Mushroom body efferent neurons responsible for aversive olfactory memory retrieval in Drosophila

Abstract

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 ...

Distinct dopamine neurons mediate reward signals for short- and long-term memories

“…All of the confocal images in the main figures underwent landmark matching-based image registration using BrainAligner (59). For in vivo calcium imaging, female flies expressing GCaMP3 (60) with R48B04-GAL4 and R15A04-GAL4 were prepared essentially as described (19,61). A droplet of 2 M sucrose, 3 M arabinose, or a mixture of 3 M arabinose and 2 M sorbitol solution in mineral water was delivered on a plastic plate controlled by a micromanipulator (19 …”

Synapsin Determines Memory Strength after Punishment- and Relief-Learning

“…We conclude that restoring Synapsin in the set of cells covered by mb247-Gal4, covering the mushroom body ␣, ␤, and ␥ lobes, and with faint if any background expression (Aso et al, 2009;Fig. 8A), is sufficient to restore the defects in both relief memory and punishment memory, which ensue upon a lack of Synapsin.…”

“…What is clear is that shock activates a subset of dopaminergic neurons mediating an internal punishment signal. These neurons provide input (mb-input neurons) to a sufficient number of mushroom body neurons to cover the full olfactory stimulus space (Ito et al, 1998;Schwaerzel et al, 2003;Kim et al, 2007;Aso et al, 2009;Claridge-Chang et al, 2009;Mao and Davis, 2009;Aso et al, 2010Aso et al, , 2012Burke et al, 2012;Pech et al, 2013;Das et al, 2014;Galili et al, 2014;larval Drosophila: Schroll et al, 2006;Selcho et al, 2009). …”

The differential requirement of mushroom body α/β subdivisions in long-term memory retrieval in Drosophila

“…Also, MB α/β subdivisions might communicate with each other through feedback loops constructed by various MB extrinsic neurons, such as DPM neurons and APL neurons (Lee et al, 1999;Keene et al, 2006;Wu et al, 2011;Pitman et al, 2011). Another possibility is that MB α/β subdivisions, being parallel with each other, release neurotransmitters to their common downstream neurons suggested for LTM storage, such as the DAL neurons (Chen et al, 2012) MB-V2 neurons (Séjourné et al, 2011), MB-V3 neurons (Pai et al, 2013) and the ellipsoid body (Wu et al, 2007), to regulate formation, consolidation and retrieval of LTM separately.…”