A comparison between the axon terminals of octopaminergic efferent dorsal or ventral unpaired median neurons in either desert locusts (Schistocerca gregaria) or fruit flies (Drosophila melanogaster) across skeletal muscles reveals many similarities. In both species the octopaminergic axon forms beaded fibers where the boutons or varicosities form type II terminals in contrast to the neuromuscular junction (NMJ) or type I terminals. These type II terminals are immunopositive for both
tyramine and
octopamine and, in contrast to the type I terminals, which possess clear synaptic vesicles, only contain dense core vesicles. These dense core vesicles contain
octopamine as shown by immunogold methods. With respect to the cytomatrix and active zone
peptides the type II terminals exhibit active zone-like accumulations of the scaffold
protein Bruchpilot (BRP) only sparsely in contrast to the many accumulations of BRP identifying active zones of NMJ type I terminals. In the fruit fly larva marked dynamic changes of octopaminergic fibers have been reported after short
starvation which not only affects the formation of new branches ("synaptopods") but also affects the type I terminals or NMJs via
octopamine-signaling (Koon et al., 2011). Our
starvation experiments of Drosophila-larvae revealed a time-dependency of the formation of additional branches. Whereas after 2 h of
starvation we find a decrease in "synaptopods", the increase is significant after 6 h of
starvation. In addition, we provide evidence that the release of
octopamine from dendritic and/or axonal type II terminals uses a similar synaptic machinery to
glutamate release from type I terminals of excitatory motor neurons. Indeed, blocking this canonical synaptic release machinery via RNAi induced downregulation of BRP in neurons with type II terminals leads to flight performance deficits similar to those observed for
octopamine mutants or flies lacking this class of neurons (Brembs et al., 2007).