Microglomerular synaptic complexes in the sky-compass network of the honeybee connect parallel pathways from the anterior optic tubercle to the central complex

• Main Authors: Martina Held, Annuska Berz, Ronja Hensgen, Thomas S Münz, Christina Scholl, Wolfgang Rössler, Uwe Homberg, Keram Pfeiffer
• Format: Article
• Language: English
• Published: Frontiers Media S.A. 2016-10-01
• Series: Frontiers in Behavioral Neuroscience
• Subjects: Honeybee; central complex; Insect brain; polarization vision; Synaptic connections; anterior optic tubercle
• Online Access: http://journal.frontiersin.org/Journal/10.3389/fnbeh.2016.00186/full

While the ability of honeybees to navigate relying on sky-compass information has been investigated in a large number of behavioral studies, the underlying neuronal system has so far received less attention. The sky-compass pathway has recently been described from its input region, the dorsal rim area of the compound eye, to the anterior optic tubercle (AOTU). The aim of this study is to reveal the connection from the AOTU to the central complex. For this purpose we investigated the anatomy of large microglomerular synaptic complexes in the medial and lateral bulbs of the lateral complex. The synaptic complexes are formed by TuLAL1 neurons of the AOTU and GABAergic tangential neurons of the central body’s lower division (TL neurons). Both TuLAL1 and TL neurons strongly resemble neurons forming these complexes in other insect species. We further investigated the ultrastructure of these synaptic complexes using transmission electron microscopy. We found that single large presynaptic terminals of TuLAL1 neurons enclose many small profiles of TL neurons. The synaptic connections between these neurons are established by two types of synapses: divergent dyads and divergent tetrads. Our data support the assumption that these complexes are a highly conserved feature in the insect brain and play an important role in reliable signal transmission within the sky-compass pathway.