elevating cAMP pharmacologically or optogenetically produced plasticity in MB neurons, altering their responses to odorants. Odor-evoked Ca²⁺ responses showed net facilitation across anatomical regions. At the single-cell level, neurons exhibited heterogeneous responses to cAMP elevation, suggesting that cAMP drives plasticity to discrete subsets of MB neurons. Olfactory appetitive conditioning enhanced MB odor responses, mimicking the cAMP-dependent plasticity in directionality and magnitude. Elevating cAMP to equivalent levels as appetitive conditioning also produced plasticity, suggesting that the cAMP generated during conditioning affects odor-evoked responses in the MB. Finally, we found that this plasticity was dependent on the Rutabaga type I adenylyl cyclase, linking cAMP-dependent plasticity to behavioral modification. Overall, these data demonstrate that learning produces robust cAMP-dependent plasticity in intrinsic MB neurons, which is biased toward naturalistic reward learning. This suggests that cAMP signaling may serve to modulate intrinsic MB responses toward salient stimuli.

A Drosophila ex vivo model of olfactory appetitive learning

During olfactory appetitive learning, animals associate an odor, or conditioned stimulus (CS), with an unconditioned stimulus (US), often a sugar reward. This association induces feeding behavior, a conditioned response (CR), upon subsequent exposure to the CS. In this study, we developed a model of this behavior in isolated Drosophila brains. Artificial activation of neurons expressing the Gr5a sugar-responsive gustatory receptor (Gr5a GRNs) induces feeding behavior in starved flies. Consistent with this, we find that in dissected brains, activation of Gr5a GRNs induces Ca²⁺ transients in motor neurons, MN11 + 12, required for ingestion. Significantly, activation of Gr5a GRNs can substitute for presentation of sugar rewards during olfactory appetitive learning. Similarly, in dissected brains, coincident stimulation of Gr5a GRNs and the antennal lobe (AL), which processes olfactory information, results in increased Ca²⁺ influx into MN11 + 12 cells upon subsequent AL stimulation. Importantly, olfactory appetitive associations are not formed in satiated flies. Likewise, AL-evoked Ca²⁺ transients in MN11 + 12 are not produced in ex vivo brains from satiated flies. Our results suggest that a starved/satiated state is maintained in dissected brains, and that this ex vivo system will be useful for identification of neural networks involved in olfactory appetitive learning.

Behavioral Modulation by Spontaneous Activity of Dopamine Neurons

Dopamine modulates a variety of animal behaviors that range from sleep and learning to courtship and aggression. Besides its well-known phasic firing to natural reward, a substantial number of dopamine neurons (DANs) are known to exhibit ongoing intrinsic activity in the absence of an external stimulus. While accumulating evidence points at functional implications for these intrinsic "spontaneous activities" of DANs in cognitive processes, a causal link to behavior and its underlying mechanisms has yet to be elucidated. Recent physiological studies in the model organism Drosophila melanogaster have uncovered that DANs in the fly brain are also spontaneously active, and that this activity reflects the behavioral/internal states of the animal. Strikingly, genetic manipulation of basal DAN activity resulted in behavioral alterations in the fly, providing critical evidence that links spontaneous DAN activity to behavioral states. Furthermore, circuit-level analyses have started to reveal cellular and molecular mechanisms that mediate or regulate spontaneous DAN activity. Through reviewing recent findings in different animals with the major focus on flies, we will discuss potential roles of this physiological phenomenon in directing animal behaviors.

Neuronal and molecular mechanisms of sleep homeostasis

Sleep is necessary for survival, and prolonged waking causes a homeostatic increase in the need for recovery sleep. Homeostasis is a core component of sleep regulation and has been tightly conserved across evolution from invertebrates to man. Homeostatic sleep regulation was first identified among insects in cockroaches several decades ago, but the characterization of sleep rebound in Drosophila melanogaster opened the use of insect model species to understand homeostatic functions and regulation of sleep. This review describes circuits in two neuropil structures, the central complex and mushroom bodies, that influence sleep homeostasis and neuromodulatory systems that influence the accrual of homeostatic sleep need.

The complete connectome of a learning and memory centre in an insect brain

Associating stimuli with positive or negative reinforcement is essential for survival, but a complete wiring diagram of a higher-order circuit supporting associative memory has not been previously available. Here we reconstruct one such circuit at synaptic resolution, the Drosophila larval mushroom body. We find that most Kenyon cells integrate random combinations of inputs but that a subset receives stereotyped inputs from single projection neurons. This organization maximizes performance of a model output neuron on a stimulus discrimination task. We also report a novel canonical circuit in each mushroom body compartment with previously unidentified connections: reciprocal Kenyon cell to modulatory neuron connections, modulatory neuron to output neuron connections, and a surprisingly high number of recurrent connections between Kenyon cells. Stereotyped connections found between output neurons could enhance the selection of learned behaviours. The complete circuit map of the mushroom body should guide future functional studies of this learning and memory centre.

The malleable brain: plasticity of neural circuits and behavior - a review from students to students

One of the most intriguing features of the brain is its ability to be malleable, allowing it to adapt continually to changes in the environment. Specific neuronal activity patterns drive long-lasting increases or decreases in the strength of synaptic connections, referred to as long-term potentiation and long-term depression, respectively. Such phenomena have been described in a variety of model organisms, which are used to study molecular, structural, and functional aspects of synaptic plasticity. This review originated from the first International Society for Neurochemistry (ISN) and Journal of Neurochemistry (JNC) Flagship School held in Alpbach, Austria (Sep 2016), and will use its curriculum and discussions as a framework to review some of the current knowledge in the field of synaptic plasticity. First, we describe the role of plasticity during development and the persistent changes of neural circuitry occurring when sensory input is altered during critical developmental stages. We then outline the signaling cascades resulting in the synthesis of new plasticity-related proteins, which ultimately enable sustained changes in synaptic strength. Going beyond the traditional understanding of synaptic plasticity conceptualized by long-term potentiation and long-term depression, we discuss system-wide modifications and recently unveiled homeostatic mechanisms, such as synaptic scaling. Finally, we describe the neural circuits and synaptic plasticity mechanisms driving associative memory and motor learning. Evidence summarized in this review provides a current view of synaptic plasticity in its various forms, offers new insights into the underlying mechanisms and behavioral relevance, and provides directions for future research in the field of synaptic plasticity. Read the Editorial Highlight for this article on page 788. Cover Image for this issue: doi: 10.1111/jnc.13815.

Long-term memory requires sequential protein synthesis in three subsets of mushroom body output neurons in Drosophila

Creating long-term memory (LTM) requires new protein synthesis to stabilize learning-induced synaptic changes in the brain. In the fruit fly, Drosophila melanogaster, aversive olfactory learning forms several phases of labile memory to associate an odor with coincident punishment in the mushroom body (MB). It remains unclear how the brain consolidates early labile memory into LTM. Here, we survey 183 Gal4 lines containing almost all 21 distinct types of MB output neurons (MBONs) and show that sequential synthesis of learning-induced proteins occurs at three types of MBONs. Downregulation of oo18 RNA-binding proteins (ORBs) in any of these MBONs impaired LTM. And, neurotransmission outputs from these MBONs are all required during LTM retrieval. Together, these results suggest an LTM consolidation model in which transient neural activities of early labile memory in the MB are consolidated into stable LTM at a few postsynaptic MBONs through sequential ORB-regulated local protein synthesis.

A connectome of a learning and memory center in the adult Drosophila brain

Understanding memory formation, storage and retrieval requires knowledge of the underlying neuronal circuits. In Drosophila, the mushroom body (MB) is the major site of associative learning. We reconstructed the morphologies and synaptic connections of all 983 neurons within the three functional units, or compartments, that compose the adult MB’s α lobe, using a dataset of isotropic 8 nm voxels collected by focused ion-beam milling scanning electron microscopy. We found that Kenyon cells (KCs), whose sparse activity encodes sensory information, each make multiple en passant synapses to MB output neurons (MBONs) in each compartment. Some MBONs have inputs from all KCs, while others differentially sample sensory modalities. Only 6% of KC>MBON synapses receive a direct synapse from a dopaminergic neuron (DAN). We identified two unanticipated classes of synapses, KC>DAN and DAN>MBON. DAN activation produces a slow depolarization of the MBON in these DAN>MBON synapses and can weaken memory recall.

DOI:http://dx.doi.org/10.7554/eLife.26975.001

Trace Conditioning in Drosophila Induces Associative Plasticity in Mushroom Body Kenyon Cells and Dopaminergic Neurons

Dopaminergic neurons (DANs) signal punishment and reward during associative learning. In mammals, DANs show associative plasticity that correlates with the discrepancy between predicted and actual reinforcement (prediction error) during classical conditioning. Also in insects, such as Drosophila, DANs show associative plasticity that is, however, less understood. Here, we study associative plasticity in DANs and their synaptic partners, the Kenyon cells (KCs) in the mushroom bodies (MBs), while training Drosophila to associate an odorant with a temporally separated electric shock (trace conditioning). In most MB compartments DANs strengthened their responses to the conditioned odorant relative to untrained animals. This response plasticity preserved the initial degree of similarity between the odorant- and the shock-induced spatial response patterns, which decreased in untrained animals. Contrary to DANs, KCs (α'/β'-type) decreased their responses to the conditioned odorant relative to untrained animals. We found no evidence for prediction error coding by DANs during conditioning. Rather, our data supports the hypothesis that DAN plasticity encodes conditioning-induced changes in the odorant's predictive power.

Parallel memory traces are built after an experience containing aversive and appetitive components in the crab Neohelice

The neurobiology of learning and memory has been mainly studied by focusing on pure aversive or appetitive experiences. Here, we challenged this approach considering that real-life stimuli come normally associated with competing aversive and appetitive consequences and that interaction between conflicting information must be intrinsic part of the memory processes. We used Neohelice crabs, taking advantage of two well-described appetitive and aversive learning paradigms and combining them in a single training session to evaluate how this affects memory. We found that crabs build separate appetitive and aversive memories that compete during retrieval but not during acquisition. Which memory prevails depends on the balance between the strength of the unconditioned stimuli and on the motivational state of the animals. The results indicate that after a mix experience with appetitive and aversive consequences, parallel memories are established in a way that appetitive and aversive information is stored to be retrieved in an opportunistic manner.

Representations of Novelty and Familiarity in a Mushroom Body Compartment

Animals exhibit a behavioral response to novel sensory stimuli about which they have no prior knowledge. We have examined the neural and behavioral correlates of novelty and familiarity in the olfactory system of Drosophila. Novel odors elicit strong activity in output neurons (MBONs) of the α'3 compartment of the mushroom body that is rapidly suppressed upon repeated exposure to the same odor. This transition in neural activity upon familiarization requires odor-evoked activity in the dopaminergic neuron innervating this compartment. Moreover, exposure of a fly to novel odors evokes an alerting response that can also be elicited by optogenetic activation of α'3 MBONs. Silencing these MBONs eliminates the alerting behavior. These data suggest that the α'3 compartment plays a causal role in the behavioral response to novel and familiar stimuli as a consequence of dopamine-mediated plasticity at the Kenyon cell-MBONα'3 synapse.

Reciprocal synapses between mushroom body and dopamine neurons form a positive feedback loop required for learning

Current thought envisions dopamine neurons conveying the reinforcing effect of the unconditioned stimulus during associative learning to the axons of Drosophila mushroom body Kenyon cells for normal olfactory learning. Here, we show using functional GFP reconstitution experiments that Kenyon cells and dopamine neurons from axoaxonic reciprocal synapses. The dopamine neurons receive cholinergic input via nicotinic acetylcholine receptors from the Kenyon cells; knocking down these receptors impairs olfactory learning revealing the importance of these receptors at the synapse. Blocking the synaptic output of Kenyon cells during olfactory conditioning reduces presynaptic calcium transients in dopamine neurons, a finding consistent with reciprocal communication. Moreover, silencing Kenyon cells decreases the normal chronic activity of the dopamine neurons. Our results reveal a new and critical role for positive feedback onto dopamine neurons through reciprocal connections with Kenyon cells for normal olfactory learning.

DOI:http://dx.doi.org/10.7554/eLife.23789.001

What can tiny mushrooms in fruit flies tell us about learning and memory?

Nervous systems have evolved to translate external stimuli into appropriate behavioral responses. In an ever-changing environment, flexible adjustment of behavioral choice by experience-dependent learning is essential for the animal's survival. Associative learning is a simple form of learning that is widely observed from worms to humans. To understand the whole process of learning, we need to know how sensory information is represented and transformed in the brain, how it is changed by experience, and how the changes are reflected on motor output. To tackle these questions, studying numerically simple invertebrate nervous systems has a great advantage. In this review, I will feature the Pavlovian olfactory learning in the fruit fly, Drosophila melanogaster. The mushroom body is a key brain area for the olfactory learning in this organism. Recently, comprehensive anatomical information and the genetic tool sets were made available for the mushroom body circuit. This greatly accelerated the physiological understanding of the learning process. One of the key findings was dopamine-induced long-term synaptic plasticity that can alter the representations of stimulus valence. I will mostly focus on the new studies within these few years and discuss what we can possibly learn about the vertebrate systems from this model organism.

Two Components of Aversive Memory in Drosophila, Anesthesia-Sensitive and Anesthesia-Resistant Memory, Require Distinct Domains Within the Rgk1 Small GTPase

Multiple components have been identified that exhibit different stabilities for aversive olfactory memory in Drosophila These components have been defined by behavioral and genetic studies and genes specifically required for a specific component have also been identified. Intermediate-term memory generated after single cycle conditioning is divided into anesthesia-sensitive memory (ASM) and anesthesia-resistant memory (ARM), with the latter being more stable. We determined that the ASM and ARM pathways converged on the Rgk1 small GTPase and that the N-terminal domain-deleted Rgk1 was sufficient for ASM formation, whereas the full-length form was required for ARM formation. Rgk1 is specifically accumulated at the synaptic site of the Kenyon cells (KCs), the intrinsic neurons of the mushroom bodies, which play a pivotal role in olfactory memory formation. A higher than normal Rgk1 level enhanced memory retention, which is consistent with the result that Rgk1 suppressed Rac-dependent memory decay; these findings suggest that rgk1 bolsters ASM via the suppression of forgetting. We propose that Rgk1 plays a pivotal role in the regulation of memory stabilization by serving as a molecular node that resides at KC synapses, where the ASM and ARM pathway may interact.SIGNIFICANCE STATEMENT Memory consists of multiple components. Drosophila olfactory memory serves as a fundamental model with which to investigate the mechanisms that underlie memory formation and has provided genetic and molecular means to identify the components of memory, namely short-term, intermediate-term, and long-term memory, depending on how long the memory lasts. Intermediate memory is further divided into anesthesia-sensitive memory (ASM) and anesthesia-resistant memory (ARM), with the latter being more stable. We have identified a small GTPase in Drosophila, Rgk1, which plays a pivotal role in the regulation of olfactory memory stability. Rgk1 is required for both ASM and ARM. Moreover, N-terminal domain-deleted Rgk1 was sufficient for ASM formation, whereas the full-length form was required for ARM formation.

Null EPAC mutants reveal a sequential order of versatile cAMP effects during Drosophila aversive odor learning

Here, we define a role of the cAMP intermediate EPAC in Drosophila aversive odor learning by means of null epac mutants. Complementation analysis revealed that EPAC acts downstream from the rutabaga adenylyl cyclase and in parallel to protein kinase A. By means of targeted knockdown and genetic rescue we identified mushroom body Kenyon cells (KCs) as a necessary and sufficient site of EPAC action. We provide mechanistic insights by analyzing acquisition dynamics and using the "performance increment" as a means to access the trial-based sequential organization of odor learning. Thereby we show that versatile cAMP-dependent mechanisms are engaged within a sequential order that correlate to individual trials of the training session.