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

Neuroprotective or neurotoxic effects of 4-aminopyridine mediated by KChIP1 regulation through adjustment of Kv 4.3 potassium channels expression and GABA-mediated transmission in primary hippocampal cells

4-Aminopyridine (4-AP) is a potassium channel blocker used for the treatment of neuromuscular disorders. Otherwise, it has been described to produce a large number of adverse effects among them cell death mediated mainly by blockage of K(+) channels. However, a protective effect against cell death has also been described. On the other hand, Kv channel interacting protein 1 (KChIP1) is a neuronal calcium sensor protein that is predominantly expressed at GABAergic synapses and it has been related with modulation of K(+) channels, GABAergic transmission and cell death. According to this KChIP1 could play a key role in the protective or toxic effects induced by 4-AP. We evaluated, in wild type and KChIP1 silenced primary hippocampal neurons, the effect of 4-AP (0.25μM to 2mM) with or without semicarbazide (0.3M) co-treatment after 24h and after 14 days 4-AP alone exposure on cell viability, the effect of 4-AP (0.25μM to 2mM) on KChIP1 and Kv 4.3 potassium channels gene expression and GABAergic transmission after 24h treatment or after 14 days exposure to 4-AP (0.25μM to1μM). 4-AP induced cell death after 24h (from 1mM) and after 14 days treatment. We observed that 4-AP modulates KChIP1 which regulate Kv 4.3 channels expression and GABAergic transmission. Our study suggests that KChIP1 is a key gene that has a protective effect up to certain concentration after short-term treatment with 4-AP against induced cell injury; but this protection is erased after long term exposure, due to KChIP1 down-regulation predisposing cell to 4-AP induced damages. These data might help to explain protective and toxic effects observed after overdose and long term exposure.

Modification of ionotropic glutamate receptor-mediated processes in the rat hippocampus following repeated, brief seizures

The seizure-induced molecular and functional alterations of glutamatergic transmission in the hippocampus have been investigated. Daily repeated epileptic seizures were induced for 12 days by intraperitoneal administration of 4-aminopyridine (4-AP; 4.5 mg/kg) in adult Wistar rats. The seizure symptoms were evaluated on the Racine's scale. One day after the last injection, the brains were removed for in vitro electrophysiological experiments and immunohistochemical analysis. The glutamate receptor subunits NR1, NR2A, NR2B, GluR1, GluR1(flop), GluR2, and KA-2 were studied using the histoblotting method. The semi-quantitative analysis of subunit immunoreactivities in hippocampal layers was performed with densitometry. In the hippocampus, increase of GluR1, GluR1(flop) and NR2B immunostaining was observed in most of the areas and layers. The significant decrease of GluR2 staining intensity was observed in the CA1 and dentate gyrus. Calcium permeability of hippocampal neurons was tested by a cobalt uptake assay in hippocampal slices. The uptake of cobalt increased in the CA1 area and dentate gyrus, but not in the CA3 region following 4-AP treatment. Effects of AMPA and NMDA (N-methyl-d-aspartate) glutamate receptor antagonists (1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466) and D-APV respectively) were measured in hippocampal slices using extracellular recording. Analysis of the population spikes revealed the reduced effectiveness of the AMPA receptor antagonist GYKI 52466, while the effect of the NMDA receptor antagonist d-(2R)-amino-5-phosphonovaleric acid was similar to controls. The results demonstrated that repeated convulsions induced structural and functional changes in AMPA receptor-mediated transmission, while NMDA and kainate receptor systems displayed only alterations in receptor subunit composition.

Blockade of AMPA-receptors attenuates 4-aminopyridine seizures, decreases the activation of inhibitory neurons but is ineffective against seizure-related astrocytic swelling

The neurotransmitter glutamate plays a pivotal role in the development of the neuropathological sequelae following acute seizures. Our previous data proved the efficacy of the NMDA-receptor antagonists on the symptoms, survival and neuronal activation in the 4-aminopyridine- (4-AP) induced seizures. In this study, we examined the effects of two different doses of a non-competitive, selective, allosteric AMPA-receptor antagonist, GYKI 52466. GYKI 52466 was effective in prolonging the latency to generalised seizures and reduction of seizure mortality. However, the effects on neuronal c-fos expression and astrocyte swelling were complex. The 25mg/kg dose of GYKI 52466 was effective in reducing the c-fos immunoreactivity (IR) in the hippocampus only. In the neocortex the overall c-fos-IR cell counts were increased significantly. Investigation of the neocortical parvalbumin-containing interneuron population proved that GYKI 52466 decreased c-fos expression. The 50mg/kg dose of GYKI 52466 significantly reduced the c-fos-IR in the neo- and allocortex, not only in principal neurons, but also in the parvalbumin-positive interneurons. The GYKI 52466-pretreatment did not prevent the astrocyte swelling in the investigated cortical areas; thus we conclude that the AMPA-receptors have little if any involvement in the in the mediation of neuropathological alterations in acute convulsions.

Effects of retigabine on the neurodegeneration and extracellular glutamate changes induced by 4-aminopyridine in rat hippocampus in vivo

We have previously shown that microdialysis perfusion of the K+ channel blocker 4-aminopyridine (4-AP) in rat hippocampus induces convulsions and neurodegeneration, due to the stimulation of glutamate release from synaptic terminals. Retigabine is an opener of the KCNQ2/Q3-type K+ channel that possesses antiepileptic action and may be neuroprotective, and we have therefore studied its effect on the hyperexcitation, the neuronal damage and the changes in extracellular glutamate induced by 4-AP. Retigabine and 4-AP were co-administered by microdialysis in the hippocampus of anesthetized rats, with simultaneous recording of the EEG, and the extracellular concentration of glutamate was measured in the microdialysis fractions. In 70-80% of the rats tested retigabine reduced the 4-AP-induced stimulation of glutamate release and prevented the neuronal damage observed at 24 h in the CA1 hippocampal region. However, retigabine did not block the EEG epileptic discharges and their duration was reduced in only 20-25% of the tested animals. We conclude that the neuroprotective action of retigabine is probably due to the blockade of the 4-AP-induced stimulation of glutamate release. This inhibition, however, was not sufficient to block the epileptic activity.

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.

Potential pleiotropic effects of Mpdz on vulnerability to seizures

We previously mapped quantitative trait loci (QTL) responsible for approximately 26% of the genetic variance in acute alcohol and barbiturate (i.e., pentobarbital) withdrawal convulsion liability to a < 1 cM (1.8 Mb) interval of mouse chromosome 4. To date, Mpdz, which encodes the multiple PSD95/DLG/ZO-1 (PDZ) domain protein (MPDZ), is the only gene within the interval shown to have allelic variants that differ in coding sequence and/or expression, making it a strong candidate gene for the QTL. Previous work indicates that Mpdz haplotypes in standard mouse strains encode distinct protein variants (MPDZ1-3), and that MPDZ status is genetically correlated with severity of withdrawal from alcohol and pentobarbital. Here, we report that MPDZ status cosegregates with withdrawal convulsion severity in lines of mice selectively bred for phenotypic differences in severity of acute withdrawal from alcohol [i.e., High Alcohol Withdrawal (HAW) and Low Alcohol Withdrawal (LAW) lines] or pentobarbital [High Pentobarbital Withdrawal (HPW) and Low Pentobarbital Withdrawal (LPW) lines]. These analyses confirm that MPDZ status is associated with severity of alcohol and pentobarbital withdrawal convulsions. Using a panel of standard inbred strains of mice, we assessed the association between MPDZ status with seizures induced by nine chemiconvulsants. Our results show that MPDZ status is genetically correlated with seizure sensitivity to pentylenetetrazol, kainate and other chemiconvulsants. Our results provide evidence that Mpdz may have pleiotropic effects on multiple seizure phenotypes, including seizures associated with withdrawal from two classes of central nervous system (CNS) depressants and sensitivity to specific chemiconvulsants that affect glutaminergic and GABAergic neurotransmission.

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.

Non-competitive NMDA receptor antagonists moderate seizure-induced c-fos expression in the rat cerebral cortex

We examined the effects of non-competitive NMDA glutamate receptor antagonists on seizures elicited by 4-aminopyridine (4-AP), and in particular, on the expression of the transcription factor c-fos induced by these seizures. Induction of c-fos mRNA due to 4-AP-elicited seizures was ascertained by reverse transcription polymerase chain reaction in samples of the neocortex. Adult rats were pretreated with the NMDA receptor antagonists amantadine (40 mg/kg), ketamine (3mg/kg), dizocilpine (MK-801; 1mg/kg) or dextrometorphan (40 mg/kg); 4-AP (5mg/kg) was then injected i.p. Controls were treated with either antagonist only or with 4-AP only. Pretreatment with the antagonists (with the exception of amantadine) increased the latency of behavioural seizures, but not all of the antagonists caused symptomatic seizure protection. In the brains which were processed for Fos immunohistochemistry, quantitative evaluation of immunostained cells was performed in the neocortex and hippocampus. Treatment with either antagonist did not induce by itself c-fos expression, with the exception of amantadine, which caused slight Fos induction in the neocortex. Pretreatment with all the antagonists resulted in decrease of seizure-induced Fos immunoreactivity with respect to non-pretreated animals. Decrease of immunostained cells was significant in the neocortex, in the granule cell layer and hilus of the dentate gyrus, in hippocampal areas CA1 and CA2. MK-801, ketamine and dextrometorphan decreased significantly Fos immunoreactivity also in area CA3. The decrease of Fos immunostaining was not directly correlated with a suppression of behavioural seizures. The results support an important role of NMDA receptors in c-fos gene induction in acute 4-AP seizures.

Superfusion of synaptosomes to study presynaptic mechanisms involved in neurotransmitter release from rat brain

Neurotransmitter release, as the primary way for neuron signaling, represents the target of a staggering number of studies in order to understand complex neural functions. The corpus striatum is a brain area especially rich in neurotransmitters where cholinergic neurons are supposed to play an associative role between different neuronal types, and therefore their activity is modulated by multiple neurotransmitter systems [Trends Neurosci. 17 (1994) 228; Trends Neurosci. 18 (1995) 527] [13,25]. In this regard, superfusion of synaptosomes is a useful in vitro approach to study the neurotransmitter release allowing an unequivocal interpretation of results obtained under accurately specified experimental conditions. Synaptosomes are sealed presynaptic nerve terminals obtained after homogenating brain tissue in iso-osmotic conditions [J. Physiol. 142 (1958) 187] [22]. Synaptosomes have been extensively used to study the mechanism of neurotransmitter release in vitro because they preserve the biochemical, morphological and electrophysiological properties of the synapse [J. Neurocytol. 22 (1993) 735] [42]. The superfusion, strictly a perfusion, allows both the continuous removal of the compounds present in the biophase of the presynaptic proteins and the easy exchange of the medium. We herein describe the method of superfusion of rat striatal synaptosomes to study the [(3)H]ACh release under basal and stimulated conditions. To depolarize the synaptosomal preparation three different strategies were employed: high extracellular concentration of K(+) (15 mM), a K(+) channel-blocker (4-aminopyridine, 1-30 microM), or veratridine (10 microM) which blocks the inactivation of voltage-dependent Na(+) channels.

Time-dependent distribution and neuronal localization of c-fos protein in the rat hippocampus following 4-aminopyridine seizures

The immunohistochemical localization of c-fos protein in the CNS neurons was studied in a model of generalized epilepsy induced by the intraperitoneal injection of 4-aminopyridine to adult Wistar rats. This specific blocker of the voltage-dependent potassium channels proved to be suitable for use in the investigation of epileptogenesis. Following the treatment of adult rats with 5 mg kg of 4-aminopyridine, the animals experienced generalized seizures. At the end of the experiment, the rats were briefly anesthetized and perfused with fixative. Frozen coronal plane sections were cut and processed for immunohistochemistry, using polyclonal c-fos antibody. The number and distribution of immunostained cell nuclei in the hippocampus were analyzed in detail with the help of a digital microscope camera and a morphometry program. The highest level of immunostaining was detected in most of the structures at 3 h, but the level had decreased to the control level by 5 h following 4-aminopyridine injection. In the dentate fascia, immunostaining was highest at 1 h and then decreased slowly until 5 h post-injection. The activated neuronal assemblies were analyzed with the aid of parvalbumin c-fos double immunostaining. These countings revealed the highest inhibitory interneuronal activation in every part of the hippocampus (including the dentate fascia) at 3 h post-injection. The results indicate that systemic 4-aminopyridine induces limbic seizures, which are probably initiated in the entorhinal cortex.

Differential effect of quinpirole and 7-OH-DPAT on the spontaneous [(3)H]-dopamine efflux from rat striatal synaptosomes

The effect of quinpirole and 7-OH-DAPT, two D(2)-like agonists, were examined using superfused rat striatal synaptosomes to study the autoregulation of spontaneous [(3)H]-dopamine ([(3)H]-DA) release. Basal [(3)H]-DA efflux was Ca(2+)-dependent by approximately 45% and was inhibited by cadmium 10 microM by 24%. Quinpirole (1 nM to 3 microM) inhibited spontaneous [(3)H]-DA efflux in a concentration-dependent manner (pEC(50) = 7.56 +/- 0.07 and E(max) = 26 +/- 0.09%) and this effect was competitively antagonized by haloperidol (0.3-1 nM) (apparent pA(2) = 9.61 +/- 0.08). In addition, activation of the D(2) DA autoreceptor by quinpirole only modulates the calcium-dependent component of [(3)H]-DA efflux. Low concentrations of a putative-selective D(3) DA agonist, (+/-)-7-OH-DPAT (0.03-0.1 microM), inhibited spontaneous [(3)H]-DA release by 13% (P < 0.05), but higher drug concentrations (> or =1 microM) increased basal [(3)H]-DA efflux in a concentration-dependent, nonsaturable, but reversible manner. Haloperidol (1-10 nM) reversed the (+/-)-7-OH-DPAT-induced inhibition, but not the increase in [(3)H]-DA outflow. The effect of (+/-)-7-OH-DPAT was mimicked by (+)-7-OH-DPAT. However, another putative D(3) DA agonist, PD 128,907 (1 nM to 3 microM), decreased spontaneous tritium efflux (maximal inhibition of 19 +/- 3.06% at 3 microM, P < 0.01). The effect of 7-OH-DPAT 10 microM was independent of the presence of extracellular Ca(2+), since its effect on basal [(3)H]-DA outflow was not significantly modified in a 200 nM free-Ca(2+) medium. In addition, the 7-OH-DPAT-induced enhancement of basal [(3)H]-DA efflux does not involve depolarization of nerve terminals or the reversal of the DA uptake system, as tetrodotoxin (1 microM) and nomifensine (1microM) did not modify the effect of 7-OH-DPAT 10 microM. The present data indicate that activation of D(2) DA autoreceptor subtype by quinpirole inhibits Ca(2+)-dependent spontaneous [(3)H]-DA efflux. 7-OH-DPAT activates the D(2) DA autoreceptor at low concentrations, whereas its action in releasing [(3)H]-DA effect is not receptor-mediated and could involve other mechanisms other than either conventional vesicular exocytosis or the DA uptake system.

Seizures and neurodegeneration induced by 4-aminopyridine in rat hippocampus in vivo: role of glutamate- and GABA-mediated neurotransmission and of ion channels

Infusion of the K(+) channel blocker 4-aminopyridine in the hippocampus induces the release of glutamate, as well as seizures and neurodegeneration. Since an imbalance between excitation and inhibition, as well as alterations of ion channels, may be involved in these effects of 4-aminopyridine, we have studied whether they are modified by drugs that block glutamatergic transmission or ion channels, or drugs that potentiate GABA-mediated transmission. The drugs were administered to anesthetized rats subjected to intrahippocampal infusion of 4-aminopyridine through microdialysis probes, with simultaneous collection of dialysis perfusates and recording of the electroencephalogram, and subsequent histological analysis. Ionotropic glutamate receptor antagonists clearly diminished the intensity of seizures and prevented the neuronal damage, but did not alter substantially the enhancement of extracellular glutamate induced by 4-aminopyridine. None of the drugs facilitating GABA-mediated transmission, including uptake blockers, GABA-transaminase inhibitors and agonists of the A-type receptor, was able to reduce the glutamate release, seizures or neuronal damage produced by 4-aminopyridine. In contrast, nipecotate, which notably increased extracellular levels of the amino acid, potentiated the intensity of seizures and the neurodegeneration. GABA(A) receptor antagonists partially reduced the extracellular accumulation of glutamate induced by 4-aminopyridine, but did not exert any protective action. Tetrodotoxin largely prevented the increase of extracellular glutamate, the electroencephalographic epileptic discharges and the neuronal death in the CA1 and CA3 hippocampal regions. Valproate and carbamazepine, also Na(+) channel blockers that possess general anticonvulsant action, failed to modify the three effects of 4-aminopyridine studied. The N-type Ca(2+) channel blocker omega-conotoxin, the K(+) channel opener diazoxide, and the non-specific ion channel blocker riluzole diminished the enhancement of extracellular glutamate and slightly protected against the neurodegeneration. However, the two former compounds did not antagonize the 4-aminopyridine-induced epileptiform discharges, and riluzole instead markedly increased the intensity and duration of the disharges. Moreover, at the highest dose tested (8mg/kg, i.p.), riluzole caused a 75% mortality of the rats. We conclude that 4-aminopyridine stimulates the release of glutamate from nerve endings and that the resultant augmented extracellular glutamate is directly related to the neurodegeneration and is involved in the generation of epileptiform discharges through the concomitant overactivation of glutamate receptors. Under these conditions, a facilitated GABA-mediated transmission may paradoxically boost neuronal hyperexcitation. Riluzole, a drug used to treat amyotrophic lateral sclerosis, seems to be toxic when combined with neuronal hyperexcitation.

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.

Effects of GYKI 52466 and some 2,3-benzodiazepine derivatives on hippocampal in vitro basal neuronal excitability and 4-aminopyridine epileptic activity

In order to determine whether the anticonvulsant effect of 2, 3-benzodiazepines is also displayed in a model of in vitro epilepsy, such as the "epileptiform" hippocampal slice, we studied the effects of 2,3-benzodiazepine 1-(4-aminophenyl)-4-methyl-7, 8-methylenedioxe-5H 2,3-benzodiazepine hydrochloride (GYKI 52466) and some new 2,3-benzodiazepine derivatives on CA1 basal neuronal excitability and on CA1 epileptiform burst activity produced by 4-aminopyridine in rat hippocampal slices. The results showed that GYKI 52466 affected basal neuronal excitability as evidenced by its influence on the magnitude of the CA1 orthodromic-evoked field potentials. 2,3-Benzodiazepines showed their antiepileptic effect also in an in vitro model of experimental epilepsy. The effects of the new 2,3-benzodiazepine derivatives suggest that the methylenedioxidation in positions 7 and 8 of the 2,3-benzodiazepine ring is the main structural modification for the antiepileptic effect of 2,3-benzodiazepines to take place.

Relationships among seizures, extracellular amino acid changes, and neurodegeneration induced by 4-aminopyridine in rat hippocampus: a microdialysis and electroencephalographic study

4-Aminopyridine is a powerful convulsant that induces the release of neurotransmitters, including glutamate. We report the effect of intrahippocampal administration of 4-aminopyridine at six different concentrations through microdialysis probes on EEG activity and on concentrations of extracellular amino acids and correlate this effect with histological changes in the hippocampus. 4-Aminopyridine induced in a concentration-dependent manner intense and frequent epileptic discharges in both the hippocampus and the cerebral cortex. The three highest concentrations used induced also a dose-dependent enhancement of extracellular glutamate, aspartate, and GABA levels and profound hippocampal damage. Neurodegenerative changes occurred in CA1, CA3, and CA4 subfields, whereas CA2 was spared. In contrast, microdialysis administration of a depolarizing K+ concentration and of tetraethylammonium resulted in increased amino acid levels but no epileptic activity and no or moderate neuronal damage. These results suggest that seizure activity induced by 4-aminopyridine is due to a combined action of excitatory amino acid release and direct stimulation of neuronal firing, whereas neuronal death is related to the increased glutamate release but is independent of seizure activity. In addition, it is concluded that the glutamate release-inducing effect of 4-aminopyridine results in excitotoxicity because it occurs at the level of nerve endings, thus permitting the interaction of glutamate with its postsynaptic receptors, which is probably not the case after K+ depolarization.

The effects of quinine and 4-aminopyridine on conditioned place preference and changes in motor activity induced by morphine in rats

1. The effects of two unselective potassium (K(+)-) channel blockers, quinine (12.5, 25 and 50 mg/kg) and 4-aminopyridine (1 and 2 mg/kg), on conditioned place preference and biphasic changes in motor activity induced by morphine (10 mg/kg) were tested in Wistar rats. Quinine is known to block voltage-, calcium- and ATP-sensitive K(+)-channels while 4-aminopyridine is known to block voltage-sensitive K(+)-channels. 2. In the counterbalanced method, quinine attenuated morphine-induced place preference, whereas 4-aminopyridine was ineffective. In the motor activity test measured with an Animex-activity meter neither of the K(+)-channel blockers affected morphine-induced hypoactivity, but both K(+)-channel blockers prevented morphine-induced secondary hyperactivity. 3. These results suggest the involvement of quinine-sensitive but not 4-aminopyridine-sensitive K(+)-channels in morphine reward. It is also suggested that the blockade of K(+)-channels sensitive to these blockers is not sufficient to prevent morphine-induced hypoactivity whereas morphine-induced hyperactivity seems to be connected to both quinine- and 4-aminopyridine-sensitive K(+)-channels.

Differential effect of protein kinase inhibitors on calcium-dependent and calcium-independent [14C]GABA release from rat brain synaptosomes

Rat brain synaptosomes were isolated to study the effects of protein kinase inhibitors (sphingosine, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride, N-(6-aminohexyl)-5-chloro-1-naphtalenesulfonamide, staurosporine) on Ca2+-dependent and Ca2+-independent [14C]GABA release. The Ca2+-dependent [14C]GABA release was stimulated by depolarization with a K+-channel blocker, 4-aminopyridine, or high K+ concentration. It has been shown that 4-aminopyridine-evoked [14C]GABA release strongly depends on extracellular Ca2+ while K+-evoked [14C]GABA release only partly decreases in the absence of calcium. The substitution of sodium by choline in Ca2+-free medium completely abolished Ca2+-independent part of K+-evoked [14C]GABA release. So the main effect of 4-aminopyridine is the Ca2+-dependent one while high K+ is able to evoke [14C]GABA release in both a Ca2+-dependent and Na+-dependent manner. In experiments with protein kinase inhibitors, 4-aminopyridine and high K+ concentration were used to study the Ca2+-dependent and the Ca2+-independent [14C]GABA release, respectively. In addition, the Ca2+-independent [14C]GABA release was studied using alpha-latrotoxin as a tool. Pretreatment of synaptosomes with protein kinase inhibitors tested, except of 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride, resulted in a marked inhibition of 4-aminopyridine-stimulated Ca2+-dependent [14C]GABA release. The inhibitory effects of N-(6-aminohexyl)-5-chloro-1-naphtalenesulfonamide and staurosporine on [14C]GABA release were not due to their effects on 4-aminopyridine-promoted 45Ca2+ influx into synaptosomes. Only sphingosine (100 microM) reduced the 45Ca2+ influx. All the inhibitors investigated were absolutely ineffective in blocking the Ca2+-independent [14C]GABA release stimulated by alpha-latrotoxin. Three of them, except for sphingosine, did not affect the Ca2+-independent [14C]GABA release stimulated by high potassium. The inhibitory effect of sphingosine was equal to 30%. Thus, if [14C]GABA release occurred in a Ca2+-independent manner irrespective of whether alpha-latrotoxin or high K+ stimulated this process, it was not inhibited by the drugs decreased the Ca2+-dependent [14C] GABA release. Given the above points it is therefore not unreasonable to assume that the absence of Ca2+ in the extracellular medium created the conditions in which the activation of neurotransmitter release was not accompanied by Ca2+-dependent dephosphorylation of neuronal phosphoproteins, and as a consequence the regulation of exocytotic process was modulated so that the inhibition of protein kinases did not disturb the exocytosis.

Characterization of the in vitro antiepileptic activity of new and old anticonvulsant drugs

1. The in vitro antiepileptiform effects of some old and new anticonvulsants in the experimental model of the "epileptiform" hippocampal slice have been reviewed. 2. On the basis of their influence on in vitro epileptogenesis and basal neuronal excitability, anticonvulsants can be classified into three main categories: (1) anticonvulsants (prototypical drug phenytoin) affecting basal neuronal excitability but not epileptogenesis; (2) anticonvulsants (prototypical drugs barbiturates) affecting basal neuronal excitability and epileptogenesis; (3) anticonvulsants (prototypical drug felbamate) affecting epileptogenesis but not basal neuronal excitability. 3. It is concluded that the model of the "epileptiform" hippocampal slices can be considered a previsional test for the study and the screening of new anticonvulsant drugs.

Potassium channel blockade does not alter the modulatory effects of dopamine in neostriatal slices

This study assessed the contribution of K+ conductances to dopamine (DA)-induced modulation of evoked depolarizing synaptic responses (DPSPs) in neostriatal slices obtained from rats. Intracellular recordings of membrane properties and DPSPs evoked by local electrical stimulation were obtained from cells bathed in standard artificial cerebrospinal fluid (ACSF), 5 mM Cs+ in ACSF, 20 mM tetraethylammonium in ACSF, or 1 mM 4-aminopyridine in ACSF. DA altered response amplitude in approximately equal proportions regardless of the presence or absence of these K(+)-channel blockers. These findings suggest that K+ conductances do not provide a major contribution to DA-induced changes in DPSPs in the neostriatum.