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

Perturbation of Ca2+ stores and store-operated Ca2+ influx by lidocaine in neuronal N2A and NG108-15 cells

In this report we examined the effects of lidocaine on Ca²⁺ homeostasis of neuronal cells using microfluorimetric measurement of cytosolic Ca²⁺ with fura 2 as probe. In mouse neuroblastoma N2A cells, 10 mM lidocaine caused Ca²⁺ release from the cyclopiazonic acid (CPA)-dischargeable pool and abolished ATP-triggered Ca²⁺ release. Lidocaine-triggered Ca²⁺ release was not affected by xestospongin C (XeC), an inositol 1,4,5-trisphosphate receptor (IP3R) inhibitor. N2A cells did not have functional ryanodine receptors (RYR) (absence of caffeine response) and we used differentiated NG108-15 cells (presence of caffeine response) for further experiments. Caffeine-triggered Ca²⁺ release was unaffected by a brief lidocaine exposure, but was eliminated after a prolonged treatment of lidocaine, suggesting lidocaine abolished caffeine action possibly not by interfering caffeine binding but via Ca²⁺ store depletion. Lidocaine-elicited Ca²⁺ release was unaffected by XeC or a high concentration of ryanodine, suggesting Ca²⁺ release was not via IP3R or RYR. Lidocaine did not affect nigericin-dischargeable lysosomal Ca²⁺ stores. Lastly, we observed that lidocaine suppressed CPA-induced store-operated Ca²⁺ influx in both N2A cells and differentiated NG108-15 cells. Our results suggest two novel actions of lidocaine in neuronal cells, namely, depletion of Ca²⁺ store (via an IP3R- and RYR-independent manner) and suppression of store-operated Ca²⁺ influx.

Dendritic sodium spikes are required for long-term potentiation at distal synapses on hippocampal pyramidal neurons

Dendritic integration of synaptic inputs mediates rapid neural computation as well as longer-lasting plasticity. Several channel types can mediate dendritically initiated spikes (dSpikes), which may impact information processing and storage across multiple timescales; however, the roles of different channels in the rapid vs long-term effects of dSpikes are unknown. We show here that dSpikes mediated by Nav channels (blocked by a low concentration of TTX) are required for long-term potentiation (LTP) in the distal apical dendrites of hippocampal pyramidal neurons. Furthermore, imaging, simulations, and buffering experiments all support a model whereby fast Nav channel-mediated dSpikes (Na-dSpikes) contribute to LTP induction by promoting large, transient, localized increases in intracellular calcium concentration near the calcium-conducting pores of NMDAR and L-type Cav channels. Thus, in addition to contributing to rapid neural processing, Na-dSpikes are likely to contribute to memory formation via their role in long-lasting synaptic plasticity.

Ca2+-sparks constitute elementary building blocks for global Ca2+-signals in myocytes of retinal arterioles

Spontaneous Ca²⁺-events were imaged in myocytes within intact retinal arterioles (diameter <40 μm) freshly isolated from rat eyes. Ca²⁺-sparks were often observed to spread across the width of these small cells, and could summate to produce prolonged Ca²⁺-oscillations and contraction. Application of cyclopiazonic acid (20 μM) transiently increased spark frequency and oscillation amplitude, but inhibited both sparks and oscillations within 60 s. Both ryanodine (100 μM) and tetracaine (100 μM) reduced the frequency of sparks and oscillations, while tetracaine also reduced oscillation amplitude. None of these interventions affected spark amplitude. Nifedipine, which blocks store filling independently of any action on L-type Ca²⁺-channels in these cells, reduced the frequency and amplitude of both sparks and oscillations. Removal of external [Ca²⁺] (1 mM EGTA) also reduced the frequency of sparks and oscillations but these reductions were slower in onset than those in the presence of tetracaine or cyclopiazonic acid. Cyclopiazonic acid, nifedipine and low external [Ca²⁺] all reduced SR loading, as indicated by the amplitude of caffeine evoked Ca²⁺-transients. This study demonstrates for the first time that spontaneous Ca²⁺-events in small arterioles of the eye result from activation of ryanodine receptors in the SR and suggests that this activation is not tightly coupled to Ca²⁺-influx. The data also supports a model in which Ca²⁺-sparks act as building blocks for more prolonged, global Ca²⁺-signals.

Inositol-1,4,5-triphosphate receptors mediate activity-induced synaptic Ca2+ signals in muscle fibers and Ca2+ overload in slow-channel syndrome

Strict control of calcium entry through excitatory synaptic receptors is important for shaping synaptic responses, gene expression, and cell survival. Disruption of this control may lead to pathological accumulation of Ca2+. The slow-channel congenital myasthenic syndrome (SCS), due to mutations in muscle acetylcholine receptor (AChR), perturbs the kinetics of synaptic currents, leading to post-synaptic Ca2+ accumulation. To understand the regulation of calcium signaling at the neuromuscular junction (NMJ) and the etiology of Ca2+ overload in SCS we studied the role of sarcoplasmic Ca2+ stores in SCS. Using fura-2 loaded dissociated fibers activated with acetylcholine puffs, we confirmed that Ca2+ accumulates around wild type NMJ and discovered that Ca2+ accumulates significantly faster around the NMJ of SCS transgenic dissociated muscle fibers. Additionally, we determined that this process is dependant on the activation, altered kinetics, and movement of Ca2+ ions through the AChR, although, surprisingly, depletion of intracellular stores also prevents the accumulation of this cation around the NMJ. Finally, we concluded that the sarcoplasmic reticulum is the main source of Ca2+ and that inositol-1,4,5-triphosphate receptors (IP3R), and to a lesser degree L-type voltage gated Ca2+ channels, are responsible for the efflux of this cation from intracellular stores. These results suggest that a signaling system mediated by the activation of AChR, Ca2+, and IP3R is responsible for localized Ca2+ signals observed in muscle fibers and the Ca2+ overload observed in SCS.

Interactions of anesthetics with their targets: non-specific, specific or both?

What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.

Effect of lidocaine on harmaline-induced tremors in the rat

The present study was undertaken to investigate the effect of lidocaine on harmaline-induced tremors in the rat. Four groups of Wistar rats weighing 45-50 g were injected with harmaline (50 mg/kg i.p.) for inducing experimental tremors. The rats in group 1 served as control, whereas the animals in groups 2, 3, and 4 were also given lidocaine i.p. at doses of 12.5, 25, and 50 mg/kg, respectively, 10 min after the onset of tremors (therapeutic study). In a separate four groups of animals intraperitoneal lidocaine injection was given 10 min before harmaline (prophylactic study) in the same dose regimen as mentioned above. The latency of onset, intensity, and duration of tremor and electromyographic responses were recorded. Lidocaine dose dependently attenuated harmaline-induced tremors in rats. The latency period was increased, and duration and intensity of harmaline-induced tremors was reduced by lidocaine. Our electromyography (EMG) study also revealed a decrease in the amplitude of harmaline-induced tremors in lidocaine-treated rats. In conclusion, the results of this study clearly suggest beneficial effects of lidocaine in harmaline-induced tremors.

Characterization of sheep brain ryanodine receptor ATP binding site by photoaffinity labeling

Two high Mr protein bands (440 and 420 kDa) in sheep brain microsomal membranes were labeled with the photoaffinity ATP analog, O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (Bz2ATP). The 420 kDa band is labeled by [alpha-32P]-Bz2ATP with about 1000-fold higher affinity than the 440 kDa band. The heavily labeled 420 kDa band is identified as dynein heavy chain based on its partial amino acid sequence, and cross-reactivity with anti-dynein antibodies. The 440 kDa protein is immunologically identified as the type-2 RyR. Bz2ATP binding is obtained in the absence of divalent cations. Bz2ATP and ATP increased the binding of ryanodine to its receptor up to 3-fold, and increased the binding affinity up to 6-fold. Other nucleotides stimulate ryanodine binding with decreasing effectiveness: Bz2ATP > ATP > ADP > AMP > AMP-PNP > GTP > cAMP. With respect to nucleotide specificity, this binding site is similar to the skeletal muscle RyR (type 1). However, the brain RyR may have additional one or more sites with lower affinity with inhibitory effect on ryanodine binding. These results suggest that the major RyR isoform in sheep brain corresponds to the type-2 isoform, and that modulation of ryanodine binding by ATP involves its binding to the RyR protein. The association of dynein with brain microsomal membranes may reflect a linkage of RyR to the cytoskeleton.

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

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

Improvement of ischemic damage in gerbil hippocampal neurons by procaine

Acute cerebral ischemia induces membrane depolarization in the neuron, thereby incurring the simultaneous influx of various ions such as Na+ and Ca2+. Since procaine possesses the ability to inhibit the release of Ca2+ from intracellular Ca2+ stores to the cytosol as well as the ability to block Na+ channels, the effects of procaine on ischemia were investigated in the present study in gerbils both in vivo and in vitro. The histologic outcome was evaluated 7 days after 3 min of transient forebrain ischemia by assessing delayed neuronal death in hippocampal CA1 pyramidal cells in animals administered procaine (0.2, 0.4, or 2 micromol) intracerebroventricularly 10 min before ischemia and in animals given saline. The changes in the direct-current potential shift in the hippocampal CA1 area were measured using an identical animal model. A hypoxia-induced intracellular Ca2+ increase was evaluated by in vitro microfluorometry in gerbil hippocampal slices, and the effects of procaine (10, 50, and 100 micromol/l) on the Ca2+ accumulation were examined. Additionally, the effect of procaine (100 micromol/l) in a Ca2+-free condition was investigated. The histologic outcome was improved and the onset of the ischemia-induced membrane depolarization was prolonged by the preischemic administration of procaine. The increase in the intracellular concentration of Ca2+ induced by the in vitro hypoxia was suppressed by the perfusion of procaine-containing mediums (50 and 100 micromol/l), regarding both the initiation and the extent of the increase. A hypoxia-induced intracellular Ca2+ elevation in the Ca2+-free condition was observed, and the perfusion with procaine (100 micromol/l) inhibited this elevation. Procaine helps protect neurons from ischemia by suppressing the direct-current potential shift and by inhibiting the release of Ca2+ from the intracellular Ca2+ stores, as well as by inhibiting the influx of Ca2+ from the extracellular space.

Evidence for postsynaptic induction and expression of NMDA receptor independent LTP

Whole cell/patch-clamp and extracellular field potential recordings were used to study the induction and expression of N-methyl-D-aspartate (NMDA) receptor independent long-term potentiation (LTP) in area CA1 of the in vitro rat hippocampus. Induction of NMDA receptor independent LTP was prevented by manipulations that inhibited postsynaptic depolarization during tetanic stimulation: direct hyperpolarization of postsynaptic neurons and bath application of an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate receptor antagonist. NMDA receptor independent LTP also was blocked by intracellular application of the lidocaine derivative, N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium bromide (QX-314), to CA1 pyramidal neurons. These results complement the previous findings that NMDA receptor independent LTP was inhibited by postsynaptic injections of the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid and also was inhibited by a L-type voltage-dependent calcium channel antagonist (nifedipine). Collectively, these data make a strong case for the postsynaptic induction of this form of LTP. This paper also provides evidence for postsynaptic expression of NMDA receptor independent LTP. In an experiment where AMPA- and NMDA-receptor-mediated excitatory postsynaptic potentials (EPSPs) were isolated pharmacologically, LTP was found for only the AMPA-receptor-mediated EPSPs. In a separate experiment, paired-pulse facilitation (PPF) was measured during NMDA receptor independent LTP. Although there was an initial decrease in PPF, suggesting a posttetanic increase in the probability of glutamate release, the change in PPF decayed within 30-40 min of the tetanic stimulation, whereas the magnitude of the LTP was constant over this same time period. In addition, the LTP, but not the corresponding change in PPF, was blocked by the metabotropic glutamate receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine. These results are accounted for most easily by a selective increase in postsynaptic AMPA receptor function, but one type of presynaptic modification-an increase in the number of release sites without an overall change in the probability of release-also could account for these results (assuming that the level of glutamate release before LTP induction fully saturated NMDA, but not AMPA, receptors). One possible presynaptic modification, an increase in axon excitability, was ruled out by analysis of the presynaptic fiber volley, which was not increased at any time after LTP induction.

Effect of intrathecally administered local anesthetics on protein phosphorylation in the spinal cord

To elucidate the biochemical mechanisms of spinal anesthesia, we studied the effects of procaine and tetracaine on protein phosphorylation in the mouse spinal cord. Mice were injected intrathecally with either procaine, tetracaine (67 mM/approximately 2%, 10 microL, N = 5/drug), or saline (N = 4/group). Five minutes after injection, animals were killed with a guillotine, and the spinal cord was removed. The caudal 3-cm cord segment was homogenized and centrifuged, and an aliquot of the supernatant was used for phosphorylation assays. Calcium-dependent phosphorylation was initiated by incubating the samples in buffer containing [gamma-32P]ATP at 37 degrees for 30 min. The proteins were electrophoresed using slab gel and two-dimensional electrophoresis, and phosphorylated proteins were visualized by autoradiography. The data demonstrated that spinal anesthesia changes the phosphorylation state of five endogenous substrate proteins with apparent molecular masses of 130 (protein-a), 105 (protein-b), 55 (protein-c), 47 (protein-d), and 33 (protein-e) kDa. In two-dimensional electrophoresis, protein-a resolved into two proteins (a1 and a2). Analysis of variance of the densitometric data suggested a significant effect for the treatment (F(2,16) 735, P < 0.00005). Post hoc comparisons with the saline-treated controls, using the Newman-Keuls test, indicated that local anesthetics significantly affected phosphoproteins (P < 0.05) except for protein-al in the tetracaine-treated group. Further characterization of these phosphoproteins should aid in determining their role in the signal transduction cascade affected by spinal anesthesia.

Ryanodine receptor/calcium release channel conformations as reflected in the different effects of propranolol on its ryanodine binding and channel activity

1. Propranolol, a beta-blocker, inhibited or stimulated ryanodine binding to both the membrane-bound and purified ryanodine receptor (RyR) depending on the assay conditions. At high NaCl concentrations, propranolol increased the number of ryanodine-binding sites (Bmax) with no effect on the binding affinity. In the presence of 0.2 M NaCl, ryanodine binding was inhibited by propranolol. Half-maximal inhibition was obtained at 1.2 mM and complete inhibition at 2 mM propranolol. The inhibitory effect of propranolol obtained at low NaCl concentration was not restored by increasing the NaCl concentration to 1 M. 2. Modulators of the RyR that are known to alter its conformational states, such as adenine nucleotides, Ca2+ concentration and pH, modified the effect of propranolol on ryanodine binding. In the presence of propranolol and at low NaCl concentrations, ryanodine binding was inhibited and showed no Ca(2+)-, pH-, or time-dependence. 3. Propranolol immediately and completely blocked the channel opening of RyR reconstituted into a planar lipid bilayer. Propranolol-modified non-active channel was reactivated to a subconductive state (about 40% of the control conductance) by ATP. 4. Competition experiments between lidocaine (a stimulatory drug) or tetracaine (an inhibitory drug) and propranolol at 0.2 or 1.0 M NaCl, respectively, suggest the existence of different interaction sites for local anaesthetics and propranolol. 5. These results suggest that propranolol interacts directly with the RyR and modifies its ryanodine binding and single-channel activities. Propranolol effects are altered by the RyR conformational state, suggesting its possible use as a conformational probe for RyR.

Local anesthetics reduce the inhibitory neurotransmitter-induced current in dissociated hippocampal neurons of the rat

The effects of local anesthetics on amino acid-induced currents were examined using the whole-cell configuration of the patch clamp technique in dissociated hippocampal pyramidal neurons of the rat. Lidocaine (3 mM) decreased the glycine-induced Cl- current (Gly-ICl) more potently (to 46% of the control value) than the gamma-aminobutyric acid-induced Cl- current (GABA-ICl; to 75%), whereas the agent had little effect on the excitatory glutamate response. The reduction in the Gly-ICl was dose-dependent, with a dissociation constant (KD) of 3 mM and a Hill coefficient of 0.96. A non-competitive inhibition was suggested by a double reciprocal plot of the effects of lidocaine on the concentration-response curve of the Gly-ICl. Benzocaine, a neutral local anesthetic at physiological pH, decreased the Gly-ICl more potently than lidocaine, while QX314, a permanently charged quaternary derivative of lidocaine, produced a much smaller inhibition, thereby indicating that the neutral form of local anesthetics is more effective in reducing the Gly-ICl. The depression of the Gly-ICl and GABA-ICl in central neurons may contribute to local anesthetic-induced convulsions.

Inhibitory action of SKPYMRFamide on acetylcholine receptors of Helix aspersa neurons: role of second messengers

1. SKPYMRFamide, a novel FMRFamide-like endogenous peptide reversibly decreases excitatory responses (depolarization and inward current) evoked by local ionophoretic application of acetylcholine (ACh) onto the soma of identified neurons F1, F2, F4 and F5/6 of the land snail, Helix aspersa. 2. Threshold concentrations of SKPYMRFamide for an inhibitory action on ACh-induced responses are 0.5-1 mumoll-1. This modulatory action of peptide is dose- and time-dependent. 3. It is concluded that SKPYMRFamide inhibits ACh receptors through activation of specific binding sites on the plasma membrane. 4. The possible role of different second messengers in the modulatory influence of SKPYMRFamide on ACh receptors was tested using 13 modulators of different second messenger systems. 5. The results indicate that SKPYMRFamide may inhibit ACh receptors through activation of one or more of the following systems: phospholipases C, A2, NO-synthase, soluble guanylate cyclase and lipoxygenases which elevate basal intracellulal levels of NO, cGMP, arachidonic acid, acyclic eicosanoids, inositol-1,4,5-trisphosphate (I(1,4,5)P3), I(1,4,5)P3-dependent Ca(2+)-mobilization followed by activation of calmodulin and Ca2+/calmodulin-dependent protein kinase II. Protein kinases A, C and cyclic eicosanoids do not appear to participate in modulatory action of SKPYMRFamide.