Hyperthermic preconditioning of presynaptic calcium regulation in Drosophila

In vivo expression of peptidylarginine deiminase in Drosophila melanogaster

Peptidylarginine deiminase (PAD) modifies peptidylarginine and converts it to peptidylcitrulline in the presence of elevated calcium. Protein modification can lead to severe changes in protein structure and function, and aberrant PAD activity is linked to human pathologies. While PAD homologs have been discovered in vertebrates-as well as in protozoa, fungi, and bacteria-none have been identified in Drosophila melanogaster, a simple and widely used animal model for human diseases. Here, we describe the development of a human PAD overexpression model in Drosophila. We established fly lines harboring human PAD2 or PAD4 transgenes for ectopic expression under control of the GAL4/UAS system. We show that ubiquitous or nervous system expression of PAD2 or PAD4 have minimal impact on fly lifespan, fecundity, and the response to acute heat stress. Although we did not detect citrullinated proteins in fly homogenates, fly-expressed PAD4-but not PAD2-was active in vitro upon Ca2+ supplementation. The transgenic fly lines may be valuable in future efforts to develop animal models of PAD-related disorders and for investigating the biochemistry and regulation of PAD function.

Dissecting cause from consequence: a systematic approach to thermal limits

Thermal limits mark the boundaries of ectotherm performance, and are increasingly appreciated as strong correlates and possible determinants of animal distribution patterns. The mechanisms setting the thermal limits of ectothermic animals are under active study and rigorous debate as we try to reconcile new observations in the lab and field with the knowledge gained from a long history of research on thermal adaptation. Here, I provide a perspective on our divided understanding of the mechanisms setting thermal limits of ectothermic animals. I focus primarily on the fundamental differences between high and low temperatures, and how animal form and environment can place different constraints on different taxa. Together, complexity and variation in animal form drive complexity in the interactions within and among levels of biological organization, creating a formidable barrier to determining mechanistic cause and effect at thermal limits. Progress in our understanding of thermal limits will require extensive collaboration and systematic approaches that embrace this complexity and allow us to separate the causes of failure from the physiological consequences that can quickly follow. I argue that by building integrative models that explain causal links among multiple organ systems, we can more quickly arrive at a holistic understanding of the varied challenges facing animals at extreme temperatures.

Opposite effects of moderate heat stress and hyperthermia on cholinergic system of soil nematodes Caenorhabditis elegans and Caenorhabditis briggsae

Cholinergic system plays important role in all functions of organisms of free-living soil nematodes C. elegans and C. briggsae. Using pharmacological analysis we showed the existence of two opposite responses of nematodes cholinergic system to moderate and extreme heat stress. Short-term (15min) noxious heat (31-32°C) caused activation of cholinergic synaptic transmission in C. elegans and C. briggsae organisms by sensitization of nicotinic ACh receptors. In contrast, hyperthermia blocked cholinergic synaptic transmission by inhibition of ACh secretion by neurons. The resistance of behavior to extreme high temperature (36-37°C) was significantly higher in C. briggsae than in C. elegans, and thermostability of cholinergic transmission correlated with resistance of behavior to hyperthermia. Activation of cholinergic transmission by moderate heat stress can be the reason of movement speed increase in such adaptive behavior as noxious heat escape. Inhibition of ACh release is one of reasons for behavior failure caused by extreme high temperature since partial inhibition of ACh-esterase by aldicarb protected C. elegans and C. briggsae behavior against hyperthermia. Antagonist of mAChRs atropine almost completely prevented the rise in behavior thermotolerance caused by aldicarb. Pilocarpine, agonist of mAChRs, protected nematodes behavior against hyperthermia similarly with aldicarb. Therefore it is evident that it is the deficiency of mAChRs activity that is the reason for nematodes' behavior failure by hyperthermia.

Ionic mechanisms maintaining action potential conduction velocity at high firing frequencies in an unmyelinated axon

The descending contralateral movement detector (DCMD) is a high‐performance interneuron in locusts with an axon capable of transmitting action potentials (AP) at more than 500 Hz. We investigated biophysical mechanisms for fidelity of high‐frequency transmission in this axon. We measured conduction velocities (CVs) at room temperature during exposure to 10 mmol/L cadmium, a calcium current antagonist, and found significant reduction in CV with reduction at frequencies >200 Hz of ~10%. Higher temperatures induced greater CV reductions during exposure to cadmium across all frequencies of ~20–30%. Intracellular recordings during 15 min of exposure to cadmium or nickel, also a calcium current antagonist, revealed an increase in the magnitude of the afterhyperpolarization potential (AHP) and the time to recover to baseline after the AHP (Medians for Control: −19.8%; Nickel: 167.2%; Cadmium: 387.2%), that could be due to a T‐type calcium current. However, the removal of extracellular calcium did not mimic divalent cation exposure suggesting calcium currents are not the cause of the AHP increase. Computational modeling showed that the effects of the divalent cations could be modeled with a persistent sodium current which could be blocked by high concentrations of divalent cations. Persistent sodium current shortened the AHP duration in our models and increased CV for high‐frequency APs. We suggest that faithful, high‐frequency axonal conduction in the DCMD is enabled by a mechanism that shortens the AHP duration like a persistent or resurgent sodium current.

cGMP-Dependent Protein Kinase Inhibition Extends the Upper Temperature Limit of Stimulus-Evoked Calcium Responses in Motoneuronal Boutons of Drosophila melanogaster Larvae

While the mammalian brain functions within a very narrow range of oxygen concentrations and temperatures, the fruit fly, Drosophila melanogaster, has employed strategies to deal with a much wider range of acute environmental stressors. The foraging (for) gene encodes the cGMP-dependent protein kinase (PKG), has been shown to regulate thermotolerance in many stress-adapted species, including Drosophila, and could be a potential therapeutic target in the treatment of hyperthermia in mammals. Whereas previous thermotolerance studies have looked at the effects of PKG variation on Drosophila behavior or excitatory postsynaptic potentials at the neuromuscular junction (NMJ), little is known about PKG effects on presynaptic mechanisms. In this study, we characterize presynaptic calcium ([Ca²⁺]_i) dynamics at the Drosophila larval NMJ to determine the effects of high temperature stress on synaptic transmission. We investigated the neuroprotective role of PKG modulation both genetically using RNA interference (RNAi), and pharmacologically, to determine if and how PKG affects presynaptic [Ca²⁺]_i dynamics during hyperthermia. We found that PKG activity modulates presynaptic neuronal Ca²⁺ responses during acute hyperthermia, where PKG activation makes neurons more sensitive to temperature-induced failure of Ca²⁺ flux and PKG inhibition confers thermotolerance and maintains normal Ca²⁺ dynamics under the same conditions. Targeted motoneuronal knockdown of PKG using RNAi demonstrated that decreased PKG expression was sufficient to confer thermoprotection. These results demonstrate that the PKG pathway regulates presynaptic motoneuronal Ca²⁺ signaling to influence thermotolerance of presynaptic function during acute hyperthermia.

Heat shock response and homeostatic plasticity

Heat shock response and homeostatic plasticity are mechanisms that afford functional stability to cells in the face of stress. Each mechanism has been investigated independently, but the link between the two has not been extensively explored. We explore this link. The heat shock response enables cells to adapt to stresses such as high temperature, metabolic stress and reduced oxygen levels. This mechanism results from the production of heat shock proteins (HSPs) which maintain normal cellular functions by counteracting the misfolding of cellular proteins. Homeostatic plasticity enables neurons and their target cells to maintain their activity levels around their respective set points in the face of stress or disturbances. This mechanism results from the recruitment of adaptations at synaptic inputs, or at voltage-gated ion channels. In this perspective, we argue that heat shock triggers homeostatic plasticity through the production of HSPs. We also suggest that homeostatic plasticity is a form of neuroprotection.

Protective effects of testosterone on presynaptic terminals against oligomeric β-amyloid peptide in primary culture of hippocampal neurons

Increasing lines of evidence support that testosterone may have neuroprotective effects. While observational studies reported an association between higher bioavailable testosterone or brain testosterone levels and reduced risk of Alzheimer's disease (AD), there is limited understanding of the underlying neuroprotective mechanisms. Previous studies demonstrated that testosterone could alleviate neurotoxicity induced by β-amyloid (Aβ), but these findings mainly focused on neuronal apoptosis. Since synaptic dysfunction and degeneration are early events during the pathogenesis of AD, we aim to investigate the effects of testosterone on oligomeric Aβ-induced synaptic changes. Our data suggested that exposure of primary cultured hippocampal neurons to oligomeric Aβ could reduce the length of neurites and decrease the expression of presynaptic proteins including synaptophysin, synaptotagmin, and synapsin-1. Aβ also disrupted synaptic vesicle recycling and protein folding machinery. Testosterone preserved the integrity of neurites and the expression of presynaptic proteins. It also attenuated Aβ-induced impairment of synaptic exocytosis. By using letrozole as an aromatase antagonist, we further demonstrated that the effects of testosterone on exocytosis were unlikely to be mediated through the estrogen receptor pathway. Furthermore, we showed that testosterone could attenuate Aβ-induced reduction of HSP70, which suggests a novel mechanism that links testosterone and its protective function on Aβ-induced synaptic damage. Taken together, our data provide further evidence on the beneficial effects of testosterone, which may be useful for future drug development for AD.

Binding of insecticidal lectin Colocasia esculenta tuber agglutinin (CEA) to midgut receptors of Bemisia tabaci and Lipaphis erysimi provides clues to its insecticidal potential

The insecticidal potential of Galanthus nivalis agglutinin-related lectins against hemipterans has been experimentally proven. However, the basis behind the toxicity of these lectins against hemipterans remains elusive. The present study elucidates the molecular basis behind insecticidal efficacy of Colocasia esculenta tuber agglutinin (CEA) against Bemisia tabaci and Lipaphis erysimi. Confocal microscopic analyses highlighted the binding of 25 kDa stable homodimeric lectin to insect midgut. Ligand blots followed by LC MS/MS analyses identified binding partners of CEA as vacuolar ATP synthase and sarcoplasmic endoplasmic reticulum type Ca(2+) ATPase from B. tabaci, and ATP synthase, heat shock protein 70 and clathrin heavy chain assembly protein from L. erysimi. Internalization of CEA into hemolymph was confirmed by Western blotting. Glycoprotein nature of the receptors was identified through glycospecific staining. Deglycosylation assay indicated the interaction of CEA with its receptors to be probably glycan mediated. Surface plasmon resonance analysis revealed the interaction kinetics between ATP synthase of B. tabaci with CEA. Pathway prediction study based on Drosophila homologs suggested the interaction of CEA with insect receptors that probably led to disruption of cellular processes causing growth retardation and loss of fecundity of target insects. Thus, the present findings strengthen our current understanding of the entomotoxic potentiality of CEA, which will facilitate its future biotechnological applications.

Genetic animal models of preconditioning

The preconditioning phenomena have been well established in heart as well as brain. In this review, we detail some of the original studies on preconditioning as well as studies from our lab using rodents and a genetic model system (fruit fly). We have used Drosophila in our lab to solve some of the questions related to tolerance or susceptibility to hypoxia. We believe that these pro-survival strategies and genetic pathways help us understand some of the preconditioning mechanisms that protect the brain from ischemia or ischemia.

Heat stress impedes development and lowers fecundity of the brown planthopper Nilaparvata lugens (Stål)

This study investigated the effects of sub-lethal high temperatures on the development and reproduction of the brown plant hopper Nilaparvata lugens (Stål). When first instar nymphs were exposed at their ULT(50) (41.8°C) mean development time to adult was increased in both males and females, from 15.2±0.3 and 18.2±0.3 days respectively in the control to 18.7±0.2 and 19±0.2 days in the treated insects. These differences in development arising from heat stress experienced in the first instar nymph did not persist into the adult stage (adult longevity of 23.5±1.1 and 24.4±1.1 days for treated males and females compared with 25.7±1.0 and 20.6±1.1 days in the control groups), although untreated males lived longer than untreated females. Total mean longevity was increased from 38.8±0.1 to 43.4±1.0 days in treated females, but male longevity was not affected (40.9±0.9 and 42.2±1.1 days respectively). When male and female first instar nymphs were exposed at their ULT(50) of 41.8°C and allowed to mate on reaching adult, mean fecundity was reduced from 403.8±13.7 to 128.0±16.6 eggs per female in the treated insects. Following exposure of adult insects at their equivalent ULT(50) (42.5°C), the three mating combinations of treated male x treated female, treated male x untreated female, and untreated male x treated female produced 169.3±14.7, 249.6±21.3 and 233.4±17.2 eggs per female respectively, all significantly lower than the control. Exposure of nymphs and adults at their respective ULT(50) temperatures also significantly extended the time required for their progeny to complete egg development for all mating combinations compared with control. Overall, sub-lethal heat stress inhibited nymphal development, lowered fecundity and extended egg development time.

Can tropical insects stand the heat? A case study with the brown planthopper Nilaparvata lugens (Stål)

The brown planthopper Nilaparvata lugens (Stål) is the most serious pest of rice across the world, especially in tropical climates. N. lugens nymphs and adults were exposed to high temperatures to determine their critical thermal maximum (CT(max)), heat coma temperature (HCT) and upper lethal temperature (ULT). Thermal tolerance values differed between developmental stages: nymphs were consistently less heat tolerant than adults. The mean (± SE) CT(max) of nymphs and adult females and males were 34.9±0.3, 37.0±0.2 and 37.4±0.2°C respectively, and for the HCT were 37.7±0.3, 43.5±0.4 and 42.0±0.4°C. The ULT₅₀ values (± SE) for nymphs and adults were 41.8±0.1 and 42.5±0.1°C respectively. The results indicate that nymphs of N. lugens are currently living at temperatures close to their upper thermal limits. Climate warming in tropical regions and occasional extreme high temperature events are likely to become important limiting factors affecting the survival and distribution of N. lugens.

Protein expression following heat shock in the nervous system of Locusta migratoria

There is a thermal range for the operation of neural circuits beyond which nervous system function is compromised. Locusta migratoria is native to the semiarid regions of the world and provides an excellent model for studying neural phenomena. In this organism previous exposure to sublethal high temperatures (heat shock, HS) can protect neuronal function against future hyperthermia but, unlike many organisms, the profound physiological adaptations are not accompanied by a robust increase of Hsp70 transcript or protein in the nervous system. We compared Hsp70 increase following HS in the tissues of isolated and gregarious locusts to investigate the effect of population density. We also localized Hsp70 in the metathoracic ganglion (MTG) of gregarious locusts to determine if HS affects Hsp70 in specific cell types that could be masked in whole ganglion assays. Our study indicated no evidence of a consistent change in Hsp70 level in the MTG of isolated locusts following HS. Also, Hsp70 was mainly localized in perineurium, neural membranes and glia and prior HS had no effect on its density or distribution. Finally, we applied 2-D gels to study the proteomic profile of MTG in gregarious locusts following HS; although these experiments showed some changes in the level of ATP-synthase β isoforms, the overall amount of this protein was found unchanged following HS. We conclude that the constitutive level of Hsps in the tissues of locusts is high. Also the thermoprotective effect of HS on the nervous system might be mediated by post-translational modifications or protein trafficking.

Hyperthermic seizures and aberrant cellular homeostasis in Drosophila dystrophic muscles

In humans, mutations in the Dystrophin Glycoprotein Complex (DGC) cause muscular dystrophies (MDs) that are associated with muscle loss, seizures and brain abnormalities leading to early death. Using Drosophila as a model to study MD we have found that loss of Dystrophin (Dys) during development leads to heat-sensitive abnormal muscle contractions that are repressed by mutations in Dys's binding partner, Dystroglycan (Dg). Hyperthermic seizures are independent from dystrophic muscle degeneration and rely on neurotransmission, which suggests involvement of the DGC in muscle-neuron communication. Additionally, reduction of the Ca(2+) regulator, Calmodulin or Ca(2+) channel blockage rescues the seizing phenotype, pointing to Ca(2+) mis-regulation in dystrophic muscles. Also, Dys and Dg mutants have antagonistically abnormal cellular levels of ROS, suggesting that the DGC has a function in regulation of muscle cell homeostasis. These data show that muscles deficient for Dys are predisposed to hypercontraction that may result from abnormal neuromuscular junction signaling.

Ca²+ buffering at a drosophila larval synaptic terminal

A quantitative analysis of Ca²+ dynamics requires knowledge of the Ca²+-binding ratio (κ(S) ); this has not been measured at Drosophila synaptic terminals or any invertebrate synaptic terminal. We measured κ(S) at a Ib motor terminal in Drosophila larvae comparing single-AP Ca²+ transients in synaptic terminals that contained varying concentrations of the Ca²+ indicator, Oregon Green 488 BAPTA-1 (OGB-1). Using a linear single-compartment model, κ(S) was calculated based upon the effect of [OGB-1] on the time constant (τ(decay) ) for the decay of intracellular free Ca²+ concentration ([Ca²+](i)). This gave a κ(S) of 77 indicating that nearly 99% of entering Ca²+ is immediately bound by endogenous fast Ca²+ buffers. Extrapolation to zero [OGB-1] gave a τ(decay) of 46 ms and a Ca²+-removal rate constant of 1641 s⁻¹ for single APs. We calculated that a single AP produced an increase in [Ca²+](i) of 196 nM and an increase in the total intracellular [Ca²+](free + bound) of 15.3 μM for measurements made in 1.0 mM external Ca²+. The increase in [Ca²+](i) for AP trains was 185 nM/ 10 Hz; this gave a Ca²+ extrusion rate constant of 827 s⁻¹, which likely reflects the activity of the plasma membrane Ca²+ ATPase. Experiments were performed to examine the effect of altering external Ca²+ or Mg²+ on Ca²+ influx at these terminals.

Peptide-induced modulation of synaptic transmission and escape response in Drosophila requires two G-protein-coupled receptors

Neuropeptides are found in both mammals and invertebrates and can modulate neural function through activation of G-protein-coupled receptors (GPCRS). The precise mechanisms by which many of these GPCRs modulate specific signaling cascades to regulate neural function are not well defined. We used Drosophila melanogaster as a model to examine both the cellular and behavioral effects of DPKQDFMRFamide, the most abundant peptide encoded by the dFMRF gene. We show that DPKQDFMRFamide enhanced synaptic transmission through activation of two G-protein-coupled receptors, Fmrf Receptor (FR) and Dromyosupressin Receptor-2 (DmsR-2). The peptide increased both the presynaptic Ca(2+) response and the quantal content of released transmitter. Peptide-induced modulation of synaptic function could be abrogated by depleting intracellular Ca(2+) stores or by interfering with Ca(2+) release from the endoplasmic reticulum through disruption of either the ryanodine receptor or the inositol 1,4,5-trisphosphate receptor. The peptide also altered behavior. Exogenous DPKQDFMRFamide enhanced fictive locomotion; this required both the FR and DmsR-2. Likewise, both receptors were required for an escape response to intense light exposure. Thus, coincident detection of a peptide by two GPCRs modulates synaptic function through effects of Ca(2+)-induced Ca(2+) release, and we hypothesize that these mechanisms are involved in behavioral responses to environmental stress.

Multifaceted role of heat shock protein 70 in neurons

Heat shock protein 70 (Hsp70) plays important roles in neural protection from stress by assisting cellular protein folding. In this review we discuss the current understanding of inducible and constitutive Hsp70 in maintaining and protecting neuronal synaptic function under normal and stressed conditions.

The nitric oxide/cGMP pathway tunes the thermosensitivity of swimming motor patterns in Xenopus laevis tadpoles

We investigated the role of the nitric oxide (NO)/cGMP pathway in setting thresholds for failure and recovery during hyperthermic stress of the swimming central pattern generator of immobilized Xenopus tadpoles (stage 42). We recorded swimming motor patterns induced by tail skin stimulation (TS) (1 ms current pulse) or by bath application of 50 microm NMDA. Swimming rhythm frequency increased in a linear manner with increasing temperature. In the presence of the NO donor S-nitroso-N-acetylpenicillamine (SNAP), recovery from hyperthermic failure was greatly slowed, often taking longer than the duration of the experiment. Pharmacological activation of the NO/cGMP pathway using SNAP or 8-bromo-cGMP (1) decreased the duration of TS-evoked swim episodes; (2) decreased the temperature threshold for hyperthermic circuit failure; (3) decreased the temperature at which the circuit recovered; and (4) increased the time taken to recover. Pharmacological inhibition of the NO/cGMP pathway using the NO scavenger CPTIO, the nitric oxide synthase (NOS) inhibitor L-NAME or the guanylyl cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) had the opposite effects. NMDA rhythms were more resistant to hyperthermic failure than TS-evoked swim episodes, but the effects of SNAP on the temperature sensitivity of swimming evoked by NMDA were similar to those on TS-evoked swimming, suggesting that drug effects occur on central pattern-generating networks rather than sensory pathways. We conclude that the NO/cGMP pathway is involved in setting the threshold temperatures for hyperthermic failure and subsequent recovery of fictive swimming in tadpoles, and we suggest that this is part of a variable response to prevent overexcitation during abiotic stress under different environmental conditions.

Chronic lead exposure alters presynaptic calcium regulation and synaptic facilitation in Drosophila larvae

Prolonged exposure to inorganic lead (Pb(2+)) during development has been shown to influence activity-dependent synaptic plasticity in the mammalian brain, possibly by altering the regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)). To explore this possibility, we studied the effect of Pb(2+) exposure on [Ca(2+)](i) regulation and synaptic facilitation at the neuromuscular junction of larval Drosophila. Wild-type Drosophila (CS) were raised from egg stages through the third larval instar in media containing either 0 microM, 100 microM or 250 microM Pb(2+) and identified motor terminals were examined in late third-instar larvae. To compare resting [Ca(2+)](i) and the changes in [Ca(2+)](i) produced by impulse activity, the motor terminals were loaded with a Ca(2+) indicator, either Oregon Green 488 BAPTA-1 (OGB-1) or fura-2 conjugated to a dextran. We found that rearing in Pb(2+) did not significantly change the resting [Ca(2+)](i) nor the Ca(2+) transient produced in synaptic boutons by single action potentials (APs); however, the Ca(2+) transients produced by 10 Hz and 20 Hz AP trains were larger in Pb(2+)-exposed boutons and decayed more slowly. For larvae raised in 250 microM Pb(2+), the increase in [Ca(2+)](i) during an AP train (20 Hz) was 29% greater than in control larvae and the [Ca(2+)](i) decay tau was 69% greater. These differences appear to result from reduced activity of the plasma membrane Ca(2+) ATPase (PMCA), which extrudes Ca(2+) from these synaptic terminals. These findings are consistent with studies in mammals showing a Pb(2+)-dependent reduction in PMCA activity. We also observed a Pb(2+)-dependent enhancement of synaptic facilitation at these larval neuromuscular synapses. Facilitation of EPSP amplitude during AP trains (20 Hz) was 55% greater in Pb(2+)-reared larvae than in controls. These results showed that Pb(2+) exposure produced changes in the regulation of [Ca(2+)](i) during impulse activity, which could affect various aspects of nervous system development. At the mature synapse, this altered [Ca(2+)](i) regulation produced changes in synaptic facilitation that are likely to influence the function of neural networks.

Role of ATP-dependent calcium regulation in modulation of Drosophila synaptic thermotolerance

Maintenance of synaptic transmission requires regulation of intracellular Ca(2+) in presynaptic nerve terminals; loss of this regulation at elevated temperatures may cause synaptic failure. Accordingly, we examined the thermosensitivity of presynaptic calcium regulation in Drosophila larval neuromuscular junctions, testing for effects of disrupting calcium clearance. Motor neurons were loaded with the ratiometric Ca(2+) indicator Fura-dextran to monitor calcium regulation as temperature increased. Block of the Na(+)/Ca(2+) exchanger or removal of extracellular Ca(2+) prevented the normal temperature-induced increase in resting calcium. Conversely, two treatments that interfered with Ca(2+) clearance-inactivation of the endoplasmic reticulum Ca(2+)-ATPase with thapsigargin and inhibition of the plasma membrane Ca(2+)-ATPase with high pH-significantly accelerated the temperature-induced rise in resting Ca(2+) concentration and reduced the thermotolerance of synaptic transmission. Disrupting Ca(2+)-ATPase function by interfering with energy production also facilitated the temperature-induced rise in resting [Ca(2+)] and reduced thermotolerance of synaptic transmission. Conversely, fortifying energy levels with extra intracellular ATP extended the operating temperature range of both synaptic transmission and Ca(2+) regulation. In each of these cases, Ca(2+) elevations evoked by an electrical stimulation of the nerve (evoked Ca(2+) responses) failed when resting Ca(2+) remained >e 200 nM for several minutes. Failure of synaptic function was correlated with the release of intracellular calcium stores, and we provide evidence suggesting that release from the mitochondria disrupts evoked calcium responses and synaptic transmission. Thus the thermal limit of synaptic transmission may be directly linked to the stability of ATP-dependent mechanisms that regulate intracellular ion concentrations in the nerve terminal.

Differences in Ca2+ regulation for high-output Is and low-output Ib motor terminals in Drosophila larvae

We determined whether two classes of Drosophila larval motor terminals with known differences in structure and transmitter release also showed differences in Ca(2+) regulation. Larval motor neurons can be separated into those producing large synaptic boutons (Ib) and those with small boutons (Is). Ib terminals release less transmitter during single action potentials (APs) than Is terminals, but show greater facilitation during high-frequency stimulation. We measured Ca(2+) transients produced by single APs and AP trains after loading the terminals with the dextran-conjugated Ca(2+) indicator Oregon Green 488 BAPTA-1 (OGB-1). The two pairs of Is and Ib terminals innervating muscle fiber 4 and fibers 6 and 7 were examined. The OGB-1 concentrations were measured in order to compare measurements from terminals with similar OGB-1 loading. For single APs, the change in OGB-1 fluorescence (DeltaF/F) in Is boutons was significantly larger than in Ib boutons due to greater Ca(2+) influx per bouton volume. The Is boutons had greater surface area and active zone number per bouton volume than Ib boutons; this could account for the differences in Ca(2+) influx and argues for similar Ca(2+) influx at Is and Ib active zones. As previously reported for the Ib boutons, the distal Is boutons had larger single-AP Ca(2+) transients than proximal ones on muscle fibers 6 and 7, but not on fiber 4. This difference was not due to proximal-distal differences in surface area or active zones per bouton volume and may be due to greater Ca(2+) influx at distal active zones. During AP trains, the Is Ca(2+) transients were larger in amplitude and had longer decay time constants than Ib ones. This can be explained by a slower rate of Ca(2+) extrusion from the Is boutons apparently due to lower plasma membrane Ca(2+) ATPase activity at Is boutons compared to Ib boutons.