Action of octopamine and tyramine on muscles ofDrosophila melanogasterlarvae

Abstract

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.

Limited distal organelles and synaptic function in extensive monoaminergic innervation

“…∼5-25 on muscles 13 and 4). This difference in innervation is easily understood given that glutamate directly elicits electrical excitation, while octopamine regulates basal and evoked larval muscle contractility (Ormerod et al, 2013) and development of glutamatergic synapses even on muscles lacking type II innervation (Koon et al, 2011). Thus, identified Drosophila octopamine neurons offer the opportunity to elucidate how organelles are distributed in an extensive field of monoaminergic innervation from a single axon associated with neurohormone action reminiscent of monoaminergic volume transmission in the mammalian brain.…”

Cell-selective modulation of theDrosophilaneuromuscular system by a neuropeptide

“…There were no qualitative differences between the two salines with regard to the ability of the peptide to enhance contractions. We have also previously demonstrated that these two salines do not alter octopamine-induced enhancements of EJPs (Ormerod et al 2013). Larvae were dissected as described above for EJP recordings.…”

“…7), force was detected using a custom force transducer composed of four silicon wafer strain gauges (Micron Instruments, Simi Valley, CA) in full Wheatstone bridge configuration and mounted about the narrowest part of a polycarbonate beam (Ormerod et al 2013;Paterson et al 2010). The transducer operates linearly between 1 N and 2 N and exhibited no temperature sensitivity between 10 and 30°C.…”

“…Invertebrate muscles, however, integrate information from synaptic inputs differently because they are often innervated by multiple excitatory axons, sometimes receive inhibitory inputs, and, in many cases, contract in response to graded electrical signals or even in response to hormones (Atwood 1976;Atwood and Cooper 1995;Atwood et al 1965;Peron et al 2009). Among invertebrates, modulation of centrally generated motor patterns by neurotransmitters or hormones can be complemented by peripheral modulation at neuromuscular synapses and/or muscle fibers by the same or similar substances (Ormerod et al 2013). In crab hearts, for example, FLRFamide peptides act centrally to increase the rate and amplitude of contractions by altering the rate of bursts generated by the cardiac ganglion, and they act peripherally to augment excitatory junctional potentials (EJPs) and muscle contractions (Fort et al 2007).…”

“…Perhaps the best example is for octopamine, which increases relaxation rate and cAMP levels more strongly in regions of the locust extensor-tibiae muscle that contain the highest proportions of slow and intermediate muscle fibers (Evans 1985). Likewise, in Drosophila larvae, octopamine increases EJP amplitude and nerve-evoked contractions more strongly in some muscle fibers than others (Ormerod et al 2013). In the crab gastric mill, allatostatin 3 decreases the initial EJP amplitude and enhances facilitation in one muscle (gm6) without altering EJP amplitude or facilitation in another (gm4), and proctolin increases EJP amplitude in muscle gm4 but not muscle gm6 (Jorge-Rivera et al 1998).…”