GLIAL DEVELOPMENT, SYNAPTIC PLASTICITY AND NEUROTRANSMITTER RECYCLING IN THE VISUAL SYSTEM OF THE FRUIT FLY DROSOPHILA MELANOGASTER

Abstract

The visual system of the fruit fly is ideal for studying the association between neurons and glia. These interact during the morphogenesis of brain neuropiles and, in the adult, work together to maintain an ideal environment for neuronal function. In this thesis I characterise the pupal metamorphosis and adult structure of glia in the optic lobe’s lamina and medulla neuropiles. Photoreceptor axons from the fly’s compound eyes terminate at locations within these neuropiles that allow them intimate contact with glia. Some neuropile glia take up and inactivate the neurotransmitter histamine after its release at photoreceptor synapses. A shuttle pathway between the glia and photoreceptors then transports inactivated histamine back to photoreceptors for reuse. The gene CG12120 encodes the protein Tan, which liberates recycled histamine within the photoreceptor cytoplasm, it is then pumped into vesicles for re-release. Histamine, however, is not exclusive to the visual system. A system of glial barriers in the lamina, and around the brain, controls the movement of histamine between neuropiles as well as between the body and the brain. How histamine is reciprocally transported between photoreceptors and glia remains unknown despite attempts to uncover candidate transporters. Photoreceptor-specific capitate projections form as invaginations from neuropile glia into photoreceptor terminals. The transmitter needs of the photoreceptor appear to dictate the dynamic structure of capitate projections, which change in shape and number after perturbations that affect terminal location, synapse number or histamine release at the photoreceptor. This dynamism suggests that capitate projections play an important role, not only in recycling synaptic vesicles, but also in recycling histamine.