Dihydropyridine receptors and type 1 ryanodine receptors constitute the molecular machinery for voltage-induced Ca2+ release in nerve terminals

Voltage-induced calcium release in Caenorhabditis elegans body muscles

Type 1 voltage-activated calcium channels (CaV1) in the plasma membrane trigger calcium release from the sarcoplasmic reticulum (SR) by two mechanisms. In voltage-induced calcium release (VICR), CaV1 voltage sensing domains are directly coupled to ryanodine receptors (RYRs), an SR calcium channel. In calcium-induced calcium release (CICR), calcium ions flowing through activated CaV1 channels bind and activate RYR channels. VICR is thought to occur exclusively in vertebrate skeletal muscle while CICR occurs in all other muscles (including all invertebrate muscles). Here, we use calcium-activated SLO-2 potassium channels to analyze CaV1-SR coupling in Caenorhabditis elegans body muscles. SLO-2 channels were activated by both VICR and external calcium. VICR-mediated SLO-2 activation requires two SR calcium channels (RYRs and IP3 Receptors), JPH-1/Junctophilin, a PDZ (PSD95, Dlg1, ZO-1 domain) binding domain (PBD) at EGL-19/CaV1's carboxy-terminus, and SHN-1/Shank (a scaffolding protein that binds EGL-19's PBD). Thus, VICR occurs in invertebrate muscles.

Subcellular localization and transcriptional regulation of brain ryanodine receptors. Functional implications

Ryanodine receptors (RyR) are intracellular Ca2+ channels localized in the endoplasmic reticulum, where they act as critical mediators of Ca2+-induced Ca2+ calcium release (CICR). In the brain, mammals express in both neurons, and non-neuronal cells, a combination of the three RyR-isoforms (RyR1-3). Pharmacological approaches, which do not distinguish between isoforms, have indicated that RyR-isoforms contribute to brain function. However, isoform-specific manipulations have revealed that RyR-isoforms display different subcellular localizations and are differentially associated with neuronal function. These findings raise the need to understand RyR-isoform specific transcriptional regulation, as this knowledge will help to elucidate the causes of neuronal dysfunction for a growing list of brain disorders that show altered RyR channel expression and function.

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.

Structure and function of STAC proteins: Calcium channel modulators and critical components of muscle excitation-contraction coupling

In skeletal muscle tissue, an intriguing mechanical coupling exists between two ion channels from different membranes: the L-type voltage-gated calcium channel (Ca_V1.1), located in the plasma membrane, and ryanodine receptor 1 (RyR1) located in the sarcoplasmic reticulum membrane. Excitable cells rely on Ca_vs to initiate Ca²⁺ entry in response to action potentials. RyRs can amplify this signal by releasing Ca²⁺ from internal stores. Although this process can be mediated through Ca²⁺ as a messenger, an overwhelming amount of evidence suggests that RyR1 has recruited Ca_V1.1 directly as its voltage sensor. The exact mechanisms that underlie this coupling have been enigmatic, but a recent wave of reports have illuminated the coupling protein STAC3 as a critical player. Without STAC3, the mechanical coupling between Ca_v1.1 and RyR1 is lost, and muscles fail to contract. Various sequence variants of this protein have been linked to congenital myopathy. Other STAC isoforms are expressed in the brain and may serve as regulators of L-type Ca_Vs. Despite the short length of STACs, several points of contacts have been proposed between them and Ca_Vs. However, it is currently unclear whether STAC3 also forms direct interactions with RyR1, and whether this modulates RyR1 function. In this review, we discuss the 3D architecture of STAC proteins, the biochemical evidence for their interactions, the relevance of these connections for functional modulation, and their involvement in myopathy.

Non-canonical Molecular Targets for Novel Analgesics: Intracellular Calcium and HCN Channels

Pain is a prevalent biopsychosocial condition that poses a significant challenge to healthcare providers, contributes substantially to a disability, and is a major economic burden worldwide. An overreliance on opioid analgesics, which primarily target the μ-opioid receptor, has caused devastating morbidity and mortality in the form of misuse and overdose-related death. Thus, novel analgesic medications are needed that can effectively treat pain and provide an alternative to opioids. A variety of cellular ion channels contribute to nociception, the response of the sensory nervous system to a noxious stimulus that commonly leads to pain. Ion channels involved in nociception may provide a suitable target for pharmacologic modulation to achieve pain relief. This narrative review summarizes the evidence for two ion channels that merit consideration as targets for non-opioid pain medications: ryanodine receptors (RyRs), which are intracellular calcium channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which belong to the superfamily of voltage-gated K+ channels. The role of these channels in nociception and neuropathic pain is discussed and suitability as targets for novel analgesics and antihyperalgesics is considered.

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.

Advances in the neurophysiology of magnocellular neuroendocrine cells

Hypothalamic magnocellular neuroendocrine cells have unique electrical properties and a remarkable capacity for morphological and synaptic plasticity. Their large somatic size, their relatively uniform and dense clustering in the supraoptic and paraventricular nuclei, and their large axon terminals in the neurohypophysis make them an attractive target for direct electrophysiological interrogation. Here, we provide a brief review of significant recent findings in the neuroplasticity and neurophysiological properties of these neurones that were presented at the symposium "Electrophysiology of Magnocellular Neurons" during the 13th World Congress on Neurohypophysial Hormones in Ein Gedi, Israel in April 2019. Magnocellular vasopressin (VP) neurones respond directly to hypertonic stimulation with membrane depolarisation, which is triggered by cell shrinkage-induced opening of an N-terminal-truncated variant of transient receptor potential vanilloid type-1 (TRPV1) channels. New findings indicate that this mechanotransduction depends on actin and microtubule cytoskeletal networks, and that direct coupling of the TRPV1 channels to microtubules is responsible for mechanical gating of the channels. Vasopressin neurones also respond to osmostimulation by activation of epithelial Na⁺ channels (ENaC). It was shown recently that changes in ENaC activity modulate magnocellular neurone basal firing by generating tonic changes in membrane potential. Both oxytocin and VP neurones also undergo robust excitatory synapse plasticity during chronic osmotic stimulation. Recent findings indicate that new glutamate synapses induced during chronic salt loading express highly labile Ca²⁺ -permeable GluA1 receptors requiring continuous dendritic protein synthesis for synapse maintenance. Finally, recordings from the uniquely tractable neurohypophysial terminals recently revealed an unexpected property of activity-dependent neuropeptide release. A significant fraction of the voltage-dependent neurohypophysial neurosecretion was found to be independent of Ca²⁺ influx through voltage-gated Ca²⁺ channels. Together, these findings provide a snapshot of significant new advances in the electrophysiological signalling mechanisms and neuroplasticity of the hypothalamic-neurohypophysial system, a system that continues to make important contributions to the field of neurophysiology.

Abscisic acid: a novel uterine stimulator in normal and diabetic rats

Diabetes is usually associated with alterations in myometrial contractility with altered oxytocin responsiveness that increase the incidence of fetal and maternal morbidity and mortality. Pancreatic β-cells release abscisic acid (ABA) in response to glucose, which in turn potentiates insulin secretion. The aim of the study was to find out the effect of ABA on the uterine contractility in normal and diabetic induced rats and tried to detect its possible underlying signaling pathway. Adult non-pregnant female rats were divided into normal nondiabetic group (n = 27) and diabetic group (n = 12). The effect of ABA on the normal and diabetic isolated myometrium was determined alone or after different blockers. Spontaneous diabetic myometrial contraction showed significant decrease and less responsiveness to oxytocin, KCL, and acetylcholine than nondiabetic samples. ABA showed 60% of oxytocin stimulatory effects on myometrial contraction in a dose-response manner in both groups. Meanwhile, this effect was decreased after blocking L-type calcium channels and completely abolished after blocking prostaglandin F (FP) and inositol trisphosphate (IP₃) receptors. ABA is found to have an uterotonic effect that is mediated mainly via FP receptor through increasing the level of IP₃. So, ABA by its novel effect could be beneficial as pre-labor prescription, especially in diabetic females.

Depolarizing, inhibitory GABA type A receptor activity regulates GABAergic synapse plasticity via ERK and BDNF signaling

γ-aminobutyric acid (GABA) begins as the key excitatory neurotransmitter in newly forming circuits, with chloride efflux from GABA type A receptors (GABA_ARs) producing membrane depolarization, which promotes calcium entry, dendritic outgrowth and synaptogenesis. As development proceeds, GABAergic signaling switches to inhibitory hyperpolarizing neurotransmission. Despite the evidence of impaired GABAergic neurotransmission in neurodevelopmental disorders, little is understood on how agonist-dependent GABA_AR activation controls the formation and plasticity of GABAergic synapses. We have identified a weakly depolarizing and inhibitory GABA_AR response in cortical neurons that occurs during the transition period from GABA_AR depolarizing excitation to hyperpolarizing inhibitory activity. We show here that treatment with the GABA_AR agonist muscimol mediates structural changes that diminish GABAergic synapse strength through postsynaptic and presynaptic plasticity via intracellular Ca²⁺ stores, ERK and BDNF/TrkB signaling. Muscimol decreases synaptic localization of surface γ2 GABA_ARs and gephyrin postsynaptic scaffold while β2/3 non-γ2 GABA_ARs accumulate in the synapse. Concurrent with this structural plasticity, muscimol treatment decreases synaptic currents while enhancing the γ2 containing benzodiazepine sensitive GABA_AR tonic current in an ERK dependent manner. We further demonstrate that GABA_AR activation leads to a decrease in presynaptic GAD65 levels via BDNF/TrkB signaling. Together these data reveal a novel mechanism for agonist induced GABAergic synapse plasticity that can occur on the timescale of minutes, contributing to rapid modification of synaptic and circuit function.

Distinct Components of Retrograde Ca(V)1.1-RyR1 Coupling Revealed by a Lethal Mutation in RyR1

The molecular basis for excitation-contraction coupling in skeletal muscle is generally thought to involve conformational coupling between the L-type voltage-gated Ca(2+) channel (CaV1.1) and the type 1 ryanodine receptor (RyR1). This coupling is bidirectional; in addition to the orthograde signal from CaV1.1 to RyR1 that triggers Ca(2+) release from the sarcoplasmic reticulum, retrograde signaling from RyR1 to CaV1.1 results in increased amplitude and slowed activation kinetics of macroscopic L-type Ca(2+) current. Orthograde coupling was previously shown to be ablated by a glycine for glutamate substitution at RyR1 position 4242. In this study, we investigated whether the RyR1-E4242G mutation affects retrograde coupling. L-type current in myotubes homozygous for RyR1-E4242G was substantially reduced in amplitude (∼80%) relative to that observed in myotubes from normal control (wild-type and/or heterozygous) myotubes. Analysis of intramembrane gating charge movements and ionic tail current amplitudes indicated that the reduction in current amplitude during step depolarizations was a consequence of both decreased CaV1.1 membrane expression (∼50%) and reduced channel Po (∼55%). In contrast, activation kinetics of the L-type current in RyR1-E4242G myotubes resembled those of normal myotubes, unlike dyspedic (RyR1 null) myotubes in which the L-type currents have markedly accelerated activation kinetics. Exogenous expression of wild-type RyR1 partially restored L-type current density. From these observations, we conclude that mutating residue E4242 affects RyR1 structures critical for retrograde communication with CaV1.1. Moreover, we propose that retrograde coupling has two distinct and separable components that are dependent on different structural elements of RyR1.

Voltage-Induced Ca²⁺ Release in Postganglionic Sympathetic Neurons in Adult Mice

Recent studies have provided evidence that depolarization in the absence of extracellular Ca²⁺ can trigger Ca²⁺ release from internal stores in a variety of neuron subtypes. Here we examine whether postganglionic sympathetic neurons are able to mobilize Ca²⁺ from intracellular stores in response to depolarization, independent of Ca²⁺ influx. We measured changes in cytosolic ΔF/F₀ in individual fluo-4 –loaded sympathetic ganglion neurons in response to maintained K⁺ depolarization in the presence (2 mM) and absence of extracellular Ca²⁺ ([Ca²⁺]_e). Progressive elevations in extracellular [K⁺]_e caused increasing membrane depolarizations that were of similar magnitude in 0 and 2 mM [Ca²⁺]_e. Peak amplitude of ΔF/F₀ transients in 2 mM [Ca²⁺]_e increased in a linear fashion as the membrane become more depolarized. Peak elevations of ΔF/F₀ in 0 mM [Ca²⁺]_e were ~5–10% of those evoked at the same membrane potential in 2 mM [Ca²⁺]_e and exhibited an inverse U-shaped dependence on voltage. Both the rise and decay of ΔF/F₀ transients in 0 mM [Ca²⁺]_e were slower than those of ΔF/F₀ transients evoked in 2 mM [Ca²⁺]_e. Rises in ΔF/F₀ evoked by high [K⁺]_e in the absence of extracellular Ca²⁺ were blocked by thapsigargin, an inhibitor of endoplasmic reticulum Ca²⁺ ATPase, or the inositol 1,4,5-triphosphate (IP₃) receptor antagonists 2-aminoethoxydiphenyl borate and xestospongin C, but not by extracellular Cd²⁺, the dihydropyridine antagonist nifedipine, or by ryanodine at concentrations that caused depletion of ryanodine-sensitive Ca²⁺ stores. These results support the notion that postganglionic sympathetic neurons possess the ability to release Ca²⁺ from IP₃-sensitive internal stores in response to membrane depolarization, independent of Ca²⁺ influx.

Functional ryanodine receptors in the membranes of neurohypophysial secretory granules

Highly localized Ca(2+) release events have been characterized in several neuronal preparations. In mouse neurohypophysial terminals (NHTs), such events, called Ca(2+) syntillas, appear to emanate from a ryanodine-sensitive intracellular Ca(2+) pool. Traditional sources of intracellular Ca(2+) appear to be lacking in NHTs. Thus, we have tested the hypothesis that large dense core vesicles (LDCVs), which contain a substantial amount of calcium, represent the source of these syntillas. Here, using fluorescence immunolabeling and immunogold-labeled electron micrographs of NHTs, we show that type 2 ryanodine receptors (RyRs) are localized specifically to LDCVs. Furthermore, a large conductance nonspecific cation channel, which was identified previously in the vesicle membrane and has biophysical properties similar to that of an RyR, is pharmacologically affected in a manner characteristic of an RyR: it is activated in the presence of the RyR agonist ryanodine (at low concentrations) and blocked by the RyR antagonist ruthenium red. Additionally, neuropeptide release experiments show that these same RyR agonists and antagonists modulate Ca(2+)-elicited neuropeptide release from permeabilized NHTs. Furthermore, amperometric recording of spontaneous release events from artificial transmitter-loaded terminals corroborated these ryanodine effects. Collectively, our findings suggest that RyR-dependent syntillas could represent mobilization of Ca(2+) from vesicular stores. Such localized vesicular Ca(2+) release events at the precise location of exocytosis could provide a Ca(2+) amplification mechanism capable of modulating neuropeptide release physiologically.

Catecholamine exocytosis during low frequency stimulation in mouse adrenal chromaffin cells is primarily asynchronous and controlled by the novel mechanism of Ca2+ syntilla suppression

Adrenal chromaffin cells (ACCs), stimulated by the splanchnic nerve, generate action potentials (APs) at a frequency near 0.5 Hz in the resting physiological state, at times described as 'rest and digest'. How such low frequency stimulation in turn elicits sufficient catecholamine exocytosis to set basal sympathetic tone is not readily explained by the classical mechanism of stimulus-secretion coupling, where exocytosis is synchronized to AP-induced Ca(2+) influx. By using simulated action potentials (sAPs) at 0.5 Hz in isolated patch-clamped mouse ACCs, we show here that less than 10% of all catecholaminergic exocytosis, measured by carbon fibre amperometry, is synchronized to an AP. The asynchronous phase, the dominant phase, of exocytosis does not require Ca(2+) influx. Furthermore, increased asynchronous exocytosis is accompanied by an AP-dependent decrease in frequency of Ca(2+) syntillas (i.e. transient, focal Ca(2+) release from internal stores) and is ryanodine sensitive. We propose a mechanism of disinhibition, wherein APs suppress Ca(2+) syntillas, which themselves inhibit exocytosis as they do in the case of spontaneous catecholaminergic exocytosis.

μ-Opioid inhibition of Ca2+ currents and secretion in isolated terminals of the neurohypophysis occurs via ryanodine-sensitive Ca2+ stores

μ-Opioid agonists have no effect on calcium currents (I(Ca)) in neurohypophysial terminals when recorded using the classic whole-cell patch-clamp configuration. However, μ-opioid receptor (MOR)-mediated inhibition of I(Ca) is reliably demonstrated using the perforated-patch configuration. This suggests that the MOR-signaling pathway is sensitive to intraterminal dialysis and is therefore mediated by a readily diffusible second messenger. Using the perforated patch-clamp technique and ratio-calcium-imaging methods, we describe a diffusible second messenger pathway stimulated by the MOR that inhibits voltage-gated calcium channels in isolated terminals from the rat neurohypophysis (NH). Our results show a rise in basal intracellular calcium ([Ca(2+)]i) in response to application of [D-Ala(2)-N-Me-Phe(4),Gly5-ol]-Enkephalin (DAMGO), a MOR agonist, that is blocked by D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a MOR antagonist. Buffering DAMGO-induced changes in [Ca(2+)]i with BAPTA-AM completely blocked the inhibition of both I(Ca) and high-K(+)-induced rises in [Ca(2+)]i due to MOR activation, but had no effect on κ-opioid receptor (KOR)-mediated inhibition. Given the presence of ryanodine-sensitive stores in isolated terminals, we tested 8-bromo-cyclic adenosine diphosphate ribose (8Br-cADPr), a competitive inhibitor of cyclic ADP-ribose (cADPr) signaling that partially relieves DAMGO inhibition of I(Ca) and completely relieves MOR-mediated inhibition of high-K(+)-induced and DAMGO-induced rises in [Ca(2+)]i. Furthermore, antagonist concentrations of ryanodine completely blocked MOR-induced increases in [Ca(2+)]i and inhibition of I(Ca) and high-K(+)-induced rises in [Ca(2+)]i while not affecting KOR-mediated inhibition. Antagonist concentrations of ryanodine also blocked MOR-mediated inhibition of electrically-evoked increases in capacitance. These results strongly suggest that a key diffusible second messenger mediating the MOR-signaling pathway in NH terminals is [Ca(2+)]i released by cADPr from ryanodine-sensitive stores.

In-VITRo effect of Ficus deltoidea on the contraction of isolated rat's uteri is mediated via multiple receptors binding and is dependent on extracellular calcium

BACKGROUND

Ficus deltoidea, is a perennial herb that is used to assist labor, firm the uterus post-delivery and to prevent postpartum bleeding. In view of its claimed uterotonic action, the mechanisms underlying plant's effect on uterine contraction were investigated.

METHODS

Adult female SD rats were injected with 2 mg/kg 17β-oestradiol (E2) to synchronize their oestrous cycle. A day after injection, uteri were removed for in-vitro contraction studies. The dose dependent effect of Ficus deltoidea aqeous extract (FDA) on the tension produced by the isolated rat's uteri was determined. The effects of atropine (2×10(-8) M), atosiban (0.5 IU), THG113.31 (10 μM), oxodipine (0.25 mM), EDTA (1 mM), 2-amino-ethoxy-diphenylborate (2-APB) (40 mM) and thapsigargin (1 mM) on the maximum force of contraction (Emax) achieved following 2 mg/ml FDA administration were also investigated.

RESULTS

FDA induced in-vitro contraction of the isolated rat's uteri in a dose-dependent manner. Administration of atropine, atosiban and THG113.31 reduced the Emax with atosiban having the greatest effect. The Emax was also reduced following oxodipine and EDTA administration. There was no significant change observed following 2-APB administration. Thapsigargin, however, augmented Emax.

CONCLUSIONS

FDA-induced contraction of the isolated rat's uteri is mediated via multiple uterotonin receptors (muscarinic, oxytocin and prostaglandin F2α) and was dependent on the extracellular Ca2+. Contraction, however, was not dependent on the Ca2+ release from the internal stores. This in-vitro study provides the first scientific evidence on the claimed effect of Ficus Deltoidea on uterine contraction.

Ryanodine Receptors Selectively Interact with L Type Calcium Channels in Mouse Taste Cells

Introduction

We reported that ryanodine receptors are expressed in two different types of mammalian peripheral taste receptor cells: Type II and Type III cells. Type II cells lack voltage-gated calcium channels (VGCCs) and chemical synapses. In these cells, ryanodine receptors contribute to the taste-evoked calcium signals that are initiated by opening inositol trisphosphate receptors located on internal calcium stores. In Type III cells that do have VGCCs and chemical synapses, ryanodine receptors contribute to the depolarization-dependent calcium influx.

Methodology/Principal Findings

The goal of this study was to establish if there was selectivity in the type of VGCC that is associated with the ryanodine receptor in the Type III taste cells or if the ryanodine receptor opens irrespective of the calcium channels involved. We also wished to determine if the ryanodine receptors and VGCCs require a physical linkage to interact or are simply functionally associated with each other. Using calcium imaging and pharmacological inhibitors, we found that ryanodine receptors are selectively associated with L type VGCCs but likely not through a physical linkage.

Conclusions/Significance

Taste cells are able to undergo calcium induced calcium release through ryanodine receptors to increase the initial calcium influx signal and provide a larger calcium response than would otherwise occur when L type channels are activated in Type III taste cells.

Tonic arterial contraction mediated by L-type Ca2+ channels requires sustained Ca2+ influx, G protein-associated Ca2+ release, and RhoA/ROCK activation

KCl-evoked sustained contraction requires L-type Ca(2+) channel activation, metabotropic Ca(2+) release from the sarcoplasmic reticulum (mechanism denoted calcium channel-induced Ca(2+) release) and RhoA/Rho associated kinase activation. Although high K(+) solutions are used to depolarize myocytes, these solutions can stimulate other signaling pathways such as those triggered by the activation of muscarinic and purinergic receptors. The present study examines the functional role of calcium channel-induced Ca(2+) release under pharmacological activation of L-type Ca(2+) channel without significant membrane depolarization. It also analyzes the role of the "steady-state" Ca(2+) influx through L-type Ca(2+) channels on myocyte sustained contraction. Measurement of contractility in arterial rings was done on a vessel myograph. Membrane potential was measured by fluorescence techniques loading intact myocytes with a membrane potential sensitive dye, and a reversible permeabilization method was used to load myocytes in intact arteries with GDPβS and Ca(v)1.2 siRNA. Application of an L-type Ca(2+) channel agonist, without effect on membrane potential, evoked sustained contraction via G-protein induced Ca(2+) release from the sarcoplasmic reticulum and RhoA/Rho associated kinase activation. Tonic myocyte contractions mediated by L-type Ca(2+) channel activation required sustained Ca(2+) influx through the channels and Ca(2+) uptake by the sarcoplasmic reticulum. Because L-type Ca(2+) channels participate in numerous pathophysiological processes mediated by maintained arterial contraction, our data could help to optimize therapeutic treatment of arterial vasospasm.

Ca(V)1.1: The atypical prototypical voltage-gated Ca²⁺ channel

Ca(V)1.1 is the prototype for the other nine known Ca(V) channel isoforms, yet it has functional properties that make it truly atypical of this group. Specifically, Ca(V)1.1 is expressed solely in skeletal muscle where it serves multiple purposes; it is the voltage sensor for excitation-contraction coupling and it is an L-type Ca²⁺ channel which contributes to a form of activity-dependent Ca²⁺ entry that has been termed Excitation-coupled Ca²⁺ entry. The ability of Ca(V)1.1 to serve as voltage-sensor for excitation-contraction coupling appears to be unique among Ca(V) channels, whereas the physiological role of its more conventional function as a Ca²⁺ channel has been a matter of uncertainty for nearly 50 years. In this chapter, we discuss how Ca(V)1.1 supports excitation-contraction coupling, the possible relevance of Ca²⁺ entry through Ca(V)1.1 and how alterations of Ca(V)1.1 function can have pathophysiological consequences. This article is part of a Special Issue entitled: Calcium channels.

Modulation/physiology of calcium channel sub-types in neurosecretory terminals

The hypothalamic-neurohypophysial system (HNS) controls diuresis and parturition through the release of arginine-vasopressin (AVP) and oxytocin (OT). These neuropeptides are chiefly synthesized in hypothalamic magnocellular somata in the supraoptic and paraventricular nuclei and are released into the blood stream from terminals in the neurohypophysis. These HNS neurons develop specific electrical activity (bursts) in response to various physiological stimuli. The release of AVP and OT at the level of neurohypophysis is directly linked not only to their different burst patterns, but is also regulated by the activity of a number of voltage-dependent channels present in the HNS nerve terminals and by feedback modulators. We found that there is a different complement of voltage-gated Ca(2+) channels (VGCC) in the two types of HNS terminals: L, N, and Q in vasopressinergic terminals vs. L, N, and R in oxytocinergic terminals. These channels, however, do not have sufficiently distinct properties to explain the differences in release efficacy of the specific burst patterns. However, feedback by both opioids and ATP specifically modulate different types of VGCC and hence the amount of AVP and/or OT being released. Opioid receptors have been identified in both AVP and OT terminals. In OT terminals, μ-receptor agonists inhibit all VGCC (particularly R-type), whereas, they induce a limited block of L-, and P/Q-type channels, coupled to an unusual potentiation of the N-type Ca(2+) current in the AVP terminals. In contrast, the N-type Ca(2+) current can be inhibited by adenosine via A(1) receptors leading to the decreased release of both AVP and OT. Furthermore, ATP evokes an inactivating Ca(2+)/Na(+)-current in HNS terminals able to potentiate AVP release through the activation of P2X2, P2X3, P2X4 and P2X7 receptors. In OT terminals, however, only the latter receptor type is probably present. We conclude by proposing a model that can explain how purinergic and/or opioid feedback modulation during bursts can mediate differences in the control of neurohypophysial AVP vs. OT release.

Type 1 ryanodine receptor knock-in mutation causing central core disease of skeletal muscle also displays a neuronal phenotype

The type 1 ryanodine receptor (RyR1) is expressed widely in the brain, with high levels in the cerebellum, hippocampus, and hypothalamus. We have shown that L-type Ca(2+) channels in terminals of hypothalamic magnocellular neurons are coupled to RyRs, as they are in skeletal muscle, allowing voltage-induced Ca(2+) release (VICaR) from internal Ca(2+) stores without Ca(2+) influx. Here we demonstrate that RyR1 plays a role in VICaR in nerve terminals. Furthermore, in heterozygotes from the Ryr1(I4895T/WT) (IT/+) mouse line, carrying a knock-in mutation corresponding to one that causes a severe form of human central core disease, VICaR is absent, demonstrating that type 1 RyR mediates VICaR and that these mice have a neuronal phenotype. The absence of VICaR was shown in two ways: first, depolarization in the absence of Ca(2+) influx elicited Ca(2+)syntillas (scintilla, spark, in a nerve terminal, a SYNaptic structure) in WT, but not in mutant terminals; second, in the presence of extracellular Ca(2+), IT/+ terminals showed a twofold decrease in global Ca(2+) transients, with no change in plasmalemmal Ca(2+) current. From these studies we draw two conclusions: (i) RyR1 plays a role in VICaR in hypothalamic nerve terminals; and (ii) a neuronal alteration accompanies the myopathy in IT/+ mice, and, possibly in humans carrying the corresponding RyR1 mutation.

A model of the physiological basis of a multivariate phenotype that is mediated by Ca(2+) signaling and controlled by ryanodine receptor composition

Calcium-signals occur in a wide variety of tissue types - from skeletal, smooth and cardiac muscle to pancreatic and brain tissues. Ca(2+) signals regulate diverse processes including muscle contraction, hormone secretion, neural communication and gene expression. Together these different tissues and processes form the basis of a multivariate trait. Calcium signals are characterized by Ca(2+) transients, which are sharp increases in Ca(2+) concentration over a short period of time. In this paper we derive and analyze a model of Ca(2+) transients for skeletal muscle, neurons and cardiac tissue based on underlying biophysical principles. Tissue differentiation in our model and in nature comes about by varying the ryanodine receptor (RyR) channel composition of tissues. In vertebrates, there are typically three types of RyR channels (labeled RyR1, RyR2 and RyR3 in mammals and α-RyR, cardiac-RyR and β-RyR in birds, amphibians and fish). Different compositions of these three RyR channels generate different Ca(2+) transient properties. There are four Ca(2+) transient properties that we measure: maximum amplitude, duration, half duration (D(50)) and integrated concentration. In agreement with experimental work, our results find that the addition of RyR3 amplifies Ca(2+) transients in skeletal muscle. An important consequence of shared molecular components between tissue types in a multivariate setting is that the shared components cause individual traits of a multivariate trait to be correlated in function. Here we show how correlations in Ca(2+) transient properties between tissues can be predicted using an underlying biophysical model.

Somatic depolarization enhances GABA release in cerebellar interneurons via a calcium/protein kinase C pathway

In cortical and hippocampal neurons, tonic somatic depolarization is partially transmitted to synaptic terminals, where it enhances transmitter release. It is not known to what extent such "analog signaling" applies to other mammalian neurons, and available evidence concerning underlying mechanisms is fragmentary and partially controversial. In this work, we investigate the presence of analog signaling in molecular layer interneurons of the rat cerebellum. GABA release was estimated by measuring autoreceptor currents in single recordings, or postsynaptic currents in paired recordings of synaptically connected neurons. We find with both assays that moderate subthreshold somatic depolarization results in enhanced GABA release. In addition, changes in the calcium concentration were investigated in the axon compartment using the calcium-sensitive dye OGB-1 (Oregon Green BAPTA-1). After a step somatic depolarization, the axonal calcium concentration and the GABA release probability rise with a common slow time course. However, the amount of calcium entry that is associated to one action potential is not affected. The slow increase in calcium concentration is inhibited by the P/Q calcium channel blocker ω-agatoxin-IVA. The protein kinase C inhibitor Ro 31-8220 (3-[3-[2,5-dihydro-4-(1-methyl-1H-indol-3-yl)-2,5-dioxo-1H-pyrrol-3-yl]-1H-indol-1-yl]propyl carbamimidothioic acid ester mesylate) did not affect the calcium concentration changes but it blocked the increase in GABA release. EGTA was a weak blocker of analog signaling, implicating a close association of protein kinase C to the site of calcium entry. We conclude that analog signaling is prominent in cerebellar interneurons and that it is triggered by a pathway involving activation of axonal P/Q channels, followed by calcium entry and local activation of protein kinase C.

Presynaptic roles of intracellular Ca(2+) stores in signalling and exocytosis

The signalling roles of Ca(2+)(ic) (intracellular Ca(2+)) stores are well established in non-neuronal and neuronal cells. In neurons, although Ca(2+)(ic) stores have been assigned a pivotal role in postsynaptic responses to G(q)-coupled receptors, or secondarily to extracellular Ca(2+) influx, the functions of dynamic Ca(2+)(ic) stores in presynaptic terminals remain to be fully elucidated. In the present paper, we review some of the recent evidence supporting an involvement of Ca(2+)(ic) in presynaptic function, and discuss loci at which this source of Ca(2+) may impinge. Nerve terminal preparations provide good models for functionally examining putative Ca(2+)(ic) stores under physiological and pathophysiological stimulation paradigms, using Ca(2+)-dependent activation of resident protein kinases as sensors for fine changes in intracellular Ca(2+) levels. We conclude that intraterminal Ca(2+)(ic) stores may, directly or indirectly, enhance neurotransmitter release following nerve terminal depolarization and/or G-protein-coupled receptor activation. During conditions that prevail following neuronal ischaemia, increased glutamate release instigated by Ca(2+)(ic) store activation may thereby contribute to excitotoxicity and eventual synaptopathy.

Minding the calcium store: Ryanodine receptor activation as a convergent mechanism of PCB toxicity

Chronic low-level polychlorinated biphenyl (PCB) exposures remain a significant public health concern since results from epidemiological studies indicate that PCB burden is associated with immune system dysfunction, cardiovascular disease, and impairment of the developing nervous system. Of these various adverse health effects, developmental neurotoxicity has emerged as a particularly vulnerable endpoint in PCB toxicity. Arguably the most pervasive biological effects of PCBs could be mediated by their ability to alter the spatial and temporal fidelity of Ca2+ signals through one or more receptor-mediated processes. This review will focus on our current knowledge of the structure and function of ryanodine receptors (RyRs) in muscle and nerve cells and how PCBs and related non-coplanar structures alter these functions. The molecular and cellular mechanisms by which non-coplanar PCBs and related structures alter local and global Ca2+ signaling properties and the possible short and long-term consequences of these perturbations on neurodevelopment and neurodegeneration are reviewed.

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.

Ionic conditions modulate stimulus-induced capacitance changes in isolated neurohypophysial terminals of the rat

Peptidergic nerve terminals of the neurohypophysis (NH) secrete both oxytocin and vasopressin upon stimulation with peptide-specific bursts of action potentials from magnocellular neurons. These bursts vary in both frequency and action potential duration and also induce in situ ionic changes both inside and outside the terminals in the NH. These temporary effects include the increase of external potassium and decrease of external calcium, as well as the increase in internal sodium and chloride concentrations. In order to determine any mechanism of action that these ionic changes might have on secretion, stimulus-induced capacitance recordings were performed on isolated terminals of the NH using action potential burst patterns of varying frequency and action potential width. The results indicate that in NH terminals: (1) increased internal chloride concentration improves the efficiency of action potential-induced capacitance changes, (2) increasing external potassium increases stimulus-induced capacitance changes, (3) decreasing external calcium decreases the capacitance induced by low frequency broadened action potentials, while no capacitance change is observed with high frequency un-broadened action potentials, and (4) increasing internal sodium increases the capacitance change induced by low frequency bursts of broadened action potentials, more than for high frequency bursts of narrow action potentials. These results are consistent with previous models of stimulus-induced secretion, where optimal secretory efficacy is determined by particular characteristics of action potentials within a burst. Our results suggest that positive effects of increased internal sodium and external potassium during a burst may serve as a compensatory mechanism for secretion, counterbalancing the negative effects of reduced external calcium. In this view, high frequency un-broadened action potentials (initial burst phase) would condition the terminals by increasing internal sodium for optimal secretion by the physiological later phase of broadened action potentials. Thus, ionic changes occurring during a burst may help to make such stimulation more efficient at inducing secretion. Furthermore, these effects are thought to occur within the initial few seconds of incoming burst activity at both oxytocin and vasopressin types of NH nerve terminals.

Ryanodine modification of RyR1 retrogradely affects L-type Ca(2+) channel gating in skeletal muscle

In skeletal muscle, there is bidirectional signalling between the L-type Ca(2+) channel (1,4-dihydropyridine receptor; DHPR) and the type 1 ryanodine-sensitive Ca(2+) release channel (RyR1) of the sarcoplasmic reticulum (SR). In the case of "orthograde signalling" (i.e., excitation-contraction coupling), the conformation of RyR1 is controlled by depolarization-induced conformational changes of the DHPR resulting in Ca(2+) release from the SR. "Retrograde coupling" is manifested as enhanced L-type current. The nature of this retrograde signal, and its dependence on RyR1 conformation, are poorly understood. Here, we have examined L-type currents in normal myotubes after an exposure to ryanodine (200 microM, 1 h at 37 degrees C) sufficient to lock RyR1 in a non-conducting, inactivated, conformational state. This treatment caused an increase in L-type current at less depolarized test potentials in comparison to myotubes similarly exposed to vehicle as a result of a approximately 5 mV hyperpolarizing shift in the voltage-dependence of activation. Charge movements of ryanodine-treated myotubes were also shifted to more hyperpolarizing potentials (approximately 13 mV) relative to vehicle-treated myotubes. Enhancement of the L-type current by ryanodine was absent in dyspedic (RyR1 null) myotubes, indicating that ryanodine does not act directly on the DHPR. Our findings indicate that in retrograde signaling, the functional state of RyR1 influences conformational changes of the DHPR involved in activation of L-type current. This raises the possibility that physiological regulators of the conformational state of RyR1 (e.g., Ca(2+), CaM, CaMK, redox potential) may also affect DHPR gating.

Homer and the ryanodine receptor

Homer proteins have recently been identified as novel high-affinity ligands that modulate ryanodine receptor (RyR) Ca(2+) release channels in heart and skeletal muscle, through an EVH1 domain which binds to proline-rich regions in target proteins. Many Homer proteins can also self-associate through a coiled-coil domain that allows their multimerisation. In other tissues, especially neurons, Homer anchors proteins embedded in the surface membrane to the Ca(2+) release channel in the endoplasmic reticulum and can anchor membrane or cytosolic proteins to the cytoskeleton. Although this anchoring aspect of Homer function has not been extensively investigated in muscle, there are consensus sequences for Homer binding in the RyR and on many of the proteins that it interacts with in the massive RyR ion channel complex. In this review we explore the potential of Homer to contribute to a variety of cell processes in muscle and neurons that also involve RyR channels.

Developmental exposure to polychlorinated biphenyls interferes with experience-dependent dendritic plasticity and ryanodine receptor expression in weanling rats

BACKGROUND

Neurodevelopmental disorders are associated with altered patterns of neuronal connectivity. A critical determinant of neuronal connectivity is the dendritic morphology of individual neurons, which is shaped by experience. The identification of environmental exposures that interfere with dendritic growth and plasticity may, therefore, provide insight into environmental risk factors for neurodevelopmental disorders.

OBJECTIVE

We tested the hypothesis that polychlorinated biphenyls (PCBs) alter dendritic growth and/or plasticity by promoting the activity of ryanodine receptors (RyRs).

METHODS AND RESULTS

The Morris water maze was used to induce experience-dependent neural plasticity in weanling rats exposed to either vehicle or Aroclor 1254 (A1254) in the maternal diet throughout gestation and lactation. Developmental A1254 exposure promoted dendritic growth in cerebellar Purkinje cells and neocortical pyramidal neurons among untrained animals but attenuated or reversed experience-dependent dendritic growth among maze-trained littermates. These structural changes coincided with subtle deficits in spatial learning and memory, increased [3H]-ryanodine binding sites and RyR expression in the cerebellum of untrained animals, and inhibition of training-induced RyR upregulation. A congener with potent RyR activity, PCB95, but not a congener with negligible RyR activity, PCB66, promoted dendritic growth in primary cortical neuron cultures and this effect was blocked by pharmacologic antagonism of RyR activity.

CONCLUSIONS

Developmental exposure to PCBs interferes with normal patterns of dendritic growth and plasticity, and these effects may be linked to changes in RyR expression and function. These findings identify PCBs as candidate environmental risk factors for neurodevelopmental disorders, especially in children with heritable deficits in calcium signaling.

CaMKII locally encodes L-type channel activity to signal to nuclear CREB in excitation-transcription coupling

Communication between cell surface proteins and the nucleus is integral to many cellular adaptations. In the case of ion channels in excitable cells, the dynamics of signaling to the nucleus are particularly important because the natural stimulus, surface membrane depolarization, is rapidly pulsatile. To better understand excitation-transcription coupling we characterized the dependence of cAMP response element-binding protein phosphorylation, a critical step in neuronal plasticity, on the level and duration of membrane depolarization. We find that signaling strength is steeply dependent on depolarization, with sensitivity far greater than hitherto recognized. In contrast, graded blockade of the Ca(2+) channel pore has a remarkably mild effect, although some Ca(2+) entry is absolutely required. Our data indicate that Ca(2+)/CaM-dependent protein kinase II acting near the channel couples local Ca(2+) rises to signal transduction, encoding the frequency of Ca(2+) channel openings rather than integrated Ca(2+) flux-a form of digital logic.

Regulation of synaptic transmission by presynaptic CaMKII and BK channels

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and the BK channel are enriched at the presynaptic nerve terminal, where CaMKII associates with synaptic vesicles whereas the BK channel colocalizes with voltage-sensitive Ca(2+) channels in the plasma membrane. Mounting evidence suggests that these two proteins play important roles in controlling neurotransmitter release. Presynaptic BK channels primarily serve as a negative regulator of neurotransmitter release. In contrast, presynaptic CaMKII either enhances or inhibits neurotransmitter release and synaptic plasticity depending on experimental or physiological conditions and properties of specific synapses. The different functions of presynaptic CaMKII appear to be mediated by distinct downstream proteins, including the BK channel.

Chronic osmotic stimuli increase salusin-beta-like immunoreactivity in the rat hypothalamo-neurohypophyseal system: possible involvement of salusin-beta on [Ca2+]i increase and neurohypophyseal hormone release from the axon terminals

Salusin-alpha and -beta were recently discovered as bioactive endogenous peptides. In the present study, we investigated the effects of chronic osmotic stimuli on salusin-beta-like immunoreactivity (LI) in the rat hypothalamo-neurohypophyseal system. We examined the effects of salusin-beta on synaptic inputs to the rat magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) and neurohypophyseal hormone release from both freshly dissociated SONs and neurohypophyses in rats. Immunohistochemical studies revealed that salusin-beta-LI neurones and fibres were markedly increased in the SON and the magnocellular division of the paraventricular nucleus after chronic osmotic stimuli resulting from salt loading for 5 days and dehydration for 3 days. Salusin-beta-LI fibres and varicosities in the internal zone of the median eminence and the neurohypophysis were also increased after osmotic stimuli. Whole-cell patch-clamp recordings from rat SON slice preparations showed that salusin-beta did not cause significant changes in the excitatory and inhibitory postsynaptic currents of the MNCs. In vitro hormone release studies showed that salusin-beta evoked both arginine vasopressin (AVP) and oxytocin release from the neurohypophysis, but not the SON. In our hands, in the neurohypophysis, a significant release of AVP and oxytocin was observed only at concentrations from 100 nm and above of salusin-beta. Low concentrations below 100 nm were ineffective both on AVP and oxytocin release. We also measured intracellular calcium ([Ca(2+)](i)) increase induced by salusin-beta on freshly-isolated single nerve terminals from the neurohypophysis devoid of pars intermedia. Furthermore, this salusin-beta-induced [Ca(2+)](i) increase was blocked in the presence of high voltage activated Ca(2+)channel blockers. Our results suggest that salusin-beta may be involved in the regulation of body fluid balance by stimulating neurohypophyseal hormone release from nerve endings by an autocrine/paracrine mechanism.

An Ryr1I4895T mutation abolishes Ca2+ release channel function and delays development in homozygous offspring of a mutant mouse line

A heterozygous Ile4898 to Thr (I4898T) mutation in the human type 1 ryanodine receptor/Ca(2+) release channel (RyR1) leads to a severe form of central core disease. We created a mouse line in which the corresponding Ryr1(I4895T) mutation was introduced by using a "knockin" protocol. The heterozygote does not exhibit an overt disease phenotype, but homozygous (IT/IT) mice are paralyzed and die perinatally, apparently because of asphyxia. Histological analysis shows that IT/IT mice have greatly reduced and amorphous skeletal muscle. Myotubes are small, nuclei remain central, myofibrils are disarranged, and no cross striation is obvious. Many areas indicate probable degeneration, with shortened myotubes containing central stacks of pyknotic nuclei. Other manifestations of a delay in completion of late stages of embryogenesis include growth retardation and marked delay in ossification, dermatogenesis, and cardiovascular development. Electron microscopy of IT/IT muscle demonstrates appropriate targeting and positioning of RyR1 at triad junctions and a normal organization of dihydropyridine receptor (DHPR) complexes into RyR1-associated tetrads. Functional studies carried out in cultured IT/IT myotubes show that ligand-induced and DHPR-activated RyR1 Ca(2+) release is absent, although retrograde enhancement of DHPR Ca(2+) conductance is retained. IT/IT mice, in which RyR1-mediated Ca(2+) release is abolished without altering the formation of the junctional DHPR-RyR1 macromolecular complex, provide a valuable model for elucidation of the role of RyR1-mediated Ca(2+) signaling in mammalian embryogenesis.

RyR1-specific requirement for depolarization-induced Ca2+ sparks in urinary bladder smooth muscle

Ryanodine receptor subtype 1 (RyR1) has been primarily characterized in skeletal muscle but several studies have revealed its expression in smooth muscle. Here, we used Ryr1-null mice to investigate the role of this isoform in Ca2+ signaling in urinary bladder smooth muscle. We show that RyR1 is required for depolarization-induced Ca2+ sparks, whereas RyR2 and RyR3 are sufficient for spontaneous or caffeine-induced Ca2+ sparks. Immunostaining revealed specific subcellular localization of RyR1 in the superficial sarcoplasmic reticulum; by contrast, RyR2 and RyR3 are mainly expressed in the deep sarcoplasmic reticulum. Paradoxically, lack of depolarization-induced Ca2+ sparks in Ryr1–/– myocytes was accompanied by an increased number of cells displaying spontaneous or depolarization-induced Ca2+ waves. Investigation of protein expression showed that FK506-binding protein (FKBP) 12 and FKBP12.6 (both of which are RyR-associated proteins) are downregulated in Ryr1–/– myocytes, whereas expression of RyR2 and RyR3 are unchanged. Moreover, treatment with rapamycin, which uncouples FKBPs from RyR, led to an increase of RyR-dependent Ca2+ signaling in wild-type urinary bladder myocytes but not in Ryr1–/– myocytes.

In conclusion, although decreased amounts of FKBP increase Ca2+ signals in Ryr1–/– urinary bladder myocytes the depolarization-induced Ca2+ sparks are specifically lost, demonstrating that RyR1 is required for depolarization-induced Ca2+ sparks and suggesting that the intracellular localization of RyR1 fine-tunes Ca2+ signals in smooth muscle.

Feedback mechanisms in the regulation of intracellular calcium ([Ca2+]i) in the peripheral nociceptive system: role of TRPV-1 and pain related receptors

Multimodal stimuli like heat, cold, bacterial or mechanical events are able to elicit pain, which is necessary to guarantee survival. However, the control of pain is of major clinical importance. The perception and transduction of pain is differentially modulated in the peripheral and central nervous system (CNS): while peripheral structures modulate these signals, the perception of pain occurs in the CNS. In recent years major advances have been made in the understanding of the processes which are involved in pain sensation. For the peripheral pain reception, the importance of specific pain receptors of the transition receptor pore (TRP)-family (e.g. the TRPV-1 receptor) has been analyzed. These receptors/channels are localized at the cell membrane of nociceptive neurones as well as in membranes of intracellular calcium stores like the endoplasmic reticulum. While the associated channel conducts different ions, a major proportion is calcium. Therefore, this review focuses on (1) the modulations of intracellular calcium ([Ca2+]i) initiated by the activation of pain receptors and (2) the consequences of [Ca2+]i changes for the processing of pain signals at the peripheral side. The possible interference of TRPV-1 induced [Ca2+]i modulations to the function of other membrane receptors and channels, like voltage gated calcium, sodium or potassium channels, or co-expressed CB1-receptors will be discussed. The latter interactions are of specific interest since the analgetic properties of endo- and exo-cannabinoids are mediated by CB1 receptors and their activation significantly modulates the calcium induced release of pain related transmitters. Furthermore, multiple cross links between different pain modulating intracellular pathways and their dependence on [Ca2+]i modulations will be illuminated. Overall, this review will summarize new insights resulting in the understanding of the prominent influence of [Ca2+]i for processes which are involved in pain sensation.

Effects of ATP, Mg2+, and redox agents on the Ca2+ dependence of RyR channels from rat brain cortex

Despite their relevance for neuronal Ca(2+)-induced Ca(2+) release (CICR), activation by Ca(2+) of ryanodine receptor (RyR) channels of brain endoplasmic reticulum at the [ATP], [Mg(2+)], and redox conditions present in neurons has not been reported. Here, we studied the effects of varying cis-(cytoplasmic) free ATP concentration ([ATP]), [Mg(2+)], and RyR redox state on the Ca(2+) dependence of endoplasmic reticulum RyR channels from rat brain cortex. At pCa 4.9 and 0.5 mM adenylylimidodiphosphate (AMP-PNP), increasing free [Mg(2+)] up to 1 mM inhibited vesicular [(3)H]ryanodine binding; incubation with thimerosal or dithiothreitol decreased or enhanced Mg(2+) inhibition, respectively. Single RyR channels incorporated into lipid bilayers displayed three different Ca(2+) dependencies, defined by low, moderate, or high maximal fractional open time (P(o)), that depend on RyR redox state, as we have previously reported. In all cases, cis-ATP addition (3 mM) decreased threshold [Ca(2+)] for activation, increased maximal P(o), and shifted channel inhibition to higher [Ca(2+)]. Conversely, at pCa 4.5 and 3 mM ATP, increasing cis-[Mg(2+)] up to 1 mM inhibited low activity channels more than moderate activity channels but barely modified high activity channels. Addition of 0.5 mM free [ATP] plus 0.8 mM free [Mg(2+)] induced a right shift in Ca(2+) dependence for all channels so that [Ca(2+)] <30 microM activated only high activity channels. These results strongly suggest that channel redox state determines RyR activation by Ca(2+) at physiological [ATP] and [Mg(2+)]. If RyR behave similarly in living neurons, cellular redox state should affect RyR-mediated CICR.

Ca(2+) sparks operated by membrane depolarization require isoform 3 ryanodine receptor channels in skeletal muscle

Stimuli are translated to intracellular calcium signals via opening of inositol trisphosphate receptor and ryanodine receptor (RyR) channels of the sarcoplasmic reticulum or endoplasmic reticulum. In cardiac and skeletal muscle of amphibians the stimulus is depolarization of the transverse tubular membrane, transduced by voltage sensors at tubular-sarcoplasmic reticulum junctions, and the unit signal is the Ca(2+) spark, caused by concerted opening of multiple RyR channels. Mammalian muscles instead lose postnatally the ability to produce sparks, and they also lose RyR3, an isoform abundant in spark-producing skeletal muscles. What does it take for cells to respond to membrane depolarization with Ca(2+) sparks? To answer this question we made skeletal muscles of adult mice expressing exogenous RyR3, demonstrated as immunoreactivity at triad junctions. These muscles showed abundant sparks upon depolarization. Sparks produced thusly were found to amplify the response to depolarization in a manner characteristic of Ca(2+)-induced Ca(2+) release processes. The amplification was particularly effective in responses to brief depolarizations, as in action potentials. We also induced expression of exogenous RyR1 or yellow fluorescent protein-tagged RyR1 in muscles of adult mice. In these, tag fluorescence was present at triad junctions. RyR1-transfected muscle lacked voltage-operated sparks. Therefore, the voltage-operated sparks phenotype is specific to the RyR3 isoform. Because RyR3 does not contact voltage sensors, their opening was probably activated by Ca(2+), secondarily to Ca(2+) release through junctional RyR1. Physiologically voltage-controlled Ca(2+) sparks thus require a voltage sensor, a master junctional RyR1 channel that provides trigger Ca(2+), and a slave parajunctional RyR3 cohort.