The reconstruction and functional mapping of a recurrent microcircuit inDrosophilamushroom body

Abstract

21In Drosophila melanogaster, mushroom body and anterior paired lateral (APL) neurons play im-22 portant roles not only in learning and memory but also in high cognitive behavior, reversal learn-23 ing. The circuit between APL neurons and Kenyon cells (KCs) in the mushroom body underlies 24 this behavior, including reversal learning, and electron microscopy (EM) methods must be used to 25 reveal this circuit. Here, we reconstructed the connections between mushroom body cells and APL 26 neurons in the vertical lobe of the mushroom body via focused ion beam scanning electron mi-27 croscopy (FIB-SEM) and sparse genetic horseradish peroxidase (HRP) labeling. We offer the first 28 EM evidence that recurrent network and lateral inhibition connections exist between APL neurons 29 and KCs in the vertical lobe of the mushroom body. This circuit is the neural basis of action selec-30 tion decision making, associative learning and reversal learning. Additionally, dopamine neurons 31 project to different areas of mushroom bodies and, together with extrinsic neurons and KC axons, 32 form a compartmental structure of mushroom body axons, thereby restricting the KC-mushroom 33 body output neuron (MBON) response to local compartments. Whether APL neurons also respond 34 locally is uncertain. We found that APL neurons exhibited input and output synapses that were 35 intermixed and arranged on enlarged and thin sections, respectively, resembling a string of beads. 36 Different KCs were found to project to APL neurons nonrepetitively, forming a local circuit struc-37 ture. Furthermore, using a single neurite calcium imaging method, we identified local calcium 38 domains on this circuit, suggestive of individual electrical compartments. The electrically recorded 39 APL neurons were nonspike neurons that selectively responded to odor in both the lobes and calyx. 40 Thus, the localized APL neuron responses coordinate with mushroom body-dopamine-MBON 41 compartmental function. 42 mushroom bodies in the MBON-β2β′2a pathway to mediate 3-h anesthesia-sensitive memory 65 (Pitman et al., 2011; Yang et al., 2016). 66 Some neuromodulator neurons innervate mushroom bodies, and dopaminergic neurons might 67 mediate unconditional stimuli, such as reward and punishment during learning. Dopamine neurons 68 couple with KCs, which encode odor information for conditioning, learning and storage of olfac-69 tory information. 70 Dopaminergic and extrinsic neurons have been shown to project to different areas of mushroom 71 body lobes and separate mushroom bodies into 15 functional compartments (Aso et al., 2014a, 72 2014b). The dopamine neurons that project locally encode innate states as well as external experi-73 ences. Together with KC axons and extrinsic neurons, the locally projecting dopamine neurons 74 form local information circuits in this area (Cohn, Morantte, & Ruta, 2015), dopamine neurons 75 constrain KC neuron responding locally. 76 Studies on APL neuron morphology have shown that these neurons innervate whole lobes and 77 the calyx (Liu &...

The anterior paired lateral neuron normalizes odour-evoked activity at the mushroom body calyx

“…As a matter of fact, similar examples of localized APL response as described here have been reported in the Drosophila MB (Amin et al 2020;Wang et al . CC-BY-NC-ND 4.0 International license perpetuity.…”

“…1981). As a matter of fact, similar examples of localized APL response as described here have been reported in the Drosophila MB (Amin et al 2020; Wang et al 2019) as well as in the APĹs homolog GGN in the locust (Ray, Aldworth, and Stopfer 2020; Leitch and Laurent 1996). An advantage of having local microcircuits is that it allows a single neuron to mimic the activity of several inhibitory interneurons, as described in amacrine cells of both mammals (Grimes et al 2010) and Drosophila (Meier and Borst 2019).…”

The anterior paired lateral neuron normalizes odour-evoked activity in the Drosophila mushroom body calyx

“…Non-spiking interneurons in insects are typically large and characterized by complex neurite branching, an ideal structure to support local microcircuits (Roberts et al 1981). As a matter of fact, similar examples of localized APL response as described here have been reported in the Drosophila MB (Amin et al 2020;Wang et al 2019) as well as in the APL´s homolog GGN in the locust (Ray, Aldworth, and Stopfer 2020;Leitch and Laurent 1996). An advantage of having local microcircuits is that it allows a single neuron to mimic the activity of several inhibitory interneurons, as described in amacrine cells of both mammals (Grimes et al 2010) and Drosophila (Meier and Borst 2019).…”

Localized inhibition in the Drosophila mushroom body

“…To compare the spatial extent of activity across flies, we normalized distances in these backbones to a ‘standard’ backbone based on the average across all flies ( Figure 5B ). Most neurites in APL run in parallel with this backbone ( Takemura et al, 2017 ; Wang et al, 2019 ; Scheffer et al, 2020 ), making this backbone a meaningful axis along which to measure spread of APL activity.…”

“…Unlike the larval APL ( Eichler et al, 2017 ; Masuda-Nakagawa et al, 2014 ), the adult APL has pre- and post-synaptic specializations throughout all its processes ( Wu et al, 2013 ), and connectomic reconstructions show that APL forms reciprocal synapses with Kenyon cells throughout the mushroom body ( Takemura et al, 2017 ; Scheffer et al, 2020 ; Zheng et al, 2018 ). There have been reports of localized activity within APL ( Inada et al, 2017 ; Wang et al, 2019 ), and compartmental modeling suggests that in the locust GGN, depolarization should diminish with distance from the site of current input, although not to zero ( Ray et al, 2020 ). However, intracellular activity propagation is difficult to measure experimentally in locusts, and the lack of action potentials in APL at the soma ( Papadopoulou et al, 2011 ) does not rule out active conductances elsewhere, given that mammalian dendrites often show dendritic spikes that do not necessarily propagate to the soma ( Branco and Häusser, 2010 ).…”