Quantal measurement and analysis methods compared for crayfish and Drosophila neuromuscular junctions, and rat hippocampus

Neuromuscular Transmission in a Biological Context

Neuromuscular transmission is the process by which motor neurons activate muscle contraction and thus plays an essential role in generating the purposeful body movements that aid survival. While many features of this process are common throughout the Animal Kingdom, such as the release of transmitter in multimolecular "quanta," and the response to it by opening ligand-gated postsynaptic ion channels, there is also much diversity between and within species. Much of this diversity is associated with specialization for either slow, sustained movements such as maintain posture or fast but brief movements used during escape or prey capture. In invertebrates, with hydrostatic and exoskeletons, most motor neurons evoke graded depolarizations of the muscle which cause graded muscle contractions. By contrast, vertebrate motor neurons trigger action potentials in the muscle fibers which give rise to all-or-none contractions. The properties of neuromuscular transmission, in particular the intensity and persistence of transmitter release, reflect these differences. Neuromuscular transmission varies both between and within individual animals, which often have distinct tonic and phasic subsystems. Adaptive plasticity of neuromuscular transmission, on a range of time scales, occurs in many species. This article describes the main steps in neuromuscular transmission and how they vary in a number of "model" species, including C. elegans , Drosophila , zebrafish, mice, and humans. © 2024 American Physiological Society. Compr Physiol 14:5641-5702, 2024.

Fluoxetine antagonizes the acute response of LPS: Blocks K2P channels

The channels responsible for maintaining resting membrane potential are known as K2P (two-P-domain K+ subunit) channels, a subset of which are known to be blocked by Fluoxetine. In this experiment, the compound's effects on the membrane potential were examined on muscles in larval Drosophila overexpressing a subtype of K2P channel (known in Drosophila as dORKA1 or ORKA1) and compared to larvae without overexpression. The compound was also observed in sequence and/or combination with a form of lipopolysaccharide (LPS) that transiently activates K2P channels. Different concentrations of Fluoxetine were tested, and it was also examined in cocktail with the LPS. At 25 μM Fluoxetine exposure, muscle in control larvae underwent depolarization, while muscles overexpressing K2P channels hyperpolarized; at 50 μM, however, much more variable responses were observed. The LPS caused hyperpolarization in both larval strains, but the effect was more transient in the Canton-S line than in the K2P overexpressors. Finally, LPS continued to cause hyperpolarization even in the presence of Fluoxetine, while Fluoxetine quickly depolarized the muscle during exposure to LPS. The cocktail showed a smaller effect on muscles overexpressing ORKA1 as compared to the controls, indicating that Fluoxetine does not block the ORKA1 subtype. This study is significant because it demonstrates how overexpression of K2P channels alters membrane response to LPS and Fluoxetine exposure.

Synapse-to-synapse plasticity variability balanced to generate input-wide constancy of transmitter release

Basal synaptic strength can vary greatly between synapses formed by an individual neuron because of diverse probabilities of action potential (AP) evoked transmitter release ( Pr ). Optical quantal analysis on large numbers of identified Drosophila larval glutamatergic synapses shows that short-term plasticity (STP) also varies greatly between synapses made by an individual type I motor neuron (MN) onto a single body wall muscle. Synapses with high and low P r and different forms and level of STP have a random spatial distribution in the MN nerve terminal, and ones with very different properties can be located within 200 nm of one other. While synapses start off with widely diverse basal P r at low MN AP firing frequency and change P r differentially when MN firing frequency increases, the overall distribution of P r remains remarkably constant due to a balance between the numbers of synapses that facilitate and depress as well as their degree of change and basal synaptic weights. This constancy in transmitter release can ensure robustness across changing behavioral conditions.

Neto proteins differentially modulate the gating properties of Drosophila NMJ glutamate receptors

The formation of functional synapses requires co-assembly of ion channels with their accessory proteins which controls where, when, and how neurotransmitter receptors function. The auxiliary protein Neto modulates the function of kainate-type glutamate receptors in vertebrates as well as at the Drosophila neuromuscular junction (NMJ), a glutamatergic synapse widely used for genetic studies on synapse development. We previously reported that Neto is essential for the synaptic recruitment and function of glutamate receptors. Here, using outside-out patch-clamp recordings and fast ligand application, we examine for the first time the biophysical properties of recombinant Drosophila NMJ receptors expressed in HEK293T cells and compare them with native receptor complexes of genetically controlled composition. The two Neto isoforms, Neto-α and Neto-β, differentially modulate the gating properties of NMJ receptors. Surprisingly, we found that deactivation is extremely fast and that the decay of synaptic currents resembles the rate of iGluR desensitization. The functional analyses of recombinant iGluRs that we report here should greatly facilitate the interpretation of compound in vivo phenotypes of mutant animals.

Examining the effect of iron (ferric) on physiological processes: Invertebrate models

Iron is a common and essential element for maintaining life in bacteria, plants and animals and is found in soil, fresh waters and marine waters; however, over exposure is toxic to organisms. Iron is used in electron transport complexes within mitochondria as well as a co-factor in many essential proteins. It is also established that iron accumulation in the central nervous system in mammals is associated with various neurological disorders. Ample studies have investigated the long-term effects of iron overload in the nervous system. However, its acute effects in nervous tissue and additional organ systems warrant further studies. This study investigates the effects of iron overload on development, behavior, survival, cardiac function, and glutamatergic synaptic transmission in the Drosophila melanogaster. Additionally, physiological responses in crayfish were examined following Fe3+ exposure. Fe3+ reduced neuronal excitability in proprioceptive neurons in a crayfish model. Thus, Fe3+ may block stretch activated channels (SACs) as well as voltage-gated Na+ channels. Exposure also rapidly reduces synaptic transmission but does not block ionotropic glutamatergic receptors, suggesting a blockage of pre-synaptic voltage-gated Ca2+ channels in both crustacean and Drosophila models. The effects are partly reversible with acute exposure, indicating the cells are not rapidly damaged. This study is relevant in demonstrating the effects of Fe3+ on various physiological functions in different organisms in order to further understand the acute and long-term consequences of overload.

Active zone structure-function relationships at the neuromuscular junction

The impact of presynaptic transmitter release site organization on synaptic function has been a vibrant area of research for synaptic physiologists. Because there is a highly nonlinear relationship between presynaptic calcium influx and subsequent neurotransmitter release at synapses, the organization and density of calcium sources (voltage-gated calcium channels [VGCCs]) relative to calcium sensors located on synaptic vesicles is predicted to play a major role in shaping the dynamics of neurotransmitter release at a synapse. Here we review the history of structure-function studies within transmitter release sites at the neuromuscular junction across three model preparations in an effort to discern the relationship between VGCC organization and synaptic function, and whether that organizational structure imparts evolutionary advantages for each species.

Focal Macropatch Recordings of Synaptic Currents from the Drosophila Larval Neuromuscular Junction

Drosophila neuromuscular junction (NMJ) is an excellent model system to study glutamatergic synaptic transmission. We describe the technique of focal macropatch recordings of synaptic currents from visualized boutons at the Drosophila larval NMJ. This technique requires customized fabrication of recording micropipettes, as well as a compound microscope equipped with a high magnification, long-distance water immersion objective, differential interference contrast (DIC) optics, and a fluorescent attachment. The recording electrode is positioned on the top of a selected synaptic bouton visualized with DIC optics, epi-fluorescence, or both. The advantage of this technique is that it allows monitoring the synaptic activity of a limited number of sites of release. The recording electrode has a diameter of several microns, and the release sites positioned outside of the electrode rim do not significantly affect the recorded currents. The recorded synaptic currents have fast kinetics and can be readily resolved. These advantages are especially important for the studies of mutant fly lines with enhanced spontaneous or asynchronous synaptic activity.

Electrophysiological analysis of synaptic transmission in Drosophila

Synaptic transmission is dynamic, plastic, and highly regulated. Drosophila is an advantageous model system for genetic and molecular studies of presynaptic and postsynaptic mechanisms and plasticity. Electrical recordings of synaptic responses represent a wide-spread approach to study neuronal signaling and synaptic transmission. We discuss experimental techniques that allow monitoring synaptic transmission in Drosophila neuromuscular and central systems. Recordings of synaptic potentials or currents at the larval neuromuscular junction (NMJ) are most common and provide numerous technical advantages due to robustness of the preparation, large and identifiable muscles, and synaptic boutons which can be readily visualized. In particular, focal macropatch recordings combined with the analysis of neurosecretory quanta enable rigorous quantification of the magnitude and kinetics of transmitter release. Patch-clamp recordings of synaptic transmission from the embryonic NMJ enable overcoming the problem of lethality in mutant lines. Recordings from the adult NMJ proved instrumental in the studies of temperature-sensitive paralytic mutants. Genetic studies of behavioral learning in Drosophila compel an investigation of synaptic transmission in the central nervous system (CNS), including primary cultured neurons and an intact brain. Cholinergic and GABAergic synaptic transmission has been recorded from the Drosophila CNS both in vitro and in vivo. In vivo patch-clamp recordings of synaptic transmission from the neurons in the olfactory pathway is a very powerful approach, which has a potential to elucidate how synaptic transmission is associated with behavioral learning. WIREs Dev Biol 2017, 6:e277. doi: 10.1002/wdev.277 For further resources related to this article, please visit the WIREs website.

Maximum-likelihood q-estimator uncovers the role of potassium at neuromuscular junctions

Recently, we demonstrated the existence of nonextensive behavior in neuromuscular transmission (da Silva et al. in Phys Rev E 84:041925, 2011). In this letter, we first obtain a maximum-likelihood q-estimator to calculate the scale factor ([Formula: see text]) and the q-index of q-Gaussian distributions. Next, we use the indexes to analyze spontaneous miniature end plate potentials in electrophysiological recordings from neuromuscular junctions. These calculations were performed assuming both normal and high extracellular potassium concentrations [Formula: see text]. This protocol was used to test the validity of Tsallis statistics under electrophysiological conditions closely resembling physiological stimuli. The analysis shows that q-indexes are distinct depending on the extracellular potassium concentration. Our letter provides a general way to obtain the best estimate of parameters from a q-Gaussian distribution function. It also expands the validity of Tsallis statistics in realistic physiological stimulus conditions. In addition, we discuss the physical and physiological implications of these findings.

Synaptic bouton properties are tuned to best fit the prevailing firing pattern

The morphology of presynaptic specializations can vary greatly ranging from classical single-release-site boutons in the central nervous system to boutons of various sizes harboring multiple vesicle release sites. Multi-release-site boutons can be found in several neural contexts, for example at the neuromuscular junction (NMJ) of body wall muscles of Drosophila larvae. These NMJs are built by two motor neurons forming two types of glutamatergic multi-release-site boutons with two typical diameters. However, it is unknown why these distinct nerve terminal configurations are used on the same postsynaptic muscle fiber. To systematically dissect the biophysical properties of these boutons we developed a full three-dimensional model of such boutons, their release sites and transmitter-harboring vesicles and analyzed the local vesicle dynamics of various configurations during stimulation. Here we show that the rate of transmission of a bouton is primarily limited by diffusion-based vesicle movements and that the probability of vesicle release and the size of a bouton affect bouton-performance in distinct temporal domains allowing for an optimal transmission of the neural signals at different time scales. A comparison of our in silico simulations with in vivo recordings of the natural motor pattern of both neurons revealed that the bouton properties resemble a well-tuned cooperation of the parameters release probability and bouton size, enabling a reliable transmission of the prevailing firing-pattern at diffusion-limited boutons. Our findings indicate that the prevailing firing-pattern of a neuron may determine the physiological and morphological parameters required for its synaptic terminals.

Serotonin and synaptic transmission at invertebrate neuromuscular junctions

The serotonergic system in vertebrates and invertebrates has been a focus for over 50 years and will likely continue in the future. Recently, genomic analysis and discovery of alternative splicing and differential expression in tissues have increased the knowledge of serotonin (5-HT) receptor types. Comparative studies can provide useful insights to the wide variety of mechanistic actions of 5-HT responsible for behaviors regulated or modified by 5-HT. To determine cellular responses and influences on neural systems as well as the efferent control of behaviors by the motor units, preparations amenable to detailed studies of synapses are beneficial as working models. The invertebrate neuromuscular junctions (NMJs) offer some unique advantages for such investigations; action of 5-HT at crustacean NMJs has been widely studied, and leech and Aplysia continue to be key organisms. However, there are few studies in insects likely due to the focus in modulation within the CNS and lack of evidence of substantial action of 5-HT at the Drosophila NMJs. There are only a few reports in gastropods and annelids as well as other invertebrates. In this review we highlight some of the key findings of 5-HT actions and receptor types associated at NMJs in a variety of invertebrate preparations in hopes that future studies will build on this knowledge base.

Quantal Analysis of Endplate Potentials in Mouse Flexor Digitorum Brevis Muscle

The isolated flexor digitorum brevis (FDB) muscle from mice is extremely well suited to rapid acquisition of data and analysis of neurotransmitter release and action at neuromuscular junctions, because the muscle and its tibial nerve supply are simple to dissect and its constituent muscle fibers are short (<1 mm) and isopotential along their length. Methods are described here for dissection of FDB, stimulation of the tibial nerve, microelectrode recording from individual muscle fibers, and quantal analysis of endplate potentials (EPPs) and miniature endplate potentials (MEPPs). Curr. Protoc. Mouse Biol. 1:429-444 © 2011 by John Wiley & Sons, Inc.

Physiological recordings of high and low output NMJs on the crayfish leg extensor muscle

We explain in detail how to expose and conduct electrophysiological recordings of synaptic responses for high (phasic) and low (tonic) output motor neurons innervating the extensor muscle in the walking leg of a crayfish. Distinct differences are present in the physiology and morphology of the phasic and tonic nerve terminals. The tonic axon contains many more mitochondria, enabling it to take a vital stain more intensely than the phasic axon. The tonic terminals have varicosities, and the phasic terminal is filiform. The tonic terminals are low in synaptic efficacy but show dramatic facilitated responses. In contrast, the phasic terminals are high in quantal efficacy but show synaptic depression with high frequency stimulation. The quantal output is measured with a focal macropatch electrode placed directly over the visualized nerve terminals. Both phasic and tonic terminals innervate the same muscle fibers, which suggests that inherent differences in the neurons, rather than differential retrograde feedback from the muscle, account for the morphological and physiological differentiation.

Structure/function assessment of synapses at motor nerve terminals

The release of transmitter at neuromuscular junctions (NMJ) of the opener muscle in crayfish is quantal in nature. This NMJ offers the advantage of being able to record quantal events at specific visually identified release sites, thus allowing measurement of the physiological parameters of vesicle release and its response to be directly correlated with synaptic structure. These experiments take advantage of areas between the varicosities on the nerve terminal that we define as "stems." Stems were chosen as the region to study because of their low synaptic output due to fewer synaptic sites. Through 3D reconstruction from hundreds of serial sections, obtained by transmission electron microscopy (TEM), at a site in which focal macropatch recordings were obtained, the number of synapses and AZs are revealed. Thus, physiological profiles with various stimulation conditions can be assessed in regards to direct synaptic structure. Here, we used the properties of the quantal shape to determine if distinct subsets of quantal signatures existed and if differences in the distributions are present depending on the frequency of stimulation. Such a quantal signature could come about by parameters of area, rise time, peak amplitude, latency, and tau decay. In this study, it is shown that even at defined sites on the stem, with few active zones, synaptic transmission is still complex and the quantal responses appear to be variable even for a given synapse over time. In this study, we could not identify a quantal signature for the conditions utilized.

Different mechanisms of Ca2+ regulation that influence synaptic transmission: comparison between crayfish and Drosophila neuromuscular junctions

A brief historical background on synaptic transmission in relation to Ca(2+) dynamics and short-term facilitation is described. This study focuses on the mechanisms responsible for the regulation of intracellular calcium concentration ([Ca(2+)](i)) in high output terminals of larval Drosophila compared to a low-output terminal of the crayfish neuromuscular junction (NMJ). Three processes; plasmalemmal Na(+)/Ca(2+) exchanger [NCX], Ca(2+)-ATPase (PMCA), and sarcoplasmic/endoplasmic Ca(2+)-ATPase (SERCA) are important in regulating the [Ca(2+)](i) are examined. When the NCX is compromised by reduced [Na(+)](o), no consistent effect occurred; but a NCX blocker KB-R7943 decreased the excitatory postsynaptic potential (EPSP) amplitudes. Compromising the PMCA with pH 8.8 resulted in an increase in EPSP amplitude but treatment with a PMCA specific inhibitor carboxyeosin produced opposite results. Thapsigargin exposure to block the SERCA generally decreases EPSP amplitude. Compromising the activity of the above Ca(2+) regulating proteins had no substantial effects on short-term depression. The Kum(170TS) strain (with dysfunctional SERCA), showed a decrease in EPSP amplitudes including the first EPSP within the train. Synaptic transmission is altered by reducing the function of the above three [Ca(2+)](i) regulators; but they are not consistent among different species as expected. Results in crayfish NMJ were more consistent with expected results as compared to the Drosophila NMJ. It is predicated that different mechanisms are used for regulating the [Ca(2+)](i) in high and low output synaptic terminals.

Historical view and physiology demonstration at the NMJ of the crayfish opener muscle

Here we present some of the key important discoveries made with the opener neuromuscular (NMJ) preparation of crustaceans and illustrate that there is still much to learn from this model preparation. In understanding the history one can appreciate why even today this NMJ still offers a rich playground to address questions regarding pre- and post-synaptic function and plasticity. The viability and ease of access to the terminal for intracellular as well as extracellular electrophysiology and imaging are significant advantages. The mechanisms behind the modulation of vesicular kinetics and fusion within the high- and low-output terminals are begging for investigation. The preparation also offers a testable model system for computational assessments and manipulations to examine key variables in theoretical models of synaptic function, for example calcium dynamics during short-term facilitation. The synaptic complexity of active zone and statistical nature of quantal release is also an open area for future investigation both experimentally and computationally.

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

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

A general model of synaptic transmission and short-term plasticity

Some synapses transmit strongly to action potentials (APs), but weaken with repeated activation; others transmit feebly at first, but strengthen with sustained activity. We measured synchronous and asynchronous transmitter release at "phasic" crayfish neuromuscular junctions (NMJs) showing depression and at facilitating "tonic" junctions, and define the kinetics of depression and facilitation. We offer a comprehensive model of presynaptic processes, encompassing mobilization of reserve vesicles, priming of docked vesicles, their association with Ca(2+) channels, and refractoriness of release sites, while accounting for data on presynaptic buffers governing Ca(2+) diffusion. Model simulations reproduce many experimentally defined aspects of transmission and plasticity at these synapses. Their similarity to vertebrate central synapses suggests that the model might be of general relevance to synaptic transmission.

Influence of PCPA and MDMA (ecstasy) on physiology, development and behavior in Drosophila melanogaster

The effects of para-chlorophenylalanine (PCPA) and 3,4 methylenedioxy-methamphetamine (MDMA, 'ecstasy') were investigated in relation to development, behavior and physiology in larval Drosophila. PCPA blocks the synthesis of serotonin (5-HT) and MDMA is known to deplete 5-HT in mammalian neurons; thus these studies were conducted primarily to target the serotonergic system. Treatment with PCPA and MDMA delayed time to pupation and eclosion. The developmental rate was investigated with a survival analysis statistical approach that is unique for Drosophila studies. Locomotion and eating were reduced in animals exposed to MDMA or PCPA. Sensitivity to exogenously applied 5-HT on an evoked sensory-central nervous system (CNS)-motor circuit showed that the CNS is sensitive to 5-HT but that when depleted of 5-HT by PCPA a decreased sensitivity occurred. A diet with MDMA produced an enhanced response to exogenous 5-HT on the central circuit. Larvae eating MDMA from the first to third instar did not show a reduction in 5-HT within the CNS; however, eating PCPA reduced 5-HT as well as dopamine content as measured by high performance liquid chromatography from larval brains. As the heart serves as a good bioindex of 5-HT exposure, it was used in larvae fed PCPA and MDMA but no significant effects occurred with exogenous 5-HT. In summary, the action of these pharmacological compounds altered larval behaviors and development. PCPA treatment changed the sensitivity in the CNS to 5-HT, suggesting that 5-HT receptor regulation is modulated by neural activity of the serotonergic neurons. The actions of acute MDMA exposure suggest a 5-HT agonist action or possible dumping of 5-HT from neurons.

Making quantal analysis more convenient, fast, and accurate: user-friendly software QUANTAN

Quantal analysis of synaptic transmission is an important tool for understanding the mechanisms of synaptic plasticity and synaptic regulation. Although several custom-made and commercial algorithms have been created for the analysis of spontaneous synaptic activity, software for the analysis of action potential evoked release remains very limited. The present paper describes a user-friendly software package QUANTAN which has been created to analyze electrical recordings of postsynaptic responses. The program package is written using Borland C++ under Windows platform. QUANTAN employs and compares several algorithms to extract the average quantal content of synaptic responses, including direct quantal counts, the analysis of synaptic amplitudes, and the analysis of integrated current traces. The integration of several methods in one user-friendly program package makes quantal analysis of action potential evoked release more reliable and accurate. To evaluate the variability in quantal content, QUANTAN performs deconvolution of the distributions of amplitudes or areas of synaptic responses employing a ridge regression method. Other capabilities of QUANTAN include the analysis of the time-course and stationarity of quantal release. In summary, QUANTAN uses digital records of synaptic responses as an input and computes the distribution of quantal content and synaptic parameters. QUANTAN is freely available to other scholars over the internet.

Synapsin maintains the reserve vesicle pool and spatial segregation of the recycling pool in Drosophila presynaptic boutons

We employed optical detection of the lipophylic dye FM1-43 and focal recordings of quantal release to investigate how synapsin affects vesicle cycling at the neuromuscular junction of synapsin knockout (Syn KO) Drosophila. Loading the dye employing high K+ stimulation, which presumably involves the recycling pool of vesicles in exo/endocytosis, stained the periphery of wild type (WT) boutons, while in Syn KO the dye was redistributed towards the center of the bouton. When endocytosis was promoted by cyclosporin A pretreatment, the dye uptake was significantly enhanced in WT boutons, and the entire boutons were stained, suggesting staining of the reserve vesicle pool. In Syn KO boutons, the same loading paradigm produced fainter staining and significantly faster destaining. When the axon was stimulated electrically, a distinct difference in dye loading patterns was observed in WT boutons at different stimulation frequencies: a low stimulation frequency (3 Hz) produced a ring-shaped staining pattern, while at a higher frequency (10 Hz) the dye was redistributed towards the center of the bouton and the fluorescence intensity was significantly increased. This difference in staining patterns was essentially disrupted in Syn KO boutons, although synapsin did not affect the rate of quantal release. Stimulation of the nerve in the presence of bafilomycin, the blocker of the transmitter uptake, produced significantly stronger depression in Syn KO boutons. These results, taken together, suggest that synapsin maintains the reserve pool of vesicles and segregation between the recycling and reserve pools, and that it mediates mobilization of the reserve pool during intense stimulation.

Self-modeling structure of evoked postsynaptic potentials

With the simplicity of the synaptic structure and physiology at neuromuscular junctions (NMJs) of crayfish and the given transmitter being released in quantal packets, a detailed assessment in the fundamental processes of chemical synaptic transmission is possible. Since the quantal event is the basic element of transmission, we consider an approach to further understand the characteristics of quantal responses. In this study, we introduce a method for combining information across excitatory postsynaptic potentials (EPSPs) that are quantal in nature. The method is called self-modeling regression, known in the statistics literature as SEMOR. This method illustrates that the differing timing and heights of EPSPs can be described with four coefficients measuring affine (shift and scale) transformations of the x and y axes. We demonstrate that this relationship allows us to provide a unified schema for the many functionals currently used in the literature, such as peak amplitude, tau, latency, area under the curve, or decay time. Computer code in R is available on the internet to perform the analysis.

Calcium sensitivity of neurotransmitter release differs at phasic and tonic synapses

The efficacy of synaptic transmission varies greatly among synaptic contacts. We have explored the origins of differences between phasic and tonic crustacean neuromuscular junctions. Synaptic boutons of a phasic motor neuron release three orders of magnitude more quanta to a single action potential and show strong depression to a train, whereas tonic synapses are nearly unresponsive to single action potentials and display an immense facilitation. Phasic and tonic synapses display a similar nonlinear dependence on extracellular [Ca2+]. We imposed similar spatially uniform intracellular [Ca2+] ([Ca2+]i) steps in phasic and tonic synapses by photolysis of presynaptic caged calcium. [Ca2+]i was measured fluorometrically while transmitter release was monitored electrophysiologically from single boutons in which the [Ca2+]i was elevated. Phasic synapses released the readily releasable pool (RRP) of vesicles at a much higher rate and with a shorter delay than did tonic synapses. Comparison of several kinetic models of molecular events showed that a difference in Ca2+-sensitive priming of vesicles in the RRP combined with a revision of the kinetic Ca2+-binding sequence to the secretory trigger produced the best fit to the markedly different responses to Ca2+ steps and action potentials and of the characteristic features of synaptic plasticity in phasic and tonic synapses. The results reveal processes underlying one aspect of synaptic diversity that may also regulate changes in synaptic strength during development and learning and memory formation.

Developmental consequences of neuromuscular junctions with reduced presynaptic calcium channel function

Evoked neurotransmitter release at the Drosophila neuromuscular junction (NMJ) is regulated by the amount of calcium influx at the presynaptic nerve terminal, as for most chemical synapses. Calcium entry occurs via voltage-gated calcium channels. The temperature-sensitive Drosophila mutant, cac(TS2), has a reduced amount of calcium entry during evoked stimulation. We have used this mutation to examine homeostatic regulatory mechanisms during development of the NMJ on muscle 6 within the developing larva. The amplitude of the excitatory postsynaptic potentials are reduced for both the Ib and Is motor neurons in 3rd instar larvae which have been raised at 33 degrees C from the 1st instar stage. Larvae raised at 25 degrees C and larvae pulsed at 33 degrees C from the late 2nd instar for various lengths of time show a reduced synaptic efficacy as a 3rd instar. The results indicate that the nerve terminal cannot fully compensate physiologically in the regulation of synaptic transmission during larval life for a reduced amount of evoked calcium entry. Morphological comparisons of Ib and Is terminals in relation to length and numbers of varicosities are significantly reduced in cac(TS2), which also suggests a lack in homeostatic ability. These findings are relevant since many deficits in synaptic transmission in various systems are compensated for either physiologically or structural over development, but not in this case for reduced calcium entry during evoked transmission.

Regulation of synaptic vesicles pools within motor nerve terminals during short-term facilitation and neuromodulation

The reserve pool (RP) and readily releasable pool (RRP) of synaptic vesicles within presynaptic nerve terminals were physiologically differentiated into distinctly separate functional groups. This was accomplished in glutamatergic nerve terminals by blocking the glutamate transporter with dl-threo-beta-benzyloxyaspartate (TBOA; 10 microM) during electrical stimulation with either 40 Hz of 10 pulses within a train or 20- or 50-Hz continuous stimulation. The 50-Hz continuous stimulation decreased the excitatory postsynaptic potential amplitude 60 min faster than for the 20-Hz continuous stimulation in the presence of TBOA (P < 0.05). There was no significant difference between the train stimulation and 20-Hz continuous stimulation in the run-down time in the presence of TBOA. After TBOA-induced synaptic depression, the excitatory postsynaptic potentials were rapidly (<1 min) revitalized by exposure to serotonin (5-HT, 1 microM) in every preparation tested (P < 0.05). At this glutamatergic nerve terminal, 5-HT promotes an increase probability of vesicular docking and fusion. Quantal recordings made directly at nerve terminals revealed smaller quantal sizes with TBOA exposure with a marked increase in quantal size as well as a continual appearance of smaller quanta upon 5-HT treatment after TBOA-induced depression. Thus 5-HT was able to recruit vesicles from the RP that were not rapidly depleted by acute TBOA treatment and electrical stimulation. The results support the notion that the RRP is selectively activated during rapid electrical stimulation sparing the RP; however, the RP can be recruited by the neuromodulator 5-HT. This suggests at least two separate kinetic and distinct regulatory paths for vesicle recycling within the presynaptic nerve terminal.

Characterization of development, behavior and neuromuscular physiology in the phorid fly, Megaselia scalaris

The Phoridae is known as 'scuttle flies' because they walk in rapid bursts of movement with short pauses between. In this study, larval locomotive behavior and development was characterized in the phorid, Megaselia scalaris. Comparison was made with the well-characterized fruit fly model, Drosophila melanogaster. Developmentally, the rate of maturation was consistently slower for Megaselia than Drosophila. This disparity was exaggerated at lower temperatures, particularly during larval development. In addition to slower growth, movements in Megaselia were also slower, as evidenced by reduced rates of larval body wall contractions and mouth hook movements. Megaselia larvae also displayed a unique behavior of swallowing air when exposed to a small pool of liquid. This permitted floating upon immersion and, therefore, might prevent drowning in the natural environment. The anatomical and physiological properties of a neuromuscular junction in the phorid larvae were also examined. The innervation of the motor nerve terminals on the ventral abdominal muscle (m6) is innervated by Type Ib and Is axons, similar to Drosophila. As in Drosophila, the Is terminals produce larger excitatory postsynaptic potentials (EPSPs) than the Ib. The amplitudes of the EPSPs in M. scalaris were reduced compared to those of D. melanogaster, but unlike D. melanogaster the EPSPs showed marked facilitation when stimulated with a 20 Hz train. We conclude that there may be differences in synaptic structure of the nerve terminals that could account for the different electrophysiological behaviors.

Actions of MDMA at glutamatergic neuromuscular junctions

3,4-Methylenedioxymethamphetamine (MDMA, "Ecstasy") compels mammalian serotonergic neurons to release serotonin (5-HT). In this study, MDMA altered synaptic transmission presynaptically by enhancing quantal release in two model glutamatergic synapses-the neuromuscular junction (NMJ) of the crayfish opener muscle, which is enhanced by exogenous 5-HT application, and the NMJ of a larval body wall muscle in Drosophila melanogaster, which is insensitive to exogenous 5-HT application. At the crayfish NMJ, MDMA mimicked the actions of 5-HT but only at a substantially higher concentration. At the Drosophila NMJ, MDMA altered synaptic transmission but not through a 5-HT receptor. Using simple invertebrate preparations, we have demonstrated an additional non-serotonergic mechanism of MDMA activity that has not yet been addressed in vertebrate systems and that may play an important role in understanding the mechanism of action for a commonly abused drug.

5-HT offsets homeostasis of synaptic transmission during short-term facilitation

In this study, we approach the topic of vesicle recruitment and recycling by perturbing neurotransmission at the crayfish neuromuscular junction with altered electrical activity and the presence of the neuromodulator serotonin (5-HT). After induction of short-term facilitation (STF) with stimulus pulse trains (40 Hz, 20 pulses), the amount of synaptic transmission can be maintained at a relatively constant level, producing a plateau in the amplitude of the excitatory postsynaptic potentials (EPSPs) throughout the remaining stimuli within a train of a few hundred milliseconds. With an increase in the frequency of the stimuli within a train (60 Hz, 20 pulses), an altered plateau of larger EPSP amplitudes occurs. This suggests that differential rates of vesicle recruitment can be rapidly reached and maintained. Exposure of nerve terminals to 5-HT further enhances the EPSP amplitudes to yet a higher plateau level. The effect of 5-HT is more pronounced for 40-Hz pulse trains than for 60-Hz trains. This suggests that 5-HT can recruit vesicles into the readily releasable pool (RRP) and that the recruitment is limited at higher stimulation frequencies. The attainment of a larger amplitude in the plateaus of the EPSPs at 60 Hz compared with 40 Hz also suggests that the rapid induction of STF enhances the entry of vesicles into the RRP. By direct quantal counts, mean quantal content increases linearly during STF, and 5-HT offsets the linear release. We propose that 5-HT and electrically induced recruitment of vesicles from a reserve pool to the RRP may share similar recruitment mechanisms.

Modulation of sensory-CNS-motor circuits by serotonin, octopamine, and dopamine in semi-intact Drosophila larva

We have introduced an in-situ preparation to induce motor unit activity by stimulating a sensory-CNS circuit, using the third instar larvae of Drosophila melanogaster. Discrete identifiable motor units that are well defined in anatomic and physiologic function can be recruited selectively and driven depending on the sensory stimulus intensity, duration, and frequency. Since the peripheral nervous system is bilaterally symmetric to coordinate bilateral symmetric segmental musculature patterns, fictive forms of locomotion are able to be induced. Monitoring the excitatory postsynaptic potentials (EPSP) on the prominent ventral longitudinal body wall muscles, such as m6 and m12, provides additional insight into how the selective motor units might be recruited within intact animals. We also introduce the actions of the neuromodulators (serotonin, octopamine (OA) and dopamine (DA)) on the inducible patterns of activity within the sensory-motor circuit. The powerful genetic manipulation in Drosophila opens many avenues for further investigations into the circuitry and cellular aspects of pattern generation and developmental issues of circuitry formation and maintenance in the model organism.

Modulatory effects of melatonin on behavior, hemolymph metabolites, and neurotransmitter release in crayfish

Melatonin affects a variety of circadian processes such as behavior and neurotransmitter release in vertebrates. Crayfish melatonin production occurs in the eyestalks, and the cycle of production may change seasonally. To date, however, melatonin's roles and mechanisms of action in crustacean physiology are unclear. We injected melatonin or saline into crayfish in scotophase and monitored activity and hemolymph glucose/lactate over 24 h in early spring. Crayfish were significantly more active in photophase versus the expected scotophase, and had concurrent glucose/lactate peaks. Melatonin reversed the activity pattern, causing a scotophase activity peak, but not the glucose/lactate patterns. This study was repeated in late summer, during which control activity and glucose/lactate levels were elevated in scotophase. Melatonin decreased the amplitude of scotophase activity and glucose/lactate, eliminating activity and glucose cycles. We also injected melatonin or saline at various times of day in early summer and monitored locomotor activity for 1 h. Controls had high activity at 1200 (mid-photophase) and 2100 h (early scotophase), and melatonin increased activity at 1200 h but decreased it at 2100 h. Melatonin also increased activity at 1500 h but not 1800 h (late photophase). Next, we examined the influence of melatonin on crayfish neurophysiology. Melatonin (10 microM) enhanced synaptic transmission at the neuromuscular junction (NMJ). The presynaptic action resulted in more vesicles being released during evoked stimulation. Our study indicates that melatonin may have a phylogenetically conserved role in the transduction of circadian information in invertebrates as in vertebrates. Behavioral and physiological effects may be mediated by modulation of central pathways, enhanced at the peripheral level via neuromodulation of the NMJ.

Effects of m-CPP in altering neuronal function: blocking depolarization in invertebrate motor and sensory neurons but exciting rat dorsal horn neurons

The compound m-chlorophenylpiperazine (m-CPP) is used clinically to manipulate serotonergic function, though its precise mechanisms of actions are not well understood. m-CPP alters synaptic transmission and neuronal function in vertebrates by non-selective agonistic actions on 5-HT(1) and 5-HT(2) receptors. In this study, we demonstrated that m-CPP did not appear to act through a 5-HT receptor in depressing neuronal function in the invertebrates (crayfish and Drosophila). Instead, m-CPP likely decreased sodium influx through voltage-gated sodium channels present in motor and primary sensory neurons. Intracellular axonal recordings showed that m-CPP reduced the amplitude of the action potentials in crayfish motor neurons. Quantal analysis of excitatory postsynaptic currents, recorded at neuromuscular junctions (NMJ) of crayfish and Drosophila, indicated a reduction in the number of presynaptic vesicular events, which produced a decrease in mean quantal content. m-CPP also decreased activity in primary sensory neurons in the crayfish. In contrast, serotonin produces an increase in synaptic strength at the crayfish NMJ and an increase in activity of sensory neurons; it produces no effect at the Drosophila NMJ. In the rat spinal cord, m-CPP enhances the occurrence of spontaneous excitatory postsynaptic potentials with no alteration in evoked currents.

Intrinsic differences in sensitivity to 5-HT between high- and low-output terminals innervating the same target

The differential action of neuromodulators on synapses of various efficacy provides additional fine tuning of synaptic regulation beyond frequency induced plasticity. We used the well-characterized high- and low-output motor nerve terminals, of the tonic and phasic neuromuscular junctions (NMJs) in the walking leg extensor muscle of the crayfish, to investigate differential actions of serotonin (5-HT) since both terminals innervate the same target. The excitatory postsynaptic potentials of the tonic NMJ are enhanced to a greater extent than for the phasic NMJs during exposure to 5-HT (100 nM). Macropatch current recordings at identified sites along the motor nerve terminals and quantal analysis indicate that mean quantal content is substantially increased by 5-HT. The overall probability of vesicular release increases to a greater extent at tonic terminals than at phasic terminals when exposed to 100 nM 5-HT. Measures in the area (i.e. charge) of spontaneous quantal currents indicate no difference in postsynaptic receptivity to the glutamatergic synaptic transmission upon exposure to 5-HT. The results provide new details concerning differential modulation of low- and high-output synapses present on the same target tissue.

Long-term in vitro maintenance of neuromuscular junction activity of Drosophila larvae

The larval Drosophila neuromuscular junction (NMJ) has proven to be an excellent system to test fundamental aspects of synaptic transmission, such as relationships among ion channel function, subtypes of glutamate receptors, and the functions of synaptic proteins in the presynaptic compartment. Recent advances in understanding bi-directional communication between nerves and muscles of Drosophila are helping uncover developmental as well as maintenance cues that could be applicable to all chemical synapses. The development of HL3 medium makes it possible to record synaptic responses at NMJs for prolonged periods of time. We demonstrate that media commonly used to culture CNS neurons and imaginal disks of Drosophila such as Schneider's and M3 completely block glutamatergic synaptic transmission at the NMJ. The depressed postsynaptic excitatory junction potentials (EJPs) partially recover from exposure to such media shortly after switching to the HL3 medium. Preliminary results from NMJs of filleted 3rd instar larvae for 4 days in vitro bathed in a modified HL3 medium show great promise. The resting membrane potential and the EJP amplitudes after 4 days in vitro are normal. These results demonstrate the possibility for chronic studies of developmental regulation in culture, which in some cases are impractical in the whole animal.

Development of Drosophila larval neuromuscular junctions: maintaining synaptic strength

In spite of the available information about the development of Drosophila neuromuscular junctions, the correlation between nerve terminal morphology and maintenance of synaptic strength has still not been systematically addressed throughout larval development. We characterized the growth of the abdominal longitudinal muscle 6 (m6) and the motor terminals Ib and Is that innervate it within segment 4. In addition, we measured the evoked excitatory junction potential (EJP) amplitudes while the Ib and Is axons were selectively recruited. Regression analysis with natural log transformation of response variables indicated that the developmental curves for m6 and the motor axons Ib and Is were best fitted as second order polynomial regressions during larval development. Initially Is terminals are longer and possess more synaptic varicosities at the first instar stage. The Is terminals also grow faster in subsequent developmental stages. The growth of nerve terminals and their target m6 are not proportional although tightly correlated. This results in a larger average muscle area innervated by a single varicosity as the animal develops. The amplitudes of the EJPs of Ib and Is neurons show no developmental difference in their amplitudes from the first to the late third larval instar. The Is axon consistently produced larger EJPs than the Ib axon at each developmental stage. The time constants for both rising and decay phases of EJPs increase exponentially throughout larval development. The results presented not only help in quantifying the normal development of Drosophila neuromuscular junctions, but also provide a framework for future investigations to properly interpret developmental abnormalities that may occur in various mutants.

Estimating the number of release sites and probability of firing within the nerve terminal by statistical analysis of synaptic charge

Investigating the function of individual synapses is essential to understanding the mechanisms that influence the efficacy of chemical synaptic transmission. The known simplicity of the synaptic structure at the crayfish neuromuscular junction (NMJ) and its quantal nature of release allows an assessment of discrete synapses within the motor nerve terminals. Our goal in this article is to investigate the effect of the stimulation frequency on the number of active release sites (n) and the probability of release (p) at those active sites. Because methods based on direct counts often provide unstable joint estimates of (n) and (p), we base our analysis on mixture modeling. In particular, the mixture modeling approach is used to estimate (n) and (p) for stimulation frequencies of 1 Hz, 2 Hz, and 3 Hz. Our results indicate that as the stimulation frequency increases, new sites are recruited (thus increasing n) and the probability of release (p) increases.

Inverse relationship between release probability and readily releasable vesicles in depressing and facilitating synapses

We tested the hypothesis that the probability of vesicular exocytosis at synapses is positively correlated with the pools of readily releasable synaptic vesicles, as shown for mammalian neurons grown in tissue culture. We compared synapses of two identified glutamatergic neurons: phasic (high-output, depressing) and tonic (low-output, facilitating) crustacean motor neurons, which differ 100- to 1000-fold in quantal content. Estimates of vesicles available for exocytosis were made from depletion during forced release and from electron microscopic observation of vesicles docked at synaptic membranes near active zones. Both measurements showed a significantly larger pool of readily releasable vesicles in facilitating synapses, despite their much lower quantal output during stimulation. Thus, the probability for release of docked vesicles is very much lower at facilitating synapses, and the presence of more docked vesicles does not predict higher synaptic release probability in these paired excitatory neurons.

Physiologically identified 5-HT2-like receptors at the crayfish neuromuscular junction

The model synaptic preparation of the crayfish opener neuromuscular junction is known to be responsive to exogenous application of 5-HT. The primary effect of 5-HT is an enhancement of vesicular release from the presynaptic motor nerve terminal. 5-HT is known to act through an IP(3) cascade which suggests the presence of a 5-HT(2) receptor subtype; however, this is based on vertebrate 5-HT receptor classification. We examined this possibility by using a selective agonist and two antagonists of the vertebrate 5-HT(2) receptor subtypes. The antagonist ketanserin and spiperone reduce the responsiveness of 5-HT in a dose-dependent manner. The broad 5-HT(2) receptor agonist, alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT) enhances synaptic transmission, in a concentration-dependent manner, but it is not as potent as 5-HT. These results support the notion that a 5-HT(2) receptor subtype is present presynaptically on the crayfish motor nerve terminals. By knowing the types of 5-HT receptors present on the presynaptic motor nerve terminals in this model synaptic preparation, a better understanding of the mechanisms of action of 5-HT on vesicular release will be forthcoming.

The effects of ethanol on pre-synaptic components of synaptic transmission in a model glutamatergic synapse: the crayfish neuromuscular junction

We have elucidated some of the mechanisms by which ethanol (EtOH) reduces synaptic efficacy at model glutamatergic synapses. The crayfish phasic and tonic neuromuscular junctions are superb models for directly assessing the effects of EtOH on pre-synaptic components of synaptic transmission. The ability to perform quantal analysis of synaptic transmission has allowed us to assess pre-synaptic alterations of release. Using this system, we report that the application of EtOH, within a range observed in intoxicated humans (44 and 88 mM), resulted in a diminution of excitatory post-synaptic potentials (EPSP) amplitudes. Additionally, using focal macro-patch recordings, quantal synaptic currents were recorded to assess the pre-synaptic component as potential target sites for EtOH's action. At the tonic neuromuscular junctions, EtOH (88 mM) reduced the probability of release (p), and in some cases, reduced the number of the release sites (n), but did not alter facilitation index nor did it affect the latency of vesicular release. At the phasic neuromuscular junction, a reduction in synaptic charge occurred during the presence of EtOH. Thus, the observed decrease in synaptic strength is at least partially attributable to a pre-synaptic alteration, specifically the release of fewer vesicles.

The neuromuscular junctions of the slow and the fast excitatory axon in the closer of the crab Eriphia spinifrons are endowed with different Ca2+ channel types and allow neuron-specific modulation of transmitter release by two neuropeptides

Most crustacean muscle fibers receive double excitatory innervation by functionally different motor neurons termed slow and fast. By using specific omega-toxins we show that the terminals of the slow closer excitor (SCE) and the fast closer excitor (FCE) at a crab muscle are endowed with different sets of presynaptic Ca(2+) channel types. omega-Agatoxin, a blocker of vertebrate P/Q-type channels, reduced the amplitude of EPSCs by decreasing the mean quantal content of transmitter release in both neurons by 70-85%, depending on the concentration. We provide the first evidence that omega-conotoxin-sensitive channels also participate in transmission at crustacean neuromuscular terminals and are colocalized with omega-agatoxin-sensitive channels in an axon-type-specific distribution. omega-Conotoxin, a blocker of vertebrate N-type channels, inhibited release by 20-25% only at FCE, not at SCE endings. Low concentrations of Ni(2+), which block vertebrate R-type channels, inhibited release in endings of the SCE by up to 35%, but had little effects in FCE endings. We found that two neuropeptides, the FMRFamide-like DF(2) and proctolin, which occur in many crustaceans, potentiated evoked transmitter release differentially. Proctolin increased release at SCE and FCE endings, and DF(2) increased release only at FCE endings. Selective blocking of Ca(2+) channels by different omega-toxins in the presence of peptides revealed that the target of proctolin-mediated modulation is the omega-agatoxin-sensitive channel (P/Q-like), that of DF(2) the omega-conotoxin-sensitive channel (N-like). The differential effects of these two peptides allows fine tuning of transmitter release at two functionally different motor neurons innervating the same muscle.

Myofibrillar protein isoform expression is correlated with synaptic efficacy in slow fibres of the claw and leg opener muscles of crayfish and lobster

In the crayfish and lobster opener neuromuscular preparations of the walking legs and claws, there are regional differences in synaptic transmission even though the entire muscle is innervated by a single excitatory tonic motor neuron. The innervation of the proximal fibres produced larger excitatory postsynaptic potentials (EPSPs) than those of the central fibres. The amplitudes of the EPSPs in the distal fibres were intermediate between those of the proximal and central regions. These differences in EPSP amplitudes were correlated with differences in short-term facilitation between the three regions. When given a 10- or 20-pulse train of stimuli, the proximal fibres showed greater short-term facilitation initially, often followed by a maximization of short-term facilitation towards the end of a train. In contrast, the central fibres showed a linear increase in short-term facilitation throughout a stimulus train. The distal fibres showed intermediate short-term facilitation compared with the other two regions. Analysis of myofibrillar isoforms showed that levels of troponin-T1 (TnT1), a 55 kDa isoform expressed in slow-tonic (S2) fibres, were correlated with synaptic properties. Proximal fibres had the highest levels of TnT1, with lower levels in distal fibres; central fibres lacked TnT1, which is characteristic of slow-twitch (S1) fibres. In addition, differences in troponin-I isoforms correlated with TnT1 levels between the proximal, central and distal regions. The correlation between slow fibre phenotype and strength of innervation suggests a relationship between synaptic structure and expression of troponin isoforms.

Effects of the ecdysoneless mutant on synaptic efficacy and structure at the neuromuscular junction in Drosophila larvae during normal and prolonged development

Hormonal regulation in development and maintenance of synaptic transmission involves examination of both the presynaptic and postsynaptic components and a system in which the hormones can be controlled. We used the ecdysoneless heat-sensitive mutation (l(3)ecd(1)/l(3)ecd(1)) of Drosophila to provide the ability to regulate endogenous ecdysone production at various larval stages. In conjunction, we used the neuromuscular junctions of Drosophila since they offer the advantage of assessable preparations for both morphological and physiological measures. The growth in the Ib and Is motor nerve terminals and the corresponding muscle 6 in segment 4 of the larval Drosophila throughout the third instar stage in the presence of normal and a much reduced endogenous ecdysone level was investigated. Muscle 6 and the motor nerve terminals parallel in growth throughout the third instar. The nerve terminals increase in length and varicosity number, thus providing an increase in the number of synaptic release sites. The ecdysoneless larvae also show an increase in muscle size, however the Is and Ib motor nerve terminals do not mature to the extent of the wild-type ecdysone producing flies. The motor nerve terminal length is shorter with fewer numbers of varicosities per terminal. In spite of a shorter nerve terminal and fewer varicosities, with an increasing muscle fiber, the compound excitatory junctional potentials of Ib and Is in the ecdysoneless flies are larger, which is suggestive of synaptic structural modification. This study demonstrates ecdysone's role in modifying nerve terminal development and neuromuscular junction function.

Alterations in development, behavior, and physiology in Drosophila larva that have reduced ecdysone production

We investigated behavior, physiology, sensitivity to exogenous application of ecdysone, and nerve terminal structure for differences between the reduced ecdysone genotype, ecd(1)/ecd(1), and wild-type control ecd(1)/TM6B animals during the early and late third instars when raised at 25 degrees C. The ecd(1) mutants were able to survive through larval development and form pupae. However, the results demonstrate that the time to pupation is lengthened by about 50 h for the ecd(1)/ecd(1) as compared with the wild-type control siblings. In addition to the lengthened larval cycle in the mutant, ecd(1)/ecd(1) animals, they also display behavioral differences as compared with controls. The rate of body wall contraction and mouth hook movements are reduced in the early third instar of ecd(1)/ecd(1) as compared with controls. The physiological measure of excitatory junction potential amplitude for the combined Is and Ib terminals did not reveal any differences among the two genotypes during the early third instar but the synaptic strength is reduced in the late third instars for controls. Application of exogenous ecdysone is still effective during the late third instar for the ecd(1)/ecd(1) but not the controls. This suggests that endogenous production of ecdysone have already taken place in the wild-type but not the ecd(1)/ecd(1) larvae, thus the rapid nongenomic responses could still be observed in the late third ecd(1)/ecd(1) larvae. Structurally the number of varicosities and the terminal length showed significant differences between ecd(1)/ecd(1) and the wild-type ecd(1)/TM6B genotype in the late third instars.

The effects of 5-HT on sensory, central and motor neurons driving the abdominal superficial flexor muscles in the crayfish

Serotonin (5-HT) induces a variety of physiological and behavioral effects in crustaceans. However, the mechanisms employed by 5-HT to effect behavioral changes are not fully understood. Among the mechanisms by which these changes might occur are alterations in synaptic drive and efficacy of sensory, interneurons and motor neurons, as well as direct effects on muscles. We investigated these aspects with the use of a defined sensory-motor system, which is entirely contained within a single abdominal segment and consists of a 'cuticular sensory neurons segmental ganglia abdominal superficial flexor motor neurons-muscles' circuit. Our studies address the role of 5-HT in altering (1) the activity of motor neurons induced by sensory stimulation; (2) the inherent excitability of superficial flexor motor neurons; (3) transmitter release properties of the motor nerve terminal and (4) input resistance of the muscle. Using en passant recordings from the motor nerve, with and without sensory stimulation, and intracellular recordings from the muscle, we show that 5-HT enhances sensory drive and output from the ventral nerve cord resulting in an increase in the firing frequency of the motor neurons. Also, 5-HT increases transmitter release at the neuromuscular junction, and alters input resistance of the muscle fibers.

Sensitivity of transformed (phasic to tonic) motor neurons to the neuromodulator 5-HT

Long-term adaptation resulting in a 'tonic-like' state can be induced in phasic motor neurons of the crayfish, Procambarus clarkii, by daily low-frequency stimulation [Lnenicka, G.A., Atwood, H.L., 1985b. Long-term facilitation and long-term adaptation at synapses of a crayfish phasic motoneuron. J. Neurobiol. 16, 97-110]. To test the hypothesis that motor neurons undergoing adaptation show increased responses to the neuromodulator serotonin (5-HT), phasic motor neurons innervating the deep abdominal extensor muscles of crayfish were stimulated at 2.5 Hz, 2 h/day, for 7 days. One day after cessation of conditioning, contralateral control and conditioned motor neurons of the same segment were stimulated at 1 Hz and the induced excitatory post-synaptic potentials (EPSPs) were recorded from DEL(1) muscle fibers innervated by each motor neuron type. Recordings were made in saline without and with 100 nM 5-HT. EPSP amplitudes were increased by 5-HT exposure in all cases. Conditioned muscles exposed to 5-HT showed a 2-fold higher percentage of increase in EPSP amplitude than did control muscles. Thus, the conditioned motor neurons behaved like intrinsically tonic motoneurons in their response to 5-HT. While these results show that long-term adaptation (LTA) extends to 5-HT neuromodulation, no phenotype switch could be detected in the postsynaptic muscle. Protein isoform profiles, including the myosin heavy chains, do not change after 1 week of conditioning their innervating motor neurons.

Neurotransmitter levels and synaptic strength at the Drosophila larval neuromuscular junction are not altered by mutation in the sluggish-A gene, which encodes proline oxidase and affects adult locomotion

The sluggish-A (slgA) gene of Drosophila melanogaster has been shown to encode for the enzyme proline oxidase, a mitochondrial enzyme which catalyzes the first step in the conversion of L-proline to L-glutamate. The slgA transcript is expressed in both larval and adult Drosophila melanogaster. Mutations in this gene lead to reduced proline oxidase activity and an elevation of free proline levels. Adult mutant flies show a striking reduction of motor activity. Since proline oxidase may contribute to the supply of the neurotransmitter glutamate in the nervous system, a reduction in proline oxidase activity could reduce neural glutamate pools and affect synaptic transmission in neurons utilizing glutamate as a transmitter, including peripheral motor neurons. We tested the hypothesis that glutamate, and synaptic transmission mediated by glutamate, are reduced at synapses of glutamatergic motor neurons in slgA mutants. Levels of glutamate and proline in different cell compartments, and functional properties of synaptic transmission were compared in slgA and control specimens. Proline is elevated in muscle cells of slgA mutants, indicating that the slgA gene regulates tissue proline levels. In nerve terminal varicosities, proline levels were low in both mutants and controls. Glutamate levels in nerve terminal varicosities of slgA mutants and controls were similar. In addition, we found that glutamatergic synaptic transmission at individual nerve endings and at the whole-cell level was similar in slgA mutants and controls. Thus, proline oxidase does not play a major role in generating neuronal glutamate pools at the Drosophila larval neuromuscular junction, and larval neuromuscular performance is not altered significantly in slgA mutants. Metabolic pathways other than that involving proline oxidase are able to sustain glutamatergic synaptic function in Drosophila larvae.