Distribution of ryanodine receptors in the chicken central nervous system

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.

Direct association of the reticulon protein RTN1A with the ryanodine receptor 2 in neurons

RTN1A is a reticulon protein with predominant localization in the endoplasmic reticulum (ER). It was previously shown that RTN1A is expressed in neurons of the mammalian central nervous system but functional information remains sparse. To elucidate the neuronal function of RTN1A, we chose to focus our investigation on identifying possible novel binding partners specifically interacting with the unique N-terminus of RTN1A. Using a nonbiased approach involving GST pull-downs and MS analysis, we identified the intracellular calcium release channel ryanodine receptor 2 (RyR2) as a direct binding partner of RTN1A. The RyR2 binding site was localized to a highly conserved 150-amino acid residue region. RTN1A displays high preference for RyR2 binding in vitro and in vivo and both proteins colocalize in hippocampal neurons and Purkinje cells. Moreover, we demonstrate the precise subcellular localization of RTN1A in Purkinje cells and show that RTN1A inhibits RyR channels in [(3)H]ryanodine binding studies on brain synaptosomes. In a functional assay, RTN1A significantly reduced RyR2-mediated Ca(2+) oscillations. Thus, RTN1A and RyR2 might act as functional partners in the regulation of cytosolic Ca(2+) dynamics the in neurons.

Differential expression of ryanodine receptor in the developing rat cochlea

Ryanodine receptors (RyRs) are one of the intracellular calcium channels involved in regulation of intracellular free calcium concentration ([Ca²⁺]i). The immunolocalization of RyRs was investigated in the developing rat cochlea at different postnatal days (PND). The change of [Ca²⁺]i in isolated outer hair cells (OHCs) was determined. Morphological results showed low expression of RyRs in the Kolliker’s organ from the PND 5 group. RyR expression in inner hair cells (IHCs) increased as the rats aged, and was mature after PND 14. RyRs in OHCs were expressed near the synaptic area of afferent and efferent nerves. RyRs in supporting cells were expressed widely and strongly. The application of ACh, ryanodine + ACh, and thapsigargin + ACh could induce a significant increase in [Ca²⁺]i in OHCs in the presence of extracellular calcium. This increase of [Ca²⁺]i induced by ACh was caused by not only the calcium influx through surface calcium channels, but also the calciuminduced calcium release (CICR) from intracellular RyR-sensitive calcium stores. Morphological and Ca imaging results suggested that RyRs expression is related to cochlear maturity, and may play an important role in its function.

Expression of FKBP12 and ryanodine receptors (RyRs) in the spinal cord of MND patients

We investigated the FKBP12 and ryanodine receptor (RyR) immunoreactivity (IR) in the spinal cords of neurological controls and patients with motor neuron disease (MND). In the neurological controls, the cytoplasm of the spinal anterior horn neurons was stained with anti-FKBP12 antibodies and anti-RyR (type 1 and type 2) antibodies. In the MND cases, the residual neurons in the anterior horn of the spinal cord showed IR for RyR (type 1 and 2) antibodies, while weak IR for anti-FKBP12 antibodies was comparable to that of controls. The numbers of neurons recognized with the anti-FKBP 12 or anti-RyR (type 1 and 2) antibodies were counted in the anterior horn of spinal cords from the MND cases and neurological controls. Frequency of neurons stained with anti-FKBP 12 antibody was significantly decreased in the MND cases compared to that in controls (48.7+/-23.2%, 71.0+/-18.5%, respectively, mean+/-SD, p<0.0005). In the MND cases, numbers of normal-appearing, chromatolytic neurons showing IR to anti-FKBP12 (N19) antibody were significantly decreased compared to those in the controls. Immunoreactivities to anti-RyR antibodies (type 1and 2) in MND cases were present and there was no difference compared to those of the controls. Neurons in the spinal cord anterior horn of Kii-ALS cases with prolonged clinical duration were immunostained with both anti-FKBP12 and anti-RyR (type 1 and 2) antibodies similar to that in the controls. The anterior horn neurons of MND cases of short clinical duration showed absent IR to FKBP 12 antibody but present IR to RyR (type 1 and 2) antibodies. The present result suggests that FKBP12 IR was decreased in the MND cases with short clinical duration. RyR (type 1 and 2) is a major component of the intracellular calcium channel, which mediates calcium-induced calcium release. FKBP12, which is an endogenous ligand for RyR, stabilizes the calcium channels preventing calcium leakage in the absence of receptor activation. Imbalance between FKBP12 and RyR IR may play an important role in degeneration due to MND. Further study of the correlation between RyR and FKBP12 should contribute to clarifying the mechanisms of neurodegeneration in MND, including calcium-induced neuronal loss.

Pharmaco-behavioural evidence indicating a complex role for ryanodine receptor calcium release channels in memory processing for a passive avoidance task

Calcium signalling is an important process underlying neuronal function and consequently behaviour. The release of calcium from intracellular stores via the ryanodine receptor calcium release (RyR) channel has been implicated in both synaptic plasticity and to a limited extent in memory processing. While past investigations have suggested a role for RyR channels in long-term memory, the present study suggests their action is more complex. Using a single trial passive avoidance task developed for the day-old chick, it is proposed that RyR channels are necessary both prior to the expression of long-term memory and also in retrieval processes. Specifically, 5 mM dantrolene (a specific RyR channel blocker) resulted in a persistent retention loss from 40 min post-training while 10 nM dantrolene produced a transient retention loss centred at 40 min post-training. We speculate that in the context of memory formation, RyR channels may be activated by nitric oxide and in the context of memory retrieval may lead to the activation of large conductance calcium-activated potassium BK(Ca) channels which, when blocked by 50 nM iberiotoxin, also demonstrated a transient retention loss centred at 40 min post-training.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Presence and functional significance of presynaptic ryanodine receptors

Ca(2+)-induced Ca(2+) release (CICR) mediated by sarcoplasmic reticulum resident ryanodine receptors (RyRs) has been well described in cardiac, skeletal and smooth muscle. In brain, RyRs are localised primarily to endoplasmic reticulum (ER) and have been demonstrated in postsynaptic entities, astrocytes and oligodendrocytes where they regulate intracellular Ca(2+) concentration ([Ca(2+)](i)), membrane potential and the activity of a variety of second messenger systems. Recently, the contribution of presynaptic RyRs and CICR to functions of central and peripheral presynaptic terminals, including neurotransmitter release, has received increased attention. However, there is no general agreement that RyRs are localised to presynaptic terminals, nor is it clear that RyRs regulate a large enough pool of intracellular Ca(2+) to be physiologically significant. Here, we review direct and indirect evidence that on balance favours the notion that ER and RyRs are found in presynaptic terminals and are physiologically significant. In so doing, it became obvious that some of the controversy originates from issues related to (i) the ability to demonstrate conclusively the physical presence of ER and RyRs, (ii) whether the biophysical properties of RyRs are such that they can contribute physiologically to regulation of presynaptic [Ca(2+)](i), (iii) how ER Ca(2+) load and feedback gain of CICR contributes to the ability to detect functionally relevant RyRs, (iv) the distance that Ca(2+) diffuses from plasma membranes to RyRs to trigger CICR and from RyRs to the Active Zone to enhance vesicle release, and (v) the experimental conditions used. The recognition that ER Ca(2+) stores are able to modulate local Ca(2+) levels and neurotransmitter release in presynaptic terminals will aid in the understanding of the cellular mechanisms controlling neuronal function.

Functional properties of ryanodine receptors from rat dorsal root ganglia

The properties of ryanodine receptors (RyRs) from rat dorsal root ganglia (DRGs) have been studied. The density of RyRs (Bmax) determined by [3H]ryanodine binding was 63 fmol/mg protein with a dissociation constant (Kd) of 1.5 nM. [3H]Ryanodine binding increased with caffeine, decreased with ruthenium red and tetracaine, and was insensitive to millimolar concentrations of Mg2+ or Ca2+. DRG RyRs reconstituted in planar lipid bilayers were Ca2+-dependent and displayed the classical long-lived subconductance state in response to ryanodine; however, unlike cardiac and skeletal RyRs, they lacked Ca2+-dependent inactivation. Antibodies against RyR3, but not against RyR1 or RyR2, detected DRG RyRs. Thus, DRG RyRs are immunologically related to RyR3, but their lack of divalent cation inhibition is unique among RyR subtypes.

Facilitation of NMDAR-independent LTP and spatial learning in mutant mice lacking ryanodine receptor type 3

To evaluate the role in synaptic plasticity of ryanodine receptor type 3 (RyR3), which is normally enriched in hippocampal area CA1, we generated RyR3-deficient mice. Mutant mice exhibited facilitated CA1 long-term potentiation (LTP) induced by short tetanus (100 Hz, 100 ms) stimulation. Unlike LTP in wild-type mice, this LTP was not blocked bythe NMDA receptor antagonist D-AP5 but was partially dependent on L-type voltage-dependent Ca2+ channels (VDCCs) and metabotropic glutamate receptors (mGluRs). Long-term depression (LTD) was not induced in RyR3-deficient mice. RyR3-deficient mice also exhibited improved spatial learning on a Morris water maze task. These results suggest that in wild-type mice, in contrast to the excitatory role of Ca2+ influx, RyR3-mediated intracellular Ca2+ ([Ca2+]i) release from endoplasmic reticulum (ER) may inhibit hippocampal LTP and spatial learning.

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.

Differential distribution and subcellular localization of ryanodine receptor isoforms in the chicken cerebellum during development

The distribution of ryanodine receptor (RyR) isoforms was examined using isoform-specific monoclonal antibodies in the developing chicken brain, from E18 through adulthood, using light and electron microscopic immunocytochemistry. Monoclonal antibody 110F is specific for the alpha-skeletal muscle form of RyR, while monoclonal antibody 110E recognizes both the beta-skeletal muscle and cardiac isoforms, but does not distinguish between the two. Significant differences in the distribution of the alpha- and beta/cardiac forms were observed. Labeling for the alpha-form was restricted to cerebellar Purkinje neurons while the beta/cardiac form was observed in neurons throughout the brain. A major finding was the presence of labeling for the beta/cardiac in presynaptic terminals of the parallel fibers in the molecular layer and the mossy fiber terminals in the granular layer glomeruli in late development and during adulthood. Labeling for the beta/cardiac, but not the alpha-form, underwent a major redistribution in the cerebellum during the course of development. At 1 day of age, beta/cardiac labeling was present mainly in Purkinje neurons. From 1 day to 4 weeks, immunolabeling for the beta/cardiac form gradually disappeared from Purkinje neurons, but increased in granule cells. Within the molecular layer, the labeling pattern changed from being primarily within Purkinje dendrites to a more diffuse pattern. Electron microscopic examination of the cerebellar molecular layer of 2-week-old chicks revealed that beta/cardiac-labeling was mainly present in the axons and presynaptic processes of the parallel fibers. No developmental changes were observed in other brain regions. This study represents the first demonstration of ryanodine receptor immunoreactivity in presynaptic boutons and suggests that the ryanodine receptor may modulate neurotransmitter release through local regulation of intracellular calcium in the parallel fiber synapse.

Distribution of inositol-1,4,5-trisphosphate and ryanodine receptors in rat neostriatum

The distribution of the inositol-1,4,5-trisphosphate (IP3) and the cardiac form of the ryanodine receptor, two intracellular calcium channels, was examined in the rat neostriatum. Both IP3 and ryanodine receptor labeling occurred within striatal medium spiny cells but only ryanodine receptor labeling was present in choline acetyltransferase- and parvalbumin-positive interneurons. IP3 receptor labeling was observed within cell bodies, dendrites and spines of spiny striatal neurons, as seen at both the light and electron microscopic levels. Subcellular labeling for the ryanodine receptor was restricted to cell bodies and proximal dendrites when a polyclonal antibody raised against a peptide sequence from the dog cardiac ryanodine receptor was employed. More extensive dendritic labeling was seen using monoclonal antibody MA3-916, also raised against the canine cardiac ryanodine receptor. At the ultrastructural level, labeled dendritic spines were observed frequently with the monoclonal but not the polyclonal antibody. Ryanodine receptor labeling also was present within astrocytic processes surrounding blood vessels and within the neuropil, regardless of the antibody used. The results of these studies suggest that the ryanodine receptor plays a general role in intracellular calcium regulation within striatal cells while the IP3 receptor plays a specialized role within spiny neurons.

Ryanodine-sensitive calcium stores involved in neurotransmitter release from sympathetic nerve terminals of the guinea-pig

1. Intracellular and focal extracellular recording techniques were used to study neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals in the guinea-pig isolated vas deferens. 2. High concentrations of the selective N-type calcium channel blocker omega-conotoxin GVIA abolished the release of the neurotransmitter ATP evoked by trains of low-frequency stimuli. However, in the presence of high concentrations of the blocker, a 'residual release' persisted at higher frequencies. 3. Residual release was dependent on calcium entry through a pharmacologically distinct voltage-dependent calcium channel. 4. Residual release was inhibited by ryanodine in a use- and time-dependent manner and this inhibitory effect was potentiated by caffeine. The inhibitory effect of ryanodine on residual release was reversed by 4-aminopyridine. 5. These findings indicate that calcium-induced calcium released from intraneuronal stores plays an important role in action potential-evoked neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals.

Functionally heterogenous ryanodine receptors in avian cerebellum

The functional heterogeneity of the ryanodine receptor (RyR) channels in avian cerebellum was defined. Heavy endoplasmic reticulum microsomes had significant levels of ryanodine and inositol 1,4,5-trisphosphate binding. Scatchard analysis and kinetic studies indicated the existence of at least two distinct ryanodine binding sites. Ryanodine binding was calcium-dependent but was not significantly enhanced by caffeine. Incorporation of microsomes into planar lipid bilayers revealed ion channels with pharmacological features (calcium, magnesium, ATP, and caffeine sensitivity) similar to the RyR channels found in mammalian striated muscle. Despite a wide range of unitary conductances (220-500 picosiemens, symmetrical cesium methanesulfonate), ryanodine locked both channels into a characteristic slow gating subconductance state, positively identifying them as RyR channels. Two populations of avian RyR channels were functionally distinguished by single channel calcium sensitivity. One population was defined by a bell-shaped calcium sensitivity analogous to the skeletal muscle RyR isoform (type I). The calcium sensitivity of the second RyR population was sigmoidal and analogous to the cardiac muscle RyR isoform (type II). These data show that there are at least two functionally distinct RyR channel populations in avian cerebellum. This leads to the possibility that these functionally distinct RyR channels are involved in different intracellular calcium signaling pathways.

Modal gating in neuronal and skeletal muscle ryanodine-sensitive Ca2+ release channels

The bursting behavior of ryanodine-sensitive single Ca2+ release channels present in chicken cerebellum endoplasmic reticulum (ER), rat hippocampus ER, and frog and rabbit skeletal muscle sarcoplasmic reticulum was established. Unconditional dwell time distributions fitted by the maximum likelihood method reveal at least three open and closed exponential components. Trains of low open probability (P(o)) bursts were interspersed with trains of high P(o) bursts (> or = 0.8) in all the ryanodine receptor isotypes tested. The gating kinetics of the Ca2+ release channels were defined in long recordings by analyzing burst sequences and gamma distributions of average intraburst open (T(o)) and closed times (Tc). The gamma distributions of T(o) had two gamma components, suggesting the existence of two distinct burst types. In contrast, the gamma distributions of Tc had only one component. The correlation between consecutive burst pairs was defined in terms of T(o) and then statistically tested by 2 x 2 matrix contingency analysis. The probability that the ubiquitous sequential burst pattern was generated by random occurrence was < 0.01 (two-tailed Fisher's exact test). Temporal correlations were observed in all ryanodine receptor isotypes under a variety of experimental conditions. These data strongly suggest that single Ca2+ release channels switch slowly between modes of gating. We propose that the effects of agonists of Ca2+ release channels such as Ca2+ itself can be explained as concentration-dependent changes in the availability of each mode.

Alpha and beta isoforms of ryanodine receptor from chicken skeletal muscle are the homologues of mammalian RyR1 and RyR3

To define the relationship between the two ryanodine receptor (RyR) isoforms present in chicken skeletal muscle, we cloned two groups of cDNAs encoding the chicken homologues of mammalian RyR1 and RyR3. Equivalent amounts of the two chicken isoform mRNAs were detected in thigh and pectoral skeletal muscles. RyR1 and RyR3 mRNAs were co-expressed in testis and cerebellum whereas RyR3 mRNA was expressed also in the cerebrum and heart. The full-length sequence of the chicken RyR3 cDNA was established. The RyR3 receptor from chicken had the same general structure as mammalian and amphibian RyRs. The 15089 nt cDNA encoded a 4869-amino-acid-long protein with a molecular mass of 552445. The predicted amino acid sequence of the chicken RyR3 showed 86.9% identity to mammalian RyR3 and 85.6% to frog RyR3. Antibodies specific for chicken RyR1 and RyR3 recognized two different proteins with an apparent molecular mass of about 500 kDa. The two proteins differ slightly in their apparent molecular mass on SDS/PAGE: the protein recognized by antibodies against RyR3 had a higher mobility than the protein recognized by the antiserum against RyR1. Antibodies against RyR1 detected a protein already present in chicken skeletal muscle from 12-day-old embryos and older, while antibodies against RyR3 isoform detected a protein in muscle from only 18-day-old embryos and older. The expression patterns of RyR1 and RyR3 superimpose with those previously reported for the alpha and beta isoforms respectively. We conclude that alpha and beta isoforms present in chicken skeletal muscle are the homologues of mammalian RyR1 and RyR3.

Subcellular localization of ryanodine receptors in rat brain

Subcellular fractions of rat brain were used to determine the subcellular localization of ryanodine receptors. [3H]Ryanodine binding in purified cortical, cerebellar and hippocampal synaptosomes was up to 3.6-fold higher compared with mitochondrial fractions. The density of sites (Bmax) in hippocampal mossy fiber synaptosomes (249 fmol/mg protein) was 3.7-fold greater than in cortical synaptosomes (67 fmol/mg protein) and binding affinity was approximately 2-fold greater in the former (KD, 6.1 nM) than the latter (KD, 3.1 nM). At single sub-saturating concentrations of [3H]ryanodine, binding was 1.6-fold higher in mossy fibers compared with total hippocampal synaptosomes. [3H]Ryanodine binding sites were distributed similarly in subfractions of cortical synaptosomes and microsomes from discontinuous sucrose density gradients. An enrichment of sites was found in the lightest fractions containing the lowest activities of plasma membrane (5'-nucleotidase) and endoplasmic reticulum (glucose 6-phosphatase) enzyme markers when data for microsomal and synaptosomal subfractions were expressed as activity/binding per mg protein and when data for synaptosomal subfractions were expressed as a percentage of total activity/binding in collected fractions. Thus, ryanodine receptors appear to be concentrated in presynaptic terminals where they may play a major role in neurotransmitter release, and appear to be localized either in a specialized endoplasmic reticulum subcompartment or a distinct subcellular organelle.

Properties of Ryr3 ryanodine receptor isoform in mammalian brain

Although the RNA for the third isoform (Ryr3) of ryanodine receptor (RyR), a Ca2+ release channel, is detected in specific regions of mammalian brain, little is known about the protein. We investigated Ryr3 in rabbit brain, using an antibody raised against the synthetic peptide corresponding to amino acid sequence 4375-4387 of rabbit Ryr3, the homologue of bullfrog beta-RyR. The antibody which reacted with bullfrog beta-RyR, but not with the other isoforms, Ryr1 or Ryr2, specifically precipitated a single polypeptide from rabbit brain microsomes having a size similar to beta-RyR. Sucrose gradient ultracentrifugation revealed that Ryr3 forms a homotetramer, as true of the other isoforms. Being consistent with the distribution of its RNA, Ryr3 was abundantly expressed in hippocampus, corpus striatum, and diencephalon. Ryr3 demonstrated Ca2+-dependent [3H]ryanodine binding, and caffeine increased its Ca2+ sensitivity. The Ca2+ sensitivity of Ryr3 was also enhanced in a medium containing 1 m NaCl, as observed with beta-RyR. [3H]Ryanodine binding gave an estimate of Ryr3 which would be only 2% or less of total RyR in rabbit brain. These results confirm the expression of functional Ryr3 in mammalian brain which is similar to nonmammalian beta-RyR.

Physiological differences between the alpha and beta ryanodine receptors of fish skeletal muscle

Two isoforms of the sarcoplasmic reticulum Ca2+ release channel (ryanodine receptor or RYR) are expressed together in the skeletal muscles of most vertebrates. We have studied physiological properties of the two isoforms (alpha and beta) by comparing SR preparations from specialized fish muscles that express the alpha isoform alone to preparations from muscles containing both alpha and beta. Regulation of channel activity was assessed through [3H]ryanodine binding and reconstitution into planar lipid bilayers. Distinct differences were observed in the calcium-activation and -inactivation properties of the two isoforms. The fish alpha isoform, expressed alone in extraocular muscles, closely resembled the rabbit skeletal muscle RYR. Maximum [3H]ryanodine binding and maximum open probability (Po) of the alpha RYR were achieved from 1 to 10 microM free Ca2+. Millimolar Ca2+ reduced [3H]ryanodine binding and Po close to zero. The beta isoform more closely resembled the fish cardiac RYR in Ca2+ activation of [3H]ryanodine binding. The most prominent difference of the beta and cardiac isoforms from the alpha isoform was the lack of inactivation of [3H]ryanodine binding and Po by millimolar free Ca2+. Differences in activation of [3H]ryanodine binding by adenine nucleotides and inhibition by Mg2+ suggest that the beta and cardiac RYRs are not identical, however. [3H]ryanodine binding by the alpha RYR was selectively inhibited by 100 microM tetracaine, whereas cardiac and beta RYRs were much less affected. Tetracaine can thus be used to separate the properties of the alpha and beta RYRs in preparations in which both are present. The distinct physiological properties of the alpha and beta RYRs that are present together in most vertebrate muscles support models of EC coupling incorporating both directly coupled and Ca(2+)-coupled channels within a single triad junction.

Intracellular channels

Intracellular channels are located on the membranes of intracellular organelles and are involved in ion transfer, within the cytosolic compartments, in response to internal stimuli. Recently, various types of inositol 1,4,5-trisphosphate- and ryanodine-sensitive Ca(2+)-release channels, mitochondrial voltage-dependent anion channels, and a vesicular Cl- channel have been molecularly cloned and characterized, and their functional roles in the central nervous system are beginning to be clarified.

Dendritic calcium dynamics

Further characterization of the biochemical components that contribute to calcium handling, together with advances in optical imaging of ion concentration, are providing quantitative information on the dynamics of calcium in the dendrites of neurons in tissue culture, brain slices and in vivo. It has recently been demonstrated that strong spatial gradients and transient calcium elevations result from the geometry and membrane properties of dendrites. These studies are adding to our understanding of calcium's role in synaptic plasticity and in shaping the electrophysiological properties of neurons.

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.