A novel type of microglomerular synaptic complex in the polarization vision pathway of the locust brain

Abstract

The lateral accessory lobes (LALs) are prominent integration centers in the insect brain. In the desert locust Schistocerca gregaria, they are connected with the anterior optic tubercles (AOTus), with the central complex, and with the ventral nerve cord. Two subcompartments of the LALs, the lateral triangle and the median olive, are easily recognized by their prominent granular texture. Both areas are part of the polarization vision pathway in the locust brain; they receive input from projection neurons of the AOTu and are the site of presumed dendritic arborizations of tangential neurons of the lower division of the central body. Both types of neuron are sensitive to polarized light and most likely play a role in sky compass navigation of the locust. We show here that neurons from the AOTu and tangential neurons of the central body form large microglomerular contacts in the median olive and lateral triangle. Presynaptic elements from the AOTu end in small numbers of large cup-shaped terminals. These cups enclose many small gamma-aminobutyric acid (GABA)-immunoreactive (-ir) profiles from tangential neurons of the lower division of the central body. Each cup-shaped profile makes numerous (>150) dyadic output synapses with the small postsynaptic GABA-ir profiles. No synaptic connections were found between the small core profiles. The microglomerular organization of the median olive and lateral triangle is unlike that of any other synaptic microglomeruli reported for the insect brain. It might provide precise spike timing information possibly used to extract spatial information by comparison of binocular inputs in the central complex.