Octopaminergic neurons have multiple targets in Drosophila larval mushroom body calyx and regulate behavioral odor discrimination

The insect mushroom bodies (MBs) are essential for associative olfactory learning, and their sensory input region, the MB calyx, is organized in discrete glomeruli, most of which receive stereotypic input from olfactory projection neurons (PNs). Odors are represented by activity of MB neurons, the Kenyon Cells (KCs) using a sparse code that allows odor discrimination during learning. Octopamine (OA) is a neurotransmitter that signals reward in associative learning and arousal, in insects including Drosophila. The calyx receives OA innervation from two neurons, sVUMmd1 and sVUMmx1, originating from their respective neuromeres in the suboesophageal zone (SEZ). To understand how these OA inputs might influence odor discrimination in the MBs, we analyzed their pattern of innervation of the MB calyx, their connectivity with the other neurons that innervate the calyx, and their influence on sensory discrimination during learning. Clonal labeling of multiple single neurons showed that the two sVUM1 neurons innervated all regions of the calyx. GRASP (GFP Reconstitution Among Synaptic Partners) revealed contacts of sVUM1 neurons in the calyx with olfactory PNs, the inhibitory neuron APL, extensively with an extrinsic neuron with dendrites throughout the calyx, but not with KCs. A GFP protein trap of the OA receptor Oamb, a Drosophila alpha-1-adrenergic receptor ortholog, localized to PN terminals in the calyx. In a behavioral odor discrimination assay, activating a set of 5 OA neurons, including the sVUM1 neurons, compromised the ability to discriminate similar odors. Our results support a model, in which OA release from sVUM1 neurons can modify release from PN terminals, thus increasing the sensitivity but decreasing discrimination in the olfactory learning pathway.