Longitudinal body wall muscles are electrically coupled across the segmental boundary in the third instar larva of Drosophila melanogaster

Miniature neurotransmission regulates Drosophila synaptic structural maturation

Miniature neurotransmission is the transsynaptic process where single synaptic vesicles spontaneously released from presynaptic neurons induce miniature postsynaptic potentials. Since their discovery over 60 years ago, miniature events have been found at every chemical synapse studied. However, the in vivo necessity for these small-amplitude events has remained enigmatic. Here, we show that miniature neurotransmission is required for the normal structural maturation of Drosophila glutamatergic synapses in a developmental role that is not shared by evoked neurotransmission. Conversely, we find that increasing miniature events is sufficient to induce synaptic terminal growth. We show that miniature neurotransmission acts locally at terminals to regulate synapse maturation via a Trio guanine nucleotide exchange factor (GEF) and Rac1 GTPase molecular signaling pathway. Our results establish that miniature neurotransmission, a universal but often-overlooked feature of synapses, has unique and essential functions in vivo.

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Miniature, but not evoked, neurotransmission is required for synapse development

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Miniature neurotransmission bidirectionally regulates synaptic terminal maturation

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Miniature events signal locally through the GEF Trio and the GTPase Rac1

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Miniature neurotransmission has unique and essential functions in vivo

Presynaptic pH and vesicle fusion in Drosophila larvae neurones

Both intracellular pH (pH_i) and synaptic cleft pH change during neuronal activity yet little is known about how these pH shifts might affect synaptic transmission by influencing vesicle fusion. To address this we imaged pH- and Ca²⁺-sensitive fluorescent indicators (HPTS, Oregon green) in boutons at neuromuscular junctions. Electrical stimulation of motor nerves evoked presynaptic Ca²⁺_i rises and pH_i falls (∼0.1 pH units) followed by recovery of both Ca²⁺_i and pH_i. The plasma-membrane calcium ATPase (PMCA) inhibitor, 5(6)-carboxyeosin diacetate, slowed both the calcium recovery and the acidification. To investigate a possible calcium-independent role for the pH_i shifts in modulating vesicle fusion we recorded post-synaptic miniature end-plate potential (mEPP) and current (mEPC) frequency in Ca²⁺-free solution. Acidification by propionate superfusion, NH₄⁺ withdrawal, or the inhibition of acid extrusion on the Na⁺/H⁺ exchanger (NHE) induced a rise in miniature frequency. Furthermore, the inhibition of acid extrusion enhanced the rise induced by propionate addition and NH₄⁺ removal. In the presence of NH₄⁺, 10 out of 23 cells showed, after a delay, one or more rises in miniature frequency. These findings suggest that Ca²⁺-dependent pH_i shifts, caused by the PMCA and regulated by NHE, may stimulate vesicle release. Furthermore, in the presence of membrane permeant buffers, exocytosed acid or its equivalents may enhance release through positive feedback. This hitherto neglected pH signalling, and the potential feedback role of vesicular acid, could explain some important neuronal excitability changes associated with altered pH and its buffering. Synapse 67:729–740, 2013.

Electrophysiological recording in the Drosophila embryo

Drosophila is a premier genetic model for the study of both embryonic development and functional neuroscience. Traditionally, these fields are quite isolated from each other, with largely independent histories and scientific communities. However, the interface between these usually disparate fields is the developmental programs underlying acquisition of functional electrical signaling properties and differentiation of functional chemical synapses during the final phases of neural circuit formation. This interface is a critically important area for investigation. In Drosophila, these phases of functional development occur during a period of <8 hours (at 25 degrees C) during the last third of embryogenesis. This late developmental period was long considered intractable to investigation owing to the deposition of a tough, impermeable epidermal cuticle. A breakthrough advance was the application of water-polymerizing surgical glue that can be locally applied to the cuticle to enable controlled dissection of late-stage embryos. With a dorsal longitudinal incision, the embryo can be laid flat, exposing the ventral nerve cord and body wall musculature to experimental investigation. Whole-cell patch-clamp techniques can then be employed to record from individually-identifiable neurons and somatic muscles. These recording configurations have been used to track the appearance and maturation of ionic currents and action potential propagation in both neurons and muscles. Genetic mutants affecting these electrical properties have been characterized to reveal the molecular composition of ion channels and associated signaling complexes, and to begin exploration of the molecular mechanisms of functional differentiation. A particular focus has been the assembly of synaptic connections, both in the central nervous system and periphery. The glutamatergic neuromuscular junction (NMJ) is most accessible to a combination of optical imaging and electrophysiological recording. A glass suction electrode is used to stimulate the peripheral nerve, with excitatory junction current (EJC) recordings made in the voltage-clamped muscle. This recording configuration has been used to chart the functional differentiation of the synapse, and track the appearance and maturation of presynaptic glutamate release properties. In addition, postsynaptic properties can be assayed independently via iontophoretic or pressure application of glutamate directly to the muscle surface, to measure the appearance and maturation of the glutamate receptor fields. Thus, both pre- and postsynaptic elements can be monitored separately or in combination during embryonic synaptogenesis. This system has been heavily used to isolate and characterize genetic mutants that impair embryonic synapse formation, and thus reveal the molecular mechanisms governing the specification and differentiation of synapse connections and functional synaptic signaling properties.

Harvesting and preparing Drosophila embryos for electrophysiological recording and other procedures

Drosophila is a premier genetic model for the study of both embryonic development and functional neuroscience. Traditionally, these fields are quite isolated from each other, with largely independent histories and scientific communities. However, the interface between these usually disparate fields is the developmental programs underlying acquisition of functional electrical signaling properties and differentiation of functional chemical synapses during the final phases of neural circuit formation. This interface is a critically important area for investigation. In Drosophila, these phases of functional development occur during a period of <8 hours (at 25°C) during the last third of embryogenesis. This late developmental period was long considered intractable to investigation owing to the deposition of a tough, impermeable epidermal cuticle. A breakthrough advance was the application of water-polymerizing surgical glue that can be locally applied to the cuticle to enable controlled dissection of late-stage embryos. With a dorsal longitudinal incision, the embryo can be laid flat, exposing the ventral nerve cord and body wall musculature to experimental investigation. This system has been heavily used to isolate and characterize genetic mutants that impair embryonic synapse formation, and thus reveal the molecular mechanisms governing the specification and differentiation of synapse connections and functional synaptic signaling properties.

Properties and possible function of a hyperpolarisation-activated chloride current in Drosophila

A chloride current, ICl,H, slowly activating on hyperpolarisation was investigated in Drosophila melanogaster larval muscles using the two-electrode voltage clamp. Sizeable currents were observed after the intracellular chloride concentration([Cl–]i) had been elevated by diffusion of Cl– from the electrodes. The time course of ICl,H was rather variable and required two exponentials to be accurately described. The reversal potential, –40 to –20 mV in Cl–-loaded fires, shifted on lowering external[Cl–] in the positive direction. Steady-state activation of ICl,H was characterised by V0.5 of≈–120 mV and a slope factor, k, of ≈10 mV at a[Cl–]i ≈35 mmol l–1. Raising[Cl–]i to ≈50 mmol l–1 caused a negative shift of V0.5 equivalent to the change of ECl and led to a nearly threefold increase in maximal steady-state conductance. ICl,H was resistant to 10 mmol l–1 Zn2+ and 1 mmol l–1Cd2+ but was greatly reduced by 1 mmol l–19-anthracenecarboxylic acid (9-AC). ICl,H was affected by changes of extracellular pH and increased on lowering extracellular osmolality. 9-AC also decreased muscle fibre resting conductance by approximately 20% and increased muscle contractions. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis confirmed the expression of all three ClC genes in muscle, and immunohistochemistry indicated location of Drosophila melanogaster chloride channel-2(DmClC-2) at the Z-lines. We conclude that DmClC-2 accounts for the channels underlying ICl,H, and in part for the resting chloride conductance. DmClC-2 may serve general homeostatic mechanisms such as pH- and osmo-regulation or may support muscle function on high motor activity or during a particular neurohormonal state of the animal.

Isoflurane depresses glutamate release by reducing neuronal excitability at the Drosophila neuromuscular junction

The mechanisms through which volatile general anaesthetics exert their behavioural effects remain unclear. The accessibility of the Drosophila larval neuromuscular junction to genetic and neurophysiological analysis has made it an attractive model system for identification of anaesthetic targets. This study provides a mechanistic basis for the genetic analysis of anaesthetic action, by analysing the neurophysiological effects of the volatile anaesthetic isoflurane on axonal and synaptic function in the Drosophila larva. The most robust effect of isoflurane was a reversible decrease in the amplitude and area of glutamatergic excitatory junctional currents (EJCs) evoked at the neuromuscular junction. Isoflurane did not affect postsynaptic glutamate receptor function detectably, in that the amplitudes, areas and decay times of spontaneous miniature EJCs were unchanged at any concentration. Therefore, decreased EJC amplitude resulted from reduction of neurotransmitter release. Reduced neurotransmitter release was associated with decreased presynaptic excitability, measured as increased delay to EJC onset and reduced axonal conduction velocity. EJC amplitude was rescued to control levels by direct electrotonic stimulation of the synapse in the presence of tetrodotoxin, indicating that isoflurane inhibits neurotransmitter release by reducing presynaptic excitability. In addition, isoflurane reduced release probability, measured as increased paired-pulse facilitation. The EC(50) for suppression of larval locomotion was similar to that for reduction of transmitter release, indicating that the axonal and synaptic effects were occurring in a behaviourally relevant range. These results provide a cellular context for ongoing genetic and neurophysiological analyses of volatile anaesthetic action in Drosophila, and suggest candidate anaesthetic target molecules.

Constant amplitude of postsynaptic responses for single presynaptic action potentials but not bursting input during growth of an identified neuromuscular junction in the lobster,Homarus americanus

As lobsters grow from early juveniles to adults their body size increases more than 20-fold, raising the question of how function is maintained during these ongoing changes in size. To address this question we studied the pyloric 1 (p1) muscle of the stomach of the lobster, Homarus americanus. The p1 muscle receives multiterminal innervation from one motor neuron, the lateral pyloric neuron of the stomatogastric ganglion. Staining with antibodies raised against synaptotagmin showed that as the muscle fibers increased in length, the spacing between the terminal innervation increased proportionally, so the number of synaptic contact regions/muscle fiber did not change. Muscle fibers were electrically coupled in both juveniles and adults. The amplitude of single intracellularly recorded excitatory junctional potentials evoked by motor nerve stimulation was the same in both juveniles and adults. Nonetheless, the peak depolarizations reached in response to ongoing pyloric rhythm activity or in response to high-frequency trains of stimuli similar to those produced during the pyloric rhythm were approximately twofold larger in juveniles than in adults. This suggests that homeostatic regulation of synaptic connections may operate at the level of the amplitude of the single synaptic potential rather than on the summed depolarization evoked during strong rhythmic activity.

Mutations in the exocyst component Sec5 disrupt neuronal membrane traffic, but neurotransmitter release persists

The exocyst (Sec6/8) complex is necessary for secretion in yeast and has been postulated to establish polarity by directing vesicle fusion to specific sites along the plasma membrane. The complex may also function in the nervous system, but its precise role is unknown. We have investigated exocyst function in Drosophila with mutations in one member of the complex, sec5. Null alleles die as growth-arrested larvae, whose neuromuscular junctions fail to expand. In culture, neurite outgrowth fails in sec5 mutants once maternal Sec5 is exhausted. Using a trafficking assay, we found impairments in the membrane addition of newly synthesized proteins. In contrast, synaptic vesicle fusion was not impaired. Thus, Sec5 differentiates between two forms of vesicle trafficking: trafficking for cell growth and membrane protein insertion depend on sec5, whereas transmitter secretion does not. In this regard, sec5 differs from the homologs of other yeast exocytosis genes that are required for both neuronal trafficking pathways.

Gap-Junctional communication between developing Drosophila muscles is essential for their normal development

Recent experiments have demonstrated that a family of proteins, known as the innexins, are structural components of invertebrate gap junctions. The shaking-B (shak-B) locus of Drosophila encodes two members of this emerging family, Shak-B(lethal) and Shak-B(neural). This study focuses on the role of Shak-B gap junctions in the development of embryonic and larval muscle. During embryogenesis, shak-B transcripts are expressed in a subset of the somatic muscles; expression is strong in ventral oblique muscles (VO4-6) but only weak in ventral longitudinals (VL3 and 4). Carboxyfluorescein injected into VO4 of wild-type early stage 16 embryos spreads, via gap junctions, to label adjacent muscles, including VL3 and 4. In shak-B2 embryos (in which the shak-B(neural) function is disrupted), dye injected into VO4 fails to spread into other muscles. In the first instar larva, when dye coupling between muscles is no longer present, another effect of the shak-B2 mutation is revealed by whole-cell voltage clamp. In a calcium-free saline, only two voltage-activated potassium currents are present in wild-type muscles; a fast IA and a slow IK current. In shak-B2 larvae, these two currents are significantly reduced in magnitude in VO4 and 5, but remain normal in VL3. Expression of shak-B(neural) in a shak-B2 background fully rescues both dye coupling in embryonic muscle and whole-cell currents in first instar VO4 and 5. Our observations show that Shak-B(neural) is one of a set of embryonic gap-junction proteins, and that it is required for the normal temporal development of potassium currents in some larval muscles.

Octopamine inhibits synaptic transmission at the larval neuromuscular junction in Drosophila melanogaster

The effect of octopamine, a biogenic amine, on synaptic transmission at the neuromuscular junction (NMJ) in first instar larvae of Drosophila melanogaster was examined using the patch clamp technique. Muscle cells were voltage-clamped at -60 mV in the whole-cell configuration, and nerve-evoked excitatory junctional currents (EJCs) and miniature excitatory junctional currents (MEJCs) were recorded. Octopamine significantly decreased the mean amplitude of nerve-evoked EJCs in a dose-dependent manner and increased the failure rate. However, the mean amplitude and amplitude distribution of MEJCs were not affected by octopamine. These results suggest that octopamine is acting presynaptically. This effect was abolished by pretreatment with the octopamine receptor blocker, yohimbine. On the other hand, octopamine significantly decreased the decay time constant of MEJCs from 6.0+/-0.3 ms (mean+/-S.E., n=16) to 4.2+/-0.3 ms (n=14) (p<0.001), which might be the effect on the kinetic properties of junctional glutamate receptor channels. However, the mean open time of extrajunctional glutamate receptor channels was not changed by octopamine. Taken together, these results suggest that octopamine inhibits synaptic transmission by affecting both pre- and postsynaptic mechanisms.

Glutamate receptor expression regulates quantal size and quantal content at the Drosophila neuromuscular junction

At the Drosophila glutamatergic neuromuscular junction, the postsynaptic cell can regulate synaptic strength by both changing its sensitivity to neurotransmitter and generating a retrograde signal that regulates presynaptic transmitter release. To investigate the molecular mechanisms underlying these forms of plasticity, we have undertaken a genetic analysis of two postsynaptic glutamate receptors that are expressed at this synapse. Deletion of both genes results in embryonic lethality that can be rescued by transgenic expression of either receptor. Although these receptors are redundant for viability, they have important differences. By transgenically rescuing the double mutant, we have investigated the relationship of receptor gene dosage and composition to synaptic function. We find that the receptor subunit composition regulates quantal size, Argiotoxin sensitivity, and receptor desensitization kinetics. Finally, we show that the activity of the receptor can regulate the retrograde signal functioning at this synapse. Thus, the diversity of receptors expressed at this synapse provides the cell with mechanisms for generating synaptic plasticity.

Selective effects of neuronal-synaptobrevin mutations on transmitter release evoked by sustained versus transient Ca2+ increases and by cAMP

Synaptobrevin is a key constituent of the synaptic vesicle membrane. The neuronal-synaptobrevin (n-syb) gene in Drosophila is essential for nerve-evoked synaptic currents, but miniature excitatory synaptic currents (mESCs) remain even in the complete absence of this gene. To further characterize the defect in these mutants, we have examined conditions that stimulate secretion. Despite the inability of an action potential to trigger fusion, high K+ saline could increase the frequency of mESCs 4- to 17-fold in a Ca2+-dependent manner, and the rate of fusion approached 25% of that seen in wild-type synapses under the same conditions. Similarly, the mESC frequency in n-syb null mutants could be increased by a Ca2+ ionophore, A23187, and by black widow spider venom. Thus, the ability of the vesicles to fuse in response to sustained increases in cytosolic Ca2+ persisted in the absence of this protein. Tetanic stimulation could also increase the frequency of mESCs, particularly toward the end of a train and after the train of stimuli. In contrast, these mutants did not respond to an elevation of cAMP induced by an activator of adenylyl cyclase, forskolin, or a membrane-permeable analog of cAMP, dibutyryl cAMP, which in wild-type synapses causes a marked increase in the mESC frequency even in the absence of external Ca2+. These results are discussed in the context of models that invoke a special role for n-syb in coupling fusion to the transient, local changes in Ca2+ and an as yet unidentified target of cAMP.

Distinct requirements for evoked and spontaneous release of neurotransmitter are revealed by mutations in the Drosophila gene neuronal-synaptobrevin

Two modes of vesicular release of transmitter occur at a synapse: spontaneous release in the absence of a stimulus and evoked release that is triggered by Ca2+ influx. These modes often have been presumed to represent the same exocytotic apparatus functioning at different rates in different Ca2+ concentrations. To investigate the mechanism of transmitter release, we have examined the role of synaptobrevin/VAMP, a protein involved in vesicular docking and/or fusion. We generated a series of mutations, including null mutations, in neuronal-synaptobrevin (n-syb), the neuronally expressed synaptobrevin gene in Drosophila. Mutant embryos completely lacking n-syb form morphologically normal neuromuscular junctions. Electrophysiological recordings from the neuromuscular junction of these mutants reveal that the excitatory synaptic current evoked by stimulation of the motor neuron is abolished entirely. However, spontaneous release of quanta from these terminals persists, although its rate is reduced by 75%. Thus, at least a portion of the spontaneous "minis" that are seen at the synapse can be generated by a protein complex that is distinct from that required for an evoked synaptic response.