The chemical component of the mixed GF-TTMn synapse in Drosophila melanogaster uses acetylcholine as its neurotransmitter

A novel class of insecticidal alkylsulfones are potent inhibitors of vesicular acetylcholine transport

Pyridine alkylsulfone derivatives typified by oxazosulfyl (Sumitomo Chemical Company Ltd.) and compound A2 (Syngenta) represent a new class of insecticides, with potent activity against several insect orders. Whilst the MOA of this class has been attributed to interaction with the voltage-gated sodium channel (VGSC), here we present strong evidence that their toxicity to insects is mediated primarily through inhibition of the vesicular acetylcholine transporter (VAChT). Alkylsulfone intoxication in insects is characterised by (i) a reduction in cholinergic synaptic transmission efficiency demonstrated by a depression of cercal afferent activity in giant-interneurone preparations of American cockroach (Periplaneta americana), (ii) selective block of cholinergic-transmission dependent post-synaptic potentials in the Drosophila giant-fibre pathway and (iii) abolition of miniature excitatory post-synaptic currents (mEPSCs) in an identified synapse in Drosophila larvae. Ligand-binding studies using a tritiated example compound ([3H]-A1) revealed a single saturable binding-site, with low nanomolar Kd value, in membrane fractions of green bottle fly (Lucilia sericata). Binding is inhibited by vesamicol and by several examples of a previously identified class of insecticidal compounds known to target VAChT, the spiroindolines. Displacement of this binding by analogues of the radioligand reveals a strong correlation with insecticidal potency. No specific binding was detected in untransformed PC12 cells but a PC12 line stably expressing Drosophila VAChT showed similar affinity for [3H]-A1 as that seen in fly head membrane preparations. Previously identified VAChT point mutations confer resistance to the spiroindoline class of insecticides in Drosophila by Gal-4/UAS directed expression in cholinergic neurones and by CRISPR gene-editing of VAChT, but none of these flies show detectable cross-resistance to this new chemical class. Oxazosulfyl was previously shown to stabilise voltage-gated sodium channels in their slow-inactivated conformation with an IC50 value of 12.3μM but inhibits binding of [3H]-A1 with approximately 5000 times greater potency. We believe this chemistry class represents a novel mode-of-action with high potential for invertebrate selectivity.

Single-cell type analysis of wing premotor circuits in the ventral nerve cord of Drosophila melanogaster

To perform most behaviors, animals must send commands from higher-order processing centers in the brain to premotor circuits that reside in ganglia distinct from the brain, such as the mammalian spinal cord or insect ventral nerve cord. How these circuits are functionally organized to generate the great diversity of animal behavior remains unclear. An important first step in unraveling the organization of premotor circuits is to identify their constituent cell types and create tools to monitor and manipulate these with high specificity to assess their function. This is possible in the tractable ventral nerve cord of the fly. To generate such a toolkit, we used a combinatorial genetic technique (split-GAL4) to create 195 sparse driver lines targeting 198 individual cell types in the ventral nerve cord. These included wing and haltere motoneurons, modulatory neurons, and interneurons. Using a combination of behavioral, developmental, and anatomical analyses, we systematically characterized the cell types targeted in our collection. Taken together, the resources and results presented here form a powerful toolkit for future investigations of neural circuits and connectivity of premotor circuits while linking them to behavioral outputs.

Differential localization of voltage-gated potassium channels during Drosophila metamorphosis

Neuronal excitability is determined by the combination of different ion channels and their sub-neuronal localization. This study utilizes protein trap fly strains with endogenously tagged channels to analyze the spatial expression patterns of the four Shaker-related voltage-gated potassium channels, K_v1-4, in the larval, pupal, and adult Drosophila ventral nerve cord. We find that all four channels (Shaker, K_v1; Shab, K_v2; Shaw, K_v3; and Shal, K_v4) each show different spatial expression patterns in the Drosophila ventral nerve cord and are predominantly targeted to different sub-neuronal compartments. Shaker is abundantly expressed in axons, Shab also localizes to axons but mostly in commissures, Shaw expression is restricted to distinct parts of neuropils, and Shal is found somatodendritically, but also in axons of identified motoneurons. During early pupal life expression of all four Shaker-related channels is markedly decreased with an almost complete shutdown of expression at early pupal stage 5 (∼30% through metamorphosis). Re-expression of K_v1-4 channels at pupal stage 6 starts with abundant channel localization in neuronal somata, followed by channel targeting to the respective sub-neuronal compartments until late pupal life. The developmental time course of tagged K_v1-4 channel expression corresponds with previously published data on developmental changes in single neuron physiology, thus indicating that protein trap fly strains are a useful tool to analyze developmental regulation of potassium channel expression. Finally, we take advantage of the large diameter of the giant fiber (GF) interneuron to map channel expression onto the axon and axon terminals of an identified interneuron. Shaker, Shaw, and Shal but not Shab channels localize to the non-myelinated GF axonal membrane and axon terminals. This study constitutes a first step toward systematically analyzing sub-neuronal potassium channel localization in Drosophila. Functional implications as well as similarities and differences to K_v1-4 channel localization in mammalian neurons are discussed.

A new method of recording from the giant fiber of Drosophila melanogaster shows that the strength of its auditory inputs remains constant with age

There have been relatively few studies of how central synapses age in adult Drosophila melanogaster. In this study we investigate the aging of the synaptic inputs to the Giant Fiber (GF) from auditory Johnston's Organ neurons (JONs). In previously published experiments an indirect assay of this synaptic connection was used; here we describe a new, more direct assay, which allows reliable detection of the GF action potential in the neck connective, and long term recording of its responses to sound. Genetic poisoning using diphtheria toxin expressed in the GF with R68A06-GAL4 was used to confirm that this signal indeed arose from the GF and not from other descending neurons. As before, the sound-evoked action potentials (SEPs) in the antennal nerve were recorded via an electrode inserted at the base of the antenna. It was noted that an action potential in the GF elicited an antennal twitch, which in turn evoked a mechanosensory response from the JONs in the absence of sound. We then used these extracellular recording techniques in males and female of different ages to quantify the response of the JONs to a brief sound impulse, and also to measure the strength of the connection between the JONs and the GF. At no age was there any significant difference between males and females, for any of the parameters measured. The sensitivity of the JONs to a sound impulse approximately doubled between 1 d and 10 d after eclosion, which corresponds to the period when most mating is taking place. Subsequently JON sensitivity decreased with age, being approximately half as sensitive at 20 d and one-third as sensitive at 50 d, as compared to 10 d. However, the strength of the connection between the auditory input and the GF itself remained unchanged with age, although it did show some variability that could mask any small changes.

Reliability of an interneuron response depends on an integrated sensory state

The central nervous system transforms sensory information into representations that are salient to the animal. Here we define the logic of this transformation in a Caenorhabditis elegans integrating interneuron. AIA interneurons receive input from multiple chemosensory neurons that detect attractive odors. We show that reliable AIA responses require the coincidence of two sensory inputs: activation of AWA olfactory neurons that are activated by attractive odors, and inhibition of one or more chemosensory neurons that are inhibited by attractive odors. AWA activates AIA through an electrical synapse, while the disinhibitory pathway acts through glutamatergic chemical synapses. AIA interneurons have bistable electrophysiological properties consistent with their calcium dynamics, suggesting that AIA activation is a stereotyped response to an integrated stimulus. Our results indicate that AIA interneurons combine sensory information using AND-gate logic, requiring coordinated activity from multiple chemosensory neurons. We propose that AIA encodes positive valence based on an integrated sensory state.

Habituation Learning Is a Widely Affected Mechanism in Drosophila Models of Intellectual Disability and Autism Spectrum Disorders

BACKGROUND

Although habituation is one of the most ancient and fundamental forms of learning, its regulators and its relevance for human disease are poorly understood.

METHODS

We manipulated the orthologs of 286 genes implicated in intellectual disability (ID) with or without comorbid autism spectrum disorder (ASD) specifically in Drosophila neurons, and we tested these models in light-off jump habituation. We dissected neuronal substrates underlying the identified habituation deficits and integrated genotype-phenotype annotations, gene ontologies, and interaction networks to determine the clinical features and molecular processes that are associated with habituation deficits.

RESULTS

We identified >100 genes required for habituation learning. For 93 of these genes, a role in habituation learning was previously unknown. These genes characterize ID disorders with macrocephaly and/or overgrowth and comorbid ASD. Moreover, individuals with ASD from the Simons Simplex Collection carrying damaging de novo mutations in these genes exhibit increased aberrant behaviors associated with inappropriate, stereotypic speech. At the molecular level, ID genes required for normal habituation are enriched in synaptic function and converge on Ras/mitogen-activated protein kinase (Ras/MAPK) signaling. Both increased Ras/MAPK signaling in gamma-aminobutyric acidergic (GABAergic) neurons and decreased Ras/MAPK signaling in cholinergic neurons specifically inhibit the adaptive habituation response.

CONCLUSIONS

Our work supports the relevance of habituation learning to ASD, identifies an unprecedented number of novel habituation players, supports an emerging role for inhibitory neurons in habituation, and reveals an opposing, circuit-level-based mechanism for Ras/MAPK signaling. These findings establish habituation as a possible, widely applicable functional readout and target for pharmacologic intervention in ID/ASD.

Postnatal Increases in Axonal Conduction Velocity of an Identified Drosophila Interneuron Require Fast Sodium, L-Type Calcium and Shaker Potassium Channels

During early postnatal life, speed up of signal propagation through many central and peripheral neurons has been associated with an increase in axon diameter or/and myelination. Especially in unmyelinated axons postnatal adjustments of axonal membrane conductances is potentially a third mechanism but solid evidence is lacking. Here, we show that axonal action potential (AP) conduction velocity in the Drosophila giant fiber (GF) interneuron, which is required for fast long-distance signal conduction through the escape circuit, is increased by 80% during the first day of adult life. Genetic manipulations indicate that this postnatal increase in AP conduction velocity in the unmyelinated GF axon is likely owed to adjustments of ion channel expression or properties rather than axon diameter increases. Specifically, targeted RNAi knock-down of either Para fast voltage-gated sodium, Shaker potassium (Kv1 homologue), or surprisingly, L-type like calcium channels counteracts postnatal increases in GF axonal conduction velocity. By contrast, the calcium-dependent potassium channel Slowpoke (BK) is not essential for postnatal speeding, although it also significantly increases conduction velocity. Therefore, we identified multiple ion channels that function to support fast axonal AP conduction velocity, but only a subset of these are regulated during early postnatal life to maximize conduction velocity. Despite its large diameter (∼7 µm) and postnatal regulation of multiple ionic conductances, mature GF axonal conduction velocity is still 20-60 times slower than that of vertebrate Aβ sensory axons and α motoneurons, thus unraveling the limits of long-range information transfer speed through invertebrate circuits.

A Computational Model of the Escape Response Latency in the Giant Fiber System of Drosophila melanogaster

The giant fiber system (GFS) is a multi-component neuronal pathway mediating rapid escape response in the adult fruit-fly Drosophila melanogaster, usually in the face of a threatening visual stimulus. Two branches of the circuit promote the response by stimulating an escape jump followed by flight initiation. A recent work demonstrated an age-associated decline in the speed of signal propagation through the circuit, measured as the stimulus-to-muscle depolarization response latency. The decline is likely due to the diminishing number of inter-neuronal gap junctions in the GFS of ageing flies. In this work, we presented a realistic conductance-based, computational model of the GFS that recapitulates the experimental results and identifies some of the critical anatomical and physiological components governing the circuit’s response latency. According to our model, anatomical properties of the GFS neurons have a stronger impact on the transmission than neuronal membrane conductance densities. The model provides testable predictions for the effect of experimental interventions on the circuit’s performance in young and ageing flies.

Human Tau isoform-specific presynaptic deficits in a Drosophila Central Nervous System circuit

Accumulation of normal or mutant human Tau isoforms in Central Nervous System (CNS) neurons of vertebrate and invertebrate models underlies pathologies ranging from behavioral deficits to neurodegeneration that broadly recapitulate human Tauopathies. Although some functional differences have begun to emerge, it is still largely unclear whether normal and mutant Tau isoforms induce differential effects on the synaptic physiology of CNS neurons. We use the oligosynaptic Giant Fiber System in the adult Drosophila CNS to address this question and reveal that 3R and 4R isoforms affect distinct synaptic parameters. Whereas 0N3R increased failure rate upon high frequency stimulation, 0N4R compromised stimulus conduction and response speed at a specific cholinergic synapse in an age-dependent manner. In contrast, accumulation of the R406W mutant of 0N4R induced mild, age-dependent conduction velocity defects. Because 0N4R and its mutant isoform are expressed equivalently, this demonstrates that the defects are not merely consequent of exogenous human Tau accumulation and suggests distinct functional properties of 3R and 4R isoforms in cholinergic presynapses.

Shaking-B misexpression increases the formation of gap junctions but not chemical synapses between auditory sensory neurons and the giant fiber of Drosophila melanogaster

The synapse between auditory Johnston's Organ neurons (JONs) and the giant fiber (GF) of Drosophila is structurally mixed, being composed of cholinergic chemical synapses and Neurobiotin- (NB) permeable gap junctions, which consist of the innexin Shaking-B (ShakB). Previous observations showed that misexpression of one ShakB isoform, ShakB(N+16), in a subset of JONs that do not normally form gap junctions results in their de novo dye coupling to the GF. Misexpression of the transcription factor Engrailed (En) in these neurons also has this effect, and in addition causes the formation of new chemical synapses. These results, along with earlier studies suggesting that gap junctions are required for the development of some chemical synapses, led to the hypothesis that ShakB would, like En, have an instructive effect on the distribution of mixed chemical/electrical contacts. To test this, we first confirmed quantitatively that ShakB(N+16) misexpression increased the dye-coupling of JONs with the GF, indicating the formation of ectopic gap junctions. Conversely, expression of the 'incorrect' isoform, ShakB(N), abolished dye coupling. Immunocytochemistry of the ShakB protein showed that ShakB(N+16) increased gap junctional plaques in JON axons but ShakB(N) did not. To test our hypothesis, fluorescently-labeled presynaptic active zone protein (Brp) was expressed in JONs and the changes in its distribution on the GF dendrites was assayed with confocal microscopy in animals with misexpression of ShakB(N+16), ShakB(N) or, as a positive control, En. Using different methods of image analysis, we confirmed our previous result that En misexpression increased the chemical synapses with the GF and the amount of GF medial dendrite branching. However, contrary to our hypothesis, misexpression of ShakB did not increase these parameters. Immunostaining showed no association between presynaptic active zones and the new ShakB plaques, further evidence against the hypothesis. We conclude that both subsets of JON form chemical synapses onto the GF dendrites but only one population forms gap junctions, comprised of ShakB(N+16). Misexpression of this isoform in all JONs does not instruct the formation of new mixed chemical/electrical synapses, but results in the insertion of new gap junctions, presumably at the sites of existing chemical synaptic contacts with the GF.

Impact of insulin signaling and proteasomal activity on physiological output of a neuronal circuit in aging Drosophila melanogaster

The insulin family of growth factors plays an important role in development and function of the nervous system. Reduced insulin and insulin-growth-factor signaling (IIS), however, can improve symptoms of neurodegenerative diseases in laboratory model organisms and protect against age-associated decline in neuronal function. Recently, we showed that chronic, moderately lowered IIS rescues age-related decline in neurotransmission through the Drosophila giant fiber escape response circuit. Here, we expand our initial findings by demonstrating that reduced functional output in the giant fiber system of aging flies can be prevented by increasing proteasomal activity within the circuit. Manipulations of IIS in neurons can also affect longevity, underscoring the relevance of the nervous system for aging.

Reduced insulin signaling maintains electrical transmission in a neural circuit in aging flies

Lowered insulin/insulin-like growth factor (IGF) signaling (IIS) can extend healthy lifespan in worms, flies, and mice, but it can also have adverse effects (the “insulin paradox”). Chronic, moderately lowered IIS rescues age-related decline in neurotransmission through the Drosophila giant fiber system (GFS), a simple escape response neuronal circuit, by increasing targeting of the gap junctional protein innexin shaking-B to gap junctions (GJs). Endosomal recycling of GJs was also stimulated in cultured human cells when IIS was reduced. Furthermore, increasing the activity of the recycling small guanosine triphosphatases (GTPases) Rab4 or Rab11 was sufficient to maintain GJs upon elevated IIS in cultured human cells and in flies, and to rescue age-related loss of GJs and of GFS function. Lowered IIS thus elevates endosomal recycling of GJs in neurons and other cell types, pointing to a cellular mechanism for therapeutic intervention into aging-related neuronal disorders.

Insulin and insulin-like growth factors play an important role in the nervous system development and function. Reduced insulin signaling, however, can improve symptoms of neurodegenerative diseases in different model organisms and protect against age-associated decline in neuronal function extending lifespan. Here, we analyze the effects of genetically attenuated insulin signaling on the escape response pathway in the fruit fly Drosophila melanogaster. This simple neuronal circuit is dominated by electrical synapses composed of the gap junctional shaking-B protein, which allows for the transfer of electrical impulses between cells. Transmission through the circuit is known to slow down with age. We show that this functional decline is prevented by systemic or circuit-specific suppression of insulin signaling due to the preservation of the number of gap junctional proteins in aging animals. Our experiments in a human cell culture system reveal increased membrane targeting of gap junctional proteins via small proteins Rab4 and Rab11 under reduced insulin conditions. We also find that increasing the level of these recycling-mediating proteins in flies preserves the escape response circuit output in old flies and suggests ways of improving the function of neuronal circuits dominated by electrical synapses during aging.

Antidromic-rectifying gap junctions amplify chemical transmission at functionally mixed electrical-chemical synapses

Neurons communicate through chemical synapses and electrical synapses (gap junctions). Although these two types of synapses often coexist between neurons, little is known about whether they interact, and whether any interactions between them are important to controlling synaptic strength and circuit functions. By studying chemical and electrical synapses between premotor interneurons (AVA) and downstream motor neurons (A-MNs) in the Caenorhabditis elegans escape circuit, we found that disrupting either the chemical or electrical synapses causes defective escape response. Gap junctions between AVA and A-MNs only allow antidromic current, but, curiously, disrupting them inhibits chemical transmission. In contrast, disrupting chemical synapses has no effect on the electrical coupling. These results demonstrate that gap junctions may serve as an amplifier of chemical transmission between neurons with both electrical and chemical synapses. The use of antidromic-rectifying gap junctions to amplify chemical transmission is potentially a conserved mechanism in circuit functions.

Tubby domain superfamily protein is required for the formation of the 7S SNARE complex in Drosophila

Tubby domain superfamily protein (TUSP) is a distant member of the Tubby-like protein (TULP) family. Although other TULPs play important roles in sensation, metabolism, and development, the molecular functions of TUSP are completely unknown. Here, we explore the function of TUSP in the Drosophila nervous system where it is expressed in all neurons. Tusp mutant flies exhibit a temperature-sensitive paralysis. This paralysis can be rescued by tissue-specific expression of Tusp in the giant fibers and peripherally synapsing interneurons of the giant fiber system, a well-characterized neuronal circuit that mediates rapid escape behavior in flies. Consistent with this paralytic phenotype, we observed a profound reduction in the assembly of the ternary 7S SNARE complex that is required for neurotransmitter release despite seeing no changes in the expression of each individual SNARE complex component. Together, these data suggest TUSP is a novel regulator of SNARE assembly and, therefore, of neurotransmitter release.

Shaking B Mediates Synaptic Coupling between Auditory Sensory Neurons and the Giant Fiber of Drosophila melanogaster

The Johnston’s Organ neurons (JONs) form chemical and electrical synapses onto the giant fiber neuron (GF), as part of the neuronal circuit that mediates the GF escape response in Drosophila melanogaster. The purpose of this study was to identify which of the 8 Drosophila innexins (invertebrate gap junction proteins) mediates the electrical connection at this synapse. The GF is known to express Shaking B (ShakB), specifically the ShakB(N+16) isoform only, at its output synapses in the thorax. The shakB² mutation disrupts these GF outputs and also abolishes JON-GF synaptic transmission. However, the identity of the innexin that forms the presynaptic hemichannels in the JONs remains unknown. We used electrophysiology, immunocytochemistry and dye injection, along with presynaptically-driven RNA interference, to investigate this question. The amplitude of the compound action potential recorded in response to sound from the base of the antenna (sound-evoked potential, or SEP) was reduced by RNAi of the innexins Ogre, Inx3, Inx6 and, to a lesser extent Inx2, suggesting that they could be required in JONs for proper development, excitability, or synchronization of action potentials. The strength of the JON-GF connection itself was reduced to background levels only by RNAi of shakB, not of the other seven innexins. ShakB knockdown prevented Neurobiotin coupling between GF and JONs and removed the plaques of ShakB protein immunoreactivity that are present at the region of contact. Specific shakB RNAi lines that are predicted to target the ShakB(L) or ShakB(N) isoforms alone did not reduce the synaptic strength, implying that it is ShakB(N+16) that is required in the presynaptic neurons. Overexpression of ShakB(N+16) in JONs caused the formation of ectopic dye coupling, whereas ShakB(N) prevented it altogether, supporting this conclusion and also suggesting that gap junction proteins may have an instructive role in synaptic target choice.

Engrailed alters the specificity of synaptic connections of Drosophila auditory neurons with the giant fiber

We show that a subset of sound-detecting Johnston's Organ neurons (JONs) in Drosophila melanogaster, which express the transcription factors Engrailed (En) and Invected (Inv), form mixed electrical and chemical synaptic inputs onto the giant fiber (GF) dendrite. These synaptic connections are detected by trans-synaptic Neurobiotin (NB) transfer and by colocalization of Bruchpilot-short puncta. We then show that misexpressing En postmitotically in a second subset of sound-responsive JONs causes them to form ectopic electrical and chemical synapses with the GF, in turn causing that postsynaptic neuron to redistribute its dendritic branches into the vicinity of these afferents. We also introduce a simple electrophysiological recording paradigm for quantifying the presynaptic and postsynaptic electrical activity at this synapse, by measuring the extracellular sound-evoked potentials (SEPs) from the antennal nerve while monitoring the likelihood of the GF firing an action potential in response to simultaneous subthreshold sound and voltage stimuli. Ectopic presynaptic expression of En strengthens the synaptic connection, consistent with there being more synaptic contacts formed. Finally, RNAi-mediated knockdown of En and Inv in postmitotic neurons reduces SEP amplitude but also reduces synaptic strength at the JON-GF synapse. Overall, these results suggest that En and Inv in JONs regulate both neuronal excitability and synaptic connectivity.

A spike-timing mechanism for action selection

We discovered a bimodal behavior in the genetically tractable organism Drosophila melanogaster that allowed us to directly probe the neural mechanisms of an action selection process. When confronted by a predator-mimicking looming stimulus, a fly responds with either a long-duration escape behavior sequence that initiates stable flight or a distinct, short-duration sequence that sacrifices flight stability for speed. Intracellular recording of the descending giant fiber (GF) interneuron during head-fixed escape revealed that GF spike timing relative to parallel circuits for escape actions determined which of the two behavioral responses was elicited. The process was well described by a simple model in which the GF circuit has a higher activation threshold than the parallel circuits, but can override ongoing behavior to force a short takeoff. Our findings suggest a neural mechanism for action selection in which relative activation timing of parallel circuits creates the appropriate motor output.

Myiasis (fly disease) and insectal disease generally are causing mental illness

Mental illness by which psychosis is meant here is known to be caused mainly by imbalances of certain neurotransmitters in the brain. But, what is causing these imbalances? There has been a recent flurry of interest focusing on the possibility of parasitical disease. The appropriateness of this is based on the fact that organisms of the animal kingdom produce the same neurotransmitters. In fact stinging insects release them in their venoms. The proposal here is that insect larval parasites acting on the human brain and body may release such neurotransmitters and cause imbalances and altered mental states and is supported by the occurrence of previously unexplained physical symptoms such as; diarrhoea, constipation, spasms, anaemia, bloating, insomnia, headache, migraine, weight loss, low blood pressure, low grade fever, amnesia and signs of allergy which may accompany mental illness. Some of these symptoms have been previously attributed to the medications prescribed to alleviate the psychotic symptoms but, many are also parasitical signs. It is proposed that the minute larvae may make sudden movements and may be highly motile and may move from pressure, hence evading the phlebotomist's needle. There is also the testimony of those with delusional parasitosis and related addictions, I propose the regularity with which humans are bitten, stung and have their foods infected with insects at all stages as a demonstration of how insectal disease may have the potential for common infection and disease; mental and physical.

New tools for targeted disruption of cholinergic synaptic transmission in Drosophila melanogaster

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels. The α7 subtype of nAChRs is involved in neurological pathologies such as Parkinson's disease, Alzheimer's disease, addiction, epilepsy and autism spectrum disorders. The Drosophila melanogaster α7 (Dα7) has the closest sequence homology to the vertebrate α7 subunit and it can form homopentameric receptors just as the vertebrate counterpart. The Dα7 subunits are essential for the function of the Giant Fiber circuit, which mediates the escape response of the fly. To further characterize the receptor function, we generated different missense mutations in the Dα7 nAChR's ligand binding domain. We characterized the effects of targeted expression of two UAS-constructs carrying a single mutation, D197A and Y195T, as well as a UAS-construct carrying a triple D77T, L117Q, I196P mutation in a Dα7 null mutant and in a wild type background. Expression of the triple mutation was able to restore the function of the circuit in Dα7 null mutants and had no disruptive effects when expressed in wild type. In contrast, both single mutations severely disrupted the synaptic transmission of Dα7-dependent but not glutamatergic or gap junction dependent synapses in wild type background, and did not or only partially rescued the synaptic defects of the null mutant. These observations are consistent with the formation of hybrid receptors, consisting of D197A or Y195T subunits and wild type Dα7 subunits, in which the binding of acetylcholine or acetylcholine-induced conformational changes of the Dα7 receptor are altered and causes inhibition of cholinergic responses. Thus targeted expression of D197A or Y195T can be used to selectively disrupt synaptic transmission of Dα7-dependent synapses in neuronal circuits. Hence, these constructs can be used as tools to study learning and memory or addiction associated behaviors by allowing the manipulation of neuronal processing in the circuits without affecting other cellular signaling.

Paired nanoinjection and electrophysiology assay to screen for bioactivity of compounds using the Drosophila melanogaster giant fiber system

Screening compounds for in vivo activity can be used as a first step to identify candidates that may be developed into pharmacological agents. We developed a novel nanoinjection/electrophysiology assay that allows the detection of bioactive modulatory effects of compounds on the function of a neuronal circuit that mediates the escape response in Drosophila melanogaster. Our in vivo assay, which uses the Drosophila Giant Fiber System (GFS, Figure 1) allows screening of different types of compounds, such as small molecules or peptides, and requires only minimal quantities to elicit an effect. In addition, the Drosophila GFS offers a large variety of potential molecular targets on neurons or muscles. The Giant Fibers (GFs) synapse electrically (Gap Junctions) as well as chemically (cholinergic) onto a Peripheral Synapsing Interneuron (PSI) and the Tergo Trochanteral Muscle neuron (TTMn. The PSI to DLMn (Dorsal Longitudinal Muscle neuron) connection is dependent on Dα7 nicotinic acetylcholine receptors (nAChRs). Finally, the neuromuscular junctions (NMJ) of the TTMn and the DLMn with the jump (TTM) and flight muscles (DLM) are glutamatergic. Here, we demonstrate how to inject nanoliter quantities of a compound, while obtaining electrophysiological intracellular recordings from the Giant Fiber System and how to monitor the effects of the compound on the function of this circuit. We show specificity of the assay with methyllycaconitine citrate (MLA), a nAChR antagonist, which disrupts the PSI to DLMn connection but not the GF to TTMn connection or the function of the NMJ at the jump or flight muscles. Before beginning this video it is critical that you carefully watch and become familiar with the JoVE video titled "Electrophysiological Recordings from the Giant Fiber Pathway of D. melanogaster" from Augustin et al, as the video presented here is intended as an expansion to this existing technique. Here we use the electrophysiological recordings method and focus in detail only on the addition of the paired nanoinjections and monitoring technique.

Constitutive activation of Ca2+/calmodulin-dependent protein kinase II during development impairs central cholinergic transmission in a circuit underlying escape behavior in Drosophila

Development of neural circuitry relies on precise matching between correct synaptic partners and appropriate synaptic strength tuning. Adaptive developmental adjustments may emerge from activity and calcium-dependent mechanisms. Calcium/calmodulin-dependent protein kinase II (CaMKII) has been associated with developmental synaptic plasticity, but its varied roles in different synapses and developmental stages make mechanistic generalizations difficult. In contrast, we focused on synaptic development roles of CaMKII in a defined sensory-motor circuit. Thus, different forms of CaMKII were expressed with UAS-Gal4 in distinct components of the giant fiber system, the escape circuit of Drosophila, consisting of photoreceptors, interneurons, motoneurons, and muscles. The results demonstrate that the constitutively active CaMKII-T287D impairs development of cholinergic synapses in giant fiber dendrites and thoracic motoneurons, preventing light-induced escape behavior. The locus of the defects is postsynaptic as demonstrated by selective expression of transgenes in distinct components of the circuit. Furthermore, defects among these cholinergic synapses varied in severity, while the glutamatergic neuromuscular junctions appeared unaffected, demonstrating differential effects of CaMKII misregulation on distinct synapses of the same circuit. Limiting transgene expression to adult circuits had no effects, supporting the role of misregulated kinase activity in the development of the system rather than in acutely mediating escape responses. Overexpression of wild-type transgenes did not affect circuit development and function, suggesting but not proving that the CaMKII-T287D effects are not due to ectopic expression. Therefore, regulated CaMKII autophosphorylation appears essential in central synapse development, and particular cholinergic synapses are affected differentially, although they operate via the same nicotinic receptor.

In vivo measurement of muscle output in intact Drosophila

We describe our methods for analysing muscle function in a whole intact small insect, taking advantage of a simple flexible optical beam to produce an inexpensive transducer with wide application. We review our previous data measuring the response to a single action potential driven muscle twitch to explore jumping behaviour in Drosophila melanogaster. In the fruitfly, where the sophisticated and powerful genetic toolbox is being widely employed to investigate neuromuscular function, we further demonstrate the use of the apparatus to analyse in detail, within whole flies, neuronal and muscle mutations affecting activation of muscle contraction in the jump muscle. We have now extended the use of the apparatus to record the muscle forces during larval and other aspects of adult locomotion. The robustness, simplicity and versatility of the apparatus are key to these measurements.

Electrophysiological recordings from the giant fiber pathway of D. melanogaster

When startled adult D. melanogaster react by jumping into the air and flying away. In many invertebrate species, including D. melanogaster, the "escape" (or "startle") response during the adult stage is mediated by the multi-component neuronal circuit called the Giant Fiber System (GFS). The comparative large size of the neurons, their distinctive morphology and simple connectivity make the GFS an attractive model system for studying neuronal circuitry. The GFS pathway is composed of two bilaterally symmetrical Giant Fiber (GF) interneurons whose axons descend from the brain along the midline into the thoracic ganglion via the cervical connective. In the mesothoracic neuromere (T2) of the ventral ganglia the GFs form electro-chemical synapses with 1) the large medial dendrite of the ipsilateral motorneuron (TTMn) which drives the tergotrochanteral muscle (TTM), the main extensor for the mesothoracic femur/leg, and 2) the contralateral peripherally synapsing interneuron (PSI) which in turn forms chemical (cholinergic) synapses with the motorneurons (DLMns) of the dorsal longitudinal muscles (DLMs), the wing depressors. The neuronal pathway(s) to the dorsovental muscles (DVMs), the wing elevators, has not yet been worked out (the DLMs and DVMs are known jointly as indirect flight muscles - they are not attached directly to the wings, but rather move the wings indirectly by distorting the nearby thoracic cuticle) (King and Wyman, 1980; Allen et al., 2006). The di-synaptic activation of the DLMs (via PSI) causes a small but important delay in the timing of the contraction of these muscles relative to the monosynaptic activation of TTM (~0.5 ms) allowing the TTMs to first extend the femur and propel the fly off the ground. The TTMs simultaneously stretch-activate the DLMs which in turn mutually stretch-activate the DVMs for the duration of the flight. The GF pathway can be activated either indirectly by applying a sensory (e.g."air-puff" or "lights-off") stimulus, or directly by a supra-threshold electrical stimulus to the brain (described here). In both cases, an action potential reaches the TTMs and DLMs solely via the GFs, PSIs, and TTM/DLM motoneurons, although the TTMns and DLMns do have other, as yet unidentified, sensory inputs. Measuring "latency response" (the time between the stimulation and muscle depolarization) and the "following to high frequency stimulation" (the number of successful responses to a certain number of high frequency stimuli) provides a way to reproducibly and quantitatively assess the functional status of the GFS components, including both central synapses (GF-TTMn, GF-PSI, PSI-DLMn) and the chemical (glutamatergic) neuromuscular junctions (TTMn-TTM and DLMn-DLM). It has been used to identify genes involved in central synapse formation and to assess CNS function.

A novel approach for in vivo screening of toxins using the Drosophila Giant Fiber circuit

Finding compounds that affect neuronal or muscular function is of great interest as potential therapeutic agents for a variety of neurological disorders. Alternative applications for these compounds include their use as molecular probes as well as insecticides. We have developed a bioassay that requires small amounts of compounds and allows for unbiased screening of biological activity in vivo. For this, we paired administering compounds in a non-invasive manner with simultaneous electrophysiological recordings from a well-characterized neuronal circuit, the Giant Fiber System of Drosophila melanogaster, which mediates the escape response of the fly. The circuit encompasses a variety of neurons with cholinergic, glutamatergic, and electrical synapses as well as neuromuscular junctions. Electrophysiological recordings from this system allow for the detection of compound-related effects against any molecular target on these components. Here, we provide evidence that this novel bioassay works with small molecules such as the cholinergic receptor blocker mecamylamine hydrochloride and the potassium channel blocker tetraethylammonium hydroxide, as well as with venom from Conus brunneus and isolated conopeptides. Conopeptides have been developed into powerful drugs, such as the painkillers Prialt™ and Xen2174. However, most conopeptides have yet to be characterized, revealing the need for a rapid and straightforward screening method. Our findings show that mecamylamine hydrochloride, as well as the α-conotoxin ImI, which is known to be an antagonist of the human α7 nicotinic acetylcholine receptor, efficiently disrupted the synaptic transmission of a Drosophila α7 nicotinic acetylcholine receptor-dependent pathway in our circuit but did not affect the function of neurons with other types of synapses. This demonstrates that our bioassay is a valid tool for screening for compounds relevant to human health.

Electrophysiological recordings from the Drosophila giant fiber system (GFS)

INTRODUCTION

The giant fiber system (GFS) of Drosophila is a well-characterized neuronal circuit that mediates the escape response in the fly. It is one of the few adult neural circuits from which electrophysiological recordings can be made routinely. This protocol describes a simple procedure for stimulating the giant fiber neurons directly in the brain of the adult fly and obtaining recordings from the output muscles of the GFS: the tergotrochanteral "jump" muscle (TTM) and the large indirect flight muscles (dorsal longitudinal muscles, or DLMs). It is a relatively noninvasive method that allows the investigator to stimulate the giant fibers in the brain and assay the function of several central synapses within this neural circuit by recording from the thoracic musculature.

A mechanism distinct from highwire for the Drosophila ubiquitin conjugase bendless in synaptic growth and maturation

The signaling mechanisms that allow the conversion of a growth cone into a mature and stable synapse are yet to be completely understood. Ubiquitination plays key regulatory roles in synaptic development and may be involved in this process. Previous studies identified the Drosophila ubiquitin conjugase bendless (ben) to be important for central synapse formation, but the precise role it plays has not been elucidated. Our studies indicate that Ben plays a pivotal role in synaptic growth and maturation. We have determined that an incipient synapse is present with a high penetrance in ben mutants, suggesting that Ben is required for a developmental step after target recognition. We used cell-autonomous rescue experiments to show that Ben has a presynaptic role in synapse growth. We then harnessed the TARGET system to transiently express UAS (upstream activating sequence)-ben in a ben mutant background and identified a well defined critical period for Ben function in establishing a full-grown, mature synaptic terminal. We demonstrate that the protein must be present at a time point before but not during the actual growth process. We also provide phenotypic evidence demonstrating that Ben is not a part of the signal transduction pathway involving the well characterized ubiquitin ligase highwire. We conclude that Bendless functions as a novel developmental switch that permits the transition from axonal growth and incipient synapse formation to synaptic growth and maturation in the CNS.

Acetylcholine beyond neurons: the non-neuronal cholinergic system in humans

Animal life is controlled by neurons and in this setting cholinergic neurons play an important role. Cholinergic neurons release ACh, which via nicotinic and muscarinic receptors (n- and mAChRs) mediate chemical neurotransmission, a highly integrative process. Thus, the organism responds to external and internal stimuli to maintain and optimize survival and mood. Blockade of cholinergic neurotransmission is followed by immediate death. However, cholinergic communication has been established from the beginning of life in primitive organisms such as bacteria, algae, protozoa, sponge and primitive plants and fungi, irrespective of neurons. Tubocurarine- and atropine-sensitive effects are observed in plants indicating functional significance. All components of the cholinergic system (ChAT, ACh, n- and mAChRs, high-affinity choline uptake, esterase) have been demonstrated in mammalian non-neuronal cells, including those of humans. Embryonic stem cells (mice), epithelial, endothelial and immune cells synthesize ACh, which via differently expressed patterns of n- and mAChRs modulates cell activities to respond to internal or external stimuli. This helps to maintain and optimize cell function, such as proliferation, differentiation, formation of a physical barrier, migration, and ion and water movements. Blockade of n- and mACHRs on non-innervated cells causes cellular dysfunction and/or cell death. Thus, cholinergic signalling in non-neuronal cells is comparable to cholinergic neurotransmission. Dysfunction of the non-neuronal cholinergic system is involved in the pathogenesis of diseases. Alterations have been detected in inflammatory processes and a pathobiologic role of non-neuronal ACh in different diseases is discussed. The present article reviews recent findings about the non-neuronal cholinergic system in humans.

A muscarinic cholinergic mechanism underlies activation of the central pattern generator for locust flight

A central question in behavioural control is how central pattern generators(CPGs) for locomotion are activated. This paper disputes the key role generally accredited to octopamine in activating the CPG for insect flight. In deafferented locusts, fictive flight was initiated by bath application of the muscarinic agonist pilocarpine, the acetylcholine analogue carbachol, and the acetylcholinesterase blocker eserine, but not by nicotine. Furthermore, in addition to octopamine, various other amines including dopamine, tyramine and histamine all induced fictive flight, but not serotonin or the amine-precursor amino acid tyrosine. However, flight initiation was not reversibly blocked by aminergic antagonists, and was still readily elicited by both natural stimulation (wind) and pilocarpine in reserpinized, amine-depleted locusts. By contrast, the muscarinic antagonists atropine and scopolamine reversibly blocked flight initiated by wind, cholinergic agonists, octopamine, and by selective stimulation of a flight-initiating interneurone (TCG). The short delay from TCG stimulation to flight onset suggests that TCG acts directly on the flight CPG, and accordingly that TCG, or its follower cell within the flight generating circuit, is cholinergic. We conclude that acetylcholine acting via muscarinic receptors is the key neurotransmitter in the mechanism underlying the natural activation of the locust flight CPG. Amines are not essential for this, but must be considered as potential neuromodulators for facilitating flight release and tuning the motor pattern. We speculate that muscarinic activation coupled to aminergic facilitation may be a general feature of behavioural control in insects for ensuring conditional recruitment of individual motor programs in accordance with momentary adaptive requirements.