No abstract

Animals discriminate stimuli, learn their predictive value and use this knowledge to modify their behavior. In Drosophila, the mushroom body (MB) plays a key role in these processes. Sensory stimuli are sparsely represented by ∼2000 Kenyon cells, which converge onto 34 output neurons (MBONs) of 21 types. We studied the role of MBONs in several associative learning tasks and in sleep regulation, revealing the extent to which information flow is segregated into distinct channels and suggesting possible roles for the multi-layered MBON network. We also show that optogenetic activation of MBONs can, depending on cell type, induce repulsion or attraction in flies. The behavioral effects of MBON perturbation are combinatorial, suggesting that the MBON ensemble collectively represents valence. We propose that local, stimulus-specific dopaminergic modulation selectively alters the balance within the MBON network for those stimuli. Our results suggest that valence encoded by the MBON ensemble biases memory-based action selection.DOI: http://dx.doi.org/10.7554/eLife.04580.001

Animals approach stimuli that predict a pleasant outcome. After the paired presentation of an odour and a reward, Drosophila melanogaster can develop a conditioned approach towards that odour. Despite recent advances in understanding the neural circuits for associative memory and appetitive motivation, the cellular mechanisms for reward processing in the fly brain are unknown. Here we show that a group of dopamine neurons in the protocerebral anterior medial (PAM) cluster signals sugar reward by transient activation and inactivation of target neurons in intact behaving flies. These dopamine neurons are selectively required for the reinforcing property of, but not a reflexive response to, the sugar stimulus. In vivo calcium imaging revealed that these neurons are activated by sugar ingestion and the activation is increased on starvation. The output sites of the PAM neurons are mainly localized to the medial lobes of the mushroom bodies (MBs), where appetitive olfactory associative memory is formed. We therefore propose that the PAM cluster neurons endow a positive predictive value to the odour in the MBs. Dopamine in insects is known to mediate aversive reinforcement signals. Our results highlight the cellular specificity underlying the various roles of dopamine and the importance of spatially segregated local circuits within the MBs.

6 These authors contributed equally to this work. 2 Aversive olfactory memory is formed in the Drosophila mushroom bodies (MB).Memory retrieval requires MB output, but it remains unknown how a memory trace in the MB drives conditioned avoidance of a learned odour. To identify neurons involved in olfactory memory retrieval, we performed an anatomical and functional screen of defined sets of MB extrinsic neurons. Here we show that MB-V2 neurons are essential for retrieval of both short-and long-lasting memory, but neither for memory formation nor for memory consolidation. We further show that MB-V2 are cholinergic efferent neurons that project from the MB vertical lobes to the middle superiormedial protocerebrum and the lateral horn (LH). Notably, the odour response of MB-V2 neurons is modified after conditioning. As the LH is implicated in innate responses to repellent odorants, we propose that during memory retrieval, MB-V2 neurons reinforce the olfactory pathway involved in innate odour avoidance.Different odours induce innate approach or avoidance behaviours in Drosophila. Innate odour responses can be modulated by experience, such as associative learning. After simultaneous exposure to an electric shock and an odorant, flies form aversive memory and show robust conditioned odour avoidance that lasts for hours to days, depending on the training protocol [1][2][3] . The neural pathways for odour or shock processing and signal integration in the fly brain have been intensely studied in recent years. Odour information is first represented in the antennal lobes in the form of olfactory receptor neuron activity 4 . Projection neurons then convey this information to higher order processing centres 4 : the mushroom bodies (MB) and the lateral horn (LH). In contrast, aversive reinforcement signals in response to electric shock are relayed to the MB via dopaminergic neurons [5][6][7] . The olfactory and 3 electric shock signals are integrated in the MB to form aversive olfactory memory 1, 2 . The MB are however dispensable for innate avoidance of the repellent odours 8,9 .In adult Drosophila, the MB consist of approximately 2000 Kenyon cells per brain hemisphere, which may be classified into three major types based on their axonal projection: γ neurons, which form only a medial lobe, α/β neurons, whose axons branch to form a vertical (α) and a medial (β) lobe, and α'/β' neurons, which also form a vertical (α') and a medial (β') lobe 10 . Functional brain imaging has revealed localised activation of cAMP/PKA signalling in the MB α lobe in response to simultaneous cholinergic and dopaminergic stimulation 11,12 , that represent respectively the odorant and electric shock pathways.Following associative conditioning, calcium imaging studies have shown that a short-term memory trace is formed in the α'/β' neurons 13, and a long-term one in α lobes 14 . Previous studies have shown that the output of the α/β neurons is necessary for the retrieval of all phases of olfactory memory 15,16 , but the neural circuits that translate ...

The mushroom bodies, substructures of the Drosophila brain, are involved in olfactory learning and short-term memory, but their role in long-term memory is unknown. Here we show that the alpha-lobes-absent (ala) mutant lacks either the two vertical lobes of the mushroom body or two of the three median lobes which contain branches of vertical lobe neurons. This unique phenotype allows analysis of mushroom body function. Long-term memory required the presence of the vertical lobes but not the median lobes. Short-term memory was normal in flies without either vertical lobes or the two median lobes studied.