Caffeine response in pyramidal neurons freshly dissociated from rat hippocampus

Calcium-induced calcium release in noradrenergic neurons of the locus coeruleus

The locus coeruleus (LC) is a nucleus within the brainstem that consists of norepinephrine-releasing neurons. It is involved in broad processes including cognitive and emotional functions. Understanding the mechanisms that control the excitability of LC neurons is important because they innervate widespread brain regions. One of the key regulators is cytosolic calcium concentration ([Ca²⁺]_c), the increases in which can be amplified by calcium-induced calcium release (CICR) from intracellular calcium stores. Although the electrical activities of LC neurons are regulated by changes in [Ca²⁺]_c, the extent of CICR involvement in this regulation has remained unclear. Here we show that CICR hyperpolarizes acutely dissociated LC neurons of the rat and demonstrate the underlying pathway. When CICR was activated by extracellular application of 10 mM caffeine, LC neurons were hyperpolarized in the current-clamp mode of patch-clamp recording, and the majority of neurons showed an outward current in the voltage-clamp mode. This outward current was accompanied by increased membrane conductance, and its reversal potential was close to the K⁺ equilibrium potential, indicating that it is mediated by opening of K⁺ channels. The outward current was generated in the absence of extracellular calcium and was blocked when the calcium stores were inhibited by applying ryanodine. Pharmacological blockers indicated that it was mediated by Ca²⁺-activated K⁺ channels of the non-small conductance type. The application of caffeine increased [Ca²⁺]_c, as visualized by fluorescence microscopy. These findings show CICR suppresses LC neuronal activity, and indicate its dynamic role in modulating the LC-mediated noradrenergic tone in the brain.

Ryanodine receptor-mediated Ca(2+) release underlies iron-induced mitochondrial fission and stimulates mitochondrial Ca(2+) uptake in primary hippocampal neurons

Mounting evidence indicates that iron accumulation impairs brain function. We have reported previously that addition of sub-lethal concentrations of iron to primary hippocampal neurons produces Ca²⁺ signals and promotes cytoplasmic generation of reactive oxygen species. These Ca²⁺ signals, which emerge within seconds after iron addition, arise mostly from Ca²⁺ release through the redox-sensitive ryanodine receptor (RyR) channels present in the endoplasmic reticulum. We have reported also that addition of synaptotoxic amyloid-β oligomers to primary hippocampal neurons stimulates RyR-mediated Ca²⁺ release, generating long-lasting Ca²⁺ signals that activate Ca²⁺-sensitive cellular effectors and promote the disruption of the mitochondrial network. Here, we describe that 24 h incubation of primary hippocampal neurons with iron enhanced agonist-induced RyR-mediated Ca²⁺ release and promoted mitochondrial network fragmentation in 43% of neurons, a response significantly prevented by RyR inhibition and by the antioxidant agent N-acetyl-L-cysteine. Stimulation of RyR-mediated Ca²⁺ release by a RyR agonist promoted mitochondrial Ca²⁺ uptake in control neurons and in iron-treated neurons that displayed non-fragmented mitochondria, but not in neurons with fragmented mitochondria. Yet, the global cytoplasmic Ca²⁺ increase induced by the Ca²⁺ ionophore ionomycin prompted significant mitochondrial Ca²⁺ uptake in neurons with fragmented mitochondria, indicating that fragmentation did not prevent mitochondrial Ca²⁺ uptake but presumably decreased the functional coupling between RyR-mediated Ca²⁺ release and the mitochondrial Ca²⁺ uniporter. Taken together, our results indicate that stimulation of redox-sensitive RyR-mediated Ca²⁺ release by iron causes significant neuronal mitochondrial fragmentation, which presumably contributes to the impairment of neuronal function produced by iron accumulation.

Sodium-calcium exchanger modulates the L-glutamate Ca(i) (2+) signalling in type-1 cerebellar astrocytes

We have previously demonstrated that rat type-1 cerebellar astrocytes express a very active Na(+)/Ca(2+) exchanger which accounts for most of the total plasma membrane Ca(2+) fluxes and for the clearance of Ca (i) (2+) induced by physiological agonist. In this chapter, we have explored the mechanism by which the reverse Na(+)/Ca(2+) exchange is involved in agonist-induced Ca(2+) signalling in rat cerebellar astrocytes. Laser-scanning confocal microscopy experiments using immunofluorescence labelling of Na(+)/Ca(2+) exchanger and RyRs demonstrated that they are highly co-localized. The most important finding presented in this chapter is that L-glutamate activates the reverse mode of the Na(+)/Ca(2+) exchange by inducing a Na(+) entry through the electrogenic Na(+)-glutamate co-transporter and not through the ionophoric L-glutamate receptors as confirmed by pharmacological experiments with specific blockers of ionophoric L-glutamate receptors, electrogenic glutamate transporters and the Na/Ca exchange.

Hypothermia reduces calcium entry via the N-methyl-D-aspartate and ryanodine receptors in cultured hippocampal neurons

Hypothermia is a powerful neuroprotective method when induced following cardiac arrest, stroke, and traumatic brain injury. The physiological effects of hypothermia are multifaceted and therefore a better knowledge of its therapeutic targets will be central to developing innovative combination therapies to augment the protective benefits of hypothermia. Altered neuronal calcium dynamics have been implicated following stroke, status epilepticus and traumatic brain injury. This study was therefore initiated to evaluate the effect of hypothermia on various modes of calcium entry into a neuron. Here, we utilized various pharmacological agents to stimulate major routes of calcium entry in primary cultured hippocampal neurons. Fluorescent calcium indicator Fura-2AM was used to compare calcium ratio under normothermic (37 °C) and hypothermic (31 °C) conditions. The results of this study indicate that hypothermia preferentially reduces calcium entry through N-methyl-D-aspartate receptors and ryanodine receptors. Hypothermia, on the other hand, did not have a significant effect on calcium entry through the voltage-dependent calcium channels or the inositol tri-phosphate receptors. The ability of hypothermia to selectively affect both N-methyl-D-aspartate receptors and ryanodine receptors-mediated calcium systems makes it an attractive intervention for alleviating calcium elevations that are present following many neurological injuries.

Levetiracetam inhibits both ryanodine and IP3 receptor activated calcium induced calcium release in hippocampal neurons in culture

Epilepsy affects approximately 1% of the population worldwide, and there is a pressing need to develop new anti-epileptic drugs (AEDs) and understand their mechanisms of action. Levetiracetam (LEV) is a novel AED and despite its increasingly widespread clinical use, its mechanism of action is as yet undetermined. Intracellular calcium ([Ca2+]i) regulation by both inositol 1,4,5-triphosphate receptors (IP3R) and ryanodine receptors (RyR) has been implicated in epileptogenesis and the maintenance of epilepsy. To this end, we investigated the effect of LEV on RyR and IP3R activated calcium-induced calcium release (CICR) in hippocampal neuronal cultures. RyR-mediated CICR was stimulated using the well-characterized RyR activator, caffeine. Caffeine (10mM) caused a significant increase in [Ca2+]i in hippocampal neurons. Treatment with LEV (33 microM) prior to stimulation of RyR-mediated CICR by caffeine led to a 61% decrease in the caffeine induced peak height of [Ca2+]i when compared to the control. Bradykinin stimulates IP3R-activated CICR-to test the effect of LEV on IP3R-mediated CICR, bradykinin (1 microM) was used to stimulate cells pre-treated with LEV (100 microM). The data showed that LEV caused a 74% decrease in IP3R-mediated CICR compared to the control. In previous studies we have shown that altered Ca2+ homeostatic mechanisms play a role in seizure activity and the development of spontaneous recurrent epileptiform discharges (SREDs). Elevations in [Ca2+]i mediated by CICR systems have been associated with neurotoxicity, changes in neuronal plasticity, and the development of AE. Thus, the ability of LEV to modulate the two major CICR systems demonstrates an important molecular effect of this agent on a major second messenger system in neurons.

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.

Calcium release from intracellular stores in rodent astrocytes and neurons in situ

Endoplasmic reticular Ca(2+) stores, instrumental for intra- and intercellular calcium signalling, can be depleted by different receptor agonists. In the present study, the functional status of ER Ca(2+) stores was probed by cyclopiazonic acid (CPA, 10-30 microM, inhibitor of SERCA-dependent ER Ca(2+) uptake) and/or caffeine (20 mM, ryanodine receptor activator) in astrocytes and neurons of rat and mouse acute hippocampal brain slices (Stratum radiatum, Stratum moleculare), and in cultured astrocytes, using confocal microscopy and conventional Ca(2+) imaging. Astrocytes and neurons in situ, identified by their Ca(2+) response in K(+)-free saline (Dallwig and Deitmer [J. Neurosci. Methods 116 (2002) 77]), had a resting cytosolic Ca(2+) level of 105 and 157 nM, respectively (P<0.05). CPA evoked a Ca(2+) transient, which was faster and larger in neurons than in astrocytes, indicating larger Ca(2+) leak of neuronal Ca(2+) stores. Caffeine evoked a Ca(2+) rise in most neurons (>80%), but only in less than 40% of astrocytes. The glial Ca(2+) transients in the presence of caffeine had a large and variable delay (>50 s), as compared to those in neurons (< or =10 s), and appeared to be spontaneous and/or secondary to the neuronal Ca(2+) response, leading to release of neuronal transmitters. Astrocytes in culture responded to CPA, but never to caffeine with a Ca(2+) rise. Our results indicate that astrocytes, in contrast to neurons, lack caffeine-sensitive Ca(2+) stores, and have a relatively smaller leak from CPA-sensitive Ca(2+) stores than neurons.

Ryanodine stores and calcium regulation in the inner segments of salamander rods and cones

Despite the prominent role played by intracellular Ca2+ stores in the regulation of neuronal Ca2+ homeostasis and in invertebrate photoreception, little is known about their contribution to the control of free Ca2+ concentration ([Ca2+]i) in the inner segments of vertebrate photoreceptors. Previously, caffeine-sensitive intracellular Ca2+ stores were shown to play a role in regulating glutamate release from photoreceptors. To understand the properties of these intracellular stores better we used pharmacological approaches that alter the dynamics of storage and release of Ca2+ from intracellular compartments. Caffeine evoked readily discernible changes in [Ca2+]i in the inner segments of rods, but not cones. Caffeine-evoked Ca2+ responses in cone inner segments were unmasked in the presence of inhibitors of the plasma membrane Ca2+ ATPases (PMCAs) and mitochondrial Ca2+ sequestration. Caffeine-evoked responses were blocked by ryanodine, a selective blocker of Ca2+ release and by cyclopiazonic acid, a blocker of Ca2+ sequestration into the endoplasmic reticulum. These two inhibitors also substantially reduced the amplitude of depolarization-evoked [Ca2+]i increases, providing evidence for Ca2+-induced Ca2+ release (CICR) in rods and cones. The magnitude and kinetics of caffeine-evoked Ca2+ elevation depended on the basal [Ca2+]i, PMCA activity and on mitochondrial function. These results reveal an intimate interaction between the endoplasmic reticulum, voltage-gated Ca2+ channels, PMCAs and mitochondrial Ca2+ stores in photoreceptor inner segments, and suggest a role for CICR in the regulation of synaptic transmission.

Spatial profiles of store-dependent calcium release in motoneurones of the nucleus hypoglossus from newborn mouse

Hypoglossal motoneurones (HMN) are selectively damaged in both human amyotrophic lateral sclerosis (ALS) and corresponding mouse models of this neurodegenerative disease, a process which has been linked to their low endogenous Ca2+ buffering capacity and an exceptional vulnerability to Ca2+-mediated excitotoxic events. In this report, we investigated local Ca2+ profiles in low buffered HMNs by utilizing multiphoton microscopy, CCD imaging and patch clamp recordings in slice preparations. Bath application of caffeine induced highly localized Ca2+ release events, which displayed an initial peak followed by a slow 'shoulder' lasting several seconds. Peak amplitudes were paralleled by Ca2+-activated, apamin-sensitive K+ currents (IKCa), demonstrating a functional link between Ca2+ stores and HMN excitability. The potential involvement of mitochondria was investigated by bath application of CCCP, which collapses the electrochemical potential across the inner mitochondrial membrane. CCCP reduced peak amplitudes of caffeine responses and consequently IKCa, indicating that functionally intact mitochondria were critical for store-dependent modulation of HMN excitability. Taken together, our results indicate localized Ca2+ release profiles in HMNs, where low buffering capacities enhance the role of Ca2+-regulating organelles as local determinants of [Ca2+]i. This might expose HMN to exceptional risks during pathophysiological organelle disruptions and other ALS-related, cellular disturbances.

Different Ca2+ source for slow AHP in completely adapting and repetitive firing pyramidal neurons

Intracellular recordings in an in vitro neocortical slice preparation from immature rats were used to investigate the Ca2 source for slow afterhyperpolarization (sAHP) generation in pyramidal neurons that exhibit complete spike frequency adaptation (CA neurons). In pyramidal neurons that maintain repetitive firing for long periods of time (RF neurons), N-, P- and Q-type Ca2+ channels supply Ca2+ for sAHP generation. In CA neurons, the sAHP was reduced by only 50% by the combination of antagonists for these Ca2+ channel types and L-type channels. Ryanodine and dantrolene, blockers of Ca2(+)-induced Ca2+ release, reduced the sAHP by approximately 45% in CA neurons, but caused no reduction of the sAHP in RF neurons. Dantrolene application caused CA neurons to fire throughout a 1s suprathreshold current injection (as do RF neurons).

The effect of quinine on outer hair cell shape, compliance and force

Quinine intoxication causes a well-described syndrome that includes tinnitus, sensorineural hearing loss and vertigo. The pathophysiology of quinine's effects on hearing is unknown, but may include a peripheral component. The cochlear outer hair cell is known to be motile and to contribute force to amplify the vibration pattern of the organ of Corti. The outer hair cell is also a target of diseases involving tinnitus and sensorineural hearing loss, including salicylate intoxication. These effects may be mediated through changes either in motile force or in mechanical properties. Quinine's effects on outer hair cell motility and mechanical properties have therefore been examined in vitro. Quinine at 5.0 mM substantially decreased active force generation in isolated guinea pig cochlear outer hair cells. Isolated cells also elongated and dilated in diameter when exposed to 5.0 mM quinine. No consistent changes in mechanical properties were observed. 1.0 mM quinine was ineffective in either force reduction or elongation. Trifluoperazine, a calmodulin inhibitor, and ML-9, a blocker of myosin light chain kinases, were ineffective in blocking quinine-induced force reduction or elongation. Deferoxamine, a hydroxyl free radical scavenger, also failed to block either the force decrease or the elongation.

Tetrahydroberberine blocks membrane K+ channels underlying its inhibition of intracellular message-mediated outward currents in acutely dissociated CA1 neurons from rat hippocampus

In the patch-clamp perforated whole-cell recording mode, tetrahydroberberine (THB), a novel dopamine (DA) receptor antagonist, inhibits not only DA-induced outward K+ currents, but also acetylcholine-, caffeine- or strychnine-induced outward current. However, THB does not affect either GABA- or glycine-induced Cl- currents, or non-NMDA receptor agonist-induced cation currents. As expected for a K+ channel blocker, THB evokes a downward current deflection accompanied by a decrease of conductance. It is concluded that the direct blockade of membrane K+ channels by THB underlies its inhibition of intracellular message-mediated outward currents.

Blockade of N-type Ca2+ current by cilnidipine (FRC-8653) in acutely dissociated rat sympathetic neurones

1 The inhibitory effects of cilnidipine (FRC-8653) and various organic Ca2+ channel blockers on high voltage-activated Ba2+ currents (HVA IBa) in rat sympathetic neurones were examined by means of the conventional whole-cell patch-clamp recording mode under voltage-clamped conditions. 2 HVA IBa was classified into three different current components with subtype selective peptide Ca2+ channel blockers. No omega-Agatoxin IVA-sensitive (P-type) or omega-conotoxin MVIIC-sensitive (Q-type) current components were observed. Most (> 85%) IBa was found to consist of omega-conotoxin GVIA-sensitive N-type components. 3 The application of cilnidipine inhibited HVA 1Ba in a concentration-dependent manner. The Kd value for cilnidipine was 0.8 microM. Cilnidipine did not shift the current-voltage (I-V) relationship for HVA IBa, as regards the threshold potential and peak potential where the amplitude reached a maximum. 4 High concentration of three hypotensive Ca2+ channel blockers, nifedipine, diltiazem and verapamil, all inhibited HVA IBa in a concentration-dependent manner. The Kd values for nifedipine, diltiazem and verapamil were 131, 151 and 47 microM, respectively. A piperazine-type Ca2+ channel blocker, flunarizine, showed a relatively potent blocking action on IBa. The Kd value was about 3 microM. 5 These results thus show that cilnidipine potently inhibits the sympathetic Ca2+ channels which predominantly consist of an omega-Cg-GVIA-sensitive component. This blockade of the N-type Ca2+ channel, as well as the L-type Ca2+ channel by cilnidipine suggests that it could be used therapeutically for treatment of hypersensitive sympathetic disorders associated with hypertension.

Release and sequestration of calcium by ryanodine-sensitive stores in rat hippocampal neurones

1. The properties of ryanodine-sensitive Ca2+ stores in CA1 pyramidal cells were investigated in rat hippocampal slices by using whole-cell patch-clamp recordings combined with fura-2-based fluorometric digital imaging of cytoplasmic Ca2+ concentration ([Ca2+]i). 2. Brief pressure applications of caffeine onto the somata of pyramidal cells caused large transient increases in [Ca2+]i (Ca2+ transients) of 50-600 nM above baseline. 3. The Ca2+ transients evoked by caffeine at -60 mV were not associated with an inward current, persisted after blocking voltage-activated Ca2+ currents and were completely blocked by bath-applied ryanodine. Similar transients were also evoked at +60 mV. Thus, these transients reflect Ca2+ release from intracellular ryanodine-sensitive Ca2+ stores. 4. The Ca2+ transients evoked by closely spaced caffeine pulses rapidly decreased in amplitude, indicating progressive depletion of the Ca2+ stores. The amplitude of the Ca2+ transients recovered spontaneously with an exponential time constant of 59 s. Recovery was accelerated by depolarization-induced elevations in [Ca2+]i and blocked by cyclopiazonic acid (CPA) and thapsigargin, indicating that store refilling is mediated by endoplasmic reticulum Ca(2+)-ATPases. 5. Even without prior store depletion the caffeine-induced Ca2+ transients disappeared after 6 min exposure to CPA, suggesting that ryanodine-sensitive Ca2+ stores are maintained at rest by continuous Ca2+ sequestration. 6. Caffeine-depleted Ca2+ stores did not refill in Ca(2+)-free saline, suggesting that the refilling of the stores depends upon Ca2+ influx through a 'capacitative-like' transmembrane influx pathway operating at resting membrane potential. The refilling of the stores was also blocked by Ni2+ and gallopamil (D600). 7. Elevations of basal [Ca2+]i produced by bath-applied KCl markedly potentiated (up to 6-fold) the caffeine-induced Ca2+ transients. The degree of potentiation was positively related to the increase in basal [Ca2+]i. The Ca2+ transients remained potentiated up to 9 min after reversing the KCl-induced [Ca2+]i increase. Thus, the ryanodine-sensitive Ca2+ stores can 'overcharge' when challenged with an increase in [Ca2+]i and slowly discharge excess Ca2+ after basal [Ca2+]i returns to its resting level. 8. Pressure applications of caffeine onto pyramidal cell dendrites evoked local Ca2+ transients similar to those separately evoked in the respective somata. Thus, dendritic ryanodine-sensitive Ca2+ stores are also loaded at rest and can function as independent compartments. 9. In conclusion, the ryanodine-sensitive Ca2+ stores in hippocampal pyramidal neurones contain a releasable pool of Ca2+ that is maintained by a Ca2+ entry pathway active at subthreshold membrane potentials. Ca2+ entry through voltage-gated Ca2+ channels transiently overcharges the stores. Thus, by acting as powerful buffers at rest and as regulated sources during activity, Ca2+ stores may control the waveform of physiological Ca2+ signals in CA1 hippocampal pyramidal neurones.

Ca(2+)-induced Ca2+ release phenomena in mammalian sympathetic neurons are critically dependent on the rate of rise of trigger Ca2+

The role of ryanodine-sensitive intracellular Ca2+ stores present in nonmuscular cells is not yet completely understood. Here we examine the physiological parameters determining the dynamics of caffeine-induced Ca2+ release in individual fura 2-loaded sympathetic neurons. Two ryanodine-sensitive release components were distinguished: an early, transient release (TR) and a delayed, persistent release (PR). The TR components shows refractoriness, depends on the filling status of the store, and requires caffeine concentrations > or = 10 mM. Furthermore, it is selectively suppressed by tetracaine and intracellular BAPTA, which interfere with Ca(2+)-mediated feedback loops, suggesting that it constitutes a Ca(2+)-induced Ca(2+)-release phenomenon. The dynamics of release is markedly affected when Sr2+ substitutes for Ca2+, indicating that Sr2+ release may operate with lower feedback gain than Ca2+ release. Our data indicate that when the initial release occurs at an adequately fast rate, Ca2+ triggers further release, producing a regenerative response, which is interrupted by depletion of releasable Ca2+ and Ca(2+)-dependent inactivation. A compartmentalized linear diffusion model can reproduce caffeine responses: When the Ca2+ reservoir is full, the rapid initial Ca2+ rise determines a faster occupation of the ryanodine receptor Ca2+ activation site giving rise to a regenerative release. With the store only partially loaded, the slower initial Ca2+ rise allows the inactivating site of the release channel to become occupied nearly as quickly as the activating site, thereby suppressing the initial fast release. The PR component is less dependent on the store's Ca2+ content. This study suggests that transmembrane Ca2+ influx in rat sympathetic neurons does not evoke widespread amplification by CICR because of its inability to raise [Ca2+] near the Ca2+ release channels sufficiently fast to overcome their Ca(2+)-dependent inactivation. Conversely, caffeine-induced Ca2+ release can undergo considerable amplification especially when Ca2+ stores are full. We propose that the primary function of ryanodine-sensitive stores in neurons and perhaps in other nonmuscular cells, is to emphasize subcellular Ca2+ gradients resulting from agonist-induced intracellular release. The amplification gain is dependent both on the agonist concentration and on the filling status of intracellular Ca2+ stores.

Kinetic characteristics of thrombin receptor-mediated responses in rat megakaryocytes

Kinetic characteristics of thrombin receptor-mediated responses on rat megakaryocytes were examined by the use of the perforated patch clamp combined with the rapid drug exchange system termed the 'Y-tube method'. The application of thrombin evoked repetitive Ca(2+)-activated K+ current (IKCa) in a concentration-dependent manner. The characteristic features for thrombin-induced response compared with purinoceptor-induced response were the long latency, long washout time and fast desensitization. The similar IKCa as thrombin was induced by trypsin. Thrombin- and trypsin-induced IKCa were both inhibited by the protease inhibitor, SBTI, and the washout time for thrombin was markedly shortened (7.4 +/- 2.2 s) when thrombin was washed out by a solution containing soybean trypsin inhibitor. A synthetic thrombin receptor agonist peptide induced IKCa oscillation with shorter latency than thrombin.

Characterization of the K+ current mediated by 5-HT1A receptor in the acutely dissociated rat dorsal raphe neurons

The action of 5-hydroxytryptamine (5-HT) via the 5-HT1A receptor on dissociated rat dorsal raphe neurons was characterized under the whole-cell mode by using the nystatin-perforated patch-clamp technique. Under voltage-clamp conditions, 5-HT induced an inwardly rectifying K+ current (I5-HT) in a concentration-dependent manner. I5-HT was mimicked by 8-OH-DPAT and buspirone, which are both 5-HT1A receptor agonists. I5-HT was reversibly blocked by such 5-HT1A receptor antagonists as (S)-UH-301 a 5-HT4 receptor antagonist. I5-HT was antagonized concentration-dependently by such K+ channel blockers as quinine, Ba2+ and 4-aminopyridine but was relatively insensitive to both CS+ and tetraethylammonium. When the neurons were loaded with guanosine 5'-O-3-thiotriphosphate through a patch pipette, the K+ current induced by 5-HT became irreversible. N-ethylmaleimide (NEM), a sulfhydryl alkylating agent, irreversibly blocked I5-HT. The intracellular perfusion with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a Ca2+ chelator, or neomycine, a phospholipase C inhibitor, never significantly affected the 5-HT-induced response. 12-Myristate 13-acetate diester (PMA), a protein kinase C (PKC) activator, had only a weak inhibitory effect on I5-HT, and staurosporine, a PKC inhibitor, failed to significantly occlude I5-HT. Therefore, the K+ conductance activated via the 5-HT1a receptor of dorsal raphe neurons was thus characterized by the sensitivity to such K+ channel blockers as quinine, Ba2+ and 4-aminopyridine. Moreover, G protein which is NEM-sensitive and can couple to the 5-HT1A receptor, is thus considered to activate the inwardly rectifying K+ conductance without being mediated by such second messengers as Ca2+ and PKC.

Two components of metabotropic glutamate responses in acutely dissociated CA3 pyramidal neurons of the rat

The excitatory and inhibitory actions of metabotropic glutamate receptor (mGluR) agonists were investigated in acutely dissociated rat hippocampal CA3 pyramidal neurons, using the conventional whole-cell and nystatin-perforated patch recording configurations under the voltage-clamp condition. With the conventional whole-cell recording, glutamate (Glu) and quisqualic acid (QA) induced only ionotropic inward currents accompanied by increased membrane conductance at a holding potential (VH) of -45 mV. The response was reversibly blocked in the presence of D-2-amino-5-phosphonopentanoic acid (D-AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the antagonists of N-methyl-D-aspartate (NMDA) receptor and non-NMDA receptor, respectively. With nystatin-perforated patch recording, mGlu responses insensitive to both D-AP5 and CNQX were observed. Fifty-five % of the cells responded by a slow inward current accompanied by conductance decrease (ImGlui) at a VH of -44 mV. One % of the neurons showed an outward current with conductance increase (ImGluo), and 34% of the neurons showed ImGluo followed by ImGlui. The onset of ImGluo occurred approximately 900 ms after the response to 30 mM K+. The time to peak of ImGluo were 32- to 79-times longer than those of ionotropic responses. ImGlui appeared at lower concentrations than ionotropic Glu responses, whereas ImGluo appeared at similar concentrations as ionotropic responses. The rank order of affinity was QA > Glu > (+/-)-1-aminocyclopentane-trans-1, 3-dicarboxylic acid (tACPD) for both ImGlui and ImGluo. Half-maximal effective concentrations (EC50) and the threshold concentrations for the three agonists were four- to tenfold lower for ImGlui than for ImGluo. The current-voltage relationship showed that the reversal potentials of ImGlui and ImGluo shifted 55 and 59 mV, respectively, for a tenfold change in extracellular K+ concentration, indicating that K+ is the charge carrier of both mGlu responses. During ImGlui, both the leakage current and muscarine-sensitive voltage-dependent K+ current (M current) were suppressed. ImGluo induced by 10(-4) M tACPD was abolished by 3.10(-7) M charybdotoxin and 10(-6) M ryanodine. These results show that there are two components of mGlu responses in CA3 pyramidal neurons and that ImGlui and ImGluo show different pharmacological properties.

Calcium-induced calcium release in neurones

Neurones express several subtypes of intracellular Ca2+ channels, which are regulated by cytoplasmic calcium concentration ([Ca2+]c) and provide the pathway for Ca(2+)-induced Ca2+ release (CICR) from endoplasmic reticulum Ca2+ stores. The initial studies of CICR which employed several pharmacological tools (and in particular caffeine and ryanodine) demonstrated that: (i) caffeine induces intracellular calcium release in various peripheral and central neurones; and (ii) inhibition of CICR affects the parameters of depolarization-triggered [Ca2+]c responses. Experiments with caffeine demonstrated also that Ca2+ release from internal pools was incremental, suggesting the coexistence of several subpopulations of Ca2+ release channels with different sensitivity to caffeine. The CICR availability in neurones is controlled by both the Ca2+ content of the internal stores and the basal [Ca2+]c. Direct comparison of transmembrane Ca2+ influx with plasmalemmal Ca2+ current and [Ca2+]c elevation performed on sympathetic, sensory and cerebellar Purkinje neurones revealed the gradual activation of CICR. The efficacy of CICR may be regulated by the newly discovered second messenger cADP ribose (cADPR), although the mechanism of signal transduction involving cADPR is still unknown. CICR in neurones may be important in creation of local [Ca2+]c signals and could be involved in a regulation of numerous neuronal functions.

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+].

3-Isobutyl-1-methylxanthine (IBMX) affects potassium permeability in rat sensory neurones via pathways that are sensitive and insensitive to [Ca2+]in

The effects of externally applied 3-isobutyl-1-methylxanthine (IBMX), in millimolar concentrations, on the membrane currents in dorsal root ganglia (DRG) neurones isolated from newborn rats were investigated using the amphotericin-based "perforated" patch-clamp technique. In some experiments, simultaneous measurements of intracellular Ca2+ concentration ([Ca2+]in) were performed using fura-2 microfluorimetry. Applications of IBMX induced elevation of [Ca2+]in resulting from Ca2+ release from caffeine-ryanodine-sensitive internal stores. In addition to Ca2+ release, IBMX produced a biphasic membrane current response comprised of an inward current transiently interrupted by outward current. The onset of the inward current slightly preceded the onset of the [Ca2+]in transient, while the interrupting outward current developed synchronously with the [Ca2+]in rise. The development of IBMX-induced outward current ultimately needed the [Ca2+]in elevation. After the depletion of Ca2+ stores by IBMX or caffeine exposure, the subsequent IBMX challenge failed to produce both the [Ca2+]in transient and outward membrane current, although the inward current remained unchanged. Both components of the IBMX-induced membrane current response had a reversal potential close to the K+ equilibrium potential and the IBMX-induced membrane current response disappeared while dialysing the cell interior with K(+)-free, Cs(+)-containing solutions suggesting their association with K+ channel activity. External administration of 10 mM tetraethylammonium chloride (TEA-Cl) evoked an inward current similar to that observed in response to IBMX; in the presence of TEA-Cl, IBMX application was almost unable to induce additional inward current. IBMX (5 mM) effectively (approximately 50%) inhibited K+ currents evoked by step depolarizations of membrane potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Imaging of caffeine-inducible release of intracellular calcium in cultured embryonic mouse telencephalic neurons

To gain a better understanding of Ca(2+)-induced Ca2+ release in central neurons, we have studied the increase in intracellular Ca2+ concentration ([Ca2+]i) induced by application of caffeine to cells cultured from embryonic mouse telencephalon (hippocampus or cortex). The magnitudes and distributions of changes in [Ca2+]i in neuron somata were measured by quantitative video microscopy. We observed that application of caffeine to pyramidally shaped neurons typically initiated an increase in [Ca2+]i in the cytoplasmic region between the nucleus and the base of a major dendrite. [Ca2+] in this region increased over a period of 3 to 6 s and was followed by, with a slight delay, a surge of Ca2+ that moved across the soma and into or over the nucleus. Similar Ca2+ responses to caffeine were observed in Ca(2+)-containing and nominally Ca(2+)-free external solutions, suggesting that caffeine was inducing Ca2+ release from intracellular stores. Ca2+ responses to caffeine were potentiated by inducing a tonic Ca2+ influx through N-methyl-D-aspartate (NMDA)-type glutamate receptors activated by 0.3 microM glutamate and multiple responses to caffeine could be elicited by using this Ca2+ influx to refill the intracellular stores. Ryanodine inhibition of caffeine-induced Ca2+ release was use- and concentration-dependent; the median effective concentration EC50 for ryanodine declined from 22 microM for the first application of caffeine to 20 nM for the fourth. We conclude, based on these responses to caffeine, that ryanodine-sensitive mechanisms of intracellular Ca2+ release are active in hippocampal and cortical neurons and may be involved in generation of directed Ca2+ waves that engulf the nucleus.

Caffeine rapidly decreases potassium conductance of dissociated outer hair cells of guinea-pig cochlea

The effects of caffeine on the outer hair cells (OHCs) freshly dissociated from guinea-pig cochlea were investigated with the whole-cell patch-clamp technique, in both the conventional and the nystatin perforated patch-clamp configurations under voltage-clamp condition. Application of caffeine (> 1 mM for 10-30 s) induced an inward current (Icaffeine) with decrease of conductance in a dose-dependent manner at a holding potential (VH) of -60 mV. The reversal potential of Icaffeine (Ecaffeine) was close to the K+ equilibrium potential. The Icaffeine was not affected by Ca(2+)-free external solution. The internal perfusion of the Ca2+ chelator BAPTA had no effect on Icaffeine. The Icaffeine was not modulated by the external application of H-8 or staurosporine and by the internal perfusion of GDP-beta S. The amplitude of Icaffeine was the largest at the basal region of OHCs when caffeine was locally applied by the 'puffer' method. These results suggest that caffeine induces a decrease in membrane potassium conductance of the OHCs mainly at the basal region without mediating the intracellular signaling pathway.

Involvement of ryanodine receptors in sphingosylphosphorylcholine-induced calcium release from brain microsomes

Sphingosylphosphorylcholine (SPC) releases Ca2+ from brain microsomes. SPC-induced CA2+ release differs from IP3-induced Ca2+ release in that it is more extensive in the cerebrum than in the cerebellum. SPC has little effect on [3H] IP3 binding but enhances [3H] ryanodine binding, as expected for an activator of ryanodine receptors. SPC-induced Ca2+ release is inhibited by ryanodine receptor blockers but not by selective blockers of IP3 receptors. We conclude that SPC releases Ca2+ from brain microsomes by activating ryanodine receptors rather than IP3 receptors. Activation of an additional SPC-sensitive pathway for releasing Ca2+ is not precluded.

Potassium currents operated by thyrotrophin-releasing hormone in dissociated CA1 pyramidal neurones of rat hippocampus

1. Membrane currents activated by thyrotrophin-releasing hormone (TRH) were investigated in the dissociated rat hippocampal CA1 pyramidal neurone using the nystatin perforated patch recording configuration. 2. Under current-clamp condition, TRH caused a transient hyperpolarization accompanied by a decrease of firing activity and a successive long-lasting depolarization. The latter induced a blockade of firing. 3. When neurones were held at a holding potential (VH) of -40 mV under voltage clamp, TRH elicited a transient outward current with an increase in the membrane conductance, which was followed by a sustained inward current with a decrease in membrane conductance. The inactive TRH metabolite, TRH free acid, did not induce any currents. 4. The reversal potential of TRH-induced outward current (ETRH) was close to the K+ equilibrium potential (EK). The change in ETRH for a 10-fold change in extracellular K+ concentration was 56.4 mV, indicating that the membrane behaves like a K+ electrode in the presence of TRH. On the other hand, the TRH-induced inward current was due to suppression of a slow inward current relaxation during hyperpolarizing voltage commands to -50 mV from a VH of -40 mV, indicating the suppression of the voltage- and time-dependent component of the K+ current (M-current). 5. The TRH-induced outward current (ITRH) increased in a concentration-dependent manner over the concentration range 10(-8)-10(-6) M. The half-maximum concentration was 7.4 x 10(-8) M and the Hill coefficient was 1.5. 6. The TRH-induced outward current (ITRH) was antagonized by K+ channel blockers such as tetraethylammonium (TEA), 4-aminopyridine (4-AP) and Ba2+ in a concentration-dependent manner. ITRH was insensitive to both apamin and iberiotoxin. 7. The first application of TRH to neurones perfused with Ca(2+)-free external solution containing 2 mM EGTA could induce ITRH but the TRH response diminished dramatically with successive applications. Intracellular perfusion with a Ca2+ chelator, 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), also diminished the TRH response. 8. The depletion of Ca2+ from the intracellular Ca2+ store by thapsigargin blocked the TRH response without affecting the caffeine response. Pretreatment with Li+ significantly enhanced ITRH, suggesting that ITRH is involved in the elevation of intracellular free Ca2+ released from the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store site but not from the caffeine-sensitive one. 9. Staurosporine, a protein kinase C (PKC) inhibitor, suppressed ITRH in a concentration-dependent manner (the half-maximum inhibitory concentration (IC50), was 2.45 x 10(-8) M).(ABSTRACT TRUNCATED AT 400 WORDS)

Heterogeneous distribution and developmental change of metabotropic glutamate receptors in rat hippocampal CA1 pyramidal neurons

The metabotropic glutamate (mGlu) response in dissociated rat hippocampal CA1 pyramidal neurons was recorded, using the nystatin-perforated patch-clamp technique. The mGlu response was localized in the ventral site of the hippocampal CA1 region. In both the ventral and dorsal sites of the CA1 region, the current amplitude of the mGlu responses as well as their induction probability (responsiveness) were reduced with development of the brain.