4-Aminopyridine-induced increase in basal and stimulation-evoked [3H]-NA release in slices from rat hippocampus: Ca2+ sensitivity and presynaptic control

Weak sinusoidal electric fields entrain spontaneous Ca transients in the dendritic tufts of CA1 pyramidal cells in rat hippocampal slice preparations

Neurons might interact via electric fields and this notion has been referred to as ephaptic interaction. It has been shown that various types of ion channels are distributed along the dendrites and are capable of supporting generation of dendritic spikes. We hypothesized that generation of dendritic spikes play important roles in the ephaptic interactions either by amplifying the impact of electric fields or by providing current source to generate electric fields. To test if dendritic activities can be modulated by electric fields, we developed a method to monitor local Ca-transients in the dendrites of a neuronal population in acute rat hippocampal slices by applying spinning-disk confocal microscopy and multi-cell dye loading technique. In a condition in which the dendrites of CA1 pyramidal neurons show spontaneous Ca-transients due to added 50 μM 4-aminopyridine to the bathing medium and adjusted extracellular potassium concentration, we examined the impact of sinusoidal electric fields on the Ca-transients. We have found that spontaneously occurring fast-Ca-transients in the tufts of the apical dendrites of CA1 pyramidal neurons can be blocked by applying 1 μM tetrodotoxin, and that the timing of the transients become entrained to sub-threshold 1-4 Hz electric fields with an intensity as weak as 0.84 mV/mm applied parallel to the somato-dendritic axis of the neurons. The extent of entrainment increases with intensity below 5 mV/mm, but does not increase further over the range of 5-20 mV/mm. These results suggest that population of pyramidal cells might be able to detect electric fields with biologically relevant intensity by modulating the timing of dendritic spikes.

Anticonvulsant Effect of Antiaris toxicaria (Pers.) Lesch. (Moraceae) Aqueous Extract in Rodents

Antiaris toxicaria (Moraceae) was evaluated for anticonvulsant activity in rodents. Animal models used include maximal electroshock test (MEST); pentylenetetrazole-induced (PTZ) convulsions; picrotoxin-induced (PCT) convulsions; strychnine- (STR-) and 4-aminopyridine-induced convulsions. Increase in latency to seizures as well as reduction in duration and frequency of seizures indicated anticonvulsant activity. The extract was more effective in all models used except the maximal electroshock test and strychnine-induced convulsions. Antiaris toxicaria aqueous extract (200, 400, and 800 mg kg(-1)) significantly (P < 0.05 - 0.01) shortened the duration of convulsions in PTZ- and PCT-induced seizures. Delay in the onset of convulsions in the two tests was significant (P < 0.001). Reduction in the frequency of seizures was also significant (P < 0.05 - 0.001) in both tests. Antiaris further delayed the onset of seizures in 4-aminopyridine model while producing 75% protection against death in mice. Diazepam (0.1, 0.3, and 1 mg kg(-1)), carbamazepine (3, 10, and 30 mg kg(-1)), and sodium valproate (100-400 mg kg(-1)) were used as reference anticonvulsant drugs for various models. Flumazenil blocked the effect of the extract in the PTZ test significantly suggesting that Antiaris toxicaria may be acting by enhancing the effects of the GABAergic system. Antiaris toxicaria aqueous extract therefore possesses anticonvulsant activity.

Gastric emptying effect by 4-aminopyridine in patients with chronic spinal cord injury

BACKGROUND

4-Aminopyridine (4-AP) given to patients with chronic spinal cord injury (SCI) has shown beneficial effects in some somatic and autonomic functions, although patients often develop dyspeptic symptoms. 4-AP is a potassium-channel blocker capable of altering gastro-pyloric functions as demonstrated experimentally. Our objective was to examine the influence of 4-AP treatment on gastric emptying in patients with chronic SCI.

METHODS

Gastric emptying was measured by the acetaminophen absorption test in 18 patients (9 with cervical and 9 with thoracic injury), and 9 healthy volunteers. Patients received increasing oral doses, 5 mg day(-1) of oral 4-AP (5-30 mg day(-1)), for 12 weeks. Patients were studied before and at the end of the last week of 4-AP treatment, whereas healthy volunteers (without 4-AP treatment) were studied only once. Whole blood samples of 2.5 mL were drawn at 0 (before 1 g of oral acetaminophen) and at 15, 30, 45, 60, 75, 90, 105 and 120 min postdose. Acetaminophen concentration in plasma was determined by high-pressure liquid chromatography.

RESULTS

Treatment with 4-AP significantly delayed stomach emptying in patients with chronic SCI, considering the significant decreasing of acetaminophen absorption (t paired test, p <0.05). This effect did not correlate either to the level or ASIA score of the injury (linear regression correlation analysis, r(2) = 0.003 and 0.015, respectively). No significant differences were observed by comparing data of patients before 4-AP treatment with healthy volunteers.

CONCLUSIONS

4-AP intake in patients with chronic spinal cord injury significantly slowed gastric emptying regardless of level and ASIA score of the injury.

Molecular mechanisms underlying the modulation of exocytotic noradrenaline release via presynaptic receptors

The release of noradrenaline from nerve terminals is modulated by a variety of presynaptic receptors. These receptors belong to one of the following three receptor superfamilies: transmitter-gated ion channels, G protein-coupled receptors (GPCR), and membrane receptors with intracellular enzymatic activities. For representatives of each of these three superfamilies, receptor activation has been reported to cause either an enhancement or a reduction of noradrenaline release. As these receptor classes display greatly diverging structures and functions, a multitude of different molecular mechanisms are involved in the regulation of noradrenaline release via presynaptic receptors. This review gives a short overview of the presynaptic receptors on noradrenergic nerve terminals and summarizes the events involved in vesicle exocytosis in order to finally delineate the most important signaling cascades that mediate the modulation via presynaptic receptors. In addition, the interactions between the various presynaptic receptors are described and the underlying molecular mechanisms are elucidated. Together, these presynaptic signaling mechanisms form a sophisticated network that precisely adapts the amount of noradrenaline being released to a given situation.

Excitatory GABAergic synaptic potentials in the mesencephalic trigeminal nucleus of adult rat in vitro

The mesencephalic trigeminal nucleus (MesV) contains the somata of primary afferent neurons innervating masticatory muscle spindles and the periodontal membrane. MesV afferent somata are unique in receiving synaptic inputs. Intracellular recordings in coronal pontine slices from adult rats were made from MesV neurons identified by having Cs-sensitive inward rectification and pseudounipolar morphology. Stimuli near the MesV evoked either a cluster of action potentials superimposed on a postsynaptic potential (PSP) or an antidromic spike at resting membrane potential (RMP). Membrane hyperpolarization revealed that each cluster of action potentials consisted of an antidromic spike and a subsequent PSP. Evoked PSPs in slices and miniature postsynaptic currents (mPSCs) recorded using whole-cell patch in dissociated MesV neurons were resistant to glutamate antagonists and strychnine but were reversibly abolished by 40 microM bicuculline. Superfusion of 1-10 mM GABA decreased input resistance and depolarized the membrane. Reversal potentials for evoked PSPs and GABA-induced depolarizations were similar and close to that for mPSCs which matched the Cl- equilibrium potential. Thus activation of synapses on MesV somata evokes GABAergic PSPs that generate action potentials at RMP in the adult. These data also indicate that primary afferent MesV neurons can act as interneurons in the central control of mastication.

Potassium channel blockers tetraethylammonium and 4-aminopyridine fail to prevent microglial activation induced by elevated potassium concentration

The effect of potassium channel blocker tetraethylammonium and 4-aminopyridine was examined on the elevated K+ concentration-induced microglial activation on rat hippocampal slice preparations. Microglial cells were detected by immunohistochemisty with a monoclonal antibody (OX 42) raised against a type 3 complement receptor. During activation the morphology of the microglial cells changes and the staining intensity increases. The degree of microglial activation was determined by measuring the integrated optical density of the cells. Tetraethylammonium and 4-aminopyridine failed to reduce the elevated K+ concentration-induced microglial activation. Both potassium channel blockers, when applied on the hippocampal slices without K+, caused significantly increased microglial activation as compared to the control slices. In order to check whether the functional alteration of the neuronal population induced by 4-aminopyridine caused the activation of the microglial cells, Schaffer collaterals were cut to block spreading of epileptiform hyperactivity of the CA3 pyramidal cells to the CA1 region. No significant differences were found in microglial activation between the CA3 and CA1 regions, indicating that the effect of 4-aminopyridine on microglial cells is independent of the epileptiform activity caused by the drug.

Antisense knockdown of the Shaker-like Kv1.1 gene abolishes the central stimulatory effects of amphetamines in mice and rats

Amphetamine (AMPH) is an indirect sympathomimetic compound classified as a substrate-type releaser that distinguishes it from other stimulants that act as uptake 1 blockers, such as cocaine (COC). In mammals, AMPH elicits central stimulation, hypermotility, anorexia, analgesia and analeptic activity, mainly through the increase of extracellular brain dopamine (DA). The inversion of vesicular transporters and/or intravesicular alkalinization is assumed to have a role in AMPH-induced exocytosis. However, the action mechanism of this compound has not yet been completely clarified. Recent evidence on the action of AMPHs indicates potassium channel-blocking properties in peripheral tissues. We investigated the possible involvement of a Shaker-like Kv1.1 channel subtype in the central effects of AMPH, using an antisense oligodeoxyribonucleotide (aODN) that specifically and reversibly inhibits the expression of these channels in the brain. The effect of aODN pretreatments was studied by evaluating the modification of behavioral effects induced in mice through the intracerebroventricular administration of AMPH, COC, or other compounds. The aODN in mice almost completely blocked the stimulatory effects of AMPH and other releasers but was ineffective in reducing the central activity of COC. In aODN-pretreated rats a strong reduction of the AMPH, but not of the COC-stimulated DA efflux from nucleus accumbens was observed. Our results suggest that the stimulant effects of AMPH and chemically related compounds, but not COC, require the presence of functionally active Kv1.1 channels in the brain.

Effects of alpha adrenoreceptor antagonists, prazosin and. yohimbine, on intrathecal lidocaine-induced antinociception in mice

BACKGROUND

The precise mechanisms involved in the spinal analgesic effect of lidocaine are not yet clear. We previously found that lidocaine releases noradrenaline, a modulator of nociception, in rat spinal cord. Here, we attempted to clarify whether or not the noradrenaline release contributes to spinal analgesia by lidocaine.

METHODS

The effects of intrathecal injections of the alpha adrenoreceptor antagonists, prazosin (0.01-0.3 nmol) and yohimbine (0.1-3 nmol), on intrathecal 2% lidocaine were assessed using the tail-flick (TF) test in mice.

RESULTS

Lidocaine significantly increased the TF latency for 15 min. Prazosin (0.03, 0.1 or 0.3 nmol) and yohimbine (0.3, 1.0 or 3.0 nmol) significantly reduced the lidocaine-induced increase of the TF latency 10 min after injection, although both drugs showed a ceiling effect.

CONCLUSION

These results suggest that stimulation of the noradrenergic systems plays an important role in spinal analgesia by lidocaine.

Investigation of the presynaptic effects of quinine and quinidine on the release and uptake of monoamines in rat brain tissue

Quinine and quinidine are reported to potentiate the behavioural effects of serotonergic agents and monoamine uptake inhibitors. We have therefore investigated the presynaptic actions of quinine and quinidine on monoamine uptake and release in rat brain tissue in vitro. Quinidine evoked the release of [3H]5-HT, [3H]noradrenaline and [3H]dopamine from pre-loaded rat brain slices in a concentration dependent manner with EC50 values of 175, 486 and 150 microM, respectively. Quinine induced [3H]monoamine release with similar potencies. Both quinine and quinidine also inhibited the active uptake of [3H]5-HT, [3H]noradrenaline and [3H]dopamine into rat brain synaptosomes with IC50 values in the range 0.13-12.4 microM. The potency of each drug to inhibit [3H]5-HT uptake was significantly higher than that for [3H]noradrenaline or [3H]dopamine. The relative potency of quinidine compared to quinine was more marked in the case of [3H]5-HT (58-fold) than for [3H]noradrenaline (3-fold) or [3H]dopamine (4-fold). The inhibition of [3H]5-HT uptake by quinine and quinidine was competitive in nature and corresponded with the potencies of these drugs to inhibit [3H]paroxetine binding. No correlation was observed between the potencies of quinine and quinidine to induce the release of [3H]monoamines and to inhibit their uptake, suggesting that these effects are mediated by two distinct mechanisms. We conclude that the presynaptic actions of quinine and quinidine on monoamine uptake and release may be implicated in their potentiation of the effects of serotonergic agents and uptake blockers.

Differential effects of potassium channel blockers on extracellular concentrations of dopamine and 5-HT in the striatum of conscious rats

1. The selective Ca(2+)-activated K+ channel blocker apamin increased extracellular 5-hydroxytryptamine (5-HT) concentrations in the striatum when administered through the microdialysis probe at doses of 0.1 mM and 1 mM. Extracellular dopamine concentrations increased only at the highest dose administered (1 mM). 2. Mast cell degranulating peptide (MCDP), which blocks the dendrotoxin sensitive delayed rectifier (DR) current, increased extracellular concentrations of dopamine at dose of 10 microM-100 microM but had no effect on 5-HT. 3. The non selective K+ channel blocker tetraethylammonium (TEA) induced a dose-dependent (1 mM-10 mM) increase in extracellular dopamine concentrations and an increase in 5-HT which showed little or no dose-dependency. 4. 4-Aminopyridine (4-AP), a blocker with some similar characteristics to MCDP, increased extracellular dopamine concentrations at doses of 10 microM-1 mM, but had no effect on 5-HT. 5. These findings suggest that dopamine release may be modulated by DR-like current and/or A-current K+ channels. However, in view of the similar effects of MCDP and 4-AP at the concentrations used it is more likely that the dendrotoxin-sensitive DR-like current is involved. In contrast, 5-HT release appears to be modulated by Ca(2+)-activated K+ channels.

Presynaptic A2-adrenoceptors and neuropeptide Y Y2 receptors inhibit [3H]noradrenaline release from rat hypothalamic synaptosomes via different mechanisms

Presynaptic receptors may reduce transmitter release with different mechanisms. Both the alpha 2-agonist, clonidine and the Y2-agonist, neuropeptide Y fragment 13-36 (NPY 13-36), induce a concentration-dependent inhibition of the 4-aminopyridine (4-AP)-evoked [3H]noradrenaline ([3H]NA) release from hypothalamic synaptosomes. Changes in alpha 2- and Y2-modulation of noradrenaline (NA) release were observed by lowering the calcium influx with the use of omega-conotoxin (omega-CgTx), a calcium-channel blocking agent. In these experimental conditions, clonidine was less active, whereas NPY 13-36 preserved its efficacy. It therefore seems possible that presynaptic alpha 2-adrenoceptors can primarily inhibit NA release by reducing calcium influx via voltage-sensitive calcium channels (VSCC), while Y2-receptors may inhibit the intracellular release process with a mechanism independent of the calcium entry.

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.

Role of NMDA, AMPA and kainate receptors in mediating glutamate- and 4-AP-induced dopamine and acetylcholine release from rat striatal slices

Striatal slices, preincubated with [3H]dopamine and [14C]choline, were superfused continuously and subjected to electrical field stimulation (3 Hz) and perfused with amino acid analogues or 4-amino pyridine (4-AP). The released radioactivity was used to monitor release of the neurotransmitters dopamine (DA) and acetylcholine (ACh). Glutamate, NMDA (in the absence of Mg2+), AMPA, kainic acid, domoate and 4-AP all induced DA and ACh release in a concentration-dependent manner. The DA and ACh release induced by NMDA (15 microM) and glutamate (1 mM) was essentially abolished by Mg2+ (1.15 mM), whereas release induced by AMPA (100 microM), kainic acid (100 microM) or 4-AP (30 microM) was not reduced. Tetrodotoxin (1 microM) essentially abolished the effects of NMDA, markedly reduced the effects of glutamate, AMPA and 4-AP, whereas the effect of kainic acid was only modestly affected. MK-801 (30 nM) reduced NMDA-induced DA release by some 70% and ACh release by 30%. MK-801 reduced 4-AP-induced DA release by 40% but not ACh release. CNQX in a concentration (10 microM) that scarcely affected NMDA-induced ACh release, but blocked that induced by AMPA, kainic acid or domoate, reduced the ACh release induced by 4-AP. In summary, DA and ACh release from rat striatum can be stimulated by activation of NMDA and non-NMDA glutamate receptors, and this mechanism is activated by the potassium channel blocker 4-AP.

On the usefulness of Fura-2 measurements of intrasynaptosomal calcium levels in rat cortical synaptosomes to study mechanisms of presynaptic function

Levels of [Ca2+]i in rat cortex synaptosomes were measured using the Ca2+ indicator Fura-2. Ca2+ influx was induced by veratridine in a concentration-dependent manner (1-10 microM). The resulting increase in [Ca2+]i was inhibited by tetrodotoxin (TTX). K+ (18 mM) increased the [Ca2+]i which was not influenced by TTX. K(+)-channel blockers such as 4-aminopyridine, alpha- and delta-dendrotoxin pre se were ineffective. The veratridine-induced Ca2+ influx in synaptosomes was reduced by L-type Ca(2+)-channel blockers, such as felodipine, nifedipine and PN-200-110, verapamil and diltiazem. omega-Conotoxin, and N-type Ca(2+)-channel blocker, did not inhibit the veratridine-stimulated [Ca2+]i increase. Bay K 8644, and L-channel agonist, stimulated an increase of [Ca2+]i in synaptosomes which was not sensitive to TTX. R-N6-Phenyl-isopropyl-adenosine (R-PIA) and clonidine, agonists at adenosine A1-receptors and alpha 2-adrenoceptors, respectively, did not influence the veratridine-stimulated [Ca2+]i increase. R-PIA did not interact with Bay K 8644-stimulated [Ca2+]i increase in synaptosomes. The results for all the substances used show major differences between the effects on Ca2+ influx in synaptosomes and on the electrically evoked neurotransmitter release in slice preparations. Thus, the synaptosome preparation is not a generally applicable experimental model for the study of Ca2+ mechanisms of presynaptic neuromodulation.

Different effects of the K+ channel blockers 4-aminopyridine and charybdotoxin on sensory nerves in guinea-pig lung

In the isolated guinea-pig bronchus, the potassium channel blocking agent 4-aminopyridine (10(-4) M) caused a contraction which was abolished by capsaicin tachyphylaxis, suggesting involvement of sensory neuropeptides. Charybdotoxin (10(-8), 5 x 10(-8) M), which is a potent blocker of the high-conductance Ca(2+)-activated K+ channel in smooth muscle, caused slowly developing and long lasting bronchoconstriction, which was resistant to capsaicin tachyphylaxis. Neither 4-aminopyridine (10(-3), 10(-4) M) nor charybdotoxin (10(-8), 5 x 10(-8) M) had any significant effect on the bronchoconstriction induced by electrical field stimulation. Furthermore, charybdotoxin had no significant influence on the inhibitory effect of the alpha 2-adrenoceptor agonist SKF 35886 (5 x 10(-7) M) on the bronchoconstriction induced by electrical field stimulation. In the isolated perfused guinea-pig lung, 4-aminopyridine (3 x 10(-5) -10(-3) M) caused bronchoconstriction and enhanced both basal and (at 3 x 10(-5) M) vagal nerve stimulation-evoked calcitonin gene-related peptide outflow from pulmonary sensory nerves. In conclusion, 4-aminopyridine stimulated capsaicin-sensitive sensory neurons and enhanced the sensory activation induced by vagal nerve stimulation in guinea-pig lung. Charybdotoxin, on the other hand, caused bronchial contraction independently of capsaicin-sensitive nerves.

Effect of K+ channel blockers on the alpha 2-adrenoceptor-coupled regulation of electrically evoked noradrenaline release in hippocampus

The question whether presynaptic alpha 2-adrenoceptors regulating noradrenaline release in hippocampus directly couple to tetraethylammonium chloride (TEA) or alpha-dendrotoxin (alpha-DTX)-sensitive K+ channels was investigated. Hippocampal slices, prelabelled with [3H] noradrenaline, were superfused in the presence of (+)-oxaprotiline and electrically stimulated with 4 pulses delivered at 100 Hz, in order to avoid autoinhibition due to released noradrenaline. TEA enhanced the evoked [3H]noradrenaline release in rabbit hippocampus in a concentration-dependent manner, yielding an approximately 4-fold increase at 30 mmol/l, whereas the spontaneous outflow of tritium was only slightly affected at this concentration. The alpha 2-adrenoceptor agonist clonidine, at 10-100 nmol/l inhibited the evoked [3H]noradrenaline release between 77% and 96%. The inhibitory effect of the alpha 2-agonist was distinctly diminished in the presence of 30 mmol/l TEA but was restored in low Ca2+/high Mg2+ buffer. Therefore, the diminution of the alpha 2-agonist effect by TEA observed in experiments with normal Ca2+ can be explained by an increase of the Ca2+ availability for the release process due to the prolongation of action potentials. In rabbit hippocampus alpha-DTX (10-200 nmol/l) did neither affect the evoked release of [3H]noradrenaline nor its alpha 2-agonist-induced modulation. However, in rat hippocampus alpha-DTX significantly increased the evoked transmitter release and diminished the effect of clonidine. Taken together, the present data for the rabbit hippocampus exclude the possibility that activation of presynaptic alpha 2-adrenoceptors inhibits depolarization-evoked [3H]noradrenaline release by inducing an outward K+ current through TEA- or alpha-DTX-sensitive K+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of an intracellular calcium chelator on the regulation of electrically evoked [3H]-noradrenaline release from rat hippocampal slices

1. The electrically (3 Hz, 5 min) evoked [3H]-noradrenaline ([3H]-NA) release from rat hippocampal slices was reduced by prior treatment of the slices with 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetraacetomethylester (BAPTA/AM) in a concentration-(10 to 500 microM) dependent manner (40% at 30 microM). Reduction of medium calcium from 1.3 to 0.5 mM caused a larger (70%) decrease. BAPTA free acid (100 mM), a non-permeable Ca(2+)-chelator had no significant effect. 2. Basal [3H]-noradrenaline release was reduced by BAPTA/AM in a concentration-dependent manner (50% at 30 microM), but reduction of external Ca2+ from 1.3 to 0.5 mM did not alter basal release. 3. About 10% of total [3H]-NA in the slices was released at 3 Hz stimulation in 1.3 mM Ca2+ buffer. Addition of the alpha 2-adrenoceptor antagonist, idazoxan (1 microM), increased electrically evoked [3H]-NA release to 26% but stimulated release was not altered by the adenosine A1-receptor antagonist, 8-cyclopentyl theophylline (8-CPT) (1 microM). 4. Evoked release was reduced by the alpha 2-receptor agonist, UK 14,304, in a concentration-dependent manner in the presence of 8-CPT (1 microM). The magnitude of this effect was not altered by the treatment of slices with 30 microM BAPTA/AM. 5. The adenosine A1 receptor agonist, N6-cyclohexyl adenosine (CHA) (1 microM) inhibited electrically evoked [3H]-NA release by about 40% in the presence of idazoxan (1 microM). The effect of CHA was not significantly altered by treatment of slices with BAPTA/AM. 7. The present results show that spontaneous [3H]-NA release is affected by reduction of intracellular Ca2+, but not by reduction of extracellular Ca2+ or by the presynaptic agonists or w-conotoxin. By contrast, electrically evoked release was affected more strongly by alterations of extracellular Ca2+ than by buffering intracellular Ca2+. The reduction of electrically evoked [3H]-NA release by agonists at the adenosine Al-receptor and a2-adrenoceptor is probably mediated through the control of Ca2+ entry via membrane ion channels or at a low affinity Ca2'-site governing evoked release.

Amitriptyline inhibits striatal efflux of neurotransmitters via blockade of voltage-dependent Na+ channels

Amitriptyline, a tricyclic antidepressant, almost completely inhibited veratridine- or scorpion toxin-evoked efflux of endogenous dopamine (DA) and gamma-aminobutyric acid (GABA) from rat striatal slices without any effects of 30 mM K+ or 4-aminopyridine on basal and evoked efflux. The effects of amitriptyline on glutamate efflux were comparable to those on DA or GABA efflux. These effects of amitriptyline can be well explained by its blockade of voltage-dependent Na+ channels and may be independent of other activities of this drug such as re-uptake inhibition of monoamines and blockade of K+ channels.

Effect of depolarizing agents on the Ca(2+)-independent and Ca(2+)-dependent release of [3H]GABA from sheep brain synaptosomes

The purpose of the present study was to compare the effects of several depolarizing agents on both the membrane potential and on the release of [3H] gamma-aminobutyric acid (GABA) from sheep brain cortex synaptosomes. We examined the effects of KCl, 4-aminopyridine (4-AP), veratridine, ouabain and tetraphenylphosphonium cation (TPP+) on Ca(2+)-independent (carrier-mediated) and Ca(2+)-dependent (exocytotic) release. We found that, in the absence of Ca2+, KCl at 40 mM releases 7.57 +/- 0.65%, veratridine at 50 microM releases 45.85 +/- 2.48%, ouabain at 1 mM releases 8.62 +/- 0.93% and TPP+ at 1 mM releases 4.09 +/- 0.37% of the total accumulated neurotransmitter, provided that the external medium contains Na+. These are about the maximal values of release obtained with each depolarizing agent in a Na+ medium and in the absence of Ca2+. Replacing external Na+ with choline blocks the release observed in the presence of the depolarizing agents in the absence of Ca2+, and this divalent ion can increase [3H]GABA release only for K+ or 4-AP. Synaptosomal depolarization requires Na+ except for K+ depolarization. Furthermore, although Ca2+ stimulates the release of [3H]GABA due to K+ depolarization (13.56 +/- 0.44%) or due to 4-AP (4.26 +/- 0.51%), it inhibits the release due to the other depolarizing agents. The amount of [3H]GABA released by 4-AP in Na+ medium (4.26 +/- 0.51%) is similar to that induced by KCl in the presence of Ca2+ in the absence of Na+ (3.39 +/- 0.29%) which represents only exocytotic release. This suggests that the Ca(2+)-dependent exocytotic release of [3H]GABA can be specifically induced by 4-AP in a Na+ medium, or by KCl in the absence of Na+, as reported by us earlier. The observation that Ca2+ inhibits the Ca(2+)-independent release is of interest because it suggests that Ca2+ may modulate the release of cytoplasmic GABA probably by inhibiting the carrier-mediated release of GABA. It is of interest as to whether Ca2+ regulation depends on intracellular Ca2+.

Comparison of the effects of four dendrotoxin peptides, 4-aminopyridine and tetraethylammonium on the electrically evoked [3H]noradrenaline release from rat hippocampus

We have examined the effects of four dendrotoxin (DaTX) peptides, alpha-, beta-, gamma- and delta-DaTX, separated from the venom of the green mamba (Dendroaspis angusticeps), on field stimulation-evoked [3H]noradrenaline (NA) release from rat hippocampus and compared their effects with those of two other inhibitors of K+ channels, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). 4-AP (10-300 microM) and TEA (0.1-5 mM) facilitated the evoked [3H]NA release in a concentration-dependent manner. The evoked [3H]NA release was reduced to about half by alpha 2-adrenoceptor stimulation (UK 14,304; 100 nM) and this reduction was antagonized by 4-AP (10-100 microM), whereas TEA even at 5 mM was a poor inhibitor of alpha 2-effects. alpha-DaTX (10-200 nM) mimicked 4-AP in increasing the electrically evoked [3H]NA release and diminishing the inhibitory effects of UK 14,304 in a concentration-dependent manner. delta-DaTX did not itself alter the electrically evoked [3H]NA release, but at 200 nM, it reduced the effects of alpha 2-receptor stimulation. beta- and gamma-DaTX (up to 200 nM) had no significant effects. 4-AP, 3,4-diaminopyridine (3,4-DAP), TEA and the four dendrotoxins displaced the binding of [3H]p-aminoclonidine ([3H]PAC) from alpha 2-receptors. The IC50 values were 6.6 x 10(-4), 1.42 x 10(-3), 5.6 x 10(-2) for 4-AP, 3,4-DAP and TEA, respectively, and 3.19 x 10(-6) M for alpha-DaTX. Thus, their potency as inhibitors of alpha 2-receptors is apparently too low to account alone for the antagonism by K+ channel inhibitors of alpha 2-effects on NA release.(ABSTRACT TRUNCATED AT 250 WORDS)