Chlormethiazole: effectiveness against toxic effects of cocaine in mice.

Chlormethiazole positively modulates the gamma-aminobutyric acid (GABA)(A) receptor complex and is primarily used to treat certain life-threatening neurological events (e.g., refractory seizures and ethanol withdrawal syndrome). On account of several experimental and clinical studies reporting effectiveness against the toxic effects of heroin and methamphetamine, chlormethiazole was systematically tested in the present study for its effectiveness against cocaine-induced seizures and lethality in mice. The protective effects of chlormethiazole were evaluated against single, submaximal convulsive (75 mg/kg) or lethal (110 mg/kg) doses of cocaine. Chlormethiazole also was tested against the expression (anticonvulsant effect) and development (antiepileptogenic effect) of cocaine-kindled seizures, and against fully developed kindled seizures. Cocaine-kindled seizures were produced by a total of five daily treatments with 60 mg/kg cocaine. The inverted-screen test was used to assess behavioral side effects of chlormethiazole. Chlormethiazole protected against acute cocaine-induced convulsions (ED(50) = 7.0 mg/kg) and lethality (ED(50)= 21.8 mg/kg) with a robust separation [protective index (PI) = TD(50)/ED(50) = 22.3 and 7.2, respectively] from doses producing behavioral side effects (TD(50) = 156 mg/kg). Chlormethiazole suppressed the behavioral expression of cocaine-kindled seizures and prevented the development of sensitization to the convulsant effects of cocaine. It was also effective in suppressing fully developed kindled seizures. Relative to cocaine seizures in naive mice, chlormethiazole was equieffective, less potent (ED(50) = 22.3 mg/kg), and had a reduced protective index (PI = 3.7) against cocaine-induced seizures in kindled mice. The protective profile and protective index of chlormethiazole were superior to those of the benzodiazepines clonazepam and diazepam, which were of limited efficacy and had low protective indices (PI = approximately 1). The results of this study predict the potential utility of chlormethiazole for the treatment of life-threatening complications of cocaine abuse for which no specific treatment has yet been identified.

Modification of behavioral effects of drugs in mice by neuroactive steroids.

RATIONALE: Neuroactive steroids represent a novel class of potential therapeutic agents (epilepsy, anxiety, migraine, drug dependence) thought to act through positive allosteric modulation of the GABA(A) receptor. A synthetically derived neuroactive steroid, ganaxolone (3alphahydroxy-3beta-methyl-5alpha-pregnan-20-one), is in phase-II clinical trials for epilepsy. Unlike traditional anticonvulsants such as diazepam and phenobarbital, ganaxolone shows equipotent suppression of both the seizure activity and the behavioral effects of pentylenetetrazol (PTZ) administration. OBJECTIVES: The present study explored possible reversal by ganaxolone and related neuroactive steroids of some behavioral effects of additional pharmacological challenges. METHODS: Direct behavioral observation and photocell-counted locomotor activity of male, Swiss-Webster mice were made with various compounds alone and in conjunction with ganaxolone. RESULTS: Ganaxolone both prevented and reversed PTZ-induced locomotor depression in mice. Further, ganaxolone reversed the locomotor depression induced by other convulsant/anxiogenic stimuli: bicuculline, picrotoxin and, to a lesser extent, yohimbine. Ganaxolone failed to reverse the locomotor stimulation induced by cocaine, methamphetamine, dizocilpine, and phencyclidine. In addition to ganaxolone, the endogenous neuroactive steroids allopregnanolone and pregnanolone and the synthetic neuroactive steroid Co 2-1068 also reversed observed behaviors and locomotor depression induced by PTZ. CONCLUSIONS: The present findings support the unique pharmacological effects of neuroactive steroids as a novel class of positive allosteric modulators of GABA.

Acute and chronic effects of the synthetic neuroactive steroid, ganaxolone, against the convulsive and lethal effects of pentylenetetrazol in seizure-kindled mice: comparison with diazepam and valproate.

A high-affinity positive modulator of the GABA(A) receptor complex, ganaxolone, is a 3beta-methylated analog of the naturally occurring neuroactive steroid allopregnanolone. In the present study, ganaxolone was tested for its ability to (1) suppress seizures (clonic and tonic) and lethality induced by pentylenetetrazol (PTZ) in PTZ-kindled mice (anticonvulsive effect) and (2) to attenuate the development of sensitization to the convulsive and lethal effects of PTZ in kindled mice (anti-epileptogenic effect) when given as a pretreatment prior to each PTZ injection during kindling acquisition. Two classical antiepileptic drugs, diazepam and valproate, were tested for comparison. All three drugs dose-dependently suppressed tonic seizures and lethality induced by PTZ in kindled mice; only ganaxolone was effective against clonic seizures. Ganaxolone showed anti-epileptogenic properties as it reduced the sensitivity of kindled mice to the convulsive (clonic and tonic seizures) and lethal effects of PTZ. Diazepam showed anti-epileptogenic effects against tonic seizures and lethality, but not clonic seizures; valproate was ineffective in preventing development of any of these effects. Sensitivity to PTZ-induced seizures and lethality was not affected in mice with a history of repeated treatment with ganaxolone, diazepam, or valproate. The drugs had effects on ambulatory activity that ranged from no effect (ganaxolone) through moderate impairment (diazepam) to marked disruption (valproate). Taken together, the results of the present study add to accumulating evidence of the unique anticonvulsive/behavioral profile of neuroactive steroids.

Pharmacological and behavioral characterization of cocaine-kindled seizures in mice.

RATIONALE: Convulsions associated with cocaine toxicity are a serious aspect of cocaine-related emergency room incidents. Seizures can result from a single high dose of cocaine, and evidence is accumulating that correlates repetitive administration of sub-convulsive doses of cocaine with a decreased seizure threshold, a phenomenon known as pharmacological kindling. A murine model of cocaine kindling has not been characterized. OBJECTIVES: To determine the necessary and sufficient conditions for generating increased sensitivity to the convulsive and lethal effects of cocaine and to characterize some of the basic pharmacological and behavioral consequences of this phenomenon in mice. METHODS: Male, Swiss-Webster mice were given repeated injections of cocaine. RESULTS: Daily administration of 60 mg/kg cocaine produced robust kindling; significant leftward shifts in the dose-effect curves for seizures were observed in cocaine-kindled mice. Cocaine kindling was enduring as these left shifts persisted for at least 20 days, indicating possible permanent synaptic changes. Induction of convulsions per se, utilizing 75 mg/kg cocaine, was not sufficient to engender kindling with a non-optimal dose (40 mg/kg). However, administration of a non-kindling dose of cocaine (40 mg/kg) for as few as four occasions produced increased seizure sensitivity to a 60-mg/kg cocaine challenge. The lethal potencies of cocaine and methamphetamine were significantly increased in cocaine-kindled mice. The baseline locomotor activity of kindled mice was not different from that of non-kindled mice. However, challenge doses of cocaine revealed significant differences in the vertically directed activity of kindled versus non-kindled mice. CONCLUSIONS: Overall, this study provides a description of important parameters for a model of cocaine kindling in mice that may be useful for the elucidation of mechanisms responsible for the long-term changes in sensitivity to cocaine and the discovery of novel pharmacological treatments.

Lethal seizures predicted after aminophylline therapy in cocaine abusers.

Mice with a history of chronic (10 days), but not acute, treatment with a non-convulsant dose of cocaine showed increased sensitivity (P<0.001) to the toxic effects of aminophylline (seizures, lethality) relative to controls even days after the cessation of cocaine treatment. The present finding suggests that individuals with a history of cocaine use may be at increased risk for convulsive and lethal complications associated with the therapeutic use of aminophylline.

Preclinical evaluation of newly approved and potential antiepileptic drugs against cocaine-induced seizures.

Seizures and status epilepticus are among the neurological complications of cocaine overdose in humans. The aim of the present study was to evaluate the protective effectiveness and therapeutic index (separation between anticonvulsive and side effect profiles) of 14 newly approved and potential antiepileptic drugs using a murine model of acute cocaine toxicity and the inverted-screen test for behavioral side effect testing. Cocaine (75 mg/kg i.p.) produces clonic seizures (approximately 90% of mice), and conventional antiepileptic drugs have been reported to be either ineffective or only effective at doses producing significant sedative/ataxic effects. Clobazam, flunarizine, lamotrigine, topiramate, and zonisamide were ineffective against seizures up to doses producing significant motor impairment. In contrast, felbamate, gabapentin, loreclezole, losigamone, progabide, remacemide, stiripentol, tiagabine, and vigabatrin produced dose-dependent protection against cocaine-induced convulsions with varied separations between their anticonvulsant and side effect profiles: the protective index values (toxic TD(50)/anticonvulsive ED(50)) ranged from 1.26 (felbamate) to 7.67 (loreclezole), and gabapentin had the highest (protective index >152). Thus, several drugs were identified with greater protective efficacy and reduced motor impairment compared with classic antiepileptic drugs. Based on the proposed mechanism of action of these new anticonvulsants, it is noteworthy that 1) drugs that enhance gamma-aminobutyric acid-mediated neuronal inhibition in a manner distinct from barbiturates and benzodiazepines offer the best protective/behavioral side effect profiles, and 2) functional antagonists of Na(+) and Ca(2+) channels are generally ineffective. Overall, this study provides the first description of the effectiveness of new antiepileptic drugs against experimentally induced cocaine seizures and points to several drugs that deserve clinical scrutiny for this indication.

Behavioral, toxic, and neurochemical effects of sydnocarb, a novel psychomotor stimulant: comparisons with methamphetamine.

Sydnocarb (3-(beta-phenylisopropyl)-N-phenylcarbamoylsydnonimine) is a psychostimulant in clinical practice in Russia as a primary and adjunct therapy for a host of psychiatric disorders, including schizophrenia and depression. It has been described as a stimulant with an addiction liability and toxicity less than that of amphetamines. The present study undertook to evaluate the psychomotor stimulant effects of sydnocarb in comparison to those of methamphetamine. Sydnocarb increased locomotor activity of mice with reduced potency (approximately 10-fold) and efficacy compared with methamphetamine. Sydnocarb blocked the locomotor depressant effects of haloperidol at doses that were inactive when given alone. The locomotor stimulant effects of both methamphetamine and sydnocarb were dose-dependently blocked by the dopamine D1 and D2 antagonists SCH 39166 and spiperone, respectively; blockade generally occurred at doses of the antagonists that did not depress locomotor activity when given alone. In mice trained to discriminate methamphetamine from saline, sydnocarb fully substituted for methamphetamine with a 9-fold lower potency. When substituted for methamphetamine under self-administration experiments in rats, 10-fold higher concentrations of sydnocarb maintained responding by its i.v. presentation. Sydnocarb engendered stereotypy in high doses with approximately a 2-fold lower potency than methamphetamine. However, sydnocarb was much less efficacious than methamphetamine in inducing stereotyped behavior. Both sydnocarb and methamphetamine increased dialysate levels of dopamine in mouse striatum; however, the potency and efficacy of sydnocarb was less than methamphetamine. The convulsive effects of cocaine were significantly enhanced by the coadministration of nontoxic doses of methamphetamine but not of sydnocarb. Taken together, the present findings indicate that sydnocarb has psychomotor stimulant effects that are shared by methamphetamine while demonstrating a reduced behavioral toxicity.

Reversal of behavioral effects of pentylenetetrazol by the neuroactive steroid ganaxolone.

Neuroactive steroids are naturally occurring or synthetically derived compounds many of which have anticonvulsant, anesthetic, anxiolytic, analgesic or hypnotic properties. The major site of neuronal activity appears to be with a specific steroid-sensitive site on the gamma-aminobutyric acidA receptor/chloride ionophore complex. Ganaxolone (3 alpha-hydroxy-3 beta-methyl-5 alpha-pregnan-20-one) is a synthetic neuroactive steroid protected from metabolic attack of the 3 alpha position. Ganaxolone is an efficacious anticonvulsant agent in a variety of acute seizure models, as well as in electrical and chemical kindling models, and is currently under Phase II clinical investigation for epilepsy. A prior observation that ganaxolone appeared to reverse the marked behavioral changes induced by the convulsant pentylenetetrazol (PTZ) was systematically examined in the present study. A model to quantify PTZ-induced behaviors is described and used to evaluate ganaxolone in comparison with the anticonvulsants valproate, ethosuximide, clonazepam, diazepam and phenobarbital. All compounds were compared using dose equivalents based on their respective ED50 values in preventing convulsions induced by 70 mg/kg PTZ. The ED50 and lower doses of ganaxolone prevented the observed behavioral effects of PTZ as well as its depressant effects on locomotor activity and rearing of mice. In contrast, the other anticonvulsants, if effective, were much less potent. Strikingly, most of the other anticonvulsants were incapable of preventing all the behavioral effects of PTZ. Only phenobarbital prevented all the behavioral effects of PTZ and only at doses 4 to 8 times the anticonvulsant ED50. Rather than normalizing behavior as ganaxolone did, however, phenobarbital resulted in supranormal behavioral responses (e.g., increases in activity). Repeated administration of PTZ did not decrease the protective efficacy of ganaxolone. The results document the unique pharmacological profile of ganaxolone and suggest additional potential benefits from its use as an antiepileptic. Furthermore, because behavioral effects of PTZ have been used to model anxiety and anxiety associated with withdrawal from drugs of abuse, ganaxolone may find additional therapeutic application in those areas.