Functional tolerance to lorazepam in the rat

Experimental and clinical evidence for loss of effect (tolerance) during prolonged treatment with antiepileptic drugs

Development of tolerance (i.e., the reduction in response to a drug after repeated administration) is an adaptive response of the body to prolonged exposure to the drug, and tolerance to antiepileptic drugs (AEDs) is no exception. Tolerance develops to some drug effects much more rapidly than to others. The extent of tolerance depends on the drug and individual (genetic?) factors. Tolerance may lead to attenuation of side effects but also to loss of efficacy of AEDs and is reversible after discontinuation of drug treatment. Different experimental approaches are used to study tolerance in laboratory animals. Development of tolerance depends on the experimental model, drug, drug dosage, and duration of treatment, so that a battery of experimental protocols is needed to evaluate fully whether tolerance to effect occurs. Two major types of tolerance are known. Pharmacokinetic (metabolic) tolerance, due to induction of AED-metabolizing enzymes has been shown for most first-generation AEDs, and is easy to overcome by increasing dosage. Pharmacodynamic (functional) tolerance is due to "adaptation" of AED targets (e.g., by loss of receptor sensitivity) and has been shown experimentally for all AEDs that lose activity during prolonged treatment. Functional tolerance may lead to complete loss of AED activity and cross-tolerance to other AEDs. Convincing experimental evidence indicates that almost all first-, second-, and third-generation AEDs lose their antiepileptic activity during prolonged treatment, although to a different extent. Because of diverse confounding factors, detecting tolerance in patients with epilepsy is more difficult but can be done with careful assessment of decline during long-term individual patient response. After excluding confounding factors, tolerance to antiepileptic effect for most modern and old AEDs can be shown in small subgroups of responders by assessing individual or group response. Development of tolerance to the antiepileptic activity of an AED may be an important reason for failure of drug treatment. Knowledge of tolerance to AED effects as a mechanism of drug resistance in previous responders is important for patients, physicians, and scientists.

Behavioral effects of acute and chronic triazolam treatments in albino rats

Previous behavioral studies on triazolam (TZ), which are small in number, could only speculate about tolerance to the anxiolytic effect of TZ, as the experiments did not cover sufficient time (of 4 to 7 days) for tolerance to develop. Therefore longer time for chronic TZ administration is used. We investigated the effects of TZ on motor activity and exploratory behavior using plus maze and open field. Three experiments were conducted. In the first, five groups of rats were acutely treated with different doses of TZ (0.25 mg/kg-4.0 mg/kg). In the second set of experiments, rats were treated chronically with a single daily dose of TZ (started with 0.25 mg/kg and increased by time to 1.0 mg/kg) for 5 weeks (representing clinical use). In the third, rats were treated chronically with three daily doses of TZ (started with 0.25 mg/kg and increased by time to 0.5 mg/kg) for 20 days (mimicking drug abuse). Acute TZ administration produced dose dependent anxiolytic effects and a decrease in motor activity with higher doses. Chronically treated rats, either once daily or three times daily doses, showed tolerance to both anxiolytic and sedative effects of TZ. It may be concluded that tolerance to the anxiolytic and sedative effects of TZ would develop after chronic administration either with clinical use or its abuse.

Behavioral analysis indicates benzodiazepine-tolerance mediated by the benzodiazepine binding-site at the GABA(A)-receptor

1. GABA(A)-receptor induced changes in locomotion and anxiety-like behaviors were studied in rats using an open-field and an elevated plus-maze. Acute and chronic doses of the benzodiazepine diazepam without and in combination with the GABA uptake inhibitor SKF-89976A were investigated. 2. Fifty-six male rats of the strain PVG/OlaHsd (PVG; 180-200 g body wt) were used to assess the influence of the benzodiazepine binding-site to the development of tolerance. Rats were divided into six groups: The first receiving saline (0.9%), the second and third diazepam (10.0 mg/kg) daily for 23 days with or without an acute challenge of 2.0 mg/kg diazepam. The fourth group received diazepam (10.0 mg/kg) daily and acutely SKF-89976A (15.0 mg/kg) plus diazepam and the fifth and sixth group received acute treatment with diazepam (2.0 mg/kg) or SKF-89976A (15.0 mg/kg). 3. Under chronic treatment with diazepam the animals became tolerant to acute doses of diazepam in activity and anxiety-related behaviors. Acute treatment with SKF-89976A increased exploration. Parameters expressing anxiolytic-like behaviors were increased, too, but not all of them significantly. In diazepam tolerant animals SKF-89976A produced anxiolytic-like behaviors 4. We conclude that the BZ- and not the GABA-binding site at the GABA(A)-receptor is involved in the development of BZ-tolerance.

Pharmacodynamic and receptor binding changes during chronic lorazepam administration

To assess pharmacodynamic and neurochemical aspects of tolerance, lorazepam (2 mg/kg/day), or vehicle was administered chronically to male Crl: CD-1(ICR)BR mice via implantable osmotic pump. Open-field behavior, benzodiazepine receptor binding in vitro, receptor autoradiography, and muscimol-stimulated chloride uptake were examined at both 1 and 14 days. Open-field activity was depressed in lorazepam-treated animals on Day 1. On Day 14, open-field parameters were indistinguishable from those of vehicle-treated animals, indicating behavioral tolerance. Benzodiazepine binding, as determined by the specific binding of [125I]diazepam, was also decreased in cortex on Day 14. Hippocampal binding was unchanged following chronic lorazepam exposure. Apparent affinity in cortical membrane preparations was unchanged, indicating that altered ligand uptake was due to decreased receptor number. Muscimol-stimulated chloride uptake into cortical synaptoneurosomes from lorazepam-treated animals was not significantly different on Day 1 or Day 14 compared to vehicle-treated animals. These results confirm that down-regulation of benzodiazepine receptor binding is closely associated with behavioral tolerance to benzodiazepines. These observed changes in binding are not necessarily associated with robust changes in receptor function.

Precipitated and spontaneous withdrawal following administration of lorazepam but not zolpidem

Radiotelemetry was utilized to compare zolpidem and lorazepam tolerance and withdrawal in rats. Locomotor activity, electromyographic activity (EMG), and body temperatures were used to assess the acute drug effects, and as measures of tolerance and withdrawal. Lorazepam, zolpidem, or vehicle was administered for 12 days, and data were recorded daily, immediately, after treatment. Data were also recorded immediately after flumazenil (25 mg/kg, IP) precipitated withdrawal and during 4 days of spontaneous withdrawal. Complete tolerance to the acute effects of lorazepam administration developed within 7 days of treatment and both flumazenil-precipitated and spontaneous withdrawal were observed. In contrast, there was no tolerance to the sedative actions of zolpidem administration after 12 days, but complete tolerance to the hypothermic and muscle relaxant effects was apparent after 8 days of treatment. Despite the presence of tolerance, no evidence of either spontaneous or flumazenil-induced withdrawal was recorded in these rats. In conclusion, this model suggests that as a sedative zolpidem has significant advantages over the classic benzodiazepines.

Development of tolerance in mice to the sedative effects of the neuroactive steroid minaxolone following chronic exposure

Minaxolone is a potent ligand for the neurosteroid binding site of the GABAA, receptor. In radioligand binding studies to rat brain membranes, minaxolone caused a 69% increase in [3H]muscimol binding and a 25% increase in [3H]flunitrazepam binding and inhibited the binding of [3H]TBOB with an IC50 of 1 microM. In mice, minaxolone (100 mg/kg, orally) had marked sedative effects as indicated by a reduction in locomotor activity. Chronic dosing with minaxolone (100 mg/kg, orally, once daily for 7 days) resulted in a loss of sedative response to an acute dose of the drug, indicating development of tolerance. Chronic dosing with temazepam (10 mg/kg, orally, once daily for 7 days) resulted in the development of tolerance to an acute dose of temazepam; however, the two drugs did not appear to be cross-tolerant, indicating that they may have a different mechanism of action at the level of the GABAA receptor.

The behavioural and neuronal effects of the chronic administration of benzodiazepine anxiolytic and hypnotic drugs

Benzodiazepine anxiolytic and hypnotic drugs are some of the most widely prescribed drugs in the Western world. Despite this fact, the mechanisms that underlie the development of tolerance to, and dependence upon, benzodiazepines are poorly understood. The aim of this review is to summarize and critically evaluate the experimental evidence relating to the chronic behavioural and neuronal effects of benzodiazepines. Behavioural studies in animals generally indicate that tolerance gradually develops to the muscle relaxant, ataxic, locomotor and anticonvulsant effects of benzodiazepines. The evidence relating to the development of tolerance to the anxiolytic effects of benzodiazepines is less clear. The literature on the possible mechanisms of benzodiazepine tolerance and dependence is large, highly complex and difficult to interpret. The effect of chronic benzodiazepine treatment varies enormously as a function of the benzodiazepine used and the treatment schedule employed. Many studies have demonstrated a down-regulation of benzodiazepine binding sites, although affinity is usually unchanged. The evidence relating to the number and affinity of GABAA binding sites is unclear. Some studies suggest that chronic benzodiazepine administration results in a reduction in the number of Cl- channels associated with the GABAA receptor complex, although it is not clear that the efficacy of the GABA binding site in operating the Cl- channel necessarily changes. There is, however, substantial evidence to support the hypothesis that chronic benzodiazepine treatment results in a reduction in the coupling between the GABAA and benzodiazepine binding sites (the "functional uncoupling hypothesis"). Although some electrophysiological studies suggest that chronic benzodiazepine treatment results in a subsensitivity to GABA, this effect seems to be highly area-specific.

Tolerance to the ataxic effects of diazepam in guinea pig is not associated with a reduced sensitivity of GABAA receptors in the vestibular nucleus

Some studies have suggested that drug tolerance observed following repeated benzodiazepine exposure may be associated with the development of a subsensitivity to gamma-aminobutyric acid (GABA) in dorsal raphe and hippocampal neurons. In other areas such as the substantia nigra such subsensitivity has not been found. The aim of the present study was to determine whether tolerance develops to the ataxic effects of diazepam on the righting reflex following low (i.e. 2 mg/kg i.p.), multiple daily doses and, if so, whether it is correlated with the development of a subsensitivity of medial vestibular nucleus neurons to the selective GABAA receptor agonist, isoguvacine. Guinea pigs which received i.p. vehicle injections three times daily for 5 days, or single daily doses of 2 or 6 mg/kg diazepam, showed increased righting reflex latencies in response to a 6 mg/kg diazepam challenge dose. However, guinea pigs which received 2 mg/kg diazepam i.p., three times daily for 5 days, exhibited minimal or no ataxia when given the same diazepam challenge dose, indicating the development of tolerance. Brain stem slices including the medial vestibular nucleus were removed from guinea pigs which had received the same diazepam and vehicle three times daily injection schedules, and recordings were made from single neurons during superfusion of isoguvacine. Although medial vestibular nucleus neurons from animals which received chronic diazepam administration showed smaller decreases in firing rate in response to 10(-8) M isoguvacine, the difference was not statistically significant compared to neurons from animals which received vehicle treatment or acute diazepam treatment. Resting activity was also similar between the diazepam and vehicle groups, in contrast to a previous study which had shown hyperexcitability in medial vestibular nucleus cells from animals which had received single daily injections for up to 60 days. These results suggest that, in contrast to studies which have employed single daily doses, tolerance to the ataxic effects of diazepam on the righting reflex occurs rapidly with divided daily doses. However, this tolerance is not correlated with significant changes in the sensitivity of GABAA receptors on medial vestibular nucleus neurons.

Antagonism of isoniazid-induced convulsions by abecarnil in mice tolerant to diazepam

The ability of the benzodiazepine receptor full agonist diazepam, the selective agonist abecarnil, and the partial agonist imidazenil to antagonize convulsions induced by isoniazid (200 mg/kg, S.C.) was studied in mice chronically treated with diazepam (3 mg/kg, i.p., three times daily) or abecarnil (0.1 or 1 mg/kg, i.p., three times daily or 6 mg/kg, S.C., daily). Diazepam induced tolerance to its own anticonvulsant effect. In contrast, chronic treatment with abecarnil failed to induce tolerance to its own anticonvulsant activity. Animals treated with abecarnil at 0.1 mg/kg developed cross-tolerance to imidazenil, whereas those treated with 1 mg/kg became less sensitive to diazepam. Mice chronically treated with abecarnil at 6 mg/kg showed almost complete tolerance to diazepam. Abecarnil was able to antagonize the convulsions elicited by isoniazid in diazepam-tolerant mice. These data indicate that chronic administration of abecarnil, unlike that of classical benzodiazepines, does not induce tolerance to its anticonvulsant effect, and that abecarnil overcomes tolerance induced by long-term treatment with the full agonist diazepam.

The effects of long-term, low-dose diazepam treatment on the guinea pig righting reflex and medial vestibular nucleus neuronal activity

Guinea pigs received a 2 mg/kg IP injection of diazepam, or an equivalent volume of vehicle, daily for 28-60 days. To determine whether tolerance developed to the ataxic effects of diazepam on the righting reflex, daily righting reflex latency (RRL) measurements were made before and 20, 30, and 40 min following the diazepam or vehicle injection for 28 days. Analyses of the RRLs for individual animals indicated that a significant decrease in RRL over time (indicating tolerance) occurred in only one out of nine animals receiving diazepam and in none of the vehicle animals. Medial vestibular nucleus (MVN) neurons in brain stem slices from animals receiving chronic diazepam treatment had a significantly higher average firing rate than those from vehicle controls. These results suggest that: a) long-term treatment with single 2 mg/kg daily IP injections of diazepam does not result in tolerance to diazepam's ataxic effects on the righting reflex in the majority of animals; b) this form of diazepam treatment may, nonetheless, induce a hyperactivity of brain stem MVN neurons that may be consistent with the occurrence of a withdrawal syndrome.

Rapid tolerance to the depressive effects of diazepam on guinea pig motor control using divided doses

The effects of acute and chronic IP injections of diazepam on the guinea pig righting reflex latency (RRL) were measured using an automated measurement system known as a "tolerometer." Single IP injections of 2.0, 6.0, 18.0, and 20.0 mg/kg diazepam significantly increased the RRL compared to no injection (naive animals), diazepam vehicle injections, or 1.0 mg/kg diazepam injections. The effects of chronic IP injection schedules on the RRL were compared: 18 or 20 mg/kg in a single, once daily injection for 5 days; 6 mg/kg in a single, once daily injection for 5 days; and 6 mg/kg, three times a day, for 5 days. Neither 20, 18, nor 6 mg/kg/day for 5 days resulted in significant tolerance to the depressive effects of diazepam on the righting reflex. By contrast, when 6 mg/kg was administered three times a day for 5 days, tolerance developed by the third day of treatment. There were no differences between the three groups in the amount of exposure to the measurement apparatus or the testing situation. These results support the view that species like guinea pig and rat that metabolise diazepam rapidly, develop tolerance more quickly if diazepam is administered in divided doses or by continuous release; this may be because the duration of the occupation of CNS benzodiazepine recognition sites is a critical factor in the development of tolerance.

Chronic administration of an anticonvulsant dose of imidazenil fails to induce tolerance of GABAA receptor function in mice

The ability of an anticonvulsant dose (0.1 mg/kg i.p.) of imidazenil, a new partial agonist of benzodiazepine receptors, to antagonize the convulsions and the increase in t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to the gamma-aminobutyric acid type A (GABAA) receptor elicited by isoniazid, an inhibitor of central GABAergic function, was evaluated in mice chronically treated (3 times daily for 30 days) with the same dose of imidazenil. The challenge dose of imidazenil, administered 36 h after the last injection of the chronic treatment protocol, reduced both isoniazid-induced convulsions and the isoniazid-induced increase in [35S]TBPS binding to the same marked extent as in control mice. These results indicate that long-term treatment with a pharmacologically effective dose of imidazenil failed to induce tolerance to both the anticonvulsant effect and the positive modulatory action on GABAA receptor function of this drug in mouse brain.

Tolerance and withdrawal after chronic lorazepam treatment in rats

The purpose of the study was to develop an animal model for benzodiazepine tolerance and dependence on the basis of oral administration, using lorazepam as the test drug. We have used the continuous measurement of locomotor activity in home cages to obtain a narrow estimation of the time course of withdrawal related hyperactivity as an observer-independent symptom. Acute administration of lorazepam (9.5-37.5 mg/kg body weight/day) resulted in the first week in a dose-dependent muscle relaxation on the accelerod and sedation in the open field. The most striking manifestation of sedation, however, was the decrease of nocturnal locomotor activity in home cages. After 5 weeks of administration tolerance to the sedative effect had developed. In a second study, using a lower dose range (2.5-9.5 mg/kg body weight/day), a decrease of nocturnal locomotor activity was also observed as was the development of tolerance. The latter can be partly explained by dispositional tolerance, i.e., decreased serum concentrations after administration of lorazepam for more than 2 weeks. Withdrawal of lorazepam in the experiment using high doses led to three symptoms, i.e., a decrease in food intake, loss of body weight and an increase in daytime locomotor activity. The decrease in food intake and the loss of body weight were maximal on the first day of withdrawal. The increase in the daytime locomotor activity was present in the high dose experiment only, with a maximum on days 2-3 and a duration of at least 1 week. The increase however, was not dependent on the dose previously administered. The symptoms 'loss of body weight' and 'decrease in food intake' appeared to be more sensitive in benzodiazepine withdrawal: they were dose dependently present over the whole dosage range (2.5-37.5 mg/kg body weight/day). It is concluded that the model represents a sensitive model to measure lorazepam tolerance and dependence in animals. Comparative studies with other benzodiazepines are in progress.

Development of tolerance to amnesic effects of chlordiazepoxide in relation to GABAergic and cholinergic neuronal systems

Chronic administration of benzodiazepines has been reported to produce tolerance in animals and humans. We investigated whether benzodiazepines produce tolerance to the amnesic effects and effects on benzodiazepine receptors, GABAergic and/or cholinergic neuronal systems of repeated administration of chlordiazepoxide, using a passive avoidance task and autoradiographic techniques. Tolerance developed to the amnesic effect of chlordiazepoxide when the drug was administered at a dose of 30 mg/kg (i.p.) once a day for 14 days. Bicuculline (1.0 and 1.5 mg/kg), a GABAA receptor antagonist, did not induce amnesia in normal mice, but did so in chlordiazepoxide-tolerant mice. Muscimol (0.25 mg/kg), a GABAA receptor agonist, in combination with a low dose of chlordiazepoxide, induced amnesia in normal mice, but not in chlordiazepoxide-tolerant mice. Scopolamine, an acetylcholine receptor antagonist, induced amnesia in normal mice, but not in chlordiazepoxide-tolerant mice. In the autoradiographical study, although repeated treatment with chlordiazepoxide had no effect on [3H]flunitrazepam and [3H]Ro 15-4513 binding to benzodiazepine receptors, it decreased [3H]muscimol binding to GABAA receptors, with a decrease in affinity in the cortex and hippocampus. Furthermore, repeated administration of chlordiazepoxide increased [3H]quinuclidinyl benzilate binding to muscarinic acetylcholine receptors in the hippocampus. These results suggest that tolerance develops to the amnesic effects of chlordiazepoxide, and that tolerance may be due to down-regulation of GABAA receptors and/or up-regulation of acetylcholine receptors.

Chronic benzodiazepine administration. XII. Anticonvulsant cross-tolerance but distinct neurochemical effects of alprazolam and lorazepam

Tolerance to the sedative and anticonvulsant effects of benzodiazepines has been reported, but cross-tolerance among benzodiazepines is poorly characterized. To evaluate cross-tolerance between lorazepam and alprazolam in a reliable anticonvulsant pharmacodynamic model, we treated mice with either drug for 14 days, and with the two drugs sequentially for 7 days each. Pentylenetetrazole-induced seizure thresholds were similar in mice treated for 14 days with lorazepam or alprazolam, 2 mg/kg/day. For both compounds, a discontinuation effect characterized by reduced seizure threshold occurred at 4 days after discontinuation. Substitution of alprazolam for lorazepam after 1 week, and vice versa, did not interrupt tolerance. [3H]flumazenil binding in vivo was downregulated in cortex after 14 days of either drug. However, binding was also reduced in hippocampus for lorazepam but not for alprazolam. Substitution of alprazolam for lorazepam resulted in downregulation in cortex only, similar to lorazepam alone. Conversely, substitution of lorazepam for alprazolam led to binding changes similar to lorazepam alone. These data demonstrate cross-tolerance to the convulsant effects of pentylenetetrazole between lorazepam and alprazolam. However, effects of the two compounds on benzodiazepine receptor binding in hippocampus remain distinct.

Chronic benzodiazepine antagonist treatment and its withdrawal upregulates components of GABA-benzodiazepine receptor ionophore complex in cerebral cortex of rat

Effect of chronic administration of benzodiazepine (BZ) receptor antagonist Ro 15-1788 (flumazenil) (4 mg/kg once daily for 14 days) treatment and its withdrawal on locomotor activity, body temperature, and the binding pattern of receptor ligands that bind to GABA-BZ receptor ionophore complex in different regions of the brain of the rat was studied. Ro 15-1788 (x 14 d) increased the specific binding of [3H]ethyl-8-fluoro-5-6-dihydro-5-methyl-6-oxo-4H- imidazo[1,5 alpha][1,4]benzodiazepine-3-carboxylate [( 3H]Ro 15-1788), [3H]ethyl-8-azido-5-6-dihydro-5-methyl-6-oxo-4H- imidazo[1,5 alpha][1,4]benzodiazepine-3-carboxylate [( 3H]Ro 15-4513), [3H]flunitrazepam, and [35S]t-butylbicyclophosphorothionate [( 35S]TBPS) in cerebral cortex, and this increase in binding remained upregulated during the drug withdrawal at 24 h. The binding of [3H]Ro 15-1788 was also found significantly increased in the hippocampus, but not in cerebellum and striatum. The chronic Ro 15-1788 treatment did not alter the specific binding of [3H]GABA. Rosenthal analysis of the saturation isotherms indicated that the observed upregulation in the binding pattern of [3H]Ro 15-1788 and [3H]Ro 15-4513 in the cerebral cortex was due to an increase in the binding capacity (Bmax). The receptor affinity (Kd) was not changed. The withdrawal of Ro 15-1788 following its chronic administration also enhanced locomotor activity. However, no apparent change in body temperature was observed either due to chronic treatment or withdrawal. These data indicate that chronic Ro 15-1788 treatment and its withdrawal may produce an upregulation of subunits which bind the positive (benzodiazepines), negative (inverse agonist), and neutral (antagonist) ligands of benzodiazepine receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

The history of benzodiazepine dependence: a review of animal studies

This article provides a historical review of the animal literature relating to the development of tolerance to the behavioral effects of benzodiazepines, and the incidence of biochemical and behavioral changes that result from termination of benzodiazepine treatment (spontaneous withdrawal responses). It charts the slow emergence of a pertinent animal literature and highlights conclusions that were prevalent in 1963 (at the introduction of diazepam), 1973 (at the introduction of lorazepam), 1980 and the present day. For 25 years the animal literature has lagged behind the clinical literature, but recent studies into the neurochemical mechanisms of benzodiazepine dependence and possible treatments for withdrawal responses suggest that, at last, animal experiments may be about to make a substantial contribution.

Long-term diazepam treatment produces changes in cholecystokinin receptor binding in rat brain

This study examined the effect of chronic diazepam administration on central benzodiazepine and CCK-8 receptor binding in rat brain. After a two-week treatment with diazepam (5 mg/kg per day) tolerance developed towards the sedative but not towards the anxiolytic action of this drug as determined using elevated plus-maze and open field tests. The % entries the rats made onto open arms and % time the rats spent in open arms were markedly decreased 24 h after the last dose of diazepam, probably indicating withdrawal anxiety. There were no changes in [3H]flunitrazepam binding either 30 min or 24 h after the last diazepam dose. However, 30 min after the last diazepam administration the apparent number of sulphated [3H]CCK-8 binding sites was significantly increased in the primary olfactory cortex. Acute diazepam treatment (5 mg/kg) had no influence on [3H]flunitrazepam or sulphated [3H]CCK-8 binding in any brain region studied. Cessation of chronic diazepam treatment was followed after 24 h by an increase in the number of CCK-8 receptors in frontal cortex and hippocampus as compared to the vehicle group. These results demonstrate that certain alterations in CCK-8 receptor characteristics may be important in the anti-anxiety effect, tolerance, and withdrawal reaction reaction after benzodiazepine administration.

Behavioral differentiation of benzodiazepine ligands after repeated administration in baboons

Baboons received repeated daily administration of saline, 5.6 mg/kg midazolam, 5.0 mg/kg flumazenil (Ro15-1788), 3.2 mg/kg 3-carboethoxy-beta-carboline hydrochloride (beta CCE) or 10 mg/kg beta CCE for 5 days. Behavioral signs of sedation and excitation were scored for 1 h after i.m. injections. Daily administration of these benzodiazepine-receptor ligands differentiated their behavioral effects; repeated midazolam resulted in tolerance to the sedative and ataxic effects; repeated beta CCE resulted in sensitization to its convulsant properties; and repeated flumazenil or saline produced no changes in behavior. In a second study, baboons received repeated injections of midazolam (5.6, 11.2 or 20 mg/kg per day) for 6 days. All three groups became tolerant to the sedative and ataxic effects of midazolam. Acute injections of flumazenil (5.0 mg/kg) on day 5 produced a dose-dependent withdrawal syndrome. This flumazenil treatment produced a slight attenuation in the degree of tolerance to midazolam on day 6, suggesting that receptor sensitivity to the benzodiazepine agonist may have partially reset.

Chronic diazepam treatment produces regionally specific changes in GABA-stimulated chloride influx

GABA-stimulated 36Cl- influx was used to investigate regional differences in response to chronic diazepam treatment by comparing cortical and cerebellar tissue from rats chronically treated with diazepam for 3 weeks. Using a treatment protocol which has previously been shown to produce behavioral tolerance and physical dependence, cortical membrane preparations from chronic diazepam-treated rats were found to exhibit a decreased responsiveness to the stimulation of 36Cl- influx by GABA and a corresponding decrease in the ability of flunitrazepam to enhance GABA-stimulated 36Cl-influx. This decrease in sensitivity to flunitrazepam, however, appears to reflect the underlying decrease in sensitivity to GABA. In contrast, in membrane vesicles prepared from cerebella of chronic diazepam-treated rats, there was no measurable effect on GABA-stimulated 36Cl--influx or on the enhancement of GABA-stimulated 36Cl- influx by flunitrazepam. These results support the suggestion that there is a regionally specific reduction in GABA/benzodiazepine receptor function following chronic benzodiazepine treatment.

Antagonizing the behavioural effects of drugs: a discussion with specific reference to benzodiazepines and alcohol

It can be extremely difficult to interpret the results of experiments in which a treatment is found to antagonize the behavioural effects of a drug. This article discusses strategies that can be used to help identify the nature of the antagonism and examines what such experiments tell us about the mechanisms underlying the drug's behavioural effects. Examples are taken from research with both benzodiazepines and ethanol. It is suggested that behavioural studies using different strategies to reduce the effects of a drug will further enhance our understanding of the mechanisms mediating the drug's effects. Such studies will include comparisons between the effects of chronic drug exposure (tolerance studies), genetic manipulations, and pharmacological treatments.

Anticonflict and rotarod impairing effects of alprazolam and diazepam in rat after acute and subchronic administration

1. The anticonflict and the rotarod impairing effects of the triazolo-benzodiazepine alprazolam (APZ) and the conventional 1,4-benzodiazepine (BDZ) diazepam (DIZ) were studied in the rat. 2. After acute administration the two drugs displayed approximately equal potency with respect to anticonflict effects in Vogel's conflict test and in Montgomery's conflict test. The results obtained in the former model indicate that also the efficacies of the two drugs are approximately equal. The shapes of the dose response curves obtained in this model differed, however, markedly. Thus, while APZ displayed a wide high efficacy dose-range (0.5-16.0 mg/kg), a maximal effect of DIZ was obtained in a narrow dose-range only (around 4.0 mg/kg). 3. Acutely, APZ was more rotarod impairing than DIZ; total impairment after 2.0 mg/kg of APZ and 3.5 mg/kg of DIZ. 4. After daily treatment with doses producing approximately equal and close to maximal anticonflict effects (APZ 1.25 mg/kg/day; DIZ 3.0 mg/kg/day) tolerance developed to the rotarod impairing (6 days of treatment) and the anticonflict (Vogel's conflict test; 9 days of treatment) effects of both drugs. It appeared, however, as if tolerance was more pronounced to the ataxic/sedative effects (approximately a four-fold rightward shift of the dose-response curve) than to the anticonflict effect (approximately a two-fold shift) of APZ, whereas tolerance developed to about the same extent to the two effects of DIZ (approximately two-fold shifts). 5. In conclusion, for APZ, but not for DIZ, after long-term administration of a dose producing marked anxiolytic-like effects, the relationship anxiolytic-like effect vs. ataxic/sedative effects was altered. These findings offer a tentative explanation to the putative advantage of APZ over conventional BDZs in the treatment of panic disorder.

Rapid induction of lorazepam dependence and reversal with flumazenil

Benzodiazepine tolerance, dependence and withdrawal are well established clinical entities although the pharmacological basis for these are still unclear. Recent data suggest that the primary event may be a change in efficacy at the benzodiazepine receptor. The present study demonstrates the rapid development of tolerance and dependence to lorazepam, defines its pharmacology in more detail, and shows that it may be rapidly reversed by treatment with the benzodiazepine antagonist flumazenil. These observations argue in favour of a receptor efficacy change underlying benzodiazepine tolerance and withdrawal and suggest a potential pharmacological treatment for this common and disabling clinical problem.

Withdrawal syndrome following subchronic treatment with anxiolytic agents

The acute administration of diazepam (0.1-2.5 mg/kg IP), sulpiride (0.5-20 mg/kg IP) and tiapride (0.5-40 mg/kg IP) to the mouse enhanced exploratory activity (rearings/line crossings) in the brightly illuminated white area of a two compartment white/black anxiety test box, with a corresponding decrease in the black, indicating an anxiolytic action. This profile of change was maintained during a twice daily administration for 7 days with diazepam (2.5 and 10 mg/kg), sulpiride (5 and 20 mg/kg) and tiapride (10 and 40 mg/kg). However, 8 and 48 hr following withdrawal of diazepam, the profile of exploratory behaviour was reversed to a preference for the black area: by 96 hr values for behaviour had returned to control levels. In contrast, an anxiolytic profile of action was maintained 8 and 48 hr following the withdrawal of sulpiride and tiapride, the values returning to control levels after 96 hr. It is concluded that a sub-chronic treatment with diazepam, sulpiride and tiapride induces an anxiolytic profile of action in the mouse model, that an anxiogenic profile follows the abrupt withdrawal of diazepam but that this is not recorded following the abrupt withdrawal of sulpiride and tiapride.

Effects of chronic diazepam exposure on GABA sensitivity and on benzodiazepine potentiation of GABA-mediated responses of substantia nigra pars reticulata neurons of rats

This study examined the effects of chronic diazepam treatment on GABA sensitivity of substantia nigra pars reticulata neurons and on the ability of benzodiazepines to enhance GABAergic responses of these neurons in rats. Chronic diazepam exposure failed to significantly alter the sensitivity of reticulata neurons to microiontophoretically applied GABA. However, following chronic diazepam treatment for 1 day, or 1, 3 or 7-11 weeks, reticulata neurons showed tolerance to additional systemically or iontophoretically applied benzodiazepines and displayed an increased firing rate following injection of Ro 15-1788. These changes were not apparent 24 h after cessation of chronic treatment. Thus, tolerance to the effects of benzodiazepines on reticulata neurons appeared to develop after a single day of diazepam exposure and to dissipate by 24 h after cessation of treatment. When compared to our previous studies on dorsal raphe neurons, these results demonstrate regional differences in neuronal responses to chronic diazepam exposure, which may help elucidate neural systems which are involved in tolerance to the various functional aspects of benzodiazepines.

Chronic treatment with lorazepam and FG 7142 may change the effects of benzodiazepine receptor agonists, antagonists and inverse agonists by different mechanisms

Treatment of mice with lorazepam 10 mg/kg p.o. or FG 7142 40 mg/kg i.p. once a day for 14 days changed the effects of benzodiazepine (BZ) receptor ligands injected acutely on the threshold of pentylenetetrazol (PTZ)-induced seizures. The effects of the two pretreatments differed qualitatively as well as quantitatively. Lorazepam elicited a shift in the effects of all BZ receptor ligands tested, whereby the agonists lorazepam and ZK 93423 now acted like partial agonists given acutely, the partial agonist ZK 91296 acted like an antagonist and the antagonists Ro 15-1788 and ZK 93426 like partial inverse agonists. The proconvulsant effects of the partial inverse agonist FG 7142 and the full inverse agonist DMCM on the PTZ-induced seizures did not change. However, FG 7142 became a full inverse agonist i.e. became convulsant, and DMCM may have increased in potency as a convulsant. After FG 7142 pretreatment lorazepam and ZK 93423 behaved like partial agonists given acutely whereas there was no change in effect for ZK 91296, Ro 15-1788 and ZK 93426. FG 7142 became convulsant (i.e. kindling occurred) and the potency of DMCM as a convulsant was non-significantly increased, while their proconvulsant effects with respect to PTZ-induced seizures were not altered. The fact that the effects of the two very different pretreatments on the BZ receptor ligand continuum were in the same direction may be explainable by assuming two different mechanisms, both of which may involve the GABA receptors.

Benzodiazepine receptor affinity alterations at physiologic temperature after chronic clonazepam exposure

Cerebral cortical cell cultures obtained from fetal mice were exposed to 200 nM clonazepam (CZP) for 14 days and benzodiazepine (BDZ) receptor binding was measured on intact cells in situ at 37 degrees C. Total, specific, and CZP-displaceable BDZ binding were significantly reduced from control values immediately after drug removal (77.7 +/- 1.4%, 75.1 +/- 3.0%, and 40.9 +/- 6.0% of control, respectively) but Ro5-4864-displaceable binding was not affected (87.6 +/- 5.1%). Binding returned to control values within 48 hours. Saturation analysis of the binding data indicated that a high-affinity binding site could not be detected in CZP-exposed cultures immediately after drug removal (162 nM versus 49 nM in controls, p less than .001), but was present 24 hours later.

Benzodiazepine cross-tolerance in mice extends to sodium valproate

Slow intravenous infusion of pentylenetetrazol was used to measure the convulsive threshold in mice. The anticonvulsant effects of clobazam, clonazepam, diazepam, lorazepam, sodium phenobarbitone and sodium valproate were assessed in naive animals and compared with the effects of the same compounds in animals which had been pretreated (twice daily for 3 days) with one of the benzodiazepines or sodium valproate. Cross-tolerance was observed between all the benzodiazepines but not between benzodiazepines and sodium phenobarbitone. Animals pretreated with the benzodiazepines were cross-tolerant to valproate, but the converse was not true; nor did sodium valproate induce tolerance to itself.

Continuous release of diazepam: electrophysiological, biochemical and behavioral consequences

Neuronal GABAergic sensitivity was assessed using electrophysiological, biochemical and behavioral techniques following the continuous release and maintenance of relatively constant brain levels of diazepam for greater than or equal to 21 days. Our studies indicate that long-term exposure to diazepam results in: (1) a decrease in iontophoretic sensitivity to GABA in the dorsal raphe nucleus, (2) an increase in the affinity of the GABA recognition site in brain tissue and (3) an increase in susceptibility to bicuculline-induced seizures in the intact animal. Since the decrease in GABAergic responsiveness was observed in the presence of measurable levels of diazepam, it was concluded that this subsensitivity phenomenon is associated with tolerance and not with withdrawal effects of the benzodiazepines.

Spontaneous and RO 15-1788-induced reversal of subsensitivity to GABA following chronic benzodiazepines

Chronic daily injections of diazepam (5 mg/kg i.p.) for 21-30 days have previously been shown to cause a selective subsensitivity to microiontophoretically applied GABA in serotonergic dorsal raphe neurons in the rat. Following termination of chronic benzodiazepine treatment, GABAergic sensitivity remained depressed for up to 96 h even though pharmacologically active concentrations of diazepam and its active metabolites could no longer be detected in brain tissue. In contrast, a single injection of the specific benzodiazepine antagonist, RO 15-1788, given 22 h prior to electrophysiological recording, restored GABAergic sensitivity to the control range. Sensitivity of dorsal raphe neurons to serotonin was not altered by either chronic treatment with or withdrawal from diazepam. These results indicate that (1) RO 15-1788 can accelerate the time course of withdrawal and (2) administration of this benzodiazepine antagonist can induce a persistent change in GABAergic responsiveness.

The amnesic action of benzodiazepines in man

A qualitative description of the amnesia produced by the benzodiazepines in man is presented. The benzodiazepines exert their greatest effects in tests of long-term episodic memory in which they cause a dose-related impairment in the acquisition of new information, do not appear to affect retention and may facilitate retrieval. Benzodiazepines do not appear to impair semantic memory or the acquisition of skills. Although state-dependent learning may be observed with benzodiazepine treatment it is a small effect and cannot account for most of the observed impairments. The amnesia appears to be characteristic of all benzodiazepines and may be related to the sedative action of these compounds but evidence on the latter point is inconclusive. The importance of the amnesic action in a population of clinically anxious outpatients taking benzodiazepines over an extended period remains to be investigated. The benzodiazepines may provide the cognitive psychologist with a useful tool to investigate the mechanisms of normal memory.

Tolerance to the behavioral actions of benzodiazepines

The evidence for tolerance to the behavioral effects in animals of benzodiazepines is reviewed. Tolerance develops rapidly (within 3-5 days) to the sedative effects and from 5 days of treatment to the anticonvulsant effects. In general, tolerance has not been found to anxiolytic effects after 7-15 days of treatment, although in the social interaction test it was found after 25 days. Tolerance has not been found to the locomotor stimulant effects up to 20 days of treatment. Dispositional tolerance does not occur following treatment with low doses and nor is there clear evidence of changes in benzodiazepine binding. Such changes could not account for the very different rates of tolerance to the different behavioral effects, but these could be explained if learned adaptation were to underlie tolerance or to influence the rate at which it develops. Whether the mechanism of learned adaptation is one of instrumental conditioning, classical conditioning or habituation will depend on the formal aspects of the test. It is therefore suggested that the different rates of tolerance are a function of the detailed arrangement of the experimental situation and not of the particular behavior measured or of the clinical effect the test is meant to reflect.

Time course for development of benzodiazepine tolerance and physical dependence

Chronic benzodiazepine treatment elicits adaptive responses in the CNS, seen behaviorally as functional tolerance and physical dependence. Experiments are described in which a radioreceptor assay is used to follow benzodiazepine activity in CSF samples during daily flurazepam treatment of cats. Tolerance is evident even after the second dose, despite increasing CSF drug activity, showing a large and rapidly developing functional tolerance. Other studies are discussed which also show tolerance within 24 hours of initiating benzodiazepine treatment. In contrast, a spontaneous withdrawal syndrome is usually seen only after prolonged treatment with high doses. However, physical dependence can also be studied by precipitating abstinence with a benzodiazepine antagonist, such as Ro15-1788. Cats were treated daily with flurazepam, then Ro15-1788 was given and abstinence signs were recorded. Abstinence could be precipitated 24 hours after beginning treatment, and dependence was nearly maximal after 7 days. Dependence developed during treatment with as little as 0.5 mg/kg flurazepam, which is near threshold for any behavioral response. Chronic diazepam caused the same dependence as flurazepam. Thus, the development of tolerance and physical dependence both show a remarkably rapid adaptation of the CNS in response to benzodiazepines.

Functional tolerance to alprazolam-induced neurologic deficits in mice

Tolerance to alprazolam-induced neurologic deficits was assessed in mice by a rotarod test. Tolerance was found following a second administration of 2 mg/kg alprazolam 24 h after the first dose and it decayed gradually over a period of several weeks. Brain and plasma concentrations of alprazolam indicated that this tolerance was functional.