GABA immunostaining in the central complex of dicondylian insects

The central complex is a group of midline-crossing neuropils in the insect brain involved in head direction coding, sky compass navigation, and spatial visual memory. To compare the neuroarchitecture and neurochemistry of the central complex in insects that differ in locomotion, ways of orientation, time of activity (diurnal, nocturnal), and evolutionary history, we studied the distribution of γ-aminobutyric acid (GABA) immunostaining in the central complex of 29 species, ranging from Zygentoma to Diptera. In all species, the lower division of the central body was densely innervated by GABA-immunoreactive tangential neurons. These neurons had additional arborizations in the bulb, a distinct region of synaptic complexes in the lateral complex, and somata in a cell cluster mediodorsally to the antennal lobe. Differences in the appearance of GABA immunostaining in the lower division of the central body corresponded to differences in neuropil architecture, such as transformation of the lower division into a toroid in certain Diptera and Heteroptera. In nearly all species two additional systems of tangential neuron of the upper division of the central body were GABA-immunoreactive. One of these systems diffusely invaded a superior layer, while the second system showed fan-like projections in an inferior layer. Sparse immunostaining in the protocerebral bridge was detected in cockroaches, a cricket, and two hemipteran species. The data show that three systems of GABA-immunoreactive tangential neurons of the central body are highly conserved and suggest that the layered organization of the upper division of the central body is, likewise, largely maintained from basal to advanced species.