Activation of metabotropic glutamate receptors enhances synaptic transmission at the Drosophila neuromuscular junction

Inhibiting Glutamate Activity during Consolidation Suppresses Age-Related Long-Term Memory Impairment in Drosophila

In Drosophila, long-term memory (LTM) formation requires increases in glial gene expression. Klingon (Klg), a cell adhesion molecule expressed in both neurons and glia, induces expression of the glial transcription factor, Repo. However, glial signaling downstream of Repo has been unclear. Here we demonstrate that Repo increases expression of the glutamate transporter, EAAT1, and EAAT1 is required during consolidation of LTM. The expressions of Klg, Repo, and EAAT1 decrease upon aging, suggesting that age-related impairments in LTM are caused by dysfunction of the Klg-Repo-EAAT1 pathway. Supporting this idea, overexpression of Repo or EAAT1 rescues age-associated impairments in LTM. Pharmacological inhibition of glutamate activity during consolidation improves LTM in klg mutants and aged flies. Altogether, our results indicate that LTM formation requires glial-dependent inhibition of glutamate signaling during memory consolidation, and aging disrupts this process by inhibiting the Klg-Repo-EAAT1 pathway.

Unraveling Synaptic GCaMP Signals: Differential Excitability and Clearance Mechanisms Underlying Distinct Ca2+ Dynamics in Tonic and Phasic Excitatory, and Aminergic Modulatory Motor Terminals in Drosophila

GCaMP is an optogenetic Ca²⁺ sensor widely used for monitoring neuronal activities but the precise physiological implications of GCaMP signals remain to be further delineated among functionally distinct synapses. The Drosophila neuromuscular junction (NMJ), a powerful genetic system for studying synaptic function and plasticity, consists of tonic and phasic glutamatergic and modulatory aminergic motor terminals of distinct properties. We report a first simultaneous imaging and electric recording study to directly contrast the frequency characteristics of GCaMP signals of the three synapses for physiological implications. Different GCaMP variants were applied in genetic and pharmacological perturbation experiments to examine the Ca²⁺ influx and clearance processes underlying the GCaMP signal. Distinct mutational and drug effects on GCaMP signals indicate differential roles of Na⁺ and K⁺ channels, encoded by genes including paralytic (para), Shaker (Sh), Shab, and ether-a-go-go (eag), in excitability control of different motor terminals. Moreover, the Ca²⁺ handling properties reflected by the characteristic frequency dependence of the synaptic GCaMP signals were determined to a large extent by differential capacity of mitochondria-powered Ca²⁺ clearance mechanisms. Simultaneous focal recordings of synaptic activities further revealed that GCaMPs were ineffective in tracking the rapid dynamics of Ca²⁺ influx that triggers transmitter release, especially during low-frequency activities, but more adequately reflected cytosolic residual Ca²⁺ accumulation, a major factor governing activity-dependent synaptic plasticity. These results highlight the vast range of GCaMP response patterns in functionally distinct synaptic types and provide relevant information for establishing basic guidelines for the physiological interpretations of presynaptic GCaMP signals from in situ imaging studies.

Octopamine and Dopamine differentially modulate the nicotine-induced calcium response in Drosophila Mushroom Body Kenyon Cells

In Drosophila associative olfactory learning, an odor, the conditioned stimulus (CS), is paired to an unconditioned stimulus (US). The CS and US information arrive at the Mushroom Bodies (MB), a Drosophila brain region that processes the information to generate new memories. It has been shown that olfactory information is conveyed through cholinergic inputs that activate nicotinic acetylcholine receptors (nAChRs) in the MB, while the US is coded by biogenic amine (BA) systems that innervate the MB. In this regard, the MB acts as a coincidence detector. A better understanding of the properties of the responses gated by nicotinic and BA receptors is required to get insights on the cellular and molecular mechanisms responsible for memory formation. In recent years, information has become available on the properties of the responses induced by nAChR activation in Kenyon Cells (KCs), the main neuronal MB population. However, very little information exists on the responses induced by aminergic systems in fly MB. Here we have evaluated some of the properties of the calcium responses gated by Dopamine (DA) and Octopamine (Oct) in identified KCs in culture. We report that exposure to BAs induces a fast but rather modest increase in intracellular calcium levels in cultured KCs. The responses to Oct and DA are fully blocked by a VGCC blocker, while they are differentially modulated by cAMP. Moreover, co-application of BAs and nicotine has different effects on intracellular calcium levels: while DA and nicotine effects are additive, Oct and nicotine induce a synergistic increase in calcium levels. These results suggest that a differential modulation of nicotine-induced calcium increase by DA and Oct could contribute to the events leading to learning and memory in flies.

Drosophila as a model organism for the study of neuropsychiatric disorders

The fruitfly Drosophila offers a model system in which powerful genetic tools can be applied to understanding the neurobiological bases of a range of complex behaviors. The Drosophila and human lineages diverged several hundred million years ago, and despite their obvious differences, flies and humans share many fundamental cellular and neurobiological processes. The similarities include fundamental mechanisms of neuronal signaling, a conserved underlying brain architecture and the main classes of neurotransmitter system. Drosophila also have a sophisticated behavioral repertoire that includes extensive abilities to adapt to experience and other circumstances, and is therefore susceptible to the same kinds of insults that can cause neuropsychiatric disorders in humans. Given the different physiologies, lifestyles, and cognitive abilities of flies and humans, many higher order behavioral features of the human disorders cannot be modeled readily in flies. However, an increasing understanding of the genetics of human neuropsychiatric disorders is suggesting parallels with underlying neurobiological mechanisms in flies, thus providing important insights into the possible mechanisms of these poorly understood disorders.

Furthering pharmacological and physiological assessment of the glutamatergic receptors at the Drosophila neuromuscular junction

Drosophila melanogaster larval neuromuscular junctions (NMJs) serve as a model for synaptic physiology. The molecular sequences of the postsynaptic glutamate receptors have been described; however, the pharmacological profile has not been fully elucidated. The postsynaptic molecular sequence suggests a novel glutamate receptor subtype. Kainate does not depolarize the muscle, but dampens evoked EPSP amplitudes. Quantal responses show a decreased amplitude and area under the voltage curve indicative of reduced postsynaptic receptor sensitivity to glutamate transmission. ATPA, a kainate receptor agonist, did not mimic kainate's action. The metabotropic glutamate receptor agonist t-ACPD had no effect. Domoic acid, a kainate/AMPA receptor agonist, blocks the postsynaptic receptors without depolarizing the muscle. However, SYM 2081, a kainate receptor agonist, did depolarize the muscle and reduce the EPSP amplitude at 1 mM but not at 0.1 mM. This supports the notion that these are generally a quisqualate subtype receptors with some oddities in the pharmacological profile. The results suggest a direct postsynaptic action of kainate due to partial antagonist action on the quisqualate receptors. There does not appear to be presynaptic auto-regulation via a kainate receptor subtype or a metabotropic auto-receptor. This study aids in furthering the pharmokinetic profiling and specificity of the receptor subtypes.

Stimulating PACalpha increases miniature excitatory junction potential frequency at the Drosophila neuromuscular junction

Photoactivated adenylate cyclase alpha (PACalpha) is a light-activated adenylate cyclase that was originally cloned from the eye spot of the protozoan Euglena gracilis. PACalpha has been shown to rapidly increase intracellular cyclic adenosine monophosphate (cAMP) in vivo in Xenopus oocytes and HEK293 cells, increase the spike width in Aplysia sensory neurons, and modify behavior in Drosophila. Using the GAL4 UAS system, we heterologously expressed PACalpha in motorneurons and quantified the effects of its activation at the neuromuscular junction of the Drosophila third instar wandering larva, a well-characterized model synapse. By recording from body-wall muscle 6, we show that the presynaptic activation of PACalpha with blue light significantly increased miniature excitatory junction potential (mEJP) frequency in the presence of calcium with a delay of about 1 minute. Similar effects have been observed in previous studies that utilized adenylate cyclase agonists (Forskolin) or membrane-permeable cAMP analogs [dibutyryl cAMP and 4-chlorophenylthio-(CPT)-cAMP] to increase presynaptic cAMP concentrations. PACalpha's efficacy in combination with its specificity make it an invaluable tool for the rapid regulation of cAMP in vivo and for investigating the mechanisms by which cAMP can modulate synaptic transmission and neuronal plasticity in Drosophila.

Delayed synaptic transmission in Drosophila cacophonynull embryos

Ca(2+) influx through the Drosophila N-type Ca(2+) channel, encoded by cacophony (cac), triggers fast synaptic transmission. We now ask whether the cac Ca(2+) channel is the Ca(2+) channel solely dedicated for fast synaptic transmission. Because the cac(null) mutation is lethal, we used cac(null) embryos to address this question. At the neuromuscular junction in HL3 solution, no fast synchronous synaptic transmission was detected on nerve stimulation. When the wild-type cac gene was introduced in the cac(null) background, fast synaptic transmission recovered. However, even in cac(null) embryos, nerve stimulation infrequently induced delayed synaptic events in the minority of cells in 1.5 mM [Ca(2+)](e) and in the majority of cells in 5 mM [Ca(2+)](e). The number of delayed quantal events per stimulus was greater in 5 mM [Ca(2+)](e) than in 1.5 mM. Thus the delayed release is [Ca(2+)](e) dependent. Plectreurys toxin II (PLTXII) (10 nM; a spider toxin analog) depressed the frequency of delayed events, suggesting that voltage-gated Ca(2+) channels, other than cac Ca(2+) channels, are contributing to them. However, delayed events were not affected by 50 microM La(3+). The frequency of miniature synaptic currents in cac(null) embryos was approximately 1/2 of control, whereas in high K(+) solutions, it was approximately 1/135. The hypertonicity response was approximately 1/10 of control. These findings indicate that the number of release-ready vesicles is smaller in cac(null) embryos. Taken together, the cac Ca(2+) channel is indispensable for fast synaptic transmission in normal conditions, and another type of Ca(2+) channel, the non-cac, PLTXII-sensitive Ca(2+) channel, is contributing to delayed release in cac(null) embryos.

Glutamate and its metabotropic receptor in Drosophila clock neuron circuits

Identification of the neurotransmitters in clock neurons is critical for understanding the circuitry of the neuronal network that controls the daily behavioral rhythms in Drosophila. Except for the neuropeptide pigment-dispersing factor, no neurotransmitters have been clearly identified in the Drosophila clock neurons. Here we show that glutamate and its metabotropic receptor, DmGluRA, are components of the clock circuitry and modulate the rhythmic behavior pattern of Drosophila. The dorsal clock neurons, DN1s in the larval brain and some DN1s and DN3s in the adult brain, were immunolabeled with antibodies against Drosophila vesicular glutamate transporter (DvGluT), suggesting that they are glutamatergic. Because the DN1s may communicate with the primary pacemaker neurons, s-LN(v)s, we tested glutamate responses of dissociated larval s-LN(v)s by means of calcium imaging. Application of glutamate dose dependently decreased intracellular calcium in the s-LN(v)s. Pharmacology of the response suggests the presence of DmGluRA on the s-LN(v)s. Antibodies against DmGluRA labeled dissociated s-LN(v)s and the LN(v) dendrites in the intact larval and adult brain. The role of metabotropic glutamate signaling was tested in behavior assays in transgenic larvae and flies with altered DmGluRA expression in the LN(v)s and other clock neurons. Larval photophobic behavior was enhanced in DmGluRA mutants. For adults, we could induce altered activity patterns in the dark phase under LD conditions and increase the period during constant darkness by knockdown of DmGluRA expression in LN(v)s. Our results suggest that a glutamate signal from some of the DNs modulates the rhythmic behavior pattern via DmGluRA on the LN(v)s in Drosophila.

nAChR-mediated calcium responses and plasticity inDrosophila Kenyon cells

In Drosophila, nicotinic acetylcholine receptors (nAChRs) mediate fast excitatory synaptic transmission in mushroom body Kenyon cells, a neuronal population involved in generation of complex behaviors, including responses to drugs of abuse. To determine whether activation of nAChRs can induce cellular changes that contribute to functional plasticity in these neurons, we examined nicotine-evoked responses in cells cultured from brains of late stage OK107-GAL4 pupae. Kenyon cells can be identified by expression of green fluorescent protein (GFP+). Nicotine activates alpha-bungarotoxin-sensitive nAChRs, causing a rapid increase in intracellular calcium levels in over 95% of the Kenyon cells. The nicotine-evoked calcium increase has a voltage-gated calcium channel (VGCC) dependent component and a VGCC-independent component that involves calcium influx directly through nAChRs. Thapsigargin treatment reduces the nicotine response consistent with amplification by calcium release from intracellular stores. The response to nicotine is experience-dependent: a short conditioning pulse of nicotine causes a transient 50% reduction in the magnitude of the response to a test pulse of nicotine when the interpulse interval is 4 h. This cellular plasticity is dependent on activation of the VGCC-component of the nicotine response and on cAMP-signaling, but not on protein synthesis. These data demonstrate that activation of nAChRs induces a calcium-dependent plasticity in Kenyon cells that could contribute to adult behaviors involving information processing in the mushroom bodies including responses to nicotine.

Nerve-evoked synchronous release and high K+ -induced quantal events are regulated separately by synaptotagmin I at Drosophila neuromuscular junctions

The distal Ca(2+)-binding domain of synaptotagmin I (Syt I), C2B, has two Ca(2+)-binding sites. To study their function in Drosophila, pairs of aspartates were mutated to asparagines and the mutated syt I was expressed in the syt I-null background (P[syt I(B-D1,2N)] and P[syt I(B-D3,4N)]). We examined the effects of these mutations on nerve-evoked synchronous synaptic transmission and high K(+)-induced quantal events at embryonic neuromuscular junctions. The P[syt I(B-D1,2N)] mutation virtually abolished synaptic transmission, whereas the P[syt I(B-D3,4N)] mutation strongly reduced but did not abolish it. The quantal content in P[syt I(B-D3,4N)] increased with the external Ca(2+) concentration, [Ca(2+)](e), with a slope of 1.86 in double-logarithmic plot, whereas that of control was 2.88. In high K(+) solutions the quantal event frequency in P[syt I(B-D3,4N)] increased progressively with [Ca(2+)](e) between 0 and 0.15 mM as in control. In contrast, in P[syt I(B-D1,2N)] the event frequency did not increase progressively between 0 and 0.15 mM and was significantly lower at 0.15 than at 0.05 mM [Ca(2+)](e). The P[syt I(B-D1,2N)] mutation inhibits high K(+)-induced quantal release in a narrow range of [Ca(2+)](e) (negative regulatory function). When Sr(2+) substituted for Ca(2+), nerve-evoked synchronous synaptic transmission was severely depressed and delayed asynchronous release was appreciably increased in control embryos. In high K(+) solutions with Sr(2+), the quantal event frequency was higher than that in Ca(2+) and increased progressively with [Sr(2+)](e) in control and in both mutants. Sr(2+) partially substitutes for Ca(2+) in synchronous release but does not support the negative regulatory function of Syt I.

Critical role of nitric oxide-cGMP cascade in the formation of cAMP-dependent long-term memory

Cyclic AMP pathway plays an essential role in formation of long-term memory (LTM). In some species, the nitric oxide (NO)-cyclic GMP pathway has been found to act in parallel and complementary to the cAMP pathway for LTM formation. Here we describe a new role of the NO-cGMP pathway, namely, stimulation of the cAMP pathway to induce LTM. We have studied the signaling cascade underlying LTM formation by systematically coinjecting various "LTM-inducing" and "LTM-blocking" drugs in crickets. Multiple-trial olfactory conditioning led to LTM that lasted for several days, while memory induced by single-trial conditioning decayed away within several hours. Injection of inhibitors of the enzyme forming NO, cGMP, or cAMP into the hemolymph prior to multiple-trial conditioning blocked LTM, whereas injection of an NO donor, cGMP analog, or cAMP analog prior to single-trial conditioning induced LTM. Induction of LTM by injection of an NO donor or cGMP analog paired with single-trial conditioning was blocked by inhibitors of the cAMP pathway, but induction of LTM by a cAMP analog was unaffected by inhibitors of the NO-cGMP pathway. Inhibitors of cyclic nucleotide-gated channel (CNG channel) or calmodulin-blocked induction of LTM by cGMP analog paired with single-trial conditioning, but they did not affect induction of LTM by cAMP analog. Our findings suggest that the cAMP pathway is a downstream target of the NO-cGMP pathway for the formation of LTM, and that the CNG channel and calcium-calmodulin intervene between the NO-cGMP pathway and the cAMP pathway.

Plasticity and second messengers during synapse development

Effective function of the locomotor system in the Drosophila larva requires a continuous adjustment of synaptic architecture and neurotransmission at the neuromuscular junction (NMJ). This feature has made the larval NMJ a favorite model to study the genetic and molecular mechanisms underlying synapse plasticity. This chapter will review experimental strategies used to study plasticity at the NMJ, the cellular parameters affected during plastic changes, and many of the known molecules involved in plastic changes. In addition, signal transduction pathways activated during plasticity will be discussed.

External Ca2+ dependency of synaptic transmission in drosophila synaptotagmin I mutants

To resolve some of differences in reports on the function of Synaptotagmin I (Syt I), we re-examined synaptic transmission at the neuromuscular junction of Drosophila embryos that have mutations in the Syt I gene (syt I). Two major questions addressed were which Ca2+ binding domain, C2A or C2B, sense Ca2+ and is Syt I a negative regulator of spontaneous vesicle fusion. Synaptic currents were induced by nerve stimulation or by high K+ treatment in external solutions containing various Ca2+ concentrations. In a null allele, syt I(AD4), synchronous synaptic currents were rarely observed but not abolished. The quantal content was about 1/60 of control but increased linearly with [Ca2+](e) with a slope of 0.95 (N) in the double logarithmic plot, in contrast to 3.01 in control. The slope of 1.06 in an allele, syt I(AD1), which lacks the second Ca2+ binding domain, C2B, was not different from in syt I(AD4). In another allele, syt I(AD3), in which one amino acid in C2B is mutated, synchronous synaptic transmission was also impaired and N was 1.54, which is significantly smaller than in control. In high K+ saline, the [Ca2+](e) dependency of vesicle release in syt I(AD4) was lower than in controls, whereas that in syt I(AD3) was even lower than in syt I(AD4), suggesting that syt I(AD3) is inhibiting vesicle fusion. These findings led us to conclude that C2B, not C2A, senses Ca2+, and Syt I is a negative regulator of vesicle fusion.

Synaptic vesicle pools and plasticity of synaptic transmission at the Drosophila synapse

Our knowledge on the Drosophila neuromuscular synapse is rapidly expanding. Thus, this synapse offers an excellent model for studies of the molecular mechanism of synaptic transmission and synaptic plasticity. Two synaptic vesicle (SV) pools have been identified and characterized using a fluorescent styryl dye, FM1-43, to stain SVs. They are termed the exo/endo cycling pool (ECP), which corresponds to the readily releasable pool (RRP) defined electrophysiologically, and the reserve pool (RP). These two pools were identified first in a temperature-sensitive paralytic mutant, shibire, and subsequently confirmed in wild-type larvae. The ECP participates in synaptic transmission during low frequency firing of presynaptic nerves and locates in the periphery of presynaptic boutons in the vicinity of release sites, while SVs in the RP spread toward the center of boutons and are recruited only during tetanic stimulation. These two pools are separately replenished by endocytosis. Cyclic AMP facilitates recruitment of SVs from the RP to the ECP. Activation of presynaptic metabotropic glutamate receptors recruits SVs from the RP and enhances SV release by elevation of the cAMP level. Memory mutants that have defects in the cAMP/PKA cascade, dunce and rutabaga, exhibit reduced levels of recruitment of synaptic SVs from the RP to the ECP and have limited short-term synaptic plasticity. SV mobilization between the two pools could be a key step for changes in synaptic efficacy. Since a variety of mutants that have distinct defects in synaptic transmission are available for detailed studies of synaptic function, this direction of approach in Drosophila seems promising.

Mutations that rescue the paralysis of Caenorhabditis elegans ric-8 (synembryn) mutants activate the G alpha(s) pathway and define a third major branch of the synaptic signaling network

To identify hypothesized missing components of the synaptic G alpha(o)-G alpha(q) signaling network, which tightly regulates neurotransmitter release, we undertook two large forward genetic screens in the model organism C. elegans and focused first on mutations that strongly rescue the paralysis of ric-8(md303) reduction-of-function mutants, previously shown to be defective in G alpha(q) pathway activation. Through high-resolution mapping followed by sequence analysis, we show that these mutations affect four genes. Two activate the G alpha(q) pathway through gain-of-function mutations in G alpha(q); however, all of the remaining mutations activate components of the G alpha(s) pathway, including G alpha(s), adenylyl cyclase, and protein kinase A. Pharmacological assays suggest that the G alpha(s) pathway-activating mutations increase steady-state neurotransmitter release, and the strongly impaired neurotransmitter release of ric-8(md303) mutants is rescued to greater than wild-type levels by the strongest G alpha(s) pathway activating mutations. Using transgene induction studies, we show that activating the G alpha(s) pathway in adult animals rapidly induces hyperactive locomotion and rapidly rescues the paralysis of the ric-8 mutant. Using cell-specific promoters we show that neuronal, but not muscle, G alpha(s) pathway activation is sufficient to rescue ric-8(md303)'s paralysis. Our results appear to link RIC-8 (synembryn) and a third major G alpha pathway, the G alpha(s) pathway, with the previously discovered G alpha(o) and G alpha(q) pathways of the synaptic signaling network.

The Drosophila metabotropic glutamate receptor DmGluRA regulates activity-dependent synaptic facilitation and fine synaptic morphology

In vertebrates, several groups of metabotropic glutamate receptors (mGluRs) are known to modulate synaptic properties. In contrast, the Drosophila genome encodes a single functional mGluR (DmGluRA), an ortholog of vertebrate group II mGluRs, greatly expediting the functional characterization of mGluR-mediated signaling in the nervous system. We show here that DmGluRA is expressed at the glutamatergic neuromuscular junction (NMJ), localized in periactive zones of presynaptic boutons but excluded from active sites. Null DmGluRA mutants are completely viable, and all of the basal NMJ synaptic transmission properties are normal. In contrast, DmGluRA mutants display approximately a threefold increase in synaptic facilitation during short stimulus trains. Prolonged stimulus trains result in very strongly increased ( approximately 10-fold) augmentation, including the appearance of asynchronous, bursting excitatory currents never observed in wild type. Both defects are rescued by expression of DmGluRA only in the neurons, indicating a specific presynaptic requirement. These phenotypes are reminiscent of hyperexcitable mutants, suggesting a role of DmGluRA signaling in the regulation of presynaptic excitability properties. The mutant phenotypes could not be replicated by acute application of mGluR antagonists, suggesting that DmGluRA regulates the development of presynaptic properties rather than directly controlling short-term modulation. DmGluRA mutants also display mild defects in NMJ architecture: a decreased number of synaptic boutons accompanied by an increase in mean bouton size. These morphological changes bidirectionally correlate with DmGluRA levels in the presynaptic terminal. These data reveal the following two roles for DmGluRA in presynaptic mechanisms: (1) modulation of presynaptic excitability properties important for the control of activity-dependent neurotransmitter release and (2) modulation of synaptic architecture.

Characterization of the two distinct subtypes of metabotropic glutamate receptors from honeybee, Apis mellifera

L-Glutamate is a major neurotransmitter at the excitatory synapses in the vertebrate brain. It is also the excitatory neurotransmitter at neuromuscular junctions in insects, however its functions in their brains remain to be established. We identified and characterized two different subtypes (AmGluRA and AmGluRB) of metabotropic glutamate receptors (mGluRs) from an eusocial insect, honeybee. Both AmGluRA and AmGluRB form homodimers independently on disulfide bonds, and bind [3H]glutamate with K(D) values of 156.7 and 80.7 nM, respectively. AmGluRB is specifically expressed in the brain, while AmGluRA is expressed in the brain and other body parts, suggesting that AmGluRA is also present at the neuromuscular junctions. Both mGluRs are expressed in the mushroom bodies and the brain regions of honeybees, where motor neurons are clustered. Their expression in the brain apparently overlaps, suggesting that they may interact with each other to modulate the glutamatergic neurotransmission.

Experience-dependent strengthening of Drosophila neuromuscular junctions

The genetic analysis of larval neuromuscular junctions (NMJs) of Drosophila has provided detailed insights into molecular mechanisms that control the morphological and physiological development of these glutamatergic synapses. However, because of the chronic defects caused by mutations, a time-resolved analysis of these mechanisms and their functional relationships has been difficult so far. In this study we provide a first temporal map of some of the molecular and cellular key processes, which are triggered in wild-type animals by natural larval locomotor activity and then mediate experience-dependent strengthening of larval NMJs. Larval locomotor activity was increased either by chronically rearing a larval culture at 29 degrees C instead of 18 or 25 degrees C or by acutely transferring larvae from a culture vial onto agar plates. Within 2 hr of enhanced locomotor activity, NMJs showed a significant potentiation of signal transmission that was rapidly reversed by an induced paralysis of the temperature-sensitive mutant parats1. Enhanced locomotor activity was also associated with a significant increase in the number of large subsynaptic translation aggregates. After 4 hr, postsynaptic DGluR-IIA glutamate receptor subunits started to transiently accumulate in ring-shaped areas around synapses, and they condensed later on, after chronic locomotor stimulation at 29 degrees C, into typical postsynaptic patches. These NMJs showed a reduced perisynaptic expression of the cell adhesion molecule Fasciclin II, an increased number of junctional boutons, and significantly more active zones. Such temporal mapping of experience-dependent adaptations at developing wild-type and mutant NMJs will provide detailed insights into the dynamic control of glutamatergic signal transmission.

Presynaptic impairment of synaptic transmission in Drosophila embryos lacking Gs(alpha)

Gs(alpha) is a subunit of the heterotrimeric G-protein complex, expressed ubiquitously in all types of cells, including neurons. Drosophila larvae, which have mutations in the Gs(alpha) gene, are lethargic, suggesting an impairment of neuronal functions. In this study, we examined synaptic transmission at the neuromuscular synapse in Gs(alpha)-null (dgsR60) embryos shortly before they hatched. At low-frequency nerve stimulation, synaptic transmission in mutant embryos was not very different from that in controls. In contrast, facilitation during tetanic stimulation was minimal in dgsR60, and no post-tetanic potentiation was observed. Miniature synaptic currents (mSCs) were slightly smaller in amplitude and less frequent in dgsR60 embryos in normal-K+ saline. In high-K+ saline, mSCs with distinctly large amplitude occurred frequently in controls at late embryonic stages, whereas those mSCs were rarely observed in dgsR60 embryos, suggesting a developmental defect in the mutant. Using the Gal4-UAS expression system, we found that these phenotypes in dgsR60 were caused predominantly by lack of Gs(alpha) in presynaptic neurons and not in postsynaptic muscles. To test whether Gs(alpha) couples presynaptic modulator receptors to adenylyl cyclase (AC), we examined the responses of two known G-protein-coupled receptors in dgsR60 embryos. Both metabotropic glutamate and octopamine receptor responses were indistinguishable from those of controls, indicating that these receptors are not linked to AC by Gs(alpha). We therefore suggest that synaptic transmission is compromised in dgsR60 embryos because of presynaptic defects in two distinct processes; one is uncoupling between the yet-to-be-known modulator receptor and AC activation, and the other is a defect in synapse formation.

Mutation and activation of Galpha s similarly alters pre- and postsynaptic mechanisms modulating neurotransmission

Constitutive activation of Galphas in the Drosophila brain abolishes associative learning, a behavioral disruption far worse than that observed in any single cAMP metabolic mutant, suggesting that Galphas is essential for synaptic plasticity. The intent of this study was to examine the role of Galphas in regulating synaptic function by targeting constitutively active Galphas to either pre- or postsynaptic cells and by examining loss-of-function Galphas mutants (dgs) at the glutamatergic neuromuscular junction (NMJ) model synapse. Surprisingly, both loss of Galphas and activation of Galphas in either pre- or postsynaptic compartment similarly increased basal neurotransmission, decreased short-term plasticity (facilitation and augmentation), and abolished posttetanic potentiation. Elevated synaptic function was specific to an evoked neurotransmission pathway because both spontaneous synaptic vesicle fusion frequency and amplitude were unaltered in all mutants. In the postsynaptic cell, the glutamate receptor field was regulated by Galphas activity; based on immunocytochemical studies, GluRIIA receptor subunits were dramatically downregulated (>75% decrease) in both loss and constitutive active Galphas genotypes. In the presynaptic cell, the synaptic vesicle cycle was regulated by Galphas activity; based on FM1-43 dye imaging studies, evoked vesicle fusion rate was increased in both loss and constitutively active Galphas genotypes. An important conclusion of this study is that both increased and decreased Galphas activity very similarly alters pre- and postsynaptic mechanisms. A second important conclusion is that Galphas activity induces transynaptic signaling; targeted Galphas activation in the presynapse downregulates postsynaptic GluRIIA receptors, whereas targeted Galphas activation in the postsynapse enhances presynaptic vesicle cycling.

Effects of calcium and glutamate receptor agonists on leaf consumption by lepidopteran neonates

Calcium and glutamate receptor (GluR) agonists affect apple leaf consumption by neonates of the apple pest, the codling moth, Cydia pomonella (L.) Initial apple leaf consumption was advanced by the presence of trans-1-amino-(1S,3R)-cyclopentanedicarboxylic acid (trans-ACPD), but not by calcium chloride or N-methyl-D-aspartate (NMDA). However, during the 3 h following hatch, CaCl(2) and NMDA increased the quantity of apple leaf tissue consumed, but trans-ACPD had no such effects. Stimulatory effects of CaCl(2) and NMDA on leaf consumption were abolished if codling moth larvae were concurrently exposed to calcium chelator EDTA. (RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid (AMPA) and kainic acid had no effects either on commencement or intensity of leaf consumption. We hypothesize that in codling moth larvae, apple leaf consumption is induced via metabotropic GluR, and sustained feeding is regulated via NMDA GluRs. Practical aspects of this finding are discussed.

Biphasic modulation of synaptic transmission by hypertonicity at the embryonic Drosophila neuromuscular junction

Puff-application of hypertonic saline (sucrose added to external saline) causes a transient increase in the frequency of spontaneous miniature synaptic currents (mSCs) at the neuromuscular junctions of Drosophila embryos. The frequency gradually returns to pre-application levels. External Ca(2+) is not needed for this response, but it may modify it. At 50 mM added sucrose, for example, enhanced spontaneous release was observed only in the presence of external Ca(2+), suggesting that Ca(2+) augments the response. In a high-K(+) solution, in which the basal mSC frequency was elevated, higher sucrose concentrations produced an increase in mSC frequency that was followed (during and after the hypertonic exposure) by depression, with the magnitude of both effects increasing with hypertonicity between 100 and 500 mM. Evoked release by nerve stimulation showed only depression in response to hypertonicity. We do not believe that the depression of spontaneous or evoked release can be explained by the depletion of releasable quanta, however, since the frequency of quantal release did not reach levels compatible with this explanation and the enhancement and depression could be obtained independent of one another. In a mutant lacking neuronal synaptobrevin, only the depression of mSC frequency was induced by hypertonicity. Conversely, only the enhancing effect was observed in wild-type embryos when the mSC frequency was elevated with forskolin in Ca(2+)-free saline. In cultured embryonic Drosophila neurons, Ca(2+) signals that were induced by high K(+) and detected by Fura-2, were reduced by hypertonicity, suggesting that the depressing response is due to a direct effect of hypertonicity on Ca(2+) influx.

The requirement of presynaptic metabotropic glutamate receptors for the maintenance of locomotion

Spinal circuits known as central pattern generators maintain vertebrate locomotion. In the lamprey, the contralaterally alternating ventral root activity that defines this behavior is driven by ipsilateral glutamatergic excitation (Buchanan and Grillner, 1987) coupled with crossed glycinergic inhibition (Buchanan, 1982; Alford and Williams, 1989). These mechanisms are distributed throughout the spinal cord. Glutamatergic excitatory synapses activate AMPA and NMDA receptors known to be necessary for the maintenance of the locomotor rhythm. Less is known of the role and location of metabotropic glutamate receptors (mGluRs), although group I mGluRs enhance transmitter release at giant synapses in the lamprey spinal cord, whereas group II/III receptors may inhibit release. In this study we show that group I mGluR antagonists block fictive locomotion, a neural correlate of locomotion, by acting at the presynaptic terminal. Under physiological conditions, synaptically released glutamate activates presynaptic group I mGluRs (autoreceptors) during the repetitive activation of glutamatergic terminals. The resulting rise in [Ca2+]i caused by the release from presynaptic intracellular stores is coincident with an enhancement of synaptic transmission. Thus, blocking mGluRs reduces glutamate release during the repetitive activity that is characteristic of locomotion, leading to the arrest of locomotor activity. We found the effects of group I mGluRs on locomotion to be inconsistent with a postsynaptic effect on the central pattern generator. Consequently, the activation of metabotropic glutamate autoreceptors is necessary to maintain rhythmic motor output. Our results demonstrate the role of presynaptic mGluRs in the physiological control of movement for the first time.

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.

Glutamate receptors on the somata of dorsal unpaired median neurons in cockroach, Periplaneta americana, thoracic ganglia

Effects of application of glutamate and glutamatergic ligands were studied to characterize the receptors for glutamate present on the soma membrane of the dorsal unpaired median (DUM) neurons in the thoracic ganglia of the cockroach, Periplaneta americana, using the intracellular recording technique. Application of L-glutamate did not block the GABA-response, and application of beta-guanidino-propionic acid, a competitive antagonist for GABA, failed to block the response to L-glutamate. These results indicate that most of L-glutamate action may not be mediated by a GABA-activated channel. To examine glutamate receptor types on the DUM neurons, glutamate receptor agonists were applied. The ionotropic glutamate receptor (iGluR) agonists evoked depolarizations with the following relative rank of order of potency: kainate > AMPA > quisqualate. Metabotropic glutamate receptor (mGluR) agonists also elicited membrane depolarizations or hyperpolarizations associated with an increase in membrane conductance. The mGluR agonists evoked depolarizations or hyperpolarizations with the following relative rank of order: L-CCG-1 > 1S, 3R-ACPD > L-AP4. Depolarization of the same DUM neuron was detected following exposure of kainate and L-CCG-I, suggesting the coexistence of distinct iGluR and mGluR types. A membrane permeable cAMP analog, CPT-cAMP, could not mimic the effect of mGluR agonists. The mGluR selective antagonists, MCCG and MCPG, failed to antagonize the response to mGluR agonists. The involvement of cAMP in the mGluR response was not confirmed in DUM neurons. Although the functional roles of these receptors are unknown, it might be possible then that these extrasynaptic receptors have a modulatory effect on the excitability of the DUM neurons.

Hypertonicity-induced transmitter release at Drosophila neuromuscular junctions is partly mediated by integrins and cAMP/protein kinase A

The frequency of quantal transmitter release increases upon application of hypertonic solutions. This effect bypasses the Ca(2+) triggering step, but requires the presence of key molecules involved in vesicle fusion, and hence could be a useful tool for dissecting the molecular process of vesicle fusion. We have examined the hypertonicity response at neuromuscular junctions of Drosophila embryos in Ca(2+)-free saline. Relative to wild-type, the response induced by puff application of hypertonic solution was enhanced in a mutant, dunce, in which the cAMP level is elevated, or in wild-type embryos treated with forskolin, an activator of adenylyl cyclase, while protein kinase A (PKA) inhibitors decreased it. The response was also smaller in a mutant, DC0, which lacks the major subunit of PKA. Thus the cAMP/PKA cascade is involved in the hypertonicity response. Peptides containing the sequence Arg-Gly-Asp (RGD), which inhibit binding of integrins to natural ligands, reduced the response, whereas a peptide containing the non-binding sequence Arg-Gly-Glu (RGE) did not. A reduced response persisted in a mutant, myospheroid, which expresses no integrins, and the response in DC0 was unaffected by RGD peptides. These data indicate that there are at lease two components in the hypertonicity response: one that is integrin mediated and involves the cAMP/PKA cascade, and another that is not integrin mediated and does not involve the cAMP/PKA cascade.

Distribution of metabotropic glutamate receptor DmGlu-A in Drosophila melanogaster central nervous system

L-glutamate is the excitatory neurotransmitter at neuromuscular junctions in insects. It may also be involved in neurotransmission within the central nervous system (CNS), but its function therein remains elusive. The roles of glutamatergic synapses in the Drosophila melanogaster CNS were investigated, with focus on the study of DmGluRA, a G-protein-coupled glutamate receptor. In a first attempt to determine the function of this receptor, we describe its distribution in the larval and adult Drosophila CNS, using a polyclonal antibody raised against the C-terminal sequence of the protein. DmGluRA is expressed in a reproducible pattern both in the larva and in the adult. In particular, DmGluRA can be found in the antennal lobes, the optic lobes, the central complex, and the median bundle in the adult CNS. However, DmGluRA-containing neurons represented only a small fraction of all CNS neurons. DmGluRA immunoreactivity was not detected at the larval neuromuscular junction nor in the body wall muscles. The correlations between DmGluRA distribution and previously described glutamate-like immunoreactivity patterns, as well as the implications of these observations concerning the possible functions of DmGluRA in the Drosophila CNS, are discussed.

Two independent pathways mediated by cAMP and protein kinase A enhance spontaneous transmitter release at Drosophila neuromuscular junctions

cAMP is thought to be involved in learning process and known to enhance transmitter release in various systems. Previously we reported that cAMP enhances spontaneous transmitter release in the absence of extracellular Ca(2+) and that the synaptic vesicle protein neuronal-synaptobrevin (n-syb), is required in this enhancement (n-syb-dependent; Yoshihara et al., 1999). In the present study, we examined the cAMP-induced enhancement of transmitter release in the presence of external Ca(2+). We raised the intracellular concentration of cAMP by application of either forskolin, an activator of adenylyl cyclase, or by 4-chlorophenylthio-(CPT)-cAMP, a membrane-permeable analog of cAMP, in the presence of external Ca(2+), while recording miniature synaptic currents (mSCs) at the neuromuscular junction in n-syb null mutant embryos. The frequency of mSCs increased in response to elevation of cAMP, and this effect of cAMP was completely blocked by Co(2+) (n-syb-independent pathway). In contrast, in wild-type embryos the cAMP-induced mSC frequency increase was partially blocked by Co(2+). In a mutant, DC0, defective in protein kinase A (PKA), nerve-evoked synaptic currents were indistinguishable from the control, but mSCs were less frequent. In this mutant the enhancement by cAMP of both nerve-evoked and spontaneous transmitter release was completely absent, even in the presence of external Ca(2+). Taken together, these results suggest that cAMP enhances spontaneous transmitter release by increasing Ca(2+) influx (n-syb-independent) as well as by modulating the release mechanism without Ca(2+) influx (n-syb-dependent) in wild-type embryos, and these two effects are mediated by PKA encoded by the DC0 gene.

Differential actions of PKA and PKC in the regulation of glutamate release by group III mGluRs in the entorhinal cortex

In a previous study we showed that activation of a presynaptically located metabotropic glutamate receptor (mGluR) with pharmacological properties of mGluR4a causes a facilitation of glutamate release in layer V of the rat entorhinal cortex (EC) in vitro. In the present study we have begun to investigate the intracellular coupling linking the receptor to transmitter release. We recorded spontaneous alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated excitatory postsynaptic currents (EPSCs) in the whole cell configuration of the patch-clamp technique, from visually identified neurons in layer V. Bath application of the protein kinase A (PKA) activator, forskolin, resulted in a marked facilitation of EPSC frequency, similar to that seen with the mGluR4a specific agonist, ACPT-1. Preincubation of slices with the PKA inhibitor H-89 abolished the effect of ACPT-1, as did preincubation with the adenylate cyclase inhibitor, SQ22536. Activation of protein kinase C (PKC) using phorbol 12 myristate 13-acetate (PMA) did not affect sEPSC frequency; however, it did abolish the facilitatory effect of ACPT-1 on glutamate release. A robust enhancement of EPSC frequency was seen in response to bath application of the specific PKC inhibitor, GF 109203X. Both H-89 and the group III mGluR antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) abolished the effects of GF 109203X. These data suggest that in layer V of the EC, presynaptic group III mGluRs facilitate release via a positive coupling to adenylate cyclase and subsequent activation of PKA. We have also demonstrated that the PKC system tonically depresses transmitter release onto layer V cells of the EC and that an interaction between mGluR4a, PKA, and PKC may exist at these synapses.

Physiological activation of presynaptic metabotropic glutamate receptors increases intracellular calcium and glutamate release

Activation of metabotropic glutamate receptors (mGluRs) has diverse effects on the functioning of vertebrate synapses. The cellular mechanisms that underlie these changes, however, are largely unknown. The role of presynaptic mGluRs in modulating Ca(2+) dynamics and regulating neurotransmitter release was investigated at the vestibulospinal-reticulospinal (VS-RS) synapse in the lamprey brain stem. Application of the specific Group I mGluRs antagonist 7-(hydroxyimino) cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) reduced the amplitude of consecutive high-frequency evoked excitatory postsynaptic currents (EPSCs). A series of experiments using techniques of electrophysiology and calcium imaging were carried out to determine the cellular mechanisms by which this phenomenon occurs. Concentration-dependent increases in the pre- and postsynaptic [Ca(2+)](i) were seen with the application of mGluR agonists. Similarly, high-frequency stimulation of axons caused a Group I mGluR-dependent enhancement in presynaptic Ca(2+) transients. Application of mGluR agonist caused a depolarization of the presynaptic elements, while thapsigargin decreased the high-frequency stimulus- and agonist-induced rises in [Ca(2+)](i). These data suggest that both membrane depolarization and the release of Ca(2+) from intracellular stores potentially play a role in mGluR-induced Ca(2+) signaling. To determine the effect of this modulation of Ca(2+) dynamics on spontaneous glutamate release, miniature EPSCs were recorded from postsynaptic reticulospinal neurons. A potent Group I mGluR agonist, (S)-homoquisqualic acid, caused a large increase in the frequency of events. These results demonstrate the presence of presynaptic Group I mGluRs at the VS-RS synapse. Activation of these receptors leads to a rise in [Ca(2+)](i) and enhances the spontaneous and evoked release of glutamate. Taken together, these studies highlight the importance of synaptic activation of these facilitatory autoreceptors in both short-term plasticity and synaptic transmission.

Tetanic stimulation recruits vesicles from reserve pool via a cAMP-mediated process in Drosophila synapses

At Drosophila neuromuscular junctions, there are two synaptic vesicle pools, namely the exo/endo cycling pool (ECP) and the reserve pool (RP). We studied the recruitment process from RP using a fluorescent dye, FMI-43. During high-frequency nerve stimulation, vesicles in RP were recruited for release, and endocytosed vesicles were incorporated into both pools, whereas with low-frequency stimulation, vesicles were incorporated into and released from ECP. Release of vesicles from RP was detected electrophysiologically after emptying vesicles in the ECP of transmitter by a H+ pump inhibitor. Recruitment from RP was depressed by inhibitors of steps in the cAMP/PKA cascade and enhanced by their activators. In rutabaga (rut) with low cAMP levels, mobilization of vesicles from RP during tetanic stimulation was depressed, while it was enhanced in dunce (dnc) with high cAMP levels.

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

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

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

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