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

Differential interactions engendered by benzodiazepine and neuroactive steroid combinations on schedule-controlled responding in rats

Benzodiazepines are positive allosteric modulators of the GABAA receptor and are prescribed as anxiolytics, hypnotics, and anticonvulsants. While these drugs clearly have clinical value, their use is associated with unwanted side effects such as sedation and motor impairment. Neuroactive steroids are endogenous modulators of GABAA receptors and recent evidence has shown that combinations of the triazolo-benzodiazepine triazolam and the endogenous neuroactive steroid pregnanolone can produce both supra-additive anxiolytic effects and infra-additive reinforcing effects. In the present study, we investigated these same combinations as well as combinations of two clinically-relevant drugs from different chemical classes, the 1, 4 substituted (7-nitro) benzodiazepine clonazepam and the synthetic neuroactive steroid ganaxolone, in rats trained under a 10-response, fixed ratio (FR) schedule of food reinforcement. All four drugs induced a significant and dose-dependent suppression of food-maintained responding. From the dose-response functions, ED50s (i.e., the doses that engendered 50% of the maximum rate-decreasing effect) were generated for each drug. Dose-response functions for combinations of triazolam/pregnanolone, clonazepam/ganaxolone, triazolam/ganaxolone, and clonazepam/pregnanolone were then determined. Isobolographic analysis of the rate-decreasing effects of these combinations revealed that the potencies of the triazolam/pregnanolone combinations were supra-additive while the clonazepam/ganaxolone combinations were additive or infra-additive in relation to predicted values based on dose-additive effects. Furthermore, mixtures of clonazepam/pregnanolone were supra-additive while triazolam/ganaxolone combinations were additive, infra-additive and supra-additive. These results suggest that the ability of benzodiazepine and neuroactive steroid combinations to attenuate rates of food-maintained responding depends critically on both the constituent drugs and the dose of drug in the mixtures.

A cycloartane glycoside derived from Actaea racemosa L. modulates GABAA receptors and induces pronounced sedation in mice

23-O-Acetylshengmanol 3-O-β-D-xylopyranoside (Ac-SM) isolated from Actaea racemosa L.-an herbal remedy for the treatment of mild menopausal disorders-has been recently identified as a novel efficacious modulator of GABAA receptors composed of α1-, β2-, and γ2S-subunits. In the present study, we analyzed a potential subunit-selective modulation of GABA-induced chloride currents (IGABA) at GABA concentrations eliciting 3-8% of the maximal GABA response (EC3-8) through nine GABAA receptor isoforms expressed in Xenopus laevis oocytes by Ac-SM with two-microelectrode voltage clamp and behavioral effects 30 minutes after intraperitoneal application in a mouse model. Efficacy of IGABA enhancement by Ac-SM displayed a mild α-subunit dependence with α2β2γ2S (maximal IGABA potentiation [Emax] = 1454 ± 97%) and α5β2γ2S (Emax = 1408 ± 87%) receptors being most efficaciously modulated, followed by slightly weaker IGABA enhancement through α1β2γ2S (Emax = 1187 ± 166%), α3β2γ2S (Emax = 1174 ± 218%), and α6β2γ2S (Emax = 1171 ± 274%) receptors and less pronounced effects on receptors composed of α4β2γ2S (Emax = 752 ± 53%) subunits, whereas potency was not affected by the subunit composition (EC50 values ranging from α1β2γ2S = 35.4 ± 12.3 µM to α5β2γ2S = 50.9 ± 11.8 µM). Replacing β2- with β1- or β3-subunits as well as omitting the γ2S-subunit affected neither efficacy nor potency of IGABA enhancement by Ac-SM. Ac-SM shifted the GABA concentration-response curve toward higher GABA sensitivity (about 3-fold) and significantly increased the maximal GABA response by 44 ± 13%, indicating a pharmacological profile distinct from a pure allosteric GABAA receptor modulator. In mice, Ac-SM significantly reduced anxiety-related behavior in the elevated plus maze test at a dose of 0.6 mg/kg, total ambulation in the open field test at doses ≥6 mg/kg, stress-induced hyperthermia at doses ≥0.6 mg/kg, and significantly elevated seizure threshold at doses ≥20 mg/kg body weight. High efficacy and long biologic half-life of Ac-SM suggest that potential cumulative sedative side effects upon repetitive intake of A. racemosa L. preparations might not be negligible.

Quantitative pharmacological analyses of the interaction between flumazenil and midazolam in monkeys discriminating midazolam: Determination of the functional half life of flumazenil

The duration of action of a drug is commonly estimated using plasma concentration, which is not always practical to obtain or an accurate estimate of functional half life. For example, flumazenil is used clinically to reverse the effects of benzodiazepines like midazolam; however, its elimination can be altered by other drugs, including some benzodiazepines, thereby altering its half life. This study used Schild analyses to characterize antagonism of midazolam by flumazenil and determine the functional half life of flumazenil. Four monkeys discriminated 0.178mg/kg midazolam while responding under a fixed-ratio 10 schedule of stimulus-shock termination; flumazenil was given at various times before determination of a midazolam dose-effect curve. There was a time-related decrease in the magnitude of shift of the midazolam dose-effect curve as the interval between flumazenil and midazolam increased. The potency of flumazenil, estimated by apparent pA2 values (95% CI), was 7.30 (7.12, 7.49), 7.17 (7.03, 7.31), 6.91 (6.72, 7.10) and 6.80 (6.67, 6.92) at 15, 30, 60 and 120min after flumazenil administration, respectively. The functional half life of flumazenil, derived from potency estimates, was 57±13min. Thus, increasing the interval between flumazenil and midazolam causes orderly decreases in flumazenil potency; however, across a broad range of conditions, the qualitative nature of the interaction does not change, as indicated by slopes of Schild plots at all time points that are not different from unity. Differences in potency of flumazenil are therefore due to elimination of flumazenil and not due to pharmacodynamic changes over time.

Discriminative stimulus effects of pregnanolone in rhesus monkeys

RATIONALE

Neuroactive steroids and benzodiazepines can positively modulate GABA by acting at distinct binding sites on synaptic GABA(A) receptors. Although these receptors are thought to mediate the behavioral effects of both benzodiazepines and neuroactive steroids, other receptors (e.g., extrasynaptic GABA(A), N-methyl-D-aspartate (NMDA), σ₁, or 5-HT₃ receptors) might contribute to the effects of neuroactive steroids, accounting for differences among positive modulators.

OBJECTIVE

The current study established the neuroactive steroid pregnanolone as a discriminative stimulus to determine whether actions in addition to positive modulation of synaptic GABA(A) receptors might contribute to its discriminative stimulus effects.

METHODS

Four rhesus monkeys discriminated 5.6 mg/kg pregnanolone while responding under a fixed-ratio 10 schedule of stimulus-shock termination.

RESULTS

Positive modulators acting at benzodiazepine, barbiturate, or neuroactive steroid sites produced ≥80 % pregnanolone-lever responding, whereas drugs acting primarily at receptors other than synaptic GABA(A) receptors, such as extrasynaptic GABA(A), NMDA, σ₁, and 5-HT₃ receptors, produced vehicle-lever responding. Flumazenil antagonized the benzodiazepines midazolam and flunitrazepam, with Schild analyses yielding slopes that did not deviate from unity and pA₂ values of 7.39 and 7.32, respectively. Flumazenil did not alter the discriminative stimulus effects of pregnanolone.

CONCLUSION

While these results do not exclude the possibility that pregnanolone acts at receptors other than synaptic GABA(A) receptors, they indicate a primary and possibly exclusive role of synaptic GABA(A) receptors in its discriminative stimulus effects. Reported differences in the effects of benzodiazepines and neuroactive steroids are not due to differences in their actions at synaptic GABA(A) receptors.

Acute tolerance to chlordiazepoxide qualitatively changes the interaction between flumazenil and pregnanolone and not the interaction between flumazenil and midazolam in rhesus monkeys discriminating midazolam

Benzodiazepines and neuroactive steroids act at distinct binding sites on γ-aminobutyric acid(A) (GABA(A)) receptors where they positively modulate GABA, resulting in similar acute behavioral effects. Tolerance to benzodiazepines can develop with repeated treatment; however, cross tolerance to neuroactive steroids does not develop, perhaps due to conformational changes in benzodiazepine, and not neuroactive steroid, binding sites. Three monkeys discriminated 0.178 mg/kg midazolam while responding under a fixed-ratio 10 schedule of stimulus-shock termination. On separate occasions, dose-effect curves for midazolam and pregnanolone were determined when monkeys had not received chlordiazepoxide and when they received 10 mg/kg chlordiazepoxide 46 hours earlier; for some tests, flumazenil was given before determination of dose-effect curves. Midazolam and pregnanolone produced ≥80% midazolam-lever responding. When administered 46 h before sessions, chlordiazepoxide did not produce pregnanolone-lever responding; under those treatment conditions, midazolam dose-effect curves were shifted 2.8-fold rightward and pregnanolone dose-effect curves were not changed. Flumazenil antagonized midazolam; Schild (linear) analyses yielded slopes that were not different from unity and pA(2) values of 7.46 when monkeys had not received chlordiazepoxide and 7.44 when they received chlordiazepoxide 46 h earlier. Flumazenil did not alter the effects of pregnanolone in chlordiazepoxide-treated monkeys. Thus, interactions between flumazenil and midazolam were not qualitatively or quantitatively changed in monkeys acutely tolerant to chlordiazepoxide, suggesting that mechanisms other than alterations of benzodiazepine binding sites account for the development of acute tolerance.

Quantitative analyses of antagonism: combinations of midazolam and either flunitrazepam or pregnanolone in rhesus monkeys discriminating midazolam

Adverse effects of benzodiazepines limit their clinical use; these effects might be reduced without altering therapeutic effects by administering other positive GABA(A) modulators (i.e., neuroactive steroids) with benzodiazepines. One concern with this strategy involves reversing these combined effects in case of overdose. The current study examined whether flumazenil can attenuate the combined effects of two benzodiazepines, midazolam and flunitrazepam, and the combined effects of midazolam and the neuroactive steroid pregnanolone, in four monkeys discriminating midazolam. Each positive modulator produced ≥80% midazolam-lever responding. Interactions between midazolam and either flunitrazepam or pregnanolone were additive. Flumazenil antagonized the benzodiazepines when they were administered alone or in combination. Schild analyses yielded slopes that did not deviate from unity, regardless of whether benzodiazepines were administered alone or together; the pA(2) value for flumazenil was 7.58. In contrast, flumazenil enhanced the effects of pregnanolone with 0.32 mg/kg flumazenil shifting the pregnanolone dose-effect curve 2-fold leftward. Flumazenil attenuated the combined effects of midazolam and pregnanolone, although antagonism was not dose-dependent. Thus, the interaction between two benzodiazepines was similar to that of a benzodiazepine and a neuroactive steroid; however, flumazenil more efficiently attenuated a combination of two benzodiazepines compared with a combination of a benzodiazepine and a neuroactive steroid. Although the magnitude of antagonism of a benzodiazepine combined with a neuroactive steroid was reduced, these results support continued exploration of the use of combinations of positive modulators to enhance therapeutic effects while reducing adverse effects.