The reconstruction and functional mapping of a recurrent microcircuit in Drosophila mushroom body

In Drosophila melanogaster, mushroom body and anterior paired lateral (APL) neurons play important roles not only in learning and memory but also in high cognitive behavior, reversal learning. The circuit between APL neurons and Kenyon cells (KCs) in the mushroom body underlies this behavior, including reversal learning, and electron microscopy (EM) methods must be used to reveal this circuit. Here, we reconstructed the connections between mushroom body cells and APL neurons in the vertical lobe of the mushroom body via focused ion beam scanning electron microscopy (FIB-SEM) and sparse genetic horseradish peroxidase (HRP) labeling. We offer the first EM evidence that recurrent network and lateral inhibition connections exist between APL neurons and KCs in the vertical lobe of the mushroom body. This circuit is the neural basis of action selection decision making, associative learning and reversal learning. Additionally, dopamine neurons project to different areas of mushroom bodies and, together with extrinsic neurons and KC axons, form a compartmental structure of mushroom body axons, thereby restricting the KC-mushroom body output neuron (MBON) response to local compartments. Whether APL neurons also respond locally is uncertain. We found that APL neurons exhibited input and output synapses that were intermixed and arranged on enlarged and thin sections, respectively, resembling a string of beads. Different KCs were found to project to APL neurons nonrepetitively, forming a local circuit structure. Furthermore, using a single neurite calcium imaging method, we identified local calcium domains on this circuit, suggestive of individual electrical compartments. The electrically recorded APL neurons were nonspike neurons that selectively responded to odor in both the lobes and calyx. Thus, the localized APL neuron responses coordinate with mushroom body-dopamine-MBON compartmental function.