ABSTRACT
In epilepsy, allegedly, a neurotransmitter imbalance between the inhibitory GABA and the excitatory glutamate prevails. Therefore, some antiepileptic drugs (AEDs) are thought to increase GABA release. Because little is known about corresponding presynaptic effects of AEDs in the human brain, this study investigated the effects of carbamazepine, lamotrigine, phenytoin, gabapentin, pregabalin, levetiracetam, and valproate on (3)H-GABA release from human neocortical synaptosomes preincubated with (3)H-GABA. To obtain information on possible species differences, rat neocortical synaptosomes were investigated concomitantly. Release was evoked by either veratridine (1, 3.2, or 10 µM), which prevents activated voltage-dependent Na(+) channels from closing, or elevation of extracellular [K(+)] from 3 to 15 mM. The exocytosis inhibitor tetanus toxin (TeT) or withdrawal of buffer Ca(2+) (Ca (e) (2+) ) reduced K(+)-evoked release in both species, while blockade of Na(+) channels with tetrodotoxin had no effect. K(+)-evoked release was characterized as predominant, Ca(2+)-dependent and Na(+)-independent, exocytosis. Carbamazepine and phenytoin in the rat and carbamazepine, phenytoin, lamotrigine, and valproate in human tissue reduced K(+)-evoked (3)H-GABA release. With respect to veratridine-evoked release, Ca (e) (2+) withdrawal did not reduce release in the rat; it even increased the release in human tissue. TeT was slightly inhibitory in the rat. Blockade of GABA transport diminished veratridine-evoked (3)H-GABA release in either species. This release was characterized as mediated mainly by transporter reversal. Carbamazepine, lamotrigine, and phenytoin in rat tissue and carbamazepine and phenytoin in human decreased veratridine-induced (3)H-GABA release. Interestingly, no AED increased (3)H-GABA release. The reduction by AEDs of veratridine-evoked release was more intense than that of K(+)-evoked release. In conclusion, reduction of GABA release by AEDs may be the actual objective in a pathologically altered neuronal network where GABA acts in a depolarizing fashion.
Subject(s)
Anticonvulsants/pharmacology , Neocortex/drug effects , Synaptosomes/metabolism , gamma-Aminobutyric Acid/metabolism , Adolescent , Adult , Aged , Amines/pharmacology , Animals , Calcium/antagonists & inhibitors , Calcium/deficiency , Carbamazepine/pharmacology , Child , Child, Preschool , Cyclohexanecarboxylic Acids/pharmacology , Female , GABA Antagonists/pharmacology , Gabapentin , Humans , Infant , Lamotrigine , Levetiracetam , Male , Middle Aged , Neocortex/metabolism , Nipecotic Acids/pharmacology , Oximes/pharmacology , Phenytoin/pharmacokinetics , Phenytoin/pharmacology , Piracetam/analogs & derivatives , Piracetam/pharmacology , Potassium/pharmacology , Pregabalin , Rats , Rats, Wistar , Synaptosomes/drug effects , Tetanus Toxin/pharmacology , Tetrodotoxin/pharmacology , Triazines/pharmacology , Valproic Acid/pharmacology , Veratridine/pharmacology , Young Adult , gamma-Aminobutyric Acid/analogs & derivatives , gamma-Aminobutyric Acid/pharmacologyABSTRACT
Aim of this study was to learn whether the antiepileptic drugs (AEDs) carbamazepine, lamotrigine, phenytoin, gabapentin, pregabalin, levetiracetam, and valproate (all at 100 µM) presynaptically influence the neurotransmission of the excitatory transmitter glutamate. The effects of these frequently used AEDs were examined on ³H-glutamate release from superfused synaptosomes of both rat and human neocortex. Release was evoked by elevation of buffer [K(+)] from 3 to 15 mM or by the Na(+) channel activator veratridine (1, 3.2, and 10 µM). Buffer [K(+)] elevation induced ³H-glutamate exocytosis, which was Ca(²+)-, but not Na(+)-, dependent and which was accompanied only in human tissue by release through transporter reversal. In rat tissue, release was diminished by the Na(+) channel inhibitors carbamazepine, lamotrigine, and phenytoin, which therefore may also affect presynaptic Ca(²+) channels. Interestingly, levetiracetam increased ³H-glutamate release. In contrast, the tested AEDs did not affect K(+)-evoked ³H-glutamate release in human tissue, neither when the transporters were operative nor when exocytosis was isolated by transporter blockade. Veratridine-evoked ³H-glutamate release was a Na(+)-dependent transmitter efflux through reversed transporters in both species which in human synaptosomes was accompanied by exocytosis. The latter depended on external Ca(²+). Carbamazepine, lamotrigine, and phenytoin reduced this release from both rat and human tissue. There is an obvious species difference in the effects of carbamazepine, lamotrigine, and phenytoin on K(+)-evoked ³H-glutamate release while their inhibitory effects on veratridine-evoked release were similar. Thus, the depression of ³H-glutamate release by carbamazepine, lamotrigine, and phenytoin may be due to inhibited synaptosomal Na(+) or Ca(²+) influx.
Subject(s)
Anticonvulsants/pharmacology , Exocytosis/drug effects , Glutamic Acid/metabolism , Neocortex/drug effects , Synaptosomes/drug effects , Animals , Cells, Cultured , Female , Humans , Male , Neocortex/cytology , Neocortex/metabolism , Rats , Rats, Wistar , Species Specificity , Synaptic Transmission/drug effects , Synaptosomes/metabolismABSTRACT
To elucidate the mechanism of action of the anticonvulsant gabapentin (GBP), we compared its effects on K+-evoked [3H]-noradrenaline ([3H]-NA) release from rat hippocampal and human neocortical slices with those of the KATP channel opener pinacidil and the Na+ channel blockers phenytoin, carbamazepine and lamotrigine. Rat hippocampal and human neocortical slices were loaded with [3H]-NA and superfused. [3H]-NA release was evoked by increasing the extracellular [K+] from 3 to 15 mM. GBP decreased [3H]-NA release from rat hippocampal with a pIC50 of 5.59 and a maximum inhibition of 44%. Concentration-dependent inhibition was also seen in human neocortical slices (39% inhibition with 100 microM GBP). These inhibitory effects were antagonized by the KATP channel antagonist glibenclamide, yielding a pA2 of 7.50 in the rat. The KATP channel opener pinacidil (10 microM), like GBP, decreased [3H]-NA release from rat hippocampal slices by 27% and this effect was also antagonized by glibenclamide. In human neocortical slices the inhibition by pinacidil (10 microM) was 31%. Although phenytoin (10 microM), carbamazepine (100 microM) and lamotrigine (10 microM) also decreased [3H]-NA release (by 25%, 57% and 22%, respectively), glibenclamide did not antagonize the effects of these classical Na+ channel blockers. These findings suggest that GBP inhibits K+-evoked [3H]-NA release through activation of KATP channels. To establish whether the KATP channels under investigation were located on noradrenergic nerve terminals or on other neuronal elements, the effects of GBP were compared in the absence and in the presence of tetrodotoxin (TTX 0.32 microM) throughout superfusion. Since the functional elimination of the perikarya of interneurons by TTX reduced the inhibitory effect of GBP, the KATP channels mediating the effect of GBP may be located on nerve terminals, probably on both noradrenergic and glutamatergic nerve endings.
Subject(s)
Acetates/pharmacology , Amines , Anticonvulsants/pharmacology , Cyclohexanecarboxylic Acids , Norepinephrine/metabolism , Potassium Channels/metabolism , Potassium Chloride/pharmacology , gamma-Aminobutyric Acid , Animals , Dose-Response Relationship, Drug , Gabapentin , Glyburide/pharmacology , Hippocampus/drug effects , Hippocampus/metabolism , Humans , Hypoglycemic Agents/pharmacology , In Vitro Techniques , Male , Neocortex/drug effects , Neocortex/metabolism , Pinacidil/pharmacology , Rats , Rats, Wistar , Retrospective Studies , Vasodilator Agents/pharmacologyABSTRACT
1. The main purpose of the present study was to investigate the effects of the neuroprotective agent riluzole on the electrically evoked release of [(3)H]-glutamate ([(3)H]-Glu) in mouse neocortical slices. The reported selectivity of riluzole for excitatory amino acids was tested in release experiments with further neurotransmitters. Also distinct species, mouse, rat and man were compared. 2. [(3)H]-Glu was formed endogenously during incubation of slices with [(3)H]-glutamine ([(3)H]-Gln). Released [(3)H]-Glu and tissue [(3)H]-Glu was separated by anion exchange chromatography. Electrically evoked [(3)H]-Glu release was strongly diminished by tetrodotoxin (TTX) and Ca(2+)-withdrawal. 3. Riluzole (100 microM) depressed the release of [(3)H]-Glu up to 77% (IC(50)=19.5 microM). Riluzole was also able to inhibit strongly the electrically evoked release of [(3)H]-acetylcholine ([(3)H]-ACh) (at 100 microM by 92%, IC(50)=3.3 microM, and [(3)H]-dopamine ([(3)H]-DA) (at 32 microM by 72%, IC(50)=6.8 microM). However, the release of [(3)H]-serotonin ([(3)H]-5-HT) was less diminished (at 100 microM by 53%, IC(50)=39.8 microM). Riluzole up to 100 microM did not affect [(3)H]-noradrenaline ([(3)H]-NA) release. 4. Between species, i.e. in mouse, rat and human neocortex, no significant differences between the effects of riluzole could be observed. 5. The NMDA-receptor blocker MK-801 (1 microM) and the AMPA/Kainate-receptor blocker NBQX (1 microM) did neither affect the electrically evoked [(3)H]-ACh release nor its inhibition by riluzole, indicating that effects of riluzole on transmitter release were neither due to modulation of ionotropic Glu receptors, nor due to indirect inhibition of Glu release through these receptors. 6. Taken together, riluzole inhibits the release of distinct neurotransmitters differently, but is not selective for the excitatory amino acid Glu.
Subject(s)
Neostriatum/drug effects , Neuroprotective Agents/pharmacology , Neurotransmitter Agents/metabolism , Riluzole/pharmacology , Acetylcholine/metabolism , Animals , Calcium/pharmacology , Dicarboxylic Acids/pharmacology , Dizocilpine Maleate/pharmacology , Dopamine/metabolism , Dose-Response Relationship, Drug , Electric Stimulation , Excitatory Amino Acid Antagonists/pharmacology , Glutamic Acid/drug effects , Glutamic Acid/metabolism , Humans , In Vitro Techniques , Mice , Neostriatum/metabolism , Norepinephrine/metabolism , Pyrrolidines/pharmacology , Quinoxalines/pharmacology , Rats , Rats, Wistar , Serotonin/metabolism , Tetrodotoxin/pharmacology , TritiumABSTRACT
The expression of brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase B are both increased after global ischemia. Therefore, a protective action of BDNF against the delayed degeneration of vulnerable neurons has been suggested. We have investigated the neuroprotective action of BDNF in global ischemia induced by a four-vessel occlusion in the rat. Following reperfusion, 0.06 microg/hr BDNF was continuously administered intracerebroventricularly with an osmotic minipump. Rats were sacrificed up to 7 days after ischemia and neuronal degeneration was identified by terminal transferase and biotin-dUTP nick end labeling (TUNEL) staining. Additionally, the glial reaction was investigated immunohistochemically and by measuring the activation of immunological nitric oxide synthase protein expression. Postischemic intracerebroventricular infusion of BDNF prevented neuronal death in the vulnerable CA1 region of the hippocampus. Additionally, astroglial activation and macrophage infiltration, which were observed in association with neuronal death, were inhibited by BDNF. This was paralleled by an inhibition of inducible nitric oxide synthase (iNOS) expression in the hippocampus. Thus, the observed neuroprotective effects of continuous BDNF administration after reperfusion suggest a therapeutic potential for BDNF in cerebral ischemia.