Regulation of <i>Drosophila</i> Visceral Muscle Contractions by Multiple, Indirect Octopaminergic and Glutamatergic Signaling Pathways

Abstract

Mammalian visceral muscles are indirectly regulated by receptors expressed in peripheral neurons and other non-contractile cells. The complex interactions between these pathways remain poorly understood. It also remains unknown whether invertebrates use pathways similar to mammals to indirectly regulate visceral contractility. To elucidate the principles of organization for the signaling pathways responsible for visceral motility and the more general logic of aminergic neuromodulation, we are using the Drosophila oviposition circuit. We find that the receptors for the adrenergic neurotransmitter octopamine as well as the metabotropic glutamate receptor are not expressed in muscle cells, but rather, in epithelial cells that line the oviduct lumen in a regionally specific pattern and in multiple subtypes of peripheral neurons, some of which co-express mechanosensory markers. We have used live imaging to demonstrate that most of the functional effects of octopamine and glutamate are mediated by indirect pathways and that contractions of the lateral and common oviducts are regulated independently. Neurons intrinsic to the lateral oviduct indirectly activate muscle cells, and mutational and optogenetic analyses indicate that Octβ2 is required for these effects. An additional indirect pathway for lateral oviduct contractions is controlled by glutamate receptors expressed on a distinct subset of neuronal processes outside of the lateral oviduct. In addition to contractions of the lateral oviduct, octopamine regulates dilation, but this occurs in both the lateral and common oviducts and is blocked by mutation of OAMB but not Octβ2. Additional, unsuspected interactions between octopamine and glutamate include contractions of the common oviduct that requires an interaction between octopamine and with glutamate, blockade of lateral oviduct contractions by preincubation with glutamate and opposing effects of octopamine and glutamate in epithelial cells. These data indicate that multiple indirect pathways regulate the oviposition circuit similar to the mechanisms described for mammalian visceral muscles. They also uncover a complex network of neuromodulatory effects that provide insight into the regulation of behavioral circuits in other systems.