Effects of Ca2+ channel blocker neurotoxins on transmitter release and presynaptic currents at the mouse neuromuscular junction

Depressed neuromuscular transmission causes weakness in mice lacking BK potassium channels

Mice lacking functional large-conductance voltage- and Ca²⁺-activated K⁺ channels (BK channels) are viable but have motor deficits including ataxia and weakness. The cause of weakness is unknown. In this study, we discovered, in vivo, that skeletal muscle in mice lacking BK channels (BK^−/−) was weak in response to nerve stimulation but not to direct muscle stimulation, suggesting a failure of neuromuscular transmission. Voltage-clamp studies of the BK^−/− neuromuscular junction (NMJ) revealed a reduction in evoked endplate current amplitude and the frequency of spontaneous vesicle release compared with WT littermates. Responses to 50-Hz stimulation indicated a reduced probability of vesicle release in BK^−/− mice, suggestive of lower presynaptic Ca²⁺ entry. Pharmacological block of BK channels in WT NMJs did not affect NMJ function, surprisingly suggesting that the reduced vesicle release in BK^−/− NMJs was not due to loss of BK channel–mediated K⁺ current. Possible explanations for our data include an effect of BK channels on development of the NMJ, a role for BK channels in regulating presynaptic Ca²⁺ current or the effectiveness of Ca²⁺ in triggering release. Consistent with reduced Ca²⁺ entry or effectiveness of Ca²⁺ in triggering release, use of 3,4-diaminopyridine to widen action potentials normalized evoked release in BK^−/− mice to WT levels. Intraperitoneal application of 3,4-diaminopyridine fully restored in vivo nerve-stimulated muscle force in BK^−/− mice. Our work demonstrates that mice lacking BK channels have weakness due to a defect in vesicle release at the NMJ.

Developmental demands contribute to early neuromuscular degeneration in CMT2D mice

Dominantly inherited, missense mutations in the widely expressed housekeeping gene, GARS1, cause Charcot-Marie-Tooth type 2D (CMT2D), a peripheral neuropathy characterised by muscle weakness and wasting in limb extremities. Mice modelling CMT2D display early and selective neuromuscular junction (NMJ) pathology, epitomised by disturbed maturation and neurotransmission, leading to denervation. Indeed, the NMJ disruption has been reported in several different muscles; however, a systematic comparison of neuromuscular synapses from distinct body locations has yet to be performed. We therefore analysed NMJ development and degeneration across five different wholemount muscles to identify key synaptic features contributing to the distinct pattern of neurodegeneration in CMT2D mice. Denervation was found to occur along a distal-to-proximal gradient, providing a cellular explanation for the greater weakness observed in mutant Gars hindlimbs compared with forelimbs. Nonetheless, muscles from similar locations and innervated by axons of equivalent length showed significant differences in neuropathology, suggestive of additional factors impacting on site-specific neuromuscular degeneration. Defective NMJ development preceded and associated with degeneration, but was not linked to a delay of wild-type NMJ maturation processes. Correlation analyses indicate that muscle fibre type nor synaptic architecture explain the differential denervation of CMT2D NMJs, rather it is the extent of post-natal synaptic growth that predisposes to neurodegeneration. Together, this work improves our understanding of the mechanisms driving synaptic vulnerability in CMT2D and hints at pertinent pathogenic pathways.

Functional Postnatal Maturation of the Medial Olivocochlear Efferent-Outer Hair Cell Synapse

The organ of Corti, the auditory mammalian sensory epithelium, contains two types of mechanotransducer cells, inner hair cells (IHCs) and outer hair cells (OHCs). IHCs are involved in conveying acoustic stimuli to the CNS, while OHCs are implicated in the fine tuning and amplification of sounds. OHCs are innervated by medial olivocochlear (MOC) cholinergic efferent fibers. The functional characteristics of the MOC-OHC synapse during maturation were assessed by electrophysiological and pharmacological methods in mouse organs of Corti at postnatal day 11 (P11)-P13, hearing onset in altricial rodents, and at P20-P22 when the OHCs are morphologically and functionally mature. Synaptic currents were recorded in whole-cell voltage-clamped OHCs while electrically stimulating the MOC fibers. A progressive increase in the number of functional MOC-OHC synapses, as well as in their strength and efficacy, was observed between P11-13 and P20-22. At hearing onset, the MOC-OHC synapse presented facilitation during MOC fibers high-frequency stimulation that disappeared at mature stages. In addition, important changes were found in the VGCC that are coupled to transmitter release. Ca²⁺ flowing in through L-type VGCCs contribute to trigger ACh release together with P/Q- and R-type VGCCs at P11-P13, but not at P20-P22. Interestingly, N-type VGCCs were found to be involved in this process at P20-P22, but not at hearing onset. Moreover, the degree of compartmentalization of calcium channels with respect to BK channels and presynaptic release components significantly increased from P11-P13 to P20-P22. These results suggest that the MOC-OHC synapse is immature at the onset of hearing.SIGNIFICANCE STATEMENT The functional expression of both VGCCs and BK channels, as well as their localization with respect to the presynaptic components involved in transmitter release, are key elements in determining synaptic efficacy. In this work, we show dynamic changes in the expression of VGCCs and Ca²⁺-dependent BK K⁺ channels coupled to ACh release at the MOC-OHC synapse and their shift in compartmentalization during postnatal maturation. These processes most likely set the short-term plasticity pattern and reliability of the MOC-OHC synapse on high-frequency activity.

R-Roscovitine Improves Motoneuron Function in Mouse Models for Spinal Muscular Atrophy

Neurotransmission defects and motoneuron degeneration are hallmarks of spinal muscular atrophy, a monogenetic disease caused by the deficiency of the SMN protein. In the present study, we show that systemic application of R-Roscovitine, a Ca_v2.1/Ca_v2.2 channel modifier and a cyclin-dependent kinase 5 (Cdk-5) inhibitor, significantly improved survival of SMA mice. In addition, R-Roscovitine increased Ca_v2.1 channel density and sizes of the motor endplates. In vitro, R-Roscovitine restored axon lengths and growth cone sizes of Smn-deficient motoneurons corresponding to enhanced spontaneous Ca²⁺ influx and elevated Ca_v2.2 channel cluster formations independent of its capability to inhibit Cdk-5. Acute application of R-Roscovitine at the neuromuscular junction significantly increased evoked neurotransmitter release, increased the frequency of spontaneous miniature potentials, and lowered the activation threshold of silent terminals. These data indicate that R-Roscovitine improves Ca²⁺ signaling and Ca²⁺ homeostasis in Smn-deficient motoneurons, which is generally crucial for motoneuron differentiation, maturation, and function.

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R-Roscovitine prolongs survival of SMA mice

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R-Roscovitine increases Ca²⁺ influx and growth cone size of SMA motoneurons

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R-Roscovitine beneficially affects neurotransmission in SMA motor nerve terminals

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R-Roscovitine wakes up dormant synapses of SMA motoneurons

Developmental Synaptic Changes at the Transient Olivocochlear-Inner Hair Cell Synapse

In the mature mammalian cochlea, inner hair cells (IHCs) are mainly innervated by afferent fibers that convey sound information to the CNS. During postnatal development, however, medial olivocochlear (MOC) efferent fibers transiently innervate the IHCs. The MOC-IHC synapse, functional from postnatal day 0 (P0) to hearing onset (P12), undergoes dramatic changes in the sensitivity to acetylcholine (ACh) and in the expression of key postsynaptic proteins. To evaluate whether there are associated changes in the properties of ACh release during this period, we used a cochlear preparation from mice of either sex at P4, P6-P7, and P9-P11 and monitored transmitter release from MOC terminals in voltage-clamped IHCs in the whole-cell configuration. The quantum content increased 5.6× from P4 to P9-P11 due to increases in the size and replenishment rate of the readily releasable pool of synaptic vesicles without changes in their probability of release or quantum size. This strengthening in transmission was accompanied by changes in short-term plasticity properties, which switched from facilitation at P4 to depression at P9-P11. We have previously shown that at P9-P11, ACh release is supported by P/Q- and N-type voltage-gated calcium channels (VGCCs) and negatively regulated by BK potassium channels activated by Ca2+ influx through L-type VGCCs. We now show that at P4 and P6-P7, release is mediated by P/Q-, R- and L-type VGCCs. Interestingly, L-type VGCCs have a dual role: they both support release and fuel BK channels, suggesting that at immature stages presynaptic proteins involved in release are less compartmentalized.SIGNIFICANCE STATEMENT During postnatal development before the onset of hearing, cochlear inner hair cells (IHCs) present spontaneous Ca2+ action potentials that release glutamate at the first auditory synapse in the absence of sound stimulation. The IHC Ca2+ action potential frequency pattern, which is crucial for the correct establishment and function of the auditory system, is regulated by the efferent medial olivocochlear (MOC) system that transiently innervates IHCs during this period. We show here that developmental changes in synaptic strength and synaptic plasticity properties at the MOC-IHC synapse upon MOC fiber activation at different frequencies might be crucial for tightly shaping the pattern of afferent activity during this critical period.

Presynaptic mechanisms controlling calcium-triggered transmitter release at the neuromuscular junction

Calcium-triggered neurotransmission underlies most communication in the nervous system. Yet, despite the conserved and essential nature of this process, the molecular underpinnings of calcium-triggered neurotransmission have been difficult to study directly and our understanding to this date remains incomplete. Here we frame more recent efforts to understand this process with a historical perspective of the study of neurotransmitter release at the neuromuscular junction. We focus on the role of calcium channel distribution and organization relative to synaptic vesicles, as well as the nature of the calcium sensors that trigger release. Importantly, we provide a framework for understanding how the function of neurotransmitter release sites, or active zones, contributes to the function of the synapse as a whole.

BDNF/trkB Induction of Calcium Transients through Cav2.2 Calcium Channels in Motoneurons Corresponds to F-actin Assembly and Growth Cone Formation on β2-Chain Laminin (221)

Spontaneous Ca²⁺ transients and actin dynamics in primary motoneurons correspond to cellular differentiation such as axon elongation and growth cone formation. Brain-derived neurotrophic factor (BDNF) and its receptor trkB support both motoneuron survival and synaptic differentiation. However, in motoneurons effects of BDNF/trkB signaling on spontaneous Ca²⁺ influx and actin dynamics at axonal growth cones are not fully unraveled. In our study we addressed the question how neurotrophic factor signaling corresponds to cell autonomous excitability and growth cone formation. Primary motoneurons from mouse embryos were cultured on the synapse specific, β2-chain containing laminin isoform (221) regulating axon elongation through spontaneous Ca²⁺ transients that are in turn induced by enhanced clustering of N-type specific voltage-gated Ca²⁺ channels (Ca_v2.2) in axonal growth cones. TrkB-deficient (trkBTK^-/-) mouse motoneurons which express no full-length trkB receptor and wildtype motoneurons cultured without BDNF exhibited reduced spontaneous Ca²⁺ transients that corresponded to altered axon elongation and defects in growth cone morphology which was accompanied by changes in the local actin cytoskeleton. Vice versa, the acute application of BDNF resulted in the induction of spontaneous Ca²⁺ transients and Ca_v2.2 clustering in motor growth cones, as well as the activation of trkB downstream signaling cascades which promoted the stabilization of β-actin via the LIM kinase pathway and phosphorylation of profilin at Tyr129. Finally, we identified a mutual regulation of neuronal excitability and actin dynamics in axonal growth cones of embryonic motoneurons cultured on laminin-221/211. Impaired excitability resulted in dysregulated axon extension and local actin cytoskeleton, whereas upon β-actin knockdown Ca_v2.2 clustering was affected. We conclude from our data that in embryonic motoneurons BDNF/trkB signaling contributes to axon elongation and growth cone formation through changes in the local actin cytoskeleton accompanied by increased Ca_v2.2 clustering and local calcium transients. These findings may help to explore cellular mechanisms which might be dysregulated during maturation of embryonic motoneurons leading to motoneuron disease.

Altered short-term synaptic plasticity and reduced muscle strength in mice with impaired regulation of presynaptic CaV2.1 Ca2+ channels

Facilitation and inactivation of P/Q-type calcium (Ca(2+)) currents through the regulation of voltage-gated Ca(2+) (CaV) 2.1 channels by Ca(2+) sensor (CaS) proteins contributes to the facilitation and rapid depression of synaptic transmission in cultured neurons that transiently express CaV2.1 channels. To examine the modulation of endogenous CaV2.1 channels by CaS proteins in native synapses, we introduced a mutation (IM-AA) into the CaS protein-binding site in the C-terminal domain of CaV2.1 channels in mice, and tested synaptic facilitation and depression in neuromuscular junction synapses that use exclusively CaV2.1 channels for Ca(2+) entry that triggers synaptic transmission. Even though basal synaptic transmission was unaltered in the neuromuscular synapses in IM-AA mice, we found reduced short-term facilitation in response to paired stimuli at short interstimulus intervals in IM-AA synapses. In response to trains of action potentials, we found increased facilitation at lower frequencies (10-30 Hz) in IM-AA synapses accompanied by slowed synaptic depression, whereas synaptic facilitation was reduced at high stimulus frequencies (50-100 Hz) that would induce strong muscle contraction. As a consequence of altered regulation of CaV2.1 channels, the hindlimb tibialis anterior muscle in IM-AA mice exhibited reduced peak force in response to 50 Hz stimulation and increased muscle fatigue. The IM-AA mice also had impaired motor control, exercise capacity, and grip strength. Taken together, our results indicate that regulation of CaV2.1 channels by CaS proteins is essential for normal synaptic plasticity at the neuromuscular junction and for muscle strength, endurance, and motor coordination in mice in vivo.

PAR1-activated astrocytes in the nucleus of the solitary tract stimulate adjacent neurons via NMDA receptors

Severe autonomic dysfunction, including the loss of control of the cardiovascular, respiratory, and gastrointestinal systems, is a common comorbidity of stroke and other bleeding head injuries. Previous studies suggest that this collapse of autonomic control may be caused by thrombin acting on astrocytic protease-activated receptors (PAR1) in the hindbrain. Using calcium imaging and electrophysiological techniques, we evaluated the mechanisms by which astrocytic PAR1s modulate the activity of presynaptic vagal afferent terminals and postsynaptic neurons in the rat nucleus of the solitary tract (NST). Our calcium-imaging data show that astrocytic and neuronal calcium levels increase after brain slices are treated with the PAR1 agonist SFLLRN-NH2. This increase in activity is blocked by pretreating the slices with the glial metabolic blocker fluorocitrate. In addition, PAR1-activated astrocytes communicate directly with NST neurons by releasing glutamate. Calcium responses to SFLLRN-NH2 in the astrocytes and neurons significantly increase after bath application of the excitatory amino acid transporter blocker DL-threo-β-benzyloxyaspartic acid (TBOA) and significantly decrease after bath application of the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (DL-AP5). Furthermore, astrocytic glutamate activates neuronal GluN2B-containing NMDA receptors. Voltage-clamp recordings of miniature EPSCs (mEPSCs) from NST neurons show that astrocytes control presynaptic vagal afferent excitability directly under resting and activated conditions. Fluorocitrate significantly decreases mEPSC frequency and SFLLRN-NH2 significantly increases mEPSC frequency. These data show that astrocytes act within a tripartite synapse in the NST, controlling the excitability of both postsynaptic NST neurons and presynaptic vagal afferent terminals.

Short-term plasticity and modulation of synaptic transmission at mammalian inhibitory cholinergic olivocochlear synapses

The organ of Corti, the mammalian sensory epithelium of the inner ear, has two types of mechanoreceptor cells, inner hair cells (IHCs) and outer hair cells (OHCs). In this sensory epithelium, vibrations produced by sound waves are transformed into electrical signals. When depolarized by incoming sounds, IHCs release glutamate and activate auditory nerve fibers innervating them and OHCs, by virtue of their electromotile property, increase the amplification and fine tuning of sound signals. The medial olivocochlear (MOC) system, an efferent feedback system, inhibits OHC activity and thereby reduces the sensitivity and sharp tuning of cochlear afferent fibers. During neonatal development, IHCs fire Ca²⁺ action potentials which evoke glutamate release promoting activity in the immature auditory system in the absence of sensory stimuli. During this period, MOC fibers also innervate IHCs and are thought to modulate their firing rate. Both the MOC-OHC and the MOC-IHC synapses are cholinergic, fast and inhibitory and mediated by the α9α10 nicotinic cholinergic receptor (nAChR) coupled to the activation of calcium-activated potassium channels that hyperpolarize the hair cells. In this review we discuss the biophysical, functional and molecular data which demonstrate that at the synapses between MOC efferent fibers and cochlear hair cells, modulation of transmitter release as well as short term synaptic plasticity mechanisms, operating both at the presynaptic terminal and at the postsynaptic hair-cell, determine the efficacy of these synapses and shape the hair cell response pattern.

Amyotrophic lateral sclerosis-immunoglobulins selectively interact with neuromuscular junctions expressing P/Q-type calcium channels

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by a gradual loss of motoneurons. The majority of ALS cases are associated with a sporadic form whose etiology is unknown. Several pieces of evidence favor autoimmunity as a potential contributor to sporadic ALS pathology. To gain understanding concerning possible antigens interacting with IgGs from sporadic ALS patients (ALS-IgGs), we studied immunoreactivity against neuromuscular junction (NMJ), spinal cord and cerebellum of mice with and without the Ca(V) 2.1 pore-forming subunit of the P/Q-type voltage-gated calcium (Ca(2+)) channel. ALS-IgGs showed a strong reactivity against NMJs of wild-type diaphragms. ALS-IgGs also increased muscle miniature end-plate potential frequency, suggesting a functional role for ALS-IgGs on synaptic signaling. In support, in mice lacking the Ca(V) 2.1 subunit ALS-IgGs showed significantly reduced NMJ immunoreactivity and did not alter spontaneous acetylcholine release. This difference in reactivity was absent when comparing N-type Ca(2+) channel wild-type or null mice. These results are particularly relevant because motoneurons are known to be early pathogenic targets in ALS. Our findings add further evidence supporting autoimmunity as one of the possible mechanisms contributing to ALS pathology. They also suggest that serum autoantibodies in a subset of ALS patients would interact with NMJ proteins down-regulated when P/Q-type channels are absent.

Vesicular ATPase inserted into the plasma membrane of motor terminals by exocytosis alkalinizes cytosolic pH and facilitates endocytosis

Key components of vesicular neurotransmitter release, such as Ca(2+) influx and membrane recycling, are affected by cytosolic pH. We measured the pH-sensitive fluorescence of Yellow Fluorescent Protein transgenically expressed in mouse motor nerve terminals, and report that Ca(2+) influx elicited by action potential trains (12.5-100 Hz) evokes a biphasic pH change: a brief acidification (∼ 13 nM average peak increase in [H(+)]), followed by a prolonged alkalinization (∼ 30 nM peak decrease in [H(+)]) that outlasts the stimulation train. The alkalinization is selectively eliminated by blocking vesicular exocytosis with botulinum neurotoxins, and is prolonged by the endocytosis-inhibitor dynasore. Blocking H(+) pumping by vesicular H(+)-ATPase (with folimycin or bafilomycin) suppresses stimulation-induced alkalinization and reduces endocytotic uptake of FM1-43. These results suggest that H(+)-ATPase, known to transfer cytosolic H(+) into prefused vesicles, continues to extrude cytosolic H(+) after being exocytotically incorporated into the plasma membrane. The resulting cytosolic alkalinization may facilitate vesicular endocytosis.

Ca(2+) and Ca(2+)-activated K(+) channels that support and modulate transmitter release at the olivocochlear efferent-inner hair cell synapse

In the mammalian auditory system, the synapse between efferent olivocochlear (OC) neurons and sensory cochlear hair cells is cholinergic, fast, and inhibitory. This efferent synapse is mediated by the nicotinic alpha9alpha10 receptor coupled to the activation of SK2 Ca(2+)-activated K(+) channels that hyperpolarize the cell. So far, the ion channels that support and/or modulate neurotransmitter release from the OC terminals remain unknown. To identify these channels, we used an isolated mouse cochlear preparation and monitored transmitter release from the efferent synaptic terminals in inner hair cells (IHCs) voltage clamped in the whole-cell recording configuration. Acetylcholine (ACh) release was evoked by electrically stimulating the efferent fibers that make axosomatic contacts with IHCs before the onset of hearing. Using the specific antagonists for P/Q- and N-type voltage-gated calcium channels (VGCCs), omega-agatoxin IVA and omega-conotoxin GVIA, respectively, we show that Ca(2+) entering through both types of VGCCs support the release process at this synapse. Interestingly, we found that Ca(2+) entering through the dihydropiridine-sensitive L-type VGCCs exerts a negative control on transmitter release. Moreover, using immunostaining techniques combined with electrophysiology and pharmacology, we show that BK Ca(2+)-activated K(+) channels are transiently expressed at the OC efferent terminals contacting IHCs and that their activity modulates the release process at this synapse. The effects of dihydropiridines combined with iberiotoxin, a specific BK channel antagonist, strongly suggest that L-type VGCCs negatively regulate the release of ACh by fueling BK channels that are known to curtail the duration of the terminal action potential in several types of neurons.

Characterization of rhythmic Ca2+ transients in early embryonic chick motoneurons: Ca2+ sources and effects of altered activation of transmitter receptors

In the nervous system, spontaneous Ca(2+) transients play important roles in many developmental processes. We previously found that altering the frequency of electrically recorded rhythmic spontaneous bursting episodes in embryonic chick spinal cords differentially perturbed the two main pathfinding decisions made by motoneurons, dorsal-ventral and pool-specific, depending on the sign of the frequency alteration. Here, we characterized the Ca(2+) transients associated with these bursts and showed that at early stages while motoneurons are still migrating and extending axons to the base of the limb bud, they display spontaneous, highly rhythmic, and synchronized Ca(2+) transients. Some precursor cells in the ependymal layer displayed similar transients. T-type Ca(2+) channels and a persistent Na(+) current were essential to initiate spontaneous bursts and associated transients. However, subsequent propagation of activity throughout the cord resulted from network-driven chemical transmission mediated presynaptically by Ca(2+) entry through N-type Ca(2+) channels and postsynaptically by acetylcholine acting on nicotinic receptors. The increased [Ca(2+)](i) during transients depended primarily on L-type and T-type channels with a modest contribution from TRP (transient receptor potential) channels and ryanodine-sensitive internal stores. Significantly, the drugs used previously to produce pathfinding errors altered transient frequency but not duration or amplitude. These observations imply that different transient frequencies may differentially modulate motoneuron pathfinding. However, the duration of the Ca(2+) transients differed significantly between pools, potentially enabling additional distinct pool-specific downstream signaling. Many early events in spinal motor circuit formation are thus potentially sensitive to the rhythmic Ca(2+) transients we have characterized and to any drugs that perturb them.

The spatial pattern of exocytosis and post-exocytic mobility of synaptopHluorin in mouse motor nerve terminals

We monitored the spatial distribution of exo- and endocytosis at 37 degrees C in mouse motor nerve terminals expressing synaptopHluorin (spH), confirming and extending earlier work at room temperature, which had revealed fluorescent 'hot spots' appearing in repeatable locations during tetanic stimulation. We also tested whether hot spots appeared during mild stimulation. Averaged responses from single shocks showed a clear fluorescence jump, but revealed no sign of hot spots; instead, fluorescence rose uniformly across the terminal. Only after 5-25 stimuli given at high frequency did hot spots appear, suggesting a novel initiation mechanism. Experiments showed that about half of the surface spH molecules were mobile, and that spH movement occurred out of hot spots, demonstrating their origin as exocytic sources, not endocytic sinks. Taken together, our results suggest that synaptic vesicles exocytose equally throughout the terminal with mild stimulation, but preferentially exocytose at specific, repeatable locations during tetanic stimulation.

Synaptic vesicle mobility in mouse motor nerve terminals with and without synapsin

We measured synaptic vesicle mobility using fluorescence recovery after photobleaching of FM 1-43 [N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide] stained mouse motor nerve terminals obtained from wild-type (WT) and synapsin triple knock-out (TKO) mice at room temperature and physiological temperature. Vesicles were mobile in resting terminals at physiological temperature but virtually immobile at room temperature. Mobility was increased at both temperatures by blocking phosphatases with okadaic acid, decreased at physiological temperature by blocking kinases with staurosporine, and unaffected by disrupting actin filaments with latrunculin A or reducing intracellular calcium concentration with BAPTA-AM. Synapsin TKO mice showed reduced numbers of synaptic vesicles and reduced FM 1-43 staining intensity. Synaptic transmission, however, was indistinguishable from WT, as was synaptic vesicle mobility under all conditions tested. Thus, in TKO mice, and perhaps WT mice, a phospho-protein different from synapsin but otherwise of unknown identity is the primary regulator of synaptic vesicle mobility.

Glycerotoxin stimulates neurotransmitter release from N-type Ca2+ channel expressing neurons

Glycerotoxin (GLTx) is capable of stimulating neurotransmitter release at the frog neuromuscular junction by directly interacting with N-type Ca2+ (Cav2.2) channels. Here we have utilized GLTx as a tool to investigate the functionality of Cav2.2 channels in various mammalian neuronal preparations. We first adapted a fluorescent-based high-throughput assay to monitor glutamate release from rat cortical synaptosomes. GLTx potently stimulates glutamate secretion and Ca2+ influx in synaptosomes with an EC50 of 50 pm. Both these effects were prevented using selective Cav2.2 channel blockers suggesting the functional involvement of Cav2.2 channels in mediating glutamate release in this system. We further show that both Cav2.1 (P/Q-type) and Cav2.2 channels contribute equally to depolarization-induced glutamate release. We then investigated the functionality of Cav2.2 channels at the neonatal rat neuromuscular junction. GLTx enhances both spontaneous and evoked neurotransmitter release causing a significant increase in the frequency of postsynaptic action potentials. These effects were blocked by specific Cav2.2 channel blockers demonstrating that either GLTx or its derivatives could be used to selectively enhance the neurotransmitter release from Cav2.2-expressing mammalian neurons.

Calcium signaling pathways mediating synaptic potentiation triggered by amyotrophic lateral sclerosis IgG in motor nerve terminals

Sporadic amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that affects particularly motoneurons. Several pieces of evidence suggested the involvement of autoimmune mechanisms mediated by antibodies in ALS. However, the significance of those antibodies in the disease and the underlying mechanisms are unknown. Here we showed that IgG purified from a group of sporadic ALS patients, but not familial ALS patients, specifically interact with the presynaptic membrane of motoneurons through an antigen-antibody interaction and modulated synaptic transmission. Immunoreactivity against nerve terminals showed strong correlation with synaptic modulation ability. In addition, several controls have ruled out the possibility for this synaptic modulation to be mediated through proteases or nonspecific effects. Effective IgG potentiated both spontaneous and asynchronous transmitter release. Application of pharmacological inhibitors suggested that activation of this increased release required a nonconstitutive Ca2+ influx through N-type (Cav2.2) channels and phospholipase C activity and that activation of IP3 and ryanodine receptors were necessary to both activate and sustain the increased release. Consistent with the notion that ALS is heterogeneous disorder, our results reveal that, in approximately 50% of ALS patients, motor nerve terminals constitutes a target for autoimmune response.

SO-3, a new O-superfamily conopeptide derived from Conus striatus, selectively inhibits N-type calcium currents in cultured hippocampal neurons

Whole-cell currents in cultured hippocampal neurons were recorded to investigate the effects of SO-3, a new O-superfamily conopeptide derived from Conus striatus, on voltage-sensitive channels. SO-3 had no effect on voltage-sensitive sodium currents, delayed rectifier potassium currents, and transient outward potassium currents. Similar to the selective N-type calcium channel blocker omega-conotoxin MVIIA (MVIIA), SO-3 could concentration-dependently inhibit the high voltage-activated (HVA) calcium currents (I(Ca)). MVIIA(3 microM), 10 microM nimodipine, and 0.5 microM omega-agatoxin IVA (Aga) could selectively block the N-, L-, and P/Q-type I(Ca), which contributed approximately 32, approximately 38, and approximately 21% of the HVA currents in hippocampal neurons, respectively. About 31% of the total HVA currents were inhibited by 3 microM SO-3. SO-3 (3 microM) and 3 microM MVIIA inhibited the overlapping components of HVA currents, whereas no overlapping component was inhibited by 3 microM SO-3 and 10 microM nimodipine, or by 3 microM SO-3 and 0.5 microM Aga. Also, 3 microM SO-3 had no effect on R-type currents. SO-3 had less inhibitory effects on non-N-type HVA currents than MVIIA at higher concentrations (30 and 100 microM). The inhibitory effects of SO-3 and MVIIA on HVA currents were almost fully reversible. However, the recovery from block by MVIIA was more rapid than recovery from block by SO-3. It is concluded that SO-3 is a new omega-conotoxin selectively targeting N-type voltage-sensitive calcium channels. Considering the significance of N-type calcium channels for pain transduction, SO-3 may have therapeutic potential as a novel analgesic agent.

Functional compensation of P/Q by N-type channels blocks short-term plasticity at the calyx of Held presynaptic terminal

Calcium channels of the P/Q subtype mediate transmitter release at the neuromuscular junction and at many central synapses, such as the calyx of Held. Transgenic mice in which alpha1A channels are ablated provide a powerful tool with which to test compensatory mechanisms at the synapse and to explore mechanisms of presynaptic regulation associated with expression of P/Q channels. Using the calyx of Held preparation from the knock-out (KO) mice, we show here that N-type channels functionally compensate for the absence of P/Q subunits at the calyx and evoke giant synaptic currents [approximately two-thirds of the magnitude of wild-type (WT) responses]. However, although evoked paired-pulse facilitation is prominent in WT, this facilitation is greatly diminished in the KO. In addition, direct recording of presynaptic calcium currents revealed that the major functional difference was the absence of calcium-dependent facilitation at the calyx in the P/Q KO animals. We conclude that one physiological function of P/Q channels is to provide additional facilitatory drive, so contributing to maintenance of transmission as vesicles are depleted during high throughput synaptic transmission.

Spontaneous muscle action potentials are blocked by N-type and P/Q-calcium channels blockers in the rat spinal cord–muscle co-culture system

This study investigated the effects of the calcium channel blockers nicardipine, calcicludine, omega-conotoxin GVIA, omega-agatoxin IVA, SNX-482, and NiCl on spontaneous muscle action potential of a rat spinal cord-muscle co-culture system. Spontaneous muscle action potential of the innervated muscle cells was blocked by d-tubocurarine (1 microM), but was not significantly affected by the L-type channel blocker nicardipine (100 nM). The neuronal L-type calcium channel blocker, calcicludine (50 and 100 nM), also had no effect on the frequency of spontaneous muscle action potential. However, nicardipine (100 nM) and calcicludine (100 nM) significantly increased the amplitude of muscle action potential. Application of the N-type calcium channel blocker, omega-conotoxin GVIA (50 and 100 nM), and the P/Q-type calcium channel blocker, omega-agatoxin IVA (10, 30, 50, and 100 nM), blocked the frequency and amplitude of spontaneous muscle action potential of the spinal cord-muscle co-cultured cells. In contrast, spontaneous muscle action potential was not affected by the R-type calcium channel blockers SNX-482 (100 nM) or NiCl (500 nM). These results indicate that blockers of N- and P/Q-type voltage-dependent calcium channels inhibit transmitter release at neuromuscular junctions in the rat spinal cord-muscle co-culture system.

Testosterone modulates Cav2.2 calcium channels’ functional expression at rat levator ani neuromuscular junction

Spinal nucleus of bulbocavernosus and its target musculature, the bulbocavernosus and levator ani muscles, are sexually dimorphic, and their sexual differentiation depends on plasmatic levels of testosterone. Electrophysiological and immunocytochemical studies have demonstrated that at mammalian adult neuromuscular junctions only P/Q-type Ca2+ channels (Ca(v2.1)), mediate evoked transmitter release. Here we report that N-type Ca2+ channel (Ca(v2.2)) blocker omega-Conotoxin GVIA, as well as Ca(v2.1) blocker omega-Agatoxin IVA, significantly reduced quantal content of transmitter release by approximately 80% and approximately 70% respectively at levator ani muscle of the adult rats, indicating that neuromuscular transmission is jointly mediated by both types of channels. In these synapses, we also observed that castration and restitution of plasmatic testosterone in rats resulted in changes in the sensitivity to omega-Conotoxin GVIA. Castration induced, whereas testosterone treatment avoided, functional loss of Ca(v2.2), as mediators of transmitter release in these synapses. Strikingly, the expression and localization of alpha1B subunits, which form the pore of the Ca(v2.2) channel, were similar at control, gonadectomized and gonadectomized testosterone-treated rats, suggesting that testosterone may regulate the coupling mechanisms between Ca(v2.2) and transmitter release at the neuromuscular junctions of these sexually dimorphic motoneurons.

Electrophysiological properties of BK channels in Xenopus motor nerve terminals

Single channel properties of Ca(2+)-activated K(+) (BK or Maxi-K) channels have been investigated in presynaptic membranes in Xenopus motoneurone-muscle cell cultures. The occurrence and density of BK channels increased with maturation/synaptogenesis and was not uniform: highest at the release face of bouton-like synaptic varicosities in contact with muscle cells, and lowest in varicosities that did not contact muscle cells. The Ca(2+) affinity of the channel (K(d)= 7.7 microM at a membrane potential of +20 mV) was lower than those of BK channels that have been characterized in other terminals. Hill coefficients varied between 1.5 and 2.8 at different potentials and open probability increased e-fold per 16 mV change in membrane potential over a range of [Ca(2+)](i) from 1 microM to 1 mM. The maximal activation rate of ensembled single BK channel currents was in the submillisecond range at > or =+20 mV. The activation rate increased approximately 10-fold in response to a [Ca(2+)](i) increase from 1 to 100 microM, but increased only approximately 2-fold with a voltage change from +20 to +130 mV. The fastest activation kinetics of BK channels in cell-attached patches resembled that in inside-out patches with [Ca(2+)](i) of 100 microM or more, suggesting that many BK channels are located very close to calcium channels. Given the low Ca(2+) affinity and rapid Ca(2+) binding/unbinding properties, we conclude that BK channels in this preparation are adapted to play an important role in regulation of neurotransmitter release, and they are ideal reporters of local [Ca(2+)] at the inner membrane surface.

Differential expression of α1 and β subunits of voltage dependent Ca2+ channel at the neuromuscular junction of normal and p/q Ca2+ channel knockout mouse

Voltage-dependent calcium channels (VDCC) have a key role in neuronal function transforming the voltage signals into intracellular calcium signals. They are composed of the pore-forming alpha(1) and the regulatory alpha(2)delta, gamma and beta subunits. Molecular and functional studies have revealed which alpha(1) subunit gene product is the molecular constituent of each class of native calcium channel (L, N, P/Q, R and T type). Electrophysiological and immunocytochemical studies have suggested that at adult mouse motor nerve terminal (MNT) only P/Q type channels, formed by alpha(1A) subunit, mediate evoked transmitter release. The generation of alpha(1A)-null mutant mice offers an opportunity to study the expression and localization of calcium channels at a synapse with complete loss of P/Q calcium channel. We have investigated the expression and localization of VDCCs alpha(1) and beta subunits at the wild type (WT) and knockout (KO) mouse neuromuscular junction (NMJ) using fluorescence immunocytochemistry. The alpha(1A) subunit was observed only at WT NMJ and was absent at denervated muscles and at KO NMJ. The subunits alpha(1B), alpha(1D) and alpha(1E) were also present at WT NMJ and they were over- expressed at KO NMJ suggesting a compensatory expression due to the lack of the alpha(1A). On the other hand, the beta(1b), beta(2a) and beta(4) were present at the same levels in both genotypes. The presence of other types of VDCC at WT NMJ indicate that they may play other roles in the signaling process which have not been elucidated and also shows that other types of VDCC are able to substitute the alpha(1A) subunit, P/Q channel under certain pathological conditions.

Promiscuous and reversible blocker of presynaptic calcium channels in frog and crayfish neuromuscular junctions from Phoneutria nigriventer spider venom

Peptide channel blockers found in venoms of many predators are useful pharmacological tools and potential therapeutic agents. The venom of the Brazilian spider Phoneutria nigriventer contains a fraction, omega-phonetoxin-IIA (omega-Ptx-IIA, 8360 MW), which blocks Ca2+ channels. At frog neuromuscular junctions (NMJ) bathed in normal Ca2+ (1.8 mM) saline, omega-Ptx IIA did not affect spontaneous transmitter release but reversibly reduced evoked transmitter release by 75 and 95% at 12 and 24 nM, respectively. In contrast, toxin effects were irreversible in low-Ca2+ (0.5 mM) saline. Ca2+ imaging in normal-Ca2+ saline showed that omega-Ptx-IIA partially blocked stimulus-dependent presynaptic Ca2+ signals, and the blockade was almost completely reversible. Increases in spontaneous release frequency induced by high extracellular K+ were blocked by omega-Ptx-IIA. Therefore omega-Ptx-IIA blocks N-type Ca2+ channels, which admit Ca2+ that triggers transmitter release at the frog NMJ. Additional evidence predicts that omega-Ptx-IIA binds to N-type Ca2+ channels at a different site from that of omega-Conotoxin-GVIA. omega-Ptx-IIA also gave a low-affinity partial blockade of transmitter release and presynaptic Ca2+ signals at crayfish NMJs where P-type channels are blocked by omega-agatoxin-IVA. The Ca2+-dependent reversibility and promiscuity of this toxin may make it highly useful experimentally and therapeutically.

Ca2+Channels and Synaptic Transmission at the Adult, Neonatal, and P/Q-Type Deficient Neuromuscular Junction

Different types of voltage-activated Ca(2+) channels have been established based on their molecular structure and pharmacological and biophysical properties. One of them, the P/Q-type, is the main channel involved in nerve-evoked neurotransmitter release at neuromuscular junctions and the immunological target in Eaton-Lambert Syndrome. At adult neuromuscular junctions, L- and N-type Ca(2+) channels become involved in transmitter release only under certain experimental or pathological conditions. In contrast, at neonatal rat neuromuscular junctions, nerve-evoked synaptic transmission depends jointly on both N- and P/Q-type channels. Synaptic transmission at neuromuscular junctions of the ataxic P/Q-type Ca(2+) channel knockout mice is also dependent on two different types of channels, N- and R-type. At both neonatal and P/Q knockout junctions, the K(+)-evoked increase in miniature endplate potential frequency was not affected by N-type channel blockers, but strongly reduced by both P/Q- and R-type channel blockers. These differences could be accounted for by a differential location of the channels at the release site, being either P/Q- or R-type Ca(2+) channels located closer to the release site than N-type Ca(2+) channels. Thus, Ca(2+) channels may be recruited to mediate neurotransmitter release where P/Q-type channels seem to be the most suited type of Ca(2+) channel to mediate exocytosis at neuromuscular junctions.

Nifedipine-mediated mobilization of intracellular calcium stores increases spontaneous neurotransmitter release at neonatal rat motor nerve terminals

The modulation of spontaneous release of acetylcholine by specific Ca2+ channel blockers was studied at neonatal rat neuromuscular junction. During early postnatal periods (0-4 days), blockers of N- and P/Q-type Ca2+ channels did not affect miniature endplate potential (MEPP) frequency. Unexpectedly, treatment with the L-type Ca2+ channel antagonist nifedipine, although not when treated with isradipine, nitrendipine, or calciseptine, resulted in strong increase in MEPP frequency. The potentiation effect of nifedipine was dose-dependent with a 56-fold maximum effect with 15 microM. The effect decreased during the first two postnatal weeks and disappeared by the third. The effect of nifedipine was not dependent on extracellular Ca2+ and was not altered by the presence of other Ca2+ channel blockers. In contrast, it was abolished by depleting intracellular Ca2+ stores with 2 microM thapsigargin and was partially inhibited by 10 microM ryanodine. In conclusion, we report a new ryanodine receptor-mediated effect of nifedipine on neonatal neuromuscular junction that may indicate the developmental expression of a specific receptor channel that interacts with intracellular Ca2+ stores. This effect of nifedipine should also be considered when using this drug as either a therapeutic or a research tool.

Adenosine inhibits N-type calcium channels at the rat neuromuscular junction

1. In earlier studies, it has been reported that under in vitro conditions transmitter release at the rat neuromuscular junction is normally suppressed due to the effect of adenosine release from the isolated tissue. In the present study we wanted to determine whether this action may involve the inhibition of calcium influx through adenosine-sensitive calcium channels. 2. In order to test this hypothesis, we examined the role of N-type calcium channels in regulating nerve-evoked transmitter release by using the N-type calcium channel-specific blocker omega-conotoxin GVIA (CTX). In order to control the inhibitory action of adenosine, we also used the adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). We tested the effect of blocking N-type calcium channels with CTX in the presence and absence of DPCPX. We examined the effects of these drugs on quantal transmitter release in the transected preparation of the phrenic nerve-hemidiaphragm of the rat using intracellular recording techniques. 3. At 10 nmol/L, CTX alone had no effect on nerve-evoked transmitter release; however, in the presence of 0.1 micro mol/L DPCPX, CTX significantly depressed nerve-evoked transmitter release. 4. These data support the view that adenosine inhibits nerve-evoked transmitter release by inhibiting N-type calcium channels on nerve terminals.

Glycerotoxin from Glycera convoluta stimulates neurosecretion by up-regulating N-type Ca2+ channel activity

We report here the purification of glycerotoxin from the venom of Glycera convoluta, a novel 320 kDa protein capable of reversibly stimulating spontaneous and evoked neurotransmitter release at the frog neuromuscular junction. However, glycerotoxin is ineffective at the murine neuromuscular junction, which displays a different subtype of voltage- dependent Ca(2+) channels. By sequential and selective inhibition of various types of Ca(2+) channels, we found that glycerotoxin was acting via Ca(v)2.2 (N-type). In neuroendocrine cells, it elicits a robust, albeit transient, influx of Ca(2+) sensitive to the Ca(v)2.2 blockers omega-conotoxin GVIA and MVIIA. Moreover, glycerotoxin triggers a Ca(2+) transient in human embryonic kidney (HEK) cells over-expressing Ca(v)2.2 but not Ca(v)2.1 (P/Q-type). Whole-cell patch-clamp analysis of Ca(v)2.2 expressing HEK cells revealed an up-regulation of Ca(2+) currents due to a leftward shift of the activation peak upon glycerotoxin addition. A direct interaction between Ca(v)2.2 and this neurotoxin was revealed by co-immunoprecipitation experiments. Therefore, glycerotoxin is a unique addition to the arsenal of tools available to unravel the mechanism controlling Ca(2+)-regulated exocytosis via the specific activation of Ca(v)2.2.

Passive transfer of Lambert-Eaton syndrome to mice induces dihydropyridine sensitivity of neuromuscular transmission

Lambert-Eaton myasthenic syndrome (LEMS) is a paraneoplastic disorder in which autoantibodies apparently target the voltage-gated Ca2+ channels that regulate acetylcholine (ACh) release at motor nerve terminals. P/Q-type Ca2+ channels are primarily involved in ACh release at mammalian neuromuscular junctions. Passive transfer of LEMS to mice by repeated administration of plasma from LEMS patients reduces the amplitude of the perineurial P/Q-type current, and unmasks a dihydropyridine (DHP)-sensitive L-type Ca2+ current at the motor nerve terminal. The present study sought to determine if this DHP-sensitive component contributes to ACh release. Mice were treated for 30 days with plasma from healthy human controls or patients with LEMS. For some studies, diaphragms from naive mice were incubated with LEMS or control human plasma for 2 or 24 h. End-plate potentials (EPPs) and miniature end-plate potentials (MEPPs) were recorded from neuromuscular junctions in the hemidiaphragm. Treatment of mice with LEMS plasma evoked the characteristic electrophysiological signs of LEMS: reduced quantal content and facilitation of EPP amplitudes at high-frequency stimulation. Quantal content was also reduced in muscles incubated acutely with LEMS plasma. Nimodipine, a DHP-type blocker of L-type Ca2+ channels, did not significantly affect the quantal content of muscles treated for 2 or 24 h with either control or LEMS plasma, or following chronic treatment with control plasma. However, following 30 days treatment with LEMS plasma, nimodipine significantly reduced the remaining quantal content to 57.7 +/- 3.3 % of pre-nimodipine control levels. Thus, DHP-sensitive Ca2+ channels become involved in synaptic transmission at the mouse neuromuscular junction after chronic, but not acute treatment with LEMS plasma. However, reductions in quantal release of ACh occur even after very short periods of exposure to LEMS plasma. As such, development of the L-type Ca2+ channel contribution to ACh release during passive transfer of LEMS appears to occur only after quantal release is significantly impaired for an extended duration, suggesting that an adaptive response of the ACh release apparatus occurs in LEMS.

Differential Ca2+-dependence of transmitter release mediated by P/Q- and N-type calcium channels at neonatal rat neuromuscular junctions

N- and P/Q-type voltage dependent calcium channels (VDCCs) mediate transmitter release at neonatal rat neuromuscular junction (NMJ). Thus the neonatal NMJ allows an examination of the coupling of different subtypes of VDCCs to the release process at a single synapse. We studied calcium dependence of transmitter release mediated by each channel by blocking with omega-conotoxin GVIA the N-type channel or with omega-agatoxin IVA the P/Q-type channel while changing the extracellular calcium concentration ([Ca2+]o). Transmitter release mediated by P/Q-type VDCCs showed steeper calcium dependence than N-type mediated release (average slope 3.6 +/- 0.09 vs. 2.6 +/- 0.03, respectively). Loading the nerve terminals with 10 microm BAPTA-AM in the extracellular solution reduced transmitter release and occluded the blocking effect of omega-conotoxin GVIA (blockade -2 +/- 9%) without affecting the action of omega-agatoxin IVA (blockade 85 +/- 4%). Both VDCC blockers were able to reduce the amount of facilitation produced by double-pulse stimulation. In these conditions facilitation was restored by increasing [Ca2+]o. The facilitation index (fi) was also reduced by loading nerve terminals with 10 microm BAPTA-AM (fi = 1.2 +/- 0.1). The control fi was 2.5 +/- 0.1. These results show that P/Q-type VDCCs were more efficiently coupled to neurotransmitter release than were N-type VDCCs at the neonatal neuromuscular junction. This difference could be accounted for by a differential location of these channels at the release site. In addition, our results indicate that space-time overlapping of calcium domains was required for facilitation.

A new omega-conotoxin that targets N-type voltage-sensitive calcium channels with unusual specificity

A new specific voltage-sensitive calcium channel (VSCC) blocker has been isolated from the venom of the fish-hunting cone snail Conus consors. This peptide, named omega-Ctx CNVIIA, consists of 27 amino acid residues folded by 3 disulfide bridges. Interestingly, loop 4, which is supposed to be crucial for selectivity, shows an unusual sequence (SSSKGR). The synthesis of the linear peptide was performed using the Fmoc strategy, and the correct folding was achieved in the presence of guanidinium chloride, potassium buffer, and reduced/oxidized glutathione at 4 degrees C for 3 days. Both synthetic and native toxin caused an intense shaking activity, characteristic of omega-conotoxins targeting N-type VSCC when injected intracerebroventricularly to mice. Binding studies on rat brain synaptosomes revealed that the radioiodinated omega-Ctx CNVIIA specifically and reversibly binds to high-affinity sites with a K(d) of 36.3 pM. Its binding is competitive with omega-Ctx MVIIA at low concentration (K(i) = 2 pM). Moreover, omega-Ctx CNVIIA exhibits a clear selectivity for N-type VSCCs versus P/Q-type VSCCs targeted respectively by radioiodinated omega-Ctx GVIA and omega-Ctx MVIIC. Although omega-Ctx CNVIIA clearly blocked N-type Ca(2+) current in chromaffin cells, this toxin did not inhibit acetylcholine release evoked by nerve stimuli at the frog neuromuscular junction, in marked contrast to omega-Ctx GVIA. omega-Ctx CNVIIA thus represents a new selective tool for blocking N-type VSCC that displays a unique pharmacological profile and highlights the diversity of voltage-sensitive Ca(2+) channels in the animal kingdom.

Synapse-glia interactions at the mammalian neuromuscular junction

Perisynaptic Schwann cells (PSCs) play critical roles in regulating and stabilizing nerve terminals at the mammalian neuromuscular junction (NMJ). However, although these functions are likely regulated by the synaptic properties, the interactions of PSCs with the synaptic elements are not known. Therefore, our goal was to study the interactions between mammalian PSCs in situ and the presynaptic terminals using changes in intracellular Ca(2+) as an indicator of cell activity. Motor nerve stimulation induced an increase in intracellular Ca(2+) in PSCs, and this increase was greatly reduced when transmitter release was blocked. Furthermore, local application of acetylcholine induced Ca(2+) responses that were blocked by the muscarinic antagonist atropine and mimicked by the muscarinic agonist muscarine. The nicotinic antagonist alpha-bungarotoxin had no effect on Ca(2+) responses induced by acetylcholine. Local application of the cotransmitter ATP induced Ca(2+) responses that were unaffected by the P2 antagonist suramin, whereas local application of adenosine induced Ca(2+) responses that were greatly reduced by the A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT). However, the presence of the A1 antagonist in the perfusate did not block responses induced by ATP. Ca(2+) responses evoked by stimulation of the motor nerve were reduced in the presence of CPT, whereas atropine almost completely abolished them. Ca(2+) responses were further reduced when both antagonists were present simultaneously. Hence, PSCs at the mammalian NMJ respond to the release of neurotransmitter induced by stimulation of the motor nerve through the activation of muscarinic and adenosine A1 receptors.

Synapse-glia interactions at the mammalian neuromuscular junction

Perisynaptic Schwann cells (PSCs) play critical roles in regulating and stabilizing nerve terminals at the mammalian neuromuscular junction (NMJ). However, although these functions are likely regulated by the synaptic properties, the interactions of PSCs with the synaptic elements are not known. Therefore, our goal was to study the interactions between mammalian PSCs in situ and the presynaptic terminals using changes in intracellular Ca(2+) as an indicator of cell activity. Motor nerve stimulation induced an increase in intracellular Ca(2+) in PSCs, and this increase was greatly reduced when transmitter release was blocked. Furthermore, local application of acetylcholine induced Ca(2+) responses that were blocked by the muscarinic antagonist atropine and mimicked by the muscarinic agonist muscarine. The nicotinic antagonist alpha-bungarotoxin had no effect on Ca(2+) responses induced by acetylcholine. Local application of the cotransmitter ATP induced Ca(2+) responses that were unaffected by the P2 antagonist suramin, whereas local application of adenosine induced Ca(2+) responses that were greatly reduced by the A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT). However, the presence of the A1 antagonist in the perfusate did not block responses induced by ATP. Ca(2+) responses evoked by stimulation of the motor nerve were reduced in the presence of CPT, whereas atropine almost completely abolished them. Ca(2+) responses were further reduced when both antagonists were present simultaneously. Hence, PSCs at the mammalian NMJ respond to the release of neurotransmitter induced by stimulation of the motor nerve through the activation of muscarinic and adenosine A1 receptors.

Amyotrophic lateral sclerosis IgG-treated neuromuscular junctions develop sensitivity to L-type calcium channel blocker

In order to search for early changes induced by the application of human immunoglobulin G (IgG) on motor nerve terminals, IgG from patients with amyotrophic lateral sclerosis (ALS) and control subjects was injected subcutaneously into the levator auris muscle of mice. A week or a month after the last injection, endplate potentials were recorded. No changes in quantal content of transmitter release were observed. In control and ALS IgG-treated muscles, neurotransmitter release remained sensitive to P/Q-type and insensitive to N-type voltage-sensitive calcium channel (VSCC) blockers as in untreated muscles. In contrast, IgG from 5 of 8 different ALS patients induced a significant reduction in quantal content of the evoked response after incubation with nitrendipine, indicating that a novel sensitivity to this calcium channel blocker appears in these motor nerve terminals. These results indicate that ALS IgG induces plastic changes at nerve terminals. The expression of transmitter release coupled to L-type VSCC indicate that ALS IgGs are capable of inducing plastic changes at the nerve terminals that may participate in the process leading to neuronal death.

Single-channel properties of BK-type calcium-activated potassium channels at a cholinergic presynaptic nerve terminal

1. A high-conductance calcium-activated potassium channel (BK KCa) was characterized at a cholinergic presynaptic nerve terminal using the calyx synapse isolated from the chick ciliary ganglion. 2. The channel had a conductance of 210 pS in a 150 mM:150 mM K+ gradient, was highly selective for K+ over Na+, and was sensitive to block by external charybdotoxin or tetraethylammonium (TEA) and by internal Ba2+. At +60 mV it was activated by cytoplasmic calcium [Ca2+]i with a Kd of approximately 0.5 microM and a Hill coefficient of approximately 2.0. At 10 microM [Ca2+]i the channel was 50 % activated (V) at -8.0 mV with a voltage dependence (Boltzmann slope-factor) of 32.7 mV. The V values hyperpolarized with an increase in [Ca2+]i while the slope factors decreased. There were no overt differences in conductance or [Ca2+]i sensitivity between BK channels from the transmitter release face and the non-release face. 3. Open and closed times were fitted by two and three exponentials, respectively. The slow time constants were strongly affected by both [Ca2+]i and membrane potential changes. 4. In cell-attached patch recordings BK channel opening was enhanced by a prepulse permissive for calcium influx through the patch, suggesting that the channel can be activated by calcium ion influx through neighbouring calcium channels. 5. The properties of the presynaptic BK channel are well suited for rapid activation during the presynaptic depolarization and Ca2+ influx that are associated with transmitter release. This channel may play an important role in terminating release by rapid repolarization of the action potential.

Calcium channels involved in the inhibition of acetylcholine release by presynaptic muscarinic receptors in rat striatum

1. The mechanism of the inhibitory action of presynaptic muscarinic receptors on the release of acetylcholine from striatal cholinergic neurons is not known. We investigated how the electrically stimulated release of [3H]-acetylcholine from superfused rat striatal slices and its inhibition by carbachol are affected by specific inhibitors of voltage-operated calcium channels of the L-type (nifedipine), N-type (omega-conotoxin GVIA) and P/Q-type (omega-agatoxin IVA). 2. The evoked release of [3H]-acetylcholine was not diminished by nifedipine but was lowered by omega-conotoxin GVIA and by omega-agatoxin IVA, indicating that both the N- and the P/Q-type (but not the L-type) channels are involved in the release. The N-type channels were responsible for approximately two thirds of the release. The release was >97% blocked when both omega-toxins acted together. 3. The inhibition of [3H]-acetylcholine release by carbachol was not substantially affected by the blockade of the L- or P/Q-type channels. It was diminished but not eliminated by the blockade of the N-type channels. 4. In experiments on slices in which cholinesterases had been inhibited by paraoxon, inhibition of [3H]-acetylcholine release by endogenous acetylcholine accumulating in the tissue could be demonstrated by the enhancement of the release after the addition of atropine. The inhibition was higher in slices with functional N-type than with functional P/Q-type channels. 5. We conclude that both the N- and the P/Q-type calcium channels contribute to the stimulation-evoked release of acetylcholine in rat striatum, that the quantitative contribution of the N-type channels is higher, and that the inhibitory muscarinic receptors are more closely coupled with the N-type than with the P/Q-type calcium channels.

Differential modulation of synaptic transmission by calcium chelators in young and aged hippocampal CA1 neurons: evidence for altered calcium homeostasis in aging

The effects of membrane-permeant Ca2+ chelators on field EPSPs (fEPSPs) were measured in the hippocampal CA1 region of brain slices from young (2-4 months) and old (24-27 months) Fischer 344 rats. BAPTA-AM depressed fEPSPs in young slices by up to 70% but enhanced fEPSPs by 30% in aged slices. EGTA-AM, with slower binding kinetics, did not affect fEPSPs from young slices but enhanced fEPSPs in aged slices. BAPTA derivatives with calcium dissociation constants (Kd) of 0.2-3.5 microM reduced or enhanced fEPSPs in young and aged slices, respectively, but 5',5'-dinitro BAPTA-AM (Kd of approximately 7000 microM) had no effect. Frequency facilitation of the fEPSPs occurred in young, but not in aged, slices, except when BAPTA-AM or EGTA-AM was perfused onto aged slices. The differential effects of BAPTA-AM in young and old slices were eliminated by perfusing with a low Ca2+-high Mg2+ saline or with the calcium blocker Co2+. These data suggest that intracellular Ca2+ regulation is altered and raised in aged neurons. Cell-permeant calcium buffers may be able to "ameliorate" deficits in synaptic transmission in the aged brain.

Differential modulation of synaptic transmission by calcium chelators in young and aged hippocampal CA1 neurons: evidence for altered calcium homeostasis in aging

The effects of membrane-permeant Ca2+ chelators on field EPSPs (fEPSPs) were measured in the hippocampal CA1 region of brain slices from young (2-4 months) and old (24-27 months) Fischer 344 rats. BAPTA-AM depressed fEPSPs in young slices by up to 70% but enhanced fEPSPs by 30% in aged slices. EGTA-AM, with slower binding kinetics, did not affect fEPSPs from young slices but enhanced fEPSPs in aged slices. BAPTA derivatives with calcium dissociation constants (Kd) of 0.2-3.5 microM reduced or enhanced fEPSPs in young and aged slices, respectively, but 5',5'-dinitro BAPTA-AM (Kd of approximately 7000 microM) had no effect. Frequency facilitation of the fEPSPs occurred in young, but not in aged, slices, except when BAPTA-AM or EGTA-AM was perfused onto aged slices. The differential effects of BAPTA-AM in young and old slices were eliminated by perfusing with a low Ca2+-high Mg2+ saline or with the calcium blocker Co2+. These data suggest that intracellular Ca2+ regulation is altered and raised in aged neurons. Cell-permeant calcium buffers may be able to "ameliorate" deficits in synaptic transmission in the aged brain.

Calcium channels coupled to neurotransmitter release at neonatal rat neuromuscular junctions

1. The effects of different calcium channel blockers (omega-agatoxin IVA (omega-Aga IVA), omega-conotoxin GVIA (omega-CgTx GVIA) and dihydropyridines) were tested on spontaneous and evoked transmitter release at embryonic and newborn rat neuromuscular junctions (NMJs). 2. The nerve-evoked transmitter release quantal content (m) was strongly reduced by the P/Q-type voltage-dependent calcium channel (VDCC) blocker omega-Aga IVA (100 nM) at newly formed endplates of embryos and 0- to 11-day-old rats, in agreement with the effect of this blocker on transmitter release at mature and reinnervating muscles. 3. omega-CgTx GVIA (1-5 microM), the N-type VDCC blocker, also caused a significant reduction in m at newly formed NMJs early in development (embryos and 0- to 4-day-old rats), while it was ineffective in more mature animals (5- to 11-day-old rats). 4. L-type channel blockers, nitrendipine (1 microM) and nifedipine (1 microM), did not significantly affect neurally evoked release at developing NMJs. However, nifedipine (10 microM) was able to increase m significantly at 0- to 4-day-old rat NMJs. 5. At developing NMJs, K+-evoked transmitter release was dependent on Ca2+ entry through VDCCs of the P/Q-type family (100 nM omega-Aga IVA reduced 70 % of the K+-evoked miniature endplate potential frequency). N- and L-type VDCC blockers did not affect this type of release. 6. We conclude that at rat neuromuscular junctions the presynaptic calcium channel types involved in transmitter release undergo developmental changes during the early postnatal period.

Single-cell RT-PCR and functional characterization of Ca2+ channels in motoneurons of the rat facial nucleus

Voltage-dependent Ca2+ channels are a major pathway for Ca2+ entry in neurons. We have studied the electrophysiological, pharmacological, and molecular properties of voltage-gated Ca2+ channels in motoneurons of the rat facial nucleus in slices of the brainstem. Most facial motoneurons express both low voltage-activated (LVA) and high voltage-activated (HVA) Ca2+ channel currents. The HVA current is composed of a number of pharmacologically separable components, including 30% of N-type and approximately 5% of L-type. Despite the dominating role of P-type Ca2+ channels in transmitter release at facial motoneuron terminals described in previous studies, these channels were not present in the cell body. Remarkably, most of the HVA current was carried through a new type of Ca2+ channel that is resistant to toxin and dihydropyridine block but distinct from the R-type currents described in other neurons. Using reverse transcription followed by PCR amplification (RT-PCR) with a powerful set of primers designed to amplify all HVA subtypes of the alpha1-subunit, we identified a highly heterogeneous expression pattern of Ca2+ channel alpha1-subunit mRNA in individual neurons consistent with the Ca2+ current components found in the cell bodies and axon terminals. We detected mRNA for alpha1A in 86% of neurons, alpha1B in 59%, alpha1C in 18%, alpha1D in 18%, and alpha1E in 59%. Either alpha1A or alpha1B mRNAs (or both) were present in all neurons, together with various other alpha1-subunit mRNAs. The most frequently occurring combination was alpha1A with alpha1B and alpha1E. Taken together, these results demonstrate that the Ca2+ channel pattern found in facial motoneurons is highly distinct from that found in other brainstem motoneurons.

Single-cell RT-PCR and functional characterization of Ca2+ channels in motoneurons of the rat facial nucleus

Voltage-dependent Ca2+ channels are a major pathway for Ca2+ entry in neurons. We have studied the electrophysiological, pharmacological, and molecular properties of voltage-gated Ca2+ channels in motoneurons of the rat facial nucleus in slices of the brainstem. Most facial motoneurons express both low voltage-activated (LVA) and high voltage-activated (HVA) Ca2+ channel currents. The HVA current is composed of a number of pharmacologically separable components, including 30% of N-type and approximately 5% of L-type. Despite the dominating role of P-type Ca2+ channels in transmitter release at facial motoneuron terminals described in previous studies, these channels were not present in the cell body. Remarkably, most of the HVA current was carried through a new type of Ca2+ channel that is resistant to toxin and dihydropyridine block but distinct from the R-type currents described in other neurons. Using reverse transcription followed by PCR amplification (RT-PCR) with a powerful set of primers designed to amplify all HVA subtypes of the alpha1-subunit, we identified a highly heterogeneous expression pattern of Ca2+ channel alpha1-subunit mRNA in individual neurons consistent with the Ca2+ current components found in the cell bodies and axon terminals. We detected mRNA for alpha1A in 86% of neurons, alpha1B in 59%, alpha1C in 18%, alpha1D in 18%, and alpha1E in 59%. Either alpha1A or alpha1B mRNAs (or both) were present in all neurons, together with various other alpha1-subunit mRNAs. The most frequently occurring combination was alpha1A with alpha1B and alpha1E. Taken together, these results demonstrate that the Ca2+ channel pattern found in facial motoneurons is highly distinct from that found in other brainstem motoneurons.