Tolerance to the anticonvulsant effects of carbamazepine, diazepam, and sodium valproate in kindled rats

Long-lasting antiseizure effects of chronic intrasubthalamic convection-enhanced delivery of valproate

Intracerebral drug delivery is an experimental approach for the treatment of drug-resistant epilepsies that allows for pharmacological intervention in targeted brain regions. Previous studies have shown that targeted pharmacological inhibition of the subthalamic nucleus (STN) via modulators of the GABAergic system produces antiseizure effects. However, with chronic treatment, antiseizure effects are lost as tolerance develops. Here, we report that chronic intrasubthalamic microinfusion of valproate (VPA), an antiseizure medication known for its wide range of mechanisms of action, can produce long-lasting antiseizure effects over three weeks in rats. In the intravenous pentylenetetrazole seizure-threshold test, seizure thresholds were determined before and during chronic VPA application (480 μg/d, 720 μg/d, 960 μg/d) to the bilateral STN. Results indicate a dose-dependent variation in VPA-induced antiseizure effects with mean increases in seizure threshold of up to 33%, and individual increases of up to 150%. The lowest VPA dose showed a complete lack of tolerance development with long-lasting antiseizure effects. Behavioral testing with all doses revealed few, acceptable adverse effects. VPA concentrations were high in STN and low in plasma and liver. In vitro electrophysiology with bath applied VPA revealed a reduction in spontaneous firing rate, increased background membrane potential, decreased input resistance and a significant reduction in peak NMDA, but not AMPA, receptor currents in STN neurons. Our results suggest an advantage of VPA over purely GABAergic modulators in preventing tolerance development with chronic intrasubthalamic drug delivery and provide first mechanistic insights in intracerebral pharmacotherapy targeting the STN.

Evolving dynamic networks: An underlying mechanism of drug resistance in epilepsy?

At least one-third of all people with epilepsy have seizures that remain poorly controlled despite an increasing number of available anti-epileptic drugs (AEDs). Often, there is an initial good response to a newly introduced AED, which may last up to months, eventually followed by the return of seizures thought to be due to the development of tolerance. We introduce a framework within which the interplay between AED response and brain networks can be explored to understand the development of tolerance. We use a computer model for seizure generation in the context of dynamic networks, which allows us to generate an 'in silico' electroencephalogram (EEG). This allows us to study the effect of changes in excitability network structure and intrinsic model properties on the overall seizure likelihood. Within this framework, tolerance to AEDs - return of seizure-like activity - may occur in 3 different scenarios: 1) the efficacy of the drug diminishes while the brain network remains relatively constant; 2) the efficacy of the drug remains constant, but connections between brain regions change; 3) the efficacy of the drug remains constant, but the intrinsic excitability within brain regions varies dynamically. We argue that these latter scenarios may contribute to a deeper understanding of how drug resistance to AEDs may occur.

Deconstructing tolerance with clobazam: Post hoc analyses from an open-label extension study

Objective:

To evaluate potential development of tolerance to adjunctive clobazam in patients with Lennox-Gastaut syndrome.

Methods:

Eligible patients enrolled in open-label extension study OV-1004, which continued until clobazam was commercially available in the United States or for a maximum of 2 years outside the United States. Enrolled patients started at 0.5 mg·kg⁻¹·d⁻¹ clobazam, not to exceed 40 mg/d. After 48 hours, dosages could be adjusted up to 2.0 mg·kg⁻¹·d⁻¹ (maximum 80 mg/d) on the basis of efficacy and tolerability. Post hoc analyses evaluated mean dosages and drop-seizure rates for the first 2 years of the open-label extension based on responder categories and baseline seizure quartiles in OV-1012. Individual patient listings were reviewed for dosage increases ≥40% and increasing seizure rates.

Results:

Data from 200 patients were included. For patients free of drop seizures, there was no notable change in dosage over 24 months. For responder groups still exhibiting drop seizures, dosages were increased. Weekly drop-seizure rates for 100% and ≥75% responders demonstrated a consistent response over time. Few patients had a dosage increase ≥40% associated with an increase in seizure rates.

Conclusions:

Two-year findings suggest that the majority of patients do not develop tolerance to the antiseizure actions of clobazam. Observed dosage increases may reflect best efforts to achieve seizure freedom. It is possible that the clinical development of tolerance to clobazam has been overstated.

ClinicalTrials.gov identifier:

NCT00518713 and NCT01160770.

Classification of evidence:

This study provides Class III evidence that the majority of patients do not develop tolerance to clobazam over 2 years of treatment.

Tolerance to the beneficial effects of prophylactic migraine drugs: a systematic review of causes and mechanisms

Loss of benefit of a previously effective treatment regimen, also known as tolerance, can be an important barrier to the successful preventive treatment of migraine. We undertook a systematic review of the literature to identify the prevalence and possible mechanisms of drug tolerance in migraine prophylaxis. Results demonstrate that the frequency of tolerance to prophylactic migraine treatment is unknown, but available data support an estimate that it occurs in 1-8% of patients receiving prophylaxis. Four broad types of tolerance were identified that are likely to be relevant to migraine prophylaxis. These are pharmacokinetic, pharmacodynamic, behavioral, and cross tolerance. The mechanisms that underlie these types of tolerance determine whether their effects can be overcome or minimized. For example, certain forms of tolerance may be affected by manipulation of environmental cues associated with drug administration, by the order in which drugs are used, and by the concomitant use of other medications. Many medications used for migraine prophylaxis exert their effects through the endogenous opioid system. The implications of this finding are explored, particularly the parallels between medication overuse headache and tolerance to migraine prophylaxis. Given the many ways in which tolerance to migraine medications may develop, in some ways it is not surprising that migraine-preventive drugs stop working; it is more surprising that in many cases they do not.

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.

Effect of intracerebroventricular continuous infusion of valproic acid versus single i.p. and i.c.v. injections in the amygdala kindling epilepsy model

Two protocols were tested to assess anticonvulsant efficacy and drug concentrations after intracerebroventricular (i.c.v.) continuous valproic acid (VPA) infusion, as compared with acute injections in the kindling epilepsy model. Protocol 1: amygdala-kindled rats were injected via intraperitoneal (i.p.) and i.c.v. routes with varying doses of VPA and tested for seizure intensity, afterdischarge and seizure duration, ataxia and sedation. Concentrations of VPA were determined by immunofluorescence in the brain, plasma, cerebrospinal fluid (CSF) and liver in matching rats. Protocol 2: amygdala-kindled rats were implanted with osmotic minipumps containing a VPA solution in saline and connected to intraventricular catheters for 7 days. Seizure threshold, latency and duration, afterdischarge duration, ataxia and sedation were recorded daily before, during, and until 5 days after VPA infusion. In matching animals, CSF, brain, plasma and liver VPA concentration was determined. Acute i.c.v. VPA injection suppressed seizures with a remarkable ataxia and sedation. However, continuous i.c.v. infusion controlled generalised and even focal seizures without producing important side effects, high plasma levels or hepatic drug concentrations. In conclusion, continuous i.c.v. VPA infusion may protect against kindled seizures by minimising ataxia and sedation, and achieving suitable intracerebral, yet low plasma or hepatic drug concentrations, thus avoiding potential systemic toxicity.

Repeated acute testing of anticonvulsant drugs in amygdala kindled rats: increase in anticonvulsant but decrease in adverse effect potential

PURPOSE

Because preparation of kindled rats is laborious, time-consuming, and expensive, such animals are often used for several experiments in the evaluation of anticonvulsant drugs (AEDs). Furthermore, for comparison with data on new drugs, often "historical" data on standard drugs obtained in previous experiments in other groups of kindled rats are used. Without knowing how factors such as repeated drug testing or seasonal variation in drug responses affect drug potencies in the kindling model, false conclusions and predictions might be drawn from such comparisons. In this study, we examined the anticonvulsant and adverse effects of the three clinically established AEDs carbamazepine (CBZ), phenobarbital (PB), and valproate (VPA) once per month in the same two groups of amygdala-kindled rats over a period of 9 (group 1) or 6 (group 2) consecutive months. To evaluate the possible effect of the season, experiments in group 1 were started in autumn, and experiments in group 2 in spring.

METHODS

For quantification of anticonvulsant activity, the focal seizure threshold (threshold for afterdischarges; ADT) was determined after each acute drug treatment and compared with a control ADT determined 2-3 days before.

RESULTS

The repeated acute (single-dose) drug testing in the same groups of amygdala-kindled rats led to three pronounced alterations in the animals: (a) a significant decrease in ADT, (b) a marked potentiation of AED effects on ADT, and (c) a striking reduction in ataxia produced by drug treatments. Drug levels in plasma, which were determined in each drug trial, showed only moderate variation over the period of the experiments, so that the observed alterations in drug responses were certainly not due to pharmacokinetic factors. PB and VPA, but not CBZ, showed a more potent anticonvulsant effect when experiments were started in October (group 1) compared with April (group 2), but this difference was rapidly overridden by the marked and progressive potentiation of anticonvulsant activity on repeated drug testing.

CONCLUSIONS

These data demonstrate that repeated use of the same kindled rats for acute drug testing significantly alters the sensitivity of the animals to the anticonvulsant and adverse effects of drugs. Because the anticonvulsant potency increases, whereas the adverse effect potential decreases during repeated acute drug testing, this may lead to false-positive data on a test compound. The mechanisms involved in these observations deserve further studies.

Modulation of sodium currents in rat CA1 neurons by carbamazepine and valproate after kindling epileptogenesis

PURPOSE

To determine the modulation of sodium currents in hippocampal CA1 neurons by carbamazepine (CBZ) and valproate (VPA), before and after kindling epileptogenesis.

METHODS

Voltage-dependent sodium current was measured in isolated hippocampal CA1 neurons, by using the whole-cell voltage-clamp technique. CBZ (15-100 microM) or VPA (0.5-5 mM) was applied by bath perfusion. Cells from fully kindled rats were compared with controls, 1 day and 5 weeks after the tenth generalized seizure.

RESULTS

CBZ did not affect sodium current activation but selectively shifted the voltage dependence of steady-state inactivation to more hyperpolarized potentials. One day after the last kindled generalized seizure, the shift induced by 15 microM CBZ was 2.1+/-0.5 mV (mean +/- SEM; n = 20) compared with 4.3+/-0.3 mV (n = 16; p<0.001) in matched controls. The EC50 of the concentration-effect relation was 57+/-6 microM compared with 34+/-2 microM (p<0.01) in controls. Five weeks after kindling, these values had recovered to a level not different from control. VPA induces at a relatively high concentration a similar but smaller shift in voltage dependence of inactivation than does CBZ. After kindling, the shift induced by 2 mM VPA (2.8+/-0.6 mV; n = 19) was not different from controls (3.0+/-0.5 mV; n = 22). The EC50 for VPA was 2.6+/-0.3 mM compared with 2.5+/-0.4 mM in controls.

CONCLUSIONS

Both CBZ and VPA selectively modulate the voltage dependence of sodium current steady-state inactivation and as a consequence reduce cellular excitability. The effect of CBZ was reduced immediately after kindling epileptogenesis, apparently by a reduced affinity of its receptor. In contrast, the shift induced by VPA was not different at any stage after kindling epileptogenesis. The change in CBZ sensitivity after kindling is related to epileptic activity rather than to the epileptic state, because it almost completely recovers in a period without seizures.

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.

A review of the preclinical pharmacology of tiagabine: a potent and selective anticonvulsant GABA uptake inhibitor

We review the neurochemical and behavioral profile of the selective gamma-aminobutyric acid (GABA) uptake inhibitor, (R)-N-(4,4-di-(3-methylthien-2-yl)but-3-enyl) nipecotic acid hydrochloride [tiagabine (TGB), previously termed NNC 05-0328, NO 05-0328, and NO-328], which is currently in phase III clinical trials for epilepsy. TGB is a potent, and specific GABA uptake inhibitor. TGB lacks significant affinity for other neurotransmitter receptor binding sites and/or uptake sites. In electrophysiological experiments in hippocampal slices in culture, TGB prolonged the inhibitory postsynaptic potentials (IPSP) and inhibitory postsynaptic currents (IPSC) in the CA1 and CA3 produced by the addition of exogenous GABA. In vivo microdialysis shows that TGB also increases extracellular GABA overflow in a dose-dependent manner. Together these biochemical data suggest that the in vitro and in vivo mechanism of action of TGB is to inhibit GABA uptake specifically, resulting in an increase in GABAergic mediated inhibition in the brain. TGB is a potent anticonvulsant agent against methyl-6,7-dimethyoxy-4-ethyl-B-carboline-3-carboxylate (DMCM)-induced clonic convulsions (mice), subcutaneous pentylenetetrazol (PTZ)-induced tonic convulsions (mice and rats), sound-induced convulsions in DBA/2 mice and genetically epilepsy-prone rats (GEPR), and electrically induced convulsions in kindled rats. TGB is partially efficacious, against subcutaneous PTZ-induced clonic convulsions, and photically induced myoclonus in Papio papio. TGB is weakly efficacious in the intravenous PTZ seizure threshold test and the maximal electroshock seizure (MES) test and produces only partial protection against bicuculline (BIC)-induced convulsions in rats. The overall biochemical and anticonvulsant profile of TGB suggests potential utility in the treatment of chronic seizure disorders such as generalized clonic-tonic epilepsy (GTCS), photomyoclonic seizures, myoclonic petit mal epilepsy, and complex partial epilepsy.

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.

Lack of tolerance to the anticonvulsant effects of tiagabine following chronic (21 day) treatment

The anticonvulsant, and side effect, profile of the gamma-aminobutyric acid (GABA) uptake inhibitor (R)-N-(4,4-di-(3-methylthien-2-yl)but-3-enyl) nipecotic acid hydrochloride (tiagabine) was examined in mice following chronic (21 day) administration. Twenty-four hours following the discontinuation of the 21 days' treatment with twice daily administration of vehicle or tiagabine at 15 or 30 mg/kg p.o., an ED50 for tiagabine was determined for the anticonvulsant effect, the rotarod performance, the traction response and the inhibition of locomotor activity in the animals treated with vehicle only, and in the groups previously treated with 15 or 30 mg/kg p.o. of tiagabine. There was no significant decrease in the anticonvulsant efficacy of acutely administered tiagabine (ED50 for inhibition of methyl 6,7-dimethoxy-4-ethyl-beta-carboline-2-carboxylate (DMCM)-induced seizures of 1.7 +/- 0.4, 1.9 +/- 0.3, and 2.0 +/- 0.50 mg/kg i.p., respectively). However, there was a significant decrease in the ability of acutely administered tiagabine to impair rotarod performance (ED50 of 5.9 +/- 1.2, 14 +/- 1.9 and 21 +/- 2.7 mg/kg i.p., respectively), inhibit a traction response (ED50 of 10 +/- 1.6, 23 +/- 3.0 and 34 +/- 4.6 mg/kg i.p., respectively), and to inhibit exploratory locomotor activity (ED50 of 13 +/- 23, 19 +/- 2.6 and 28 +/- 38 mg/kg i.p. respectively). Following the discontinuation of chronic tiagabine administration there was no change in pentylenetetrazol (PTZ) seizure threshold, animal weight or gross behavior, suggesting the lack of a behavioral withdrawal syndrome. The production of tolerance to the sedative and ataxic effects, but not the anticonvulsant effects, of tiagabine suggests that tiagabine may be a useful agent for the long-term treatment of epilepsy.

Dissipation of contingent tolerance to the anticonvulsant effect of diazepam: effect of the criterion response

The effect of convulsive stimulations on the dissipation of tolerance to the anticonvulsant effect of diazepam was investigated using the kindled-convulsion model. Amygdala-kindled rats were rendered tolerant to diazepam's anticonvulsant effect by 25 "bidaily" (one/48 h) diazepam injections (2.5 mg/kg), each followed 1 h later by a convulsive stimulation. They were then divided into nine groups for the tolerance-dissipation phase of the experiment. Of the nine groups, three received bidaily control handling for one trial, three trials, or seven trials; three received bidaily saline injections, each 1 h before a convulsive stimulation, for one, three, or seven trials; and three received bidaily diazepam injections, each 1 h after a convulsive stimulation, for one, three, or seven trials. Finally, each rat received a tolerance-retention test (i.e., a diazepam injection followed 1 h later by a convulsive stimulation) 48 h after its last tolerance-dissipation trial. The tolerance dissipated gradually but completely over the 4-, 8-, and 16-day test intervals in the rats that received a convulsive stimulation before each injection during the tolerance-dissipation phase, whether they were injected with saline or diazepam; in contrast, tolerance did not dissipate in the rats that received saline injections but no stimulations. Remarkably, the discontinuance of the bidaily diazepam injections, even for 16 days, was not sufficient to dissipate the tolerance that had developed to diazepam's anticonvulsant effect; nor was the continuation of the bidaily diazepam injections sufficient to keep tolerance from dissipating.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of the hippocampal GABAA receptor system in kindled rats by autoradiographic and in situ hybridization techniques: contingent tolerance to carbamazepine

Tolerance to the anticonvulsant effects of carbamazepine (CBZ) in the amygdala kindling paradigm is a contingent process, since it only develops in rats treated with CBZ before the kindling stimulation and not in those animals treated after the stimulation. The present study was designed to investigate the GABAA receptor system in CBZ contingent tolerance. Receptor autoradiography utilizing various radioligands that bind to different components of the GABAA receptor system and in situ hybridization with oligonucleotides that recognize different subunits of the GABAA receptor were performed. Kindling increased binding to benzodiazepine, picrotoxin, and GABA recognition sites selectively in the dentate gyrus of the hippocampus. Kindling also increased levels of mRNA for the alpha 4, beta 1, and beta 3 subunits but did not change alpha 1, alpha 2, or gamma 2 subunit levels. Rats tolerant to CBZ showed decreased [3H]muscimol binding, diazepam-insensitive [3H]Ro 15-4513 binding, and decreased alpha 4 subunit mRNA content compared to non-tolerant rats, whereas [3H]flunitrazepam binding, [35S]TBPS binding, and the levels of beta 1, and beta 3 subunit mRNAs remained elevated. The data suggest an indirect interaction of CBZ with the GABAA receptor system, since CBZ reportedly does not bind to this receptor system.

Bidirectional contingent cross tolerance between the anticonvulsant effects of pentobarbital and ethanol

In Experiment 1, two groups of kindled rats received a pentobarbital injection (15 mg/kg, IP) and a convulsive amygdala stimulation once every 48 h. In one group, pentobarbital was injected 1 h before each stimulation; in the other, it was injected 1 h after each stimulation. Only the rats that received pentobarbital before each stimulation became tolerant to pentobarbital's anticonvulsant effect. Cross tolerance to the anticonvulsant effect of ethanol (1.5 g/kg, IP) was also found to be greater in the pentobarbital-before-stimulation rats. Experiment 2 was designed to assess the transfer of tolerance in the opposite direction, that is, from ethanol to pentobarbital, and the results mirrored those of Experiment 1: convulsive stimulation during the periods of ethanol exposure facilitated the development of tolerance to the anticonvulsant effect of ethanol and its transfer to pentobarbital. These results support the theory that functional drug tolerance and cross tolerance are adaptations to the effects of drugs on concurrent patterns of neural activity rather than to drug exposure per se.

Tolerance to the anticonvulsant effects of phenobarbital, trimethadione, and clonazepam in kindled rats: cross tolerance to carbamazepine

The kindled-convulsion model was used to assess the development of tolerance and cross tolerance to the anticonvulsant effects of antiepileptic drugs. In Experiment 1, tolerance developed to the anticonvulsant effects of bidaily (one every 48 h) IP injections of phenobarbital, trimethadione, and clonazepam on convulsions elicited 1 h after each injection in kindled rats by amygdala stimulation. In Experiment 2, kindled rats that were tolerant to the anticonvulsant effects of phenobarbital, trimethadione, or clonazepam received bidaily IP injections of carbamazepine, each followed 1 h later by a convulsive amygdala stimulation. There was a statistically significant transfer of tolerance from phenobarbital to carbamazepine, but not from either trimethadione or clonazepam to carbamazepine. Apparently, tolerance to anticonvulsant drugs is most likely to transfer between drugs that are effective against similar kinds of clinical and experimental seizures and have similar putative mechanisms of action.

Contingent tolerance to the anticonvulsant effects of carbamazepine, diazepam, and sodium valproate in kindled rats

The effect of convulsive stimulation during periods of drug exposure on the development of tolerance to the anticonvulsant effects of carbamazepine (CBZ), diazepam (DZP), or sodium valproate (VPA) was studied in three similar experiments. In each experiment, amygdala-kindled rats were assigned to one of three groups: one group received a drug injection (CBZ, 70 mg/kg, IP; DZP, 2 mg/kg, IP; VPA, 250 mg/kg, gavage) 1 h before each of a series of 10 bidaily (one every 48 h) convulsive stimulations, a second group received the same dose of the drug 1 h after each of the 10 stimulations, and a third group served as a vehicle control. The drug tolerance test occurred in each experiment 48 h after the 10th tolerance-development trial; every rat received the appropriate dose of CBZ, DZP, or VPA 1 h before being stimulated. In each experiment, only the rats from the drug-before-stimulation group displayed a significant amount of tolerance to the drug's anticonvulsant effect. Thus the development of tolerance to the anticonvulsant effects of CBZ, DZP, and VPA was not an inevitable consequence of drug exposure; the development of tolerance was contingent upon the occurrence of convulsive stimulation during the periods of drug exposure. These results support the idea that functional drug tolerance is an adaptation to a drug's effects on ongoing patterns of neural activity, rather than to drug exposure per se.