Assessment of direct gating and allosteric modulatory effects of meprobamate in recombinant GABA(A) receptors.

Meprobamate is a schedule IV anxiolytic and the primary metabolite of the muscle relaxant carisoprodol. Meprobamate modulates GABAA (γ-aminobutyric acid Type A) receptors, and has barbiturate-like activity. To gain insight into its actions, we have conducted a series of studies using recombinant GABAA receptors. In αxßzγ2 GABAA receptors (where x=1-6 and z=1-3), the ability to enhance GABA-mediated current was evident for all α subunit isoforms, with the largest effect observed in α5-expressing receptors. Direct gating was present with all α subunits, although attenuated in α3-expressing receptors. Allosteric and direct effects were comparable in α1ß1γ2 and α1ß2γ2 receptors, whereas allosteric effects were enhanced in α1ß2 compared to α1ß2γ2 receptors. In "extrasynaptic" (α1ß3δ and α4ß3δ) receptors, meprobamate enhanced EC20 and saturating GABA currents, and directly activated these receptors. The barbiturate antagonist bemegride attenuated direct effects of meprobamate. Whereas pentobarbital directly gated homomeric ß3 receptors, meprobamate did not, and instead blocked the spontaneously open current present in these receptors. In wild type homomeric ρ1 receptors, pentobarbital and meprobamate were ineffective in direct gating; a mutation known to confer sensitivity to pentobarbital did not confer sensitivity to meprobamate. Our results provide insight into the actions of meprobamate and parent therapeutic agents such as carisoprodol. Whereas in general actions of meprobamate were comparable to those of carisoprodol, differential effects of meprobamate at some receptor subtypes suggest potential advantages of meprobamate may be exploited. A re-assessment of previously synthesized meprobamate-related carbamate molecules for myorelaxant and other therapeutic indications is warranted.

Carisoprodol-mediated modulation of GABAA receptors: in vitro and in vivo studies.

Carisoprodol is a frequently prescribed muscle relaxant. In recent years, this drug has been increasingly abused. The effects of carisoprodol have been attributed to its metabolite, meprobamate, a controlled substance that produces sedation via GABA(A) receptors (GABA(A)Rs). Given the structural similarities between carisoprodol and meprobamate, we used electrophysiological and behavioral approaches to investigate whether carisoprodol directly affects GABA(A)R function. In whole-cell patch-clamp studies, carisoprodol allosterically modulated and directly activated human alpha1beta2gamma2 GABA(A)R function in a barbiturate-like manner. At millimolar concentrations, inhibitory effects were apparent. Similar allosteric effects were not observed for homomeric rho1 GABA or glycine alpha1 receptors. In the absence of GABA, carisoprodol produced picrotoxin-sensitive, inward currents that were significantly larger than those produced by meprobamate, suggesting carisoprodol may directly produce GABAergic effects in vivo. When administered to mice via intraperitoneal or oral routes, carisoprodol elicited locomotor depression within 8 to 12 min after injection. Intraperitoneal administration of meprobamate depressed locomotor activity in the same time frame. In drug discrimination studies with carisoprodol-trained rats, the GABAergic ligands pentobarbital, chlordiazepoxide, and meprobamate each substituted for carisoprodol in a dose-dependent manner. In accordance with findings in vitro, the discriminative stimulus effects of carisoprodol were antagonized by a barbiturate antagonist, bemegride, but not by the benzodiazepine site antagonist, flumazenil. The results of our studies in vivo and in vitro collectively suggest the barbiturate-like effects of carisoprodol may not be due solely to its metabolite, meprobamate. Furthermore, the functional traits we have identified probably contribute to the abuse potential of carisoprodol.

Evaluation of CNS activities of aerial parts of Cynodon dactylon Pers. in mice.

The dried extracts of aerial parts of Cynodon dactylon Pers. (Graminae) were evaluated for CNS activities in mice. The ethanol extract of aerial parts of C. dactylon (EECD) was found to cause significant depression in general behavioral profiles in mice. EECD significantly potentiated the sleeping time in mice induced by standard hypnotics viz. pentobarbitone sodium, diazepam, and meprobamate in a dose dependant manner. EECD showed significant analgesic properties as evidenced by the significant reduction in the number of writhes and stretches induced in mice by 1.2% acetic acid solution. It also potentiated analgesia induced by morphine and pethidine in mice. EECD inhibited the onset and the incidence of convulsion in a dose dependent manner against pentylenetetrazole (PTZ)-induced convulsion. The present study indicates that EECD has significant CNS depressant activities.

Stimulant and relaxant drugs combined with stimulant and relaxant information: a study of active placebo.

RATIONALE: The active placebo hypothesis states that placebo effects are potentiated when an active drug is administered. OBJECTIVE: This hypothesis was tested in an experiment where information about the effect of a drug was combined with administration of an active drug or placebo. METHODS: Information that a drug acted as a relaxant, a stimulant, or as a placebo was crossed with oral administration of a relaxant drug (700 mg carisoprodol), a stimulant drug (400 mg caffeine) or placebo (lactose) in healthy volunteers ( n=94). Dependent variables were subjective and physiological measures of arousal, as well as serum carisoprodol and caffeine levels. Data were collected from 15 to 280 min after administration of drug or placebo. RESULTS: Caffeine increased alertness, systolic and diastolic blood pressure, startle blink reflexes, and skin conductance responses. Subjects were calmer after carisoprodol, and heart rate was increased. There was a positive correlation between increased arousal and carisoprodol levels when stimulant information had been provided. However, this was only seen when carisoprodol levels were very low. There was no evidence that caffeine modulated the placebo response. CONCLUSIONS: Active placebo responses were seen only transiently when carisoprodol levels were low, and only in the subjective arousal data. Caffeine did not support active placebo responses. The overall findings did not favour the active placebo hypothesis.

Comparative abuse liability and pharmacological effects of meprobamate, triazolam, and butabarbital.

Implementation of regulations to control the prescribing of benzodiazepines in New York State in 1989 resulted in a 55% decrease in benzodiazepine prescribing, with a concomitant increase in the rates of prescribing older sedative-hypnotic compounds such as butabarbital (30% increase) and meprobamate (125% increase). In a double-blind, crossover, placebo-controlled study, we compared the behavioral and pharmacological effects of triazolam, meprobamate, and butabarbital in 14 recreational drug users. Placebo and three doses each of triazolam, meprobamate, and butabarbital were administered to each subject in a random order. Objective tests (motor performance, concentration) and subjective response questionnaires measured drug effects. Triazolam, meprobamate, and butabarbital showed comparable negative dose-response slopes on the objective measures. On the basis of these objective data, equivalent doses for the three compounds were determined to be as follows: 0.5 mg triazolam = 2,400 mg meprobamate = 400 mg butabarbital. Subjective effects data on equivalent doses show that butabarbital produced the highest peak score on Cole/ARCI Abuse Potential, ARCI Pentobarbital Chlorpromazine Alcohol Group (PCAG), and "drug strength" scales. Triazolam and butabarbital produced equivalent results on ARCI Morphine Benzedrine Group (MBG), Cole/ARCI Euphoria, and "drug liking" scales. Meprobamate was indistinguishable from placebo on euphoria and abuse potential scales. Behavioral economics analysis indicated a price crossover point two times higher for butabarbital (400 mg) than for any other drug condition. These data indicate a comparative abuse liability of butabarbital > triazolam > or = meprobamate, suggesting that the prescribing restrictions on benzodiazepines had little net benefit on abuse risk in the population and may have increased the risk of overdose morbidity and mortality.

Carisoprodol, meprobamate, and driving impairment.

This paper considers the pharmacology of the centrally acting muscle relaxant carisoprodol, and its metabolite meprobamate, which is also administered as an anxiolytic in its own right. Literature implicating these drugs in impaired driving is also reviewed. A series of 104 incidents in which these drugs were detected in the blood of drivers involved in accidents or arrested for impaired driving was considered, with respect to the analytical toxicology results, patterns of drug use in these subjects, the driving behaviors exhibited, and the symptoms observed in the drivers. Symptomatology and driving impairment were consistent with other CNS depressants, most notably alcohol. Reported driving behaviors included erratic lane travel, weaving, driving slowly, swerving, stopping in traffic, and hitting parked cars and other stationary objects. Drivers on contact by the police displayed poor balance and coordination, horizontal gaze nystagmus, bloodshot eyes, unsteadiness, slurred speech, slow responses, tendency to doze off or fall asleep, difficulty standing, walking or exiting their vehicles, and disorientation. Many of these cases had alcohol or other centrally acting drugs present also, making difficult the attribution of the documented impairment specifically to carisoprodol and meprobamate. In 21 cases, however, no other drugs were detected, and similar symptoms were present. Impairment appeared to be possible at any concentration of these two drugs; however, the most severe driving impairment and most overt symptoms of intoxication were noted when the combined concentration exceeded 10 mg/L, a level still within the normal therapeutic range.

Naloxone potentiates anxiolytic-like actions of diazepam, pentobarbital and meprobamate but not those of Ro19-8022 in the rat.

The elevated plus-maze test was used to determine if the opiate antagonist naloxone could potentiate the anxiolytic-like effects of the benzodiazepine diazepam, the barbiturate pentobarbital, the propanediol carbamate meprobamate and the partial benzodiazepine receptor agonist [R]-1-[(10-chloro-4-oxo-3-phenyl-4H-benzo[a]quinolizin-1-yl) carbonyl]-2-pyrrolidine-methanol (Ro19-8022) in the rat. A subeffective dose of each of these compounds was combined with naloxone, 10 mg/kg. Naloxone had no effect by itself, but potentiated all drugs except Ro19-8022. The proportion of entries on the open arm increased while the total number of arms entries was not modified. These results coincide with and extend data previously obtained in the mouse. One possible explanation for naloxone's effect could be that it blocks opioid inhibition of GABAergic (gamma-aminobutyric acid) neurons thereby enhancing the effects of benzodiazepines. Another possibility is that naloxone blocks opioid effects on adenosinergic systems.

Anxiolytic-like effects of meprobamate. Interactions with an opiate antagonist in Swiss and BALB/c mice.

Naloxone has previously been shown to block the effects of benzodiazepines in the Swiss but not in the BALB/c strain. We have also reported that naloxone potentiates subeffective doses of benzodiazepines in Swiss mice. In the present studies we first determined whether naloxone could block anxiolytic-like effects of meprobamate in Swiss and BALB/c mice. Then we evaluated if subeffective doses of meprobamate could be potentiated in Swiss as well as in BALB/c mice. The elevated plus-maze test and the light/dark choice procedure were used. The lowest dose of meprobamate with anxiolytic-like effects was 60 mg/kg in the BALB/c mice. This dose was effective in both the plus-maze and in the light/dark choice procedure. In Swiss mice the same dose was effective in the plus-maze, whereas 120 mg/kg was required in the light/dark choice procedure. When an effective dose of meprobamate was combined with naloxone, 10 mg/kg, no blockade of anxiolytic-like effects was obtained in any strain in any procedure. To the contrary, when a subeffective dose of meprobamate was combined with naloxone, 10 mg/kg, an anxiolytic-like effect was obtained in both strains in both procedures. The present series of experiment shows that the ability of naloxone to block anxiolytic-like drug effects do not apply to meprobamate. However, the naloxone-induced potentiation of subeffective doses previously observed after treatment with benzodiazepines or buspirone was present also after treatment with meprobamate. Moreover, although blockade of anxiolytic-like drug effects with naloxone has not been observed in BALB/c mice, potentiation was as evident in that strain as in the Swiss. This suggests that the mechanisms behind naloxone's blockade of anxiolytic-like effects are independent from those behind its potentiation of such effects.

Selectivity in the generalization profile in baboons trained to discriminate lorazepam: benzodiazepines, barbiturates and other sedative/anxiolytics.

The discriminative stimulus effects of benzodiazepines often have been indistinguishable from those of barbiturates or other sedative/anxiolytics. However, baboons and rats trained to discriminate lorazepam did not reliably generalize to pentobarbital in previous studies, although animals comparably trained to discriminate pentobarbital reliably generalized to lorazepam. The present study investigated the generalization profile for a variety of anxiolytic, sedative and other drugs in baboons trained to discriminate oral lorazepam (1.8 mg/kg). Triazolam, alprazolam, diazepam, midazolam, bromazepam, temazepam and nordiazepam occasioned >80% of total responses on the lorazepam-paired lever, in that order of potency, 60 min after oral dosing; chlordiazepoxide did so in three of five baboons. However, barbiturates (amobarbital, hexobarbital, methohexital, pentobarbital, phenobarbital, secobarbital) and methypryIon occasioned lorazepam-appropriate responding in only one or two baboons. Testing barbiturates at different pretreatment times (amobarbital, hexobarbital, pentobarbital or secobarbital) or by an i.m. route of administration (methohexital, pentobarbital) did not produce an increase in generalization. Neither other classic sedatives/anxiolytics (chloral hydrate, clomethiazole, ethanol, methaqualone, meprobamate, triclofos), nor anticonvulsants (phenytoin, valproic acid), nor drugs from other pharmacological classes shared discriminative-stimulus effects with lorazepam. These results, together with those from previous studies in which lorazepam or another benzodiazepine served as the training stimulus, indicate that lorazepam training results in a more selective generalization profile with respect to sedative/anxiolytic drugs than does training with other benzodiazepines.

Felbamate and meprobamate: a comparison of their anticonvulsant properties.

The anticonvulsant effect of felbamate and meprobamate were compared in a series of models for seizure activity and regarding their neurotoxic action. In the MES test, felbamate was active below neurotoxic doses. Meprobamate had an ED50 in the range of neurotoxic doses. The s.c. PTZ test was influenced by meprobamate in a fairly low dosage (ED50 66 mg/kg), but for felbamate no clearly dose-related effect could be shown up to 150 mg/kg. Reflex epilepsy in gerbils was stronger suppressed by meprobamate (ED50 34 mg/kg) than by felbamate (ED50 63 mg/kg). In amygdala kindled rats, meprobamate proved to be the most active compound, both regarding treatment of fully kindled rats, development of kindling and independent discharges from a mirror focus (secondary epileptogenesis), which were fully suppressed by oral treatment with 80 mg/kg for 30 days. Both drugs were weakly effective in a model for absence epilepsy in rats. The unexpectedly high activity of meprobamate justifies a second look at the anticonvulsant properties of the drug, especially since it was extensively used as an anxiolytic drug in the past with few obvious serious side effects.

Barbiturate-like actions of the propanediol dicarbamates felbamate and meprobamate.

Felbamate and meprobamate are structurally related propanediol dicarbamates that possess distinct pharmacological profiles. Felbamate is a minimally sedative, broad-spectrum anticonvulsant, whereas meprobamate is a strong sedative-anxiolytic agent. Previously, we reported that felbamate potentiates gamma-aminobutyric acid(A) (GABA(A)) receptor Cl- currents and inhibits N-methyl-D-aspartate (NMDA) receptor currents. Here we further characterized the interaction of the two dicarbamates with GABA(A) receptors to determine the basis for their pharmacological differences. In whole-cell voltage-clamp recordings from cultured rat hippocampal neurons, meprobamate enhanced GABA-evoked responses in a concentration-dependent manner and, at high concentrations (>1 mM), exhibited a separate channel-blocking effect that limited the magnitude of GABA(A) receptor potentiation. At equivalent concentrations, meprobamate produced substantially greater potentiation than did felbamate. Furthermore, meprobamate (but not felbamate), in the absence of GABA, directly activated Cl- currents that could be attenuated by the GABA(A) receptor antagonists bicuculline and picrotoxin. The mean deactivation time constant of whole-cell currents evoked by 10 mM meprobamate (110 ms) or 1 and 3 microM GABA (180 ms) were faster than the deactivation time constant of 10 mM meprobamate (490 ms) or 3 mM felbamate (470 ms) in the presence of GABA. Meprobamate and felbamate prolonged the mean burst duration of GABA-activated unitary currents in excised outside-out membrane patches. In addition, at high (supratherapeutic) concentrations, meprobamate blocked NMDA-activated currents. We conclude that felbamate and meprobamate have barbiturate-like modulatory actions on GABA(A) receptors, but meprobamate has greater activity and, unlike felbamate, is able to directly activate the receptor.

[The effect of phytin, benzonal and their combination administered prophylactically on the pharmacodynamics of drugs metabolized in the liver in hypokinesia]. / Vliianie fitina, benzonala i ikh kombinatsii pri profilakticheskom vvedenii na farmakodinamiku lekarstvennykh veshchestv, metaboliziruiushchikhsia v pecheni pri gipokinezii.

Hexenal, meprobamate, amidopyrine and ethylmorphine produced a significantly marked effect in animals under hypokinesia as compared with normal rats. When phytin, benzonal and their combination were used for preventive purposes, impaired pharmacodynamics of the tested drugs metabolizing in the liver disappeared. The investigations demonstrated that the preventive use of phytin in combination with benzonal is the most optimal in correcting the impairments of drug pharmacodynamics in hypokinesia.

[Effects of tranquilizing agents on bioelectrical activity of the rat brain]. / Vliianie trankviliziruiushchikh sredstv na bioélektricheskuiu aktivnost' mozga krys.

The action of diazepam, meprobamate, trioxazine and mexidol on bioelectrical activity of sensorimotor cortex and dorsal hippocamp of the left and right hemisphere of the brain in conscious rat in free behavior has been studied. All the drugs produced a decline in the frequency of the dominant peak of EEG power spectra. Diazepam and meprobamate increased beta-activity. It is concluded that the decreased frequency may be due to an anxiolytic effect of the tranquilizers, whereas high beta-activity is related to muscle relaxant effect of some drugs.

Use of antianxiety drugs as countermeasures in the detection of guilty knowledge.

To evaluate whether antianxiety drugs enable guilty subjects to appear innocent on polygraph tests, we compared the effects of diazepam, meprobamate, and propranolol on the outcome of a guilty knowledge test (GKT). Seventy-five undergraduate students were evenly divided among one innocent and four guilty groups. Subjects in each of the guilty groups received either one of the drugs or a placebo prior to the administration of the GKT and after viewing a videotape that depicted a burglary as seen from the perspective of the burglar. The results showed that drug status had no influence on the outcome of the GKT. Innocent subjects who coincidentally obtained high scores on a recognition memory test covering details of the mock crime tended to obtain higher guilt scores on the GKT.

Effects of olfactory stimulation on the sleep time induced by pentobarbital administration in mice.

The effect on the pentobarbital sleep time by olfactory stimulation with various odorants was investigated using mice to appraise the physiological or psychological significance of olfactory information. The sleep time was determined as the time elapsed between intraperitoneal pentobarbital administration and the first time that the animal was able to spontaneously right itself. The sleep time was affected by inhalation of some odorants compared to pure air controls, but not by others. The sleep time was prolonged by terpinyl acetate and phenethyl alcohol, and was shortened by lemon oil and jasmin oil. However, neither potentiation nor attenuation of pentobarbital action by odorant inhalation was observed when using anosmic mice produced by intranasal zinc sulphate treatment. In conclusion, olfactory stimulation associated with odorant inhalation influences the pentobarbital sleep time, suggesting that olfactory information may have a more potent influence on the physiological and psychological status than has previously been thought.

Effects of anxiolytic drugs on escape behavior induced by dorsal central gray stimulation in rats.

Each animal was chronically implanted with bipolar electrodes in dorsal central gray matter (DCG) and was trained to press a lever to decrease the DCG-stimulation current. Chlordiazepoxide (5-20 mg/kg, PO), diazepam (2-10 mg/kg, PO) and bromazepam (1-5 mg/kg, PO) produced dose-dependent increases in the DCG-stimulation threshold 1-4 h after administration without affecting motor performance. Meprobamate (200 mg/kg, PO) and pentobarbital (10 mg/kg, PO) also slightly increased the stimulation threshold. Their potency was in the order of bromazepam greater than diazepam greater than chlordiazepoxide greater than pentobarbital greater than meprobamate. The increase in the threshold induced by diazepam (10 mg/kg, PO) was inhibited by the GABA antagonists, bicuculline (1 mg/kg, IP) and picrotoxin (0.1 mg/kg, IP). These results suggest that decreased susceptibility to brain stimulation is involved in suppressing effects of anxiolytic drugs on the escape behavior, and also that the antiaversive action of benzodiazepines may be related to a GABAergic mechanism.

[Characteristics of drug pharmacodynamics in irradiated rats]. / Osobennosti farmakodinamiki lekarstvennykh veshchestv u obluchennykh krys.

A whole-body exposure of rats to 8 Gy radiation is ineffective in 3 days, and in 6 days, it prolongs considerably the effect and increases the pharmacological activity of hexenal, meprobamate, ethylmorphine, and amidopyrine, inhibits the activity of amidopyrine demethylase, aniline hydroxylase, NADPH-cytochrome c reductase, and reduces the content of protein, cytochromes P-450 and b5 in a microsomal liver fraction.

Interactive effects of acute ethanol administration on meprobamate levels in blood and brain of rabbit and rat.

In the simultaneous administration of meprobamate and ethanol to rabbits, the blood meprobamate concentration (BMC) increased greatly when the maximum blood ethanol concentration (BECmax) exceeded 1.0 mg/ml. Thus, we subjected the rabbits to continuous infusion of ethanol so as to make the blood ethanol concentration (BEC) constant and administered meprobamate by intravenous injection. Elimination of meprobamate became slow at about the BEC of 0.5 mg/ml and the degree reached almost maximum around the BEC of 1.0 mg/ml. The elimination rate did not change any more even when the BEC was raised higher. In the study conducted to elucidate the relationship between the BMC and brain meprobamate concentration (BrMC) using rats, it was found that meprobamate would show similar movements and its level would rise extremely by an acute administration of ethanol. It was indicated that the effect of ethanol on reinforcement of meprobamate activity would appear strongly by potentiation effect.

A new sensitive method which unlike the Vogel test detects the anxiolytic effect of tofisopam.

A new, more sensitive than previously used anxiolytic test is described. The test consists in measuring of inhibition by punishment of drinking water necessary to swallow dry food by very hungry rats. This test reveals the anxiolytic properties of tofisopam, a clinically effective benzodiazepine anxiolytic, and of very low doses of chlordiazepoxide, both ineffective in the Vogel test, as well as anxiolytic properties of high doses of chlordiazepoxide and other anxiolytics, and confirms the lack of anxiolytic effects of major tranquilizers. The results suggest that chlordiazepoxide (and possibly other benzodiazepines) acts on two subgroups of benzodiazepine receptors, named BRI (high affinity) and BR2 (low affinity), while tofisopam acts specifically on BRI receptors. The new test is proposed as a tool for a search for drugs specifically acting on high affinity benzodiazepine BRI receptors.

Psychopharmacological analysis of hypothalamically-induced emotions.

The effect of minor tranquilizers and neuroleptics was compared on self-stimulation and escape behaviourelicited by electrical stimulation of the hypothalamic nuclei in rabbits. It was shown that while tranquilizers (diazepam, oxazepam and meprobamate) increased the rate of self-stimulation elicited from the lateral hypothalamus, neuroleptics considerably suppressed such behaviour. Tranquilizers caused a remarkable reversal of the escape behaviour into a high-rate self-stimulation, both responses being induced from the same electrodes within the medial hypothalamus. Neuroleptics (chlorpromazine, reserpine and haloperidol) had not such an influence, though they somewhat increased the general activity of the animals. The reversing effect of the tranquilizers was compared with similar findings obtained after electrolytic ablation of the ventral hippocampus. It is suggested that the hippocampus has an inhibitory influence on the hypothalamic motivational system thus providing substantially for the animals' survival in a hostile environment.