Oviposition is induced upon mating in most insects. Ovulation is a primary step in oviposition, representing an important target to control insect pests and vectors, but limited information is available on the underlying mechanism. Here we report that the beta adrenergic-like octopamine receptor Octβ2R serves as a key signaling molecule for ovulation and recruits protein kinase A and Ca2+/calmodulin-sensitive kinase II as downstream effectors for this activity. We found that the octβ2r homozygous mutant females are sterile. They displayed normal courtship, copulation, sperm storage and post-mating rejection behavior but were unable to lay eggs. We have previously shown that octopamine neurons in the abdominal ganglion innervate the oviduct epithelium. Consistently, restored expression of Octβ2R in oviduct epithelial cells was sufficient to reinstate ovulation and full fecundity in the octβ2r mutant females, demonstrating that the oviduct epithelium is a major site of Octβ2R’s function in oviposition. We also found that overexpression of the protein kinase A catalytic subunit or Ca2+/calmodulin-sensitive protein kinase II led to partial rescue of octβ2r’s sterility. This suggests that Octβ2R activates cAMP as well as additional effectors including Ca2+/calmodulin-sensitive protein kinase II for oviposition. All three known beta adrenergic-like octopamine receptors stimulate cAMP production in vitro. Octβ1R, when ectopically expressed in the octβ2r’s oviduct epithelium, fully reinstated ovulation and fecundity. Ectopically expressed Octβ3R, on the other hand, partly restored ovulation and fecundity while OAMB-K3 and OAMB-AS that increase Ca2+ levels yielded partial rescue of ovulation but not fecundity deficit. These observations suggest that Octβ2R have distinct signaling capacities in vivo and activate multiple signaling pathways to induce egg laying. The findings reported here narrow the knowledge gap and offer insight into novel strategies for insect control.
Associative learning is a fundamental form of behavioral plasticity. Octopamine plays central roles in various learning types in invertebrates, however the target receptors and underlying mechanisms are poorly understood. Drosophila provides a powerful system to uncover the mechanisms for learning and memory. Here, we report that OAMB in the mushroom body neurons mediates the octopamine’s signal for appetitive olfactory learning. The octopamine receptor OAMB has two isoforms (OAMB-K3 and OAMB-AS), differing in the third cytoplasmic loop and downstream sequence. The activation of each OAMB isoform increases intracellular Ca2+ similar to the alpha1 adrenergic receptor, while OAMB-K3 additionally stimulates cAMP production. The oamb null mutants showed severely impaired learning in appetitive olfactory conditioning that tests flies’ capacity to learn and remember the odor associated with sugar reward. This deficit was also seen in the hypomorphic mutant with reduced OAMB expression in the mushroom bodies, the brain structure crucial for olfactory conditioning. Consistently, the oamb mutant’s learning phenotype was fully rescued by conditional expression of either OAMB isoform in the mushroom body αβ and γ neurons. These results indicate that the OAMB receptor is a key molecule mediating the octopamine’s signal for appetitive olfactory learning and its functional site is the mushroom body αβ and γ neurons. This study represents a critical step forward in understanding the cellular mechanism and neural circuit mediating reward learning and memory.
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