Behavior-dependent activity labeling in the central complex of Drosophila during controlled visual stimulation

3H-2-deoxyglucose (2-DG) is used as a histological marker to study the spatial distribution of neuronal activity in the central brain of Drosophila melanogaster under controlled stimulus conditions. Tethered flies injected with the label are kept for 45-90 minutes in the center of a rotating drum carrying on its periphery a single black vertical stripe or a periodic grating. In either stimulus situation, one-half of the flies walk on top of a small black styrofoam ball floating on a jet of air, and one-half are kept flying 1.5 mm above the ball. 1) During walking on the styrofoam ball little 2-DG accumulates in the central brain. The area in and around the ellipsoid body as well as several tracts are weakly labeled. 2) After flight, labeling patterns are more distinct. The following labeled structures can be identified: ventral bodies, ring neurons of the ellipsoid body, the region of the inferior bridge, fiber tracts connecting the central complex to other parts of the protocerebrum, and tracts in the subesophageal and dorsal brain. These contrast with fan-shaped body and protocerebral lobes, which are only weakly labeled. 3) Not only flight and walking lead to distinct staining patterns but also the two visual stimuli. With the single stripe, in most flies the caudal layers of the ellipsoid body accumulate more 2-DG than the rostral ones. With the striped drum, the ellipsoid body is uniformly labeled. 4) Other stimulus-related features are found less regularly: Unilateral distribution of 2-DG in or around the noduli was observed in several flies that had been exposed to a rotating single stripe. In one preparation, two strongly marked individual neurons of the protocerebral bridge can be tentatively identified. The labeling patterns support earlier notions that the central complex is involved in the initiation and organization of behavior and that it integrates visual data of the two brain hemispheres.