Dendrotoxin, 4-aminopyridine, and beta-bungarotoxin act at common loci but by two distinct mechanisms to induce Ca2+-dependent release of glutamate from guinea-pig cerebrocortical synaptosomes

Diverse β-bungarotoxin isoforms manifest different affinities to voltage-gated potassium channels of Kv1.x subfamily

β-Bungarotoxins (β-BuTx), consisting of covalently bound phospholipase A2 subunit (A-chain), a member of group Ia of secretory phospholipases A2, and non-enzymatic subunit (B-chain) structurally related to Kunitz-type protease inhibitors, block presynaptic neuromuscular transmission via a not completely defined mechanism of action. In vivo physiological studies revealed that the B-chain is targeting voltage-gated potassium channels of not identified subtypes. In our work, six β-BuTx isoforms were isolated from Bungarus multicinctus krait venom and characterized by mass spectrometry revealing that isoforms differ in the A- and B-chain composition. Their secondary structures determined by a circular dichroism spectroscopy were similar, while phospholipase activities differed by 1.4-2.6 times between isoforms. The β-BuTx isoforms were found to bind to the extracellular pore blocker binding site of human Kv1.1, Kv1.3 and Kv1.6 channels with submicromolar and micromolar affinities depending on the type of constituent chains. Electrophysiology data demonstrated the ability of β-BuTx isoforms to block human Kv1.3 channels with different efficiency. Though the properties of β-BuTx can depend on their subunit composition, the presented data identify human Kv1.1, Kv1.3 and Kv1.6 channels as pharmacological targets for the particular β-BuTx isoforms.

Albiflorin Decreases Glutamate Release from Rat Cerebral Cortex Nerve Terminals (Synaptosomes) through Depressing P/Q-Type Calcium Channels and Protein Kinase A Activity

The purpose of this study was to investigate whether and how albiflorin, a natural monoterpene glycoside, affects the release of glutamate, one of the most important neurotransmitters involved in neurotoxicity, from cerebrocortical nerve terminals (synaptosomes) in rats. The results showed that albiflorin reduced 4-aminopyridine (4-AP)-elicited glutamate release from synaptosomes, which was abrogated in the absence of extracellular Ca2+ or in the presence of the vesicular glutamate transporter inhibitor or a P/Q-type Ca2+ channel inhibitor, indicating a mechanism of action involving Ca2+-dependent depression of vesicular exocytotic glutamate release. Albiflorin failed to alter the increase in the fluorescence intensity of 3,3-diethylthiacarbocyanine iodide (DiSC3(5)), a membrane-potential-sensitive dye. In addition, the suppression of protein kinase A (PKA) abolished the effect of albiflorin on glutamate release. Albiflorin also reduced the phosphorylation of PKA and synaptosomal-associated protein of 25 kDa (SNAP-25) and synapsin I at PKA-specific residues, which correlated with decreased available synaptic vesicles. The results of transmission electron microscopy (TEM) also observed that albiflorin reduces the release competence of synaptic vesicles evoked by 4-AP in synaptosomes. In conclusion, by studying synaptosomally released glutamate, we suggested that albiflorin reduces vesicular exocytotic glutamate release by decreasing extracellular Ca2+ entry via P/Q-type Ca2+ channels and reducing PKA-mediated synapsin I and SNAP-25 phosphorylation.

Neuronal Voltage Gated Potassium Channels May Modulate Nitric Oxide Synthesis in Corpus Cavernosum

Potassium channels (K⁺Ch) in corpus cavernosum play an important role in the regulation of erection. Nitric oxide (NO) acts through opening of K⁺Ch leading to hyperpolarization and relaxation.

Aim : This study aims to update knowledge about the role of voltage-gated K⁺Ch (K_V) channels in erectile machinery and investigate their role in the control of NO action &/or synthesis in the corpus cavernosum.

Methods : Tension studies using isolated rabbit corpus cavernosum (CC) strips and rat anococcygeus muscle were conducted. Results are expressed as mean ± SEM.

Results : Electric field stimulation (EFS, 2–16 Hz) evoked frequency-dependent relaxations of the PE (phenylephrine)-precontracted CC strips. At 2 Hz, EFS-induced relaxation amounted to 73.17 ± 2.55% in presence 4-AP (10⁻³ M) compared to 41.98 ± 1.45% as control. None of the other selective K⁺Ch blockers tested inhibited EFS-induced relaxation. 4-AP (10⁻³M) significantly attenuated ACh-induced relaxation of rabbit CC where dose-response curve was clearly shifted upward, and attenuated SNP- induced relaxation, for example, to 49.28 ± 4.52% compared to 65.53 ± 3.01% as control at 10⁻⁶ M SNP. The potentiatory effect of 4-AP on EFS was abolished or reversed in presence of N^G-nitro-L-arginine (L-NNA, non-selective nitric oxide synthase inhibitor, 10⁻⁵M, and 2 × 10⁻⁴M). Same results were observed in rat anococcygeus muscle which is a part of the erectile machinery in rats.

Conclusion : This study provides evidence for the presence of prejunctional voltage-gated K⁺Ch in CC, the blockade of which may increase the neuronal synthesis of NO.

Gambierol inhibition of voltage-gated potassium channels augments spontaneous Ca2+ oscillations in cerebrocortical neurons

Gambierol is a marine polycyclic ether toxin produced by the marine dinoflagellate Gambierdiscus toxicus and is a member of the ciguatoxin toxin family. Gambierol has been demonstrated to be either a low-efficacy partial agonist/antagonist of voltage-gated sodium channels or a potent blocker of voltage-gated potassium channels (Kvs). Here we examined the influence of gambierol on intact cerebrocortical neurons. We found that gambierol produced both a concentration-dependent augmentation of spontaneous Ca(2+) oscillations, and an inhibition of Kv channel function with similar potencies. In addition, an array of selective as well as universal Kv channel inhibitors mimicked gambierol in augmenting spontaneous Ca(2+) oscillations in cerebrocortical neurons. These data are consistent with a gambierol blockade of Kv channels underlying the observed increase in spontaneous Ca(2+) oscillation frequency. We also found that gambierol produced a robust stimulation of phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2). Gambierol-stimulated ERK1/2 activation was dependent on both inotropic [N-methyl-d-aspartate (NMDA)] and type I metabotropic glutamate receptors (mGluRs) inasmuch as MK-801 [NMDA receptor inhibitor; (5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate], S-(4)-CGP [S-(4)-carboxyphenylglycine], and MTEP [type I mGluR inhibitors; 3-((2-methyl-4-thiazolyl)ethynyl) pyridine] attenuated the response. In addition, 2-aminoethoxydiphenylborane, an inositol 1,4,5-trisphosphate receptor inhibitor, and U73122 (1-[6-[[(17b)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione), a phospholipase C inhibitor, both suppressed gambierol-induced ERK1/2 activation, further confirming the role of type I mGluR-mediated signaling in the observed ERK1/2 activation. Finally, we found that gambierol produced a concentration-dependent stimulation of neurite outgrowth that was mimicked by 4-aminopyridine, a universal potassium channel inhibitor. Considered together, these data demonstrate that gambierol alters both Ca(2+) signaling and neurite outgrowth in cerebrocortical neurons as a consequence of blockade of Kv channels.

5-HT2 receptors-mediated modulation of voltage-gated K+ channels and neurophysiopathological correlates

The activity of voltage-gated K(+) channels (Kv) can be dynamically modulated by several events, including neurotransmitter stimulated biochemical cascades mediated by G protein-coupled receptors such as 5-HT2 receptors (5-HT2Rs). Activation of 5-HT2A/CR inhibits the Shaker-like K(+) channels Kv1.1 and Kv1.2, and this modulation involves the dual coordination of both RPTPα and distinct tyrosine kinases coupled to this receptor; 5-HT2Rs-mediated modulation of Kv channels controls glutamate release onto prefrontal cortex neurons that might play critical roles in neurophysiological, neurological, and psychiatric conditions. Noticeably, hallucinogens modulate Kv channel activity, acting at 5-HT2R. Hence, comprehensive knowledge of 5-HT2R signaling through modulation of distinct K(+) channels is a pivotal step in the direction that will enable scientists to discover novel 5-HT functions and dysfunctions in the brain and to identify original therapeutic targets.

Impaired spare respiratory capacity in cortical synaptosomes from Sod2 null mice

Presynaptic nerve terminals require high levels of ATP for the maintenance of synaptic function. Failure of synaptic mitochondria to generate adequate ATP has been implicated as a causative event preceding the loss of synaptic networks in neurodegenerative disease. Endogenous oxidative stress has often been postulated as an etiological basis for this pathology, but has been difficult to test in vivo. Inactivation of the superoxide dismutase gene (Sod2) encoding the chief defense enzyme against mitochondrial superoxide radicals results in neonatal lethality. However, intervention with an SOD mimetic extends the life span of this model and uncovers a neurodegenerative phenotype providing a unique model for the examination of in vivo oxidative stress. We present here studies on synaptic termini isolated from the frontal cortex of Sod2 null mice demonstrating impaired bioenergetic function as a result of mitochondrial oxidative stress. Cortical synaptosomes from Sod2 null mice demonstrate a severe decline in mitochondrial spare respiratory capacity in response to physiological demand induced by mitochondrial respiratory chain uncoupling with FCCP or by plasma membrane depolarization induced by 4-aminopyridine treatment. However, Sod2 null animals compensate for impaired oxidative metabolism in part by the Pasteur effect allowing for normal neurotransmitter release at the synapse, setting up a potentially detrimental energetic paradigm. The results of this study demonstrate that high-throughput respirometry is a facile method for analyzing specific regions of the brain in transgenic models and can uncover bioenergetic deficits in subcellular regions due to endogenous oxidative stress.

Bioenergetic analysis of isolated cerebrocortical nerve terminals on a microgram scale: spare respiratory capacity and stochastic mitochondrial failure

Pre-synaptic nerve terminals (synaptosomes) require ATP for neurotransmitter exocytosis and recovery and for ionic homeostasis, and are consequently abundantly furnished with mitochondria. Pre-synaptic mitochondrial dysfunction is implicated in a variety of neurodegenerative disorders, although there is no precise definition of the term 'dysfunction'. In this study, we test the hypothesis that partial restriction of electron transport through Complexes I and II in synaptosomes to mimic possible defects associated with Parkinson's and Huntington's diseases respectively, sensitizes individual terminals to mitochondrial depolarization under conditions of enhanced proton current utilization, even though these stresses are within the respiratory capacity of the synaptosomes when averaged over the entire population. We combine two novel techniques, firstly using a modification of a plate-based respiration and glycolysis assay that requires only microgram quantities of synaptosomal protein, and secondly developing an improved method for fluorescent imaging and statistical analysis of single synaptosomes. Conditions are defined for optimal substrate supply to the in situ mitochondria within mouse cerebrocortical synaptosomes, and the energetic demands of ion cycling and action-potential firing at the plasma membrane are additionally determined.

Mechanism of β-bungarotoxin in facilitating spontaneous transmitter release at neuromuscular synapse

The mechanism of the action of beta-bungarotoxin (beta-BuTx) in the facilitation of spontaneous transmitter release at neuromuscular synapse was investigated in Xenopus cell culture using whole-cell patch clamp recording. Exposure of the culture to beta-BuTx dose-dependently enhances the frequency of spontaneous synaptic currents (SSCs). Buffering the rise of intracellular Ca2+ with BAPTA-AM hampered the facilitation of SSC frequency induced by beta-BuTx. The beta-BuTx-enhanced SSC frequency was reduced when the pharmacological Ca2+ -ATPase inhibitor thapsigargin was used to deplete intracellular Ca2+ store. Application of membrane-permeable inhibitors of inositol 1,4,5-trisphosphate (IP3) but not ryanodine receptors effectively occluded the increase of SSC frequency elicited by beta-BuTx. Treating cells with either wortmannin or LY294002, two structurally different inhibitors of phosphatidylinositol 3-kinase (PI3K) and with phospholipase C (PLC) inhibitor U73122, abolished the beta-BuTx-induced facilitation of synaptic transmission. The beta-BuTx-induced synaptic facilitation was completely abolished while there was presynaptic loading of the motoneuron with GDPbetaS, a non-hydrolyzable GDP analogue and inhibitor of G protein. Taken collectively, these results suggest that beta-BuTx elicits Ca2+ release from the IP3 sensitive intracellular Ca2+ stores of the presynaptic nerve terminal. This is done via PI3K/PLC signaling cascades and G protein activation, leading to an enhancement of spontaneous transmitter release.

PP2 inhibits glutamate release from nerve endings by affecting vesicle mobilization

Src kinase is widely expressed in the brain and its inhibition with PP2 has previously been shown to depress depolarization-evoked glutamate release from rat cerebrocortical synaptosomes by reducing voltage-dependent Ca2+ entry. In this study, we further showed that the inhibitory effect of PP2 on 4-aminopyridine-evoked glutamate release results from a reduction of vesicular exocytosis and not from an inhibition of non-vesicular release. In addition, PP2 significantly inhibited ionomycin-induced or hypertonic sucrose-induced glutamate release. Also, disruption of cytoskeleton organization with cytochalasin D occluded the inhibitory action of PP2 on 4-aminopyridine and ionomycin-evoked glutamate release. These results suggest that PP2-mediated inhibition of glutamate release involves the modulation of some exocytotic steps, possibly through a regulation of actin cytoskeleton dynamics.

Voltage-dependent ebselen and diorganochalcogenides inhibition of 45Ca2+ influx into brain synaptosomes

By mediating the Ca(2+) influx, Ca(2+) channels play a central role in neurotransmission. Chemical agents that potentially interfere with Ca(2+) homeostasis are potential toxic agents. In the present investigation, changes in Ca(2+) influx into synaptosomes by organic forms of selenium and tellurium were examined under nondepolarizing and depolarizing conditions induced by high KCl concentration (135 mM) or by 4-aminopyridine (4-AP). Under nondepolarizing conditions, ebselen (400 micro M) increased Ca(2+) influx; diphenyl ditelluride (40-400 micro M) decreased Ca(2+) in all concentrations tested; and diphenyl diselenide decreased Ca(2+) influx at 40 and 100 micro M, but had no effect at 400 micro M. In the presence of KCl as depolarizing agent, ebselen and diphenyl ditelluride decreased Ca(2+) influx in a linear fashion. In contrast, diphenyl diselenide did not modify Ca(2+) influx into isolated nerve terminals. In the presence of 4-AP (3 mM) as depolarizing agent, ebselen (400 micro M) caused a significant increase, whereas diphenyl diselenide and diphenyl ditelluride inhibited Ca(2+) influx into synaptosomes. The results can be explained by the fact that the mechanism through which 4-AP and high K(+) induced elevation of intracellular Ca(2+) is not exactly coincident. The mechanism by which diphenyl ditelluride and ebselen interact with Ca(2+) channel is unknown, but may be related to reactivity with critical sulfhydryl groups in the protein complex. The results of the present study indicate that the effects of organochalcogenides were rather complex depending on the condition and the depolarizing agent used.

Bioenergetics and transmitter release in the isolated nerve terminal

The isolated nerve terminal (or synaptosome) is the simplest preparation that allows mitochondrial bioenergetics to be studied in a physiological milieu, as well as facilitating investigation of the protein chemistry and regulation of synaptic vesicle exocytosis and recovery and providing a target for the study of the mechanism of action of numerous neurotoxins. This brief review discusses studies from our laboratory that may have provided some insight into these aspects of nerve terminal function.

Seizure, neurotransmitter release, and gene expression are closely related in the striatum of 4-aminopyridine-treated rats

The present experiments aimed to compare the length of seizure activity with the time-related increase of transmitter release and the induction of c-fos gene expression in the striatum of the rat. Anesthetized Wistar rats were intraperitoneally treated with 7 mg/kg 4-aminopyridine, and the transmitter levels in the striatum were measured by means of in vivo microdialysis, 30, 60, 90, 120, and 150 min following the treatment. Striatal and neocortical electric activity was monitored with depth and surface electrodes, respectively. The expression level of the c-fos gene was estimated by counting the striatal c-fos-immunostained cell nuclei at the time intervals of the microdialysis. 4-aminopyridine elicited high-frequency seizure discharges in the EEG and significantly increased glutamate, aspartate, GABA, serotonin, noradrenaline, and dopamine levels in the extracellular dialysates. The number of c-fos-stained cell nuclei in the striatum displayed a prolonged increase, showing significantly elevated numbers throughout the experiment. The increase of c-fos expression in time correlated best with the increase of glutamate release, which was also significantly elevated at every sampling time. The GABA release, culminating at 60 min after the seizure onset, correlated best with the cessation of the electrographic seizure. Aspartate, norepinephrine, serotonin, and dopamine displayed transient but significant elevations. We conclude that glutamate plays the essential role (most probably through ionotropic and metabotropic receptors) in the extracellular signaling, which eventually leads to intracellular cascades and c-fos gene expression in the striatum during convulsions.

Suramin inhibits beta-bungarotoxin-induced activation of N-methyl-D-aspartate receptors and cytotoxicity in primary neurons

We demonstrated that beta-bungarotoxin (beta-BuTX), a snake presynaptic neurotoxin, exhibited a potent cytotoxic effect on cultured cerebellar granule neurons. The mechanism of action of beta-BuTX and the cytoprotective agents against beta-BuTX were studied. The neuronal death of cerebellar granule neurons induced by beta-BuTX was manifested with apoptosis and necrosis processes as revealed by neurite fragmentation, morphological alterations, and staining apoptotic bodies with the fluorescent dye Hoechst 33258. By means of microspectrofluorimetry and fura-2, we measured intracellular Ca2+ concentration, [Ca2+]i and found that [Ca2+]i was increased markedly prior to the morphological changes and cytotoxicity. The downstream pathway of the increased [Ca2+]i was investigated: there was increased production of free radicals, decreased mitochondrial membrane potential, and depleted cellular ATP content. MK801 and suramin effectively suppressed these detrimental effects of beta-BuTX. Furthermore, the [3H]MK801 binding was reduced by unlabeled MK801, beta-BuTX, and suramin. Thus, activation of N-methyl-D-aspartate (NMDA) receptors appeared to play a crucial role in the cytotoxic effects following betaBuTX exposure. In conclusion, the novel finding of this study was that a polypeptide beta-BuTX exerted a potent cytotoxic effect through sequential events, including activating NMDA receptors followed by increasing [Ca2+]i, ROS production, and impaired mitochondrial energy metabolism. Suramin, clinically used as a trypanocidal agent, was an effective antagonist against beta-BuTX. Data suggest that suramin might have value to detect the possible pathway of certain neuropathological disorders.

Neuronal death signaling by beta-bungarotoxin through the activation of the N-methyl-D-aspartate (NMDA) receptor and L-type calcium channel

The aim of this study was to elucidate the mechanism of the neurotoxic effect of beta-bungarotoxin (beta-BuTX, a snake presynaptic neurotoxin isolated from the venom of Bungarus multicinctus) on cultured cerebellar granule neurons. beta-BuTX exerted a potent, time-dependent, neurotoxic effect on mature granule neurons. Mature neurons, with an abundance of neurite outgrowths, were obtained after 7-8 days in culture. By means of microspectrofluorimetry and fura-2, we measured the intracellular Ca(2+) concentration ([Ca(2+)](i)) and found it to be increased markedly. BAPTA-AM [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tertrakis(acetoxymethyl ester)], EGTA, MK801 (dizocilpine maleate), and diltiazem prevented not only the elevation of [Ca(2+)](i), but also the beta-BuTX-induced neurotoxic effect. The signaling pathway involved in the elevation of [Ca(2+)](i) in beta-BuTX-induced neurotoxicity was studied. The results obtained indicated that beta-BuTX initially increased the production of reactive oxygen species and subsequently reduced mitochondrial membrane potential and depleted ATP. All of these events in the signaling pathway were blocked by MK801, diltiazem, EGTA, and BAPTA-AM. These findings suggest that the neurotoxic effect of beta-BuTX is mediated, at least in part, by a cascade of events that include the direct or indirect activation of N-methyl-D-aspartate (NMDA) receptors and L-type calcium channels that, in turn, lead to Ca(2+) influx, oxidative stress, mitochondrial dysfunction, and ATP depletion. Therefore, we suggest that this polypeptide neurotoxin, as a result of its high potency and irreversible properties, is a useful tool to elucidate the mechanisms of neurodegenerative diseases.

Long-term lithium treatment prevents neurotoxic effects of beta-bungarotoxin in primary cultured neurons

Lithium is the most commonly used drug for the treatment of manic-depressive illness. The precise mechanisms underlying its clinical efficacy remain unknown. In this study, we found that long-term exposure to lithium chloride protected cultured cerebellar granule neurons (CGNs) against beta-bungarotoxin (beta-BuTX)-induced neurotoxicity. This neuroprotection was exhibited at the therapeutically relevant concentration of 1.2 mM lithium. Pretreatments for 3-5 days (long-term) were required for protection to occur; but a 3 hr treatment (short-term) was ineffective. In contrast, a longer treatment for 6-7 days or a higher concentration of 3 mM lithium led not only to loss of the neuroprotective effect but also to a neurotoxic effect. These findings suggest that lithium protection is limited to its narrow window of concentration and apparently relevant to its narrow therapeutic index in clinical application. Measurement of intracellular calcium [Ca(2+)](i) revealed that neurotoxic concentrations of beta-BuTX markedly increased [Ca(2+)](i), which could be attenuated by long-term, but not short-term, lithium treatment. Thus, the protection induced by lithium in CGNs was attributed to its inhibition of calcium overload. In addition, the Ca(2+) signaling pathway, including reactive oxygen species production and mitochondrial membrane potential reduction, along with the neurotoxic effect of beta-BuTX was blocked by long-term, but not short-term, lithium treatment. All of these results indicate that a crucial step for lithium protection is modulation of [Ca(2+)](i) homeostasis and that lithium neurotoxicity possibly, at least in part, is due to calcium overload. In conclusion, our results suggest that lithium, in addition to its use in treatment of bipolar depressive illness, may have an expanded use in intervention for neurotoxicity.

Paired pulse facilitation is turned into paired pulse depression in hippocampal slices after epilepsy induced by 4-aminopyridine in vivo

Modifications in synaptic plasticity seem to play a key role in the origin and persistence of epilepsy. 4-Aminopyridine (4-AP) induces intense and long lasting epileptic seizures and neurodegeneration when applied into the hippocampus in vivo, effects that seem to be mediated by overactivation of glutamate receptors due to the enhancement of glutamate release from nerve endings. We have studied presynaptic modifications of CA1 responses, using the paired pulse paradigm, in hippocampal slices obtained from 4-AP-treated rats killed during epileptic activity (ex vivo). The paired pulse facilitation (PPF) observed in control slices with interstimulus intervals of 10-30 ms was changed into paired pulse depression (PPD) after 100 microM 4-AP added in vitro. A strikingly similar change was observed in the ex vivo slices even though 4-AP was no longer present in the tissue. We conclude that the facilitation of glutamate release induced by 4-AP becomes chronic after a transient exposure to the drug. This suggests that the facilitated neurotransmitter release induced by 4-AP triggers a more permanent plastic change that may be responsible for the persistence of epilepsy.

Dendrotoxin sensitive potassium channels modulate GABA but not glutamate release in the rat entorhinal cortex in vitro

We have previously shown that the anticonvulsant drug, phenytoin, increases the frequency and amplitude of spontaneous inhibitory postsynaptic currents at GABA synapses on principal neurones in the rat entorhinal cortex. This effect is similar to that seen at other GABA synapses following blockade of voltage-gated potassium channels (Kv1.1, 1.2 and 1.6) with alpha-dendrotoxin. In the present study we examined whether dendrotoxins can alter GABA release at synapses in the entorhinal cortex. We recorded spontaneous inhibitory postsynaptic currents using whole cell voltage clamp techniques in slices of rat entorhinal cortex in vitro. alpha-Dendrotoxin evoked an increase in frequency and amplitude of spontaneous inhibitory postsynaptic currents, an effect that was blocked by prior perfusion with tetrodotoxin. The effect of the toxin did not occlude the increase in spontaneous inhibitory postsynaptic currents seen with phenytoin. Indeed, the effect of the two drugs together was, at least, additive on GABA release. Perfusion with the specific Kv1.1 blocker, dendrotoxin-K had no effect on GABA release. In addition, alpha-dendrotoxin had no effect on frequency or amplitude of spontaneous excitatory postsynaptic currents at glutamate synapses on entorhinal cortex neurones. We conclude that K-channels containing the Kv1.2 and/or 1.6 subunits modulate the release of GABA, but not glutamate in the entorhinal cortex. The modulation of GABA release by phenytoin is unlikely to be due to an effect on these channels.

The role of Kv1.2-containing potassium channels in serotonin-induced glutamate release from thalamocortical terminals in rat frontal cortex

Serotonin 5-HT(2A) receptors have been implicated in psychiatric illness and the psychotomimetic effects of hallucinogens. In brain slices, focal stimulation of 5-HT(2A) receptors in rat prefrontal cortex results in dramatically increased glutamate release onto layer V pyramidal neurons, as measured by an increase in "spontaneous" (nonelectrically evoked) EPSCs. This glutamate release is blocked by tetrodotoxin (TTX) and is thought to involve local spiking in thalamocortical axon terminals; however, the detailed mechanism has remained unclear. Here, we investigate parallels in EPSCs induced by either serotonin or the potassium channel blockers 4-aminopyridine (4-AP) or alpha-dendrotoxin (DTX). DTX, a selective blocker of Kv1.1-, Kv1.2-, and Kv1.6-containing potassium channels, has been shown to release glutamate in cortical synaptosomes, presumably by inhibiting a subthreshold-activated, slowly inactivating potassium conductance. By comparing DTX with other potassium channel blockers, we found that the ability to induce EPSCs in cortical pyramidal neurons depends on affinity for Kv1.2 subunits. DTX-induced EPSCs are similar to 5-HT-induced EPSCs in terms of sensitivity to TTX and omega-agatoxin-IVA (a blocker of P-type calcium channels) and laminar selectivity. The involvement of thalamocortical terminals in DTX-induced EPSCs was confirmed by suppression of these EPSCs by micro-opiates and thalamic lesions. More directly, DTX-induced EPSCs substantially occlude those induced by 5-HT, suggesting a common mechanism of action. No occlusion by DTX was seen when EPSCs were induced by a nicotinic mechanism. These results indicate that blockade of Kv1.2-containing potassium channels is part of the mechanism underlying 5-HT-induced glutamate release from thalamocortical terminals.

What does beta-bungarotoxin do at the neuromuscular junction?

beta-Bungarotoxin from the Taiwan banded krait, Bungarus multicinctus is a basic protein (pI=9.5), with a molecular weight of 21,800 consisting of two different polypeptide subunits. A phospholipase A(2) subunit named the A-chain and a non-phospholipase A(2) subunit named the B-chain, which is homologous to Kunitz protease inhibitors. The A-chain and the B-chain are covalently linked by one disulphide bridge. On mouse hemi-diaphragm nerve-muscle preparations, partially paralysed by lowering the external Ca(2+) concentration, beta-bungarotoxin classically produces triphasic changes in the contraction responses to indirect nerve stimulation. The initial transient inhibition of twitches (phase 1) is followed by a prolonged facilitatory phase (phase 2) and finally a blocking phase (phase 3). These changes in twitch tension are mimicked, to some extent, by similar changes to end plate potential amplitude and miniature end plate potential frequency. The first and second phases are phospholipase-independent and are thought to be due to the B-chain (a dendrotoxin mimetic) binding to or near to voltage-dependent potassium channels. The last phase (phase 3) is phospholipase dependent and is probably due to phospholipase A(2)-mediated destruction of membrane phospholipids in motor nerve terminals.

Presynaptic kappa-opioid and muscarinic receptors inhibit the calcium-dependent component of evoked glutamate release from striatal synaptosomes

In addition to cytosolic efflux, reversal of excitatory amino acid (EAA) transporters evokes glutamate exocytosis from the striatum in vivo. Both kappa-opioid and muscarinic receptor agonists suppress this calcium-dependent response. These data led to the hypothesis that the calcium-independent efflux of striatal glutamate evoked by transporter reversal may activate a transsynaptic feedback loop that promotes glutamate exocytosis from thalamo- and/or corticostriatal terminals in vivo and that this activation is inhibited by presynaptic kappa and muscarinic receptors. Corollaries to this hypothesis are the predictions that agonists for these putative presynaptic receptors will selectively inhibit the calcium-dependent component of glutamate released from striatal synaptosomes, whereas the calcium-independent efflux evoked by an EAA transporter blocker, L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-PDC), will be insensitive to such receptor ligands. Here we report that a muscarinic agonist, oxotremorine (0.01-10 microM), and a kappa-opioid agonist, U-69593 (0.1-100 microM), suppressed the calcium-dependent release of glutamate that was evoked by exposing striatal synaptosomes to the potassium channel blocker 4-aminopyridine. The presynaptic inhibition produced by these ligands was concentration dependent, blocked by appropriate receptor antagonists, and not mimicked by the delta-opioid agonist [D-Pen2,5]-enkephalin. The finding that glutamate efflux evoked by L-trans-PDC from isolated striatal nerve endings was entirely calcium independent supports the notion that intact basal ganglia circuitry mediates the calcium-dependent effects of this agent on glutamate efflux in vivo. Furthermore, because muscarinic or kappa-opioid receptor activation inhibits calcium-dependent striatal glutamate release in vitro as it does in vivo, it is likely that both muscarinic and kappa receptors are inhibitory presynaptic heteroceptors expressed by striatal glutamatergic terminals.

Induction of seizures by the potent K+ channel-blocking scorpion venom peptide toxins tityustoxin-K(alpha) and pandinustoxin-K(alpha)

The scorpion venom peptide toxins tityustoxin-K(alpha) (TsTx-K(alpha)) and pandinustoxin-K(alpha) (PiTx-K(alpha)) are novel, highly potent and selective blockers of voltage-activated K+ channels. PiTx-K(alpha) preferentially blocks rapidly inactivating (A-type) K+ channels whereas TsTx-K(alpha) is selective for slowly inactivating (delayed rectifier-type) channels. K+ channel blockers are known to induce seizures, but the specific K channel types that can serve as convulsant targets are not well defined. To address this issue, we examined for convulsant activity the K+ channel type-specific scorpion toxins and the selective K+ channel antagonists 4-aminopyridine (4-AP), an inhibitor of A-type voltage-activated K+ channels, and paxilline, a selective blocker of large conductance (maxi K) Ca(2+)-activated K+ channels. Intracerebroventricular injection of recombinant TsTx-K(alpha) and PiTx-K(alpha) in mice produced limbic and clonic-tonic seizures. The severity of the seizures increased during the 60-min period following injection, culminating in continuous clonic seizure activity (status epilepticus), tonic hindlimb extension, and eventually in death. The estimated doses producing limbic and clonic seizures in 50% of animals (CD50) for TsTx-K(alpha) and PiTx-K(alpha) were 9 and 33 ng, respectively. 4-AP produced seizure activity similar to the toxins (CD50, 76 ng) whereas paxilline failed to induce seizures at doses up to 13.5 microg. Carbamazepine protected fully against the toxin- and 4-AP-induced seizures whereas phenytoin had variable activity against the clonic component although it was protective against tonic hindlimb extension. The AMPA receptor antagonist GYKI 52466 also conferred full protection against toxin-induced seizures, but the NMDA receptor antagonists (R)-CPP and dizocilpine failed to affect limbic and clonic seizures, although they protected against hindlimb extension. We conclude that selective blockade of delayed rectifier- or A-type voltage-activated K+ channels can produce limbic, clonic and tonic seizures, whereas blockade of maxi K-type Ca(2+)-activated K+ channels does not. The convulsant effects may be related to enhanced glutamate release and, in the case of the limbic and clonic convulsions, activation of AMPA receptors.

Effects of 4-aminopyridine on extracellular concentrations of glutamate in striatum of the freely moving rat

4-aminopyridine (4-AP) is a voltage-sensitive K(+)-channel blocker extensively used in in vitro experiments as a depolarizing agent for the release of glutamate (GLU). This research investigated whether 4-AP could be used in in vivo experiments using microdialysis. For that, the effects of 4-AP on the extracellular concentrations of glutamate (GLU), glutamine (GLN), taurine (TAU) and citrulline (CIT) in striatum of the freely moving rat were investigated. The effects of 4-AP were compared with those produced by perfusion with a high K+ (100 mM) medium. Intrastriatal perfusion with 4-AP (1, 5 and 10 mM) produced no effects on extracellular [GLU], [TAU] and [CIT], but decreased extracellular [GLN]. Perfusion with a high K+ (100 mM) medium increased extracellular [GLU] and [TAU], decreased extracellular [GLN], and had no effects on [CIT]. To test whether the lack of effects of 4-AP on extracellular [GLU] was due to GLU uptake mechanisms, 4-AP was perfused after a previous inhibition of GLU uptake with L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC). Under the effects of PDC (1 mM), 4-AP (1 mM) had no effects on extracellular [GLU], [TAU] and [CIT], but decreased extracellular [GLN]. These results show that 4-AP decreased extracellular [GLN] but failed to produce a significant release of GLU in striatum of the freely moving rat. Thus, 4-AP can not be used as a depolarizing agent for stimulating the release of GLU in in vivo studies using microdialysis.

Effects of excitatory amino acid antagonists on dendrotoxin-induced increases in neurotransmitter release and epileptiform bursting in rat hippocampus in vitro

Alpha-dendrotoxin (alpha-DTx), a snake venom toxin which blocks several types of fast-activating voltage-dependent potassium channels, induces limbic seizures and neuronal damage when injected into the brain. The mechanisms underlying these convulsant and neuropathological actions are not fully understood. We have studied the effects of alpha-DTx on neurotransmitter release and electrical activity in rat hippocampal brain slices and the role of excitatory amino acid receptors in mediating these actions of the toxin. alpha-DTx increased the basal release of acetylcholine, glutamate, aspartate, and GABA in a concentration-dependent manner and induced epileptiform bursting in the CA1 and CA3 regions of the slice. The increase in neurotransmitter release was evident during the first 4 min after toxin addition, whereas the bursting appeared after a concentration-dependent delay (20-40 min with 250 nM toxin). The N-methyl-D-aspartate (NMDA) receptor antagonists AP5 and MK-801 had no effect on the frequency or amplitude of dendrotoxin-induced epileptiform bursts, but the non-NMDA antagonists CNQX and DNQX abolished bursting in both CA1 and CA3 within 4-6 min. In contrast, the toxin-induced increases in neurotransmitter release were not blocked by DNQX. This study has demonstrated that, following exposure to alpha-DTx, there is a rapid increase in the release of neurotransmitters which precedes the onset of epileptiform bursting in the hippocampus. Since DNQX abolished the bursting but had no effect on the increase in neurotransmitter release, these results suggest that DNQX blocks alpha-DTx-induced epileptiform activity by antagonism of postsynaptic non-NMDA receptors.

IA-type K+ channel blockers promote survival of cortical neurons in culture: involvement of L-type Ca2+ channels

Dendrotoxin (DTX), a well characterized IA-type potassium channel blocker, directly added to the culture medium had no effect on survival of cultured cortical neurons at 6 or 14 days in vitro. On the contrary, neurons exposed to DTX remained in better condition than untreated ones. In an attempt to demonstrate the mechanisms by which DTX may affect neuronal survival we studied its effect in co-cultures of cortical neurons and astrocytes submitted to successive medium changes. After the second change of medium, at 9 days in vitro, the neuronal number in controls decreased by 43%, while in cultures receiving astrocyte-conditioned medium the cell loss was significantly reduced (15%, p < 0.01) with respect to control conditions. When DTX was added to the culture medium neuronal loss was also significantly prevented (25% for 1 microM DTX, p < 0.01) with respect to control conditions. 4-Aminopyridine (4-AP) and 21 mM K+ also preserved neurons. The L-type calcium channel antagonist nifedipine (5 microM) abolished the protective effect of DTX and 4-AP. These results show that K+ channel blockade induces protection against damage produced by repetitive medium change and that this effect is mediated by L-type Ca2+ channels.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

Presynaptic receptors and the control of glutamate exocytosis

When a typical glutamate-containing neurone fires, an action potential is propagated down the branching axon through more than a thousand varicosities. At each of these release sites the probability that a synaptic vesicle will be exocytosed into the synaptic cleft is individually controlled by means of presynaptic receptors: autoreceptors responding by positive or negative feedback to previously released transmitter, or heteroreceptors under the influence of other neurotransmitters or modulators. The simplest system in which to investigate presynaptic modulation is the isolated nerve terminal or synaptosome; studies with this preparation have revealed a complex interplay of signal-transduction pathways.

Molecular properties of voltage-gated K+ channels

Subfamilies of voltage-activated K+ channels (Kv1-4) contribute to controlling neuron excitability and the underlying functional parameters. Genes encoding the multiple alpha subunits from each of these protein groups have been cloned, expressed and the resultant distinct K+ currents characterized. The predicted amino acid sequences showed that each alpha subunit contains six putative membrane-spanning alpha-helical segments (S1-6), with one (S4) being deemed responsible for the channels' voltage sensing. Additionally, there is an H5 region, of incompletely defined structure, that traverses the membrane and forms the ion pore; residues therein responsible for K+ selectively have been identified. Susceptibility of certain K+ currents produced by the Shaker-related subfamily (Kv1) to inhibition by alpha-dendrotoxin has allowed purification of authentic K+ channels from mammalian brain. These are large (M(r) approximately 400 kD), octomeric sialoglycoproteins composed of alpha and beta subunits in a stoichiometry of (alpha)4(beta)4, with subtypes being created by combinations of subunit isoforms. Subsequent cloning of the genes for beta 1, beta 2 and beta 3 subunits revealed novel sequences for these hydrophilic proteins that are postulated to be associated with the alpha subunits on the inner side of the membrane. Coexpression of beta 1 and Kv1.4 subunits demonstrated that this auxiliary beta protein accelerates the inactivation of the K+ current, a striking effect mediate by an N-terminal moiety. Models are presented that indicate the functional domains pinpointed in the channel proteins.

N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors mediate seizures and CA1 hippocampal damage induced by dendrotoxin-K in rats

The epileptogenic and neurodegenerative effects of dendrotoxin K, from Dendroaspis polylepis, a specific blocker of a non-inactivating, voltage-sensitive K+ channel, were studied after focal injection into one dorsal hippocampus in rats. Administration of 35 pmol dendrotoxin K elicited motor seizures and bilateral electrocortical discharges after a latent period (5.3 +/- 2.1 min), in all of the treated animals (n = 6). At 24 h, histological examination of brain (n = 5) coronal sections (10 microns; n = 6 per brain) detected bilateral damage to the hippocampal formation which extended 300 microns rostral and caudal to the injection tract. Quantitation of the damage revealed significant bilateral neuronal cell loss in the CA1 and CA4 pyramidal cell layer relative to the corresponding brain regions of rats (n = 3) injected with bovine serum albumin (105 pmol), which per se was ineffective in all respects. Dendrotoxin K (35 pmol) also caused a significant loss of CA3 pyramidal neurons and dentate gyrus granule cells ipsilateral to the site of toxin injection. In one out of six rats, a lower dose (3.5 pmol) of dendrotoxin K produced convulsive behaviour and electrocortical seizures but after a longer latency and these were accompanied by significant neuronal loss in the CA1, CA3 and CA4 pyramidal cell layer ipsilateral to the injected side. The lowest dose (0.35 pmol; n = 6 rats) of dendrotoxin K used failed to induce seizures and did not cause hippocampal damage (n = 6 rats). Systemic (i.p.) treatment with dizocilpine maleate (3 mg/kg) or LY 274614 (5 mg/kg i.p.), two N-methyl-D-aspartate receptor antagonists (given 15 min beforehand), prevented dendrotoxin K (35 pmol)-induced motor seizures and electrocortical epileptogenic discharges in 100% of the animals (n = 6 per group) treated. Similarly, these antagonists minimized the damage typically produced in the rat hippocampus, with no significant neuronal loss being observed. By contrast, NBQX (30 mg/kg, i.p. given 15 min previously), a non-N-methyl-D-aspartate antagonist, failed to prevent seizures normally evoked by dendrotoxin K (35 pmol; n = 6 rats); also, this treatment was unable to abolish CA1 pyramidal cell loss but minimized the loss in hippocampal sectors distant to the site of dendrotoxin K injection. However, complete protection against motor and electrocortical seizures and hippocampal damage was afforded by GYKI 52466 (10 mg/kg i.p.; n = 6 rats), a more effective non-N-methyl-D-aspartate receptor antagonist. These findings differ from the reported lack of protection by N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor antagonists to rats receiving intra-hippocampal injection of alpha-dendrotoxin; this difference may stem from the ability of alpha-dendrotoxin to block predominantly a slowly inactivating K+ current whereas dendrotoxin K inhibits a non inactivating variant. In conclusion, the present data on dendrotoxin K, together with the previously described pattern of neurotoxicity for alpha-dendrotoxin, show that these homologues act via different mechanisms and, thus, can be used effectively as complementary tools to study seizures and neuronal cell death.

4-Aminopyridine differentially affects the spontaneous release of radiolabelled transmitters from rat brain slices in vitro

4-Aminopyridine increased the release of [3H]noradrenaline from dorsal hippocampus slices in vitro in a concentration-dependent manner. When the slices were exposed to 4-aminopyridine for 5 min, the overflow of radioactivity returned to pre-exposure values within 20-25 min. When the exposure of the slices was continued, a sustained enhancement of the release of [3H]noradrenaline was observed for the duration of the exposure. 4-Aminopyridine, 10(-4) M, had an effect of similar magnitude, or an even more pronounced effect, on the release of [3H]catecholamine from cortex, septum, periaqueductal gray and striatum slices. The effects of the compound on the release of [3H]5-hydroxytryptamine and [14C]acetylcholine were less pronounced. At this concentration 4-aminopyridine had no effect on the release of [3H]D-aspartate from hippocampus or septum slices, whereas the effect on the release of this transmitter in striatal slices was marginal. The effect of 4-aminopyridine on the release of [3H]noradrenaline in hippocampus slices was largely dependent on the presence of Ca2+ in the superfusion medium. This was also the case for the effect on the release of [3H]noradrenaline from preloaded dorsal hippocampus synaptosomes. In the presence of nitrendipine the effect of 4-aminopyridine was dose-dependently reduced, but the maximal reduction, at a nitrendipine concentration of 10(-4) M, was only 40%. Cd2+ completely abolished the effect of 4-aminopyridine on the release of [3H]noradrenaline. These results confirm that the enhancing effect of 4-aminopyridine on the release of [3H]noradrenaline depends on the entry of extracellular Ca2+ into the nerve terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

3,4-Diaminopyridine induced hydrolysis of phosphoinositide in cultured neurons from embryo chick forebrain

The effect of 3,4-diaminopyridine (DAP) on phosphoinositide hydrolysis in cultured neurons from embryo chick forebrain has been studied. DAP produced a dose- and time-dependent accumulation of inositol phosphates. At 1 mM DAP a maximal effect was obtained. In Ca2+ free medium, DAP-activated turnover of phosphoinositide was reduced, but was still significant. Blocking Ca2+ entry with 200 microM Cd2+ also did not abolish the DAP-induced accumulation of inositol phosphates. As a comparison the effect of high K+ exposure was investigated. High K+ enhanced phosphoinositide hydrolysis, and this effect was also reduced by excluding Ca2+ influx. Moreover, DAP had no additional effect on the high K(+)-induced hydrolysis of phosphoinositide. Using oxonol-V, a depolarization of the membrane potential was seen in the neurons bathed in DAP containing medium. It is suggested that the depolarization may play a role in DAP-activated phosphoinositide turnover in cultured neurons of the embryo chick forebrain, but that Ca2+ entry is not necessary for this effect.

Glutamate efflux from rat brain slices and cultures: a comparison of the depolarizing agents potassium, 4-aminopyridine, and veratrine

The major excitatory amino acid neurotransmitter in the mammalian brain is glutamate (GLU). GLU release from nerve terminals is both calcium-dependent and -independent, yet these mechanisms of release are not fully understood. Potassium, 4-aminopyridine (4-AP) and veratrine are commonly used depolarizing agents that were studied for their ability to stimulate GLU efflux from brain slices. These agents produced significant regional variations in GLU efflux from rat brain slices. Potassium was the most potent of the three secretogogues tested. 4-AP produced a significant GLU efflux only in the cerebellum. Veratrine produced consistent stimulation of GLU efflux from all brain regions tested. Potassium was the only depolarizing agent tested that stimulated GLU release from primary astroglial cultures of rat cerebral cortex. All three agents also demonstrated an ability to inhibit GLU reuptake in brain slice preparations. This data suggest that both GLU release and uptake are modulated in a regionally selective manner, and that commonly used depolarizing agents affect not only calcium-dependent neuronal release, but also uptake and glial responses.

Relation of [Ca2+]i to dopamine release in striatal synaptosomes: role of Ca2+ channels

We compared the effects of KCl and 4-aminopyridine (4-AP) stimulation on the coupling of Ca2+ channel activation to [3H]dopamine ([3H]DA) release in rat striatal synaptosomes and used specific Ca2+ channel blockers to discriminate between the different VSCC's activated by the two stimulatory agents. We found that whereas [3H]DA release is strictly Ca(2+)-dependent in the case of KCl depolarization, 4-AP, at concentrations above 100 microM, progressively causes a large Ca(2+)-independent release of [3H]DA. Thus, at 1 to 3 mM 4-AP, as much as 80-95% of the [3H]DA release is Ca(2+)-independent and can be partially blocked by nomifensine, indicating that some [3H]DA release is occurring through reversal of the DA carrier. Therefore, in the studies relating [Ca2+]i to [3H]DA release we selected 4-AP concentrations lower than 100 microM and corrected for the Ca(2+)-independent release. Under these conditions, we determined that: (1) Ca2+ entry through N-type VSCC's is involved in [3H]DA release both in the case of KCl depolarization (35% inhibition by omega-CgTx) and in 4-AP stimulation (23% inhibition by omega-CgTx); (2) Ca2+ entering through P-type and/or Q-type VSCC's is also involved in [3H]DA release due to 4-AP stimulation (26% inhibition by 200 nM omega-Aga IVA); (3) Neomycin (0.35 mM) inhibited the [3H]DA release due to 4-AP stimulation by about 20% and decreased the KCl induced [3H]DA release by 55%; the effects of neomycin (0.35 mM) and omega-CgTx were additive in both cases, indicating that, at this concentration, the antibiotic does not affect significantly N-type Ca2+ channels; (4) When applied together, omega-CgTx and omega-Aga IVA inhibited the 4-AP stimulated [3H]DA release by about 40-50%, suggesting that the remaining large fraction of the VSCC's activated by 4-AP stimulation are non-N, non-P VSCC's and are coupled to Ca(2+)-dependent [3H]DA release; (5) The contribution of L-type VSCC's is uncertain, since there seemed to be a small contribution in the case of KCl depolarization, but not in the case of 4-AP stimulation. On the whole, the results suggest that the release of [3H]DA in the rat striatal nerve terminals depends on Ca2+ entry through N-, P-, possibly Q-, and other non-N-, non-P-type VSCC's when either KCl or 4-AP stimulation is utilized.

Barium evokes glutamate release from rat brain synaptosomes by membrane depolarization: involvement of K+, Na+, and Ca2+ channels

During K(+)-induced depolarization of isolated rat brain nerve terminals (synaptosomes), 1 mM Ba2+ could substitute for 1 mM Ca2+ in evoking the release of endogenous glutamate. In addition, Ba2+ was found to evoke glutamate release in the absence of K(+)-induced depolarization. Ba2+ (1-10 mM) depolarized synaptosomes, as measured by voltage-sensitive dye fluorescence and [3H]-tetraphenylphosphonium cation distribution. Ba2+ partially inhibited the increase in synaptosomal K+ efflux produced by depolarization, as reflected by the redistribution of radiolabeled 86Rb+. The release evoked by Ba2+ was inhibited by tetrodotoxin (TTX). Using the divalent cation indicator fura-2, cytosolic [Ca2+] increased during stimulation by approximately 200 nM, but cytosolic [Ba2+] increased by more than 1 microM. Taken together, our results indicate that Ba2+ initially depolarizes synaptosomes most likely by blocking a K+ channel, which then activates TTX-sensitive Na+ channels, causing further depolarization, and finally enters synaptosomes through voltage-sensitive Ca2+ channels to evoke neurotransmitter release directly. Though Ba(2+)-evoked glutamate release was comparable in level to that obtained with K(+)-induced depolarization in the presence of Ca2+, the apparent intrasynaptosomal level of Ba2+ required for a given amount of glutamate release was found to be several-fold higher than that required of Ca2+.

Presynaptic A-current based on heteromultimeric K+ channels detected in vivo

A wide variety of voltage-gated K+ channels are involved in the regulation of neuronal excitability and synaptic transmission. Their heterogeneity arises in part from the large number of genes encoding different K+ channel subunits (reviewed in ref. 1). In addition, heterologous expression studies indicate that assembly of distinct subunits into heteromultimeric channels may contribute further to K+ channel diversity. A question has been whether heteromeric K+ channels actually form in vivo, and if so, whether specific combinations of subunits could account for major K+ currents identified in neurons. We present here biochemical evidence that Kv1.4 and Kv1.2, two K+ channel subunits of the Shaker subfamily, co-assemble in rat brain. The Kv1.4/Kv1.2 heteromultimer combines features of both parent subunits, resulting in an A-type K+ channel. Immunocytochemical evidence suggests that the heteromultimers are localized in axons and nerve terminals. We propose that Kv1.4/Kv1.2 heteromultimers may form the molecular basis of a presynaptic A-type K+ channel involved in the regulation of neurotransmitter release.

Thirty years of synaptosome research

Detached synapses (synaptosomes), first isolated by the author in 1958 and identified as such in 1960, are sealed presynaptic nerve terminals often with a portion of the target cell--sometimes amounting to a complete dendritic spine--adhering to their external surface. They can be prepared in high yield from brain tissue and also in decreasing yield from spinal cord, retina, sympathetic ganglia, myenteric plexus and electric organs. They are sealed structures which, under metabolizing conditions, respire, take up oxygen and glucose, extrude Na+, accumulate K+, maintain a normal membrane potential and, on depolarization, release transmitter in a Ca(2+)-dependent manner. They thus provide an excellent preparation with which to investigate synaptic function without the complications encountered with synapses in situ. They also serve as the parent fraction for preparations of synaptic vesicles and other synaptic components.

Barium-evoked glutamate release from guinea-pig cerebrocortical synaptosomes

Ba2+ has multiple effects on presynaptic terminals. The ion inhibits the K+ channels responsible for stabilizing the plasma membrane potential in the same way as previously reported for dendrotoxin and 4-aminopyridine. Secondly, the ion can substitute fully for Ca2+ in supporting KCl-evoked release of glutamate from guinea-pig cerebrocortical synaptosomes. In the latter case, the kinetics of glutamate release in the presence of saturating Ca2+ or Ba2+ are essentially identical. Substantially lower external concentrations of Ba2+ are required to achieve the same release kinetics as with Ca2+. The average internal free Ba2+ concentration attained during KCl depolarization is some 10-fold higher than that for Ca2+. However, because the fura-2 signal reflects predominantly the overflow of divalent cation after dissociation from the release trigger, it is not the valid parameter to compare effectiveness of the cations in triggering glutamate exocytosis. In view of the established inability of Ba2+ to interact with calmodulin, these results are discussed in relation to theories in which Ca2+/calmodulin-dependent protein kinase-mediated phosphorylation is a prerequisite for synaptic vesicle exocytosis.

Action of alpha-dendrotoxin on K+ currents in nerve terminal regions of axons in rat olfactory cortex

1. In the rat olfactory cortex, unmyelinated axons give rise to synapses en passant. This tissue was used to study the pharmacology of axonal K(+)-currents. Responses were measured from a group of these axons as unclamped field currents, with a polarizable suction electrode. 2. A single stimulus to the axons elicited a tetrodotoxin-sensitive Na(+)-dependent transient K(+)-currents were revealed by positive polarization of the suction electrode and were manifest as a negative current following the Na(+)-component. 3. In the presence of tetraethylammonium (TEA, 5 mM) and Cd2+ (100 microM), the K(+)-component was depressed by 3,4-diaminopyridine (3,4-DAP; 1 to 20 microM; IC50 2.0 +/- 0.4 microM). alpha-Dendrotoxin (DTX; 15-1500 nM) also attenuated the aminopyridine-sensitive component (IC50 93 +/- 4 nM). At the highest DTX concentration, depression of the K(+)-current was incomplete, the residual K+ current being reduced by 3,4-DAP (0.1 to 5 microM). 4. These results indicate the presence of two aminopyridine-sensitive K+ currents in this preparation distinguished by their susceptibility to DTX.

The effect of toxin I, a K+ channel inhibitor, on [3H]noradrenaline release from rat cerebral cortex

Toxin I, a K+ channel blocker found in the venom of the black mamba snake with close sequence homology to the dendrotoxins, produced a concentration-related enhancement of both spontaneous and electrically evoked [3H]noradrenaline ([3H]NA) release from slices of rat cerebral cortex. The effect of toxin I on spontaneous [3H]NA release was blocked by tetrodotoxin and reduced in the presence of either CPP ((+-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid) or CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) and was abolished in the presence of both antagonists. The results suggest that the enhancement of spontaneous [3H]NA release produced by toxin I may be mediated via the release of glutamate acting on both NMDA (N-methyl-D-aspartate) and non-NMDA receptors.

Effect of a toxin isolated from the sponge Haliclona viridis on the release of gamma-aminobutyric acid from rat olfactory bulb

A partially purified toxin from the marine sponge Haliclona viridis was studied for its effect on the presynaptic release mechanism of 3H-gamma-aminobutyric acid from nerve terminals of the external plexiform layer of rat olfactory bulb. Previously, the toxin of H. viridis was shown to block the resting potassium conductance in frog muscle. In the present study, the toxin induced a reversible release of 3H-gamma-aminobutyric acid in the external plexiform layer. This effect was similar to that induced by 25 mM K+. The toxin-induced outflow of 3H-gamma-aminobutyric acid was concentration dependent. The action of the toxin was specific for gamma-aminobutyric acid secretion from the external plexiform layer, and dopamine liberation from the frontal cortex; the toxin did not release 3H-valine, a non-neurotransmitter amino acid, from the external plexiform layer. Toxin- and high K(+)-induced neurotransmitter release were both drastically reduced when Ca2+ was removed from the saline. The addition of 0.3 microM tetrodotoxin or the removal of Na+ from the saline did not reduce the toxin's ability to release neurotransmitters. The effect of toxin was enhanced by the addition of valinomycin. Although Haliclona toxin and 4-aminopyridine induced the release of neurotransmitters, they antagonized each other's effect on gamma-aminobutyric acid secretion when added simultaneously.

Neurotensin modulates differently potassium, veratridine and 4-aminopyridine-evoked release of dopamine in rat striatal slices

We have studied the effects of neurotensin (NT) on the release of [3H]dopamine ([3H]DA) evoked by terminal depolarization with either K+, veratridine or 4-aminopyridine (4-AP). NT (1-1000 nM) induced a net potentiation (up to 170%) of the K+ (25 mM)-evoked release of [3H]DA. The capacity of NT to potentiate the effect of K+ ions decreased as the K+ concentration rose from 25 to 50 mM and totally disappeared at this high K+ concentration. NT (100 nM; 1,000 nM) had no significant effect on the veratridine (1.5; 5 microM) or 4-AP (20 microM) -evoked release of [3H]DA. The relevance of these experimental models of DA release to physiological transmitter release remains to be established. Those data highlight the complexity of the modulation of evoked neurotransmitter release by pharmacological agents.

Inhibition of phosphorylation of synapsin I and other synaptosomal proteins by beta-bungarotoxin, a phospholipase A2 neurotoxin

Some snake venom neurotoxins, such as beta-bungarotoxin (beta-BuTX), which possess relatively low phospholipase A2 (PLA2) activity, act presynaptically to alter acetylcholine (ACh) release both in the periphery and in the CNS. In investigating the mechanism of this action, we found that beta-BuTX (5 and 15 nM) inhibited phosphorylation, in both resting and depolarized synaptosomes, of a wide range of proteins, including synapsin I. Naja naja atra PLA2, which has higher PLA2 activity, also inhibited phosphorylation but was less potent than beta-BuTX. At 1 nM, beta-BuTX and N. n. atra PLA2 inhibited phosphorylation of synapsin I only in depolarized synaptosomes. Synaptosomal ATP levels were not affected by 5 or 15 nM beta-BuTX or by 5 nM N. n. atra PLA2. Limited proteolysis, using Staphylococcus aureus V-8 protease, indicated that beta-BuTX inhibited phosphorylation of synapsin I in both the head and the tail regions. The inhibition of phosphorylation was not antagonized by nordihydroguaiaretic acid or indomethacin, suggesting that arachidonic acid derivatives do not mediate this inhibition. Furthermore, inhibition of phosphorylation by beta-BuTX and N. n. atra PLA2 was not altered in the presence of the phosphatase inhibitor okadaic acid, suggesting that stimulation of phosphatase activity is not responsible for this inhibition. Inhibition of protein phosphorylation by PLA2 neurotoxins and enzymes may be associated with an inhibition of ACh release.

Production of seizures and brain damage in rats by alpha-dendrotoxin, a selective K+ channel blocker

alpha-Dendrotoxin (Dtx), a snake polypeptide, increases neuronal excitability by blocking certain fast-activating, voltage-dependent K+ channels. Thus, the behavioural, electrocortical (ECoG) and neuropathological effects of Dtx, injected into rat brain areas, were studied. A unilateral injection of 35 pmol of Dtx into the CA1 hippocampal area or the dendate gyrus (DG; upper blade) immediately produced motor and ECoG seizures, followed at 24 h by multi-focal brain damage and significant neuronal loss. Whilst brain damage was seen bilaterally, significant neuronal loss occurred only in regions (CA1, CA3, CA4 and DG) ipsilateral to the site of injection. A lower dose (3.5 pmol) of toxin elicited motor and ECoG seizures but failed to produce brain damage. Seizures were observed 50 min after injecting Dtx (35 pmol) into the amygdala, though significant neuronal loss was not evident. 4-Aminopyridine (100 nmol), given into the CA1 area elicited a similar motor and ECoG pattern to that of Dtx except no brain damage could be seen at 24 h. Systemic pretreatment with antagonists of N-methyl-D-aspartate receptors (MK-801 or CGP 37849) did not protect against the effects typically evoked by injecting Dtx into the CA1 area.

Specificity of action of beta-bungarotoxin on acetylcholine release from synaptosomes

Presynaptically acting phospholipase A2 (PLA2) neurotoxins such as beta-bungarotoxin (beta-BuTX) specifically modify the release of acetylcholine (ACh) in the periphery, whereas in the central nervous system (CNS) the release of other neurotransmitters such as norepinephrine (NE) and serotonin (5-HT) are also modified. In addition, ACh release in the periphery is modified in a triphasic manner (decrease, then increase, then block), while in the CNS only the increase has been demonstrated. To determine the specificity of the central effects of beta-BuTX we compared the effects of beta-BuTX and N. n. atra PLA2 on the release from rat cerebrocortical synaptosomes of ACh, NE, and 5-HT. We also measured the leakage of lactate dehydrogenase (LDH) in order to determine whether membrane permeablization was responsible for neurotransmitter leakage. Both the PLA2 neurotoxin (5.0 nM) and the non-neurotoxic enzyme (0.5 nM) stimulated the loss of NE and 5-HT, but only at concentrations which induced leakage of LDH. Conversely, beta-BuTX stimulated the release of ACh at a concentration (0.5 nM) which caused no leakage of LDH, while N. n. atra PLA2 (0.5 nM) did not stimulate ACh release. beta-Bungarotoxin thus exerts a specific effect on cholinergic nerve terminals, while the leakage of NE and 5-HT induced by beta-BuTX and N. n. atra PLA2 correlates with membrane disruption due to their PLA2 activities. Within 20 min, 0.5 nM beta-BuTX increased the resting release of ACh and decreased the stimulated release induced by depolarization with 4-aminopyridine, while N. n. atra PLA2 (0.5 nM) did not stimulate ACh release and required 45 min to exert an inhibitory effect. beta-BuTX (5.0 nM) also exerted an inhibitory effect on ACh release stimulated by veratridine, but not by high KCl. It is concluded that in low concentrations that do not disrupt membrane permeability, beta-BuTX acts specifically on cholinergic terminals in rat synaptosomes, where it exerts both stimulatory and inhibitory effects.