A study on the metamorphosis of neural circuitry in the mushroom body calyx in Drosophila melanogaster

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The mushroom bodies (MBs) of Drosophila play an important part in olfactory learning and memory. However, the synaptic circuitry of MBs has so far not been investigated in any detail. The calyces of MBs are the main input regions, where the intrinsic Kenyon cells receive olfactory information carried by the projection neurons (PNs) from the antennal lobes. To obtain fundamental information about the synaptic circuits involved in olfactory information processing we have studied the synaptic connections in the calyces of the Drosophila MB, with special reference to the reorganization of neural circuits during metamorphosis. The adult MB, with calyces contained a pattern of repeated microglomeruli, each of which comprised a large cholinergic PN-bouton immunoreactive to choline acetyltransferase (ChA T-ir), at its core, encircled by a number of tiny Kenyon cell dendrites as well as by the terminals of GABAergic extrinsic neuron. ChAT-ir boutons formed divergent synapses upon multitudinous … More surrounding Kenyon cell dendrites. GABAergic terminals also contributed divergent synaptic input to these dendrites as well as to PN-boutons, indicating that GABAergic elements may play a role in pre- and postsynaptic inhibition of Kenyon cell dendrites. The structure of microglomeruli was also found in larval calyces. In early pupae (12 hours after puparium formation; APF), the calyces showed a degenerated appearance. The PN-boutons became involuted and, as a result, the microglomeruli disappeared. Interestingly, however, a small number of ChAT-ir PNs still continued to maintain the synaptic connections with the dendritic fibers. In the middle stage of pupae (48hours APF), the PN-terminals swelled out again and formed immature microglomeruli loosely surrounded by the dendritic fibers. The PN-boutons formed divergent synapses on the surrouding fibers. These observations suggest that the specific synapses between the PNs and Kenyon cells, which persist through the pupal stage, might be responsible for memory retention during metamorphosis Less