Functional coupling of Ca(2+) channels to ryanodine receptors at presynaptic terminals. Amplification of exocytosis and plasticity

Quantal size increase induced by the endocannabinoid 2-arachidonoylglycerol requires activation of CGRP receptors in mouse motor synapses

In mouse motor synapses, the exogenous application of the endocannabinoid (EC) 2-arachidonoylglycerol (2-AG) increases acetylcholine (ACh) quantal size due to the activation of CB1 receptors and the stimulation of ACh vesicular uptake. In the present study, microelectrode recordings of miniature endplate potentials (MEPP) revealed that this effect of 2-AG is independent of brain-derived neurotrophic factor (BDNF) signaling but involves the activation of calcitonin gene-related peptide (CGRP) receptors along with CB1 receptors. Potentiation of MEPP amplitude in the presence of 2-AG was prevented by blockers of CGRP receptors and ryanodine receptors (RyR) and by inhibitors of phospholipase C (PLC) and Ca2+ /calmodulin-dependent protein kinase II (CaMKII). Therefore, we suggest a hypothetical chain of events, which starts from the activation of presynaptic CB1 receptors, involves PLC, RyR, and CaMKII, and results in CGRP release with the subsequent activation of presynaptic CGRP receptors. Activation of CGRP receptors is probably a part of a complex molecular cascade leading to the 2-AG-induced increase in ACh quantal size and MEPP amplitude. We propose that the same chain of events may also take place if 2-AG is endogenously produced in mouse motor synapses, because the increase in MEPP amplitude that follows after prolonged tetanic muscle contractions (30 Hz, 2 min) was prevented by the blocking of CB1 receptors. This work may help to unveil the previously unknown aspects of the functional interaction between ECs and peptide modulators aimed at the regulation of quantal size and synaptic transmission.

Spatially non-overlapping Ca2+ signals drive distinct forms of neurotransmission

Calcium (Ca2+) signaling is tightly regulated within a presynaptic bouton. Here, we visualize Ca2+ signals within hippocampal presynaptic boutons using GCaMP8s tagged to synaptobrevin, a synaptic vesicle protein. We identify evoked presynaptic Ca2+ transients (ePreCTs) that derive from synchronized voltage-gated Ca2+ channel openings, spontaneous presynaptic Ca2+ transients (sPreCTs) that originate from ryanodine sensitive Ca2+ stores, and a baseline Ca2+ signal that arises from stochastic voltage-gated Ca2+ channel openings. We find that baseline Ca2+, but not sPreCTs, contributes to spontaneous glutamate release. We employ photobleaching as a use-dependent tool to probe nano-organization of Ca2+ signals and observe that all three occur in non-overlapping domains within the synapse at near-resting conditions. However, increased depolarization induces intermixing of these Ca2+ domains via both local and non-local synaptic vesicle turnover. Our findings reveal nanosegregation of Ca2+ signals within a presynaptic terminal that derive from multiple sources and in turn drive specific modes of neurotransmission.

The Role of Ryanodine Receptors in Regulating Neuronal Activity and Its Connection to the Development of Alzheimer's Disease

Research into the early impacts of Alzheimer's disease (AD) on synapse function is one of the most promising approaches to finding a treatment. In this context, we have recently demonstrated that the Abeta42 peptide, which builds up in the brain during the processing of the amyloid precursor protein (APP), targets the ryanodine receptors (RyRs) of mouse hippocampal neurons and potentiates calcium (Ca²⁺) release from the endoplasmic reticulum (ER). The uncontrolled increase in intracellular calcium concentration ([Ca²⁺]_i), leading to the development of Ca²⁺ dysregulation events and related excitable and synaptic dysfunctions, is a consolidated hallmark of AD onset and possibly other neurodegenerative diseases. Since RyRs contribute to increasing [Ca²⁺]_i and are thought to be a promising target for AD treatment, the goal of this review is to summarize the current level of knowledge regarding the involvement of RyRs in governing neuronal function both in physiological conditions and during the onset of AD.

Distinct pools of synaptic vesicles are released by different calcium channels

Mueller et al. [1] uncover distinct roles for Ca_V1 and Ca_V2 channels in neurotransmitter release at the C. elegans neuromuscular junction. Although nanodomain coupling occurs via clustered Ca_V2 channels, evidence is also presented that release of a separate vesicular pool is mediated by more peripheral, dispersed Ca_V1 channels, requiring obligatory coupling with RYR to amplify the Ca²⁺ signal.

Roles and Sources of Calcium in Synaptic Exocytosis

Calcium ions (Ca2+) play a critical role in triggering neurotransmitter release. The rate of release is directly related to the concentration of Ca2+ at the presynaptic site, with a supralinear relationship. There are two main sources of Ca2+ that trigger synaptic vesicle fusion: influx through voltage-gated Ca2+ channels in the plasma membrane and release from the endoplasmic reticulum via ryanodine receptors. This chapter will cover the sources of Ca2+ at the presynaptic nerve terminal, the relationship between neurotransmitter release rate and Ca2+ concentration, and the mechanisms that achieve the necessary Ca2+ concentrations for triggering synaptic exocytosis at the presynaptic site.

Effects Of Doxycycline On Mice Neuromuscular Junction, In Situ

BACKGROUND

The antibacterial mechanism of doxycycline is known, but on the nerve-muscle apparatus is yet unclear.

OBJECTIVE

To combine molecular targets of the neuromuscular machinery using the neuronal blocker effect doxycycline, a semisynthetic second-generation tetracycline derivative, on mice neuromuscular preparations, in situ.

METHODS

Doxycycline was assessed at the neurotransmission; presynaptic; synaptic cleft; and postsynaptic, including the muscle fiber, using the traditional myographic technique. Preliminarily, doxycycline showed an "all or nothing" effect, being "all" obtained with 4 µM and "nothing", with 1-3 µM. The rationale of this study was to apply known pharmacological tools against the blocker effect of 4 µM doxycycline such as F55-6 (Casearia sylvestris), CaCl2 (or Ca2+), atropine, neostigmine, polyethylene glycol (PEG 400), and d-Tubocurarine. The evaluation of cholinesterase enzyme activity, the diaphragm muscle histology, and protocols on the neuromuscular preparation submitted to indirect or direct stimuli were complementary.

RESULTS

Doxycycline does not affect cholinesterase activity nor cause damage to skeletal muscle diaphragm; acts on ryanodine receptor, sarcolemmal membrane, and on neuronal sodium channel with a postjunctional consequence due to the decreased availability of muscle nicotinic acetylcholine receptors.

CONCLUSIONS

In conclusion, using the blocker effect we showed that doxycycline acts on multiple targets, among them, is antagonized by F55-6, a neuronal Na+-channel agonist and Ca2+, but not by neostigmine.

Delayed increase of acetylcholine quantal size induced by the activity-dependent release of endogenous CGRP but not ATP in neuromuscular junctions

In mouse motor synapses tetanic neuromuscular activity (30 Hz, 2 min) led to a delayed posttetanic potentiation of amplitude and duration of spontaneous miniature endplate potentials (MEPPs). Microelectrode recordings of MEPPs before and after nerve stimulation showed an increase in MEPP amplitude and time course by 30% and 15%, respectively, without changes in their frequency. Peak effect was detected 20 min after tetanic activity and progressively faded throughout the next 40 min of recording. The revealed potentiation of MEPPs was fully preserved in preparations from pannexin 1 knockout mice. It means, that myogenic ATP released via pannexin 1 channels from contracting muscle fibers is not likely to participate in the described phenomenon. But posttetanic potentiation of MEPPs was fully prevented by competitive antagonist of calcitonin gene-related peptide (CGRP) receptors CGRP_8-37 , ryanodine receptors inhibitor ryanodine and by vesicular acetylcholine transporter inhibitor vesamicol. It is suggested that the combination of intensive synaptic and contractile activity in neuromuscular junctions is required to induce Ca²⁺ -dependent exocytosis of endogenous CGRP. The accumulation of CGRP in the synaptic cleft and its presynaptic activity may induce posttetanic potentiation of MEPP amplitude due to CGRP-stimulated acetylcholine loading into vesicles and subsequent increase of quantal size.

Voltage-induced Ca2+ release by ryanodine receptors causes neuropeptide secretion from nerve terminals

Depolarisation-secretion coupling is assumed to be dependent only on extracellular calcium ([Ca²⁺ ]_o ). Ryanodine receptor (RyR)-sensitive stores in hypothalamic neurohypophysial system (HNS) terminals produce sparks of intracellular calcium ([Ca²⁺ ]_i ) that are voltage-dependent. We hypothesised that voltage-elicited increases in intraterminal calcium are crucial for neuropeptide secretion from presynaptic terminals, whether from influx through voltage-gated calcium channels and/or from such voltage-sensitive ryanodine-mediated calcium stores. Increases in [Ca²⁺ ]_i upon depolarisation in the presence of voltage-gated calcium channel blockers, or in the absence of [Ca²⁺ ]_o , still give rise to neuropeptide secretion from HNS terminals. Even in 0 [Ca²⁺ ]_o , there was nonetheless an increase in capacitance suggesting exocytosis upon depolarisation. This was blocked by antagonist concentrations of ryanodine, as was peptide secretion elicited by high K⁺ in 0 [Ca²⁺ ]_o . Furthermore, such depolarisations lead to increases in [Ca²⁺ ]_i . Pre-incubation with BAPTA-AM resulted in > 50% inhibition of peptide secretion elicited by high K⁺ in 0 [Ca²⁺ ]_o . Nifedipine but not nicardipine inhibited both the high K⁺ response for neuropeptide secretion and intraterminal calcium, suggesting the involvement of CaV1.1 type channels as sensors in voltage-induced calcium release. Importantly, RyR antagonists also modulate neuropeptide release under normal physiological conditions. In conclusion, our results indicate that depolarisation-induced neuropeptide secretion is present in the absence of external calcium, and calcium release from ryanodine-sensitive internal stores is a significant physiological contributor to neuropeptide secretion from HNS terminals.

Single Amino Acid Changes in the Ryanodine Receptor in the Human Population Have Effects In Vivo on Caenorhabditis elegans Neuro-Muscular Function

The ryanodine receptor mediates intracellular calcium ion release with excitation of nerve and muscle cells. Ryanodine receptor missense variants cause a number of myopathologies, such as malignant hyperthermia, and have been linked with various neuropathologies, including Alzheimer's disease. We characterized the consequences of ryanodine receptor variants in vivo. Eight Caenorhabditis elegans strains, with ryanodine receptor modifications equivalent to human myopathic RYR1 variants, were generated by genome editing. In humans, these variants are rare and confer sensitivity to the inhalational anaesthetic halothane when heterozygous. Increased sensitivity to halothane was found in both homozygous and heterozygous C. elegans. Close analysis revealed distinct subtle locomotion defects, due to the different single amino acid residue changes, even in the absence of the external triggering agent. Distinct pre- and postsynaptic consequences of the variants were characterized through the responses to cholinergic pharmacological agents. The range of phenotypes reflects the complexity of the regulatory inputs to the ryanodine receptor and the criticality of the calcium ion channel opening properties, in different cell types and with age. Ryanodine receptors with these single amino acid residue changes still function as calcium ion channels, but with altered properties which are likely to have subtle consequences for human carriers of such variants. The long-term consequences of subtly altered calcium ion signalling could be cumulative and may be focussed in the smaller nerve cells rather than the more robust muscle cells. It was important to assess phenotypes in vivo to properly appreciate consequences for a whole organism.

The Ever-Growing Puzzle of Asynchronous Release

Invasion of an action potential (AP) to presynaptic terminals triggers calcium dependent vesicle fusion in a relatively short time window, about a millisecond, after the onset of the AP. This allows fast and precise information transfer from neuron to neuron by means of synaptic transmission and phasic mediator release. However, at some synapses a single AP or a short burst of APs can generate delayed or asynchronous synaptic release lasting for tens or hundreds of milliseconds. Understanding the mechanisms underlying asynchronous release (AR) is important, since AR can better recruit extrasynaptic metabotropic receptors and maintain a high level of neurotransmitter in the extracellular space for a substantially longer period of time after presynaptic activity. Over the last decade substantial work has been done to identify the presynaptic calcium sensor that may be involved in AR. Several models have been suggested which may explain the long lasting presynaptic calcium elevation a prerequisite for prolonged delayed release. However, the presynaptic mechanisms underlying asynchronous vesicle release are still not well understood. In this review article, we provide an overview of the current state of knowledge on the molecular components involved in delayed vesicle fusion and in the maintenance of sufficient calcium concentration to trigger AR. In addition, we discuss possible alternative models that may explain intraterminal calcium dynamics underlying AR.

Effects of intravesicular loading of a Ca2+ chelator and depolymerization of actin fibers on neurotransmitter release in frog motor nerve terminals

Ca²⁺ -induced Ca²⁺ release (CICR) via type-3 ryanodine receptor enhances neurotransmitter release in frog motor nerve terminals. To test a possible role of synaptic vesicle in CICR, we examined the effects of loading of EGTA, a Ca²⁺ chelator, into synaptic vesicles and depolymerization of actin fibers. Intravesicular EGTA loading via endocytosis inhibited the ryanodine sensitive enhancement of transmitter release induced by tetanic stimulation and the associated rises in intracellular-free Ca²⁺ ([Ca²⁺ ]_i : Ca²⁺ transients). Latrunculin A, a depolymerizer of actin fibers, enhanced both spontaneous and stimulation-induced transmitter release, but inhibited the enhancement of transmitter release elicited by successive tetanic stimulation. The results suggest a possibility that the activation of CICR from mobilized synaptic vesicles caused the enhancement of neurotransmitter release.

Effect of 2,5-dimethylphenol on Ca(2+) movement and viability in PC3 human prostate cancer cells

The phenolic compound 2,5-dimethylphenol is a natural product. 2,5-Dimethylphenol has been shown to affect rat hepatic and pulmonary microsomal metabolism. However, the effect of 2,5-dimethylphenol on Ca(2+ )signaling and cyotoxicity has never been explored in any culture cells. This study explored the effect of 2,5-dimethylphenol on cytosolic free Ca(2+ )levels ([Ca(2+)]i) and cell viability in PC3 human prostate cancer cells. 2,5-Dimethylphenol at concentrations between 500 μM and 1000 μM evoked [Ca(2+)]i rises in a concentration-dependent manner. This Ca(2+ )signal was inhibited by approximately half by the removal of extracellular Ca(2+). 2,5-Dimethylphenol-induced Ca(2+ )influx was confirmed by Mn(2+)-induced quench of fura-2 fluorescence. Pretreatment with the protein kinase C (PKC) inhibitor GF109203X, nifedipine or the store-operated Ca(2+ )entry inhibitors (econazole or SKF96365) inhibited 2,5-dimethylphenol-induced Ca(2+ )signal in Ca(2+)-containing medium by ∼30%. Treatment with the endoplasmic reticulum Ca(2+ )pump inhibitor thapsigargin in Ca(2+)-free medium abolished 2,5-dimethylphenol-induced [Ca(2+)]i rises. Conversely, treatment with 2,5-dimethylphenol abolished thapsigargin-induced [Ca(2+)]i rises. Inhibition of phospholipase C (PLC) with U73122 reduced 2,5-dimethylphenol-evoked [Ca(2+)]i rises by ∼80%. 2,5-Dimethylphenol killed cells at concentrations of 350-1000 μM in a concentration-dependent fashion. Chelation of cytosolic Ca(2+ )with 1,2-bis(2-aminophenoxy)ethane-N, N, N', N'-tetraacetic acid/AM (BAPTA/AM) did not prevent 2,5-dimethylphenol's cytotoxicity. Together, in PC3 cells, 2,5-dimethylphenol induced [Ca(2+)]i rises that involved Ca(2+ )entry through PKC-regulated store-operated Ca(2+ )channels and PLC-dependent Ca(2+ )release from the endoplasmic reticulum. 2,5-Dimethylphenol induced cytotoxicity in a Ca(2+)-independent manner.

Mechanisms of hydrogen sulfide (H2S) action on synaptic transmission at the mouse neuromuscular junction

Hydrogen sulfide (H2S) is a widespread gasotransmitter also known as a powerful neuroprotective agent in the central nervous system. However, the action of H2S in peripheral synapses is much less studied. In the current project we studied the modulatory effects of the H2S donor sodium hydrosulfide (NaHS) on synaptic transmission in the mouse neuromuscular junction using microelectrode technique. Using focal recordings of presynaptic response and evoked transmitter release we have shown that NaHS (300 μM) increased evoked end-plate currents (EPCs) without changes of presynaptic waveforms which indicated the absence of NaHS effects on sodium and potassium currents of motor nerve endings. Using intracellular recordings it was shown that NaHS increased the frequency of miniature end-plate potentials (MEPPs) without changing their amplitudes indicating a pure presynaptic effect. Furthermore, NaHS increased the amplitude of end-plate potentials (EPPs) without influencing the resting membrane potential of muscle fibers. L-cysteine, a substrate of H2S synthesis induced, similar to NaHS, an increase of EPC amplitudes whereas inhibitors of H2S synthesis (β-cyano-L-alanine and aminooxyacetic acid) had the opposite effect. Inhibition of adenylate cyclase using MDL 12,330A hydrochloride (MDL 12,330A) or elevation of cAMP level with 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (pCPT-cAMP) completely prevented the facilitatory action of NaHS indicating involvement of the cAMP signaling cascade. The facilitatory effect of NaHS was significantly diminished when intracellular calcium (Ca(2+)) was buffered by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis acetoxymethyl ester (BAPTA-AM) and ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid acetoxymethyl ester (EGTA-AM). Activation of ryanodine receptors by caffeine or ryanodine increased acetylcholine release and prevented further action of NaHS on transmitter release, likely due to an occlusion effect. Inhibition of ryanodine receptors by ryanodine or dantrolene also reduced the action of NaHS on EPC amplitudes. Our results indicate that in mammalian neuromuscular synapses endogenously produced H2S increases spontaneously and evoked quantal transmitter release from motor nerve endings without changing the response of nerve endings. The presynaptic effect of H2S appears mediated by intracellular Ca(2+) and cAMP signaling and involves presynaptic ryanodine receptors.

Ryanodine and inositol triphosphate receptors modulate facilitation and tetanic depression at the frog neuromuscular junction

INTRODUCTION

Short-term plasticity of synaptic function is an important physiological control of transmitter release. Short-term plasticity can be regulated by intracellular calcium released by ryanodine and inositol triphosphate (IP3) receptors, but the role of these receptors at the neuromuscular junction is understood incompletely.

METHODS

We measured short-term plasticity of evoked endplate potential (EPP) amplitudes from frog neuromuscular junctions treated with ryanodine, 2-aminoethoxydiphenylborane (2-APB), or 1-[6-[[(17β)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U- 73122).

RESULTS

Ryanodine decreases paired-pulse facilitation for intervals <20 ms and markedly decreases tetanic depression. Treatment with 2-APB reduces EPP amplitude, increases paired-pulse facilitation for intervals of <20 ms, and significantly reduces tetanic depression. U-73122 decreases EPP amplitude and decreases paired-pulse depression for intervals <20 ms.

CONCLUSIONS

Ryanodine, IP3 receptors, and phospholipase C modulate short-term plasticity of transmitter release at the neuromuscular junction. These results suggest possible targets for improving the safety factor of neuromuscular transmission during repetitive activity of the neuromuscular junction.

The appetite-inducing peptide, ghrelin, induces intracellular store-mediated rises in calcium in addiction and arousal-related laterodorsal tegmental neurons in mouse brain slices

Ghrelin, a gut and brain peptide, has recently been shown to be involved in motivated behavior and regulation of the sleep and wakefulness cycle. The laterodorsal tegmental nucleus (LDT) is involved in appetitive behavior and control of the arousal state of an organism, and accordingly, behavioral actions of ghrelin could be mediated by direct cellular actions within this nucleus. Consistent with this interpretation, postsynaptically mediated depolarizing membrane actions of ghrelin on LDT neurons have been reported. Direct actions were ascribed solely to closure of a potassium conductance however this peptide has been shown in other cell types to lead to rises in calcium via release of calcium from intracellular stores. To determine whether ghrelin induced intracellular calcium rises in mouse LDT neurons, we conducted calcium imaging studies in LDT brain slices loaded with the calcium binding dye, Fura-2AM. Ghrelin elicited TTX-insensitive changes in dF/F indicative of rises in calcium, and a portion of these rises were independent of membrane depolarization, as they persisted in conditions of high extracellular potassium solutions and were found to involve SERCA-pump mediated intracellular calcium stores. Involvement of the ghrelin receptor (GHR-S) in these actions was confirmed. Taken together with other studies, our data suggest that ghrelin has multiple cellular actions on LDT cells. Ghrelin's induction of calcium via intracellular release in the LDT could play a role in behavioral actions of this peptide as the LDT governs processes involved in stimulation of motivated behavior and control of cortical arousal.

Role of membrane cholesterol in spontaneous exocytosis at frog neuromuscular synapses: reactive oxygen species-calcium interplay

Using electrophysiological and optical techniques, we studied the mechanisms by which cholesterol depletion stimulates spontaneous transmitter release by exocytosis at the frog neuromuscular junction. We found that methyl-β-cyclodextrin (MCD, 10 mM)-mediated exhaustion of cholesterol resulted in the enhancement of reactive oxygen species (ROS) production, which was prevented by the antioxidant N-acetyl cysteine (NAC) and the NADPH oxidase inhibitor apocynin. An increase in ROS levels occurred both extra- and intracellularly, and it was associated with lipid peroxidation in synaptic regions. Cholesterol depletion provoked a rise in the intracellular Ca(2+) concentration, which was diminished by NAC and transient receptor potential vanilloid (TRPV) channel blockers (ruthenium red and capsazepine). By contrast, the MCD-induced rise in [Ca(2+)]i remained unaffected if Ca(2+) release from endoplasmic stores was blocked by TMB8 (8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride). The effects of cholesterol depletion on spontaneous release and exocytosis were significantly reduced by the antioxidant, intracellular Ca(2+) chelation with BAPTA-AM and blockers of TRPV channels. Bath application of the calcineurin antagonist cyclosporine A blocked MCD-induced enhancement of spontaneous release/exocytosis, whereas okadaic acid, an inhibitor of phosphatases PP1 and PP2A, had no effect. Thus, our findings indicate that enhancement of spontaneous exocytosis induced by cholesterol depletion may depend on ROS generation, leading to an influx of Ca(2+) via TRPV channels and, subsequently, activation of calcineurin.

Endotoxemia enhances catecholamine secretion from male mouse adrenal chromaffin cells through an increase in Ca(2+) release from the endoplasmic reticulum

Enhanced epinephrine secretion from adrenal chromaffin cells (ACCs) is an important homeostatic response to severe systemic inflammation during sepsis. Evidence suggests that increased activation of ACCs by preganglionic sympathetic neurons and direct alterations in ACC function contribute to this response. However, the direct effects of sepsis on ACC function have yet to be characterized. We hypothesized that sepsis enhances epinephrine secretion from ACCs by increasing intracellular Ca(2+) signaling. Plasma epinephrine concentration was increased 5-fold in the lipopolysaccharide-induced endotoxemia model of sepsis compared with saline-treated control mice. Endotoxemia significantly enhanced stimulus-evoked epinephrine secretion from isolated ACCs in vitro. Carbon fiber amperometry revealed an increase in the number of secretory events during endotoxemia, without significant changes in spike amplitude, half-width, or quantal content. ACCs isolated up to 12 hours after the induction of endotoxemia exhibited larger stimulus-evoked Ca(2+) transients compared with controls. Similarly, ACCs from cecal ligation and puncture mice also exhibited enhanced Ca(2+) signaling. Although sepsis did not significantly affect ACC excitability or voltage-gated Ca(2+) currents, a 2-fold increase in caffeine (10 mM)-stimulated Ca(2+) transients was observed during endotoxemia. Depletion of endoplasmic reticulum Ca(2+) stores using cyclopiazonic acid (10 μM) abolished the effects of endotoxemia on catecholamine secretion from ACCs. These findings suggest that sepsis directly enhances catecholamine secretion from ACCs through an increase in Ca(2+) release from the endoplasmic reticulum. These alterations in ACC function are likely to amplify the effects of increased preganglionic sympathetic neuron activity to further enhance epinephrine levels during sepsis.

Molecular mechanisms for synchronous, asynchronous, and spontaneous neurotransmitter release

Most neuronal communication relies upon the synchronous release of neurotransmitters, which occurs through synaptic vesicle exocytosis triggered by action potential invasion of a presynaptic bouton. However, neurotransmitters are also released asynchronously with a longer, variable delay following an action potential or spontaneously in the absence of action potentials. A compelling body of research has identified roles and mechanisms for synchronous release, but asynchronous release and spontaneous release are less well understood. In this review, we analyze how the mechanisms of the three release modes overlap and what molecular pathways underlie asynchronous and spontaneous release. We conclude that the modes of release have key fusion processes in common but may differ in the source of and necessity for Ca(2+) to trigger release and in the identity of the Ca(2+) sensor for release.

The role of intracellular calcium stores in synaptic plasticity and memory consolidation

Memory processing requires tightly controlled signalling cascades, many of which are dependent upon intracellular calcium (Ca(2+)). Despite this, most work investigating calcium signalling in memory formation has focused on plasma membrane channels and extracellular sources of Ca(2+). The intracellular Ca(2+) release channels, ryanodine receptors (RyRs) and inositol (1,4,5)-trisphosphate receptors (IP3Rs) have a significant capacity to regulate intracellular Ca(2+) signalling. Evidence at both cellular and behavioural levels implicates both RyRs and IP3Rs in synaptic plasticity and memory formation. Pharmacobehavioural experiments using young chicks trained on a single-trial discrimination avoidance task have been particularly useful by demonstrating that RyRs and IP3Rs have distinct roles in memory formation. RyR-dependent Ca(2+) release appears to aid the consolidation of labile memory into a persistent long-term memory trace. In contrast, IP3Rs are required during long-term memory. This review discusses various functions for RyRs and IP3Rs in memory processing, including neuro- and glio-transmitter release, dendritic spine remodelling, facilitating vasodilation, and the regulation of gene transcription and dendritic excitability. Altered Ca(2+) release from intracellular stores also has significant implications for neurodegenerative conditions.

An ultra fast detection method reveals strain-induced Ca(2+) entry via TRPV2 in alveolar type II cells

A commonly used technique to investigate strain-induced responses of adherent cells is culturing them on an elastic membrane and globally stretching the membrane. However, it is virtually impossible to acquire microscopic images immediately after the stretch with this method. Using a newly developed technique, we recorded the strain-induced increase of the cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) in rat primary alveolar type II (ATII) cells at an acquisition rate of 30ms and without any temporal delay. We can show that the onset of the mechanically induced rise in [Ca(2+)](c) was very fast (<30 ms), and Ca(2+) entry was immediately abrogated when the stimulus was withdrawn. This points at a direct mechanical activation of an ion channel. RT-PCR revealed high expression of TRPV2 in ATII cells, and silencing TRPV2, as well as blocking TRPV channels with ruthenium red, significantly reduced the strain-induced Ca(2+) response. Moreover, the usually homogenous pattern of the strain-induced [Ca(2+)](c) increase was converted into a point-like response after both treatments. Also interfering with actin/myosin and integrin binding inhibited the strain-induced increase of [Ca(2)](c). We conclude that TRPV2 participates in strain-induced Ca(2+) entry in ATII cells and suggest a direct mechanical activation of the channel that depends on FAs and actin/myosin. Furthermore, our results underline the importance of cell strain systems that allow high temporal resolution.

The number of components of enhancement contributing to short-term synaptic plasticity at the neuromuscular synapse during patterned nerve Stimulation progressively decreases as basal release probability is increased from low to normal levels by changing extracellular Ca2+

Presynaptic short-term plasticity (STP) dynamically modulates synaptic strength in a reversible manner on a timescale of milliseconds to minutes. For low basal vesicular release probability (prob0), four components of enhancement, F1 and F2 facilitation, augmentation (A), and potentiation (P), increase synaptic strength during repetitive nerve activity. For release rates that exceed the rate of replenishment of the readily releasable pool (RRP) of synaptic vesicles, depression of synaptic strength, observed as a rundown of postsynaptic potential amplitudes, can also develop. To understand the relationship between enhancement and depression at the frog (Rana pipiens) neuromuscular synapse, data obtained over a wide range of prob0 using patterned stimulation are analyzed with a hybrid model to reveal the components of STP. We find that F1, F2, A, P, and depletion of the RRP all contribute to STP during repetitive nerve activity at low prob0. As prob0 is increased by raising Ca(o)(2+) (extracellular Ca2+), specific components of enhancement no longer contribute, with first P, then A, and then F2 becoming undetectable, even though F1 continues to enhance release. For levels of prob0 that lead to appreciable depression, only F1 and depletion of the RRP contribute to STP during rundown, and for low stimulation rates, F2 can also contribute. These observations place prob0-dependent limitations on which components of enhancement contribute to STP and suggest some fundamental mechanistic differences among the components. The presented model can serve as a tool to readily characterize the components of STP over wide ranges of prob0.

Synaptic mitochondria in synaptic transmission and organization of vesicle pools in health and disease

Cell types rich in mitochondria, including neurons, display a high energy demand and a need for calcium buffering. The importance of mitochondria for proper neuronal function is stressed by the occurrence of neurological defects in patients suffering from a great variety of diseases caused by mutations in mitochondrial genes. Genetic and pharmacological evidence also reveal a role of these organelles in various aspects of neuronal physiology and in the pathogenesis of neurodegenerative disorders. Yet the mechanisms by which mitochondria can affect neurotransmission largely remain to be elucidated. In this review we focus on experimental data that suggest a critical function of synaptic mitochondria in the function and organization of synaptic vesicle pools, and in neurotransmitter release during intense neuronal activity. We discuss how calcium handling, ATP production and other mitochondrial mechanisms may influence synaptic vesicle pool organization and synaptic function. Given the link between synaptic mitochondrial function and neuronal communication, efforts toward better understanding mitochondrial biology may lead to novel therapeutic approaches of neurological disorders including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and psychiatric disorders that are at least in part caused by mitochondrial deficits.

Individual calcium syntillas do not trigger spontaneous exocytosis from nerve terminals of the neurohypophysis

Recently, highly localized Ca(2+) release events, similar to Ca(2+) sparks in muscle, have been observed in neuronal preparations. Specifically, in murine neurohypophysial terminals (NHT), these events, termed Ca(2+) syntillas, emanate from a ryanodine-sensitive intracellular Ca(2+) pool and increase in frequency with depolarization in the absence of Ca(2+) influx. Despite such knowledge of the nature of these Ca(2+) release events, their physiological role in this system has yet to be defined. Such localized Ca(2+) release events, if they occur in the precise location of the final exocytotic event(s), may directly trigger exocytosis. However, directly addressing this hypothesis has not been possible, since no method capable of visualizing individual release events in these CNS terminals has been available. Here, we have adapted an amperometric method for studying vesicle fusion to this system which relies on loading the secretory granules with the false transmitter dopamine, thus allowing, for the first time, the recording of individual exocytotic events from peptidergic NHT. Simultaneous use of this technique along with high-speed Ca(2+) imaging has enabled us to establish that spontaneous neuropeptide release and Ca(2+) syntillas do not display any observable temporal or spatial correlation, confirming similar findings in chromaffin cells. Although these results indicate that syntillas do not play a direct role in eliciting spontaneous release, they do not rule out indirect modulatory effects of syntillas on secretion.

Ca2+ dependence of the binomial parameters p and n at the mouse neuromuscular junction

The Ca(2+) dependence of synaptic quantal release is generally thought to be restricted to probability of vesicular release. However, some studies have suggested that the number of release sites (n) at the neuromuscular junction (NMJ) is also Ca(2+) dependent. In this study, we recorded endplate currents over a wide range of extracellular Ca(2+) concentrations and found the expected Ca(2+) dependency of release. A graphical technique was used to estimate p (probability of release) and n using standard binomial assumptions. The results suggested n was Ca(2+) dependent. The data were simulated using compound binomial statistics with variable n (Ca(2+) dependent) or fixed n (Ca(2+) independent). With fixed n, successful simulation of increasing Ca(2+) required that p increase abruptly at some sites from very low to high values. Successful simulation with variable n required the introduction of previously silent release sites (p = 0) with high values of p. Thus the success of both simulations required abrupt, large increases of p at a subset of release sites with initially low or zero p. Estimates of the time course of release obtained by deconvolving evoked endplate currents with average miniature endplate currents decreased slightly as Ca(2+) increased, thus arguing against sequential release of multiple quanta at higher Ca(2+) levels. Our results suggest that the apparent Ca(2+) dependence of n at the NMJ can be explained by an underlying Ca(2+) dependence of a spatially variable p such that p increases abruptly at a subset of sites as Ca(2+) is increased.

Application of caffeine reveals input frequency-dependent determination of signal-traveling routes between primary and secondary visual cortices in rats

Upon entering the neocortex, neural signals are required to select which neocortical circuits to propagate through. The present study focused attention on use-dependent selection of signal-traveling routes. Rat brain slices including primary visual cortex (Oc1) and the medial part of the secondary visual cortex (Oc2M) were prepared. Electrical stimulation was delivered to white matter in Oc1 and spatiotemporal aspects of traveling signals were observed using optical recording methods under caffeine application. With an interstimulus interval (ISI) of 4-8s, signals traveled horizontally along deep layers from Oc1 to Oc2M, climbed within Oc2M, then returned along layer II/III from Oc2M to Oc1. Conversely, with an ISI of 40-64s, signals climbed within Oc1 and traveled horizontally along layer II/III from Oc1 to Oc2M in parallel with signals traveling along deep layers. Pharmacological experiments with antagonists for ionotropic glutamate receptors revealed that signal-traveling routes under higher-frequency stimulation were N-methyl-d-aspartate (NMDA) receptor activity-dependent, while those at the lower-frequency were non-NMDA receptor activity-dependent. These results suggest that neural circuits between Oc1 and Oc2M possess an input frequency-dependent gating system, in which signal-traveling routes might be affected by the relative balance of receptor activities between NMDA and non-NMDA receptors.

Maturation and long-term hypoxia alters Ca2+-induced Ca2+ release in sheep cerebrovascular sympathetic neurons

The contribution of sympathetic nerves arising from the superior cervical ganglia (SCG) toward the growth and function of cerebral blood vessels is pertinent throughout maturation as well as in response to cardiovascular stress imposed by high-altitude long-term hypoxia (LTH). The function of SCG sympathetic neurons is dependent on intracellular Ca2+ concentration ([Ca2+]i) signaling, which is strongly influenced by a process known as Ca(2+)-induced Ca2+ release (CICR) from the smooth endoplasmic reticulum (SER). In this study, we used the sheep SCG neuronal model to test the hypotheses that maturation decreases CICR and high-altitude LTH depresses CICR in fetal SCG neurons but not in those of the adult. We found that the contribution of CICR to electric field stimulation (EFS)-evoked [Ca2+]i transients was greatest in SCG cells from normoxic fetuses and was abolished by LTH. The decline in CICR was associated with a reduction in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) function in fetal SCG cells during LTH, reducing SER Ca2+ levels below the threshold needed for the coupling of Ca2+ influx and CICR. With respect to the maturation from the fetus to adult, the decrease in CICR may reflect both a reduction in the levels of ryanodine receptor isoforms 2 and 3 and SERCA function. In response to LTH and in contrast to the fetus, CICR function in adult SCG cells is maintained and may reflect alterations in other mechanisms that modulate the CICR process. As CICR is instrumental in the function of sympathetic neurons within the cerebrovasculature, the loss of this signaling mechanism in the fetus may have consequences for the adaptation to LTH in terms of fetal susceptibility to vascular insults.

Excitatory and inhibitory synaptic transmission is differentially influenced by two ortho-substituted polychlorinated biphenyls in the hippocampal slice preparation

Exposure to polychlorinated biphenyls impairs cognition and behavior in children. Two environmental PCBs 2,2',3,3',4,4',5-heptachlorobiphenyl (PCB170) and 2,2',3,5',6-pentachlorobiphenyl (PCB95) were examined in vitro for influences on synaptic transmission in rat hippocampal slices. Field excitatory postsynaptic potentials (fEPSPs) were recorded in the CA1 region using a multi-electrode array. Perfusion with PCB170 (10 nM) had no effect on fEPSP slope relative to baseline period, whereas (100 nM) initially enhanced then depressed fEPSP slope. Perfusion of PCB95 (10 or 100 nM) persistently enhanced fEPSP slope >200%, an effect that could be inhibited by dantrolene, a drug that attenuates ryanodine receptor signaling. Perfusion with picrotoxin (PTX) to block GABA neurotransmission resulted in a modest increase in fEPSP slope, whereas PTX+PCB170 (1-100 nM) persistently enhanced fEPSP slope in a dose dependent manner. fEPSP slope reached >250% of baseline period in the presence of PTX+100 nM PCB170, conditions that evoked marked epileptiform after-potential discharges. PCB95 and PCB170 were found to differentially influence the Ca(2+)-dependence of [(3)H]ryanodine-binding to hippocampal ryanodine receptors. Non-coplanar PCB congeners can differentially alter neurotransmission in a manner suggesting they can elicit imbalances between inhibitory and excitatory circuits within the hippocampus. Differential sensitization of ryanodine receptors by Ca(2+) appears to mediate, at least in part, hippocampal excitotoxicity by non-coplanar PCBs.

Advancing age alters the contribution of calcium release from smooth endoplasmic reticulum stores in superior cervical ganglion cells

In superior cervical ganglion (SCG) neurons calcium-induced calcium release (CICR), mediated by ryanodine receptors (RyRs), contributes to stimulation-evoked intracellular calcium ([Ca²⁺]_i) transients. Hypothesis: The contribution of CICR to electrical field stimulation (EFS)–evoked [Ca²⁺]_i transients in SCG cells declines with senescence and may be partially recovered in the presence of caffeine. We measured EFS-evoked [Ca²⁺]_i transients in isolated fura-2–loaded SCG cells from Fischer-344 rats aged 6, 12, and 24 months with either the RyR antagonist ryanodine to block the contribution of CICR to [Ca²⁺]_i transients or caffeine to sensitize CICR to EFS. EFS-evoked [Ca²⁺]_i transients increased from 6 to 12 months and declined at 24 months and ryanodine decreased [Ca²⁺]_i transients in SCG cells from 6- and 12-month-old animals only. Caffeine significantly increased EFS-evoked [Ca²⁺]_i transients in all age groups. These data suggest that CICR declines with senescence and residual CICR function may be reclaimed in senescent cells with caffeine.

Delayed synaptic transmission in Drosophila cacophonynull embryos

Ca(2+) influx through the Drosophila N-type Ca(2+) channel, encoded by cacophony (cac), triggers fast synaptic transmission. We now ask whether the cac Ca(2+) channel is the Ca(2+) channel solely dedicated for fast synaptic transmission. Because the cac(null) mutation is lethal, we used cac(null) embryos to address this question. At the neuromuscular junction in HL3 solution, no fast synchronous synaptic transmission was detected on nerve stimulation. When the wild-type cac gene was introduced in the cac(null) background, fast synaptic transmission recovered. However, even in cac(null) embryos, nerve stimulation infrequently induced delayed synaptic events in the minority of cells in 1.5 mM [Ca(2+)](e) and in the majority of cells in 5 mM [Ca(2+)](e). The number of delayed quantal events per stimulus was greater in 5 mM [Ca(2+)](e) than in 1.5 mM. Thus the delayed release is [Ca(2+)](e) dependent. Plectreurys toxin II (PLTXII) (10 nM; a spider toxin analog) depressed the frequency of delayed events, suggesting that voltage-gated Ca(2+) channels, other than cac Ca(2+) channels, are contributing to them. However, delayed events were not affected by 50 microM La(3+). The frequency of miniature synaptic currents in cac(null) embryos was approximately 1/2 of control, whereas in high K(+) solutions, it was approximately 1/135. The hypertonicity response was approximately 1/10 of control. These findings indicate that the number of release-ready vesicles is smaller in cac(null) embryos. Taken together, the cac Ca(2+) channel is indispensable for fast synaptic transmission in normal conditions, and another type of Ca(2+) channel, the non-cac, PLTXII-sensitive Ca(2+) channel, is contributing to delayed release in cac(null) embryos.

Use-dependent amplification of presynaptic Ca2+ signaling by axonal ryanodine receptors at the hippocampal mossy fiber synapse

Presynaptic Ca(2+) stores have been suggested to regulate Ca(2+) dynamics within the nerve terminals at certain types of the synapse. However, little is known about their mode of activation, molecular identity, and detailed subcellular localization. Here, we show that the ryanodine-sensitive stores exist in axons and amplify presynaptic Ca(2+) accumulation at the hippocampal mossy fiber synapses, which display robust presynaptic forms of plasticity. Caffeine, a potent drug inducing Ca(2+) release from ryanodine-sensitive stores, causes elevation of presynaptic Ca(2+) levels and enhancement of transmitter release from the mossy fiber terminals. The blockers of ryanodine receptors, TMB-8 or ryanodine, reduce presynaptic Ca(2+) transients elicited by repetitive stimuli of mossy fibers but do not affect those evoked by single shocks, suggesting that ryanodine receptors amplify presynaptic Ca(2+) dynamics in an activity dependent manner. Furthermore, we generated the specific antibody against the type 2 ryanodine receptor (RyR2; originally referred to as the cardiac type) and examined the cellular and subcellular localization using immunohistochemistry. RyR2 is highly expressed in the stratum lucidum of the CA3 region and mostly colocalizes with axonal marker NF160 but not with terminal marker VGLUT1. Immunoelectron microscopy revealed that RyR2 is distributed around smooth ER within the mossy fibers but is almost excluded from their terminal portions. These results suggest that axonal localization of RyR2 at sites distant from the active zones enables use dependent Ca(2+) release from intracellular stores within the mossy fibers and thereby facilitates robust presynaptic forms of plasticity at the mossy fiber-CA3 synapse.

Calcium release via activation of presynaptic IP3 receptors contributes to kainate-induced IPSC facilitation in rat neocortex

We examined the mechanisms of kainate (KA) induced modulation of GABA release in rat prefrontal cortex. Pharmacologically isolated IPSCs were recorded from visually identified layer II/III pyramidal cells using whole-cell patch clamp techniques. KA produced an increase in evoked IPSC amplitude at low nanomolar concentrations (100-500 nM). The frequency but not the amplitude of miniature (m) IPSCs was also increased. The GluR5 subunit selective agonist (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA) caused an increase in mIPSC frequency whereas (3S,4aR,6S,8aR)-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid (LY382884), a selective GluR5 subunit antagonist, inhibited this facilitation. Philanthotoxin-433 (PhTx) blocked the effect of KA, indicating involvement of Ca(2+)-permeable GluR5 receptors. No IPSC facilitation was seen when Ca(2+) was omitted from the bathing solution. Facilitation was observed when slices were preincubated in ruthenium red or high concentrations of ryanodine, but was inhibited with application of thapsigargin. The IP3 receptor (IP3R) antagonists diphenylboric acid 2-amino-ethyl ester (2-APB) (15 microM) and Xestospongin C (XeC) blocked IPSC facilitation. These results show that activation of KA receptors (KARs) on GABAergic nerve terminals results is linked to intracellular Ca(2+) release via activation of IP3, but not ryanodine, receptors. This represents a new mechanism of presynaptic modulation whereby Ca(2+) entry through Ca(2+)-permeable GluR5 subunit containing KARs activates IP3Rs receptors leading to an increase in GABA release.

A constitutive, transient receptor potential-like Ca2+ influx pathway in presynaptic nerve endings independent of voltage-gated Ca2+ channels and Na+/Ca2+ exchange

Calcium levels in the presynaptic nerve terminal are altered by several pathways, including voltage-gated Ca(2+) channels, the Na(+)/Ca(2+) exchanger, Ca(2+)-ATPase, and the mitochondria. The influx pathway for homeostatic control of [Ca(2+)](i) in the nerve terminal has been unclear. One approach to detecting the pathway that maintains internal Ca(2+) is to test for activation of Ca(2+) influx following Ca(2+) depletion. Here, we demonstrate that a constitutive influx pathway for Ca(2+) exists in presynaptic terminals to maintain internal Ca(2+) independent of voltage-gated Ca(2+) channels and Na(+)/Ca(2+) exchange, as measured in intact isolated nerve endings from mouse cortex and in intact varicosities in a neuronal cell line using fluorescence spectroscopy and confocal imaging. The Mg(2+) and lanthanide sensitivity of the influx pathway, in addition to its pharmacological and short hairpin RNA sensitivity, and the results of immunostaining for transient receptor potential (TRP) channels indicate the involvement of TRPC channels, possibly TRPC5 and TRPC1. This constitutive Ca(2+) influx pathway likely serves to maintain synaptic function under widely varying levels of synaptic activity.

Critical role of type 2 ryanodine receptor in mediating activity-dependent neurogenesis from embryonic stem cells

Activity-induced neurogenesis via Ca(2+) entry may be important for establishing Hebbian neural network. However, it remains unclear whether intracellular Ca(2+) mobilization is required and which subtypes of Ca(2+) release channels expressed in Ca(2+) store organelles are involved in the activity-dependent neurogenesis. Here, we demonstrated that the activity of intracellular Ca(2+) signaling, expression of neuronal transcription factor NeuroD, and the rate of neurogenesis were significantly inhibited in neuronal cells derived from embryonic stem (ES) cells deficient in the Ca(2+) release channel type 2 ryanodine receptors (RyR2(-/-)). In wild-type (RyR2(+/+)) but not in RyR2(-/-) ES cells, activation of L-type Ca(2+) channels, GABA(A) receptors, or RyRs promoted neuronal differentiation, while inhibition of these channels/receptors had an opposite effect. Moreover, neuronal differentiation promoted by activation of GABA(A) receptors or L-type Ca(2+) channels in RyR2(+/+) cells was prevented by RyR inhibitors. No significant difference was detected in the expression level of GABA(A) receptors and L-type channels between neuronal cells derived from two types of ES cells. Thus, activity-induced Ca(2+) influx through L-type Ca(2+) channels alone is not sufficient in promoting neurogenesis. Instead, an intimate cooperation of L-type Ca(2+) channels with RyR2 is crucial for the activity-dependent neurogenesis induced by paracrine and/or autocrine GABA signaling.

4-Chloro-m-cresol, an activator of ryanodine receptors, inhibits voltage-gated K(+) channels at the rat calyx of Held

4-Chloro-m-cresol (4-CmC) is thought to be a specific activator of ryanodine receptors (RyRs). Using this compound, we examined whether the RyR-mediated Ca(2+) release is involved in transmitter release at the rat calyx of Held synapse in brainstem slices. Bath application of 4-CmC caused a dramatic increase in the amplitude of excitatory postsynaptic currents (TIFCs) with the half-maximal effective concentration of 0.12 mm. By making direct patch-clamp whole-cell recordings from presynaptic terminals, we investigated the mechanism by which 4-CmC facilitates transmitter release. 4-CmC markedly prolonged the duration of action potentials, with little effect on their rise time kinetics. In voltage-clamp recordings, 4-CmC inhibited voltage-gated presynaptic K(+) currents (I(pK)) by 53% (at +20 mV) but had no effect on voltage-gated presynaptic Ca(2+) currents (I(pCa)). In simultaneous pre- and postsynaptic recordings, 4-CmC had no effect on the TIFC evoked by I(pCa). Although immunocytochemical study of the calyceal terminals showed immunoreactivity to type 3 RyRs, ryanodine (0.02 mm) had no effect on the 4-CmC-induced TIFC potentiation. We conclude that the facilitatory effect of 4-CmC on nerve-evoked transmitter release is mediated by its inhibitory effect on I(pK).

Potentiation phase of spike timing-dependent neuromodulation by a serotonergic interneuron involves an increase in the fraction of transmitter release

In the mollusk, Tritonia diomedea, the serotonergic dorsal swim interneuron (DSI) produces spike timing-dependent neuromodulation (STDN) of the synaptic output of ventral swim interneuron B (VSI) resulting in a biphasic, bidirectional change of synaptic strength characterized by a rapid heterosynaptic potentiation followed by a more prolonged heterosynaptic depression. This study examined the mechanism underlying the potentiation phase of STDN. In the presence of 4-aminopyridine, which blocks the depression phase and enhances transmitter release from VSI, rapidly stimulating VSI led to a steady-state level of transmitter depletion during which potentiation by DSI or serotonin (5-HT) was eliminated. Cumulative plots of excitatory postsynaptic currents were used to estimate changes in the size and replenishment rate of the readily releasable pool (RRP) and the fraction of release. 5-HT application increased transmitter release without altering replenishment rate. The magnitude of 5-HT-evoked potentiation correlated with the increase in the fraction of release. A phenomenological model of the synapse further supported the hypothesis that 5-HT-induced potentiation was caused by an increase in the fraction of release and correctly predicted no change in frequency facilitation. A dynamic version of the model correctly predicted the effect of DSI stimulation under a variety of conditions. Finally, depletion of internal Ca(2+) stores with cyclopiazonic acid showed that Ca(2+) from internal stores is necessary for the 5-HT-induced potentiation. The data indicate that 5-HT released from DSI increases the fraction of the RRP discharged during VSI action potentials using a mechanism that involves Ca(2+) extrusion from internal stores, resulting in time- and state-dependent neuromodulation.

Perinatal exposure to a noncoplanar polychlorinated biphenyl alters tonotopy, receptive fields, and plasticity in rat primary auditory cortex

Noncoplanar polychlorinated biphenyls (PCBs) are widely dispersed in human environment and tissues. Here, an exemplar noncoplanar PCB was fed to rat dams during gestation and throughout three subsequent nursing weeks. Although the hearing sensitivity and brainstem auditory responses of pups were normal, exposure resulted in the abnormal development of the primary auditory cortex (A1). A1 was irregularly shaped and marked by internal nonresponsive zones, its topographic organization was grossly abnormal or reversed in about half of the exposed pups, the balance of neuronal inhibition to excitation for A1 neurons was disturbed, and the critical period plasticity that underlies normal postnatal auditory system development was significantly altered. These findings demonstrate that developmental exposure to this class of environmental contaminant alters cortical development. It is proposed that exposure to noncoplanar PCBs may contribute to common developmental disorders, especially in populations with heritable imbalances in neurotransmitter systems that regulate the ratio of inhibition and excitation in the brain. We conclude that the health implications associated with exposure to noncoplanar PCBs in human populations merit a more careful examination.

High potassium-induced facilitation of glycine release from presynaptic terminals on mechanically dissociated rat spinal dorsal horn neurons in the absence of extracellular calcium

The high potassium-induced potentiation of spontaneous glycine release in extracellular Ca2+-free conditions was studied in mechanically dissociated rat spinal dorsal horn neurons using whole-cell patch-clamp technique. Elevating extracellular K+ concentration reversibly increased the frequency of spontaneous glycinergic inhibitory postsynaptic currents (IPSCs) in the absence of extracellular Ca2+. Blocking voltage-dependent Na+ channels (tetrodotoxin) and Ca2+ channels (nifedipine and omega-grammotoxin-SIA) had no effect on this potassium-induced potentiation of glycine-release. The high potassium-induced increase in IPSC frequency was also observed in the absence of extracellular Na+, although the recovery back to baseline levels of release was prolonged under these conditions. The action of high potassium solution on glycine release was prevented by BAPTA-AM, by depletion of intracellular Ca2+ stores by thapsigargin and by the phospholipase C inhibitor U-73122. The results suggest that the elevated extracellular K+ concentration causes Ca2+ release from internal stores which is independent of extracellular Na+- and Ca2+-influx, and may reveal a novel mechanism by which the potassium-induced depolarization of presynaptic nerve terminals can regulate intracellular Ca2+ concentration and exocytosis.

Enhancement of Ca2+-induced Ca2+ release by cyclic ADP-ribose in frog motor nerve terminals

Ca2+-induced Ca2+ release (CICR) occurs via activation of ryanodine receptors (RyRs) in frog motor nerve terminals after RyRs are primed for activation by repetitive Ca2+ entries, thereby contributing to synaptic plasticity. To clarify how the mechanism of CICR becomes activable by repetitive Ca2+ entries, we studied effects of a RyR modulator, cyclic ADP-ribose (cADPr), on CICR by Ca2+ imaging techniques. Use-dependent binding of fluorescent ryanodine and its blockade by ryanodine revealed the existence of RyRs in the terminals. Repetition of tetani applied to the nerve produced repetitive rises in intracellular Ca2+ ([Ca2+]i) in the terminals. The amplitude of each rise slowly waxed and waned during the course of the stimulation. These slow rises and decays were blocked by ryanodine, indicating the priming, activation and inactivation of CICR. Uncaging of caged-cADPr loaded in the terminals increased the amplitude of short tetanus-induced rises in [Ca2+]i and the amplitude, time to peak and half decay time of the slow waxing and waning rises in [Ca2+]i evoked by repetitive tetani. A cADPr blocker, 8-amino-cADPr, loaded in the terminals decreased the slow waxing and waning component of rises and blocked all the actions of exogenous cADPr. It is concluded that cADPr enhances the priming and activation of CICR. The four-state model for RyRs suggests that cADPr inhibits the inactivation of CICR and increases the activation efficacy of RyR.

Bioluminescence imaging of mitochondrial Ca2+ dynamics in soma and neurites of individual adult mouse sympathetic neurons

Changes in the cytosolic Ca(2+) concentration ([Ca(2+)](c)) are essential for triggering neurotransmitter release from presynaptic nerve terminals. Calcium-induced Ca(2+) release (CICR) from the endoplasmic reticulum (ER) may amplify the [Ca(2+)](c) signals and facilitate neurotransmitter release in sympathetic neurons. In adrenal chromaffin cells, functional triads are formed by voltage-operated Ca(2+) channels (VOCCs), CICR sites and mitochondria. In fact, mitochondria take up most of the Ca(2+) load entering the cells and are essential for shaping [Ca(2+)](c) signals and exocytosis. Here we have investigated the existence of such functional triads in sympathetic neurons. The mitochondrial Ca(2+) concentration ([Ca(2+)](m)) in soma and neurites of individual mouse superior cervical ganglion (SCG) neurons was monitored by bioluminescence imaging of targeted aequorins. In soma, Ca(2+) entry through VOCCs evoked rapid, near millimolar [Ca(2+)](m) increases in a subpopulation of mitochondria containing about 40% of the aequorin. Caffeine evoked a similar [Ca(2+)](m) increase in a mitochondrial pool containing about 30% of the aequorin and overlapping with the VOCC-sensitive pool. These observations suggest the existence of functional triads similar to the ones described in chromaffin cells. In neurites, mitochondria were able to buffer [Ca(2+)](c) increases resulting from activation of VOCCs but not those mediated by caffeine-induced Ca(2+) release from the ER. The weaker Ca(2+) buffering by mitochondria in neurites could contribute to facilitate Ca(2+)-induced exocytosis at the presynaptic sites.

Calcium regulation in individual peripheral sensory nerve terminals of the rat

Ca2+ is vital for release of neurotransmitters and trophic factors from peripheral sensory nerve terminals (PSNTs), yet Ca2+ regulation in PSNTs remains unexplored. To elucidate the Ca2+ regulatory mechanisms in PSNTs, we determined the effects of a panel of pharmacological agents on electrically evoked Ca2+ transients in rat corneal nerve terminals (CNTs) in vitro that had been loaded with the fluorescent Ca2+ indicator, Oregon Green 488 BAPTA-1 dextran or fura-2 dextran in vivo. Inhibition of the sarco(endo)plasmic reticulum Ca2+-ATPase, disruption of mitochondrial Ca2+ uptake, or inhibition of the Na+-Ca2+ exchanger did not measurably alter the amplitude or decay kinetics of the electrically evoked Ca2+ transients in CNTs. By contrast, inhibition of the plasma membrane Ca2+-ATPase (PMCA) by increasing the pH slowed the decay of the Ca2+ transient by 2-fold. Surprisingly, the energy for ion transport across the plasma membrane of CNTs is predominantly from glycolysis rather than mitochondrial respiration, as evidenced by the observation that Ca2+ transients were suppressed by iodoacetate but unaffected by mitochondrial inhibitors. These observations indicate that, following electrical activity, the PMCA is the predominant mechanism of Ca2+ clearance from the cytosol of CNTs and glycolysis is the predominant source of energy.