Chemical neuroanatomy of the Drosophila central complex: distribution of multiple neuropeptides in relation to neurotransmitters.

The central complex of the insect brain is an integration center, receiving inputs from many parts of the brain. In Drosophila it has been associated with the control of both locomotor and visually correlated behaviors. The central complex can be divided into several substructures and is comprised of a large number of neuronal types. These neurons produce classical neurotransmitters, biogenic amines, and different neuropeptides. However, the distribution of neurotransmitters and neuromodulators in central-complex circuits of Drosophila is poorly known. By immunolabeling and GAL4-directed expression of marker proteins, we analyzed the distribution of acetylcholine, glutamate, GABA, monoamines, and eight different neuropeptides; Drosophila tachykinin, short neuropeptide F, myoinhibitory peptide, allatostatin A, proctolin, SIFamide, neuropeptide F, and FMRFamide. All eight neuropeptides were localized to the fan-shaped body, the largest substructure of the central complex, and were mapped to different layers within this structure. Several populations of peptide-immunoreactive tangential and columnar neurons were identified, of which some colocalized acetylcholine. Fewer peptides were found to be expressed in the other substructures: the ellipsoid body, the protocerebral bridge, and the noduli. The ellipsoid body and the protocerebral bridge were innervated by extrinsic peptide expressing neurons. Our findings reveal that numerous neuropeptides are expressed in the central complex and that each peptide has a distinct distribution pattern, suggesting important roles for neuropeptides as neuromediators and cotransmitters in this brain area.