High-dose discrimination training with midazolam: context determines generalization profile

Benzodiazepine and neuroactive steroid combinations in rats: anxiolytic-like and discriminative stimulus effects

RATIONALE

Benzodiazepines are effective anxiolytics, hypnotics, and anticonvulsants but unwanted side effects, including abuse potential, limit their use. A possible strategy to increase the therapeutic index of this drug class is to combine benzodiazepines with neuroactive steroids.

OBJECTIVES

The present study evaluated the extent to which combinations of benzodiazepines (triazolam, clonazepam) and neuroactive steroids (pregnanolone, ganaxolone) induced additive, supra-additive, or infra-additive effects in an elevated zero maze and a drug discrimination procedure in rats.

METHODS

Male Sprague-Dawley rats (N = 7/group) were placed into an elevated zero maze apparatus following injections of multiple doses of triazolam and pregnanolone, alone and combined, or clonazepam and ganaxolone, alone and combined. These drugs/drug combinations also were evaluated in rats (N = 8) trained to discriminate triazolam (0.1 mg/kg, i.p.) from vehicle. Drug interactions were evaluated using isobolographic and dose-addition analysis.

RESULTS

In the elevated zero maze, all drugs engendered dose-dependent increases in time spent in the open quadrant when administered alone. Triazolam and pregnanolone, as well as clonazepam and ganaxolone combinations produced additive or supra-additive effects depending on the fixed-proportion that was tested. In triazolam discrimination, all drugs engendered dose-dependent increases in triazolam-lever responding. In combination, triazolam and pregnanolone and clonazepam and ganaxolone produced predominantly additive discriminative stimulus effects, except for one fixed proportion of clonazepam and ganaxolone which had supra-additive effects.

CONCLUSIONS

Although drug interactions depended on the constituent drugs, the combination tested, and the behavioral endpoint; a combination was identified that would be predicted to result in supra-additive anxiolytic-like effects with predominantly additive discriminative stimulus effects.

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.

The behavioral pharmacology of zolpidem: evidence for the functional significance of α1-containing GABA(A) receptors

RATIONALE

Zolpidem is a positive allosteric modulator of γ-aminobutyric acid (GABA) with preferential binding affinity and efficacy for α1-subunit containing GABA(A) receptors (α1-GABA(A)Rs). Over the last three decades, a variety of animal models and experimental procedures have been used in an attempt to relate the behavioral profile of zolpidem and classic benzodiazepines (BZs) to their interaction with α1-GABA(A)Rs.

OBJECTIVES

This paper reviews the results of rodent and non-human primate studies that have evaluated the effects of zolpidem on motor behaviors, anxiety, memory, food and fluid intake, and electroencephalogram (EEG) sleep patterns. Also included are studies that examined zolpidem's discriminative, reinforcing, and anticonvulsant effects as well as behavioral signs of tolerance and withdrawal.

RESULTS

The literature reviewed indicates that α1-GABA(A)Rs play a principle role in mediating the hypothermic, ataxic-like, locomotor- and memory-impairing effects of zolpidem and BZs. Evidence also suggests that α1-GABA(A)Rs play partial roles in the hypnotic, EEG sleep, anticonvulsant effects, and anxiolytic-like of zolpidem and diazepam. These studies also indicate that α1-GABA(A)Rs play a more prominent role in mediating the discriminative stimulus, reinforcing, hyperphagic, and withdrawal effects of zolpidem and BZs in primates than in rodents.

CONCLUSIONS

The psychopharmacological data from both rodents and non-human primates suggest that zolpidem has a unique pharmacological profile when compared with classic BZs. The literature reviewed here provides an important framework for studying the role of different GABA(A)R subtypes in the behavioral effects of BZ-type drugs and helps guide the development of new pharmaceutical agents for disorders currently treated with BZ-type drugs.

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.

Self-administration of bretazenil under progressive-ratio schedules: behavioral economic analysis of the role intrinsic efficacy plays in the reinforcing effects of benzodiazepines

Previous research suggests that intrinsic efficacy of benzodiazepines is an important determinant of their behavioral effects. We evaluated the reinforcing effects of the benzodiazepine partial agonist bretazenil using behavioral economic models referred to as "consumer demand" and "labor supply". Four rhesus monkeys were trained under a progressive-ratio (PR) schedule of i.v. midazolam injection. A range of doses of bretazenil (0.001-0.03 mg/kg/injection and vehicle) was evaluated for self-administration with an initial response requirement of 40 that doubled to 640; significant self-administration was maintained at doses of 0.003-0.03 mg/kg/injection. Next, a dose of bretazenil that maintained peak injections/session was made available with initial response requirements doubling from 10 to 320 (maximum possible response requirements of 160 and 5120, respectively), and increasing response requirements decreased self-administration (mean number of injections/session) of a peak dose (0.01 mg/kg/injection). Analyses based on consumer demand revealed that a measure of reinforcing strength termed "essential value", for bretazenil was similar to that previously obtained with midazolam (non-selective full agonist), but less than that observed for zolpidem (full agonist, selective for α1 subunit-containing GABA(A) receptors). According to labor supply analysis, the reinforcing effects of bretazenil were influenced by the economic concept referred to as a "price effect", similar to our previous findings with midazolam but not zolpidem. In general, behavioral economic indicators of reinforcing effectiveness did not differentiate bretazenil from a non-selective full agonist. These findings raise the possibility that degree of intrinsic efficacy of a benzodiazepine agonist may not be predictive of relative reinforcing effectiveness.

Discriminative stimulus effects of L-838,417 (7-tert-butyl-3-(2,5-difluoro-phenyl)-6-(2-methyl-2H-[1,2,4]triazol-3-ylmethoxy)-[1,2,4]triazolo[4,3-b]pyridazine): role of GABA(A) receptor subtypes

Previous reports suggest that gamma-aminobutyric acid type A (GABA(A)) receptors containing alpha1 subunits may play a pivotal role in mediating the discriminative stimulus effects of benzodiazepines (BZs). L-838,417 (7-tert-Butyl-3-(2,5-difluoro-phenyl)-6-(2-methyl-2H-[1,2,4]triazol-3-ylmethoxy)-[1,2,4]triazolo[4,3-b]pyridazine) is a GABA(A) receptor modulator with intrinsic efficacy in vitro at alpha2, alpha3, and alpha5 subunit-containing GABA(A) receptors, and little demonstrable intrinsic efficacy in vitro at alpha1 subunit-containing GABA(A) receptors. The present study evaluated the discriminative stimulus effects of L-838,417 in order to determine the extent to which the alpha2, alpha3, and alpha5 subunit-containing GABA(A) receptors contribute to the interoceptive effects of BZ-type drugs. Squirrel monkeys (Saimiri sciureus) were trained to discriminate L-838,417 (0.3 mg/kg, i.v.) from vehicle under a 5-response fixed-ratio schedule of food reinforcement. Under test conditions, L-838,417 administration resulted in dose-dependent increases in drug-lever responding that were antagonized by the BZ-site antagonist, flumazenil. Administration of non-selective BZs, compounds with 10-fold greater affinity for alpha1 subunit-containing GABA(A) receptors compared to alpha2, alpha3, and alpha5 subunit-containing GABA(A) receptors, barbiturates and ethanol (which modulate the GABA(A) receptor via a non-BZ site), all resulted in a majority of responses on the L-838,417-paired lever (65-100% drug-lever responding). betaCCT, an antagonist that binds with 20-fold greater affinity for alpha1 subunit-containing GABA(A) receptors relative to alpha2, alpha3, and alpha5-containing GABA(A) receptors, had no significant effect on the discriminative stimulus effects of L-838,417 or the L-838,417-like effects of diazepam or zolpidem. These data suggest that efficacy at alpha2, alpha3, and/or alpha5 subunit-containing GABA(A) receptors likely are sufficient for engendering BZ-like discriminative stimulus effects.

Lack of generalisation between the GABAA receptor agonist, gaboxadol, and allosteric modulators of the benzodiazepine binding site in the rat drug discrimination procedure

INTRODUCTION

The binding sites for gamma-aminobutyric acid (GABA) and GABA(A) receptor agonists are located differently from the binding sites for benzodiazepine receptor agonists. Furthermore, the major pharmacological effects of benzodiazepine receptor agonists and the GABA(A) receptor agonist gaboxadol (4,5,6,7-tetrahydrroisoxazolo(5,4-c)pyridin-3-ol, THIP) are mediated by different GABA(A) receptor subunit compositions; that is, gaboxadol may interact primarily with extra-synaptically located alpha(4)beta(2/3)delta-containing receptors and benzodiazepines with the synaptically located alpha(1)beta(2/3)gamma(2)-containing receptors.

OBJECTIVES

The aim of the present study was to address if this different receptor subtype selectivity was reflected in vivo.

MATERIALS AND METHODS

A two-lever liquid reinforced operant discrimination procedure was conducted. Three groups of rats were trained to discriminate gaboxadol, diazepam and zolpidem 5.5, 1.5 and 0.7 mg/kg i.p., respectively, from vehicle.

RESULTS

Substitution tests showed that gaboxadol-trained animals failed to recognize diazepam (0.75-1.5 mg/kg), zolpidem (0.4-0.7 mg/kg), zopiclone (2.5 mg/kg), zaleplon (1.0-1.5 mg/kg) or indiplon (0.31 mg/kg). In contrast, all benzodiazepine receptor agonists, but not gaboxadol (4.5-5.5 mg/kg), generalised to the discriminative stimulus in diazepam- and zolpidem-trained animals.

DISCUSSION

In agreement with these data, the competitive benzodiazepine receptor antagonist flumazenil (10 mg/kg s.c.) antagonised the discriminative stimulus of zolpidem but not of gaboxadol. Interaction tests showed no synergistic interaction of concomitant administration of gaboxadol and zolpidem or diazepam.

CONCLUSION

Previous studies have shown that gaboxadol and benzodiazepines interact with different receptor populations, and the present study confirms that in vivo functional consequences of this receptor selectivity exist in the form of differential behavioural responses in rats.

Comparison of the behavioral effects of bretazenil and flumazenil in triazolam-dependent and non-dependent baboons

Behavioral effects of the benzodiazepine receptor partial agonist bretazenil were compared with those of the benzodiazepine receptor antagonist flumazenil under conditions in which three baboons received continuous intragastric (i.g.) infusion of vehicle and then continuous i.g. infusion of triazolam (1.0 mg/kg/day). In each condition, acute doses of flumazenil (0.01-3.2 mg/kg) and bretazenil (0.01-10.0 mg/kg) were administered every 2 weeks (beginning after 30 days of treatment in the triazolam-dependent condition). Food pellets were available during daily 20-h sessions. Following test injections, 60-min behavioral observations were conducted followed by a fine motor assessment. During chronic vehicle administration, neither drug produced changes in observed behaviors. Bretazenil increased pellets earned and time to complete the fine-motor task (10.0 mg/kg dose). During chronic triazolam dosing, both bretazenil and flumazenil precipitated benzodiazepine withdrawal syndromes, characterized by vomiting, tremors/jerks, and a decrease in pellets earned. Thus, bretazenil can function as an antagonist under conditions of benzodiazepine physical dependence.

Discriminative stimulus effects of ethanol in rats using a three-choice ethanol???midazolam???water discrimination

Three-choice discrimination procedures are used to characterize how similar the discriminative stimulus effects of two drugs are in relation to each other. This procedure has suggested similarities between ethanol and ligands that positively modulate the gamma-aminobutyric acid type A (GABAA) receptor complex. As an extension to these studies, male Long-Evans rats were trained to discriminate midazolam (3.0 mg/kg, i.p.) from ethanol (1.0 g/kg, i.g.) from water (2.3 ml, i.g.) in a three-lever, food reinforced task. Substitution tests were conducted following administration of GABAA-positive modulators, noncompetitive N-methyl-D-aspartate (NMDA) antagonists, 5-HT1B agonists and isopropanol. Among the GABAA-positive modulators, diazepam was the only drug that completely substituted for midazolam; both pentobarbital and the neurosteroid allopregnanolone showed partial midazolam substitution. The NMDA antagonist dizocilpine substituted for ethanol, while phencyclidine showed no substitution for either ethanol or midazolam. The serotonin agonists tested also showed no substitution for either ethanol or midazolam. Isopropanol was the only other drug that completely substituted for ethanol. These data extend previous findings from an ethanol-pentobarbital-water discrimination and further define training conditions that result in a conditional basis for the ethanol discrimination where only those drugs with pharmacological heterogeneous effects similar to ethanol produce a full ethanol-like effect.

Selectivity in generalization to GABAergic drugs in midazolam-trained baboons

When barbiturates have been tested in animals trained to discriminate the intravenous benzodiazepine (Bz) anesthetic midazolam, squirrel monkeys and pigeons did not reliably generalize to barbiturates but rats did. To explore this unexpected phenomenon in another species and to extend the midazolam generalization profile to GABAergic compounds not previously tested, five baboons were trained to discriminate midazolam maleate (0.32 mg/kg i.v.) from saline under a two-lever procedure. In tests 10 min after dose delivery, the partial agonist imidazenil, the full agonist chlordiazepoxide, and the receptor-subtype-selective hypnotic zolpidem fully shared discriminative effects with midazolam. The barbiturate pentobarbital did so in only one of five baboons, and the intravenous anesthetic propofol failed to do so in the three baboons tested. Testing 1 min after dose delivery shifted midazolam and zolpidem curves to the left and increased generalization to propofol but not pentobarbital. Taken together with previous published data, partial or full agonism at the Bz binding site appears sufficient for midazolam-like discriminative effects in nonhuman primates, pigeons, and rodents, and modulation through the anesthetic site is sufficient in baboons. However, to date, positive modulation of GABA through the barbiturate site is not generally sufficient for this effect in nonhuman primates and pigeons although it is in rodents.

Differentiating benzodiazepine- and barbiturate-like discriminative stimulus effects of lorazepam, diazepam, pentobarbital, imidazenil and zaleplon in two- versus three-lever procedures

Previous studies found that animals trained to discriminate pentobarbital show a relatively inclusive generalization profile. They generalize to sedative-hypnotics and anxiolytics, regardless of differences among such drugs in molecular mechanism of action. In contrast, animals trained to discriminate lorazepam have shown a generalization profile that appears selective for compounds with in-vitro profiles as full agonists at the benzodiazepine modulatory site. The present study investigated whether benzodiazepine receptor ligands, to which pentobarbital-trained rats had generalized under a two-lever procedure, would occasion pentobarbital- or lorazepam-appropriate responding when the rats were retrained to discriminate among pentobarbital, lorazepam and the no-drug condition under a three-lever procedure. A second group of rats was trained first to discriminate lorazepam and then retrained under the same three-lever procedure. Under the two-lever procedure, all pentobarbital-trained rats showed dose-dependent generalization to lorazepam, but not all lorazepam-trained rats showed full generalization to pentobarbital. Both groups showed full generalization to diazepam and zaleplon, a novel hypnotic that is selective for alpha, 1-subunit-containing subtypes of the gamma-aminobutyric acid (GABA)A receptor. Pentobarbital-trained rats, but not all lorazepam-trained rats, generalized to imidazenil. Under the three-lever procedure, dose-dependent generalization to lorazepam and pentobarbital was demonstrated on the appropriate levers. Diazepam shared discriminative effects with pentobarbital, zaleplon shared discriminative effects with lorazepam, and imidazenil shared discriminative effects with lorazepam and pentobarbital. These results show that when the opportunity for finer differentiation of discriminative effects of GABAergic drugs is provided, a generalization profile more in line with differential in-vitro profiles can be revealed.