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Antimicrobial Agents and Chemotherapy, July 1998, p. 1831-1836, Vol. 42, No. 7
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Determination of the Excitatory Potencies of
Fluoroquinolones in the Central Nervous System by an In Vitro
Model
Gabriele
Schmuck,*
Anja
Schürmann, and
Gerhard
Schlüter
BAYER AG, Institute of Toxicology, 42096 Wuppertal, Germany
Received 29 December 1997/Returned for modification 25 February
1998/Accepted 6 May 1998
 |
ABSTRACT |
Fluoroquinolones have been reported to induce central nervous
system side effects, including seizures and psychiatric events. Although relatively rare in patients up to now, the proconvulsant activity depends on the chemical structure and might be a critical endpoint of some new representatives of this valuable class
of antimicrobials. The electrophysiological determination of
field potentials in the CA1 region of the rat
hippocampus slice allowed an assessment of the excitatory
potential of fluoroquinolones and might be predictive for their
neurotoxic potency in vivo. An optimization of this method and its
extension to other fluoroquinolones resulted in a defined rank order.
Well-known already-marketed quinolones as well as some fluoroquinolones
under evaluation and development were used. The dose range
tested was between 0.5 and 4 µmol/liter, which was
comparable to the therapeutic concentration in the brain. All
tested compounds increased the population spike amplitude in a
concentration-dependent manner, and the resulting excitatory potency
was highly dependent on the chemical structure, with compounds ranging
from least to most excitatory as follows: ofloxacin, ciprofloxacin,
nalidixic acid, moxifloxacin (= BAY × 8843), fleroxacin,
lomefloxacin, enoxacin, clinafloxacin (much more excitatory than
enoxacin), tosufloxacin, trovafloxacin, BAY 15-7828, and BAY × 9181 (much more excitatory than BAY 15-7828). The proposed hippocampus
slice model not only is suitable for giving valuable alerts as to
convulsive potential during candidate selection but also enables
mechanistic investigations. These investigations pointed to the
N-methyl-D-aspartate receptor as the probable
target of the fluoroquinolone effects.
| |
INTRODUCTION |
Quinolones have been reported to
induce central nervous system (CNS) reactions in humans with an
incidence of 0.9 to 2.1%. However, severe psychiatric or neurological
reactions like hallucinations, periods of depression, nightmares, or
convulsions are relatively rare (5). Convulsive seizures are
reported after quinolone treatment mostly in the elderly or in
patients with a history of epilepsy, cerebral trauma, or alcohol abuse
(5). The proconvulsant activity of fluoroquinolones depends
on the chemical structure and might be a critical endpoint of some new
representatives of this valuable class of antimicrobials. It therefore
has to be considered during preclinical development, either in animal
studies or in vitro. Up to now, most animal models have failed to
provide a reliable ranking of these compounds that is relevant
to humans. As an in vitro model for assessing the excitatory potential
of fluoroquinolones, the determination of evoked potentials, measured by extracellular recordings in the hippocampus slice of the rat, has been proposed (9). In this model, enoxacin, nalidixic
acid, and ofloxacin were the most potent convulsants, whereas
ciprofloxacin, norfloxacin, and pefloxacin were less active.
With the exception of pefloxacin, these results were in agreement
with the results of studies in DBA/2 mice, a strain genetically
susceptible to sound-induced seizures (8), and also with
data for CNS side effects in patients (20).
Different drugs, such as methylxanthine derivatives or nonsteroidal
anti-inflammatory drugs, potentiate the convulsant activity of
fluoroquinolones. The adenosine or 
-aminobutyric acid
(GABAA) receptor has therefore been proposed as a possible
target for quinolones (12). In addition, interactions of
quinolones with the dopamine and opiate receptors were also
postulated (5, 27, 31). However, receptor binding studies
using radioactive ligands for these receptors failed to identify a rank
order for the different quinolones that was predictive of the in vivo
situation (1, 12).
The structural similarities of fluoroquinolones to kynurenic acid and
other similar compounds which are endogenous ligands of the glutamate
receptor might suggest an interaction of quinolones with ligand-gated
glutamate receptors. Accordingly, the proconvulsive action of
quinolones is antagonized by AP-5 or AP-7, selective antagonists
of the glutamate binding site of the
N-methyl-D-aspartate (NMDA) receptor (27,
39). However, in receptor binding studies with
[3H]glutamate, [3H]kainate,
-[3H]amino-3-hydroxy-5-methylisoxazole-4-propionic
acid), and [3H]NMDA, no specific affinity of quinolones
for the ion- or ligand-gated glutamate receptors has been found
(12, 17, 34).
Thus, receptor binding studies of the relevant neuronal receptor types
failed to predict the convulsive potency of the different fluoroquinolones up to now. A more sophisticated mechanistic model for
investigating this effect, one which reflects the cell-cell interaction
within the CNS properly, has been needed. The electrophysiological determination of the evoked field potentials in the CA1
pyramidal cell layer of the hippocampus slice may represent such a
mechanistic model, because (i) the complex functional architecture of
the hippocampus is retained, (ii) it contains a well-defined
principal cell population expressing the receptor types (GABA and NMDA) in question, and (iii) the endpoint determined in vitro is closely related to the seizure threshold responsible for some of the CNS side
effects. The hippocampus slice model was therefore used, after some
technical improvements, to investigate the excitatory potency of a
broad range of fluoroquinolones in order to obtain a more-reliable
ranking with respect to the different chemical structures and to get
more insight into the possible mechanism of these effects.
| |
MATERIALS AND METHODS |
Chemicals.
The tested fluoroquinolones
ofloxacin,
ciprofloxacin, fleroxacin, clinafloxacin, lomefloxacin, tosufloxacin,
moxifloxacin, BAY × 9181 {7-[1-ami- nomethyl-2-oxa-7-aza-bicyclo(3.3.0)oct-7-yl]-1-cyclopropyl-6,8-difluoro-1,4- dihydro-4-oxo-3-quinolinecarboxylic acid hydrochloride}, BAY × 8843 {1-cyclopropyl-7-[(S,S)-2,8-diazabicyclo[4.3.0]non-8-yl]-6,8-difluoro-1,4- dihydro-4-oxo-3-quinolinecarboxylic acid hydrochloride}, BAY 15-7828 {7-[(3aS,7aR)-3a-amino-1,2,3,7a-tetrahydro-isoindol-2-yl]-8-chloro-6-fluoro- 1-[(1R,2S]-2-fluorocyclopropyl]-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid}, and
trovafloxacin (mesylate and hydrochloride)were obtained from Bayer
(Leverkusen, Germany). Enoxacin, nalidixic acid, and norfloxacin were
purchased from Sigma (Deisenhofen, Germany). The purity of all
fluoroquinolones was 
99%. All compounds were dissolved in 0.1 N HCl to a concentration of 2 mmol/liter; afterwards, the stock
solutions were diluted in artificial cerebrospinal fluid (ACSF) (see
below) to their end concentrations (0.5 to 4 µmol/liter).
Equipment.
The orthodromic recordings of the evoked
potentials in the hippocampus slice were driven by a specific computer
program (Institute for Neurobiology, Madgeburg, Germany). Via a
stimulation isolator (Hi-Med Isolator HG 203; List Electronics, Tamm,
Germany), the electric stimulation occurred. The evoked field
potentials were amplified by two systems (SEC-10L and EXT-01
preamplifiers and DPA 2F amplifier; npi Electronic GmbH, Tamm,
Germany), were converted by a 1401 interface (Cambridge Electronic
Design via Science Products, Hofheim, Germany), and were
visualized by an oscilloscope (DRO 1604; Gould Electronics GmbH,
Diezenbach, Germany).
Evoked potentials.
Hippocampus slice preparations were
obtained from female rats (Wistar; Harlan Winkelmann). The animals were
anesthetized with halothane (Hoechst, Frankfurt, Germany). The brains
were removed immediately, the hippocampus was isolated, and the middle
part was cut immediately in 450-µm-thick slices (four to six slices per middle part of each hippocampus). The slices were transferred to a
chamber where they remained for 1 h before the experiment was
started. The experiment was done in a superfusion chamber (modified
McIlwain chamber; Fine Science Tools, Heidelberg, Germany) under
defined conditions (34°C; 2-ml/min superfusion rate). The slices were
stored and superfused with ACSF medium containing 124 mM NaCl, 5 mM
KCl, 1.25 mM NaH2PO4, 26 mM NaHCO3,
10 mM glucose, 2 mM MgSO4, and 2 mM CaCl2 (all
constituents from Merck, Darmstadt, Germany). The medium was saturated
with a 95% O2-5% CO2 mixture. Stimulation
(stimulation electrode, SNEX-100; Science Products) occurred in the
area of the Schaffer collaterals in the CA2 area; the
recording (glass capillaries, GB150TC-10; Science Products) was
performed from the pyramidal cell layer of CA1.
The electric stimulation was performed by 10 pulses with a pulse width
of 200 µs, a pulse interval of 10 s, a potential of 3 to 8 V,
and an amplitude of ~1 mV. The following time schedule was used for
every compound: 30 min of treatment with ACSF, 30 min of treatment with
quinolone (0.5 to 4 µM), and 30 min in ACSF as wash out phase.
Additional experiments with different concentrations of
Mg
2+ (1.75 to 3.5 mM), MK 801 (dizocilpine) (0.1 to 10 µg/ml), or
D-serine
(100 and 400 µM) alone or in
combination with clinafloxacin were
done according to the same time
schedule described above: 30 min
of treatment with ACSF, 30 min under
experimental conditions (Mg
2+, MK 801, or clinafloxacin),
and 30 to 60 min in ACSF as wash
out phase. Each experiment was done
with six slices from two individual
animals (three slices from each
animal).
The influence of fluoroquinolones on the evoked field potentials in the
hippocampus slice of rats was investigated according
to the method of
Dimpfel et al. (
9). In principle, the Schaffer
collaterals
projecting to the neurons in the CA
1 region of the
hippocampus were stimulated extracellularly in the CA
2
region
by single pulses. The response of the neurons in the
CA
1 region,
the amplitude of the field potential
(population spike), was recorded
by an extracellular electrode. An
increase of the amplitude compared
to that with untreated control was
indicative of increased excitability
of these neurons.
We modified the method in order to obtain improved reproducibility. The
main differences from the original method described
by Dimpfel et al.
(
9) were as follows. We used a modified McIlwain
chamber
instead of a Haas chamber, which allowed a more intense
superfusion of
the slice with the substance-containing buffer.
As a consequence, a
deeper recording position (250 to 300 µm from
the surface) in the
center of the slice was possible, and its
reproducibility was improved
by searching the optimal somatic
layer stepwise with a nanostepper
system (SPI, Oppenheim, Germany).
Additionally, the study design was
modified. After a stabilization
period of 30 min, a control phase of 30 min with ACSF superfusion
was recorded. After this time, the
superfusion with the quinolone-containing
ACSF was started and lasted
for a period of 30 min, followed by
a wash out phase of 30 to 60 min
with ACSF. Depending on the physicochemical
properties of the
substance, the peak of the excitatory effect
was even reached in the
wash out period.
Statistical analysis.
A statistical analysis was performed
using a one-way analysis of variance (ANOVA) followed by a t
test (Student-Newman-Keuls method) (Sigma Stat; Jandel, Erkrath,
Germany).
| |
RESULTS |
Extracellular evoked field potentials.
The fluoroquinolone
concentrations in the in vitro model were defined according to
pharmacokinetic considerations. The maximum therapeutic concentration
of the fluoroquinolones in plasma is <30 µmol/liter. In
cerebrospinal fluid approximately 10% of this concentration was
reached (6, 23, 25, 30). Therefore, concentrations of 0.5 to
4 µmol/liter were used in our investigations.
The fluoroquinolones tested increased the population spike amplitude of
the pyramidal cells in the CA
1 region of the
hippocampus
in a concentration-dependent manner (Fig.
1). At 4 µmol/liter,
some of the
compounds (clinafloxacin, trovafloxacin, and BAY ×
9181) were
strongly active in this model and irreversibly damaged
the system or
were unsoluble in the superfusion fluid (BAY 15-7828).
For comparing an
extended number of compounds in the screening
model, a fixed
concentration of 2 µmol/liter was therefore used
(Table
1). Most of the fluoroquinolones as well
as the new 8-methoxyfluoroquinolone
moxifloxacin increased the
population spike amplitude only moderately.
The resulting ranking was
ofloxacin, ciprofloxacin, nalidixic
acid, moxifloxacin, BAY × 8843, fleroxacin, lomefloxacin, and
enoxacin. Some other
fluoroquinolone derivatives increase the
population spike amplitude
more distinctly, with the ranking clinafloxacin,
tosufloxacin,
trovafloxacin (mesylate and hydrochloride), and
BAY 15-7828. The
greatest population spike (increase of more than
400% of the
control) was observed for the experimental fluoroquinolone
BAY × 9181, indicating that its convulsive potential might be
extremely high. This also demonstrates that the effect range
of
the hippocampus slice model (130 to 400%) with different
fluoroquinolones
was markedly higher than reported before
(
9).
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FIG. 1.
Dose-response curves of selected fluoroquinolones. The
concentration range tested was 0.5 to 4 µmol/liter. Each compound and
concentration was tested with six individual brain slices from two
animals. A statistically significant (P < 0.01)
increase of the population spike amplitude (more than 125 to 130% in
relation to the control level) could be shown in all experiments.
Statistical analysis was done by one-way ANOVA followed by a
t test.
|
|
Mechanistic investigations of fluoroquinolone effects
on field potentials.
The increase of the population spike
amplitude could in principle be due to activation of the NMDA receptor
on the neurons of the CA1 region or, alternatively, to a
reduction of the activity of the GABAergic inhibitory interneurons by
GABAA antagonism.
The activity of the NMDA receptor-gated Ca
2+ channel is
modulated by Mg
2+ and may be blocked by the specific ligand
MK 801. In order to
substantiate the influence of fluoroquinolones on
the NMDA receptor,
the effects of Mg
2+ and MK 801 in
combination with fluoroquinolones were investigated.
As expected, a reduced concentration of Mg
2+ alone in the
ACSF increased the population spike amplitude, whereas an increased
concentration depressed the response (Fig.
2a). Clinafloxacin
(2 µmol/liter)
induced an increase of the population spike amplitude
of 233% related
to the control level (100%) at the physiological
Mg
2+
concentrations of 2 mmol/liter. In combination with increasing
concentrations of Mg
2+, the population spike amplitude
induced by clinafloxacin (2 µmol/liter)
was reduced to the control
level (Fig.
2b). It is important that
this happened already at
Mg
2+ concentrations, which affected the control population
spike amplitude
only slightly (Fig.
2a). In contrast, a slight decrease
of the
Mg
2+ concentrations (1.75 mmol/liter) potentiated
very strongly the
clinafloxacin effect.
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FIG. 2.
Effects of Mg2+ concentration on population
spike amplitude. The standard concentration of Mg2+ in ACSF
was 2 mM. The Mg2+ concentration was varied between 1.75 and 3.5 mmol/liter. Each experiment was done with six individual brain
slices from two animals. Statistical analysis was done by one-way ANOVA
followed by a t test. (a) Mg2+ alone; (b)
Mg2+ with 2 µM clinafloxacin.
|
|
D-Serine (100 and 400 µmol/l) alone and in combination
with different concentrations of Mg
2+ (1.75 and 3 mmol/liter) (Fig.
3) was used to
demonstrate the
effect of Mg
2+ on the effects of ligands of
the glycine binding site of the
NMDA receptor. A possible effect of
fluoroquinolones on this binding
site was postulated by Dimpfel and
coworkers (
10). In contrast
to these authors, we detected no
effects with
D-serine alone,
even at very high
concentrations. This was in agreement with the
findings of Peeters and
Vanderheyden (
29) and Huettner (
21),
who
demonstrated that glycine or
D-serine had no effect by
itself
but could modulate the NMDA receptor channel opening times. Only
in combination with low Mg
2+ concentrations and at high
D-serine concentrations could a slight
modulative
action of
D-serine be demonstrated. This effect was
not comparable to the results with clinafloxacin in combination
with low Mg
2+ concentrations (Fig.
2b), where a threefold
increase of the population
spike amplitude was observed. This might
indicate that the underlying
mechanisms were different.
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FIG. 3.
Effects of D-serine and Mg2+ on
population spike amplitude. D-Serine concentrations of 100 and 400 µmol/liter and control concentrations were tested. Each
experiment was done with six individual slices from two animals.
Statistical analysis was done by one-way ANOVA followed by a
t test.
|
|
Alternatively MK 801 was used to block the NMDA receptor-gated ion
channel. MK 801 at 1 and 10 µg/ml had no effect on the
control
population spike amplitude, whereas at a concentration
of 50 µg/ml it
decreased the amplitude strongly (Fig.
4a). Therefore,
MK 801 concentrations of
0.1 to 10 µg/ml were used in order to
influence the excitatory
effects of clinafloxacin (2 µmol/l) (Fig.
4b). Blocking the ion
channel of the NMDA receptor by MK 801 counteracts
the effects of
quinolones in a concentration-dependent manner.
MK 801 at 1 and 10 µg/ml abolished the excitatory effect of clinafloxacin
completely,
whereas MK 801 at 0.1 µg/ml already reduced it by
about 50%.
Although not conclusive, both experiments point to
a direct involvement
of the NMDA-gated ion channel in the exitatory
effects of
fluoroquinolones.
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FIG. 4.
Effects of MK 801 on population spike amplitude. Each
experiment was done with six individual slices from two animals.
Statistical analysis was done by one-way ANOVA followed by a
t-test. (a) MK 801 alone (at 1, 10, and 50 mg/kg); (b) MK
801 (at 0.1, 1, and 10 µg/ml) with clinafloxacin (2 µmol/liter).
|
|
| |
DISCUSSION |
The hippocampus exhibits a unique sensitivity to cell injury
resulting from hypoxia, seizures, and neurotoxic compounds.
Hippocampal vulnerability has been shown anatomically,
biochemically, electrophysiologically, and behaviorally (13,
14). Therefore, the hippocampus is considered to be a
relevant model for toxicological or pharmacological
investigations in vitro and in vivo. The method using mammalian
brain slices as physiologically intact models dates back to 1966 (40). The interest in the technique has increased in
recent years, because in this in vitro model the complex
physiology, e.g., the interconnections between neurons as well as
between neurons and glial cells, is maintained. The hippocampus is
thought to be the structure in the brain with the lowest
seizure threshold (3). It has therefore seen widespread use
as a fairly simple model for the study of cellular mechanisms of acute
epilepsy by measuring extracellular field potentials.
Field potentials reflect the summated output of an entire population of
neurons, including both excitatory and inhibitory influences.
Excitatory neurotransmission in the hippocampus is mediated mainly by
glutamate, for which different receptors exist. The most abundant
excitatory acting receptor in the CA1 region of the
hippocampus is the NMDA subtype. The NMDA ionophore is blocked by
Mg2+ in a voltage-dependent fashion. The Mg2+
block is relieved by a large or prolonged cell depolarization (11,
26). The inhibitory interneurons in the hippocampus are predominantly GABAergic, with two subclasses, GABAA
and GABAB receptors, both hyperpolarizing the cells
(4).
The CA1 region of the hippocampus is considered an optimal
region for studying the excitatory potency of test substances, because
the neurotransmitter receptors involved in the generation of field
potentials are well characterized and mechanistically related to the
endpoint in question in vivo.
By an extensive number of fluoroquinolones tested in this model it was
shown that all fluoroquinolones dose dependently increased the
population spike amplitude of the neurons in the CA1 region of the hippocampus. The observation is qualitatively in agreement with
the observed convulsant potential of some fluoroquinolones in humans
(20). It is, however, important to state that proconvulsant activity might be only one aspect of the CNS effects of
fluoroquinolones. With respect to this endpoint, a ranking of the
substances tested based on the in vitro data (population spike, 130 to
400% of control) was possible. This ranking indicated that
considerable differences between the fluoroquinolones exist. However, a
detailed correlation of the in vitro activity to the in vivo findings
is hampered by the fact that in vivo data under comparable
experimental conditions are largely lacking. Therefore, for two
fluoroquinolones, moxifloxacin and BAY 15-7828 (population
spikes, 170 and 281%, respectively, of control in vitro) an
orientating study in a rhesus monkey was performed as an example.
Following intravenous administration of 30 mg of BAY 15-7828/kg
of bodyweight, the monkey showed clear signs of CNS toxicity, with
somnolence, nystagmus, and seizures, whereas intravenously administered
moxifloxacin at 45 mg/kg induced no clinically detectable signs of
neurotoxicity, indicating that the ranking found in vitro is also of
relevance in vivo. Unfortunately, results in humans are available only
for narrow-spectrum fluoroquinolones, which showed a very tight
clustering of population spike amplitudes in vitro (130 to 192% of
control) and in fact are also relatively similar in their incidence of
CNS side effects (<2%) (20). Given the narrow range of the
intrinsic excitatory potency of these compounds, other factors, such as
the steepness of the concentration-response curve and pharmacokinetics,
significantly influence their in vivo response. Therefore, the in vitro
model used allowed no further differentiation of the expected in vivo
convulsant activity for these compounds with relatively similar
intrinsic activities. This similarity may also account for some
inconsistencies in the ranking obtained in different studies either in
vitro or in vivo with these fluoroquinolones (8, 10, 20).
However, the hippocampus slice model indicated a higher excitatory
potency for some newer fluoroquinolones, such as clinafloxacin and
trovafloxacin. By accumulating clinical experience with these
compounds, further insight into the relevance of this model to the
situation in humans should become available. For the time being, the
proposed hippocampus slice model represents a rapid in vitro model for
quantifying the excitatory potency of fluoroquinolones, which may give
valuable alerts as to convulsive potential during drug candidate
selection. In addition, this model will in principle allow
more-detailed studies for structure-activity relationship for this
effect. Unfortunately, the marked structural diversity of the
fluoroquinolones tested did not yet allow final conclusions on the
structural prerequisites influencing the excitatory potency.
As outlined above, the hippocampus slice might be used for mechanistic
studies on the excitatory effects of fluoroquinolones since the
relevant receptors are present and functionally active. The increased
excitability may in principle be due to inhibition of the activity of
the GABAergic interneurons or to activation of the NMDA receptor. It
has been shown in various receptor binding studies as well as
patch-clamp investigations that quinolones have no or only a weak
affinity to the GABAergic system (2, 15, 16, 18, 22, 33,
37). In contrast it was shown by our experiments that MK 801, a
selective channel blocker of the NMDA receptor, abolishes the
excitatory effects of clinafloxacin, thus strongly suggesting the
involvement of the NMDA channel in its effects in the hippocampus
slice. MK 801 has also been reported to antagonize the proconvulsive
action of fluoroquinolones in male mice (19, 28, 39).
However, it has also been shown that fluoroquinolones did not bind to
the glutamate or glycine binding site of the NMDA receptor (12,
17). A functional agonism to the glycine binding site has
therefore been postulated for the fluoroquinolones (10) but
could not be reproduced in the present study. In contrast to
Dimpfel's findings, no excitatory effects of glycine or other agonists
of this site, such as D-serine or D-alanine,
could be demonstrated electrophysiologically (21, 29).
With very high concentrations (10 mmol/liter), a reduction of the
population spike amplitude occurred (38). The modulatory
action of agonists to the glycine binding site should become obvious by
enhancing normal signals related to the channel. This was shown by the
slight increase of the population spike amplitude under low
Mg2+ concentrations. However, this effect was not
comparable at all to the strong effect of clinafloxacin under these
conditions. Interestingly, MK 801 failed to antagonize the effects of
the agonists of the glycine site, whereas 7-chlorokynurenic acid
was an excellent antagonist of this site (32, 38).
Therefore, based on the observations that (i) fluoroquinolones do not
bind to the glycine site, (ii) glycine site agonists do not or only
slightly potentiate the population spike, even under low
Mg2+ concentrations, and (iii) fluoroquinolone effects but
not glycine site agonist effects are completely antagonized by MK 801, we conclude that fluoroquinolones probably do not act via an agonism of
the glycine binding site.
Interestingly, Tanaka et al. (36) showed that
fluoroquinolones decreased the blocking effects of Mg2+ and
MK 801 binding to the receptor channel. They therefore characterized the fluoroquinolones as "open channel blockers." This is supported by our findings on the effect of Mg2+ on the population
spike amplitude, again underlining the involvement of the NMDA receptor
in the fluoroquinolones' convulsive action. Considering the
Mg2+ chelating properties of fluoroquinolones, which have
been also postulated as a mechanism for fluoroquinolone action in
juvenile cartilage (7, 24, 35), it is tempting to speculate
that the excitatory potency of fluoroquinolones might be based on
activation of the NMDA receptor by abolishing the Mg2+
block in the ion channel. This would prolong the opening time of the
channel, thus increasing intracellular Ca2+ concentrations
and the excitability of the neuron.
In summary, the determination of field potentials in the hippocampus
slice offers a fairly simple mechanistic model for the convulsive
properties of fluoroquinolones. It not only is useful for a rapid
screening of fluoroquinolones with regard to this relevant endpoint but
also enables more-detailed insight into the underlying mechanisms of
this effect.
| |
ACKNOWLEDGMENTS |
We thank Tanja Schubert for her excellent technical assistance
and H. J. Ahr for carefully reading the manuscript. Additionally, we thank U. Petersen for providing most of the compounds tested.
| |
FOOTNOTES |
*
Corresponding author. Mailing address: BAYER AG,
Institute of Toxicology, Aprather Weg, 42096 Wuppertal,
Germany. Phone: 49-(0)202-368830. Fax: 49-(0)202-364137.
E-mail: GABRIELE.SCHMUCK.GS{at}bayer-ag.de.
| |
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Abstract
Introduction
