Effects of morphine dependence and withdrawal on levels of neurosteroids in rat brain

AIM: To investigate the effects of morphine dependence and withdrawal on the concentrations of neurosteroids in rat brain. METHODS: A method of simultaneous quantification of neurosteroids by gas chromatography-mass spectrometry (GC-MS) had been established. RESULTS: The chronic morphine administration (ip) resulted in a marked decrease in the brain concentrations of pregnenolone (PREG), progesterone (PROG), and pregenenolone sulfate (PREGS) in rats killed 6 h after the last treatment. In contrast, there were no significant effects of morphine dependence on the brain concentrations of allopregnanolone (AP), dihydroepiandrosterone (DHEA), and dihydroepiandrosterone sulfate (DHEAS). Naloxone-induced withdrawal produced a significant increase in the concentrations of PREG, PROG, AP, DHEA, PREGS, and DHEAS as compared with the control group. CONCLUSION: Morphine dependence and withdrawal affected the concentrations of neurosteroids in rat brain, which suggests that endogenous neurosteroids in brain might be     

The effects of neurosteroids on rat behavior and 3H-muscimol binding in the brain.

The effects of ICV administration of metabolites of progesterone and deoxycorticosterone [i.e., neurosteroids: AP (3alpha-hydroxy-5alpha-pregnan-20-one, allopregnanolone), 5alpha(-THDOC (3alphat-21-dihydroxy-5alpha-pregnan-20-one, 5alpha-tetrahydrodeoxycorticosterone), 5beta-THDOC (3alpha-21-dihydroxy-5beta-pregnan-20-one, 5beta-tetrahydrodeoxycorticosterone), and PS (3beta-hydroxy-5-pregnen-20-one sulfate, pregnenolone sulfate] were studied in the open-field test of neophobia and Vogel's test of conflict behavior in rats. The influence of in vivo administered 5beta-THDOC, a positive allosteric modulator of the GABA(A) receptor complex, on 3H-muscimol binding in different brain structures, was also studied with the help of quantitative autoradiography. The presented data did not reveal any anxioselective effects for a range of centrally active neurosteroids, in the ethologically orientated and conflict models of anxiety, after intracerebral drug administration. Their central effects appeared secondary to changes in rat gross behavior. It is possible that high local concentration of neurosteroids after ICV injection and production of a narrower range of behavioral effects than that of benzodiazepines, precluded manifestation of the antianxiety effects of AP, 5alpha-THDOC and 5beta-THDOC. Autoradiography did not reveal any significant changes in the specific binding of 3H-muscimol in brain structures after in vivo ICV administration of 5beta-THDOC at the behaviorally active dose. Thus, the possibility that neuroactive neurosteroids may provide a novel potential site for therapeutic interventions in anxiety disorders is not supported. The part of the experiment with 5beta-THDOC is interpreted as contributing to other results, suggesting the existence of a new category of neurosteroids acting as partial agonists of the GABA(A) receptor.     

酒精依赖和戒断时大鼠伏隔核、杏仁核区内神经甾体水平的变化

目的:采用♂大鼠酒精依赖模型,观察酒精依赖及戒断对大鼠伏隔核、杏仁核中不同神经甾体水平的影响。方法:通过大鼠自由饮含6%的乙醇溶液,连续42d使大鼠形成酒精依赖,并撤除酒精使其自然戒断。断头取脑分离取出伏隔核、杏仁核脑区。使用液液萃取和固相萃取两步法提取脑组织中的孕烯醇酮(PREG)、脱氢表雄酮(DHEA)、别孕烯醇酮(AP)、脱氢表雄酮硫酸酯(DHEAS)、孕烯醇酮硫酸酯(PREGS),以高效液相色谱-质谱联用法测定神经甾体含量。结果:与对照组相比,酒精依赖大鼠伏隔核DHEA,PREG,AP,DHEAS,PREGS的水平显著降低(P<0.05),杏仁核AP,PREGS的水平显著降低(P<0.05);酒精戒断6h大鼠伏隔核DHEA,DHEAS,PREGS的水平显著降低(P<0.05),杏仁核PREGS的水平显著降低(P<0.05);酒精戒断24h大鼠杏仁核DHEA的水平显著上升(P<0.05),伏隔核、杏仁核DHEAS、PREGS的水平显著下降(P<0.01)。结论:酒精依赖形成和戒断对伏隔核、杏仁核中的神经甾体水平有不同的影响,具有区域特异性。     

谷氨酸和γ-氨基丁酸对原代培养星形胶质细胞神经甾体合成释放的影响

目的研究氨基酸类神经递质对原代培养大鼠大脑皮质星形胶质细胞神经甾体合成释放的影响。方法采用原代培养的大鼠大脑皮质星形胶质细胞,分别加入不同浓度的谷氨酸和γ-氨基丁酸处理48h;采用固相萃取结合高效液相色谱质-谱联用分析方法提取分离和测定细胞培养液中游离型(脱氢表雄酮,DHEA;孕烯醇酮,PREG;别孕烯醇酮,AP)及结合型神经甾体(脱氢表雄酮硫酸酯,DHEAS;孕烯醇酮硫酸酯,PREGS)。结果与生理盐水对照组比较,谷氨酸处理使PREG和PREGS水平明显下降,DHEAS水平明显升高;γ-氨基丁酸处理使PREG水平明显降低,AP水平增加。结论谷氨酸和γ-氨基丁酸两种神经递质对原代培养的星形胶质细胞PREG合成释放均呈抑制作用;谷氨酸对DHE-AS、γ-氨基丁酸对AP的合成释放分别呈现明显促进作用;高剂量的谷氨酸还可以抑制PRGES的合成和释放。     

MK-801 and AP5 impair acquisition, but not retention, of the Morris milk maze

The effects of the NMDA blockers, AP5 and MK-801, were assessed in two spatial tests. AP5 (10 micrograms in 2 microliters ICV, N = 6), or MK-801 (0.07 mg/kg IP, N = 6), significantly increased open-field activity in male Long-Evans rats in two 3-min tests (Days 1 and 2) compared to control groups receiving equal volume saline injections (N = 12). In the Morris milk maze, NMDA blockade significantly impaired acquisition performance on two blocks of six trials, which followed each open-field test. Only control animals showed evidence of acquisition on a drug-free retention test assessing latency to reach the expected platform area and number of crossings in the area on Day 4. Retention was tested in control animals under NMDA blockade on Day 6. There was no effect of NMDA blockade on retention in the Morris milk maze. These results support the hypothesis that NMDA receptors are critical for the initiation of synaptic modification underlying place learning, but are not necessary in synaptic transmission during retrieval of place information.     

Effect of phosphamidon on convulsive behavior and biochemical parameters: modulation by progesterone and 4′-chlorodiazepam in rats

Phosphamidon (PHOS) has been shown to affect nervous system adversely. The present study was designed to explore the modulation of the effects of PHOS on convulsions by neurosteroids, progesterone (PROG), and 4′-chlorodiazepam (4′-CD), in both acute and chronic seizure models. In acute study, seizures were induced by either pentylenetetrazole (PTZ) injection or maximal electroshock seizures, while in the chronic study, kindling was induced by injecting PTZ (30 mg/kg, s.c.) on alternate days three times in a week. Oxidative stress was assessed in the brain by measuring the levels of malondialdehyde (MDA), acetylcholinesterase (AChE), and non-protein thiol (NP-SH). PROG and 4′-CD were able to modulate the PHOS-induced convulsions in acute PTZ convulsions as well as in chronic kindling model. However, they failed to reverse the derangements in oxidative stress parameters of MDA and NP-SH produced by PHOS in kindled animals. PROG significantly increased the AChE activity in untreated rats, while PROG and 4′-CD reversed the AChE activity inhibition induced by PHOS. The study indicates a possible anticonvulsive mechanism of neurosteroids, since both PROG and 4′-CD reversed PHOS-induced inhibition of AChE activity. The neurosteroids seem to play a protective role in PHOS-induced convulsions besides their antioxidant property.     

Antagonism of caffeine-induced convulsions by ethanol and dizocilpine (MK-801) in mice

Ethanol (1 and 2 g/kg, i.p.) and MK-801 (1 mg/kg, i.p.), N-methyl-D-aspartate (NMDA) receptor antagonist, offered protection against caffeine-induced convulsions in mice. Subeffective doses of ethanol (0.5-g/kg, i.p.) and MK-801 (0.5 mg/kg, i.p.) when administered concurrently, did not provide a facilitatory anticonvulsant action against caffeine-induced convulsions. Ethanol (0.5 g/kg, i.p.) when administered concurrently with adenosine (100 mg/kg, i.p.) or dipyridamole (5 mg/kg, i.p.) elicited a facilitatory anticonvulsant action. However, concurrent administration of subeffective doses of MK-801 (0.5 mg/kg i.p.) with adenosine (100 mg/kg, i.p.) did not offer a facilitatory anticonvulsant action against caffeine-induced convulsions. The protective effect of ethanol (1 g/kg, i.p.) against caffeine-induced convulsions was reversed by an imidazobenzodiazepine, Ro 15-4513 (4 mg/kg, i.p.). Ro 15-4513 did not produce any proconvulsant effect with caffeine. It is suggested that ethanol and MK-801 elicit their anticonvulsant actions against caffeine-induced convulsions through different receptor mechanisms and that the anticonvulsant action of ethanol may be partly attributed to its ability to act via central adenosinergic mechanisms.     

Antagonism of cocaine, amphetamine, and methamphetamine toxicity

The effect of diazepam, haloperidol, MK-801, and propranolol in antagonizing behavioral symptoms induced by lethal doses of cocaine, amphetamine, and methamphetamine were studied in a rat model. Animals were first pretreated IP with potential antagonists, diazepam (2, 5, and 10 mg/kg), haloperidol (5, 10, and 20 mg/kg), propranolol (5, 10, and 20 mg/kg), MK-801 (0.5, 1.0, and 2.5 mg/kg), and then were challenged IP with cocaine (70 mg/kg) (LD85), d-amphetamine (75 mg/kg) (LD100), and methamphetamine (100 mg/kg) (LD90). Diazepam, at all doses, provided significant protection against cocaine- (p less than or equal to 0.01) and methamphetamine- (p less than or equal to 0.05) induced seizures and produced a dose-dependent effect against amphetamine-induced seizures. MK-801, at all doses, reduced seizures in all groups (p less than or equal to 0.01). Propranolol altered the incidence of methamphetamine-induced seizures. Significant protection against cocaine-induced death was afforded by diazepam (p less than or equal to 0.01) and propranolol (p less than or equal to 0.05). Significant protection against amphetamine-induced death was provided by haloperidol (all doses, p less than or equal to 0.1), MK-801 (all doses, p less than or equal to 0.1), and propranolol (10 and 20 mg/kg, p less than or equal to 0.1). No agent reduced the incidence of methamphetamine- (50 or 100 mg/kg) induced death. The failure of d-amphetamine antagonists to protect against methamphetamine-induced toxicity and death suggest that different mechanisms of toxicity may exist between these drugs.     

Picrotoxin-induced tonic-clonic seizures and lethality are decreased by MK-801 in developing rats.

The action of MK-801 (NMDA antagonist; 0.1 and 0.5 mg/kg, IP) was tested against picrotoxin-induced seizures (3-6 mg/kg, IP) in rats aged 7, 12, 18, 25, and 90 days. We found MK-801 only inconsistently affected clonic seizures in 12- and 25-day-old rats, whereas tonic-clonic seizures were suppressed or delayed in almost all age groups. In addition, the lethality of picrotoxin was diminished by the higher dose of MK-801 in all age groups. The results suggest: a) different generators for both seizure patterns (clonic and tonic-clonic), b) an involvement of NMDA receptors in the genesis of tonic-clonic seizure pattern, and c) an interaction of MK-801 with GABAergic transmission throughout the entire development studied.     

Anticonvulsant activity of (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (MK-801), a substance with potent anticonvulsant,central sympathomimetic,and apparent anxiolytic properties

MK-801 prevented tonic extensor seizures in the rat induced by bicuclline with the ED₅₀ being 23 μg/kg p.o. Clonazepam, phenobarbital, diazepam, phenytoin, γ-acetylenic GABA, sodium valproate, and trimethadione were all less potent. In mice, MK-801 was likewise the most potent (ED₅₀ = 0.35 mg/kg p.o.) compound in protecting against tonic seizures induced by electroshock. Clonazepam (ED₅₀ = 0.41 mg/kg p.o.) and MK-801 (ED₅₀ = 0.67 mg/kg p.o.) were by far more potent than any of the other anticonvulsants tested versus bicuculline-elicited seizures in mice. Besides being a potent anticonvulsant, MK-801 demonstrated selectivity, since much higher doses were required in mice to block clonic convulsions produced by pentylenetetrazol (ED₅₀ = 11 mg/kg p.o.) and tonic seizures caused by strychnine (ED₅₀ > 15 mg/kg p.o.) than were needed against electroshock or bicuculline. The anticonvulsant (electroshock) effect of MK-801 in mice was unaffected by pretreating the animals with various receptor antagonists (atropine, mecamylamine, chlorpheniramine, tripelennamine, cyproheptadine, cinanserin, methysergide, cimetidine, and propranolol). MK-801 was slightly, but significantly, antagonized by methergoline, naloxone, and theophylline, whereas haloperidol and especially α-adrenoceptor blockers (prazosin, HEAT, phenoxybenzamine) markedly reduced the anticonvulsant effect of MK-801. Haloperidol was selective for MK-801, not affecting the anticonvulsant actions of phenytoin or phenobarbital. Prazosin antagonized phenytoin and phenobarbital, but to a much lesser extent than it antagonized MK-801. MK-801 is an extremely potent and selective anticonvulsant acting at least partly via a catecholaminergic mechanism.     

吗啡依赖对大鼠不同脑区内神经甾体水平的影响

目的建立大鼠条件性位置偏爱(CPP)模型，探讨吗啡精神依赖对大鼠脑内神经甾体水平的影响。方法 大鼠连续10 d腹腔注射吗啡5 mg·kg^-1，诱导CPP形成。高效液相色谱-质谱法测定大鼠伏隔核、杏仁核、下丘脑和血浆中脱氢表雄酮、孕烯醇酮、别孕烯醇酮、脱氢表雄酮硫酸酯及孕烯醇酮硫酸酯的含量。结果经10 d吗啡训练后，吗啡组大鼠在伴药侧的停留时间显著长于对照组，吗啡诱导的大鼠CPP形成。与对照组相比，吗啡组大鼠下丘脑内孕烯醇酮明显降低，伏隔核和血浆中的脱氢表雄酮明显降低。结论吗啡诱导CPP形成，吗啡处理影响大鼠脑内某些神经甾体的水平，表明内源性神经甾体可能参与吗啡依赖的形成。     

NMDA receptors in mice bred to be prone or resistant to ethanol withdrawal seizures

Selective breeding has produced replicate lines of mice that are prone (WSP) or resistant (WSR) to ethanol withdrawal seizures. Ethanol-naive WSP mice inherently have a greater number of hippocampal binding sites for the NMDA receptor-gated ion channel blocker, MK-801, than ethanol-naive WSR mice. After chronic ethanol ingestion, hippocampal (but not cerebral cortical) MK-801 binding sites increase in both lines of mice. However, the number of MK-801 binding sites in the ethanol-treated WSR mice does not exceed the number of MK-801 binding sites in untreated WSP mice. At the time of ethanol withdrawal, the number of hippocampal MK-801 binding sites in each line of WSP mice is 50-70% higher than the number of such sites in WSR mice. Given the past evidence for a role of the NMDA receptor in seizures, the results implicate hippocampal NMDA receptor-gated channels in the generation of ethanol withdrawal seizures.     

Subjective effects and changes in steroid hormone concentrations in humans following acute consumption of alcohol

Background: GABA_A receptors are an important site of action of endogenous neurosteroids and an important mediator of several behavioral effects of alcohol. This study examined the effects of alcohol on plasma steroid hormone concentrations on the hypothesis that the endocrine effects mediate some of the subjective effects of alcohol. Methods: Thirty-two healthy subjects (17 men) with no history of a substance use disorder participated in this human laboratory study. All subjects consumed three standard drinks of grain alcohol. Subjective measures and blood samples for steroid concentrations were collected at baseline and 40 min after alcohol consumption. Results: Alcohol increased self-reported stimulation, alcohol liking, and desire for more alcohol. Alcohol also increased pregnenolone (PREG) and dehydroepiandrosterone (DHEA) concentrations, while it decreased progesterone (PROG) and allopregnanolone (ALLO) concentrations, as well as ALLO/PREG and PROG/PREG ratios. In men, the change in PREG concentration was significantly correlated with alcohol liking, while the alcohol-induced change in ALLO concentration correlated significantly with both alcohol liking and desire for more alcohol. Discussion: These findings provide preliminary support for the hypothesis that endogenous neurosteroids mediate some of the subjective effects of alcohol. Efforts to replicate these findings should aim to specify more clearly the nature and time course of the effects.     

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis

MK-801 is a non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist with anticonvulsant and neuroprotective properties. The action of MK-801 (0.05–10 mg/kg IP) was assessed against pentylenetetrazol-induced seizures (PTZ; 100 mg/kg SC; 30 min after MK-801) in rats aged 7, 12, 18, 25, and 90 days (N=263). We observed pronounced ataxia and hypermobility after MK-801 pretreatment during the whole ontogenesis, and the animals exhibited head-weaving and body-rolls. After the combination of MK-801 and PTZ wet dog shakes were detected in 18-, 25-, and 90-day-old rats (never seen in controls receiving PTZ only). MK-801 only insignificantly modified the latencies of minimal (clonic) seizures in 18-day-old and older rats where this seizure type is regularly elicited. In 12-day-old rats an increased incidence of minimal seizures was detected. MK-801 nearly completely blocked or strongly delayed major (generalized tonic-clonic) seizures and attenuated the seizure severity during ontogenesis in a dose-dependent manner. Present results suggest the important role of NMDA receptors in the genesis of generalized tonic-clonic seizures whilst the role of NMDA receptors in minimal seizures appears to be negligible during the whole ontogenetic development.     

Continuous de novo synthesis of neurosteroids is required for normal synaptic transmission and plasticity in the dentate gyrus of the rat hippocampus

Both in vivo and in vitro studies have shown that neurosteroids promote learning and memory by modulating synaptic functions in the hippocampus. However, we do not know to what degree endogenously synthesized neurosteroids contribute to the hippocampal synaptic functions. Cytochrome P450scc is the enzyme that converts cholesterol to pregnenolone (PREG), which is required for the biosynthesis of all other neurosteroids. To investigate the physiological roles of endogenous neurosteroids in synaptic functions, we electrophysiologically examined the effects of aminoglutethimide (AG), a selective inhibitor of P450scc, on the synaptic transmission and plasticity in the dentate gyrus of rat hippocampal slices. The application of AG (100 μM) decreased the slope of the field excitatory postsynaptic potentials (fEPSPs) in granule cells by 20-30% in 20 min through the modulation of postsynaptic AMPA receptors, while it did not affect the presynaptic properties, including the paired-pulse ratio and the probability of glutamate release from presynaptic terminals. The AG-induced depression was nearly completely rescued by exogenously applied 500 nM PREG or by 1 nM dehydroepiandrosterone sulfate (DHEAS), one of the neurosteroids synthesized from PREG, suggesting that the AG-induced depression was caused by the loss of DHEAS. AG also reduced NMDA receptor activity, and suppressed high-frequency stimulation (HFS)-induced long-term potentiation (LTP). These findings provide novel evidence that the endogenous neurosteroids locally synthesized in the brain are required to maintain the normal excitatory synaptic transmission and plasticity in the dentate gyrus of the rat hippocampus.     

Effects of drugs on the convulsions induced by the combination of a new quinolone antimicrobial, enoxacin, and a nonsteroidal anti-inflammatory drug, fenbufen, in mice]

The effects of drugs on the convulsions induced by the combination of a new quinolone antimicrobial, enoxacin, and a nonsteroidal anti-inflammatory drug, fenbufen, were studied in mice. The combination of enoxacin at 30 or 100 mg/kg, p.o. and fenbufen at 100 mg/kg, p.o. induced convulsions; and the mice died as a result of the convulsions. Pretreatment with either phenobarbital, phenytoin, valproic acid intraperitoneally, or morphine intravenously did not influence the convulsions. A high dose of diazepam or clonazepam prolonged the survival time, but could not prevent the mice from dying. After the occurrence of convulsions induced by enoxacin with fenbufen, administration of the excitatory amino acid antagonist MK-801 at 1 mg/kg, i.v. extended the survival time, even though all the mice died as a result of the convulsions. Simultaneous intravenous injections of MK-801 and diazepam suppressed the convulsions. This suppression was stronger than that produced by MK-801 or diazepam, injected separately. However, no mouse survived at the end. From these results, participation of both GABA-ergic and excitatory amino acidergic systems in the convulsions induced by enoxacin and fenbufen was discussed.     

Aβ_(25-35)对大鼠大脑皮质神经元神经甾体水平的影响

目的研究Aβ25-35对大鼠大脑皮质神经元合成神经甾体的影响。方法采用Aβ25-35(1μmol·L-1)处理原代培养的大鼠大脑皮质神经元,固相萃取结合高效液相色谱-质谱联用法测定细胞培养液中神经甾体的浓度,四甲基偶氮唑(MTT)法检测细胞活性。结果与对照组相比,Aβ25-35处理后神经元存活率降低约50%(P<0.01),胆固醇处理组神经元存活率未见改变(P>0.05),但Aβ+胆固醇组的神经元存活率明显高于Aβ处理组(P<0.01)。与胆固醇组相比,Aβ25-35处理后神经甾体PREG水平略有降低(P>0.05),PROG水平明显下降(P<0.01),AP水平明显增高(P<0.01)。结论神经甾体PREG-PROG-AP代谢通路在Aβ25-35引起的大鼠大脑皮质神经元损伤时发生明显改变,并能部分地对抗Aβ的神经毒性作用。     

Potentiation of the Ambulation-increasing effect induced by combined administration of MK-801 with ethanol in mice

Although both MK-801 (dizocilpine: 0.1 mg/kg, IP) and ethanol (1.6 nd 2.4 g/kg, PO) only slightly increased ambulatory activity in mice, their combination produced a marked enhancement of the ambulation-increasing effect. The combination of MK-801 (0.03 mg/kg) with ethanol (1.6 and 2.4 g/kg) also elicited a significant increase in the mouse's ambulation. A significant enhancement of the effect was produced by the combination of ketamine (3 and 10 mg/kg) with ethanol only (2.4 g/kg). The ambulation increment induced by the combination of MK-801 (0.1 mg/kg) plus ethanol (1.6 g/kg) was dose-dependently inhibited by YM-09151-2 (0.0001–0.01 mg/kg IP), SCH 23390 (0.001–1 mg/kg IP), reserpine (0.1–1 mg/kg, IP) and ceruletide (0.00001–0.001 mg/kg, IP), and the highest dose of each drug was effective for complete inhibition of the ambulation. Naloxone (0.05–5 mg/kg IP), apomorphine (0.001–0.1 mg/kg IP) and -methyl-p-tyrosine (50–200 mg/kg, IP) partially reduced the ambulatory activity induced by the combination of MK-801 with ethanol. These results suggest that the dopaminergic system, particularly via presynaptic changes in the release of stored dopamine, as well as the opioid system, are involved in the interaction of MK-801 with ethanol.     

吗啡对原代培养大鼠皮质神经元神经甾体水平的影响

目的探讨吗啡慢性处理对大鼠大脑皮质神经元合成释放神经甾体水平的影响及其机制。方法建立原代培养大鼠大脑皮质神经元吗啡依赖样模型,固相萃取结合高效液相色谱-质谱联用法测定细胞培养液中神经甾体水平;免疫印迹法检测神经元中p-CREB水平。结果与盐水对照组相比,吗啡(1μmol.L-1)处理使PREG和DS的水平降低(P<0.01);μ-受体激动剂DAMGO使PREG、DS和PS水平下降,AP水平增高;与吗啡单独处理组相比,μ-阿片受体拮抗剂CTAP与吗啡联合处理使PREG增加(P<0.01)。与盐水对照组相比,吗啡或DAMGO慢性处理均可增加神经元内p-CREB的水平(P<0.01);与吗啡处理组相比,CTAP抑制吗啡诱导的p-CREB的增加。结论μ-阿片受体参与了介导吗啡慢性处理对皮质神经元神经甾体合成释放的影响;神经元内p-CREB水平的变化反映了吗啡引起的神经元适应性改变,提示神经甾体水平的变化可能与吗啡依赖有关。     

Anticonvulsant activity of MK-801 and nimodipine alone and in combination against pentylenetetrazole and strychnine

The effects of the N-methyl-D-aspartate receptor antagonist MK-801 and the dihydropyridine calcium channel antagonist nimodipine were assessed for their anticonvulsant activity alone and in combination against clonic convulsions to pentylenetetrazole (PTZ) and strychnine (STR) in mice. Nimodipine (2-20 mg/kg) and MK-801 (0.1 and 0.5 mg/kg) did not affect the number of mice displaying clonic convulsions to PTZ. However, nimodipine in a dose-dependent manner increased (100%) the latency to clonic convulsions and lethality (mortality from tonic extension convulsions and respiratory failure) following PTZ. In contrast, MK-801 did not increase the latency to PTZ convulsions, but prevented the lethal effects of PTZ. When combined, MK-801 and nimodipine produced a significant reduction in the number of animals (40-60%) displaying PTZ convulsions and a greater increase in the latency to PTZ convulsions than did nimodipine alone. In contrast, MK-801 decreased the onset time, and increased the severity of STR convulsions. A combination of MK-801 and nimodipine which afforded significant protection against PTZ convulsions did not affect STR convulsions. These findings suggest that MK-801 and nimodipine, while possessing significant anticonvulsant activity on their own, produce a potent anticonvulsant synergism against PTZ but not STR.