Octopamine (OA) and tyramine (TA) play important roles in homeostatic mechanisms, behavior, and modulation of neuromuscular junctions in arthropods. However, direct actions of these amines on muscle force production that are distinct from effects at the neuromuscular synapse have not been well studied. We utilize the technical benefits of the Drosophila larval preparation to distinguish the effects of OA and TA on the neuromuscular synapse from their effects on contractility of muscle cells. In contrast to the slight and often insignificant effects of TA, the action of OA was profound across all metrics assessed. We demonstrate that exogenous OA application decreases the input resistance of larval muscle fibers, increases the amplitude of excitatory junction potentials (EJPs), augments contraction force and duration, and at higher concentrations (10(-5) and 10(-4) M) affects muscle cells 12 and 13 more than muscle cells 6 and 7. Similarly, OA increases the force of synaptically driven contractions in a cell-specific manner. Moreover, such augmentation of contractile force persisted during direct muscle depolarization concurrent with synaptic block. OA elicited an even more profound effect on basal tonus. Application of 10(-5) M OA increased synaptically driven contractions by ≈ 1.1 mN but gave rise to a 28-mN increase in basal tonus in the absence of synaptic activation. Augmentation of basal tonus exceeded any physiological stimulation paradigm and can potentially be explained by changes in intramuscular protein mechanics. Thus we provide evidence for independent but complementary effects of OA on chemical synapses and muscle contractility.

Structural and functional plasticity induced by neuronal competition is a common feature of developing nervous systems. However, the rules governing how postsynaptic cells differentiate between presynaptic inputs are unclear. In this study, we characterized synaptic interactions following manipulations of tonic Ib or phasic Is glutamatergic motoneurons that coinnervate postsynaptic muscles of male or female Drosophila melanogaster larvae. After identifying drivers for each neuronal subtype, we performed ablation or genetic manipulations to alter neuronal activity and examined the effects on synaptic innervation and function at neuromuscular junctions. Ablation of either Ib or Is resulted in decreased muscle response, with some functional compensation occurring in the Ib input when Is was missing. In contrast, the Is terminal failed to show functional or structural changes following loss of the coinnervating Ib input. Decreasing the activity of the Ib or Is neuron with tetanus toxin light chain resulted in structural changes in muscle innervation. Decreased Ib activity resulted in reduced active zone (AZ) number and decreased postsynaptic subsynaptic reticulum volume, with the emergence of filopodial-like protrusions from synaptic boutons of the Ib input. Decreased Is activity did not induce structural changes at its own synapses, but the coinnervating Ib motoneuron increased the number of synaptic boutons and AZs it formed. These findings indicate that tonic Ib and phasic Is motoneurons respond independently to changes in activity, with either functional or structural alterations in the Ib neuron occurring following ablation or reduced activity of the coinnervating Is input, respectively.

Glial-neuronal signaling at synapses is widely studied, but how glia interact with neuronal somas to regulate their activity is unclear. Drosophila cortex glia are restricted to brain regions devoid of synapses, providing an opportunity to characterize interactions with neuronal somas. Mutations in the cortex glial NCKXzydeco elevate basal Ca2+, predisposing animals to seizure-like behavior. To determine how cortex glial Ca2+ signaling controls neuronal excitability, we performed an in vivo modifier screen of the NCKXzydeco seizure phenotype. We show that elevation of glial Ca2+ causes hyperactivation of calcineurin-dependent endocytosis and accumulation of early endosomes. Knockdown of sandman, a K2P channel, recapitulates NCKXzydeco seizures. Indeed, sandman expression on cortex glial membranes is substantially reduced in NCKXzydeco mutants, indicating enhanced internalization of sandman predisposes animals to seizures. These data provide an unexpected link between glial Ca2+ signaling and the well-known role of glia in K+ buffering as a key mechanism for regulating neuronal excitability.

Maintenance of neural circuit activity requires appropriate regulation of excitatory and inhibitory synaptic transmission. Recently, glia have emerged as key partners in the modulation of neuronal excitability; however, the mechanisms by which glia regulate neuronal signaling are still being elucidated. Here, we describe an analysis of how Ca2+ signals within Drosophila astrocyte-like glia regulate excitability in the nervous system. We find that Drosophila astrocytes exhibit robust Ca2+ oscillatory activity manifested by fast, recurrent microdomain Ca2+ fluctuations within processes that infiltrate the synaptic neuropil. Unlike the enhanced neuronal activity and behavioral seizures that were previously observed during manipulations that trigger Ca2+ influx into Drosophila cortex glia, we find that acute induction of astrocyte Ca2+ influx leads to a rapid onset of behavioral paralysis and a suppression of neuronal activity. We observe that Ca2+ influx triggers rapid endocytosis of the GABA transporter (GAT) from astrocyte plasma membranes, suggesting that increased synaptic GABA levels contribute to the neuronal silencing and paralysis. We identify Rab11 as a novel regulator of GAT trafficking that is required for this form of activity regulation. Suppression of Rab11 function strongly offsets the reduction of neuronal activity caused by acute astrocyte Ca2+ influx, likely by inhibiting GAT endocytosis. Our data provide new insights into astrocyte Ca2+ signaling and indicate that distinct glial subtypes in the Drosophila brain can mediate opposing effects on neuronal excitability.

Diet profoundly influences the behavior of animals across many phyla. Despite this, most laboratories using model organisms, such as Drosophila, use multiple, different, commercial or custom-made media for rearing their animals. In addition to measuring growth, fecundity and longevity, we used several behavioral and physiological assays to determine if and how altering food media influence wild-type (Canton S) Drosophila melanogaster, at larval, pupal, and adult stages. Comparing 2 commonly used commercial food media we observed several key developmental and morphological differences. Third-instar larvae and pupae developmental timing, body weight and size, and even lifespan significantly differed between the 2 diets, and some of these differences persisted into adulthood. Diet was also found to produce significantly different thermal preference, locomotory capacity for geotaxis, feeding rates, and lower muscle response to hormonal stimulation. There were no differences, however, in adult thermal preferences, in the number or viability of eggs laid, or in olfactory learning and memory between the diets. We characterized the composition of the 2 diets and found particularly significant differences in cholesterol and (phospho)lipids between them. Notably, diacylglycerol (DAG) concentrations vary substantially between the 2 diets, and may contribute to key phenotypic differences, including lifespan. Overall, the data confirm that 2 different diets can profoundly influence the behavior, physiology, morphology and development of wild-type Drosophila, with greater behavioral and physiologic differences occurring during the larval stages.

The neuropeptide proctolin (RYLPT) plays important roles as both a neurohormone and a cotransmitter in arthropod neuromuscular systems. We used third-instar Drosophila larvae as a model system to differentiate synaptic effects of this peptide from its direct effects on muscle contractility and to determine whether proctolin can work in a cell-selective manner on muscle fibers. Proctolin did not appear to alter the amplitude of excitatory junctional potentials but did induce sustained muscle contractions in preparations where the CNS had been removed and no stimuli were applied to the remaining nerves. Proctolin-induced contractions were dose-dependent, were reduced by knocking down expression of the Drosophila proctolin receptor in muscle tissue, and were larger in some muscle cells than others (i.e., larger in fibers 4, 12, and 13 than in 6 and 7). Proctolin also increased the amplitude of nerve-evoked contractions in a dose-dependent manner, and the magnitude of this effect was also larger in some muscle cells than others (again, larger in fibers 4, 12, and 13 than in 6 and 7). Increasing the intraburst impulse frequency and number of impulses per burst increased the magnitude of proctolin's enhancement of nerve-evoked contractions and decreased the threshold and EC50 concentrations for proctolin to enhance nerve-evoked contractions. Reducing proctolin receptor expression decreased the velocity of larval crawling at higher temperatures, and thermal preference in these larvae. Our results suggest that proctolin acts directly on body-wall muscles to elicit slow, sustained contractions and to enhance nerve-evoked contractions, and that proctolin affects muscle fibers in a cell-selective manner.

The Drosophila neuromuscular system is widely used to characterize synaptic development and function. However, little is known about how specific synaptic alterations effect neuromuscular transduction and muscle contractility, which ultimately dictate behavioural output. Here we develop and use a force transducer system to characterize excitation‐contraction coupling at Drosophila larval neuromuscular junctions (NMJs), examining how specific neuronal and muscle manipulations disrupt muscle contractility. Muscle contraction force increased with motoneuron stimulation frequency and duration, showing considerable plasticity between 5 and 40 Hz and saturating above 50 Hz. Endogenous recordings of fictive contractions revealed average motoneuron burst frequencies of 20–30 Hz, consistent with the system operating within this plastic range of contractility. Temperature was also a key factor in muscle contractility, as force was enhanced at lower temperatures and dramatically reduced with increasing temperatures. Pharmacological and genetic manipulations of critical components of Ca2+ regulation in both pre‐ and postsynaptic compartments affected the strength and time course of muscle contractions. A screen for modulators of muscle contractility led to identification and characterization of the molecular and cellular pathway by which the FMRFa peptide, TPAEDFMRFa, increases muscle performance. These findings indicate Drosophila NMJs provide a robust system to correlate synaptic dysfunction, regulation and modulation to alterations in excitation‐contraction coupling. Key points Larval muscle contraction force increases with stimulation frequency and duration, revealing substantial plasticity between 5 and 40 Hz. Fictive contraction recordings demonstrate endogenous motoneuron burst frequencies consistent with the neuromuscular system operating within the range of greatest plasticity. Genetic and pharmacological manipulations of critical components of pre‐ and postsynaptic Ca2+ regulation significantly affect the strength and time course of muscle contractions. A screen for modulators of the excitation‐contraction machinery identified a FMRFa peptide, TPAEDFMRFa and its associated signalling pathway, that dramatically increases muscle performance. Drosophila serves as an excellent model for dissecting components of the excitation‐contraction coupling machinery.

Drosophila melanogaster is a model system for examining the mechanisms of action of neuropeptides. DPKQDFMRFamide was previously shown to induce contractions in Drosophila body wall muscle fibres in a Ca 2+ -dependent manner. The present study examined the possible involvement of a G-protein-coupled receptor and second messengers in mediating this myotropic effect after removal of the central nervous system. DPKQDFMRFamide-induced contractions were reduced by 70% and 90%, respectively, in larvae with reduced expression of the Drosophila Fmrf receptor (FR) either ubiquitously or specifically in muscle tissue, compared with the response in control larvae in which expression was not manipulated. No such effect occurred in larvae with reduced expression of this gene only in neurons. The myogenic effects of DPKQDFMRFamide do not appear to be mediated through either of the two Drosophila myosuppressin receptors (DmsR-1 and DmsR-2). DPKQDFMRFamide-induced contractions were not reduced in Ala1 transgenic flies lacking activity of calcium/calmodulin-dependent protein kinase (CamKII), and were not affected by the CaMKII inhibitor KN-93. Peptide-induced contractions in the mutants of the phospholipase C-β (PLCβ) gene (norpA larvae) and in IP 3 receptor mutants were similar to contractions elicited in control larvae. The peptide failed to increase cAMP and cGMP levels in Drosophila body wall muscles. Peptide-induced contractions were not potentiated by 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, and were not antagonized by inhibitors of cAMP-dependent or cGMPdependent protein kinases. Additionally, exogenous application of arachidonic acid failed to induce myogenic contractions. Thus, DPKQDFMRFamide induces contractions via a G-protein coupled FMRFamide receptor in muscle cells but does not appear to act via cAMP, cGMP, IP 3 , PLC, CaMKII or arachidonic acid.