Serotonin is an important neuromodulator associated with a wide range of physiological effects in the central nervous system. The exact mechanisms for how serotonin influences brain development are not well understood, although studies in invertebrate and vertebrate model organisms are beginning to unravel a regulatory role for serotonin in neuronal morphology and circuit formation. Recent data suggests a developmental window during which altered serotonin levels permanently impact circuitry, however, the temporal constraints and molecular mechanisms responsible are still under investigation. Growing evidence suggests that alterations in early serotonin signaling contribute to a number of neurodevelopmental and neuropsychiatric disorders. Thus, understanding how altered serotonin signaling affects neuronal morphology and plasticity, and ultimately animal physiology and pathophysiology, will be of great significance.

Administration of certain serotonin-releasing amphetamine derivatives (fenfluramine and/or 3,4-methylenedioxymethamphetamine, MDMA, 'ecstasy') results in dystrophic serotonergic morphology in the mammalian brain. In addition to drug administration, dystrophic serotonergic neurites are also associated with neurodegenerative disorders. We demonstrate here that endogenously elevated serotonin in the Drosophila CNS induces aberrant enlarged varicosities, or spheroids, that are morphologically similar to dystrophic mammalian serotonergic fibers. In Drosophila these spheroids are specific to serotonergic neurons, distinct from typical varicosities, and form only after prolonged increases in cytoplasmic serotonin. Our results also suggest that serotonin levels during early development determine later sensitivity of spheroid formation to manipulations of the serotonin transporter (SERT). Elevated serotonin also interacts with canonical protein aggregation and autophagic pathways to form spheroids. The data presented here support a model in which excess cytoplasmic neurotransmitter triggers a cell-specific pathway inducing aberrant morphology in fly serotonergic neurons that may be shared in certain mammalian pathologies.

INTRODUCTIONThis protocol describes a method for mounting and immunostaining Drosophila larval tissue in preparation for high-resolution fluorescent imaging of fine structures in the central nervous system (CNS). Affixing the tissue directly to the coverslip and then moving the coverslip between wash solutions provides a simple solid-phase method of immunostaining that assists in preserving fine structures. This method also easily allows for manipulations and/or viewing of the live sample prior to fixation if desired. Finally, putting the tissue in direct contact with the coverslip places fine structures immediately adjacent to the objective lens. We also briefly describe a method to create three-dimensional (3D) models of confocal Z-stacks in order to better characterize fine structures by measuring their volume and obtaining 3D Cartesian coordinates in space.

INTRODUCTIONThis protocol describes a simple behavioral assay designed to test the response of Drosophila larvae to a homeostatic insult: bright light. Treating normally photophobic larvae with bright light before placing them near a target food source reveals a greater latency to reach the target when compared to controls not receiving light treatment. This effect is reversible given a recovery period after light treatment, and it may reveal a method by which to measure behavioral plasticity and stress responses in fruit fly larvae.