Changes in relative potency among positive GABA(A) receptor modulators upon discontinuation of chronic benzodiazepine treatment in rhesus monkeys

Tolerance to the rate-increasing and not rate-decreasing effects of pregnanolone in rats

Chronic treatment with benzodiazepines, which positively modulate γ-aminobutyric acidA (GABAA) receptors, can lead to the development of tolerance. Similar effects might also occur during chronic treatment with positive modulators acting at other sites on GABAA receptors (e.g. neuroactive steroids). In this study, tolerance and cross tolerance were examined in seven rats treated daily with the neuroactive steroid pregnanolone (25.6 mg/kg/day) and responding under a fixed ratio 10 schedule of food presentation. Dose-effect curves were determined for positive GABAA modulators (pregnanolone, flunitrazepam, midazolam, and pentobarbital), and other drugs (ketamine and morphine) before, during, and after chronic treatment. Initially, daily pregnanolone administration increased responding; although tolerance developed to the rate-increasing effects after 14 weeks, tolerance did not develop to the rate-decreasing effects. The potencies of pregnanolone, midazolam, and morphine to decrease responding did not change during treatment, whereas flunitrazepam was more potent and pentobarbital and ketamine were less potent during treatment as compared to before treatment. Pregnanolone and midazolam were more potent after treatment than before treatment. The development of tolerance to the rate-increasing effects of pregnanolone indicates that neuroadaptations occur during chronic treatment; the fact that tolerance develops to only some effects suggests that the behavioral consequences of these neuroadaptations are limited.

Chronic benzodiazepine treatment does not alter interactions between positive GABA(A) modulators and flumazenil or pentylenetetrazole in monkeys

Benzodiazepines and neuroactive steroids are positive c-aminobutyric acid(A) (GABA(A)) modulators acting at distinct binding sites; during benzodiazepine treatment, tolerance develops to many behavioral effects of benzodiazepines, although cross tolerance typically does not develop to neuroactive steroids. To determine whether differential changes in binding sites contribute to these behavioral differences, interactions between GABA(A) modulators were studied in two groups of four monkeys: one otherwise untreated group discriminated 0.178 mg/kg of the benzodiazepine midazolam; the other received 5.6 mg/kg/day of diazepam and discriminated 0.1 mg/kg of flumazenil, which binds to benzodiazepine sites without modulating GABA(A) receptors. In untreated monkeys, flumazenil antagonized midazolam but not the neuroactive steroid pregnanolone, whereas pentylenetetrazole (a negative modulator acting at a third site) antagonized both positive modulators. In diazepam-treated monkeys, 0.1 mg/kg of flumazenil or 32 mg/kg of pentylenetetrazole produced flumazenil-lever responding, which was reversed by midazolam and pregnanolone. As the flumazenil dose increased, larger doses of midazolam, but not pregnanolone, were needed to reverse flumazenil-lever responding. When the pentylenetetrazole dose increased, larger doses of both positive modulators were needed. Thus, interactions between GABA(A) modulators were not different between diazepam-treated and untreated monkeys and do not reveal changes in binding sites that could account for reported differences between benzodiazepines and neuroactive steroids.

Comparing the discriminative stimuli produced by either the neuroactive steroid pregnanolone or the benzodiazepine midazolam in rats

RATIONALE

Neuroactive steroids might be therapeutic alternatives for benzodiazepines because they have similar anxiolytic, sedative, and anticonvulsant effects, and their actions at different modulatory sites on γ-aminobutyric acid(A) (GABA(A)) receptors might confer differences in adverse effects.

OBJECTIVES

This study used drug discrimination to compare discriminative stimuli produced by positive GABA(A) modulators that vary in their site of action on GABA(A) receptors.

METHODS

Two groups of rats discriminated either 3.2 mg/kg of pregnanolone or 0.56 mg/kg of midazolam from vehicle while responding under a fixed ratio 10 schedule of food presentation.

RESULTS

Pregnanolone, midazolam, and flunitrazepam produced ≥ 80% drug-lever responding in both groups; each drug was more potent in rats discriminating pregnanolone. Pentobarbital produced ≥ 80% drug-lever responding in all rats discriminating pregnanolone, and in 1/3 of the rats discriminating midazolam with larger doses decreasing response rates to <20% of control. Morphine and ketamine produced predominantly saline-lever responding in both groups. Flumazenil antagonized midazolam and flunitrazepam in both groups; slopes of Schild plots were not different from unity, and pA (2) values for flumazenil ranged from 5.86 to 6.09. Flumazenil did not attenuate the discriminative stimulus effects of pregnanolone.

CONCLUSIONS

The midazolam and pregnanolone discriminative stimuli were qualitatively similar, although the effects of pentobarbital were not identical in the two groups. Although acute effects of midazolam and pregnanolone are similar, suggesting that neuroactive steroids might retain the therapeutic effects of benzodiazepines, differences emerge during chronic treatment, indicating that neuroactive steroids might produce fewer adverse effects than benzodiazepines.

Quantification of rimonabant (SR 141716A) in monkey plasma using HPLC with UV detection

Using an isocratic high-performance liquid chromatography (HPLC) system and UV detection, a simple and precise analytical procedure was developed to quantify levels of the CB(1) receptor antagonist rimonabant in the plasma of rhesus monkeys. Rimonabant was extracted from plasma samples into 5% isopropanol in hexane. After separation, the isopropanol-hexane fractions were dried to residue, redissolved in mobile phase, and then injected into the HPLC. The HPLC system included an acetonitrile-phosphate buffer (62:38, v/v) mobile phase (pH 6.7), flow rate of 1.5 mL/min, C(18) column (4.6 mm i.d. x 150 mm length, 5 microm), and UV detection at 280 nm. Retention times for rimonabant and doxepin (internal standard) were 9.9 and 2.4 min, respectively. The regression of the spiked calibrator curve was linear from 60 to 4000 ng/mL (r(2) = 0.996). The lower limit of quantification was 60 ng/mL, and recovery was 83.6%. Rimonabant was stable in stock solutions and monkey plasma across a range of temperatures and concentrations. To demonstrate utility, plasma rimonabant was measured in six rhesus monkeys at 60 and 240 min after intramuscular administration of 1 mg/kg rimonabant. Rimonabant levels ranged from 175 to 1290 ng/mL. The analytical assay described here provides a simple and accurate procedure for multiple within-subject measurements of the CB(1) antagonist rimonabant.

Tolerance, sensitization and dependence to diazepam in Balb/c mice exposed to a novel open space anxiety test

Balb/c mice were exposed to an elevated platform that is extended on two opposite sides with lowered steep slopes. They were tested for 12min per session in 6 successive days. They received i.p. administration of either saline or one dose of diazepam (DZP 0.5, 1, 3mg/kg) in sessions 1-3, and saline in sessions 4 and 5. All groups of mice received a single dose of DZP (1mg/kg) in session 6. DZP produced inverted U-shaped dose-responses on the number of entries into different areas of the apparatus, with a peak in mean response at 1mg/kg whereas its effect on the duration of entries was mostly comparable between the 3 doses. It increased the number of crossings on the surface of the platform and facilitated entries onto the slopes. DZP-treated mice crossed frequently onto and spent longer time on the slopes in sessions 1-3 whereas saline-treated mice remained on the platform in sessions 1-6. Withdrawal of DZP in sessions 4-5 increased the latency of first entry and decreased the number and duration of entries onto the slopes which was reversed with the administration of 1mg/kg of DZP in the next session. This ON-OFF the drug may be due to the half-life of DZP which is very short in mice and rats ( approximately 0.88h). It also indicates that DZP-treated mice did not benefit from previous experience of entries onto the slopes which suggests a possible "state-dependent" effect. Administration of DZP after repeated exposures to the test did not facilitate entries onto the slopes but instead increased significantly the number of crossings on the surface of the platform; this increase was much higher than that observed in mice initially treated with DZP and exposed to the test. There is no evidence of habituation in saline-treated mice: the number of crossings on the platform was comparable between the first 5 sessions of the test. These results demonstrate that repeated exposures to the same anxiogenic environment resulted in avoidance responses developing tolerance and approach responses developing sensitization. They suggest that tolerance and sensitization are two opposite sides of the habituation process to the same stimulus and may account for the maintained state of anxiety.

Selective changes in sensitivity to benzodiazepines, and not other positive GABA(A) modulators, in rats receiving flunitrazepam chronically

RATIONALE

Tolerance and dependence can develop during chronic benzodiazepine treatment; however, cross tolerance and cross dependence to positive modulators acting at other sites on GABA(A) receptors might not occur.

OBJECTIVES

The current study evaluated changes in sensitivity to positive GABA(A) modulators during chronic treatment with the benzodiazepine flunitrazepam to determine whether cross tolerance and cross dependence varied as a function of site of action.

METHODS

Eight rats responded under a fixed ratio 20 schedule of food presentation. Dose-effect curves were determined before, during and after chronic treatment with one or two daily injections of 1 mg/kg of flunitrazepam.

RESULTS

Prior to chronic treatment, benzodiazepines (flunitrazepam, midazolam), a barbiturate (pentobarbital), a neuroactive steroid (pregnanolone), and drugs with primary mechanisms of action at receptors other than GABA(A) receptors (morphine, ketamine) dose-dependently decreased responding. Twice daily treatment with flunitrazepam produced 9.5- and 23-fold shifts to the right in the flunitrazepam and midazolam dose-effect curves, respectively. In contrast, dose-effect curves for other drugs either were not changed or were shifted less than or equal to fourfold to the right.

CONCLUSIONS

Decreased sensitivity to benzodiazepines and not to a barbiturate or a neuroactive steroid during chronic flunitrazepam treatment indicates that tolerance and cross tolerance developed only to benzodiazepines. Despite similar acute behavioral effects among positive GABA(A) modulators, the differential development of cross tolerance suggests that adaptations at GABA(A) receptors produced by chronic benzodiazepine treatment differentially affect positive modulators depending on their site of action; such differences might be exploited to benefit patients treated daily with positive GABA(A) modulators.

Acute cross tolerance to midazolam, and not pentobarbital and pregnanolone, after a single dose of chlordiazepoxide in monkeys discriminating midazolam

Although cross tolerance can develop among positive gamma-aminobutyric acidA (GABAA) modulators acting at the same modulatory site, cross tolerance does not always develop to drugs acting at sites that are different from the site of action of the drug administered chronically. To examine the relationship between cross tolerance and site of action, four rhesus monkeys discriminated midazolam and, on separate occasions, received 32 mg/kg of chlordiazepoxide 24 h before dose-effect determinations for drugs acting at different sites. Midazolam, pentobarbital, and pregnanolone produced >80% midazolam-lever responding. Although monkeys responded on the midazolam lever 2-4 h after 32 mg/kg of chlordiazepoxide, they responded on the saline lever 24 h later. Twenty-four hours after an acute injection of 32 mg/kg of chlordiazepoxide, midazolam dose-effect curves were shifted 4.6-fold to the right, whereas pregnanolone dose-effect curves were shifted three-fold to the left. Sensitivity to pentobarbital increased in one monkey and decreased in others 24 h after chlordiazepoxide administration. Decreased sensitivity to midazolam shows that acute cross tolerance develops after chlordiazepoxide administration, although it does not develop to drugs acting at other sites on GABAA receptors. These differences among positive GABAA modulators suggest that even short-term benzodiazepine administration changes GABAA receptors, and those changes impact modulatory sites differently.