Dual muscarinic and nicotinic action on a motor program in Drosophila

Neurotransmitters Affect Larval Development by Regulating the Activity of Prothoracicotropic Hormone-Releasing Neurons in Drosophila melanogaster

Ecdysone, an essential insect steroid hormone, promotes larval metamorphosis by coordinating growth and maturation. In Drosophila melanogaster, prothoracicotropic hormone (PTTH)-releasing neurons are considered to be the primary promoting factor in ecdysone biosynthesis. Recently, studies have reported that the regulatory mechanisms of PTTH release in Drosophila larvae are controlled by different neuropeptides, including allatostatin A and corazonin. However, it remains unclear whether neurotransmitters provide input to PTTH neurons and control the metamorphosis in Drosophila larvae. Here, we report that the neurotransmitters acetylcholine (ACh) affect larval development by modulating the activity of PTTH neurons. By downregulating the expression of different subunits of nicotinic ACh receptors in PTTH neurons, pupal volume was significantly increased, whereas pupariation timing was relatively unchanged. We also identified that PTTH neurons were excited by ACh application ex vivo in a dose-dependent manner via ionotropic nicotinic ACh receptors. Moreover, in our Ca2+ imaging experiments, relatively low doses of OA caused increased Ca2+ levels in PTTH neurons, whereas higher doses led to decreased Ca2+ levels. We also demonstrated that a low dose of OA was conveyed through OA β-type receptors. Additionally, our electrophysiological experiments revealed that PTTH neurons produced spontaneous activity in vivo, which provides the possibility of the bidirectional regulation, coming from neurons upstream of PTTH cells in Drosophila larvae. In summary, our findings indicate that several different neurotransmitters are involved in the regulation of larval metamorphosis by altering the activity of PTTH neurons in Drosophila.

Characterization of an A-Type Muscarinic Acetylcholine Receptor and Its Possible Non-neuronal Role in the Oriental Armyworm, Mythimna separata Walker (Lepidoptera: Noctuidae)

Muscarinic acetylcholine receptor (mAChR) regulates many neurophysiological functions in insects. In this report, a full-length cDNA encoding an A-type mAChR was cloned from the oriental armyworm, Mythimna separata. Pharmacological properties studies revealed that nanomolar to micromolar concentrations of carbachol or muscarine induced an increase of intracellular Ca²⁺ concentration ([Ca²⁺] _i ), with the EC₅₀ values of 124.6 and 388.1 nM, respectively. The increases of [Ca²⁺] _i can be greatly blocked by the antagonist atropine, with an IC₅₀ value of 0.09 nM. The receptor mRNA is expressed in all developmental stages, with great differential expression between male and female adults. The tissue expression analysis identified novel target tissues for this receptor, including ovaries and Malpighian tubules. The distribution of Ms A-type mAChR protein in the male brain may suggest the neurophysiological roles that are mediated by this receptor. However, the receptor protein was found to be distributed on the membranes of oocytes that are not innervated by neurons at all. These results indicate that Ms A-type mAChR selectively mediates intracellular Ca²⁺ mobilization. And the high level of receptor protein in the membrane of oocytes may indicate a possible non-neuronal role of A-type mAChR in the reproductive system of M. separata.

Overexpression of the vesicular acetylcholine transporter disrupts cognitive performance and causes age-dependent locomotion decline in Drosophila

Acetylcholinergic (ACh) neurotransmission is essential for key organismal functions such as locomotion and cognition. However, the mechanism through which ACh is regulated in the central nervous system is not fully understood. The vesicular acetylcholine transporter (VAChT) mediates the packaging and transport of ACh for exocytotic release and is a critical component of the ACh release machinery. Yet its precise role in the maintenance of cholinergic tone remains a subject of active investigation. Here we use the overexpression of VAChT as a tool to investigate the role of changes in ACh exocytosis on the regulation of synaptic activity and its downstream consequences. We measured the effect of an increase in VAChT expression on locomotion and cognitive performance as well as on organismal survival across the lifespan. We report the surprising finding that increased VAChT expression results in a significantly shorter lifespan in comparison to control flies. Moreover, constructs overexpressing VAChT demonstrate an age-dependent decrease in locomotion performance. Importantly, we report clear deficits in learning and memory which we measured through a courtship conditioning assay. Together, these data provide evidence for the adverse effects of overexpression of the vesicular acetylcholine transporter in the maintenance of normal behavioral abilities in Drosophila and demonstrates for the first time a role for ACh in the regulation of organismal survival.

The Effects of a Bacterial Endotoxin on Behavior and Sensory-CNS-Motor Circuits in Drosophila melanogaster

The effect of bacterial sepsis on animal behavior and physiology is complex due to direct and indirect actions. The most common form of bacterial sepsis in humans is from gram-negative bacterial strains. The endotoxin (lipopolysaccharide, LPS) and/or associated peptidoglycans from the bacteria are the key agents to induce an immune response, which then produces a cascade of immunological consequences. However, there are direct actions of LPS and associated peptidoglycans on cells which are commonly overlooked. This study showed behavioral and neural changes in larval Drosophila fed commercially obtained LPS from Serratia marcescens. Locomotor behavior was not altered, but feeding behavior increased and responses to sensory tactile stimuli were decreased. In driving a sensory-central nervous system (CNS)-motor neural circuit in in-situ preparations, direct application of commercially obtained LPS initially increased evoked activity and then decreased and even stopped evoked responses in a dose-dependent manner. With acute LPS and associated peptidoglycans exposure (10 min), the depressed neural responses recovered within a few minutes after removal of LPS. Commercially obtained LPS induces a transitory hyperpolarization of the body wall muscles within seconds of exposure and alters activity within the CNS circuit. Thus, LPS and/or associated peptidoglycans have direct effects on body wall muscle without a secondary immune response.

Localization of Motor Neurons and Central Pattern Generators for Motor Patterns Underlying Feeding Behavior in Drosophila Larvae

Motor systems can be functionally organized into effector organs (muscles and glands), the motor neurons, central pattern generators (CPG) and higher control centers of the brain. Using genetic and electrophysiological methods, we have begun to deconstruct the motor system driving Drosophila larval feeding behavior into its component parts. In this paper, we identify distinct clusters of motor neurons that execute head tilting, mouth hook movements, and pharyngeal pumping during larval feeding. This basic anatomical scaffold enabled the use of calcium-imaging to monitor the neural activity of motor neurons within the central nervous system (CNS) that drive food intake. Simultaneous nerve- and muscle-recordings demonstrate that the motor neurons innervate the cibarial dilator musculature (CDM) ipsi- and contra-laterally. By classical lesion experiments we localize a set of CPGs generating the neuronal pattern underlying feeding movements to the subesophageal zone (SEZ). Lesioning of higher brain centers decelerated all feeding-related motor patterns, whereas lesioning of ventral nerve cord (VNC) only affected the motor rhythm underlying pharyngeal pumping. These findings provide a basis for progressing upstream of the motor neurons to identify higher regulatory components of the feeding motor system.

Drosophila modifier screens to identify novel neuropsychiatric drugs including aminergic agents for the possible treatment of Parkinson's disease and depression

Small molecules that increase the presynaptic function of aminergic cells may provide neuroprotection in Parkinson's disease (PD) as well as treatments for attention deficit hyperactivity disorder (ADHD) and depression. Model genetic organisms such as Drosophila melanogaster may enhance the detection of new drugs via modifier or 'enhancer/suppressor' screens, but this technique has not been applied to processes relevant to psychiatry. To identify new aminergic drugs in vivo, we used a mutation in the Drosophila vesicular monoamine transporter (dVMAT) as a sensitized genetic background and performed a suppressor screen. We fed dVMAT mutant larvae ∼ 1000 known drugs and quantitated rescue (suppression) of an amine-dependent locomotor deficit in the larva. To determine which drugs might specifically potentiate neurotransmitter release, we performed an additional secondary screen for drugs that require presynaptic amine storage to rescue larval locomotion. Using additional larval locomotion and adult fertility assays, we validated that at least one compound previously used clinically as an antineoplastic agent potentiates the presynaptic function of aminergic circuits. We suggest that structurally similar agents might be used to development treatments for PD, depression and ADHD, and that modifier screens in Drosophila provide a new strategy to screen for neuropsychiatric drugs. More generally, our findings demonstrate the power of physiologically based screens for identifying bioactive agents for select neurotransmitter systems.

Release of a single neurotransmitter from an identified interneuron coherently affects motor output on multiple time scales

Neurotransmitters can have diverse effects that occur over multiple time scales often making the consequences of neurotransmission difficult to predict. To explore the consequences of this diversity, we used the buccal ganglion of Aplysia to examine the effects of GABA release by a single interneuron, B40, on the intrinsic properties and motor output of the radula closure neuron B8. B40 induces a picrotoxin-sensitive fast IPSP lasting milliseconds in B8 and a slow EPSP lasting seconds. We found that the excitatory effects of this slow EPSP are also mediated by GABA. Together, these two GABAergic actions structure B8 firing in a pattern characteristic of ingestive programs. Furthermore, we found that repeated B40 stimulation induces a persistent increase in B8 excitability that was occluded in the presence of the GABA B receptor agonist baclofen, suggesting that GABA affects B8 excitability over multiple time scales. The phasing of B8 activity during the feeding motor programs determines the nature of the behavior elicited during that motor program. The persistent increase in B8 excitability induced by B40 biased the activity of B8 during feeding motor programs causing the motor programs to become more ingestive in nature. Thus, a single transmitter released from a single interneuron can have consequences for motor output that are expressed over multiple time scales. Importantly, despite the differences in their signs and temporal characteristics, the three actions of B40 are coherent in that they promote B8 firing patterns that are characteristic of ingestive motor outputs.

Glial processes at the Drosophila larval neuromuscular junction match synaptic growth

Glia are integral participants in synaptic physiology, remodeling and maturation from blowflies to humans, yet how glial structure is coordinated with synaptic growth is unknown. To investigate the dynamics of glial development at the Drosophila larval neuromuscular junction (NMJ), we developed a live imaging system to establish the relationship between glia, neuronal boutons, and the muscle subsynaptic reticulum. Using this system we observed processes from two classes of peripheral glia present at the NMJ. Processes from the subperineurial glia formed a blood-nerve barrier around the axon proximal to the first bouton. Processes from the perineurial glial extended beyond the end of the blood-nerve barrier into the NMJ where they contacted synapses and extended across non-synaptic muscle. Growth of the glial processes was coordinated with NMJ growth and synaptic activity. Increasing synaptic size through elevated temperature or the highwire mutation increased the extent of glial processes at the NMJ and conversely blocking synaptic activity and size decreased the presence and size of glial processes. We found that elevated temperature was required during embryogenesis in order to increase glial expansion at the nmj. Therefore, in our live imaging system, glial processes at the NMJ are likely indirectly regulated by synaptic changes to ensure the coordinated growth of all components of the tripartite larval NMJ.

Cloning and expression analysis of a muscarinic cholinergic receptor from the brain of ant, Polyrhachis vicina

Muscarinic acetylcholine receptors (mAchRs) are the predominant cholinergic receptors in the central and peripheral nervous systems of animals. They also have been found in various insect nervous systems. In this article, a full-length cDNA of a pupative mAchR (PmAchR) was obtained from the brains of ant Polyrhachis vicina by homology cloning in combination with rapid amplification of cDNA ends. PmAchR encodes a 599-amino acid protein that exhibits a high degree of homology with other mAchRs. Real-time quantitative RT-PCR analysis showed that PmAchR is differentially expressed in the brains of workers, males, and females. By in situ hybridization, it is revealed that PmAchR is widely expressed in different soma clusters of the brain, including the mushroom bodies, the antennal lobes, as well as the optic lobes (OL), and the most intensely staining is found in Kenyon cells. Nonetheless, there are more positive nerve fibers in the OL of males' brains than in females' and workers' brains.

Modulatory action of acetylcholine on the Na+-dependent action potentials in Kenyon cells isolated from the mushroom body of the cricket brain

Kenyon cells, intrinsic neurons of the insect mushroom body, have been assumed to be a site of conditioning stimulus (CS) and unconditioned stimulus (US) association in olfactory learning and memory. Acetylcholine (ACh) has been implicated to be a neurotransmitter mediating CS reception in Kenyon cells, causing rapid membrane depolarization via nicotinic ACh receptors. However, the long-term effects of ACh on the membrane excitability of Kenyon cells are not fully understood. In this study, we examined the effects of ACh on Na(+) dependent action potentials (Na(+) spikes) elicited by depolarizing current injection and on net membrane currents under the voltage clamp condition in Kenyon cells isolated from the mushroom body of the cricket Gryllus bimaculatus. Current-clamp studies using amphotericin B perforated-patch recordings showed that freshly dispersed cricket Kenyon cells could produce repetitive Na(+) spikes in response to prolonged depolarizing current injection. Bath application of ACh increased both the instantaneous frequency and the amplitudes of Na(+) spikes. This excitatory action of ACh on Kenyon cells is attenuated by the pre-treatment of the cells with the muscarinic receptor antagonists, atropine and scopolamine, but not by the nicotinic receptor antagonist mecamylamine. Voltage-clamp studies further showed that bath application of ACh caused an increase in net inward currents that are sensitive to TTX, whereas outward currents were decreased by this treatment. These results indicate that in order to mediate CS, ACh may modulate the firing properties of Na(+) spikes of Kenyon cells through muscarinic receptor activation, thus increasing Na conductance and decreasing K conductance.

The thoracic muscular system and its innervation in third instar Calliphora vicina Larvae. I. Muscles of the pro- and mesothorax and the pharyngeal complex

An anatomical description is given by the muscles in the pro- and mesothorax, and those associated with the feeding apparatus (cephalopharyngeal skeleton, CPS) that participate in feeding behavior in third instar Calliphora larvae. The body wall muscles in the pro- and mesothoracic segments are organized in three layers: internal, intermedial, and external. The muscles were labeled with roman numerals according to the nomenclature in use for the abdominal segments. Muscles associated with the CPS are labeled according to their function. The prothorax bears five pairs of lateral symmetrically longitudinal segmental body wall muscles and lacks the transversal muscle group present in the mesothorax and abdominal segments. Additionally, four pairs of intersegmental muscles project from the prothorax to the second, fourth, and fifth segment. The mesothorax bears 15 pairs of segmental longitudinal and 18 pairs of transversal muscles. The accessory pharyngeal muscles span the CPS and the cuticle. Three pairs of protractors and retractors and two pairs of mouth hook accessors (MH(AC)) exist, which move the CPS relative to the body. The pharyngeal muscles are exclusively attached to the structures of the CPS. The mouth hook elevators and depressors, which mediate the hooks rotation are attached to the ventral arm of the CPS and project to a dorsal (elevators) or ventral (depressors) protuberance of the mouth hooks. The cibarial dilator muscles (CDM) span the dorsal arms of the CPS and the dorsal surface of the esophagus and mediate food ingestion. The labial retractors (LRs) lack antagonists and project from the ventral surface of the CPS to the unpaired labium. Contractions of these muscles open the mouth cavity.

The thoracic muscular system and its innervation in third instar Calliphora vicina Larvae. II. Projection patterns of the nerves associated with the pro- and mesothorax and the pharyngeal complex

We describe the anatomy of the nerves that project from the central nervous system (CNS) to the pro- and mesothoracic segments and the cephalopharyngeal skeleton (CPS) for third instar Calliphora larvae. Due to the complex branching pattern we introduce a nomenclature that labels side branches of first and second order. Two fine nerves that were not yet described are briefly introduced. One paired nerve projects to the ventral arms (VAs) of the CPS. The second, an unpaired nerve, projects to the ventral surface of the cibarial part of the esophagus (ES). Both nerves were tentatively labeled after the structures they innervate. The antennal nerve (AN) innervates the olfactory dorsal organ (DO). It contains motor pathways that project through the frontal connectives (FC) to the frontal nerve (FN) and innervate the cibarial dilator muscles (CDM) which mediate food ingestion. The maxillary nerve (MN) innervates the sensory terminal organ (TO), ventral organ (VO), and labial organ (LO) and comprises the motor pathways to the mouth hook (MH) elevator, MH depressor, and the labial retractor (LR) which opens the mouth cavity. An anastomosis of unknown function exists between the AN and MN. The prothoracic accessory nerve (PaN) innervates a dorsal protractor muscle of the CPS and sends side branches to the aorta and the bolwig organ (BO) (stemmata). In its further course, this nerve merges with the prothoracic nerve (PN). The architecture of the PN is extremely complex. It innervates a set of accessory pharyngeal muscles attached to the CPS and the body wall musculature of the prothorax. Several anastomoses exist between side branches of this nerve which were shown to contain motor pathways. The mesothoracic nerve (MeN) innervates a MH accessor and the longitudinal and transversal body wall muscles of the second segment.

From behavior to fictive feeding: anatomy, innervation and activation pattern of pharyngeal muscles of Calliphora vicina 3rd instar larvae

A description of the muscles and nerves involved in feeding of larval Calliphora vicina is given as a prerequisite to establish fictive feeding patterns recorded from the isolated central nervous system. Feeding Diptera larvae show a repetitive sequence of pro- and retraction of the cephalopharyngeal skeleton (CPS), elevation and depression of the mouth hooks and food ingestion. The corresponding pharyngeal muscles are protractors, mouth hook elevators and depressors, the labial retractor and cibarial dilator muscles. These muscles are innervated by the prothoracic accessory nerve (PaN), maxillary nerve (MN) and antennal nerve (AN) as shown electrophysiologically by recording action potentials from the respective nerve that correlate to post-synaptic potentials on the muscles. All three nerves show considerably more complex branching patterns than indicated in the literature. Extracellular recordings from the stumps of PaN, MN and AN connected to an isolated CNS show spontaneous rhythmic motor patterns that reflect the feeding sequence in intact larvae. Variability of the feeding pattern observed in behavioral experiments is also evident from the level of motor output from an isolated CNS. The data obtained from Calliphora will facilitate electrophysiological investigations dealing with the genetic background of feeding behavior in Drosophila larvae.

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.

Tissue-specific targeting of Hsp26 has no effect on heat resistance of neural function in larval Drosophila

Hsp26 belongs to the small heat-shock protein family and is normally expressed in all cells during heat stress. We aimed to determine if overexpression of this protein protects behavior and neural function in Drosophila melanogaster during heat stress, as has previously been shown for Hsp70. We used the UAS-GAL4 expression system to drive expression of Hsp26 in the whole animal (ubiquitously), in the motoneurons, and in the muscles of wandering third-instar larvae. There were slight increases in time to crawling failure and normalized excitatory junction potential (EJP) area for some of the transgenic lines, but these were not consistent. In addition, Hsp26 had no effect on the temperature at failure of EJPs, normalized EJP peak amplitude, and normalized EJP half-width. Overexpression larvae had a similar number of motoneuronal boutons and length of nerve terminals as controls, indicating that the occasional protective effects on locomotion were not due to changes at the synapse. We conclude that overexpression had a small thermoprotective effect on locomotion and no effect on neural function. As it has been shown that Hsp26 requires action of other Hsps to reactivate the denatured proteins to which it binds, we propose that at least in larvae, the function of Hsp26 was masked in the relative absence of other Hsps.

The emergence of patterned movement during late embryogenesis of Drosophila

Larval behavioral patterns arise in a gradual fashion during late embryogenesis as the innervation of the somatic musculature and connectivity within the central nervous system develops. In this paper, we describe in a quantitative manner the maturation of behavioral patterns. Early movements are locally restricted "twitches" of the body wall, involving single segments or parts of segments. These twitches occur at a low frequency and have low amplitude, reflecting weak muscle contractions. Towards later stages twitches increase in frequency and amplitude and become integrated into coordinated movements of multiple segments. Most noticeable among these is the peristaltic wave of longitudinal segmental contractions by which the larva moves forward or backward. Besides becoming more complex as development proceeds, embryonic movements also acquire a pronounced rhythm. Thus, late embryonic movements occur in bursts, with phases of frequent movement separated by phases of no movement at all; early movements show no such periodicity. These data will serve as a baseline for future studies that address the function of embryonic lethal genes controlling neuronal connectivity and larval behavior. We have analyzed behavioral abnormalities in two embryonic lethal mutations with severe neural defects, tailless (tll), which lacks the protocerebrum, and glial cells missing (gcm), in which glial cells are absent. Our results reveal prominent alterations in embryonic motility for both of these mutations, indicating that the protocerebrum and glial cells play a crucial role in the neural mechanism controlling larval movement in Drosophila.

A specific synaptic pathway activates a conditional plateau potential underlying protraction phase in the Aplysia feeding central pattern generator

A common feature in the architecture of neuronal networks is a high degree of seemingly redundant synaptic connectivity. In many cases, the synaptic inputs converging on any particular neuron all use the same neurotransmitter and appear to be fundamentally equivalent. Here, we analyze a striking counterexample in which such inputs are not equivalent and, as a result, play very different roles in the generation of the pattern of activity produced by the network. In the feeding central pattern generator of Aplysia, the pattern-initiating neuron B50 elicits motor programs by exciting the plateauing neuron B31/B32 in two ways: directly and indirectly through neuron B63. All of the synaptic connections use ACh. Despite the direct input of B50 to B31/B32, the indirect pathway of exciting B31/B32 through B63 is required for B50 to elicit the B31/B32 plateau potential and the motor program. We dissect this requirement using the muscarinic cholinergic antagonist pirenzepine. Pirenzepine blocks the B50-elicited motor program, the plateau potential in B31/B32, and, notably, a slow component of the EPSP elicited in B31/B32 by B63 but not that elicited by B50. The muscarinic agonist oxotremorine restores the plateau potential in B31/B32 and eliminates the necessity for B63 in B50-elicited motor programs. Together, our analysis shows that the plateau potential in B31/B32 is not endogenous but conditional, furthermore conditional on one particular synaptic input, that from B63. Thus, among several inputs to B31/B32 that use the same transmitter, the input from B63 is functionally distinct in its preferential access to the plateau potential that represents the committed step toward the initiation of a motor program.

Electrophysiological and Morphological Characterization of Identified Motor Neurons in theDrosophilaThird Instar Larva Central Nervous System

We have used dye fills and electrophysiological recordings to identify and characterize a cluster of motor neurons in the third instar larval ventral ganglion. This cluster of neurons is similar in position to the well-studied embryonic RP neurons. Dye fills of larval dorsomedial neurons demonstrate that individual neurons within the cluster can be reproducibly identified by observing their muscle targets and bouton morphology. The terminal targets of these five neurons are body wall muscles 6/7, 1, 14, and 30 and the intersegmental nerve (ISN) terminal muscles (1, 2, 3, 4, 9, 10, 19, 20). All cells except the ISN neuron, which has a type Is ending, display type Ib boutons. Two of these neurons appear to be identical to the embryonic RP3 and aCC cells, which define the most proximal and distal innervations within a hemisegment. The targets of the other neurons in the larval dorsomedial cluster do not correspond to embryonic targets of the neurons in the RP cluster, suggesting rewiring of this circuit during early larval stages. Electrophysiological studies of the five neurons in current clamp revealed that type Is neurons have a longer delay in the appearance of the first spike compared with type Ib neurons. Genetic, biophysical, and pharmacological studies in current and voltage clamp show this delay is controlled by the kinetics and voltage sensitivity of inactivation of a current whose properties suggest that it may be the Shal IAcurrent. The combination of genetic identification and whole cell recording allows us to directly explore the cellular substrates of neural and locomotor behavior in an intact system.

GABAergic modulation of motor-driven behaviors in juvenileDrosophila and evidence for a nonbehavioral role for GABA transport

We have identified specific GABAergic-modulated behaviors in the juvenile stage of the fruit fly, Drosophila melanogaster via systemic treatment of second instar larvae with the potent GABA transport inhibitor DL-2,4-diaminobutyric acid (DABA). DABA significantly inhibited motor-controlled body wall and mouth hook contractions and impaired rollover activity and contractile responses to touch stimulation. The perturbations in locomotion and rollover activity were reminiscent of corresponding DABA-induced deficits in locomotion and the righting reflex observed in adult flies. The effects were specific to these motor-controlled behaviors, because DABA-treated larvae responded normally in olfaction and phototaxis assays. Recovery of these behaviors was achieved by cotreatment with the vertebrate GABA(A) receptor antagonist picrotoxin. Pharmacological studies performed in vitro with plasma membrane vesicles isolated from second instar larval tissues verified the presence of high-affinity, saturable GABA uptake mechanisms. GABA uptake was also detected in plasma membrane vesicles isolated from behaviorally quiescent stages. Competitive inhibition studies of [3H]-GABA uptake into plasma membrane vesicles from larval and pupal tissues with either unlabeled GABA or the transport inhibitors DABA, nipecotic acid, or valproic acid, revealed differences in affinities. GABAergic-modulation of motor behaviors is thus conserved between the larval and adult stages of Drosophila, as well as in mammals and other vertebrate species. The pharmacological studies reveal shared conservation of GABA transport mechanisms between Drosophila and mammals, and implicate the involvement of GABA and GABA transporters in regulating physiological processes distinct from neurotransmission during behaviorally quiescent stages of development.

mAChRs in the grasshopper brain mediate excitation by activation of the AC/PKA and the PLC second-messenger pathways

The species-specific sound production of acoustically communicating grasshoppers can be stimulated by pressure injection of both nicotinic and muscarinic agonists into the central body complex and a small neuropil situated posterior and dorsal to it. To determine the role of muscarinic acetylcholine receptors (mAChRs) in the control of acoustic communication behavior and to identify the second-messenger pathways affected by mAChR-activation, muscarinic agonists and membrane-permeable drugs known to interfere with specific mechanisms of intracellular signaling pathways were pressure injected to identical sites in male grasshopper brains. Repeated injections of small volumes of muscarine elicited stridulation of increasing duration associated with decreased latencies. This suggested an accumulation of excitation over time that is consistent with the suggested role of mAChRs in controlling courtship behavior: to provide increasing arousal leading to higher intensity of stridulation and finally initiating a mating attempt. At sites in the brain where muscarine stimulation was effective, stridulation could be evoked by forskolin, an activator of adenylate cyclase (AC); 8-Br-cAMP-activating protein kinase A (PKA); and 3-isobuty-1-methylxanthine, leading to the accumulation of endogenously generated cAMP through inhibition of phosphodiesterases. This suggested that mAChRs mediate excitation by stimulating the AC/cAMP/PKA pathway. In addition, muscarine-stimulated stridulation was inhibited by 2'-5'-dideoxyadenonsine and SQ 22536, two inhibitors of AC; H-89 and Rp-cAMPS, two inhibitors of PKA; and by U-73122 and neomycin, two agents that inhibit phospholipase C (PLC) by independent mechanisms. Because the inhibition of AC, PKA, or PLC by various individually applied substances entirely suppressed muscarine-evoked stridulation in a number of experiments, activation of both pathways, AC/cAMP/PKA and PLC/IP(3)/diacylglycerine, appeared to be necessary to mediate the excitatory effects of mAChRs. With these studies on an intact "behaving" grasshopper preparation, we present physiological relevance for mAChR-evoked excitation mediated by sequential activation of the AC- and PLC-initiated signaling pathways that has been reported in earlier in vitro studies.

A role for muscarinic excitation: control of specific singing behavior by activation of the adenylate cyclase pathway in the brain of grasshoppers

Muscarinic acetylcholine receptors exert slow and prolonged synaptic effects in both vertebrate and invertebrate nervous systems. Through activation of G proteins, they typically decrease intracellular cAMP levels by inhibition of adenylate cyclase or stimulate phospholipase C and the turnover of inositol phosphates. In insects, muscarinic receptors have been credited with two main functions: inhibition of transmitter release from sensory neuron terminals and regulation of the excitability of motoneurons and interneurons. Our pharmacological studies with intact and behaving grasshoppers revealed a functional role for muscarinic acetylcholine receptors as being the basis for specific arousal in defined areas of the brain, underlying the selection and control of acoustic communication behavior. Periodic injections of acetylcholine into distinct areas of the brain elicited songs of progressively increasing duration. Coinjections of the muscarinic receptor antagonist scopolamine and periodic stimulations with muscarine identified muscarinic receptor activation as being the basis for the underlying accumulation of excitation. In contrast to reports from other studies on functional circuits, muscarinic excitation was apparently mediated by activation of the adenylate cyclase pathway. Stimulation of adenylate cyclase with forskolin and of protein kinase A with 8-Br-cAMP mimicked the stimulatory effects of muscarine whereas inhibition of adenylate cyclase with SQ22536 and of protein kinase A with H-89 and Rp-cAMPs suppressed muscarine-stimulated singing behavior. Activation of adenylate cyclase by muscarinic receptors has previously been reported from studies on membrane preparations and heterologous expression systems, but a physiological significance of this pathway remained to be demonstrated in an in vivo preparation.

Blockade of the central generator of locomotor rhythm by noncompetitive NMDA receptor antagonists in Drosophila larvae

The noncompetitive antagonists of the vertebrate N-methyl-D-aspartate (NMDA) receptor dizocilpine (MK 801) and phencyclidine (PCP), delivered in food, were found to induce a marked and reversible inhibition of locomotor activity in Drosophila melanogaster larvae. To determine the site of action of these antagonists, we used an in vitro preparation of the Drosophila third-instar larva, preserving the central nervous system and segmental nerves with their connections to muscle fibers of the body wall. Intracellular recordings were made from ventral muscle fibers 6 and 7 in the abdominal segments. In most larvae, long-lasting (>1 h) spontaneous rhythmic motor activities were recorded in the absence of pharmacological activation. After sectioning of the connections between the brain and abdominal ganglia, the rhythm disappeared, but it could be partially restored by perfusing the muscarinic agonist oxotremorine, indicating that the activity was generated in the ventral nerve cord. MK 801 and PCP rapidly and efficiently inhibited the locomotor rhythm in a dose-dependent manner, the rhythm being totally blocked in 2 min with doses over 0.1 mg/mL. In contrast, more hydrophilic competitive NMDA antagonists had no effect on the motor rhythm in this preparation. MK 801 did not affect neuromuscular glutamatergic transmission at similar doses, as demonstrated by monitoring the responses elicited by electrical stimulation of the motor nerve or pressure applied glutamate. The presence of oxotremorine did not prevent the blocking effect of MK 801. These results show that MK 801 and PCP specifically inhibit centrally generated rhythmic activity in Drosophila, and suggest a possible role for NMDA-like receptors in locomotor rhythm control in the insect CNS.

Pharmacological identification of acetylcholine receptor subtypes in echinoderm smooth muscle (Sclerodactyla briareus)

Contractions of an echinoderm (sp. Sclerodactyla briareus) smooth muscle, the longitudinal muscle of the body wall (LMBW), were evoked by acetylcholine (ACh) and agonists: epibatidine, muscarine and nicotine (in order of force generation: ACh>muscarine=epibatidine>nicotine). ACh-induced contractions were blocked by atropine by 50%, and methoctramine, by 30%. ACh responses were also blocked by 25% by methyllycaconitine (MLA) but not by D-tubocurarine (dTC). Muscarine initiated large contractions that were completely blocked by atropine. To elucidate possible muscarinic ACh receptor (mAChR) subtypes, muscarinic agonists (oxotremorine, pilocarpine) and antagonists (methoctramine, pirenzepine) were tested. Oxotremorine, pilocarpine, and pirenzepine each enhanced resting tonus and potentiated ACh-induced contractions (order of potency: pilocarpine>oxotremorine=pirenzepine). Muscarine, oxotremorine or pirenzepine generated phasic, rhythmic contractions. Nicotine-induced contractions were almost completely blocked by dTC but were not altered by atropine. Large contractions evoked by epibatidine were potentiated by dTC whereas atropine had no effect on them. MLA blocked spontaneous rhythmicity. Cholinesterase inhibitors, neostigmine or physostigmine, caused marked potentiation of ACh-induced contractions and initiated rhythmic slow wave contractions in previously quiescent muscles. The present pharmacological evidence points to the co-existence of excitatory nicotinic ACh receptor (nAChRs) and mAChRs where nAChRs possibly modulate tone, and the mAChRs initiate and enhance rhythmicity.

Localization of choline acetyltransferase-expressing neurons in Drosophila nervous system

A variety of approaches have been developed to localize neurons and neural elements in nervous system tissues that make and use acetylcholine (ACh) as a neurotransmitter. Choline acetyltransferase (ChAT) is the enzyme catalyzing the biosynthesis of ACh and is considered to be an excellent phenotypic marker for cholinergic neurons. We have surveyed the distribution of choline acetyltransferase (ChAT)-expressing neurons in the Drosophila nervous system detected by three different but complementary techniques. Immunocytochemistry, using anti-ChAT monoclonal antibodies results in identification of neuronal processes and a few types of cell somata that contain ChAT protein. In situ hybridization using cRNA probes to ChAT messenger RNA results in identification of cell bodies transcribing the ChAT gene. X-gal staining and/or beta-galactosidase immunocytochemistry of transformed animals carrying a fusion gene composed of the regulatory DNA from the ChAT gene controlling expression of a lacZ reporter has also been useful in identifying cholinergic neurons and neural elements. The combination of these three techniques has revealed that cholinergic neurons are widespread in both the peripheral and central nervous system of this model genetic organism at all but the earliest developmental stages. Expression of ChAT is detected in a variety of peripheral sensory neurons, and in the brain neurons associated with the visual and olfactory system, as well as in neurons with unknown functions in the cortices of brain and ganglia.

Coupling of efferent neuromodulatory neurons to rhythmical leg motor activity in the locust

The spike activity of neuromodulatory dorsal unpaired median (DUM) neurons was analyzed during a pilocarpine-induced motor pattern in the locust. Paired intracellular recordings were made from these octopaminergic neurons during rhythmic activity in hindleg motor neurons evoked by applying pilocarpine to an isolated metathoracic ganglion. This motor pattern is characterized by two alternating phases: a levator phase, during which levator, flexor, and common inhibitor motor neurons spike, and a depressor phase, during which depressor and extensor motor neurons spike. Three different subpopulations of efferent DUM neurons could be distinguished during this rhythmical motor pattern according to their characteristic spike output. DUM 1 neurons, which in the intact animal do not innervate muscles involved in leg movements, showed no change apart from a general increase in spike frequency. DUM 3 and DUM 3,4 neurons produced the most variable activity but received frequent and sometimes pronounced hyperpolarizations that were often common to both recorded neurons. DUM 5 and DUM 3,4,5 neurons innervate muscles of the hindleg and showed rhythmical excitation leading to bursts of spikes during rhythmic activity of the motor neurons, which innervate these same muscles. Sometimes the motor output was coordinated across both sides of the ganglion so that there was alternating activity between levators of both sides. In these cases, the spikes of DUM 5 and DUM 3,4,5 neurons and the hyperpolarization of DUM 3 and DUM 3,4 neurons occurred at particular phases in the motor pattern. Our data demonstrate a central coupling of specific types of DUM neurons to a rhythmical motor pattern. Changes in the spike output of these particular efferent DUM neurons parallel changes in the motor output. The spike activity of DUM neurons thus may be controlled by the same circuits that determine the action of the motor neurons. Functional implications for real walking are discussed.

The shaking-B2 mutation disrupts electrical synapses in a flight circuit in adult Drosophila

The shaking-B2 mutation was used to analyze synapses between haltere afferents and a flight motoneuron in adult Drosophila. We show that the electrical synapses among many neurons in the flight circuit are disrupted in shaking-B2 flies, suggesting that shaking-B expression is required for electrical synapses throughout the nervous system. In wild-type flies haltere afferents are dye-coupled to the first basalar motoneuron, and stimulation of these afferents evokes electromyograms from the first basalar muscle with short latencies. In shaking-B2 flies dye coupling between haltere afferents and the motoneuron is abolished, and afferent stimulation evokes electromyograms at abnormally long latencies. Intracellular recordings from the motoneuron confirm that the site of the defect in shaking-B2 flies is at the synapses between haltere afferents and the flight motoneuron. The nicotinic cholinergic antagonist mecamylamine blocks the haltere-to-flight motoneuron synapses in shaking-B2 flies but does not block those synapses in wild-type flies. Together, these results show that the haltere-to-flight motoneuron synapses comprise an electrical component that requires shaking-B and a chemical component that is likely to be cholinergic.

Characterization of buccal motor programs elicited by a cholinergic agonist applied to the cerebral ganglion of Aplysia californica

Applying the non-hydrolyzable cholinergic agonist carbachol (CCh) to the cerebral ganglion of Aplysia elicits sustained, regular bursts of activity in the buccal ganglia resembling those seen during biting. The threshold for bursting is approximately 10(-4) M. Bursting begins after a 2 to 5 min delay. The burst frequency increases over the first 5 bursts, reaching a plateau value of approximately 3 per minute. Bursting is maintained for over 10 min. Some of the effects of CCh may be attributed to its ability to depolarize and fire CBI-2, a command-like neuron in the cerebral ganglion that initiates biting. CBI-2 is also depolarized by ACh, and by stimulating peripheral sensory nerves. Excitation of CBI-2 caused by carbachol is partially blocked by the muscarinic antagonist atropine. We examined whether CCh-induced bursting is modified in ganglia taken from Aplysia that previously experienced treatments inhibiting feeding, such as satiation, head shock contingent or non-contingent with food, and training animals with an inedible food. No treatment consistently and repeatedly affected the latency, the peak burst period, the length of time that bursting was maintained, or the threshold CCh concentration for eliciting bursting. However, there was a decrease in the rate of the build-up of the buccal ganglion program in previously satiated animals.

Temporal and spatial expression patterns of two G-protein coupled receptors in Drosophila melanogaster

Temporal and spatial expression patterns of a muscarinic acetylcholine receptor (Acr60C) and an octopamine/tyramine receptor (Octyr) were determined in Drosophila melanogaster using quantitative Northern analysis and in situ hybridization to tissue sections. Expression of mRNA encoding both of these G-protein coupled receptors peaks initially in 18 to 21 hour embryos following the formation of the mature larval nervous system. Levels of mRNA then decline during larval stages, rising to a second peak in 3 to 4-day-old pupae after a period of major nervous system reorganization. The muscarinic acetylcholine receptor mRNA is expressed throughout the cortical regions of the central nervous system in adults and embryos. Particularly high levels of expression of Acr60C are observed in cell bodies adjacent to the antennal lobes, suggesting a major role for this muscarinic receptor in the processing of olfactory information. In contrast, the octopamine/tyramine receptor mRNA is distributed diffusely throughout the adult brain, with patches of signal concentrated in the cortex of the dorsal protocerebrum near the mushroom bodies. These patches may represent individual cells expressing Octyr receptors.

Localization in the nervous system of Drosophila melanogaster of a C-terminus anti-peptide antibody to a cloned Drosophila muscarinic acetylcholine receptor

Localization in the nervous system of Drosophila melanogaster of a cloned Drosophila muscarinic acetylcholine receptor (mAChR) was investigated using a polyclonal antiserum raised against a peptide corresponding to the predicted receptor carboxyl terminal domain. Immunocytochemical studies on fly sections indicated that the product of the Dm1 mAChR gene was localized in the antennal lobes and in other regions of the brain and thoracic nervous system. Intense staining in the glomeruli of the antennal lobes, the region of the nervous system containing terminals of antennal olfactory sensory neurones and mechanosensory neurones, indicates possible roles for this mAChR gene product in the processing of olfactory and mechanosensory signals in the fly. The staining of a discrete group of neurosecretory cells in the pars intercerebralis of the brain indicates a possible new role for this mAChR in the regulation of neurosecretion. Very little staining is detected in the thoracic nervous system.

Muscarinic acetylcholine receptors on an identified motor neurone in the cockroach, Periplaneta americana

Muscarinic acetylcholine receptors (mAChRs) on the cell body of the fast coxal depressor motor neurone (Df) in the metathoracic ganglion of the cockroach Periplaneta americana were investigated using electrophysiological methods. Muscarinic agonists, arecoline and oxotremorine, induced dose-dependent depolarizations on motor neurone Df. McN-A-343, a vertebrate mAChR M1 subtype-selective agonist, failed to induce any responses when tested on the same neurone at concentrations of up to 1.0 x 10(-4) M. The order of effectiveness of a series of muscarinic antagonists on the mAChRs of motor neurone Df is as follows: scopolamine > atropine > pirenzepine. 4-DAMP (1.0 x 10(-5) M) had only a weak blocking effect and AF-DX 116 (1.0 x 10(-5) M) was completely inactive. The pharmacological profile of muscarinic responses on motor neurone Df reveals a novel type of insect mAChR.

Immunohistochemical localization of a ligand-binding and a structural subunit of nicotinic acetylcholine receptors in the central nervous system of Drosophila melanogaster

The distribution of two subunits of nicotinic acetylcholine receptors in the developing and the differentiated central nervous system of Drosophila melanogaster was studied. With subunit-specific antibodies raised against the ligand-binding alpha-like subunit ALS and the putative non-ligand-binding subunit ARD, we find both ALS-like and ARD-like immunoreactivity widely distributed in most neuropiles of the optic lobes, the protocerebrum, the deutocerebrum and the thoracic ganglion of the adult fly. With a single exception, namely in the lamina of the visual system, the antigens recognized by the two types of antibodies are colocalized. This observation is consistent with previous immunoprecipitation data indicating that the ALS and ARD proteins are integral components of the same hetero-oligomeric receptor that binds the nicotinic antagonist alpha-bungarotoxin with high affinity. During embryonic development ARD-like immunoreactivity is first detectable in approximately 10 hour old embryos. Both subunits are consistently detected in the central nervous system of the late embryo, the three larval stages, and all prepupal and pupal stages. During metamorphosis the optic stalk is transiently immunoreactive with anti-ARD, but not with anti-ALS antiserum. Although in larvae and adults, immunoreactivity with both types of antibodies is most abundant in synaptic regions, in embryos and pupae strong staining of cortical cell body layers is observed, in particular with anti-ARD antisera. As these developmental periods coincide with strong accumulation of ARD transcripts, the cell body staining may reflect newly synthesized and assembled receptors, while the functional ARD- and ALS-containing receptor may be destined for synapses.

Muscarinic acetylcholine receptors in invertebrates: comparisons with homologous receptors from vertebrates

The pharmacology, physiology and molecular biology of invertebrate muscarinic acetylcholine receptors are compared with current knowledge concerning vertebrate muscarinic acetylcholine receptors. Evidence for the existence of multiple receptor subtypes in invertebrates is examined, emphasizing what is presently known about the sensitivity of invertebrate preparations to subtype selective ligands previously defined in vertebrate studies. Other evidence for muscarinic receptor subtypes which is examined includes: heterogeneous responses to classical muscarinic ligands and evidence for coupling of invertebrate muscarinic receptors to several different classes of second messenger systems. Clues regarding possible functions for invertebrate muscarinic receptors are discussed, including evidence from both physiological studies and in situ localization studies which reveal patterns of receptor protein and mRNA expression. A detailed analysis of the structural similarities between a cloned Drosophila muscarinic receptor and vertebrate muscarinic receptors is also presented. Regions of the receptors that may be involved in ligand binding, effector coupling and receptor regulation are identified in this comparison. Future directions for invertebrate muscarinic receptor research are considered including: methods for cloning other receptor subtypes, methods for cloning homologous receptors from other species and genetic approaches for determining the physiological roles of muscarinic receptors.

How complex is the nicotinic receptor system of insects?

In insects, nicotinic acetylcholine receptors (nAChRs) are confined to the nervous system. It is a long-standing open question whether the insect nicotinic cholinergic receptor system is less complex than that of the vertebrate nervous system. Simplicity can be conceived in two ways. (1) Fewer receptor subtypes may exist. (2) Single receptors may have a more primitive (homo-oligomeric) quaternary structure. Recent approaches to the molecular cloning of insect nAChRs may contribute valuable new information to this issue. Thus, the identification of multiple genes encoding proteins similar to vertebrate nAChR subunits implicates a remarkable heterogeneity for these receptors. The discovery of putatively non-ligand-binding subunits hints to the existence of vertebrate-like hetero-oligomeric nAChRs. However, the simultaneous occurrence of homo-oligomeric receptors must still be considered.

Acetylcholine receptors of thoracic dorsal midline neurones in the cockroach, Periplaneta Americana

The actions of acetylcholine and cholinergic ligands have been studied using dorsal midline neurones from the rnetathoracic ganglion of the cockroach Periplaneta americana.Both nicotine and oxotremorine depolarized dorsal midline neuronal cell bodies.Dose-response curves for nicotine and oxotremorine saturated at different levels. Nicotine-induced depolarizations were completely or partially blocked by mecamylamine, d-tubocurarine, strychnine, and bicuculline, but were insensitive to alpha-bungarotoxin(100 nM), atropine (100 micronM),Scopolamine (10 micronM), and pirenzepine (50 micronM). Following pretreatment with collagenase, the dorsal midline neurones were sensitive to high doses of alpha-bungarotoxin (3 micronM). Oxotremorine-induced depolarizations were blocked by scopolamine (10 micronM) atropine (100 micronM), and pirenzepine (50 micronM) and were insensitive to mecamylamine (10 micronM) and d-tubocurarine (100 micronM). The results indicate the coexistence of at least two distinct acetylcholine receptors on dorsal midline neuronal cell bodies in the cockroach metathoracic ganglion.