Morphological and molecular characterization of columnar cholinergic neurotransmission onto T5 cells of Drosophila

Samara, Eleni (2025): Morphological and molecular characterization of columnar cholinergic neurotransmission onto T5 cells of Drosophila. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)

Abstract

Sensory feedback initiates bottom-up brain processing, which in line activates top-down processing. Vision is a major sensory input in this processing loop, that eventually leads to perception and decision making. Navigating the environment while choosing attractive visual cues over aversive ones, evaluating ego-motion to stabilize a course, chromatically and achromatically detect objects, are some of the tasks that visual systems perform. To elucidate the cellular and molecular mechanisms of such neural computations and the resulting behaviors, more simplistic, yet powerful, visual systems have been explored. Being a genetically tractable model organism that exhibits high neuronal and behavioral diversity, Drosophila melanogaster serves this purpose well. Neuronal connectivity is now accessible thanks to connectomic approaches, but the morphological and molecular identities of each chemical connection are still quite elusive. This cumulative-style doctoral thesis consists of two manuscripts that explore chemical synapses in the OFF-motion vision circuitry of Drosophila. The first manuscript investigates the polyadic morphology of chemical synapses between the primary motion detectors in the OFF pathway, T5 cells, and their columnar cholinergic inputs Tm1, Tm2, Tm4 and Tm9. In this work, I used the open-data connectomics database FlyWire and identified the Tm-to-T5 wiring via various polyadic synapse types. Then, I focused on the T5 dendritic distribution of the different polyadic types and found differences in their spatial patterns. Lastly, I showed that the polyadic morphology is setting a directional wiring architecture at the T5 network level. This work demonstrated the subsynaptic level of complexity in Tm-to-T5 connectivity. In the second manuscript of my doctoral thesis, we investigated the molecular identity of each Tm-to-T5 chemical connection. Fast ionotropic nicotinic (nAChRs) and slow metabotropic muscarinic (mAChRs) acetylcholine receptors are expressed in the brain of Drosophila, but the contribution of different AChRs to visual information processing remained poorly understood. We used a suit of genetic tools and gained accessibility to AChRs, thus finding the nAChRα1, nAChRα3, nAChRα4, nAChRα5, nAChRα7 and nAChβ1 subunits and the mAChR-B receptor localizing on T5 dendrites. Mapping the most highly expressed nAChR subunits across Tm-to-T5 synapses showed the nAChRα5 prevalence in Tm1-, Tm2- and Tm4-to-T5 synapses and of nAChRα7 in Tm9-to-T5 synapses. In vivo functional characterization of nAChRα4, nAChRα5, nAChRα7 and mAChR-B revealed alterations in the fly optomotor response and T5 directional tuning after AChR knock-down. Collectively, this work exhibited the complexity of cholinergic neurotransmission and consequently of preferred direction enhancement in T5 cells, which is introduced by the different receptor categories, subunit stoichiometries, isoforms and their synaptic localization.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Drosophila, motion vision, acetylcholine receptors
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Graduate School of Systemic Neurosciences (GSN)
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 23. Mai 2025
• 1. Bericht­erstatter:in: Borst, Alexander