Characteristics of [3H]ryanodine binding to the rabbit cerebral microsomes

Inhibitory ryanodine prevents ryanodine receptor-mediated Ca²⁺ release without affecting endoplasmic reticulum Ca²⁺ content in primary hippocampal neurons

Ryanodine is a cell permeant plant alkaloid that binds selectively and with high affinity to ryanodine receptor (RyR) Ca(2+) release channels. Sub-micromolar ryanodine concentrations activate RyR channels while micromolar concentrations are inhibitory. Several reports indicate that neuronal synaptic plasticity, learning and memory require RyR-mediated Ca(2+)-release, which is essential for muscle contraction. The use of micromolar (inhibitory) ryanodine represents a common strategy to suppress RyR activity in neuronal cells: however, micromolar ryanodine promotes RyR-mediated Ca(2+) release and endoplasmic reticulum Ca(2+) depletion in muscle cells. Information is lacking in this regard in neuronal cells; hence, we examined here if addition of inhibitory ryanodine elicited Ca(2+) release in primary hippocampal neurons, and if prolonged incubation of primary hippocampal cultures with inhibitory ryanodine affected neuronal ER calcium content. Our results indicate that inhibitory ryanodine does not cause Ca(2+) release from the ER in primary hippocampal neurons, even though ryanodine diffusion should produce initially low intracellular concentrations, within the RyR activation range. Moreover, neurons treated for 1 h with inhibitory ryanodine had comparable Ca(2+) levels as control neurons. These combined findings imply that prolonged incubation with inhibitory ryanodine, which effectively abolishes RyR-mediated Ca(2+) release, preserves ER Ca(2+) levels and thus constitutes a sound strategy to suppress neuronal RyR function.

The structure, function, and cellular regulation of ryanodine-sensitive Ca2+ release channels

The fundamental biological process of Ca2+ signaling is known to be important in most eukaryotic cells, and inositol 1,2,5-trisphosphate and ryanodine receptors, intracellular Ca2+ release channels encoded by two distantly related gene families, are central to this phenomenon. Ryanodine receptors in the sarcoplasmic reticulum of skeletal and cardiac muscle have a predominant role in excitation-contraction coupling, but the channels are also present in the endoplasmic reticulum of noncontractile tissues including the central nervous system and the immune system. In all, three highly homologous ryanodine receptor isoforms have been identified, all very large proteins which assemble as (homo)tetramers of approximately 2 MDa. They contain large cytoplasmically disposed regulatory domains and are always associated with other structural or regulatory proteins, including calmodulin and immunophilins, which can have marked effects on channel function. The type 1 isoform in skeletal muscle is electromechanically coupled to surface membrane voltage sensors, whereas the remaining isoforms appear to be activated solely by endogenous cytoplasmic second messengers or other ligands, including Ca2+ itself ("Ca(2+)-induced Ca2+ release"). This review concentrates on ryanodine receptor structure-function relationships as probed by a variety of methods and on the molecular mechanisms of channel modulation at the cellular level (including evidence for the regulation of gene expression and transcription). It also touches on the relevance of ryanodine receptors to complex cellular functions and disease.

Postischemic changes of

Sequential alterations of [3H]nimodipine and [3H]ryanodine binding in gerbils were investigated in selectively vulnerable regions, such as the striatum and hippocampus, 1 h to 7 days after 10 min of transient cerebral ischemia. [3H]Nimodipine binding showed no significant changes in the striatum and hippocampus up to 48 h after ischemia. Seven days after ischemia, however, a severe reduction in [3H]nimodipine binding was observed in the dorsolateral striatum, hippocampal CA1 (stratum oriens, stratum pyramidale and stratum radiatum) and hippocampal CA3 sector. On the other hand, [3H]ryanodine binding showed a significant increase in the hippocampus 1 h after ischemia. Five hours after ischemia, a significant reduction in [3H]ryanodine binding was observed only in the hippocampal CA1 sector. Thereafter, the striatum and hippocampus showed no significant alterations in [3H]ryanodine binding up to 48 h after ischemia. After 7 days, a marked reduction in [3H]ryanodine binding was observed in the striatum and hippocampus which were particularly vulnerable to ischemia. These results demonstrate that postischemic alteration in [3H]nimodipine and [3H]ryanodine binding is produced with different processes in the hippocampus. They also suggest that the mechanism for striatal cell damage caused by transient cerebral ischemia may, at least in part, differ from that for hippocampal neuronal damage. Furthermore, our findings suggest that abnormal calcium release from intracellular stores may play a pivotal role in the development of hippocampal neuronal damage. Copyright Rapid Science Ltd

Properties of the ryanodine-sensitive release channels that underlie caffeine-induced Ca2+ mobilization from intracellular stores in mammalian sympathetic neurons

The most compelling evidence for a functional role of caffeine-sensitive intracellular Ca2+ reservoirs in nerve cells derives from experiments on peripheral neurons. However, the properties of their ryanodine receptor calcium release channels have not been studied. This work combines single-cell fura-2 microfluorometry, [3H]ryanodine binding and recording of Ca2+ release channels to examine calcium release from these intracellular stores in rat sympathetic neurons from the superior cervical ganglion. Intracellular Ca2+ measurements showed that these cells possess caffeine-sensitive intracellular Ca2+ stores capable of releasing the equivalent of 40% of the calcium that enters through voltage-gated calcium channels. The efficiency of caffeine in releasing Ca2+ showed a complex dependence on [Ca2+]i. Transient elevations of [Ca2+]i by 50-500 nM were facilitatory, but they became less facilitatory or depressing when [Ca2+]i reached higher levels. The caffeine-induced Ca2+ release and its dependence on [Ca2+]i was further examined by [3H]ryanodine binding to ganglionic microsomal membranes. These membranes showed a high-affinity binding site for ryanodine with a dissociation constant (KD = 10 nM) similar to that previously reported for brain microsomes. However, the density of [3H]ryanodine binding sites (Bmax = 2.06 pmol/mg protein) was at least three-fold larger than the highest reported for brain tissue. [3H]Ryanodine binding showed a sigmoidal dependence on [Ca2+] in the range 0.1-10 microM that was further increased by caffeine. Caffeine-dependent enhancement of [3H]ryanodine binding increased and then decreased as [Ca2+] rose, with an optimum at [Ca2+] between 100 and 500 nM and a 50% decrease between 1 and 10 microM. At 100 microM [Ca2+], caffeine and ATP enhanced [3H]ryanodine binding by 35 and 170% respectively, while binding was reduced by > 90% with ruthenium red and MgCl2. High-conductance (240 pS) Ca2+ release channels present in ganglionic microsomal membranes were incorporated into planar phospholipid bilayers. These channels were activated by caffeine and by micromolar concentrations of Ca2+ from the cytosolic side, and were blocked by Mg2+ and ruthenium red. Ryanodine (2 microM) slowed channel gating and elicited a long-lasting subconductance state while 10 mM ryanodine closed the channel with infrequent opening to the subconductance level. These results show that the properties of the ryanodine receptor/Ca2+ release channels present in mammalian peripheral neurons can account for the properties of caffeine-induced Ca2+ release. Our data also suggest that the release of Ca2+ by caffeine has a bell-shaped dependence on Ca2+ in the physiological range of cytoplasmic [Ca2+].

Ionic strength dependence of calcium, adenine nucleotide, magnesium, and caffeine actions on ryanodine receptors in rat brain

[3H]Ryanodine binding studies of ryanodine receptors in brain membrane preparations typically require the presence of high salt concentrations in assay incubations to yield optimal levels of binding. Here, radioligand binding measurements on rat cerebral cortical tissues were conducted under high (1.0 M KCl) and low (200 mM KCl) salt buffer conditions to determine the effects of ionic strength on receptor binding properties as well as on modulation of ligand binding by Ca2+, Mg2+, beta, gamma-methylene-adenosine 5'-triphosphate (AMP-PCP), and caffeine. In 1.0 M KCl buffer, labeled titration/equilibrium analyses yielded two classes of binding sites with apparent KD (nM) and Bmax (fmol/mg of protein) values of 2.4 and 34, respectively, for the high-affinity site and 19.9 and 157, respectively, for the low-affinity site. Unlabeled titration/equilibrium measurements gave a single high-affinity site with a KD value of 1.9 nM and a Bmax value of 95 fmol/mg of protein. The apparent KD value derived from association and dissociation studies was 20 pM. Equilibrium binding was activated by Ca2+ (KD/Ca2+ = 14 nM), inhibited by Mg2+ (IC50 = 5.0 mM), and unaffected by AMP-PCP or caffeine. In 200 mM KCl buffer conditions, labeled titration analyses gave only a single site with a KD value similar to and a Bmax value 1.8-fold greater than those obtained for the low-affinity site in 1.0 M KCl buffer. In unlabeled titration measurements, the KD value was fivefold lower, whereas the Bmax value was unaffected. The KD value derived from association and dissociation analysis was 2.4-fold greater in 200 mM KCl compared with 1.0 M KCl buffer conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of ryanodine receptors

Recent findings on the ryanodine receptor of vertebrates, a Ca-release channel protein for the caffeine- and ryanodine-sensitive Ca pools, are reviewed in this article. Three distinct genes, i.e., ryr1, ryr2, and ryr3, express different isoforms in specific locations: Ryr1 in skeletal muscle and Purkinje cells of cerebellum; Ryr2 in cardiac muscle and brain, especially cerebellum; Ryr3 in skeletal muscle of nonmammalian vertebrates, the corpus striatum, and limbic cortex of brain, smooth muscles, and the other cells in vertebrates. While only one isoform (Ryr1) is expressed in mammalian skeletal muscles, two isoforms (alpha- and beta-isoforms expressed by ryr1 and ryr3, respectively) are found in nonmammalian vertebrate skeletal muscles. Although the coexistence of two isoforms may merely be related to differentiation and specialization, the biological significance remains to be clarified. Ryanodine receptors in vertebrate skeletal muscles are believed to mediate two different modes of Ca release: Ca(2+)-induced Ca release and action potential-induced Ca release. All results obtained so far with any isoform of ryanodine receptor are related to Ca(2+)-induced Ca release and show very similar characteristics. Ca(2+)-induced Ca release, however, cannot be the underlying mechanism of Ca release on skeletal muscle activation. Susceptibility of the ryanodine receptor's ryanodine-binding activity to modification by physical factors, such as osmolality of the medium, might be related to action potential-induced Ca release. A hypothesis of molecular interaction in view of the plunger model of action potential-induced Ca release is discussed, suggesting that the model could be compatible with Ryr1 and Ryr3, but incompatible with Ryr2. The functional relevance of ryanodine receptor isoforms, especially Ryr3, in brain also remains to be clarified. Among ryr1 gene-related diseases, malignant hyperthermia was the first to be identified; however, there is still the possibility of involvement of the other genes. Central core disease has been added to the list recently. A molecular approach for the diagnosis and treatment of diseases is now in progress.

The effect of local anaesthetics on the ryanodine receptor/Ca2+ release channel of brain microsomal membranes

The effects of various local anaesthetics (LAs) on ryanodine binding of the sheep brain ryanodine receptor were tested. Tetracaine and dibucaine inhibit the binding with half-maximal inhibition (CI50) of 0.12 mM and 0.7 mM, respectively. Lidocaine and its analog QX-314, on the other hand, stimulate the binding up to 3-fold with half-maximal stimulation occurring with about 2 mM of the drugs. Lidocaine increases both the receptor affinity for ryanodine by about 5-fold and the rate of ryanodine association with its binding site by about 6-fold. Tetracaine and lidocaine also interact with the purified brain ryanodine receptor and produce inhibitory and stimulatory effects similar to those obtained with the membrane-bound receptor. The interaction of the LAs with the brain ryanodine receptor, as well as with the skeletal muscle receptor [J. Memb. Biol. 133 (1993) 171-182], suggest that ryanodine receptor possesses intrinsic binding site(s) for LAs.

Characterisation and distribution of inositol polyphosphate and Ryanodine receptors in the rat brain

The regional distribution of inositol 1,4,5-trisphosphate (InsP3), inositol 1,3,4,5-tetrakisphosphate (InsP4), and ryanodine binding sites has been characterised and compared in the rat brain using radioligand binding assays. Cortical [3H]InsP3 binding indicated similar positional and stereospecificity as observed in other tissues, with 100-fold selectivity for InsP3 over InsP4. Similarly, high-affinity [32P]InsP4 binding also showed a high degree of positional specificity, with a 1,000-fold selectivity for InsP4 over InsP3. Initial characterisation of [3H]ryanodine binding to cortical membranes demonstrated that specific binding was highly dependent on high salt and micromolar Ca2+ concentrations and inhibited by Ca2+ levels of > 1 mM. [3H]-Ryanodine binding was also enhanced by beta, gamma-methylene-adenosine 5'-trisphosphate and caffeine and inhibited by magnesium and ruthenium red (Ki = 0.81 microM). However, dantrolene (300 microM) was ineffective on the binding. Therefore, although the results indicate a greater similarity to the binding properties of the Ca(2+)-induced Ca2+ release channel isoform present in skeletal, rather than cardiac, muscle, it does not appear to be identical. Detailed binding analysis of ryanodine and polyphosphate sites, with the exception of ruthenium red, indicated no interaction between binding sites. Ruthenium red markedly enhanced the binding of both [3H]InsP3 and [32P]InsP4, an effect most probably due to nonspecific complex formation. Regional binding of InP3, InsP4, and ryanodine in the rat brain was of similar affinity for each ligand in each area, but the density profile for each ligand was clearly different. The highest density of InsP3 sites was in the cerebellum, whereas the highest density of ryanodine sites was in the hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

Two types of ryanodine receptors in mouse brain: skeletal muscle type exclusively in Purkinje cells and cardiac muscle type in various neurons

Two types of ryanodine receptors, channels for Ca2+ release from intracellular stores, are known. We detected the skeletal muscle type only in cerebellum by immunoblot analysis of microsomes and partially purified proteins. The cardiac muscle type was found in all parts of the mouse brain. Immunohistochemical study showed that the cardiac muscle type was localized mainly at the somata of most neurons. Analysis of mutant cerebella suggested that the skeletal muscle type was present exclusively in Purkinje cells. These results suggest that Ca(2+)-induced Ca2+ release, probably mediated by the cardiac muscle receptor, functions generally in various neurons, whereas depolarization-induced Ca2+ release, probably mediated by the skeletal muscle receptor, functions specifically in Purkinje cells.

High affinity [3H]ryanodine binding sites in postmortem human brain: regional distribution and effects of calcium, magnesium and caffeine

The pharmacological properties, regional distribution and autoradiographic localization of [3H]ryanodine binding sites were examined in postmortem human brain. Analyses of binding data from labeled ryanodine titration experiments conducted in frontal cortex revealed a single class of high affinity binding sites with a Kd value of 3.6 nM and a Bmax value of 99 fmol/mg protein. In unlabeled ryanodine titration experiments, Kd and Bmax values were 6.5 nM and 132 fmol/mg protein, respectively. Binding was found to be dependent on free Ca2+ (ED50 value, 89 microM) and was decreased by 35% in the presence of 5 mM Mg2+. This Mg2+ inhibition was abolished by the addition of 10 mM caffeine. Analysis of the regional distribution of [3H]ryanodine binding in membrane preparations revealed high levels of sites in putamen and caudate nucleus, intermediate levels in hippocampus and cortex, and low levels in cerebellum. Autoradiographically, the hippocampus displayed a high density of binding sites in the CA3 region and the dentate gyrus. Ryanodine binding sites in human brain exhibit similar, but not identical binding and pharmacological characteristics to ryanodine receptors previously identified in muscle and more recently in rat and rabbit brain and accordingly may be involved in the regulation of intracellular calcium.

Autoradiographic analysis of [3H]Ryanodine binding sites in rat brain: regional distribution and the effects of lesions on sites in the hippocampus

Quantitative and qualitative autoradiographic methods together with lesion approaches were used to determine the distribution of [3H]ryanodine binding sites in rat brain and the neuronal localization of these sites in the hippocampus. In normal animals, levels of [3H]ryanodine binding sites ranged from a low of about 1 fmol/mg tissue in subcortical structures to a high of 12-18 fmol/mg tissue in subregions of the hippocampus and the olfactory bulb. Relatively high densities of sites (5-9 fmol/mg tissue) were also seen in the olfactory tubercle, most areas of the cerebral cortex, accumbens nucleus, striatum, lateral septal nuclei, pontine nucleus, superior colliculus and granule cell layer of the cerebellum. Specific binding was undetectable in white matter. In experimental animals, intracerebral injections of kainic acid caused neuronal degeneration and a near total depletion of [3H]ryanodine binding sites in the dentate gyrus and in fields CA1, CA2 and CA3 of the hippocampus. Injections of kainic acid that left dentate granule cells largely intact while destroying all neurons in field CA3 had no effect on binding sites in the dentate gyrus. However, these lesions substantially reduced the density of binding in field CA3, leaving a narrow band of sites outlining the position of the degenerated CA3 pyramidal cells. Mechanical knife-cut lesions that severed the granule cell mossy fiber input to field CA3 reduced the density of binding sites in the CA3 region. The results indicate that [3H]ryanodine binding sites in brain are heterogeneously distributed and suggest that a proportion of these sites in the hippocampus may be contained in mossy fiber terminals where a presumptive calcium channel/ryanodine receptor complex may be involved in the regulation of calcium mobilization and/or neurotransmitter release.

Characteristics and function of Ca(2+)- and inositol 1,4,5-trisphosphate-releasable stores of Ca2+ in neurons

Molecular, biochemical and physiological evidence for the existence of releasable Ca2+ stores in neurons is strong. There are two separate molecules that function as release channels from those Ca2+ stores, the RyanR and InsP3R, and both have multiple regulatory sites for positive and negative control. Perhaps most intriguing is the biphasic, concentration-dependent action of cytosolic Ca2+ on both channels, first to stimulate release then, at higher concentration, to depress release. Whether the InsP3R and RyanR channels regulate Ca2+ release from different or identical functional compartments will need to be defined for each neuron type and perhaps even for each intracellular region within neurons since the evidence for functional separation of stores is mixed. The identification of Ca2+ storage and releasing capacity throughout all subcellular regions of neurons and the increasing evidence for a role for Ca2+ stores in neuronal plasticity suggests that the further characterization of the functional properties of Ca2+ stores will be an increasingly important and expanding area of interest in neurobiology.

Pharmacological characterization of the specific binding of [3H]ryanodine to rat brain microsomal membranes

High-affinity binding of [3H]ryanodine has been characterized in rat brain microsomal fractions. Membrane fractions from 4 brain regions (cerebral cortex, cerebellum, hippocampus and brainstem) have been isolated using sucrose density gradient purification. Sodium dodecyl sulfate polyacrylamide gel electrophoresis showed the presence of a high-molecular weight protein (Mr approximately 320 kDa), similar to that of ryanodine receptor from muscle sarcoplasmic reticulum (SR). In the presence of high salt (1 M KCl), [3H]ryanodine binds to low density (0.8 M sucrose) cortical microsomal fraction with high affinity (Kd 1.5 nM), and with the highest capacity (Bmax 330 fmol/mg protein). Kinetic analysis of the binding suggests multiple available binding sites for ryanodine. Binding of ryanodine is Ca2+ dependent (ED50 1 microM) and inhibited by Mg2+ and Ruthenium red. Adenine nucleotides have a biphasic effect on the binding of [3H]ryanodine. At low Ca2+ concentration caffeine and daunorubicin enhance the binding of [3H]ryanodine. The inositol 1,4,5-trisphosphate (IP3) binding inhibitor, heparin, has no effect on ryanodine binding, and ryanodine and caffeine do not influence the binding of [3H]IP3, which is enriched in the cerebellar fractions. These data demonstrate significant quantitative differences in the pharmacology of brain and muscle receptors and raise the question as to the physiological role of ryanodine binding proteins in the central nervous system and whether it is coupled to an endoplasmatic reticulum (ER) Ca2+ release channel.