Tolerance to the effects of diazepam, clonazepam and bretazenil on GABA-stimulated Cl- influx in flurazepam tolerant rats

To what extent is it possible to dissociate the anxiolytic and sedative/hypnotic properties of GABAA receptors modulators?

The relatively common view indicates a possible dissociation between the anxiolytic and sedative/hypnotic properties of benzodiazepines (BZs). Indeed, GABAA receptor (GABAAR) subtypes have specific cerebral distribution in distinct neural circuits. Thus, GABAAR subtype-selective drugs may be expected to perform distinct functions. However, standard behavioral test assays provide limited direction towards highlighting new action mechanisms of ligands targeting GABAARs. Automated behavioral tests, lack sensitivity as some behavioral characteristics or subtle behavioral changes of drug effects or that are not considered in the overall analysis (Ohl et al., 2001) and observation-based analyses are not always performed. In addition, despite the use of genetically engineered mice, any possible dissociation between the anxiolytic and sedative properties of BZs remains controversial. Moreover, the involvement the different subtypes of GABAAR subtypes in the anxious behavior and the mechanism of action of anxiolytic agents remains unclear since there has been little success in the pharmacological investigations so far. This raises the question of the involvement of the different subunits in anxiolytic-like and/or sedative effects; and the actual implication of these subunits, particularly, α-subunits in the modulation of sedation and/or anxiety-related disorders. This present review was prompted by several conflicting studies on the degree of involvement of these subunits in anxiolytic-like and/or sedative effects. To this end, we explored the GABAergic system, particularly, the role of different subunits containing synaptic GABAARs. We report herein the targeting gene encoding the different subunits and their contribution in anxiolytic-like and/or sedative actions, as well as, the mechanism underlying tolerance to BZs.

Flumazenil Attenuates Development of Tolerance to Diazepam After Chronic Treatment of Mice with Either Isoflurane or Diazepam

In an effort to clarify the mechanism of action of isoflurane, we studied the effect of flumazenil on mice chronically treated with isoflurane or diazepam. Mice were pretreated with diazepam, isoflurane, or saline, with and without flumazenil. After 2 wk, responses to isoflurane and diazepam were assessed, and central benzodiazepine receptor (CBR) binding characteristics were assayed. Mice pretreated with isoflurane failed the horizontal wire test at a larger isoflurane concentration (0.5%) compared with saline-pretreated mice (0.4%) (P < 0.05). These differences did not occur when flumazenil was added to the pretreatment. After the administration of diazepam, 20% of diazepam- and 11% of isoflurane-pretreated mice failed the horizontal wire test, versus 50% and 44% when flumazenil was added to either drug (P < 0.002) and 80% and 100% in the saline and saline plus flumazenil-treated mice. The increased CBR density due to flumazenil was attenuated by the coadministration of isoflurane or diazepam. Flumazenil attenuated the development of tolerance to diazepam after chronic treatment with diazepam or isoflurane and attenuated the development of tolerance to isoflurane. Isoflurane, like diazepam, attenuated the effect of flumazenil on CBR ligand binding. These findings suggest that isoflurane shares a mechanism of action with diazepam, probably via the gamma-aminobutyric acid system, most probably the CBR.

IMPLICATIONS

Flumazenil attenuates the development of tolerance to isoflurane and diazepam after chronic isoflurane pretreatment. Isoflurane, like diazepam, attenuates the increase in central benzodiazepine receptor (CBR) density caused by flumazenil. These findings suggest that isoflurane and diazepam share a mechanism of action, most probably via the gamma-aminobutyric acid system and the CBR.

Lack of allosteric modulation of striatal GABA(A) receptor binding and function after cocaine sensitization

GABA(A) receptor binding after repeated cocaine has been shown to be either increased as indicated by benzodiazepine binding or decreased as indicated by convulsant-site binding. We measured the GABA binding site with [3H]-muscimol binding to GABA(A) receptors and found no differences between saline- and cocaine-sensitized rats. Allosteric modulation of [3H]-muscimol binding with flunitrazepam was also unchanged after cocaine sensitization. In addition, [3H]-flunitrazepam binding and allosteric modulation of [3H]-flunitrazepam binding with GABA was unchanged after 1 day withdrawal from repeated cocaine. GABA(A) receptor function and allosteric modulation of GABA(A) receptor function measured by GABA-stimulated Cl(-) uptake was also unchanged after withdrawal from repeated cocaine. Finally, in vitro cocaine reduced GABA(A) receptor function in striatal microsacs of saline- and cocaine-treated rats. In conclusion, repeated cocaine did not change the coupling of the GABA(A) receptor between the GABA and benzodiazepine (BZD) binding site after 1 day withdrawal.

GABA(A) receptor gene expression in rat cortex: differential effects of two chronic diazepam treatment regimes

Diazepam is widely prescribed as an anxiolytic but its therapeutic application is limited because with daily use tolerance develops to certain aspects of its pharmacological profile. We compared the effects of two dosing paradigms on GABA(A) receptor gene expression and benzodiazepine binding characteristics. Equivalent daily doses of 15 mg/kg/day diazepam were delivered either via constant infusion or daily subcutaneous injection for 14 days. The two distinct treatment regimes produced significantly different changes in GABA(A) receptor alpha4-, beta2-, beta3- and gamma1-subunit mRNA steady-state levels. Similar changes in the GABA enhancement of flunitrazepam binding and the BZ3/BZ2 subtype ratio determined ex vivo were produced, however, significant differences were found in [(3)H]-Ro 15-4513 binding between cortical tissue from diazepam injected animals compared with diazepam infused animals. Our data suggest that it is the diurnal fluctuations in receptor occupancy that are responsible for the different effects produced by these two dosing regimes.

Cross-sensitivity between isoflurane and diazepam: evidence from a bidirectional tolerance study in mice

We examined in mice the effect of chronic diazepam treatment on the sensitivity to isoflurane, and that of repeated isoflurane exposure on the sensitivity to diazepam. Mice were divided into four groups: group 1, treated with diazepam, 10 mg/kg i.p. twice daily; group 2, vehicle-treated controls; group 3, exposed to 3% isoflurane for 25 min twice daily; and group 4, untreated controls. After 14 days the effect of the treatment was assessed. Twenty-four hours after the last 10 mg/kg diazepam treatment, groups 1 and 2 received diazepam, 5 mg/kg i.p., and were subjected to the horizontal wire test (HWT). All control mice but only 10% of the diazepam-treated mice failed the HWT. Groups 1 and 2 were then exposed to increasing concentrations of isoflurane. Diazepam-treated mice (group 1) lost the HWT at 0.7+/-0.7%, compared with 0.6+/-0.1% in controls (group 2) (P<0.001); the ED50 was 0.75% vs. 0.65%. Group 1 mice lost the righting reflex at 0.94+/-0.07% isoflurane vs. 0.87+/-0.06% in group 2 (P<0.01); the ED50 was 0.93% vs. 0.82%. Recovery time was 175+/-161 s in group 1 vs. 343+/-275 s in group 2 (P<0.02). Twenty-four hours after the last of the repeated exposures to isoflurane, we examined the responses of groups 3 and 4 to increasing concentrations of isoflurane. Mice in group 3 lost the righting reflex at 1.0+/-0.06% isoflurane vs. 0.9+/-0.04% in controls (group 4) (P<0.001); the ED50 was 0.96% vs. 0.85%. Recovery time was 113+/-124 s vs. 208+/-126 s in groups 3 and 4 (P<0.09). Diazepam, 3 mg/kg i.p. administered to groups 3 and 4, caused loss of the HWT reflex in 33% of group 3 mice and in 82% of controls (group 4) (P<0.001). It appears that prolonged exposure to both diazepam and isoflurane caused reduced sensitivity to each drug separately, as well as to the other drug. This finding may strengthen the theory that inhalational anesthetics may act via the same mechanism as the benzodiazepines.

Lack of tolerance to the anxiolytic effect of diazepam and pentobarbital following chronic administration in perinatally undernourished rats

Adult female rats, undernourished at perinatal age, were evaluated for anxiolytic action in the plus-maze test after acute and chronic administration of diazepam (DZP) and pentobarbital (PTB). Deprived (D) rats chronically treated with vehicle showed an increased anxiety as compared with control (C) animals. A single intraperitoneal (i.p.) administration of DZP (1 mg/kg) or PTB (7.5 mg/kg) produced similar anticonflict effect in both C and D rats. Tolerance to the anxiolytic effect of DZP and PBT developed in C rats after a 15-day administration schedule, whereas no tolerance was observed in D animals. Drug disposition was not altered after chronic treatment either in C or in D rats. Gamma-aminobutyric acid (GABA)-mediated chloride uptake in microsacs of cerebral cortex of naive D rats was decreased as compared with naive C rats. After chronic DZP administration (1 mg/kg/day i.p. for 15 days), GABA-mediated 36Cl- influx in brain cortex microsacs of C rats did not change; however, GABA efficacy was increased in microsacs of D animals. In addition, chronic DZP treatment induced GABA-benzodiazepine uncoupling in brain cortex of C rats, but not in D animals, as assessed by chloride uptake in microsacs. Chronic PTB treatment (7.5 or 30 mg/kg/day i.p. for 15 days) did not modify GABA stimulation or GABA-PTB interaction in cortical microsacs of C or D rats.

Diazepam physical dependence and withdrawal in rats is associated with alteration in GABAA receptor function

Alteration in the function of the GABAA receptor complex and its relation to changes in withdrawal signs in diazepam (DZP)-dependent rats were studied. Physical dependence on DZP was induced in male F344 rats by using the drug-admixed food method. After cessation of treatment, withdrawal signs such as spontaneous convulsions were observed and withdrawal scores were maximal at 39 approximately 45 hr after the DZP withdrawal. Furthermore, these withdrawal signs almost disappeared by 159 approximately 168 hr after the DZP withdrawal. GABA-stimulated 36Cl- influx into cerebral cortical membrane vesicles was significantly decreased in rats 0 hr after DZP withdrawal and significantly increased in rats 42 hr after DZP withdrawal compared with control rats Flunitrazepam (FZ)-induced potentiation and an antagonistic effect of Ro 15-1788 on GABA-stimulated 36Cl- influx were observed in control rats. No FZ-potentiated GABA-stimulated 36Cl- influx was observed in rats 0 hr after DZP withdrawal: however, such an effect of FZ was recognized in rats 42 hr and 162 hr after DZP withdrawal. No antagonistic effect of Ro15-1788 on the FZ-induced stimulation was recognized in rats 0 hr and 42 hr after DZP withdrawal but was recognized at 162 hr after DZP treatment, although it was not significant. In a [3H]FZ assay of binding to benzodiazepine (BZ) receptors. Bmax values were significantly decreased in rats 0 hr after DZP withdrawal, but increased at 42 hr after DZP withdrawal, compared with control rats Bmax had almost returned to the control level at 162 hr after DZP treatment rats. In conclusion, these results indicate that functional changes in the GABAA/BZ receptor/CI- channel complex, i.e. increased sensitivity in GABAA receptors and impairment in the functional coupling between BZ receptors and GABAA receptors, may possibly be involved in the biochemical mechanism of the severe withdrawal symptoms appearing after chronic treatment with DZP.

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.

Effects of GABA uptake inhibitors on posthypoxic myoclonus in rats

Male Sprague-Dawley rats developed posthypoxic myoclonus following 10-min cardiac arrest and resuscitation. Previous results showed that dysfunction of central GABAergic neurotransmission may contribute to the disease. In current studies, effects of GABA uptake inhibitors, guvacine hydrochloride (1,2,5,6-tetrahydro-3-pyridine carboxylic acid hydrochloride) and (+/-)-cis-4-hydroxynipecotic acid ([+/-]-cis-4-hydroxy-3-piperidine carboxylic acid), in the pathophysiology of posthypoxic myoclonus were investigated. Administration of guvacine (1 or 10 mg/kg, IP) or nipecotic acid (0.5 or 5 mg/kg, IP) significantly attenuated myoclonus scores of the animals. Tolerance to antimyoclonus effects of these two compounds did not develop after chronic administration (twice a day for 14 days) of guvacine (10 mg/kg, IP) or nipecotic acid (5 mg/kg, IP). On the other hand, tolerance was noticed with clonazepam (2.5 mg/kg, IP twice a day for 7 days). The results indicate that guvacine or nipecotic acid may be used in combination with (at reduced doses) or as alternatives to clonazepam to treat patients with the disease so as to reduce tolerance phenomenon usually associated with clonazepam.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Rapid down-regulation of [3H]zolpidem binding to rat brain benzodiazepine receptors during flurazepam treatment

In a previous study, it was found that down-regulation of benzodiazepine (BZ) binding in rats treated 4 weeks with flurazepam was relatively greater and more widespread when measured with [3H]zolpidem, a selective 'BZ1 receptor' ligand, than that measured with the non-selective ligand, [3H]flunitrazepam. In the present study, the time course for down-regulation of [3H]zolpidem binding was studied in rats treated with flurazepam. [3H]Zolpidem binding was also studied in rats given a midazolam treatment shown to cause tolerance. Rats were chronically treated with flurazepam for 1 or 2 weeks, or with midazolam for 3 weeks, then killed immediately after the treatment. Another group of rats was acutely treated with desalkyl-flurazepam and killed 30 min later. After 2 weeks of flurazepam treatment, the Bmax of [3H]zolpidem binding was decreased by 22% in cerebral cortex, 26% in cerebellum and 33% in hippocampus, with no change in the Kd in any region. After 1 week of flurazepam treatment, the Bmax was decreased by 23% in cerebellum and 14% in hippocampus, but not changed in cerebral cortex. The Kd was increased in cerebral cortex, but not in cerebellum or hippocampus. Neither the Bmax nor the Kd of [3H]zolpidem binding was affected by acute desalkyl-flurazepam treatment, or by 3 weeks of midazolam treatment. These results, in combination with previous findings, which showed no change in [3H]flunitrazepam binding after 1 or 2 week flurazepam treatment, and no change in cerebellum even after the 4 week treatment, may indicate a shift in BZ receptor subtypes in flurazepam-tolerant rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Zinc inhibition of GABA-stimulated Cl- influx in rat brain regions is unaffected by acute or chronic benzodiazepine

Zinc modulation of GABAA receptor function was studied using GABA-stimulated 36Cl- influx into microsacs prepared from rat cerebral cortex, cerebellum and hippocampus. Zinc (10-100 microM) did not affect the basal influx, but significantly inhibited GABA-stimulated 36Cl- influx. The inhibition appeared to be noncompetitive. Zinc produced differing degrees of inhibition of GABA-stimulated 36Cl- influx in different brain regions. The order of sensitivity to zinc inhibition of GABA-stimulated 36Cl- influx was hippocampus > cerebral cortex > cerebellum. These regional differences may reflect the structural heterogeneity of GABAA receptors among brain areas. Zinc inhibition was not affected by the short-term addition of three benzodiazepines, diazepam, bretazenil and triazolam. The effect of diazepam and bretazenil to potentiate GABA-stimulated 36Cl- influx was not affected by zinc, but the effect of triazolam was decreased by zinc. In brain tissue prepared from flurazepam-treated rats, there was no difference compared with controls in zinc inhibition of GABA-stimulated 36Cl- influx. The results indicate that the effects of zinc on the GABAA receptor are largely independent of drugs acting on the benzodiazepine binding site.

Decreased expression of gamma-aminobutyric acid type A/benzodiazepine receptor beta subunit mRNAs in brain of flurazepam-tolerant rats

The expression of GABAA/benzodiazepine beta subunit mRNAs was studied in cerebral cortex, hippocampus, and cerebellum of flurazepam-treated rats. Immediately following 4 wk of treatment, beta 2 and beta 3 subunit mRNAs were significantly reduced in cerebellum and hippocampus, whereas only beta 2 was decreased in cortex. These decreases had largely reversed 48 h following flurazepam treatment. After 2 wk of treatment, both beta 2 and beta 3 mRNAs were reduced in cerebellum, and beta 3 mRNA was reduced in hippocampus, but neither was changed in cortex. Four hours after an acute flurazepam treatment, the only change was a decrease in beta 3 mRNA in hippocampus. These results indicate that the expression of GABAA receptor beta subunit mRNAs in different brain regions is differentially regulated during chronic flurazepam treatment, and some changes occur within hours after a single large dose.