Biomedical subjects

M P Charlton

Publications and source records attributed to M P Charlton.

At least 19 recordsLinked to original sources

PubMed2006

The Drosophila cacts2 mutation reduces presynaptic Ca2+ entry and defines an important element in Cav2.1 channel inactivation.

Voltage-gated Ca2+ channels in nerve terminals open in response to action potentials and admit Ca2+, the trigger for neurotransmitter release. The cacophony gene encodes the primary presynaptic voltage-gated Ca2+ channel in Drosophila motor-nerve terminals. The cac(ts2) mutant allele of cacophony is associated with paralysis and reduced neurotransmission at non-permissive temperatures but the basis for the neurotransmission deficit has not been established. The cac(ts2) mutation occurs in the cytoplasmic carboxyl tail of the alpha1-subunit, not within the pore-forming trans-membrane domains, making it difficult to predict the mutation's impact. We applied a Ca2+-imaging technique at motor-nerve terminals of mutant larvae to test the hypothesis that the neurotransmission deficit is a result of impaired Ca2+ entry. Presynaptic Ca2+ signals evoked by single and multiple action potentials showed a temperature-dependent reduction. The amplitude of the reduction was sufficient to account for the neurotransmission deficit, indicating that the site of the cac(ts2) mutation plays a role in Ca2+ channel activity. As the mutation occurs in a motif conserved in mammalian high-voltage-activated Ca2+ channels, we used a heterologous expression system to probe the effect of this mutation on channel function. The mutation was introduced into rat Ca(v)2.1 channels expressed in human embryonic kidney cells. Patch-clamp analysis of mutant channels at the physiological temperature of 37 degrees C showed much faster inactivation rates than for wild-type channels, demonstrating that the integrity of this motif is critical for normal Ca(v)2.1 channel inactivation.

Amino Acid Sequence↗

PubMed2003

Single neuron activity in the Drosophila larval CNS detected with calcium indicators.

Although the Drosophila larva has been extensively used for genetic studies of synaptic transmission and locomotion, neurophysiological studies have lagged because it is difficult to investigate circuitry and synaptic function in the larval central nervous system (CNS). Here we introduce an optical technique to monitor neuronal activity in the intact Drosophila larval CNS. We loaded neurons retrogradely through cut axons with dextran-conjugated calcium indicators. Fluorescence responses to changes in the concentration of intracellular calcium are sufficiently fast and large to monitor electrical activity in single neurons. Responses to action potentials were detected in motor neuron cell bodies, axons, neurites, dendrites and sensory neuron afferents identified by genetically targeted green fluorescent protein expression. Our findings provide an experimental procedure for testing synaptic function and connectivity within the intact larval CNS.

Animals↗

PubMed2002

Fast calcium signals in Drosophila motor neuron terminals.

Drosophila is a powerful model for neuroscientists, but physiological techniques have not kept pace with advances in molecular genetics. We introduce a reliable assay for intracellular calcium dynamics in Drosophila larval motor neuron terminals, and a new physiological solution that improves the longevity of the larval preparation. By loading calcium indicators into motor neuron terminals through cut axons, we obtained a high signal-to-noise ratio with confocal microscopy, and good temporal resolution of calcium-dependent fluorescence changes. We provide an estimate for the resting intracellular calcium concentration, the first description of calcium kinetics for a single action potential (AP), and improved resolution of calcium kinetics during AP trains. The very rapid decay of the calcium signal following a single AP (tau ~60 ms) indicates a previously unreported fast calcium extrusion mechanism in Drosophila motor neuron terminals well suited for sustaining physiological processes during the high rates of impulse activity which drive locomotor activity.

Animals↗

PubMed2000

alpha-Latrotoxin releases calcium in frog motor nerve terminals.

alpha-Latrotoxin (alpha-LTX) is a neurotoxin that accelerates spontaneous exocytosis independently of extracellular Ca(2+). Although alpha-LTX increases spontaneous transmitter release at synapses, the mechanism is unknown. We tested the hypothesis that alpha-LTX causes transmitter release by mobilizing intracellular Ca(2+) in frog motor nerve terminals. Transmitter release was measured electrophysiologically and with the vesicle marker FM1-43; presynaptic ion concentration dynamics were measured with fluorescent ion-imaging techniques. We report that alpha-LTX increases transmitter release after release of a physiologically relevant concentration of intracellular Ca(2+). Neither the blockade of Ca(2+) release nor the depletion of Ca(2+) from endoplasmic reticulum affected Ca(2+) signals produced by alpha-LTX. The Ca(2+) source is likely to be mitochondria, because the effects on Ca(2+) mobilization of CCCP (which depletes mitochondrial Ca(2+)) and of alpha-LTX are mutually occlusive. The release of mitochondrial Ca(2+) is partially attributable to an increase in intracellular Na(+), suggesting that the mitochondrial Na(+)/Ca(2+) exchanger is activated. Effects of alpha-LTX were not blocked when Ca(2+) increases were reduced greatly in saline lacking both Na(+) and Ca(2+) and by application of intracellular Ca(2+) chelators. Therefore, although increases in intracellular Ca(2+) may facilitate the effects of alpha-LTX on transmitter release, these increases do not appear to be necessary. The results show that investigations of Ca(2+)-independent alpha-LTX mechanisms or uses of alpha-LTX to probe exocytosis mechanisms would be complicated by the release of intracellular Ca(2+), which itself can trigger exocytosis.

Animals↗

PubMed1999

Calcium entry related to active zones and differences in transmitter release at phasic and tonic synapses.

Synaptic functional differentiation of crayfish phasic and tonic motor neurons is large. For one impulse, quantal release of neurotransmitter is typically 100-1000 times higher for phasic synapses. We tested the hypothesis that differences in synaptic strength are determined by differences in synaptic calcium entry. Calcium signals were measured with the injected calcium indicator dyes Calcium Green-1 and fura-2. Estimated Ca(2+) entry increased almost linearly with frequency for both axons and was two to three times larger in phasic terminals. Tonic terminal Ca(2+) at 10 Hz exceeded phasic terminal Ca(2+) at 1 Hz, yet transmitter release was much higher for phasic terminals at these frequencies. Freeze-fracture images of synapses revealed on average similar numbers of prominent presynaptic active zone particles (putative ion channels) for both neurons and a two- to fourfold phasic/tonic ratio of active zones per terminal volume. This can account for the larger calcium signals seen in phasic terminals. Thus, differences in synaptic strength are less closely linked to differences in synaptic channel properties and calcium entry than to differences in calcium sensitivity of transmitter release.

Animals↗

PubMed1999

Muscarinic control of cytoskeleton in perisynaptic glia.

Similar to astrocytes at CNS synapses, perisynaptic Schwann cells (PSCs) surround nerve terminals at the neuromuscular junction (NMJ). These special teloglial cells are sensitive to neurotransmitters and upregulate glial fibrillary acidic protein (GFAP) when deprived of synaptic activity. We found that activation of muscarinic acetylcholine receptors (mAChRs) at PSCs, but not purinergic (ATP and adenosine) or peptidergic [substance P (SP) and calcitonin gene-related peptide (CGRP)] receptors, prevented this upregulation. When applied onto single PSCs, muscarine evoked Ca2+ responses that fatigued but prevented upregulation of this glial cytoskeletal protein. Application of ATP onto single PSCs evoked Ca2+ signals that showed little fatigue, and GFAP upregulation occurred. Thus, Ca2+ signals alone cannot prevent GFAP upregulation in the PSCs. After blockade of cholinergic receptors by gallamine, neuronal activity was not effective in maintaining low GFAP levels in the perisynaptic glia. Last, immunohistochemistry disclosed mAChRs on PSCs and nearby fibroblasts. Thus, acetylcholine secreted by the nerve terminal acts on the PSCs via mAChRs to regulate GFAP. Cytoskeletal changes may influence perisynaptic glial functions, including growth, remodeling, and modulation of the synapse.

Animals↗

PubMed1999

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.

Animals↗

PubMed1999

Non-myelin-forming perisynaptic schwann cells express protein zero and myelin-associated glycoprotein.

Perisynaptic Schwann cells (PSCs) envelop axonal terminals and are physiologically distinct from the nearby myelinating Schwann cells (MSCs), which surround the same innervating motor axons. PSCs have special functions at the neuromuscular synapse, where they detect and can modulate neurotransmitter release. Although PSCs are similar to non-myelinating Schwann cells in that they do not form multiple myelin wrappings around nerve terminals, they do wrap around single nerve terminals. These differences, as well as others, lead us to question whether PSCs are truly of Schwann cell origin. We thus characterized the expression of molecules, classically associated with myelin and Schwann cells, in PSCs at the frog neuromuscular junction. We wondered whether PSCs express the Schwann cell marker protein zero (P(0)) and whether their lack of myelination was related to an absence of myelin-associated glycoprotein (MAG), a protein found in myelinating cells that is considered important in myelination. Instead, we found that PSCs express both P(0) and MAG, and other myelinating glial markers such as galactocerebroside and 2',3'-cyclic nucleotide 3'-phosphodiesterase. In denervated preparations, P(0) and MAG expression persisted, including at newly formed PSC extensions. Because PSCs do not myelinate, it is clear that expression of these proteins alone is not sufficient for myelin formation. It is possible that factors present at synapses may prevent myelination, while P(0) and MAG may mediate adhesion between nerve terminals and the surrounding PSCs. The results indicate that PSCs are of Schwann cell origin.

2',3'-Cyclic-Nucleotide Phosphodiesterases↗

PubMed1999

Activity-dependent changes in partial VAMP complexes during neurotransmitter release.

The temporal sequence of SNARE protein interactions that cause exocytosis is unknown. Blockade of synaptic neurotransmitter release through cleavage of VAMP/synaptobrevin by tetanus toxin light chain (TeNT-LC) was accelerated by nerve stimulation. Botulinum/B neurotoxin light chain (BoNT/B-LC), which cleaves VAMP at the same site as TeNT-LC, did not require stimulation. Because TeNT-LC requires the N-terminal coil domain of VAMP for binding but BoNT/B-LC requires the C-terminal coil domain, it seems that, before nerve activity, the N-terminal domain is shielded in a protein complex, but the C-terminal domain is exposed. This N-terminal complex lasts until nerve activity occurs and may serve to cock synaptic vesicles for immediate exocytosis upon Ca2+ entry.

Animals↗

PubMed1999

alpha-latrocrustatoxin increases neurotransmitter release by activating a calcium influx pathway at crayfish neuromuscular junction.

alpha-latrocrustatoxin (alpha-LCTX), a component of black widow spider venom (BWSV), produced a 50-fold increase in the frequency of spontaneously occurring miniature excitatory postsynaptic potentials (mEPSPs) at crayfish neuromuscular junctions but did not alter their amplitude distribution. During toxin action, periods of high-frequency mEPSP discharge were punctuated by periods in which mEPSP frequency returned toward control levels. EPSPs were increased in amplitude during periods of enhanced mEPSP discharge. alpha-LCTX had no effect when applied in Ca(2+)-free saline, but subsequent addition of Ca(2+) caused an immediate enhancement of mEPSP frequency even when alpha-LCTX was previously washed out of the bath with Ca(2+)-free saline. Furthermore removal of Ca(2+) from the saline after alpha-LCTX had elicited an effect immediately blocked the action on mEPSP frequency. Thus alpha-LCTX binding is insensitive to Ca(2+), but toxin action requires extracellular Ca(2+) ions. Preincubation with wheat germ agglutinin prevented the effect of alpha-LCTX but not its binding. These binding characteristics suggest that the toxin may bind to a crustacean homologue of latrophilin/calcium-independent receptor for latrotoxin, a G-protein-coupled receptor for alpha-latrotoxin (alpha-LTX) found in vertebrates. alpha-LCTX caused "prefacilitation" of EPSP amplitudes, i.e., the first EPSP in a train was enhanced in amplitude to a greater degree than subsequent EPSPs. A similar alteration in the pattern of facilitation was observed after application of the Ca(2+) ionophore, A23187, indicating that influx of Ca(2+) may mediate the action of alpha-LCTX. In nerve terminals filled with the Ca(2+) indicator, calcium green 1, alpha-LCTX caused increases in the fluorescence of the indicator that lasted for several minutes before returning to rest. Neither fluorescence changes nor toxin action on mEPSP frequency were affected by the Ca(2+) channel blockers omega-agatoxin IVA or Cd(2+), demonstrating that Ca(2+) influx does not occur via Ca(2+) channels normally coupled to transmitter release in this preparation. The actions of alpha-LCTX could be reduced dramatically by intracellular application of the Ca(2+) chelator, bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid. We conclude that induction of extracellular Ca(2+) influx into nerve terminals is sufficient to explain the action of alpha-LCTX on both spontaneous and evoked transmitter release at crayfish neuromuscular junctions.

Animals↗

PubMed1999

Retention of cleaved synaptosome-associated protein of 25 kDa (SNAP-25) in neuromuscular junctions: a new hypothesis to explain persistence of botulinum A poisoning.

Botulinum neurotoxins can block neurotransmitter release for several months. The molecular mechanism of these toxins' action is known, but the persistence of neuromuscular paralysis that they cause is unexplained. At frog neuromuscular junctions, application of botulinum toxin type A caused paralysis and reduced the C-terminus immunoreactivity of SNAP-25, but not that of the remaining N-terminus fragment. Botulinum toxin type C caused paralysis and reduced syntaxin immunoreactivity without affecting that of SNAP-25. Co-application of botulinum A and C reduced syntaxin immunoreactivity, and that of both C and N termini of SNAP-25. Application of hydroxylamine to de-palmitoylate SNAP-25 resulted in a slight reduction of the immunoreactivity of SNAP-25 N terminus, while it had no effect on immunoreactivity of botulinum A cleaved SNAP-25. In contrast, application of hydroxylamine to nerve terminals where syntaxin had been cleaved by botulinum C caused a considerable reduction in SNAP-25 N-terminus immunoreactivity. Hence the retention of immunoreactive SNAP-25 at the neuromuscular junction depends on its interactions with syntaxin and plasma membrane. Persistence of cleaved SNAP-25 in nerve terminals may prevent insertion of new SNAP-25 molecules, thereby contributing to the longevity of botulinum A effects.

Animals↗

PubMed1999

Effects of adenosine on Ca2+ entry in the nerve terminal of the frog neuromuscular junction.

This study aimed to test whether nerve-evoked and adenosine-induced synaptic depression are due to reduction in Ca2+ entry in nerve terminals of the frog neuromuscular junction. Nerve terminals were loaded with the fluorescent Ca2+ indicator fluo 3 (fluo 3-AM) or loaded with dextran-coupled Ca2+ green-1 transported from the cut end of the nerve. Adenosine (10-50 microM) did not change the resting level of Ca2+ in the presynaptic terminal, whereas it induced large Ca2+ responses in perisynaptic Schwann cells, indicating that adenosine was active and might have induced changes in the level of Ca2+ in the nerve terminal. Ca2+ responses in nerve terminals could be induced by nerve stimulation (0.5 or 100 Hz for 100 ms) over several hours. In the presence of adenosine (10 microM), the size and duration of the nerve-evoked Ca2+ responses were unchanged. When extracellular Ca2+ concentration was lowered to produce the same reduction in transmitter release as the application of adenosine, Ca2+ responses induced by nerve stimulations were reduced by 40%. This indicates that changes in Ca2+ responsible for the decrease in release should have been detected if the mechanism of adenosine depression involved partial block of Ca2+ influx. Ca2+ responses evoked by prolonged high frequency trains of stimuli (50 Hz for 10 or 30 s), which caused profound depression of transmitter release, were sustained during the whole duration of the stimulation, and adenosine had no effect on these responses. These data indicate that neither adenosine induced synaptic depression nor stimulation-induced synaptic depression are caused by reductions in Ca2+ entry into the presynaptic terminal in the frog neuromuscular junction.

Adenosine↗

PubMed1998

Protein interactions implicated in neurotransmitter release.

Biochemical evidence indicates that the exocytotic release of neurotransmitters involves both evolutionary conserved membrane proteins, the SNAREs, as well as ubiquitous cytosolic fusion proteins, NSF and SNAPs. We have analyzed the biochemical properties and the physiological effects of these proteins. Our data suggest models how NSF, SNAPs and SNAREs may function in neurotransmitter exocytosis.

Adenosine Triphosphatases↗

PubMed1998

Presynaptic protein interactions in vivo: evidence from botulinum A, C, D and E action at frog neuromuscular junction.

The present study examines the paralytic action of botulinum neurotoxins at their natural target, the neuromuscular junction. We asked whether syntaxin, synaptosome-associated protein of 25 kDa (SNAP-25) and vesicle-associated membrane protein (VAMP/synaptobrevin), the proteins proteolysed by botulinum, are susceptible to cleavage in frog nerve terminals, and whether they form complexes in vivo. In control terminals, the three SNAREs were distributed in broad bands at 1 micrometer intervals, at sites consistent with presynaptic Ca2+ channels. Within 3 h, botulinum A, C, D and E (BoNT/A/C/D/E) blocked nerve-evoked muscle contractions but their effects on substrate immunoreactivity varied. The effect of BoNT/A on either C-terminus or N-terminus immunoreactivity of SNAP-25 was undetectable after 3-h incubation, although C-terminus immunoreactivity was reduced after 24 h; N-terminus immunoreactivity was not affected even after 36 h. BoNT/E reduced C-terminus immunoreactivity of SNAP-25 1.5 h after toxin application when transmitter release was blocked, but required 24 h to reduce N-terminus immunoreactivity. BoNT/C reduced syntaxin immunoreactivity after 24-h incubation but did not affect SNAP-25. BoNT/D reduced VAMP immunoreactivity at 3 h while it increased SNAP-25 C-terminal staining fourfold. BoNT/A and BoNT/C applied together for 24 h reduced syntaxin immunoreactivity and that of both C- and N-terminus of SNAP-25, indicating that retention of SNAP-25 N-terminus after cleavage by BoNT/A depended on intact syntaxin. Therefore, we infer that SNAP-25 interacts with VAMP and with syntaxin in vivo. Neurotoxin action abolished only 40-60% of SNAP-25, VAMP or syntaxin immunoreactivity suggesting that distinct pools of these proteins, not immediately involved in triggered exocytosis, are resistant to proteolysis.

Animals↗

PubMed1998

Distinct influx pathways, not calcium load, determine neuronal vulnerability to calcium neurotoxicity.

Many forms of neurodegeneration are ascribed to excessive cellular Ca2+ loading (Ca2+ hypothesis). We examined quantitatively whether factors other than Ca2+ loading were determinants of excitotoxic neurodegeneration. Cell survival, morphology, free intracellular Ca2+ concentration ([Ca2+]i), and 45Ca2+ accumulation were measured in cultured cortical neurons loaded with known quantities of Ca2+ through distinct transmembrane pathways triggered by excitatory amino acids, cell membrane depolarization, or Ca2+ ionophores. Contrary to the Ca2+ hypothesis, the relationships between Ca2+ load and cell survival, free [Ca2+]i, and Ca2+-induced morphological alterations depended primarily on the route of Ca2+ influx, not the Ca2+ load. Notably, Ca2+ loading via NMDA receptor channels was toxic, whereas identical Ca2+ loads incurred through voltage-sensitive Ca2+ channels were completely innocuous. Furthermore, accounting quantitatively for Ca2+ loading via NMDA receptors uncovered a previously unreported component of L-glutamate neurotoxicity apparently not mediated by ionotropic or metabotropic glutamate receptors. It was synergistic with toxicity attributable to glutamate-evoked Ca2+ loading, and correlated with enhanced cellular ATP depletion. This previously unrecognized toxic action of glutamate constituted a chief excitotoxic mechanism under conditions producing submaximal Ca2+ loading. We conclude that (a) Ca2+ neurotoxicity is a function of the Ca2+ influx pathway, not Ca2+ load, and (b) glutamate toxicity may not be restricted to its actions on glutamate receptors.

Animals↗

PubMed1998

Different VAMP/synaptobrevin complexes for spontaneous and evoked transmitter release at the crayfish neuromuscular junction.

Different VAMP/synaptobrevin complexes for spontaneous and evoked transmitter release at the crayfish neuromuscular junction. J. Neurophysiol. 80: 3233-3246, 1998. Although vesicle-associated membrane protein (VAMP/synaptobrevin) is essential for evoked neurotransmitter release, its role in spontaneous transmitter release remains uncertain. For instance, many studies show that tetanus toxin (TeNT), which cleaves VAMP, blocks evoked transmitter release but leaves some spontaneous transmitter release. We used recombinant tetanus and botulinum neurotoxin catalytic light chains (TeNT-LC, BoNT/B-LC, and BoNT/D-LC) to examine the role of VAMP in spontaneous transmitter release at neuromuscular junctions (nmj) of crayfish. Injection of TeNT-LC into presynaptic axons removed most of the VAMP immunoreactivity and blocked evoked transmitter release without affecting nerve action potentials or Ca2+ influx. The frequency of spontaneous transmitter release was little affected by the TeNT-LC when the evoked transmitter release had been blocked by >95%. The spontaneous transmitter release left after TeNT-LC treatment was insensitive to increases in intracellular Ca2+. BoNT/B-LC, which cleaves VAMP at the same site as TeNT-LC but uses a different binding site, also blocked evoked release but had minimal effect on spontaneous release. However, BoNT/D-LC, which cleaves VAMP at a different site from the other two toxins but binds to the same position on VAMP as TeNT, blocked both evoked and spontaneous transmitter release at similar rates. The data indicate that different VAMP complexes are employed for evoked and spontaneous transmitter release; the VAMP used in spontaneous release is not readily cleaved by TeNT or BoNT/B. Because the exocytosis that occurs after the action of TeNT cannot be increased by increased intracellular Ca2+, the final steps in neurotransmitter release are Ca2+ independent.

Animals↗

PubMed1997

Disruption of syntaxin-mediated protein interactions blocks neurotransmitter secretion.

The membrane protein syntaxin participates in several protein-protein interactions that have been implicated in neurotransmitter release. To probe the physiological importance of these interactions, we microinjected into the squid giant presynaptic terminal botulinum toxin C1, which cleaves syntaxin, and the H3 domain of syntaxin, which mediates binding to other proteins. Both reagents inhibited synaptic transmission yet did not affect the number or distribution of synaptic vesicles at the presynaptic active zone. Recombinant H3 domain inhibited the interactions between syntaxin and SNAP-25 that underlie the formation of stable SNARE complexes in vitro. These data support the notion that syntaxin-mediated SNARE complexes are necessary for docked synaptic vesicles to fuse.

Amino Acid Sequence↗

PubMed1997

Muscarinic Ca2+ responses resistant to muscarinic antagonists at perisynaptic Schwann cells of the frog neuromuscular junction.

1. Acetylcholine causes a rise of intracellular Ca2+ in perisynaptic Schwann cells (PSCs) of the frog neuromuscular junction. The signalling pathway was characterized using the fluorescent Ca2+ indicator fluo-3 and fluorescence microscopy. 2. Nicotinic antagonists had no effect on Ca2+ responses evoked by ACh and no Ca2+ responses were evoked with the nicotinic agonist nicotine. The muscarinic agonists muscarine and oxotremorine-M induced Ca2+ signals in PSCs. 3. Ca2+ responses remained unchanged when extracellular Ca2+ was removed, indicating that they are due to the release of Ca2+ from internal stores. Incubation with pertussis toxin did not alter the Ca2+ signals induced by muscarine, but did block depression of transmitter release induced by adenosine and prevented Ca2+ responses in PSCs induced by adenosine. 4. The general muscarinic antagonists atropine, quinuclidinyl benzilate and N-methyl-scopolamine failed to block Ca2+ responses to muscarinic agonists. Atropine (at 20,000-fold excess concentration) also failed to reduce the proportion of cells responding to a threshold muscarine concentration sufficient to cause responses in less than 50% of cells. Only the allosteric, non-specific blocker, gallamine (1-10 microM) was effective in blocking muscarine-induced Ca2+ responses. 5. In preparations denervated 7 days prior to experiments, low concentrations of atropine reversibly and completely blocked Ca2+ responses to muscarine. 6. The lack of blockade by general muscarinic antagonists in innervated, in situ preparations suggests that muscarinic Ca2+ responses at PSCs are not mediated by any of the five known muscarinic receptors or that post-translational modification prevented antagonist binding.

Acetylcholine↗