Altered synaptic plasticity in Drosophila memory mutants with a defective cyclic AMP cascade

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.

Synaptotagmin I, a Ca2+ sensor for neurotransmitter release

The discovery that Ca(2+) triggers rapid neurotransmitter release has prompted the search for the Ca(2+) sensor. There is now general agreement that the vesicle-associated Ca(2+)-binding protein, synaptotagmin I, is required for the tight temporal coupling between Ca(2+) influx and synaptic vesicle fusion. However, the precise mechanism of Ca(2+)-sensing by synaptotagmin I is still under debate despite intensive investigation using genetic, biochemical and electrophysiological tools. Here, we discuss many of the genetic manipulations from the past few years that have shed light on the Ca(2+)-sensing function of synaptotagmin I. We also present our view as to how the Ca(2+) signal is translated rapidly into membrane fusion at fast chemical synapses.

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.

Synaptic Drosophila UNC-13 is regulated by antagonistic G-protein pathways via a proteasome-dependent degradation mechanism

UNC-13 is a highly conserved plasma membrane-associated synaptic protein implicated in the regulation of neurotransmitter release through the direct modulation of the SNARE exocytosis complex. Previously, we characterized the Drosophila homologue (DUNC-13) and showed it to be essential for neurotransmitter release immediately upstream of vesicular fusion ("priming") at the neuromuscular junction (NMJ). Here, we show that the abundance of DUNC-13 in NMJ synaptic boutons is regulated downstream of GalphaS and Galphaq pathways, which have inhibitory and facilitatory roles, respectively. Both cAMP modulation and PKA function are required for DUNC-13 synaptic up-regulation, suggesting that the cAMP pathway enhances synaptic efficacy via DUNC-13. Similarly, PLC function and DAG modulation also regulate the synaptic levels of DUNC-13, through a mechanism that appears independent of PKC. Our results suggest that proteasome-mediated protein degradation is the primary mechanism regulating DUNC-13 levels at the synapse. Both PLC- and PKA-mediated pathways appear to regulate synaptic levels of DUNC-13 through controlling the rate of proteasome-dependent DUNC-13 degradation. We conclude that the functional abundance of DUNC-13 at the synapse, a key determinant of synaptic vesicle priming and neurotransmitter release probability, is primarily regulated by the rate of protein degradation, rather than translocation or transport, convergently controlled via both cAMP and DAG signal transduction pathways.

Cellular bases of behavioral plasticity: establishing and modifying synaptic circuits in the Drosophila genetic system

Genetic malleability and amenability to behavioral assays make Drosophila an attractive model for dissecting the molecular mechanisms of complex behaviors, such as learning and memory. At a cellular level, Drosophila has contributed a wealth of information on the mechanisms regulating membrane excitability and synapse formation, function, and plasticity. Until recently, however, these studies have relied almost exclusively on analyses of the peripheral neuromuscular junction, with a smaller body of work on neurons grown in primary culture. These experimental systems are, by themselves, clearly inadequate for assessing neuronal function at the many levels necessary for an understanding of behavioral regulation. The pressing need is for access to physiologically relevant neuronal circuits as they develop and are modified throughout life. In the past few years, progress has been made in developing experimental approaches to examine functional properties of identified populations of Drosophila central neurons, both in cell culture and in vivo. This review focuses on these exciting developments, which promise to rapidly expand the frontiers of functional cellular neurobiology studies in Drosophila. We discuss here the technical advances that have begun to reveal the excitability and synaptic transmission properties of central neurons in flies, and discuss how these studies promise to substantially increase our understanding of neuronal mechanisms underlying behavioral plasticity.

Effects of chronic lead exposure on the neuromuscular junction in Drosophila larvae

Long term or chronic exposure to lead is associated with cognitive and other deficits in humans, which may reflect lead-induced changes in synaptic development and function. We believe that Drosophila has great potential as a model system for studying such changes. To test this, we compared the structure of single, identified synapses between identified axons (axons 1 and 2) and muscle fibers (fibers 6 and 7) in untreated 3rd instar larvae, and in larvae reared on medium made with 100 microM lead acetate in distilled water. We used three approaches to examine the motor terminals on muscle fibers 6 and 7 in segment 2: (1) all terminals were stained with an antibody to HRP; (2) only the terminals of axon 1 were stained by injecting biotinylated Lucifer yellow into it; and (3) the regions of the terminal containing synaptic vesicles were stained with an antibody to synaptotagmin, which provides an estimate of "synaptic" terminal area. Lead burdens were determined by inductively coupled plasma mass spectrometry; hemolymph lead levels at the neuromuscular junction were likely to be micromolar. We observed that lead exposure did not significantly affect the average terminal area or the average muscle fiber area, but did significantly affect the uniformity of the matching between muscle area and motor terminal size that normally occurs during development. There was a significant positive correlation between motor terminal size and muscle area in control, but not in lead-exposed larvae. The sensitivity of Drosophila larval synaptic development to lead opens the way to using the powerful genetic and molecular tools available for this system to study the underlying mechanisms of this sensitivity. We would hope that from such an understanding may come strategies for dealing with lead-induced deficits in children.

Synaptotagmin I functions as a calcium sensor to synchronize neurotransmitter release

To characterize Ca(2+)-mediated synaptic vesicle fusion, we analyzed Drosophila synaptotagmin I mutants deficient in specific interactions mediated by its two Ca(2+) binding C2 domains. In the absence of synaptotagmin I, synchronous release is abolished and a kinetically distinct delayed asynchronous release pathway is uncovered. Synapses containing only the C2A domain of synaptotagmin partially recover synchronous fusion, but have an abolished Ca(2+) cooperativity. Mutants that disrupt Ca(2+) sensing by the C2B domain have synchronous release with normal Ca(2+) cooperativity, but with reduced release probability. Our data suggest the Ca(2+) cooperativity of neurotransmitter release is likely mediated through synaptotagmin-SNARE interactions, while phospholipid binding and oligomerization trigger rapid fusion with increased release probability. These results indicate that synaptotagmin is the major Ca(2+) sensor for evoked release and functions to trigger synchronous fusion in response to Ca(2+), while suppressing asynchronous release.

Reduced delayed-rectifier K+ current in the learning mutant rutabaga

In the Drosophila mutant rutabaga, short-term memory is deficient and intracellular cyclic adenosine monophosphate (cAMP) concentration is reduced. We characterized the delayed-rectifier potassium current (IK(DR)) in rutabaga as compared with the wild-type. The conventional whole-cell patch-clamp technique was applied to cultured Drosophila neurons derived from embryonic neuroblasts. IK(DR) was smaller in rutabaga (368 +/- 11 pA) than in wild-type (541 +/- 14 pA) neurons, measured in a Ca(2+)-free solution. IK(DR) was clearly activated at approximately 0 mV in the two genotypes. IK(DR) typically reached its peak within 10-20 msec after the start of the pulse (60 mV). There was no difference in inactivation of IK(DR) for wild-type (14 +/- 3%) and rutabaga (19 +/- 3%). After application of 10 mM TEA, in wild-type, IK(DR) was reduced by 46 +/- 5%, whereas in rutabaga, IK(DR) was reduced by 28 +/- 3%. Our results suggest that IK(DR) is carried by two different types of channels, one which is TEA-sensitive, whereas the other is TEA-insensitive. Apparently, the TEA-sensitive channel is less expressed in rutabaga neurons than in wild-type neurons. Conceivably, altered neuronal excitability in the rutabaga mutant could disrupt the processing of neural signals necessary for learning and memory.

Postsynaptic kinase signaling underlies inhibitory synaptic plasticity in the lateral superior olive

In the auditory system, inhibitory transmission from the medial nucleus of the trapezoid body (MNTB) to neurons of the lateral superior olivary nucleus (LSO) undergoes activity-dependent long-term depression, and may be associated with developmental elimination of these synapses [Sanes DH, Friauf E (2000).

REVIEW

development and influence of inhibition in the laterial superior olivary nucleus. Hear Res 147:46-58]. Although GABA(B) receptor activation and postsynaptic free calcium are implicated in this depression, little is known about intracellular signaling mechanisms in this or other forms of inhibitory plasticity. In this study, we asked whether the calcium dependency of inhibitory depression was associated with the activation of calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase C (PKC), and/or cAMP-dependent protein kinase A (PKA). Whole-cell voltage-clamp recordings were obtained from LSO neurons in a brain slice preparation, permitting for the selective pharmacologic manipulation of individual postsynaptic LSO neurons. Inclusion of a CaMKII antagonist (KN-62) in the internal pipet solution blocked inhibitory synaptic depression. A second CaMKII inhibitor (autocamtide peptide fragment) significantly decreased inhibitory depression. Inclusion of a specific antagonist of protein kinase C (PKC fragment 19-36) in the internal recording solution also blocked inhibitory depression. To test involvement of a cAMP-dependent intracellular cascade, two different manipulations were performed. Inclusion of PKA antagonists (Rp-cAMPS or a cAMP dependent protein kinase inhibitor peptide) prevented inhibitory depression. In contrast, when a nonhydrolyzable cAMP analog (Sp-cAMPS) was permitted to enter the postsynaptic cell, the MNTB-evoked IPSCs became depressed in the absence of low-frequency stimulation. Thus, three key postsynaptic kinases, CaMKII, PKC, and PKA, participate in the activity-dependent depression of inhibitory MNTB-LSO synapses during postnatal development.

Endophilin is critically required for synapse formation and function in Drosophila melanogaster

Studies in cell-free systems and the lamprey giant synapse have implicated crucial roles for amphiphysin and endophilin in synaptic transmission. However, null mutants at the amphiphysin locus of Drosophila are viable and have no demonstrable synaptic vesicle-recycling defect. This has necessitated a re-examination of the role of Src homology 3 domain-containing proteins in synaptic vesicle recycling. In this report, we show that endophilin-deficient eye clones in Drosophila have an altered electroretinogram. A characteristic of this defect is its aggravation during heightened visual stimulation. It is shown that endophilin is primarily required in the nervous system. Decreased endophilin activity results in alterations in the neuromuscular junction structure and physiology. Immunofluorescence studies show colocalization of endophilin with dynamin consistent with a possible role in synaptic vesicle recycling.

Regulation of synaptic plasticity and synaptic vesicle dynamics by the PDZ protein Scribble

The Drosophila tumor suppressor Scribble (Scrib) is a PDZ-containing protein required for maintaining epithelial cell polarity. At the larval neuromuscular junction, Scrib colocalizes and indirectly interacts with another tumor suppressor and PDZ protein, Discs-Large (Dlg). Previous studies demonstrate that Dlg is critical for development of normal synapse structure and function, as well as for normal synaptic Scrib localization. Here we show that Scrib is also an important regulator of synaptic architecture and physiology. The most notable ultrastructural defect in scrib mutants is an increase in the number of synaptic vesicles in an area of the synaptic bouton thought to contain the reserve vesicle pool. Additionally, the number of active zones is reduced in scrib mutants. Functionally, the scrib synapse behaves relatively normally at low-frequency stimulation. However, several forms of plasticity at this synapse are drastically altered in the mutants. Specifically, scrib mutants exhibit loss of facilitation and post-tetanic potentiation, and faster synaptic depression. In addition, FM1-43 imaging of recycling synaptic vesicles shows that vesicle dynamics are impaired in scrib mutants. These results identify Scrib as an essential regulator of short-term synaptic plasticity. Taken together, our results are consistent with a model in which Scrib is required to sustain synaptic vesicle concentrations at their sites of release.

Electrophysiological analysis of synaptic transmission in central neurons of Drosophila larvae

We report functional neuronal and synaptic transmission properties in Drosophila CNS neurons. Whole cell current- and voltage-clamp recordings were made from dorsally positioned neurons in the larval ventral nerve cord. Comparison of neuronal Green Fluorescent Protein markers and intracellular dye labeling revealed that recorded cells consisted primarily of identified motor neurons. Neurons had resting potentials of -50 to -60 mV and fired repetitive action potentials (APs) in response to depolarizing current injection. Acetylcholine application elicited large excitatory responses and AP bursts that were reversibly blocked by the nicotinic receptor antagonist D-tubocurarine (dtC). GABA and glutamate application elicited similar inhibitory responses that reversed near normal resting potential and were reversibly blocked by the chloride channel blocker picrotoxin. Multiple types of endogenous synaptically driven activity were present in most neurons, including fast spontaneous synaptic events resembling unitary excitatory postsynaptic currents (EPSCs) and sustained excitatory currents and potentials. Sustained forms of endogenous activity ranged in amplitude from smaller subthreshold "intermediate" sustained events to large "rhythmic" events that supported bursts of APs. Electrical stimulation of peripheral nerves or focal stimulation of the neuropil evoked sustained responses and fast EPSCs similar to endogenous events. Endogenous activity and evoked responses required external Ca(2+) and were reversibly blocked by dtC application, indicating that cholinergic synaptic transmission directly underlies observed activity. Synaptic current amplitude and frequency were reduced in shibire conditional dynamin mutants and increased in dunce cAMP phosphodiesterase mutants. These results complement and advance those of recent functional studies in Drosophila embryonic neurons and demonstrate the feasibility of in-depth synaptic transmission and plasticity studies in the Drosophila CNS.

Regional calcium regulation within cultured Drosophila neurons: effects of altered cAMP metabolism by the learning mutations dunce and rutabaga

The dunce (dnc) and rutabaga (rut) mutations of Drosophila affect a cAMP-dependent phosphodiesterase and a Ca(2+)/CaM-regulated adenylyl cyclase, respectively. These mutations cause deficiencies in several learning paradigms and alter synaptic transmission, growth cone motility, and action potential generation. The cellular phenotypes either are Ca(2+) dependent (neurotransmission and motility) or mediate a Ca(2+) rise (action potential generation). However, interrelations among these defects have not been addressed. We have established conditions for fura-2 imaging of Ca(2+) dynamics in the "giant" neuron culture system of Drosophila. Using high K(+) depolarization of isolated neurons, we observed a larger, faster, and more dynamic response from the growth cone than the cell body. This Ca(2+) increase depended on an influx through Ca(2+) channels and was suppressed by the Na(+) channel blocker TTX. Altered cAMP metabolism by the dnc and rut mutations reduced response amplitude in the growth cone while prolonging the response within the soma. The enhanced spatial resolution of these larger cells allowed us to analyze Ca(2+) regulation within distinct domains of mutant growth cones. Modulation by a previous conditioning stimulus was altered in terms of response amplitude and waveform complexity. Furthermore, rut disrupted the distinction in Ca(2+) responses observed between the periphery and central domain of growth cones with motile filopodia.

Dominant-negative NSF2 disrupts the structure and function of Drosophila neuromuscular synapses

N-ethylmaleimide sensitive fusion protein (NSF) is an ATPase necessary for vesicle trafficking, including exocytosis. Current models hold that NSF is required in a step that readies vesicles for fusion by disassembling postfusion SNARE protein complexes allowing them to participate in further rounds of vesicle cycling. Whereas most organisms have only one NSF isoform, Drosophila has two. dNSF1 is the predominant functional isoform in the adult nervous system. Conditional mutations in the dNSF1 gene, comatose, are paralytic and lead to disruption of synaptic transmission and the rapid accumulation of SNARE complexes in adult flies. This isoform is not required for synaptic transmission in larvae. In contrast, dNSF2 is important at earlier developmental stages, and its broad expression indicates its importance in neural and non-neural tissues alike. To study dNSF2, and to circumvent the lethality of dNSF2 null mutants, we have constructed transgenic flies carrying a dominant negative form of dNSF2. When this construct was expressed in neurons we observed suppression of synaptic transmission, activity-dependent fatigue of transmitter release, and a reduction in the number of releasable vesicles. However, we unexpectedly found that there was no accumulation of SNARE complexes accompanying these physiological phenotypes. Intriguingly, we also found that expression of mutant dNSF2 induced pronounced overgrowth of the neuromuscular junction and some misrouting of axons. These results support the idea that dNSF2 has multiple roles in cellular function and adds that not all of its functions require disassembly of the SNARE complex.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

SLO-1 potassium channels control quantal content of neurotransmitter release at the C. elegans neuromuscular junction

Six mutants of SLO-1, a large-conductance, Ca(2+)-activated K(+) channel of C. elegans, were obtained in a genetic screen for regulators of neurotransmitter release. Mutants were isolated by their ability to suppress lethargy of an unc-64 syntaxin mutant that restricts neurotransmitter release. We measured evoked postsynaptic currents at the neuromuscular junction in both wild-type and mutants and observed that the removal of SLO-1 greatly increased quantal content primarily by increasing duration of release. The selective isolation of slo-1 as the only ion channel mutant derived from a whole genomic screen to detect regulators of neurotransmitter release suggests that SLO-1 plays an important, if not unique, role in regulating neurotransmitter release.

Drosophila: genetics meets behaviour

Genes are understandably crucial to physiology, morphology and biochemistry, but the idea of genes contributing to individual differences in behaviour once seemed outrageous. Nevertheless, some scientists have aspired to understand the relationship between genes and behaviour, and their research has become increasingly informative and productive over the past several decades. At the forefront of behavioural genetics research is the fruitfly Drosophila melanogaster, which has provided us with important insights into the molecular, cellular and evolutionary bases of behaviour.

A post-docking role for active zone protein Rim

Rim1 was previously identified as a Rab3 effector localized to the presynaptic active zone in vertebrates. Here we demonstrate that C. elegans unc-10 mutants lacking Rim are viable, but exhibit behavioral and physiological defects that are more severe than those of Rab3 mutants. Rim is localized to synaptic sites in C. elegans, but the ultrastructure of the presynaptic densities is normal in Rim mutants. Moreover, normal levels of docked synaptic vesicles were observed in mutants, suggesting that Rim is not involved in the docking process. The level of fusion competent vesicles at release sites was reduced fivefold in Rim mutants, but calcium sensitivity of release events was unchanged. Furthermore, expression of a constitutively open form of syntaxin suppressed the physiological defects of Rim mutants, suggesting Rim normally acts to regulate conformational changes in syntaxin. These data suggest Rim acts after vesicle docking likely via regulating priming.

Drosophila CAPS is an essential gene that regulates dense-core vesicle release and synaptic vesicle fusion

Calcium-activated protein for secretion (CAPS) is proposed to play an essential role in Ca2+-regulated dense-core vesicle exocytosis in vertebrate neuroendocrine cells. Here we report the cloning, mutation, and characterization of the Drosophila ortholog (dCAPS). Null dCAPS mutants display locomotory deficits and complete embryonic lethality. The mutant NMJ reveals a 50% loss in evoked glutamatergic transmission, and an accumulation of synaptic vesicles at active zones. Importantly, dCAPS mutants display a highly specific 3-fold accumulation of dense-core vesicles in synaptic terminals, which was not observed in mutants that completely arrest synaptic vesicle exocytosis. Targeted transgenic CAPS expression in identified motoneurons fails to rescue dCAPS neurotransmission defects, demonstrating a cell nonautonomous role in synaptic vesicle fusion. We conclude that dCAPS is required for dense-core vesicle release and that a dCAPS-dependent mechanism modulates synaptic vesicle release at glutamatergic synapses.

Genetic analysis of the Drosophila Gs(alpha) gene

One of the best understood signal transduction pathways activated by receptors containing seven transmembrane domains involves activation of heterotrimeric G-protein complexes containing Gs(alpha), the subsequent stimulation of adenylyl cyclase, production of cAMP, activation of protein kinase A (PKA), and the phosphorylation of substrates that control a wide variety of cellular responses. Here, we report the identification of "loss-of-function" mutations in the Drosophila Gs(alpha) gene (dgs). Seven mutants have been identified that are either complemented by transgenes representing the wild-type dgs gene or contain nucleotide sequence changes resulting in the production of altered Gs(alpha) protein. Examination of mutant alleles representing loss-of-Gs(alpha) function indicates that the phenotypes generated do not mimic those created by mutational elimination of PKA. These results are consistent with the conclusion reached in previous studies that activation of PKA, at least in these developmental contexts, does not depend on receptor-mediated increases in intracellular cAMP, in contrast to the predictions of models developed primarily on the basis of studies in cultured cells.

Differential distribution of cAMP-specific phosphodiesterase 7A mRNA in rat brain and peripheral organs

We investigated the regional distribution and cellular localization of mRNA coding for the cAMP-specific phosphodiesterase 7A (PDE7A) in rat brain and several peripheral organs by in situ hybridization histochemistry. The regional expression of two splice variants, PDE7A1 and PDE7A2, was examined by RT-PCR using RNA extracted from several brain regions. PDE7A mRNA was found to be widely distributed in rat brain in both neuronal and nonneuronal cell populations. The highest levels of hybridization were observed in the olfactory bulb, olfactory tubercle, hippocampus, cerebellum, medial habenula nucleus, pineal gland, area postrema, and choroid plexus. Positive hybridization signals were also detected in other areas, such as raphe nuclei, temporal and entorhinal cortex, pontine nuclei, and some cranial nerve motor nuclei. Both mRNA splice forms were differentially distributed in several areas of the brain with the striatum expressing only PDE7A1 and the olfactory bulb and spinal cord expressing PDE7A2 exclusively. In peripheral organs the highest levels of PDE7A hybridization were seen in kidney medulla, although testis, liver, adrenal glands, thymus, and spleen also presented high hybridization signal. These results are consistent with PDE7A being involved in the regulation of cAMP signaling in many brain functions. The consistent colocalization with PDE4 mRNAs suggests that PDE7A could have an effect on memory, depression, and emesis. The results offer clear anatomical and functional systems in which to investigate future specific PDE7 inhibitors.

Distinct roles of CaMKII and PKA in regulation of firing patterns and K(+) currents in Drosophila neurons

The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and the cAMP-dependent protein kinase A (PKA) cascades have been implicated in neural mechanisms underlying learning and memory as supported by mutational analyses of the two enzymes in Drosophila. While there is mounting evidence for their roles in synaptic plasticity, less attention has been directed toward their regulation of neuronal membrane excitability and spike information coding. Here we report genetic and pharmacological analyses of the roles of PKA and CaMKII in the firing patterns and underlying K(+) currents in cultured Drosophila central neurons. Genetic perturbation of the catalytic subunit of PKA (DC0) did not alter the action potential duration but disrupted the frequency coding of spike-train responses to constant current injection in a subpopulation of neurons. In contrast, selective inhibition of CaMKII by the expression of an inhibitory peptide in ala transformants prolonged the spike duration but did not affect the spike frequency coding. Enhanced membrane excitability, indicated by spontaneous bursts of spikes, was observed in CaMKII-inhibited but not in PKA-diminished neurons. In wild-type neurons, the spike train firing patterns were highly reproducible under consistent stimulus conditions. However, disruption of either of these kinase pathways led to variable firing patterns in response to identical current stimuli delivered at a low frequency. Such variability in spike duration and frequency coding may impose problems for precision in signal processing in these protein kinase learning mutants. Pharmacological analyses of mutations that affect specific K(+) channel subunits demonstrated distinct effects of PKA and CaMKII in modulation of the kinetics and amplitude of different K(+) currents. The results suggest that PKA modulates Shaker A-type currents, whereas CaMKII modulates Shal-A type currents plus delayed rectifier Shab currents. Thus differential regulation of K(+) channels may influence the signal handling capability of neurons. This study provides support for the notion that, in addition to synaptic mechanisms, modulations in spike activity patterns may represent an important mechanism for learning and memory that should be explored more fully.

The products of the Drosophila stoned locus interact with synaptic vesicles via synaptotagmin

The stoned locus of Drosophila melanogaster encodes two novel proteins, stonedA (STNA) and stonedB (STNB), both of which are expressed in the nervous system. Flies with defects at the stoned locus have abnormal behavior and altered synaptic transmission. Genetic interactions, in particular with the shibire (dynamin) mutation, indicated a presynaptic function for stoned and suggested an involvement in vesicle cycling. Immunological studies revealed colocalization of the stoned proteins at the neuromuscular junction with the integral synaptic vesicle protein synaptotagmin (SYT). We show here that stoned interacts genetically with synaptotagmin to produce a lethal phenotype. The STNB protein is found by co-immunoprecipitation to be associated with synaptic vesicles, and glutathione S-transferase pull-downs demonstrate an in vitro interaction between the micro2-homology domain of STNB and the C2B domain of the SYTI isoform. The STNA protein is also found in association with vesicles, and it too exhibits an in vitro association with SYTI. However, we find that the bulk of STNA is in a nonmembranous fraction. By using the shibire mutant to block endocytosis, STNB is shown to be present on some synaptic vesicles before exocytosis. However, STNB is not associated with all synaptic vesicles. We hypothesize that STNB specifies a subset of synaptic vesicles with a role in the synaptic vesicle cycle that is yet to be determined.

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.

Genetic studies in Drosophila: vesicle pools and cytoskeleton-based regulation of synaptic transmission

Presynaptic plasticity mechanisms rely on modulation of the synaptic vesicle fusion machinery and the regulated mobilization of synaptic vesicles at the active zone. This review discusses recent evidence suggesting that the relative proportions of synaptic vesicles in the reserve and ready releasable pools is the primary determinant of synaptic transmission strength, and that transport of vesicles between these pools is mediated by cytoskeletal mechanisms. Recent efforts to identify the molecules required for regulation of the presynaptic cytoskeleton suggest that common mechanisms may exist to regulate synaptic vesicle pools in widely divergent neuronal types, ranging from synaptic modulation at the Drosophila neuromuscular junction to the synaptic plasticity required for learning and memory in the mammalian brain.

SNARE proteins contribute to calcium cooperativity of synaptic transmission

A hallmark of calcium-triggered synaptic transmission is the cooperative relationship between calcium and the amount of transmitter released. This relationship is thought to be important for improving the efficiency of synaptic vesicle exocytosis. Although it is generally held that cooperativity arises from the interaction of multiple calcium ions with a single calcium-sensing molecule, the precise molecular basis of this phenomenon is not known. The SNARE proteins are known to be critical for synaptic vesicle exocytosis. We therefore tested for a contribution of SNARE proteins to cooperativity by genetically reducing the levels of syntaxin IA and neuronal-synaptobrevin in Drosophila. Surprisingly, we found that reducing these SNARE proteins also reduced Ca(2+) cooperativity. Thus, SNARE proteins are important for determining the cooperative relationship between calcium and synaptic transmission.

Size of vesicle pools, rates of mobilization, and recycling at neuromuscular synapses of a Drosophila mutant, shibire

Two vesicle pools, readily releasable (RRP) and reserve (RP) pools, are present at Drosophila neuromuscular junctions. Using a temperature-sensitive mutant, shibire(ts), we studied pool sizes and vesicle mobilization rates. In shibire(ts), due to lack of endocytosis at nonpermissive temperatures, synaptic currents continuously declined during tetanic stimulation until they ceased as the result of vesicle depletion. By then, approximately 84,000 quanta were released. Vesicles were mobilized from RP at a rate 1/7-1/10 of RRP. Cytochalasin D inhibited mobilization of vesicles from RP, allowing us to estimate the size of RRP as 14%-19% of all vesicles. Vesicle recycling supports synaptic transmission during prolonged tetanic stimulation and the maximum recycling rate was 1000 vesicles/s.

Phosphodiesterase type 4 isozymes expression in human brain examined by in situ hybridization histochemistry and[3H]rolipram binding autoradiography. Comparison with monkey and rat brain

We have examined the distribution of four different cyclic AMP-specific phosphodiesterase isozyme (PDE4A, PDE4B, PDE4C and PDE4D) mRNAs in the brain of different species by in situ hybridization histochemistry and by autoradiography with [3H]rolipram. We have compared the localization of each isozyme in human brain with that in rat and monkey brain. We have found that the four PDE4 isoforms display a differential expression pattern at both regional and cellular level in the three species. PDE4A, PDE4B and PDE4D are widely distributed in human brain, with the two latter appearing more abundant. In contrast, PDE4C in human brain, presents a more restricted distribution, limited to cortex, some thalamic nuclei and cerebellum. This is at variance with the distribution of PDE4C in rat brain, where it is found exclusively in olfactory bulb. In monkey brain, the highest expression for this isoform is found in the claustrum, and at lower levels in cortical areas and cerebellum. PDE4B presented a broad distribution, being expressed in both neuronal and non neuronal cell populations. In general, the distribution of binding sites visualized with [3H]rolipram correlated well with the expression of each PDE4 isozyme.

Surprises from Drosophila: genetic mechanisms of synaptic development and plasticity

Drosophila are excellent models for the study of synaptic development and plasticity, thanks to the availability and applicability of a wide variety of powerful molecular, genetic, and cell-biology techniques. Three decades of study have led to an intimate understanding of the sequence of events leading to a functional and plastic synapse, yet many of the molecular mechanisms underlying these events are still poorly understood. Here, we provide a review of synaptogenesis at the Drosophila glutamatergic neuromuscular junction (NMJ). Next, we discuss the role of two proteins that forward genetic screens in Drosophila have revealed to play crucial-and completely unexpected-roles in NMJ development and plasticity: the origin of replication complex protein Latheo, and the enzyme glutamate decarboxylase. The requirement for these proteins at the NMJ highlights the fact that synaptic development and plasticity involves intense inter- and intracellular signaling about which we know almost nothing.

Drosophila endosomal proteins hook and deep orange regulate synapse size but not synaptic vesicle recycling

To study the function of endosomes at synapses we analyzed the localization and function of two Drosophila endosomal proteins, Hook and Deep orange (Dor), at the larval neuromuscular junction. Hook, a negative regulator of endocytic trafficking, and Dor, a positive regulator of endocytic trafficking, are highly enriched at synapses, especially close to postsynaptic membranes. Mutations in hook (hk) and dor do not affect synaptic vesicle recycling, as assessed by electrophysiological analysis of synaptic transmission and behavioral studies of double mutants with shi(ts) mutations that alter vesicle recycling. However, hk and dor mutations alter the number of presynaptic varicosities (synapse size) in opposing ways. Synapse size is increased in hk(11) mutants and is decreased in dor(4) mutants. Double mutants for dor and hk show a dor-like phenotype. These effects on synapse size parallel known functions of Hook and Dor in endocytosis and strongly indicate a role for endocytic trafficking in the regulation of synapse size in vivo. Our observations suggest a model in which Hook and Dor function in later stages of endocytosis is essential for regulating synaptic plasma membrane composition but not synaptic vesicle recycling.

Integrin-mediated regulation of synaptic morphology, transmission, and plasticity

Volado, the gene encoding the Drosophila alphaPS3-integrin, is required for normal short-term memory formation (Grotewiel et al., 1998), supporting a role for integrins in synaptic modulation mechanisms. We show that the Volado protein (VOL) is localized to central and peripheral larval Drosophila synapses. VOL is strongly concentrated in a subpopulation of synaptic boutons in the CNS neuropil and to a variable subset of synaptic boutons at neuromuscular junctions (NMJs). Mutant morphological and functional synaptic phenotypes were analyzed at the NMJ. Volado mutant synaptic arbors are structurally enlarged, suggesting VOL negatively regulates developmental synaptic sprouting and growth. Mutant NMJs exhibit abnormally large evoked synaptic currents and reduced Ca(2+) dependence of transmission. Strikingly, multiple forms of Ca(2+)- and activity-dependent synaptic plasticity are reduced or absent. Conditional Volado expression in mutant larvae largely rescues normal transmission and plasticity. Pharmacologicially disrupting integrin function at normal NMJs phenocopies features of mutant transmission and plasticity within 30-60 min, demonstrating that integrins acutely regulate functional transmission. Our results provide direct evidence that Volado regulates functional synaptic plasticity processes and support recent findings implicating integrins in rapid changes in synaptic efficacy and in memory formation.

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.

Role of cAMP cascade in synaptic stability and plasticity: ultrastructural and physiological analyses of individual synaptic boutons in Drosophila memory mutants

Mutations of the genes rutabaga (rut) and dunce (dnc) affect the synthesis and degradation of cAMP, respectively, and disrupt learning in Drosophila. Combined ultrastructural analysis and focal electrophysiological recording in the larval neuromuscular junction revealed a loss of stability and fine tuning of synaptic structure and function in both mutants. Increased ratios of docked/undocked vesicles and poorly defined synaptic specializations characterized dnc synapses. In contrast, rut boutons possessed fewer, although larger, synapses with lower proportions of docked vesicles. At reduced Ca(2+) levels, decreased quantal content coupled with an increase in failure rate was seen in rut boutons and reduced pair-pulse facilitation were found in both rut and dnc mutants. At physiological Ca(2+) levels, strong enhancement, instead of depression, in evoked release was observed in some dnc and rut boutons during 10 Hz tetanus. Furthermore, increased variability of synaptic transmission, including fluctuation and asynchronicity of evoked release, paralleled an increase in synapse size variation in both dnc and rut boutons, which might impose problems for effective signal processing in the nervous system. Pharmacological and genetic studies indicated broader ranges of physiological alteration by dnc and rut mutations than either the acute effects of cAMP analogs or the available mutations that affect cAMP-dependent protein kinase (PKA) activity. This is consistent with previous reports of more severe learning defects in dnc and rut mutations than these PKA mutants and allows identification of the phenotypes involving long-term developmental regulation and those conferred by PKA.

Facilitation of spontaneous acetylcholine release induced by activation of cAMP in rat neuromuscular junctions

Regulation of neurotransmitter release is thought to involve modulation of the release probability by protein phosphorylation. Activation of the cAMP-protein kinase A (PKA) pathway has been shown to facilitate synaptic transmission in mammalian neuromuscular synapses, although the relevant phosphorylation targets are mostly unknown. We found that the inhibitor of the phosphodiesterase aminophylline (1 mM AMIN), the membrane-permeable analog of cAMP, 8-Br-cAMP (5 mM) and, the direct adenylate cyclase activator, forskolin (20 microM), induced an increase of miniature end-plate potentials (MEPPs) frequency in rat neuromuscular junctions. We investigated the possible involvement of the voltage-dependent calcium channels (VDCC), since these proteins are known to be phosphorylated by PKA. But this possibility was ruled out, since the increase in MEPPs frequency was not attenuated by the VDCC blocker Cd2+ (100 microM) and it was observed when AMIN was studied on hyperosmotic response, which is independent of [Ca2+]o and of Ca2+ influx through the VDCC. The lack of action of AMIN on MEPPs frequency when [Ca2+]i was diminished by exposing the preparations to zero Ca2+-EGTA solution (isotonic condition) or when nerve terminals were loaded with a permeant Ca2+ chelator (BAPTA-AM) (hypertonic condition), indicate that cAMP-mediated presynaptic facilitation is a function of nerve terminal Ca2+ concentration. We also found that AMIN exerted a comparable increase in MEPPs frequency in control and high K+ (10 and 15 mM), suggesting a single mechanism of action for spontaneous and K+-induced secretion.

Spontaneous acetylcholine secretion from developing growth cones of Drosophila central neurons in culture: effects of cAMP-pathway mutations

We describe a novel bioassay system that uses Xenopus embryonic myocytes (myoballs) to detect the release of acetylcholine from Drosophila CNS neurons. When a voltage-clamped Xenopus myoball was manipulated into contact with cultured Drosophila "giant" neurons, spontaneous synaptic current-like events were registered. These events were observed within seconds after contact and were blocked by curare and alpha-bungarotoxin, but not by TTX and Cd(2+), suggesting that they are caused by the spontaneous quantal release of acetylcholine (ACh). The secretion occurred not only at the growth cone, but also along the neurite and at the soma, with significantly different release parameters among various regions. The amplitude of these currents displayed a skewed distribution. These features are distinct from synaptic transmission at more mature synapses or autapses formed in this culture system and are reminiscent of the transmitter release process during early development in other preparations. The usefulness of this coculture system in studying presynaptic secretion mechanisms is illustrated by a series of studies on the cAMP pathway mutations, dunce (dnc) and PKA-RI, which disrupt a cAMP-specific phosphodiesterase and the regulatory subunit of cAMP-dependent protein kinase A, respectively. We found that these mutations affected the ACh current kinetics, but not the quantal ACh packet, and that the release frequency was greatly enhanced by repetitive neuronal activity in dnc, but not wild-type, growth cones. These results suggest that the cAMP pathway plays an important role in the activity-dependent regulation of transmitter release not only in mature synapses as previously shown, but also in developing nerve terminals before synaptogenesis.

Highwire regulates synaptic growth in Drosophila

The formation, stabilization, and growth of synaptic connections are dynamic and highly regulated processes. The glutamatergic neuromuscular junction (NMJ) in Drosophila grows new boutons and branches throughout larval development. A primary walking behavior screen followed by a secondary anatomical screen led to the identification of the highwire (hiw) gene. In hiw mutants, the specificity of motor axon pathfinding and synapse formation appears normal. However, NMJ synapses grow exuberantly and are greatly expanded in both the number of boutons and the extent and length of branches. These synapses appear normal ultrastructurally but have reduced quantal content physiologically. hiw encodes a large protein found at presynaptic terminals. Within presynaptic terminals, HIW is localized to the periactive zone surrounding active zones; Fasciclin II (Fas II), which also controls synaptic growth, is found at the same location.

Localization of a short-term memory in Drosophila

Memories are thought to be due to lasting synaptic modifications in the brain. The search for memory traces has relied predominantly on determining regions that are necessary for the process. However, a more informative approach is to define the smallest sufficient set of brain structures. The rutabaga adenylyl cyclase, an enzyme that is ubiquitously expressed in the Drosophila brain and that mediates synaptic plasticity, is needed exclusively in the Kenyon cells of the mushroom bodies for a component of olfactory short-term memory. This demonstrates that synaptic plasticity in a small brain region can be sufficient for memory formation.

cAMP-dependent plasticity at excitatory cholinergic synapses in Drosophila neurons: alterations in the memory mutant dunce

It is well known that cAMP signaling plays a role in regulating functional plasticity at central glutamatergic synapses. However, in the Drosophila CNS, where acetylcholine is thought to be a primary excitatory neurotransmitter, cellular changes in neuronal communication mediated by cAMP remain unexplored. In this study we examined the effects of elevated cAMP levels on fast excitatory cholinergic synaptic transmission in cultured embryonic Drosophila neurons. We report that chronic elevation in neuronal cAMP (in dunce neurons or wild-type neurons grown in db-cAMP) results in an increase in the frequency of cholinergic miniature EPSCs (mEPSCs). The absence of alterations in mEPSC amplitude or kinetics suggests that the locus of action is presynaptic. Furthermore, a brief exposure to db-cAMP induces two distinct changes in transmission at established cholinergic synapses in wild-type neurons: a short-term increase in the frequency of spontaneous action potential-dependent synaptic currents and a long-lasting, protein synthesis-dependent increase in the mEPSC frequency. A more persistent increase in cholinergic mEPSC frequency induced by repetitive, spaced db-cAMP exposure in wild-type neurons is absent in neurons from the memory mutant dunce. These data demonstrate that interneuronal excitatory cholinergic synapses in Drosophila, like central excitatory glutamatergic synapses in other species, are sites of cAMP-dependent plasticity. In addition, the alterations in dunce neurons suggest that cAMP-dependent plasticity at cholinergic synapses could mediate changes in neuronal communication that contribute to memory formation.

Synaptic ultrastructure in nerve terminals of Drosophila larvae overexpressing the learning gene dunce

We investigated synaptic ultrastructure of individual nerve ending varicosities at the Drosophila larval neuromuscular junction in transgenic larvae overexpressing the learning gene dunce (dnc) in the nervous system. It was previously shown that cAMP is reduced to one-third normal in these larvae and that they have fewer nerve terminal varicosities and smaller junction potentials, although transmitter release from individual nerve ending varicosities is not significantly altered. We tested the hypothesis that synaptic ultrastructure is modified to compensate for possible reduced efficacy of synaptic transmission resulting from lower than normal cAMP. Synaptic size and number of presynaptic dense bodies (active zone structures) per synapse are modestly enhanced in transgenic larvae overexpressing the dnc gene product and in rutabaga (rut(1)) mutant larvae, which have reduced adenylyl cyclase activity and reduced neural cAMP. The incidence of complex synapses (possessing 2 or more presynaptic dense bodies) was not consistently different in experimental larvae compared to controls. The observations suggest that chronic reduction of cAMP levels in the nervous system of Drosophila larvae, although leading to a modest compensatory change in synaptic structure, does not markedly alter several synaptic ultrastructural parameters which are thought to influence the strength of transmitter release; thus, homeostatic mechanisms do not act to maintain normal-sized junction potentials by altering synaptic structure.

Drosophila larval neuromuscular junction: molecular components and mechanisms underlying synaptic plasticity

Understanding the mechanisms that mediate synaptic plasticity is a primary goal of molecular neuroscience. The Drosophila larval neuromuscular junction provides a particularly useful model for investigating the roles of synaptic components in both structural and functional plasticity. The powerful molecular genetics of this system makes it possible to uncover new synaptic components and signaling molecules, as well as their function in the intact organism. Together with the mouse hippocampus and Aplysia dissociated cell culture, the Drosophila larval neuromuscular junction has been among the most valuable model systems for examining the molecular and cellular basis of neuronal plasticity.

Tissue-specific expression of a type I adenylyl cyclase rescues the rutabaga mutant memory defect: in search of the engram

Most attempts to localize physical correlates of memory in the central nervous system (CNS) rely on ablation techniques. This approach has the limitation of defining just one of an unknown number of structures necessary for memory formation. We have used the Drosophila rutabaga type I Ca(2+)/CaM-dependent adenylyl cyclase (AC) gene to determine in which CNS region AC expression is sufficient for memory formation. Using pan-neural and restricted CNS expression with the GAL4 binary transcription activation system, we have rescued the memory defect of the rutabaga mutant in a fast robust spatial learning paradigm. The ventral ganglion, antennal lobes, and median bundle are likely the CNS structures sufficient for rutabaga AC- dependent spatial learning.

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.

The stoned proteins regulate synaptic vesicle recycling in the presynaptic terminal

The Drosophila stoned locus was identified 25 years ago on the basis of stress-sensitive behavioral mutants (Grigliatti et al., 1973). The locus is dicistronic and encodes two distinct proteins, stoned A and stoned B, which are expressed specifically in presynaptic terminals at central and peripheral synapses. Several stoned mutant alleles cause embryonic lethality, suggesting that these proteins are essential for synaptic function. Physiological analyses at the stoned synapse reveal severe neurotransmission defects, including reduced and asynchronous neurotransmitter release and rapid fatigue after repetitive stimulation. At the EM level, stoned synapses show a depletion of synaptic vesicles and a concomitant increase in membrane-recycling intermediates. Mutant terminals also display a specific mislocalization of the synaptic vesicle protein synaptotagmin. These results suggest that the stoned proteins are essential for the recycling of synaptic vesicle membrane and are required for the proper sorting of synaptotagmin during endocytosis.

Altered outward K(+) currents in Drosophila larval neurons of memory mutants rutabaga and amnesiac

K(+) currents in cultured Drosophila larval neurons have been classified into four categories according to their inactivation time constants, relative amplitude, and response to K(+) channel blockers 4-AP and tetraethylammonium. The percentage (65%) of neurons displaying K(+) currents which were reduced to 30% in amplitude by 5 mM cyclic adenosine monophosphate (cAMP) analog 8-bromo-cAMP in both Drosophila memory mutants rutabaga (rut) and amnesiac (amn) was significantly larger than that (50%) in wild type. This initial characterization provides evidence for altered K(+) currents in both rut and amn mutants. Arachidonic acid, a specifical inhibitor of Kv4 family (shal) K(+) channels, was found to inhibit K(+) currents in cultured Drosophila neurons, suggesting the presence of shal channels in these neurons.

Fast excitatory synaptic transmission mediated by nicotinic acetylcholine receptors in Drosophila neurons

Difficulty in recording from single neurons in vivo has precluded functional analyses of transmission at central synapses in Drosophila, where the neurotransmitters and receptors mediating fast synaptic transmission have yet to be identified. Here we demonstrate that spontaneously active synaptic connections form between cultured neurons prepared from wild-type embryos and provide the first direct evidence that both acetylcholine and GABA mediate fast interneuronal synaptic transmission in Drosophila. The predominant type of fast excitatory transmission between cultured neurons is mediated by nicotinic acetylcholine receptors (nAChRs). Detailed analysis of cholinergic transmission reveals that spontaneous EPSCs (sEPSCs) are composed of both evoked and action potential-independent [miniature EPSC (mEPSC)] components. The mEPSCs are characterized by a broad, positively skewed amplitude histogram in which the variance is likely to reflect differences in the currents induced by single quanta. Biophysical characteristics of the cholinergic mEPSCs include a rapid rise time (0.6 msec) and decay (tau = 2 msec). Regulation of mEPSC frequency by external calcium and cobalt suggests that calcium influx through voltage-gated channels influences the probability of ACh release. In addition, brief depolarization of the cultures with KCl can induce a calcium-dependent increase in sEPSC frequency that persists for up to 3 hr after termination of the stimulus, illustrating one form of plasticity at these cholinergic synapses. These data demonstrate that cultured embryonic neurons, amenable to both genetic and biochemical manipulations, present a unique opportunity to define genes/signal transduction cascades involved in functional regulation of fast excitatory transmission at interneuronal cholinergic synapses in Drosophila.

Drosophila larval neuromuscular junction's responses to reduction of cAMP in the nervous system

We investigated the effects of chronically lowered cyclic adenosine monophosphate (cAMP) on the morphology and physiology of the Drosophila larval neuromuscular junction, using two fly lines in which cAMP was significantly lower than normal in the nervous system: (a) transgenic flies in which the dunce (dnc) gene product was overexpressed in the nervous system, and (b) flies mutant for the rutabaga gene (rut1) which have reduced adenylyl cyclase activity. In comparison with controls, larvae with reduced cAMP exhibited a smaller number of synaptic varicosities. This effect was more pronounced in transgenic larvae, in which the reduction of neural cAMP was more pronounced. Synaptic transmission was also reduced in both cases, as evidenced by smaller excitatory junctional potentials (EJPs). Synaptic currents recorded from individual synaptic varicosities of the neuromuscular junction indicated almost normal transmitter release properties in transgenic larvae and a modest impairment in rut1 larvae. Thus, reduction in EJP amplitude in transgenic larvae is primarily due to reduced innervation, while in rut1 larvae it is attributable to the combined effects of reduced innervation and a mild impairment of transmitter release. We conclude that the major effect of chronically lowered cAMP is reduction of innervation rather than impairment of transmitter release properties.

Modulation of dihydropyridine-sensitive calcium channels in Drosophila by a cAMP-mediated pathway

Drosophila has proved to be a valuable system for studying the structure and function of ion channels. However, relatively little is known about the regulation of ion channels, particularly that of Ca2+ channels, in Drosophila. Physiological and pharmacological differences between invertebrate and mammalian L-type Ca2+ channels raise questions on the extent of conservation of Ca2+ channel modulatory pathways. We have examined the role of cyclic adenosine monophosphate (cAMP) cascade in modulating the dihydropyridine (DHP)-sensitive Ca2+ channels in the larval muscles of Drosophila, using mutations and drugs that disrupt specific steps in this pathway. The L-type (DHP-sensitive) Ca2+ channel current was increased in the dunce mutants, which have high cAMP concentration owing to cAMP-specific phosphodiesterase (PDE) disruption. The current was decreased in the rutabaga mutants, where adenylyl cyclase (AC) activity is altered thereby decreasing the cAMP concentration. The dunce effect was mimicked by 8-Br-cAMP, a cAMP analog, and IBMX, a PDE inhibitor. The rutabaga effect was rescued by forskolin, an AC activator. H-89, an inhibitor of protein kinase-A (PKA), reduced the current and inhibited the effect of 8-Br-cAMP. The data suggest modulation of L-type Ca2+ channels of Drosophila via a cAMP-PKA mediated pathway. While there are differences in L-type channels, as well as in components of cAMP cascade, between Drosophila and vertebrates, main features of the modulatory pathway have been conserved. The data also raise questions on the likely role of DHP-sensitive Ca2+ channel modulation in synaptic plasticity, and learning and memory, processes disrupted by the dnc and the rut mutations.

latheo, a Drosophila gene involved in learning, regulates functional synaptic plasticity

Mutations in the latheo (lat) gene disrupt associative learning in Drosophila , but a role for LAT in regulating neuronal function has not been demonstrated. Here, we report that LAT plays a central role in regulating Ca2(+)- and activity-dependent synaptic plasticity. Immunological localization of the LAT protein indicates it is present at synaptic connections of the larval neuromuscular junction (NMJ) and is enriched in presynaptic boutons. Basal synaptic transmission amplitude at the lat mutant NMJ is elevated 3- to 4-fold, and Ca2+ dependence of transmission is significantly reduced. Multiple forms of synaptic facilitation and posttetanic potentiation (PTP) are strongly depressed or absent at the mutant synapse. Our results suggest that LAT is a novel presynaptic protein with a role in the Ca2(+)-dependent synaptic modulation mechanisms necessary for behavioral plasticity.