Immunocytochemistry of GABA in the central complex of the locust Schistocerca gregaria: Identification of immunoreactive neurons and colocalization with neuropeptides

The central complex is a highly organized neuropil structure in the insect brain and plays a role in motor control and visual orientation. We describe the distribution of gamma-aminobutyric acid (GABA) immunostaining in the c entral complex of the locust Schistocerca gregaria in an effort to analyze inhibitory neural circuits within this brain area. Antisera against GABA an d the GABA-synthesizing enzyme glutamic acid decarboxylase resulted in iden tical patterns of immunostaining. Cell counts revealed about 100 bilateral pairs of GABA-immunoreactive neurons with arborizations in the central comp lex. Five types of immunostained neurons could be identified through recons truction of the staining pattern, comparison with individually stained neur ons, and double labeling experiments with Neurobiotin-injected neurons. All of these GABA-immunostained neurons are tangential neurons that connect th e lateral accessory lobes to distinct layers of the central body. Three typ es of immunostained neurons (TL2, TL3, TL4) invade the lower division of th e central body, and two additional types of neurons (TU1, TU2) have ramific ations in layers I and II of the upper division of the central body. Double -labeling experiments with peptide antisera suggest that peptides related t o Phe-Met-Arg-Phe-NH2/bovine pancreatic polypeptide and Dip-allatostatin mi ght act as cotransmitters with GABA in TL4 neurons of the lower division an d (Dip-allatostatin only) in TU2 neurons of the upper division of the centr al body. The high conservation in the pattern of GABA immunostaining in all insect species investigated so far suggests that GABA plays an essential r ole in the basic neural circuitry of the central complex in insects. (C) 19 99 Wiley-Liss, Inc.