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Publications (18)66.04 Total impact

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    ABSTRACT: The efficacy of the subchronically administered cholinesterase-inhibitor physostigmine (PHY) (0.025 mg/kg/h) either with or without the muscarinergic receptor antagonist scopolamine (SCO) (0.018 mg/kg/h) in counteracting soman-induced lethality and incapacitation were determined in guinea pigs. This was tested in animals that either received atropine sulphate (AS, 17.4 mg/kg IM) or no postintoxication therapy. Behavioral and neurophysiological readout systems were used to measure postintoxication incapacitation. Only the pretreatment with PHY alone did not offer any protection against 2× LD50 soman intoxication. Animals that received the complete treatment (PHY + SCO + AS) did not show any abberations in the performance of learned behavior. The use of AS after soman intoxication resulted in an increase of the startle response, whereas the addition of SCO to the pretreatment led to a more persistent duration of the effect in time. In case one has to rely completely on the pretreatment, the addition of SCO to PHY is life-saving. However, some postintoxication incapacitation is still present. Therefore, the pretreatment regime may perhaps further be improved by the addition of a nicotinic antagonist.
    Pharmacology Biochemistry and Behavior 02/2000; 65(1-65):175-182. DOI:10.1016/S0091-3057(99)00171-9 · 2.82 Impact Factor
  • Pharmacology Biochemistry and Behavior 01/2000; 65(1). · 2.82 Impact Factor
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    ABSTRACT: The behavioral and neurophysiological effects of the subchronically administered cholinesterase-inhibitor physostigmine (PHY) (0.025 mg/kg/h) either with or without the muscarinergic antagonist scopolamine (SCO) (0.018 mg/kg/h) were determined in guinea pigs. In contrast to a single injection of PHY, subchronic application by osmotic minipumps of PHY, even without SCO, caused no behavioral or neurophysiological side effects. Also, the efficacy of such a pretreatment in counteracting soman-induced lethality and apparent symptoms of intoxication were determined. After subchronically administered PHY or PHY + SCO, the treated animals were protected against a 3 × LD50 dose of soman.
    Pharmacology Biochemistry and Behavior 05/1998; 59(4-59):1061-1067. DOI:10.1016/S0091-3057(97)00511-X · 2.82 Impact Factor
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    ABSTRACT: To prevent incapacitation following nerve agent intoxications, it is proposed to replace pyridostigmine by the centrally active carbamate physostigmine (PHY). Behavioral and neurophysiological effects of PHY were determined and whether these effects would be counteracted by scopolamine. In addition, we compared them with the effects of another reversible cholinesterase (ChE) inhibitor ethyl-p-nitrophenylphosphoramidate (PNF). At similar levels of blood AChE inhibition, PHY caused a larger shuttlebox performance decrement than PNF, which was antagonized by scopolamine (0.1 mg/kg). SCO enhanced the PHY-induced increase of the auditory startle response, whereas PNF, with or without scopolamine, had no effect. In the EEG, PHY led to a power increase at the theta2-alphal band, also found after PNF, and at the thetal band. SCO antagonized all EEG effects, but not the effects of PHY on visual evoked responses, in contrast to those of PNF. Based on the different effects of both inhibitors, it is suggested that at relevant doses several PHY-induced phenomena occur that are unrelated to AChE inhibition.
    Pharmacology Biochemistry and Behavior 10/1996; 55(1-55):99-105. DOI:10.1016/0091-3057(96)83115-7 · 2.82 Impact Factor
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    ABSTRACT: The increased international concern about the threat of military and terroristic use of nerve agents, prompted us to critically consider the expected value of the currently available oxime treatment of nerve agent poisoning. Although oximes have been designed to reactivate the inhibited acetylcholinesterase (AChE), clinical experience has indicated that they are not always very effective as reactivators and at this very moment none of them can be regarded as a broad-spectrum antidote. In spite of this drawback, oximes are worth further investigating, since recent data derived from soman or tabun lethally intoxicated non-human primates suggest that the oxime HI-6 may exert a pharmacological effect that is not related to reactivation of inhibited AChE, but still leads to survival. This pharmacological effect causes recovery of neuronal transmission in the respiratory centres of the brain and recovery of neuromuscular transmission in the diaphragm. These findings have stimulated research to reveal the pharmacological basis of these effects in order to find drugs which could be more effective and less toxic than the available oximes. Since cholinergic drugs were able to exert this effect, a new concept for further treatment is suggested: maintenance of neuronal transmission in spite of continued AChE-inhibition by pharmacological manipulation of the cholinergic receptor. This should renew interest in the diverse pharmacological effects of oximes to reach a more effective treatment in the future.
    Archive für Toxikologie 02/1996; 70(12):779-86. DOI:10.1007/s002040050340 · 5.08 Impact Factor
  • Bert P.C. Melchers · Ingrid H.C.H.M. Philippens · Otto L. Wolthuis
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    ABSTRACT: The therapeutic efficacy of the oximes HI-6 and HLö-7 (132.5 μmol/kg), in combination with atropine, in soman- or tabunintoxicated guinea pigs was compared, particularly with respect to recovery of shuttlebox performance and electroencephalograms (EEGs). After 1.5 × LD50 soman SC, therapy with HI-6 or HLö-7 resulted in survival of 87.5% of the animals in each group. In both groups postintoxication performance decrements and EEG abnormalities lasted approximately 2 weeks after intoxication. After 3 × LD50 soman all HLö-7-treated animals died within 5 h; 70% of the HI-6-treated animals were still alive after 8 h; however, only 10% survived more than 24 h. After 2 × LD50 tabun 36% of the HI-6-treated animals died; HLö-7 prevented lethality and led to faster recovery of performance and EEG than after HI-6. Even after 7.5 × LD50 tabun, followed by HLö-7, full recovery was reached within 1 week in the surviving animals (82%). In soman-intoxicated guinea pigs HI-6 is therapeutically slightly more effective than HLö-7. HLö-7 is far more effective, under similar conditions, against tabun intoxication than HI-6.
    Pharmacology Biochemistry and Behavior 01/1995; 49(4-49):781-788. DOI:10.1016/0091-3057(94)90223-2 · 2.82 Impact Factor
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    ABSTRACT: The oximes HI-6, HLö-7, HGG-12, HGG-42 and obidoxime were used in a previously developed rat model to evaluate the therapeutic effects of oximes other than acetylcholinesterase (AChE) reactivation (so-called "non-reactivating effects"). To test this anaesthetized, atropinized and artificially ventilated rats (n = 8 or 16) were poisoned with a three times LD50 dose of the potent AChE-inhibitor crotylsarin (CRS, i.v.). CRS-inhibited rat AChE dealkylates instantaneously, thereby excluding AChE reactivation by the oximes. Five minutes after poisoning the rats were treated (i.v.) with an oxime or saline and 10 min later artificial ventilation was terminated. Survival times were determined. Saline-treated animals died within 15 min. In comparison, treatment with HI-6, HLö-7, HGG-12, HGG-42 or obidoxime resulted in a significant prolongation of survival time. In the groups treated with HLö-7, HI-6 or HGG-12, 12-37% of the animals survived more than 24 h. It was investigated whether differences in therapeutic effectiveness are caused by differences in pharmacokinetics of the oximes. The plasma half-lives of HI-6, HLö-7, HGG-12, HGG-42 and obidoxime amounted to 67, 63, 27, 55 and 179 min, respectively. At doses of 75 or 150 mumol/kg, all oximes could be detected in brain and medulla oblongata in similar amounts (6-10 nmol/g tissue). In vitro, all oximes were effective in restoring failure of neuromuscular transmission (NMT) caused by CRS, albeit with varying potency. All oximes bound with affinities in the micromolar range to rat brain muscarinic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)
    Archive für Toxikologie 02/1994; 68(4):224-30. DOI:10.1007/s002040050061 · 5.08 Impact Factor
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    ABSTRACT: This mini-review mainly describes a part of the pharmacological research carried out in our laboratory during the past decades, aimed at finding a therapy against intoxication by cholinesterase-inhibiting organophosphates, in particular against the nerve agent soman. In particular soman, because this is one of the nerve agents that consistently appears to be very resistant to treatment. Various experimental approaches are described. Yet, even after all these years of research an adequate (pre)treatment against poisoning by soman is still not available.
    Neuroscience & Biobehavioral Reviews 02/1994; 18(4):469-86. DOI:10.1016/0149-7634(94)90002-7 · 10.28 Impact Factor
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    ABSTRACT: Effects of the oxime HI-6, unrelated to reactivation of acetylcholinesterase (AChE), on field potentials in the dentate gyrus of the rat hippocampus following AChE inhibition, were investigated both in vitro and in vivo. In hippocampal slices, AChE inhibition decreased the perforant path evoked population spike amplitude (PSA). This effect could be prevented by pre-incubation of the slices with atropine (0.1-1 microM) or with the M1 muscarinic receptor antagonist pirenzepine (1 microM). A similar preventive effect was found after pre-incubation with the GABAA antagonist picrotoxin (20 microM), suggesting that the effects of AChE inhibition in vitro may be due to an enhancement of GABAergic inhibitory activity via activation of M1-muscarinic receptors. The effects of AChE inhibition in vivo were variable; both increases and decreases of the PSA were found. Following AChE inhibition, HI-6 increased the PSA dose-dependently, both in the in vivo and in the in vitro hippocampus. At higher oxime doses the perforant path stimulation elicited multiple population spikes. The effects of the oxime were presumably not mediated by an antagonism of cholinergic receptors, since they could not be mimicked with cholinergic antagonists like atropine, mecamylamine or gallamine. Further testing of the nature of the HI-6 effect in hippocampal slices in vitro, using a paired antidromic-orthodromic stimulation protocol, showed that HI-6 may interfere with GABAergic inhibition.
    Archive für Toxikologie 02/1994; 69(2):118-26. DOI:10.1007/s002040050146 · 5.08 Impact Factor
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    ABSTRACT: It is known that rats poisoned with near-lethal doses of pinacolyl methylphosphonofluoridate (soman) develop brain lesions, particularly when convulsions are induced. When rats were intoxicated with a LD50 of soman and treated immediately thereafter with a combination of low doses of atropine and diazepam (LOW AS/DZ treatment), large decrements in performance of an earlier acquired shuttle-box task were found 6 days after intoxication. In contrast, no such decrements were found in soman-intoxicated animals treated similarly with a combination of high doses of these drugs (HIGH AS/DZ treatment). Surprisingly, surviving LOW AS/DZ animals acquired the same task again at a speed that was almost as fast as before intoxication. Similarly treated animals were examined lightmicroscopically 24 h after intoxication; in LOW-AS/DZ-treated animals, neuropathology was only observed in animals that had exhibited convulsions, whereas in HIGH AS/DZ animals neither convulsions nor brain damage were observed. Power spectra, obtained from electroencephalograms (EEGs) 6 days after intoxication, revealed significant differences between both treatment groups, particularly in the δ-, θ-, and β-frequencies. After the HIGH AS/DZ treatment, a significant increase in δ activity was found compared to control values, suggestive of neuropathology. It is concluded that, in contrast with the LOW AS/DZ combination, HIGH AS/DZ prevents active avoidance deficits, convulsions, and lightmicroscopically detectable neuropathology after soman intoxication. However, the results of EEG measurements suggest that some aberrations may still remain even after the HIGH AS/DZ treatment.
    Pharmacology Biochemistry and Behavior 09/1992; 42(4-42):711-719. DOI:10.1016/0091-3057(92)90019-C · 2.82 Impact Factor
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    ABSTRACT: The therapeutic efficacy of the oxime HI-6 against intoxication with the irreversible cholinesterase (ChE) inhibitor soman was tested in marmoset monkeys. Five out of six marmosets, intoxicated with 5 x LD50 soman and treated immediately with diazepam (0.2 iv) and 15 sec later with atropine (0.5 im) and HI-6 (50 im), survived for more than 24 hr. One of these animals died after 4 days. In the HI-6-treated marmosets blood ChE activity was inhibited at a rate slower than that in three animals treated similarly but with saline instead of HI-6. The latter marmosets died within 8 min after soman. HI-6 achieved its plasma peak 5 min after injection and was eliminated with a t1/2 of about 40 min. In a second experiment similarly treated marmosets were euthanized at 5 min (three saline-treated animals) or at 10 min (three HI-6-treated animals) after the soman intoxication to enable the determination of acetylcholinesterase (AChE) activities in diaphragm and brain tissue. In addition, in these animals blood AChE and butyrylcholine esterase (BuChE) activities were determined. Low AChE activities were encountered in diaphragms and brains. These levels were not significantly different between saline- and HI-6-treated marmosets. In vitro treatment with HI-6 at 40 min after soman still led to an increase of the AChE activity, which was significant in diaphragm, suggesting that postmortem AChE inhibition had occurred. The ratio of AChE to BuChE in blood was significantly enhanced in HI-6-treated animals, indicating that HI-6 preferentially reactivated AChE. It is concluded that (i) HI-6 is an effective treatment against soman poisoning in marmosets and (ii) AChE reactivation or protection by HI-6 contributed to the survival of the animals.
    Toxicology and Applied Pharmacology 08/1992; 115(1):50-6. DOI:10.1016/0041-008X(92)90366-Z · 3.63 Impact Factor
  • Ingrid H.C.H.M. Philippens · Bert P.C. Melchers · Otto L. Wolthuis
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    ABSTRACT: Behavioral training of guinea pigs by conventional methods, such as used for rats and mice, appears difficult. Hence, only a few behavioral experiments with guinea pigs have been described in the literature. An active avoidance technique in an automated two-way shuttlebox is described using sound as a conditioned (CS) and a tactile stimulus (a stream of air ruffling their fur) as an unconditioned (UCS) stimulus. Acquisition is fairly rapid and reproducible. Doses of physostigmine that caused moderate blood acetylcholinesterase inhibition induced dose-dependent performance decrements. These decrements were counteracted by a sign-free dose of scopolamine.
    Pharmacology Biochemistry and Behavior 07/1992; 42(2-42):285-289. DOI:10.1016/0091-3057(92)90528-N · 2.82 Impact Factor
  • Bert P.C. Melchers · Anton L. Van der Laaken
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    ABSTRACT: Neuromuscular transmission shows a significant degree of spontaneous recovery after being impeded by acetylcholinesterase inhibition. Part of this recovery can be ascribed to de novo synthesis of acetylcholinesterase but another part is independent of enzyme activity. To unravel the mechanism underlying this synaptic adaptation to acetylcholinesterase inhibition we have compared a number of electrophysiological parameters in diaphragms taken from animals that were sacrificed within 15 min after a 2 x LD50 dose of the acetylcholinesterase inhibitor diisopropylfluorophosphate and from similarly treated animals killed after being kept alive for 3 h under artificial respiration. We found no differences in the quantal content. There was a significantly smaller degree of endplate potential rundown at tetanic stimulation and the miniature endplate potential amplitude was smaller in the 3-h adapted animals. In addition, the desensitization induced by carbachol appeared to be less in this group. It is likely that these synaptic changes, demonstrating the plasticity of the neuromuscular synapse, are involved in the spontaneous recovery of neuromuscular transmission after acetylcholinesterase inhibition.
    Brain Research 12/1991; 563(1-2):49-56. DOI:10.1016/0006-8993(91)91514-2 · 2.83 Impact Factor
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    ABSTRACT: Experiments were performed to elucidate the mechanism of action by which the oxime HI-6 causes a recovery of neuromuscular function after oxime-resistant inhibition of acetylcholinesterase by the organophosphate S27. In the presence of HI-6 (1-3 mM), the ability of isolated rat diaphragm muscle strips to sustain tetanic contractions after inhibition by S27 was markedly improved, as was the electrophysiological response to indirect tetanic stimulation. At lower concentrations (0-1 mM), HI-6 reduced the amplitude of the miniature endplate potentials and their decay time constant in a dose-dependent manner without having any effect on the resting membrane potential. In addition, HI-6 dose dependently increased the quantal content. It is likely that these post- and presynaptic effects of HI-6 are responsible for the improvement of muscle contractions after inhibition of acetylcholinesterase and they could well be of value in the therapy of organophosphate poisoning.
    European Journal of Pharmacology 09/1991; 200(2-3):331-7. DOI:10.1016/0014-2999(91)90591-D · 2.68 Impact Factor
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    ABSTRACT: A method was developed to study exclusively those therapeutic effects of oximes that are not related to reactivation of organophosphate-inhibited acetylcholinesterase (AChE). The model uses the organophosphorus compound crotylsarin (CRS), which proved to be a potent, irreversible, peripherally and centrally active AChE inhibitor with a very short biological half-life. CRS-inhibited AChE appeared to age very rapidly, because in vitro addition of oximes immediately following inhibition, did not result in any AChE reactivation. Anaesthetized, atropinized and artificially ventilated rats were intoxicated with 3 x LD50 CRS and treated 5 min later with the bispyridinium oxime HI-6. Fifty percent of these animals survived more than 24 h following termination of artificial ventilation at 10 min after oxime treatment. The mean survival time of the remaining animals was 66 min, whereas all untreated animals died within 4 min. HI-6, when added in vitro to isolated intact hemidiaphragms, or to diaphragm or brain homogenates from rats which had been killed 1 min following 3 x LD50 CRS, failed to reactivate the inhibited AChE. If blood was sampled (before and) after HI-6 administration to CRS-intoxicated rats, no HI-6-induced AChE reactivation was observed. Yet, a clear improvement of the neuromuscular transmission in the hindleg muscles of these animals was found following HI-6 injection. With this model, decisive evidence is obtained that non-reactivating effects of HI-6 by themselves are therapeutically relevant.
    Toxicology 02/1991; 69(3):331-44. DOI:10.1016/0300-483X(91)90191-3 · 3.75 Impact Factor
  • Herman P. M. van Helden · Jan de Lange · Ruud W. Busker · Bert P. C. Melchers
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    ABSTRACT: Isolated rat diaphragm preparations treated with soman or with the irreversible and oxime resistant cholinesterase (ChE) inhibitor S27 showed a considerable recovery of neuromuscular transmission (NMT) during incubation with the (bis)pyridinium oximes HI-6, HGG-12, P2S and obidoxime. In the soman-treated preparations this NMT recovery was predominantly caused by reactivation of acetylcholinesterase (AChE) but in the S27-treated preparations it was caused by a direct (pharmacological) effect unrelated to enzyme reactivation. Atropinized rats were artificially ventilated after injection with 3 x LD50 soman for 3 h and then treated with HI-6, i.e. at a time when oxime reactivation of soman inhibited ChE is no longer possible. Nevertheless, these rats started to breathe spontaneously and 50-60% survived more than 24 h, whereas all control animals (saline instead of HI-6) died within 10 min after artificial ventilation was terminated. In such animals no significant reactivation of ChE activity at various time intervals following HI-6 treatment was found, either in the diaphragms or in the brains. There was a significant amount of NMT (50%) in vitro in diaphragms obtained from these animals. This NMT did not improve in vitro in the presence of HI-6 and was not inhibited by soman administered to the medium. It is concluded that in this case the NMT found was based on synaptic adaptation to the continued inhibition of ChE and that the survival of the animals might be due to a combination of this synaptic adaptation and central direct effects of HI-6.
    Archive für Toxikologie 02/1991; 65(7):586-93. DOI:10.1007/BF01973721 · 5.08 Impact Factor
  • B.P.C. Melchers · H.P.M. van Helden
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    ABSTRACT: In a recent study (9) it was found in rats that chronic treatment with the irreversible cholinesterase inhibitors DFP or soman led to behavioral tolerance in the case of DFP, but not in the case of soman. Biochemically, no explanation was found for this difference between these two inhibitors. Notably, chronic administration of each of these inhibitors did not affect the availability of the nicotinic receptors at the motor endplate, in spite of very low cholinesterase activity. In an attempt to explain the different effects of these inhibitors a neurophysiological approach seemed appropriate. The spontaneous quantal release of acetylcholine from diaphragm muscles in vitro from animals chronically treated with each inhibitor showed a similar trend; compared with controls the MEPP frequency was decreased, which was significant for DFP, and the MEPP amplitude was increased, which was significant for soman. Neuromuscular function of muscle strips obtained from both DFP- or soman-treated animals appeared significantly more sensitive to additional inhibitor added in vitro. This could simply be explained by the high preexisting level of cholinesterase inhibition, but seems in contrast with the phenomenon of tolerance.
    Pharmacology Biochemistry and Behavior 03/1990; 35(2):321-5. DOI:10.1016/0091-3057(90)90163-C · 2.82 Impact Factor
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    ABSTRACT: In earlier studies the efficacy of atropine, diazepam and the oxime HI-6 was investigated in soman intoxicated marmoset monkeys. In one of these studies the monkeys were treated with all three drugs at 15 sec after soman and most of them survived. In another study, in which the animals received atropine and HI-6 at 1 min after soman, therapy was unsuccessful. It remained unclear whether these different results were caused by diazepam or by the time difference between soman and therapy administration. This was currently investigated in guinea pigs. The half-life of ageing in the marmoset in 1 min; in the guinea pig it is 8 min. To study the influence of the mentioned time difference, guinea pigs were either treated with HI-6 at 1 min or at 8 min after soman. Atropine and diazepam was in all cases given after 1 min. To establish the effect of diazepam on survival, this drug was omitted in some of the groups. The survival rate appeared to be the same in the 1 min and 8 min HI-6 groups. Apparently, waiting for one half-life of ageing did not deteriorate the therapeutic efficacy. Diazepam appeared to have a beneficial effect on the survival. From both results, it was concluded that the presence of diazepam in the first mentioned marmoset study was essential for the therapeutic success.