Mode of action of a Drosophila FMRFamide in inducing muscle contraction

Investigating postsynaptic effects of a Drosophila neuropeptide on muscle contraction

The Drosophila neuropeptide, DPKQDFMRFamide, was previously shown to enhance excitatory junctional potentials (EJPs) and muscle contraction by both presynaptic and postsynaptic actions. Since the peptide acts on both sides of the synaptic cleft, it has been difficult to examine postsynaptic modulatory mechanisms, particularly when contractions are elicited by nerve stimulation. Here, postsynaptic actions are examined in 3rd instar larvae by applying peptide and the excitatory neurotransmitter, l-glutamate, in the bathing solution to elicit contractions after silencing motor output by removing the central nervous system (CNS). DPKQDFMRFamide enhanced glutamate-evoked contractions at low concentrations (EC50 1.3 nM), consistent with its role as a neurohormone, and the combined effect of both substances was supra-additive. Glutamate-evoked contractions were also enhanced when transmitter release was blocked in temperature-sensitive (Shibire) mutants, confirming the peptide's postsynaptic action. The peptide increased membrane depolarization in muscle when co-applied with glutamate, and its effects were blocked by nifedipine, an L-type channel blocker, indicating effects at the plasma membrane involving calcium influx. DPKQDFMRFamide also enhanced contractions induced by caffeine in the absence of extracellular calcium, suggesting increased calcium release from the sarcoplasmic reticulum (SR) or effects downstream of calcium release from the SR. The peptide's effects do not appear to involve calcium/calmodulin-dependent protein kinase II (CaMKII), previously shown to mediate presynaptic effects. The approach used here might be useful for examining postsynaptic effects of neurohormones and cotransmitters in other systems.NEW & NOTEWORTHY Distinguishing presynaptic and postsynaptic effects of neurohormones is a long-standing challenge in many model organisms. Here, postsynaptic actions of DPKQDFMRFamide are demonstrated by assessing its ability to potentiate contractions elicited by direct application of the neurotransmitter, glutamate, when axons are silent and when transmitter release is blocked. The peptide acts at multiple sites to increase contraction, increasing glutamate-induced depolarization at the cell membrane, acting on L-type channels, and acting downstream of calcium release from the sarcoplasmic reticulum.

FMRF-related peptides in Aedes aegypti midgut: neuromuscular connections and enteric nervous system

FMRFamide-related peptides (FaRPs) are a class of neuropeptides that participate in a variety of physiological processes in invertebrates. They occur in nerves of stomatogastric ganglia and enteroendocrine cells of the insect digestive tract, where they may control muscle functions. However, their direct involvement in muscle function has never been shown in situ. We studied the relationship between FaRPs and midgut muscle during larval-pupal transition of the mosquito Aedes aegypti. In late L4, FaRP-positive neuronal extensions attach to the bundles of the external circular muscle layer, and muscle stem cells start to undergo mitosis in the internal circular layer. Thereafter, the external muscle layer degenerates, disappearing during early pupal development, and is completely absent in the adult mosquito. Our results indicate that FaRP-based neural signals are involved in the reorganization of the muscle fibers of the mosquito midgut during the larval-pupal transition. In addition to confirming FaRP involvement in muscle function, we show that the mosquito midgut muscles are largely innervated, and that circular and longitudinal muscle have specific neuron bodies associated with them.

FMRFamide-Related Peptides Signaling Is Involved in the Regulation of Muscle Contractions in Two Tenebrionid Beetles

Peptidergic signaling regulates various physiological processes in insects. Neuropeptides are important messenger molecules that act as neurotransmitters, neuromodulators or hormones. Neuropeptides with myotropic properties in insects are known as FMRFamide-like peptides (FaLPs). Here, we describe the myotropic effects of the endogenous FaLPs in the regulation of contractile activity of the heart, ejaculatory duct, oviduct and the hindgut in two beetle species, Tenebrio molitor and Zophobas atratus. A putative receptor was identified in silico in both species. Using RT-PCR these putative FaLPs receptors were found in the various tissues of both beetles, including visceral organs. Analysis of the amino acid sequence of the receptor indicated that it is similar to other insect FaLPs receptors and belongs to G-protein coupled receptors. A synthetic FaLP (NSNFLRFa) found as the bioanalogue of both species demonstrated concentration-dependent and organ-specific myoactive properties. The peptide had species–specific cardioactivity, in that it stimulated Z. atratus heart contractions, while slightly inhibiting that of T. molitor and had mainly myostimulatory effect on the examined visceral organs of both beetle species, with the lowest activity in the ejaculatory duct of these beetles. The peptide was the most active in the hindgut of both species, but only at high concentration of 10^–5 M. The results suggest that FaLPs are potent modulators of endogenous contractile activity of the visceral muscles in beetles and may indirectly affect various physiological processes.

Modulation of neuromuscular synapses and contraction in Drosophila 3rd instar larvae

Over the past four decades, Drosophila melanogaster has become an increasingly important model system for studying the modulation of chemical synapses and muscle contraction by cotransmitters and neurohormones. This review describes how advantages provided by Drosophila have been utilized to investigate synaptic modulation, and it discusses key findings from investigations of cotransmitters and neurohormones that act on body wall muscles of 3rd instar Drosophila larvae. These studies have contributed much to our understanding of how neuromuscular systems are modulated by neuropeptides and biogenic amines, but there are still gaps in relating these peripheral modulatory effects to behavior.

Octopamine and tyramine regulate the activity of reproductive visceral muscles in the adult female blood-feeding bug, Rhodnius prolixus

The role of octopamine and tyramine in regulating spontaneous contractions of reproductive tissues was examined in the female Rhodnius prolixus Octopamine decreased the amplitude of spontaneous contractions of the oviducts and reduced RhoprFIRFa-induced contractions in a dose-dependent manner, whereas tyramine only reduced the RhoprFIRFa-induced contractions. Both octopamine and tyramine decreased the frequency of spontaneous bursal contractions and completely abolished the contractions at 5×10^-7 mol l^-1 and above. Phentolamine, an octopamine receptor antagonist, attenuated the inhibition induced by octopamine on the oviducts and the bursa. Octopamine also increased the levels of cAMP in the oviducts, and this effect was blocked by phentolamine. Dibutyryl cyclic AMP mimicked the effects of octopamine by reducing the frequency of bursal contractions, suggesting that the octopamine receptor may act by an Octβ receptor. The tyramine receptor antagonist yohimbine failed to block the inhibition of contractions induced by tyramine on the bursa, suggesting that tyramine may be acting on the Octβ receptor in the bursa.

Characterizing the physiological and behavioral roles of proctolin in Drosophila melanogaster

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.

Gene Regulatory Mechanisms Underlying the Spatial and Temporal Regulation of Target-Dependent Gene Expression in Drosophila Neurons

Neuronal differentiation often requires target-derived signals from the cells they innervate. These signals typically activate neural subtype-specific genes, but the gene regulatory mechanisms remain largely unknown. Highly restricted expression of the FMRFa neuropeptide in Drosophila Tv4 neurons requires target-derived BMP signaling and a transcription factor code that includes Apterous. Using integrase transgenesis of enhancer reporters, we functionally dissected the Tv4-enhancer of FMRFa within its native cellular context. We identified two essential but discrete cis-elements, a BMP-response element (BMP-RE) that binds BMP-activated pMad, and a homeodomain-response element (HD-RE) that binds Apterous. These cis-elements have low activity and must be combined for Tv4-enhancer activity. Such combinatorial activity is often a mechanism for restricting expression to the intersection of cis-element spatiotemporal activities. However, concatemers of the HD-RE and BMP-RE cis-elements were found to independently generate the same spatiotemporal expression as the Tv4-enhancer. Thus, the Tv4-enhancer atypically combines two low-activity cis-elements that confer the same output from distinct inputs. The activation of target-dependent genes is assumed to 'wait' for target contact. We tested this directly, and unexpectedly found that premature BMP activity could not induce early FMRFa expression; also, we show that the BMP-insensitive HD-RE cis-element is activated at the time of target contact. This led us to uncover a role for the nuclear receptor, seven up (svp), as a repressor of FMRFa induction prior to target contact. Svp is normally downregulated immediately prior to target contact, and we found that maintaining Svp expression prevents cis-element activation, whereas reducing svp gene dosage prematurely activates cis-element activity. We conclude that the target-dependent FMRFa gene is repressed prior to target contact, and that target-derived BMP signaling directly activates FMRFa gene expression through an atypical gene regulatory mechanism.

Cell-selective modulation of the Drosophila neuromuscular system by a neuropeptide

Neuropeptides can modulate physiological properties of neurons in a cell-specific manner. The present work examines whether a neuropeptide can also modulate muscle tissue in a cell-specific manner using identified muscle cells in third-instar larvae of fruit flies. DPKQDFMRFa, a modulatory peptide in the fruit fly Drosophila melanogaster, has been shown to enhance transmitter release from motor neurons and to elicit contractions by a direct effect on muscle cells. We report that DPKQDFMRFa causes a nifedipine-sensitive drop in input resistance in some muscle cells (6 and 7) but not others (12 and 13). The peptide also increased the amplitude of nerve-evoked contractions and compound excitatory junctional potentials (EJPs) to a greater degree in muscle cells 6 and 7 than 12 and 13. Knocking down FMRFamide receptor (FR) expression separately in nerve and muscle indicate that both presynaptic and postsynaptic FR expression contributed to the enhanced contractions, but EJP enhancement was mainly due to presynaptic expression. Muscle ablation showed that DPKQDFMRFa induced contractions and enhanced nerve-evoked contractions more strongly in muscle cells 6 and 7 than cells 12 and 13. In situ hybridization indicated that FR expression was significantly greater in muscle cells 6 and 7 than 12 and 13. Taken together, these results indicate that DPKQDFMRFa can elicit cell-selective effects on muscle fibers. The ability of neuropeptides to work in a cell-selective manner on neurons and muscle cells may help explain why so many peptides are encoded in invertebrate and vertebrate genomes.