Caffeine-sensitive calcium stores in presynaptic nerve endings: a physiological role?

Subcellular localization of hippocampal ryanodine receptor 2 and its role in neuronal excitability and memory

Ryanodine receptor 2 (RyR2) is abundantly expressed in the heart and brain. Mutations in RyR2 are associated with both cardiac arrhythmias and intellectual disability. While the mechanisms of RyR2-linked arrhythmias are well characterized, little is known about the mechanism underlying RyR2-associated intellectual disability. Here, we employed a mouse model expressing a green fluorescent protein (GFP)-tagged RyR2 and a specific GFP probe to determine the subcellular localization of RyR2 in hippocampus. GFP-RyR2 was predominantly detected in the soma and dendrites, but not the dendritic spines of CA1 pyramidal neurons or dentate gyrus granular neurons. GFP-RyR2 was also detected within the mossy fibers in the stratum lucidum of CA3, but not in the presynaptic terminals of CA1 neurons. An arrhythmogenic RyR2-R4496C^+/− mutation downregulated the A-type K⁺ current and increased membrane excitability, but had little effect on the afterhyperpolarization current or presynaptic facilitation of CA1 neurons. The RyR2-R4496C^+/− mutation also impaired hippocampal long-term potentiation, learning, and memory. These data reveal the precise subcellular distribution of hippocampal RyR2 and its important role in neuronal excitability, learning, and memory.

A mouse model containing a GFP-tagged ryanodine receptor 2 (RyR2) has shed light on the precise subcellular localization of hippocampal RyR2 and mechanisms underlying neuronal excitability, learning, and memory.

The Pathophysiology and Treatment of Essential Tremor: The Role of Adenosine and Dopamine Receptors in Animal Models

Essential tremor (ET) is one of the most common neurological disorders that often affects people in the prime of their lives, leading to a significant reduction in their quality of life, gradually making them unable to independently perform the simplest activities. Here we show that current ET pharmacotherapy often does not sufficiently alleviate disease symptoms and is completely ineffective in more than 30% of patients. At present, deep brain stimulation of the motor thalamus is the most effective ET treatment. However, like any brain surgery, it can cause many undesirable side effects; thus, it is only performed in patients with an advanced disease who are not responsive to drugs. Therefore, it seems extremely important to look for new strategies for treating ET. The purpose of this review is to summarize the current knowledge on the pathomechanism of ET based on studies in animal models of the disease, as well as to present and discuss the results of research available to date on various substances affecting dopamine (mainly D3) or adenosine A1 receptors, which, due to their ability to modulate harmaline-induced tremor, may provide the basis for the development of new potential therapies for ET in the future.

Activation of pre-synaptic M-type K+ channels inhibits [3H]D-aspartate release by reducing Ca2+ entry through P/Q-type voltage-gated Ca2+ channels

In this study, the functional consequences of the pharmacological modulation of the M-current (I(KM)) on cytoplasmic Ca(2+) intracellular Ca(2+)concentration ([Ca(2+)](i)) changes and excitatory neurotransmitter release triggered by various stimuli from isolated rat cortical synaptosomes have been investigated. K(v)7.2 immunoreactivity was identified in pre-synaptic elements in cortical slices and isolated glutamatergic cortical synaptosomes. In cerebrocortical synaptosomes exposed to 20 mM [K(+)](e), the I(KM) activator retigabine (RT, 10 microM) inhibited [(3)H]D-aspartate ([(3)H]D-Asp) release and caused membrane hyperpolarization; both these effects were prevented by the I(KM) blocker XE-991 (20 microM). The I(KM) activators RT (0.1-30 microM), flupirtine (10 microM) and BMS-204352 (10 microM) inhibited 20 mM [K(+)](e)-induced synaptosomal [Ca(2+)](i) increases; XE-991 (20 microM) abolished RT-induced inhibition of depolarization-triggered [Ca(2+)](i) transients. The P/Q-type voltage-sensitive Ca(2+)channel (VSCC) blocker omega-agatoxin IVA prevented RT-induced inhibition of depolarization-induced [Ca(2+)](i) increase and [(3)H]D-Asp release, whereas the N-type blocker omega-conotoxin GVIA failed to do so. Finally, 10 microM RT did not modify the increase of [Ca(2+)](i) and the resulting enhancement of [(3)H]D-Asp release induced by [Ca(2+)](i) mobilization from intracellular stores, or by store-operated Ca(2+)channel activation. Collectively, the present data reveal that the pharmacological activation of I(KM) regulates depolarization-induced [(3)H]D-Asp release from cerebrocortical synaptosomes by selectively controlling the changes of [Ca(2+)](i) occurring through P/Q-type VSCCs.

The role of extracellular calcium in exo- and endocytosis of synaptic vesicles at the frog motor nerve terminals

In the present study we combined FM 1-43 imaging and electrophysiological recording of miniature end-plate currents (MEPCs) to determine the role of extracellular calcium in synaptic vesicle exo- and endocytosis at the frog motor nerve terminals. We replaced extracellular Ca2+ ions with other bivalent cations (Sr2+, Ba2+, Cd2+, Mg2+) or used a calcium-free solution and monitored fluorescent staining of the nerve terminals in the presence of caffeine, which promotes the release of Ca2+ from intracellular stores. Caffeine has induced FM1-43 internalization only in the presence of bivalent cations in the external solution. The exposure of the neuromuscular junction to caffeine in a calcium-free solution caused a reversible failure of FM 1-43 loading and an increase in the nerve terminal width. This effect of a calcium-free solution was not due to a decrease in exocytosis, because caffeine-induced FM1-43 unloading from the previously loaded nerve terminals, as well as a degree of the MEPCs frequency increase, was unchanged. We conclude that the presence of Ca2+ or other bivalent cations in extracellular space is necessary for endocytosis but not for exocytosis of synaptic vesicles, while transmitter release is promoted by efflux of Ca2+ from intracellular stores. The effect of extracellular Ca2+ on endocytosis might be driven by the non-specific interactions with membrane lipids.

Effects of high-potassium solutions and caffeine on synaptic vesicle exoendocytosis processes in the frog neuromuscular junction

Studies on frog skin-pectoris muscle preparations using vital fluorescent microscopy showed that stimulation of transmitter secretion using high-potassium solutions with the endocytosis marker FM 1-43 induced bright spots in all motor nerve terminals, these representing accumulations of vesicles undergoing the exoendocytic cycle in the active zones of nerve endings. Stimulation of transmitter secretion with caffeine evoked bright spots only in some nerve terminals and only in some parts of the terminals. In summer, the number of bright spots on stimulation of transmitter secretion by caffeine increased sharply. Extracellular recording of spontaneous synaptic signals showed that high-potassium solutions, like caffeine, produced dose-dependent increases in the frequency of miniature endplate currents. However, while high-potassium solutions always increased the frequency, this occurred with caffeine in only a proportion of experiments. This leads to the conclusion that exoendocytosis processes can occur both because of the influx of Ca(2+) ions into nerve endings as a result of depolarization (high-potassium solutions) and because of the release of Ca(2+) ions from the endoplasmic reticulum (caffeine). The possible spatial localization of the endoplasmic reticulum in nerve endings is discussed. The endoplasmic reticulum is suggested to have a role in synapse remodeling processes.

Post-Trauma Administration of Caffeine Plus Ethanol Reduces Contusion Volume and Improves Working Memory in Rats

It has been demonstrated that ethanol exerts dose-dependent effects, both beneficial and detrimental, on the outcome of traumatic brain injury (TBI). Recently, it has been reported that co-administration of caffeine (10 mg/kg) and a low amount of alcohol (0.65 g/kg; caffeinol) reduces cortical infarct volume up to 80%, and improves motor coordination, following a rodent model of reversible common carotid/middle cerebral artery occlusion. However, the protective effects of caffeinol following other CNS insults, nor its influence on cognitive function, have been examined. Using a controlled cortical impact model of brain injury, the effect of caffeinol administration on TBI-associated motor and cognitive deficits was assessed. When given 15 min following injury, caffeinol reduced cortical tissue loss and improved working memory. However, no influence on motor skills, Morris water maze performance or associative learning and memory was observed. Delayed administration (6 h post-injury) of caffeinol containing a dose of ethanol (1 g/kg) previously demonstrated to improve motor performance eliminated the working memory benefit and cortical protection. These results indicate that early administration of caffeinol may be beneficial in lessening some of the deficits and cortical tissue loss associated with brain trauma.

Effect of acute caffeine on severity of harmaline induced tremor in rats

Recent studies suggest an association between caffeine consumption and tremor. However, the available literature is scanty and inconclusive. The present study was undertaken to investigate the effect of acute caffeine treatment on harmaline induced tremors in the rat. Four groups of male Sprague-Dawley rats (six animals in each group) weighing 88+/-2 g were administered harmaline (10 mg/kg, intraperitoneally (i.p.)) for inducing experimental tremors. The rats in group 1 served as controls and received normal saline, whereas the animals in groups 2, 3 and 4 were given caffeine (i.p.) at doses of 50, 100 and 150 mg/kg, respectively 60 min after harmaline administration. The latency of onset, intensity and duration of tremor and electromyographic (EMG) responses were recorded. Treatment of rats with caffeine resulted in a significant increase in the intensity and duration of harmaline induced tremors. Caffeine also enhanced the EMG amplitude in harmaline treated animals. In conclusion, the results of this study suggest that acute treatment with caffeine significantly potentiates the severity of harmaline induced tremors in rats.

Studies on the mechanism of enhancement of purinergic transmission by caffeine in the guinea-pig isolated vas deferens

1. Purinergic transmission from sympathetic nerves in the guinea-pig vas deferens was monitored using intracellular recording techniques. Stimulation of the hypogastric nerve with trains of 15 pulses at 1 Hz evoked excitatory junction potentials (EJPs) which increased in amplitude from the first pulse and reached a maximum after 6-8 pulses. 2. Caffeine (3 and 10 mm), depolarized cells by 5-10 mV and increased the amplitude of the first few EJPs in each train but reduced the maximum amplitude of EJPs late in the train. 3. The adenosine receptor antagonist 8-p-sulphophenyl-theophylline (8-SPT; 30 microm) had no effect on either the resting membrane potential or the EJP amplitude; however, at 100 microm it reduced the amplitude of all EJPs by 5-10%. 4. Adenosine (10 and 30 microm) reduced the amplitude of EJPs in a concentration-dependent manner. The inhibitory effect of adenosine on EJP amplitude was prevented by pretreatment with either caffeine (3 mm) or 8-SPT (30 microm). 5. Ryanodine (30 microm) did not alter EJP amplitude and did not inhibit the enhancement of the first EJP by caffeine (3 mm). Incubation of the tissue with the cell permeable calcium chelator 1-2-bis(o-aminophenoxy)ethane-N,N-N',N'-tetraacetic acid (BAPT-AM) resulted in a depression of EJP amplitude and a longer time to reach maximum amplitude. In cells that had been exposed to BAPT-AM, caffeine 3 mm still increased amplitude of EJP early in the train. 6. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX; 500 microm), hyperpolarized cells and increased the amplitude of EJP throughout the train of stimulation. In the presence of IBMX, caffeine 3 mm still depolarized the cells and enhanced the EJP early in the train of stimulation. 7. The findings in this study confirm that caffeine and 8-SPT are effective inhibitors of the actions of adenosine. However, caffeine has an additional action to enhance EJP early during a train of stimulation, which cannot be attributed to blockade of adenosine receptors, but which may be related to inhibition of phosphodiesterase.

beta-Amyloid peptide induces ultrastructural changes in synaptosomes and potentiates mitochondrial dysfunction in the presence of ryanodine

In Alzheimer's disease (AD), loss of synapses exceeds neuronal loss and some evidence suggests a role of beta-amyloid protein (Abeta) in synaptic degeneration through a mechanism which may involve intraneuronal Ca2+ dyshomeostasis. Emerging evidence points to the participation of the internal Ca2+ stores in the pathophysiology of neurodegeneration in AD. To test the involvement of intrasynaptic Ca2+ mobilization in A toxicity, we explored the role of ryanodine receptor activation in rat cortical synaptosomes taken as a model system for the central presynapses. Evaluation of synaptosomal mitochondrial redox capacity was assessed by the MTT reduction technique, and ultrastructural changes of synaptosomes after exposure to Abeta and ryanodine were evaluated by electron microscopy. Our results show that Abeta potentiates mitochondrial dysfunction in the presence of ryanodine and induces morphological changes consisting of mitochondrial swelling and intense small synaptic vesicles depletion. These changes were accompanied by a reduction in the content of synaptophysin and actin proteins. The reduction of actin immunoreactivity was reversed in the presence of a wide range caspase inhibitors, suggesting the activation of synaptic apoptotic mechanisms.

Functional IP3- and ryanodine-sensitive calcium stores in presynaptic varicosities of NG108-15 (rodent neuroblastoma x glioma hybrid) cells

Presynaptic varicosities of the model neuronal cell line NG108-15, a cholinergic neuroblastoma cell x glioma cell hybrid capable of innervating striated myotubes, were examined for the presence of inositol 1,4,5-trisphosphate (IP3)-sensitive and Ca2+-activated (ryanodine-sensitive) Ca2+ stores using confocal microscopic imaging of Ca2+-sensitive fluorescent dye loaded into the cells. Initial demonstration of the presence of IP3 receptors and ryanodine receptors in the NG108-15 varicosities was obtained using immunocytochemistry. Treatment of NG108-15 cells with bradykinin (0.1 microM), whose receptor is linked to IP3 generation, and separately, caffeine (10 mM), an activator of endoplasmic reticulum ryanodine receptors, resulted in substantial increases in [Ca2+]i in the varicosities. K+-evoked changes in [Ca2+]i in the varicosities were reduced (52 %) after emptying the ryanodine-sensitive Ca2+ store using caffeine (10 mM), but were not affected by prior depletion of the IP3-sensitive Ca2+ store using thapsigargin (1 microM). Bradykinin-induced changes in [Ca2+]i were abolished following depletion of the IP3-sensitive Ca2+ store using thapsigargin (1 microM) and were reduced (72 %) by prior emptying of the ryanodine-sensitive Ca2+ store with caffeine (10 mM). The same results were obtained when the varicosities of the NG108-15 cells had formed synaptic junctions with co-cultured rat hindlimb myotubes. Taken together, the results suggest that, in the varicosities, activation of the IP3 pathway evoked the release of Ca2+ from the IP3-sensitive store, which, in turn, secondarily induced the release of Ca2+ from the ryanodine-sensitive store via Ca2+-induced Ca2+ release, and that depolarization-induced Ca2+ entry evoked Ca2+-induced Ca2+ release only from the ryanodine-sensitive store. Thus, functional internal Ca2+ stores are inherent components of presynaptic varicosities in this neural cell line.

Calcium signaling in the ER: its role in neuronal plasticity and neurodegenerative disorders

Endoplasmic reticulum (ER) is a multifaceted organelle that regulates protein synthesis and trafficking, cellular responses to stress, and intracellular Ca2+ levels. In neurons, it is distributed between the cellular compartments that regulate plasticity and survival, which include axons, dendrites, growth cones and synaptic terminals. Intriguing communication networks between ER, mitochondria and plasma membrane are being revealed that provide mechanisms for the precise regulation of temporal and spatial aspects of Ca2+ signaling. Alterations in Ca2+ homeostasis in ER contribute to neuronal apoptosis and excitotoxicity, and are being linked to the pathogenesis of several different neurodegenerative disorders, including Alzheimer's disease and stroke.

Trimethyltin and triethyltin differentially induce spontaneous noradrenaline release from rat hippocampal slices

The environmental contaminants trimethyltin (TMT) and triethyltin (TET) stimulated the spontaneous release of [(3)H]noradrenaline ([(3)H]NA) from hippocampal slices in a time- and concentration-dependent manner. TMT was the most potent compound, exhibiting an EC50 value 10-fold lower (3.8 microM) than that of TET (39.5 microM). Metal-evoked [(3)H]NA release did not increase in the absence of desipramine and was completely blocked by reserpine preincubation, indicating a vesicular origin of [(3)H]NA release but not a mechanism involving reversal of the transmitter transporter. The voltage-gated Na(+) channel blocker tetrodotoxin (TTX) did not affect metal-evoked [(3)H]NA release. [(3)H]NA release elicited by TMT was partially extracellular Ca(2+)-dependent, since it was significantly decreased in a Ca(2+)-free EGTA-containing medium, whereas TET induced an extracellular Ca(2+)-independent release of [(3)H]NA. Neither inhibitors of Ca(2+)-entry through Na(+)/Ca(2+)exchanger and voltage-gated calcium channels, nor agents that interfere with Ca(2+)-mobilization from intracellular stores affected [(3)H]NA release induced by TMT. TET-evoked [(3)H]NA release was reduced by ruthenium red, which depletes mitochondrial Ca(2+)stores, but was not modified by caffeine and thapsigargin, which interfere with Ca(2+)mobilization from endoplasmic reticulum. The fact that TET effect was also attenuated by DIDS, an inhibitor of anion exchange, indicates that the effect of TET on spontaneous [(3)H]NA release may be mediated by intracellular mobilization of Ca(2+) from mitochondrial stores through a Cl(-) dependent mechanism.

Ortho-substituted polychlorinated biphenyls alter microsomal calcium transport by direct interaction with ryanodine receptors of mammalian brain

A stringent structure-activity relationship among polychlorinated biphenyls (PCBs) possessing two or more ortho-chlorine substituents is observed for activation of ryanodine receptors in mammalian brain, revealing an arylhydrocarbon receptor-independent mechanism through which non-coplanar PCBs disrupt neuronal Ca2+ signaling. Of the congeners assayed, non-coplanar PCB 95 exhibits the highest potency (EC50 = 12-24 microM) toward activating high affinity [3H]ryanodine-binding in rat hippocampus, cerebellum, and cerebral cortex. Coplanar PCB 66 and PCB 126 have no ryanodine receptor activity in all brain regions examined. PCB 95 enhances [3H]ryanodine-binding affinity and capacity by significantly altering modulation by Ca2+ and Mg2+, thereby stabilizing a high affinity conformation of the ryanodine receptor. Ca2+ transport measurements using cortical microsomes reveal that PCB 95 discriminates between inositol 1,4,5-trisphosphate- and ryanodine-sensitive stores. PCB 95 selectively mobilizes Ca2+ from ryanodine-sensitive stores in a dose-dependent manner (EC50 = 3.5 microM) and is completely inhibited by ryanodine receptor blockers, whereas coplanar PCBs are inactive. These data demonstrate that ortho-substituted PCBs disrupt Ca2+ transport in central neurons by direct interaction with ryanodine receptors, showing high selectivity and specificity. Alteration of Ca2+ signaling mediated by ryanodine receptors in specific regions of the central nervous system may account, at least in part, for the significant impact of these agents toward neurodevelopment and neuroplasticity in mammals.

Activation of Ca2+-dependent currents in dorsal root ganglion neurons by metabotropic glutamate receptors and cyclic ADP-ribose precursors

Cultured dorsal root ganglion neurons were voltage clamped at -90 mV to study the effects of intracellular application of nicotinamide adenine dinucleotide (betaNAD+), intracellular flash photolysis of caged 3',5'-cyclic guanosine monophosphate (cGMP), and metabotropic glutamate receptor activation. The activation of metabotropic glutamate receptors evoked inward Ca2+-dependent currents in most cells. This was mimicked both by intracellular flash photolysis of the caged axial isomer of cGMP [P-1-(2-nitrophenyl)ethyl cGMP] and intracellular application of betaNAD+. Whole cell Ca2+-activated inward currents were used as a physiological index of raised intracellular Ca2+ levels. Extracellular application of 10 microM glutamate evoked the activation of Ca2+-dependent inward currents, thus reflecting a rise in intracellular Ca2+ levels. Similar inward currents were also activated after isolation of metabotropic glutamate receptor activation by application of 10 microM glutamate in the presence of 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione and 20 microM dizocilpine maleate (MK 801), or by extracellular application of 10 microM trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid. Intracellular photorelease of cGMP, from its caged axial isomer, in the presence of betaNAD+ was also able to evoke similar Ca2+-dependent inward currents. Intracellular application of betaNAD+ alone produced a concentration-dependent effect on inward current activity. Responses to both metabotropic glutamate receptor activation and cGMP were suppressed by intracellular ryanodine, chelation of intracellular Ca2+ by bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid, and depletion of intracellular Ca2+ stores, but were insensitive to the removal of extracellular Ca2+. Therefore both cGMP, possibly via a mechanism that involves betaNAD+ and/or cyclic ADP-ribose, and glutamate can mobilize intracellular Ca2+ from ryanodine-sensitive stores in sensory neurons.

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.

Caffeine enhances sympathetic purinergic and noradrenergic transmission in the guinea-pig isolated vas deferens

Intracellular recording techniques were used to monitor the resting membrane potential of smooth muscle cells and the excitatory junction potentials (EJPs) evoked by stimulation of the hypogastric nerve. Stimulation with trains of 15 pulses at 1 Hz or 0.33 Hz evoked individual EJPs which increased in amplitude from the first pulse and reached a plateau after 6-8 pulses. Stimulation at 1 Hz resulted in EJPs facilitating to a plateau level of approximately 25 mV, whereas with stimulation at 0.33 Hz the EJPs only facilitated to a plateau level of about 12 mV. With stimulation at 1 Hz, caffeine (3 mM and 10 mM), increased the amplitude of the first few EJPs in each train and decreased the extent of facilitation and reduced the amplitude of fully facilitated EJPs. In comparison, the amplitude of all EJPs evoked by stimulation at 0.33 Hz was increased by caffeine (3 mM and 10 mM). With 0.33 Hz stimulation, facilitation of the first few EJPs was observed in the presence of 3 mM caffeine but not in the presence of 10 mM caffeine. In the presence of the alpha 2-adrenoceptor antagonist idazoxan, caffeine (3 mM and 10 mM) still enhanced the amplitude of EJPs early in trains of stimulation but there was no depression of EJPs later in the trains. Similarly, in reserpine-treated vasa deferentia, caffeine (3 mM) enhanced EJPs early in the train of stimulation at 1 Hz and there was no depression of EJPs at the end of the train. In addition to electrophysiological experiments, the effect of caffeine (0.1-30 mM) on the resting and stimulation-induced (S-I) efflux of radioactivity was investigated in guinea-pig isolated vasa deferentia previously incubated with [3H]-noradrenaline. Caffeine (10 mM) did not affect the resting efflux of [3H]-noradrenaline but significantly enhanced the S-I efflux by 150-160%. The present findings suggest that caffeine enhances sympathetic purinergic and noradrenergic transmission at the sympathetic neuroeffector junction in the guinea-pig vas deferens. Moreover, the increased release of transmitter noradrenaline can modulate purinergic transmission by activation of alpha 2-adrenoceptors located at sympathetic neuroeffector sites.

[3H]9-Methyl-7-bromoeudistomin D, a caffeine-like powerful Ca2+ releaser, binds to caffeine-binding sites distinct from the ryanodine receptors in brain microsomes

[3H]9-Methyl-7-bromoeudistomin D ([3H]MBED), the most powerful Ca2+ releaser from sarcoplasmic reticulum, specifically bound to the brain microsomes. Caffeine competitively inhibited [3H]MBED binding. [3H]MBED binding was markedly blocked by procaine, whereas that was enhanced by adenosine-5'-(beta,gamma-methylene)triphosphate. The Bmax value was 170 times more than that of [3H]ryanodine binding. The profile of sucrose-density gradient centrifugation of solubilized microsomes indicated that [3H]MBED binding protein was different from [3H]ryanodine binding protein. These results suggest that there are MBED/caffeine-binding sites in brain that are distinct from the ryanodine receptor and that MBED becomes an essential molecular probe for characterizing caffeine-binding protein in the central nervous system.

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+].

Dual action of thapsigargin on calcium mobilization in sensory neurons: inhibition of Ca2+ uptake by caffeine-sensitive pools and blockade of plasmalemmal Ca2+ channels

The action of thapsigargin on intracellular calcium homeostasis and voltage-activated calcium currents was studied on freshly isolated adult mouse dorsal root ganglia neurons. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured using indo-1-based microfluorimetry; transmembrane Ca2+ currents were recorded under voltage-clamp in the whole-cell configuration of the patch-clamp technique. Extracellular applications of thapsigargin at concentrations of 20-2000 nM did not cause substantial changes of basal [Ca2+]i level in the majority of neurons studied. However, 5-10 min incubation of neurons with 20 nM thapsigargin completely and almost irreversibly inhibited caffeine-mediated Ca2+ release from intracellular pools. This inhibition was associated with deceleration of the recovery of depolarization-induced [Ca2+]i transients, presumably due to the inhibition of Ca2+ uptake by intracellular calcium stores. At concentrations between 200 and 2000 nM, thapsigargin markedly depressed the amplitudes of depolarization-triggered [Ca2+]i transients due to the inhibition of transmembrane Ca2+ entry through voltage-activated Ca2+ channels. We found that thapsigargin discriminates between low- and high-voltage-activated Ca2+ channels: 2000 nM of thapsigargin decreased the amplitudes of high-voltage-activated currents by 60%, while the amplitudes of low-voltage-activated Ca2+ currents were reduced by only 25%. Thus, thapsigargin exerts a dual action on [Ca2+]i handling mechanisms in mouse sensory neurons: at low concentrations (< 50 nM) it inhibits Ca2+ accumulation by endoplasmic reticulum pools, whereas at higher concentrations (200-2000 nM) thapsigargin blocks high-voltage-activated Ca2+ currents, reducing Ca2+ entry into the cell.

Analysis of rapid calcium signals in synaptosomes

A combination of the stopped-flow technology with dual channel spectrofluorometry of Ca(2+)-indicators was utilized for the measurement of rapid Ca(2+)-signals in rat cerebral cortical synaptosomes evoked by K(+)-depolarization. There was no observable contribution of Ca(2+)-ions from intracellular stores to the rise in [Ca2+]i. The kinetics of the fast increase in intracellular Ca2+ concentration was analysed in relation to the depolarization strength. The maximal increase in [Ca2+]i and the time course of Ca(2+)-channel inactivation were determined for depolarizations obtained by different extracellular K(+)-concentrations ([K+]o). An apparent threshold was observed at about 18 mM [K+]o; a maximal Ca(2+)-signal amplitude was estimated at about 40 mM [K+]o. Pharmacological properties of the involved Ca(2+)-channels were determined using selective Ca(2+)-channel blockers (Dihydropyridines, omega-Conotoxin, omega-Agatoxins); the results suggest that a P-type voltage-dependent Ca(2+)-channel is the relevant channel type, generating the evoked Ca(2+)-signals in rat cerebral cortical synaptosomes.

Age-related changes in calcium homeostatic mechanisms in synaptosomes in relation with working memory deficiency

Aging is associated with alterations in different systems that govern neuronal calcium homeostasis. This study was designed to determine whether any of these alterations may contribute to the decline in spatial working memory that is observed in old rats. Several parameters [initial (5 s) and steady state (15 min) 45Ca2+ uptake, FCCP-releaseable 45Ca2+, [Ca2+]i levels, depolarization-induced phosphoprotein (P97, PP65, P42) dephosphorylation and acetylcholine levels and release) involved in calcium homeostasis/signaling were determined in whole brain synaptosomes derived from adult (9-month-old) and old (24-month-old) rats that were evaluated for spatial memory performance in the eight-arm radial maze. The neurochemical analysis indicated that both the 9- and 24-month-old rats were impaired with respect to 3-month-old animals. When learners (animals reaching criterion; RC) were compared to memory impaired rats (MI), it was found that the FCCP-releaseable 45Ca2+ of synaptosomes, that reflects mitochondrial calcium, was lower in the MI than the RC rats and was correlated with the behavioral performance of the rats in their first testing sessions. The results suggest that the loss of calcium uptake capacity in synaptic mitochondria during aging may be associated with impaired working memory in old animals.

Norepinephrine release from guinea pig cardiac sympathetic nerves is insensitive to ryanodine under physiological conditions

The activation of neurotransmitter release in nerve cells appears to be primarily dependent upon influx of extracellular Ca2+, most of which is thought to cross nerve terminal membranes through N-type Ca2+ channels. Events in skeletal and cardiac muscle, in contrast, are regulated to a greater extent by intracellular Ca2+ exchange between cytosol and intracellular organelles such as sarcoplasmic reticulum. It is not known to what extent corresponding intracellular organelles, i.e. endoplasmic reticulum (ER), contribute to cytosolic Ca2+ transients and norepinephrine (NE) release from cardiac sympathetic nerves. Heart rate and NE release were measured in isolated perfused guinea pig hearts during 1-min stimulations (5 V, 4 Hz, 2 ms) of the right stellate ganglia prior to (S1), during the administration of (S2), and after (S3) the removal of ryanodine (1 microM) from the perfusate. Ryanodine is a selective modulator of caffeine-sensitive Ca2+ stores in ER. Baseline heart rates decreased significantly in the presence of ryanodine, documenting its physiological effect on cardiac cells. However, there was no detectable effect of ryanodine on nerve-stimulated increase in heart rate or NE release. These results indicate that the ryanodine-sensitive intracellular Ca2+ stores do not play a major role in cardiac sympathetic neurotransmission.

Relation of exocytotic release of gamma-aminobutyric acid to Ca2+ entry through Ca2+ channels or by reversal of the Na+/Ca2+ exchanger in synaptosomes

The specific inhibitor of the gamma-aminobutyric acid (GABA) carrier, NNC-711, (1-[(2-diphenylmethylene)amino]oxyethyl)- 1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride, blocks the Ca(2+)-independent release of [3H]GABA from rat brain synaptosomes induced by 50 mM K+ depolarization. Thus, in the presence of this inhibitor, it was possible to study the Ca(2+)-dependent release of [3H]GABA in the total absence of carrier-mediated release. Reversal of the Na+/Ca2+ exchanger was used to increase the intracellular free Ca2+ concentration ([Ca2+]i) to test whether an increase in [Ca2+]i alone is sufficient to induce exocytosis in the absence of depolarization. We found that the [Ca2+]i may rise to values above 400 nM, as a result of Na+/Ca2+ exchange, without inducing release of [3H]GABA, but subsequent K+ depolarization immediately induced [3H]GABA release. Thus, a rise of only a few nanomolar Ca2+ in the cytoplasm induced by 50 mM K+ depolarization, after loading the synaptosomes with Ca2+ by Na+/Ca2+ exchange, induced exocytotic [3H]GABA release, whereas the rise in cytoplasmic [Ca2+] caused by reversal of the Na+/Ca2+ exchanger was insufficient to induce exocytosis, although the value for [Ca2+]i attained was higher than that required for exocytosis induced by K+ depolarization. The voltage-dependent Ca2+ entry due to K+ depolarization, after maximal Ca2+ loading of the synaptosomes by Na+/Ca2+ exchange, and the consequent [3H]GABA release could be blocked by 50 microM verapamil. Although preloading the synaptosomes with Ca2+ by Na+/Ca2+ exchange did not cause [3H]GABA release under any conditions studied, the rise in cytoplasmic [Ca2+] due to Na+/Ca2+ exchange increased the sensitivity to external Ca2+ of the exocytotic release of [3H]GABA induced by subsequent K+ depolarization. Thus, our results show that the vesicular release of [3H]GABA is rather insensitive to bulk cytoplasmic [Ca2+] and are compatible with the view that GABA exocytosis is triggered very effectively by Ca2+ entry through Ca2+ channels near the active zones.

Frequency modulation of transmitter release

In 1952 Fatt and Katz recorded at a frog neuromuscular junction while stimulating the nerve and found "... that successive endplate potential responses varied in a step-like manner, corresponding to units of miniature endplate potentials" (J Physiol 117, 109-128). This led them to propose that fast neuromuscular transmission is 'quantal'. Quantal release is now commonly ascribed to a vesicular form of neurosecretion since vesicles have routinely been visualized in presynaptic terminals. The vesicular hypothesis (Del Castillo and Katz, 1955) assumes that quanta, or 'transmitter packets of standard size', are assembled and stored in the numerous vesicles routinely identified in micrographs of virtually all central and peripheral presynaptic nerve terminals. Simply stated, this model predicts that each one of the miniature synaptic signals (MSSs) follows from the exocytosis of one vesicle's contents. However, the time required for membrane fusion preceding exocytosis (Almers and Tse, 1990) and the variability in MSS amplitude and time course (Vautrin et al, 1992a,b) cannot readily be reconciled by a simple, exocytotic model of quantal release from preloaded vesicles. These difficulties with the original model have led us to re-evaluate MSSs generated at the classical peripheral synapse, the cholinergic neuromuscular junction of the mouse diaphragm, as well as at central synapses between embryonic hippocampal neurons mediated by gamma-aminobutyric acid (GABA). At these synapses, the release of GABA is also assumed to have classical quantal properties like peripheral acetylcholine release (Edwards et al, 1990). Our results show that at both synapses, progressive alterations in elementary signal properties can be induced in a remarkably rapid manner. The original report of preferred amplitudes and intervals in the spontaneous miniature signals (Fatt and Katz, 1952) has repeatedly been confirmed and is here incorporated into a dynamic model of fast synaptic transmission. Although MSSs exhibit variable rise-times and peak amplitudes, they can both be described in terms of synchronization of transmitter release. We have reviewed many experimental findings, which together strongly suggest that the original interpretation of Fatt and Katz (1952) regarding MSSs as reflecting the non-propagated 'neurogenic' activity of 'terminal spots' may be a useful concept to pursue since it may help to explain part of the underlying molecular basis of quantal release.(ABSTRACT TRUNCATED AT 400 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.

Involvement of Ca2+ uptake by a reticulum-like store in the control of transmitter release

At an identified neuro-neuronal synapse of Aplysia, 2,5-diterbutyl 1,4-benzohydroquinone, a selective blocker of the reticulum Ca2+ pump, was found to potentiate evoked quantal release of acetylcholine through an increased accumulation of Ca2+ in the presynaptic neuron during depolarization without any accompanying changes in the presynaptic Ca2+ current. We conclude that a rapid Ca2+ buffering system, similar to that associated with the endoplasmic reticulum, must be present in the nerve terminal and play a role in the control of Ca2+ which reaches the release system.

Regulation of cytosolic free calcium concentration by intrasynaptic mitochondria

By the use of digitonin permeabilized presynaptic nerve terminals (synaptosomes), we have found that intrasynaptic mitochondria, when studied "in situ," i.e., surrounded by their cytosolic environment, are able to buffer calcium in a range of calcium concentrations close to those usually present in the cytosol of resting synaptosomes. Adenine nucleotides and polyamines, which are usually lost during isolation of mitochondria, greatly improve the calcium-sequestering activity of mitochondria in permeabilized synaptosomes. The hypothesis that the mitochondria contributes to calcium homeostasis at low resting cytosolic free calcium concentration ([Ca2+]i) in synaptosomes has been tested; it has been found that in fact this is the case. Intrasynaptic mitochondria actively accumulates calcium at [Ca2+]i around 10(-7) M, and this activity is necessary for the regulation of [Ca2+]i. When compared with other membrane-limited calcium pools, it was found that depending on external concentration the calcium pool mobilized from mitochondria is similar or even greater than the IP3- or caffeine-sensitive calcium pools. In summary, the results presented argue in favor of a more prominent role of mitochondria in regulating [Ca2+]i in presynaptic nerve terminals, a role that should be reconsidered for other cellular types in light of the present evidence.

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.

The brain ryanodine receptor: a caffeine-sensitive calcium release channel

The release of stored Ca2+ from intracellular pools triggers a variety of important neuronal processes. Physiological and pharmacological evidence has indicated the presence of caffeine-sensitive intracellular pools that are distinct from the well-characterized inositol 1,4,5,-trisphosphate (IP3)-gated pools. Here we report that the brain ryanodine receptor functions as a caffeine- and ryanodine-sensitive Ca2+ release channel that is distinct from the brain IP3 receptor. The brain ryanodine receptor has been purified 6700-fold with no change in [3H]ryanodine binding affinity and shown to be a homotetramer composed of an approximately 500 kd protein subunit, which is identified by anti-peptide antibodies against the skeletal and cardiac muscle ryanodine receptors. Our results demonstrate that the brain ryanodine receptor functions as a caffeine-sensitive Ca2+ release channel and thus is the likely gating mechanism for intracellular caffeine-sensitive Ca2+ pools in neurons.

Conditions restricting depolarization-dependent calcium influx in synaptosomes reveal a graded response of P96 dephosphorylation and a transient dephosphorylation of P65

Temporal changes in the phosphorylation level of synaptosomal phosphoproteins following depolarization of synaptosomes were investigated under conditions restricting calcium influx. High-K+ depolarization in media of low [Na+]o (32 mM during preincubation and depolarization) at pH 6.5 resulted in a pronounced fall in the cytosolic free calcium concentration transient, and in a reduction in the initial K(+)-stimulated 45Ca2+ uptake and endogenous acetylcholine release relative to the values obtained with control synaptosomes (preincubated and depolarized in Na(+)-based media). This reduction was paralleled by a decrease in the rate of dephosphorylation of the synaptosomal protein P96. A slower dephosphorylation of P96 also was observed on exposure to 20 microM veratridine at 0.5 mM external calcium. Our results indicate that, similar to synapsin I phosphorylation, P96 dephosphorylation shows a graded response to the amount of calcium entering the presynaptic terminal. Depolarization of synaptosomes under conditions restricting the influx of calcium revealed a transient dephosphorylation (reversed within 10 s) of the phosphoprotein P65. The possible significance of this finding to the process of neurotransmitter release is discussed.

Depolarization promotes caffeine induced [3H]-noradrenaline release in calcium-free solution from peripheral sympathetic nerves

The transmitter releasing action of caffeine was studied in the absence of extracellular Ca2+ from the peripheral sympathetic nerves of the rabbit main pulmonary artery. Caffeine (10 mM) increased the release of [3H]-noradrenaline moderately, but not significantly in Ca2(+)-free (+1 mM EGTA) Krebs solution. When peripheral nerve endings/varicosities were depolarized by elevating extracellular K+ to 47.2 mM and 70.8 mM in Ca2(+)-free solution, the transmitter releasing effect of 10 mM caffeine became significant. Ca2+ removal itself transiently increased the [3H]-noradrenaline outflow. In the individual experiments the amount of the caffeine evoked transmitter release at 47.2 mM and 70.8 mM K(+)-depolarization was inversely correlated to the release evoked by Ca2(+)-removal. Our results suggest that caffeine-sensitive calcium stores are present in peripheral nerve terminals of rabbit pulmonary artery, and part of the caffeine sensitive calcium stores may discharge during Ca2(+)-removal from the extracellular solution.