Transsynaptic mapping of Drosophila mushroom body output neurons

The mushroom body (MB) is a well-characterized associative memory structure within the Drosophila brain. Analyzing MB connectivity using multiple approaches is critical for understanding the functional implications of this structure. Using the genetic anterograde transsynaptic tracing tool, trans-Tango, we identified divergent projections across the brain and convergent downstream targets of the MB output neurons (MBONs). Our analysis revealed at least three separate targets that receive convergent input from MBONs: other MBONs, the fan-shaped body (FSB), and the lateral accessory lobe (LAL). We describe, both anatomically and functionally, a multilayer circuit in which inhibitory and excitatory MBONs converge on the same genetic subset of FSB and LAL neurons. This circuit architecture enables the brain to update and integrate information with previous experience before executing appropriate behavioral responses. Our use of trans-Tango provides a genetically accessible anatomical framework for investigating the functional relevance of components within these complex and interconnected circuits.

Encoding and control of orientation to airflow by a set of Drosophila fan-shaped body neurons

The insect central complex (CX) is thought to underlie goal-oriented navigation but its functional organization is not fully understood. We recorded from genetically-identified CX cell types in Drosophila and presented directional visual, olfactory, and airflow cues known to elicit orienting behavior. We found that a group of neurons targeting the ventral fan-shaped body (ventral P-FNs) are robustly tuned for airflow direction. Ventral P-FNs did not generate a ‘map’ of airflow direction. Instead, cells in each hemisphere were tuned to 45° ipsilateral, forming a pair of orthogonal bases. Imaging experiments suggest that ventral P-FNs inherit their airflow tuning from neurons that provide input from the lateral accessory lobe (LAL) to the noduli (NO). Silencing ventral P-FNs prevented flies from selecting appropriate corrective turns following changes in airflow direction. Our results identify a group of CX neurons that robustly encode airflow direction and are required for proper orientation to this stimulus.

Transsynaptic mapping of Drosophila mushroom body output neurons

The Mushroom Body (MB) is a well-characterized associative memory structure within the Drosophila brain. Although previous studies have analyzed MB connectivity and provided a map of inputs and outputs, a detailed map of the downstream targets is missing. Using the genetic anterograde transsynaptic tracing tool, trans-Tango, we identified divergent projections across the brain and convergent downstream targets of the MB output neurons (MBONs). Our analysis revealed at least three separate targets that receive convergent input from MBONs: other MBONs, the fan shaped body (FSB), and the lateral accessory lobe (LAL). We describe, both anatomically and functionally, a multilayer circuit in which inhibitory and excitatory MBONs converge on the same genetic subset of FSB and LAL neurons. This circuit architecture provides an opportunity for the brain to update information and integrate it with previous experience before executing appropriate behavioral responses.Highlights-The postsynaptic connections of the output neurons of the mushroom body, a structure that integrates environmental cues with associated valence, are mapped using trans-Tango.-Mushroom body circuits are highly interconnected with several points of convergence among mushroom body output neurons (MBONs).-The postsynaptic partners of MBONs have divergent projections across the brain and convergent projections to select target neuropils outside the mushroom body important for multimodal integration.-Functional connectivity suggests the presence of multisynaptic pathways that have several layers of integration prior to initiation of an output response.

Encoding and control of airflow orientation by a set of Drosophila fan-shaped body neurons

How brain circuits convert sensory signals into goal-oriented movement is a central question in neuroscience. In insects, a region known as the Central Complex (CX) is believed to support navigation, but how its compartments process diverse sensory cues to guide navigation is not fully clear. To address this question, we recorded from genetically-identified CX cell types in Drosophila and presented directional visual, olfactory, and airflow cues known to elicit orienting behavior. We found that a group of columnar neurons targeting the ventral fan-shaped body (ventral P-FNs) are robustly tuned for airflow direction. Unlike compass neurons (E-PGs), ventral P-FNs do not generate a “map” of airflow direction; rather they are tuned to two directions – approximately 45° to the right or left of the midline – depending on the hemisphere of the cell body. Ventral P-FNs with both direction preferences innervate each CX column, potentially forming a basis for constructing representations of airflow in various directions. We explored two possible sources for ventral P-FN airflow tuning, and found that they mostly likely inherit these responses via a pathway from the lateral accessory lobe (LAL) to the noduli (NO). Silencing ventral P-FNs prevented flies from adopting stable orientations relative to airflow in closed-loop flight. Specifically, silenced flies selected improper corrective turns following changes in airflow direction, but not after airflow pauses, suggesting a specific deficit in sensory-motor action selection. Our results identify a group of central complex neurons that robustly encode airflow direction and are required for proper orientation to this stimulus.

Descending pathways from the lateral accessory lobe and posterior slope in the brain of the silkmoth Bombyx mori

A population of descending neurons connect the brain and thoracic motor cener, playing a critical role in controlling behavior. We examined the anatomical organization of descending neurons (DNs) in the brain of the silkmoth Bombyx mori. Moth pheromone orientation is a good model to investigate the neuronal mechanisms of olfactory behavior. Based on mass staining and single-cell staining, we evaluated the anatomical organization of neurite distribution by DNs in the brain. Dense innervation was observed in the posterior–ventral part of the brain, called the posterior slope (PS). We examined the morphology of DNs innervating the lateral accessory lobe (LAL), which is assumed to be important for moth olfactory behavior. We observed that the LAL DNs also innervate the PS, suggesting the integration of signals from the LAL and PS. We also identified a set of DNs innervating the PS, but not the LAL. These DNs were sensitive to sex pheromones, suggesting a role of the PS in motor control for pheromone orientation. The organization of descending pathways for pheromone orientation is discussed.