Antagonism of triazolam self-administration in rhesus monkeys responding under a progressive-ratio schedule: In vivo apparent pA2 analysis

GABAA receptor subtypes and benzodiazepine use, misuse, and abuse

Benzodiazepines have been in use for over half a century. While they remain highly prescribed, their unfavorable side-effect profile and abuse liability motivated a search for alternatives. Most of these efforts focused on the development of benzodiazepine-like drugs that are selective for specific GABA_A receptor subtypes. While there is ample evidence that subtype-selective GABA_A receptor ligands have great potential for providing symptom relief without typical benzodiazepine side-effects, it is less clear whether subtype-selective targeting strategies can also reduce misuse and abuse potential. This review focuses on the three benzodiazepine properties that are relevant to the DSM-5-TR criteria for Sedative, Hypnotic, or Anxiolytic Use Disorder, namely, reinforcing properties of benzodiazepines, maladaptive behaviors related to benzodiazepine use, and benzodiazepine tolerance and dependence. We review existing evidence regarding the involvement of different GABA_A receptor subtypes in each of these areas. The reviewed studies suggest that α1-containing GABA_A receptors play an integral role in benzodiazepine-induced plasticity in reward-related brain areas and might be involved in the development of tolerance and dependence to benzodiazepines. However, a systematic comparison of the contributions of all benzodiazepine-sensitive GABA_A receptors to these processes, a mechanistic understanding of how the positive modulation of each receptor subtype might contribute to the brain mechanisms underlying each of these processes, and a definitive answer to the question of whether specific chronic modulation of any given subtype would result in some or all of the benzodiazepine effects are currently lacking from the literature. Moreover, how non-selective benzodiazepines might lead to the maladaptive behaviors listed in DSM and how different GABA_A receptor subtypes might be involved in the development of these behaviors remains unexplored. Considering the increasing burden of benzodiazepine abuse, the common practice of benzodiazepine misuse that leads to severe dependence, and the current efforts to generate side-effect free benzodiazepine alternatives, there is an urgent need for systematic, mechanistic research that provides a better understanding of the brain mechanisms of benzodiazepine misuse and abuse, including the involvement of specific GABA_A receptor subtypes in these processes, to establish an informed foundation for preclinical and clinical efforts.

Blockade of α1 subtype GABAA receptors attenuates the development of tolerance to the antinociceptive effects of midazolam in rats

Benzodiazepines bind to and act on α1-3 and α5-containing GABAA receptors. Previous studies suggest that different GABAA receptor α-subtypes mediate the various behavioral effects of benzodiazepines, which raises the possibility of combining benzodiazepines with subtype-selective GABAA receptor antagonists to improve the therapeutic profiles of benzodiazepines. This study examined the GABAA receptor subtype mediation of the tolerance to midazolam-induced antinociception in rats. Midazolam (3.2 mg/kg) significantly reduced the locomotion in rats which was prevented by the selective α1-preferring GABAA receptor antagonist β-carboline-3-carboxylate-t-butyl ester (βCCt) (3.2 mg/kg). Midazolam increased the paw withdrawal threshold as tested by the von Frey filament assay in the complete Freund's adjuvant-induced inflammatory pain model in rats, and this effect was not altered by βCCt or another α1-preferring GABAA receptor antagonist 3-propoxy-β-carboline hydrochloride (3PBC). Repeated treatment with midazolam in combination with vehicle, βCCt or 3PBC (twice daily) for 7 days led to a progressive increase of the ED50 values in the midazolam- and vehicle-treated rats, but not in other rats, suggesting the development of tolerance to midazolam but not to the combination of midazolam with α1-preferring GABAA receptor antagonists. These results suggest the essential role of the α1-subtype of GABAA receptors in mediating the development of tolerance to midazolam-induced antinociceptive effects and raise the possibility of increasing therapeutic profiles of benzodiazepines by selectively blocking specific α-subtypes of GABAA receptors.

Clonazepam: Indications, Side Effects, and Potential for Nonmedical Use

After participating in this activity, learners should be better able to:• Assess the misuse potential of clonazepam• Characterize the nonmedical use of clonazepam• Identify the health problems associated with long-term use of clonazepam ABSTRACT: Clonazepam, a benzodiazepine, is commonly used in treating various conditions, including anxiety disorders and epileptic seizures. Due to its low price and easy availability, however, it has become a commonly misused medication, both in medical and recreational contexts. In this review, we aim to highlight the behavioral and pharmacological aspects of clonazepam and its history following its approval for human use. We examine the circumstances commonly associated with the nonmedical use of clonazepam and raise points of particular concern. Clonazepam, alone or in combination with other psychoactive substances, can lead to unwanted effects on health, such as motor and cognitive impairment, sleep disorders, and aggravation of mood and anxiety disorders. Prolonged use of clonazepam may lead to physical dependence and tolerance. There is therefore a need to find safer therapeutic alternatives for treating seizures and anxiety disorders. Greater awareness of its frequent nonmedical use is also needed to achieve safer overall use of this medication.

GABAA Receptor Subtypes and the Reinforcing Effects of Benzodiazepines in Remifentanil-Experienced Rhesus Monkeys

BACKGROUND

Opioid-use disorder is associated with a high degree of co-abuse with benzodiazepines. While the mechanisms underlying the co-abuse of opioids and benzodiazepines remain unknown, α1 subunit-containing GABA_A receptors may play a critical role in the reinforcing effects of benzodiazepine-type compounds, depending on whether the monkeys have a history of benzodiazepine or stimulant self-administration. The present study extended our prior research by comparing the reinforcing effects of a compound lacking activity at α1 subunit-containing GABA_A receptors with the reinforcing effects of non-selective GABA_A receptor positive allosteric modulators in monkeys with a history of opioid self-administration.

METHODS

The reinforcing effects of L-838,417 (partial intrinsic efficacy at α2, α3, and α5 subunit-containing GABA_A receptors, but no efficacy at α1 subunit-containing GABA_A receptors, i.e., "α1-sparing compound") were compared with those of the non-selective GABA_A receptor partial modulator MRK-696, and non-selective GABA_A receptor full modulators, triazolam and lorazepam, in rhesus monkeys (n = 3) experienced in remifentanil self-administration under a progressive-ratio schedule of intravenous drug injection.

RESULTS

Neither the partial modulator nor the α1-sparing compound were self-administered above vehicle levels. The full modulators triazolam and lorazepam were self-administered significantly above vehicle levels, albeit at lower levels than remifentanil.

CONCLUSIONS

Our findings suggest that relatively high efficacy at one or more GABA_A receptor subtypes is required for a compound to have reinforcing effects in monkeys with a history of remifentanil self-administration, in contrast to monkeys with benzodiazepine or stimulant self-administration histories.

Evaluation of the anti-conflict, reinforcing, and sedative effects of YT-III-31, a ligand functionally selective for α3 subunit-containing GABAA receptors

BACKGROUND

In recent years, pharmacological strategies have implicated α3 subunit-containing GABA_A (α3GABA_A) receptor subtypes in the anxiety-reducing effects of benzodiazepines, whereas transgenic mouse approaches have implicated α2 or α5 subunit-containing GABA_A receptors.

AIMS

We investigated the role of α3GABA_A subtypes in benzodiazepine-induced behaviors by evaluating the anti-conflict, reinforcing, and sedative-motor effects of the novel compound YT-III-31, which has functional selectivity for α3GABA_A receptors.

METHODS

Female and male rhesus monkeys were trained under a conflict procedure (n = 3), and a progressive-ratio schedule of reinforcement with midazolam as the training drug (n = 4). Sedative-like behavior was assessed using a quantitative behavioral observation procedure (n = 4). A range of doses of YT-III-31 was administered in all tests using the i.v. route of administration.

RESULTS

In the conflict procedure, increasing doses of YT-III-31 resulted only in dose-dependent attenuation of non-suppressed responding. In the progressive-ratio model of self-administration, YT-III-31 maintained average injections/session above vehicle levels at 0.1 and 0.18 mg/kg/injection. In quantitative observation procedures, YT-III-31 engendered mild sedative effects ("rest/sleep posture"), and deep sedation at the highest dose tested (5.6 mg/kg, i.v.), along with a suppression of tactile/oral exploration and increased observable ataxia. In contrast to other benzodiazepine-like ligands, YT-III-31 uniquely engendered a biphasic dose-response function for locomotion and suppressed self-groom.

CONCLUSIONS

The finding that YT-III-31 lacked anti-conflict properties is in accordance with transgenic mouse research indicating no role for α3GABA_A subtypes in benzodiazepine-mediated anxiety reduction. Instead, our results raise the possibility of a role for α3GABA_A receptors in the abuse potential and sedative effects of benzodiazepine-type drugs.

Self-administration of benzodiazepine and cocaine combinations by male and female rhesus monkeys in a choice procedure: role of α1 subunit-containing GABAA receptors

RATIONALE

Compounds lacking efficacy at the α1 subunit-containing GABA_A (α1GABA_A) receptor appear to have reduced abuse potential compared with those having measurable efficacy at this receptor, though their self-administration in nonhuman primates is dependent upon past drug experience.

OBJECTIVES

We used a drug vs. drug choice procedure to evaluate the hypothesis that L-838,417, a compound lacking efficacy at αGABA_A receptors, would not enhance cocaine choice in monkeys trained to self-administer cocaine. We also hypothesized that zolpidem, a compound with preferential modulation of ⍺1GABA_A receptors and midazolam, a nonselective benzodiazepine, would enhance cocaine choice in this procedure.

METHODS

One female and three male rhesus monkeys chose between cocaine alone (0.1 mg/kg/injection) vs. the same dose of cocaine combined with midazolam (0.003-0.1 mg/kg/injection), zolpidem (0.003-0.3 mg/kg/injection), or L-838-417 (0.01-0.1 mg/kg/injection). In addition, we evaluated choice between saline and L-838,417 at select doses to determine whether L-838,417 would function as a reinforcer on its own.

RESULTS

Consistent with our hypotheses, midazolam- and zolpidem-cocaine mixtures were chosen over cocaine alone at sufficiently high doses. However, L-838,417-cocaine mixtures also were chosen over cocaine alone in three of four subjects with at least one dose. When available alone vs. saline, L-838,417 did not function as a reinforcer in any subject.

CONCLUSION

Compounds that lack efficacy at α1GABA_A receptors may have low abuse potential compared to classic benzodiazepines, but self-administration of these compounds is context-dependent.

GABAA receptor positive allosteric modulators modify the abuse-related behavioral and neurochemical effects of methamphetamine in rhesus monkeys

GABA_A receptor positive allosteric modulators (GABA_A receptor modulators) are commonly used for the treatment of insomnia. Nevertheless, the effects of these compounds on psychostimulant-induced sleep impairment are poorly understood. Because GABA_A receptor modulators have been shown to decrease the abuse-related effects of psychostimulants, the aim of the present study was to evaluate the effects of temazepam (0.3, 1.0 or 3.0 mg/kg) and eszopiclone (0.3, 1.0 or 3.0 mg/kg), two GABA_A receptor modulators, on the behavioral neuropharmacology of methamphetamine in adult rhesus macaques (n = 5). Sleep-like measures and general daytime activity were evaluated with Actiwatch monitors. Methamphetamine self-administration (0.03 mg/kg/inf) was evaluated during morning sessions. Methamphetamine-induced dopamine overflow was assessed through in vivo microdialysis targeting the nucleus accumbens. Nighttime treatment with either temazepam or eszopiclone was ineffective in improving sleep-like measures disrupted by methamphetamine self-administration. Acute pretreatment with a low dose of temazepam before self-administration sessions increased methamphetamine self-administration without affecting normal daytime home-cage activity. At a high dose, acute temazepam pretreatment decreased methamphetamine self-administration and attenuated methamphetamine-induced increases in dopamine in the nucleus accumbens, without decreasing general daytime activity. Acute eszopiclone treatment exerted no effects on methamphetamine intake or drug-induced increases in dopamine. Our study suggests that treatments based on GABA_A receptor modulators are not effective for the treatment of sleep disruption in the context of psychostimulant use. In addition, distinct GABA_A receptor modulators differentially modulated the abuse-related effects of methamphetamine, with acute treatment with the high efficacy GABA_A receptor modulator temazepam decreasing the behavioral and neurochemical effects of methamphetamine.

Preference for an Opioid/Benzodiazepine Mixture over an Opioid Alone Using a Concurrent Choice Procedure in Rhesus Monkeys

Increased abuse of opioids is contributing to an escalation in overdose deaths. Benzodiazepines are frequently abused with opioids, possibly because they increase the potency and/or effectiveness of opioids to produce reinforcing effects. This study used a concurrent-choice procedure to determine whether monkeys would choose to self-administer a mixture of the opioid remifentanil and the benzodiazepine midazolam over remifentanil alone. Initially, three monkeys could respond on one lever for saline and on a second lever for either remifentanil alone or midazolam alone. Thereafter, monkeys chose between a dose of remifentanil (0.32 µg/kg/infusion) that did not change and a dose of remifentanil that varied across sessions; for some sessions, midazolam was combined with varying doses of remifentanil. All monkeys received more infusions of remifentanil (0.0032-0.32 µg/kg/infusion) than saline, whereas only two monkeys responded more for midazolam than for saline. When 0.32 µg/kg/infusion remifentanil was available on one lever and a dose of remifentanil that varied across sessions (0.1-1 µg/kg/infusion) was available on the other lever, monkeys chose the larger dose. Combining 3.2 µg/kg/infusion midazolam with 0.32 µg/kg/infusion remifentanil increased responding for the mixture over 0.32 µg/kg/infusion remifentanil alone, although monkeys chose remifentanil alone over mixtures containing smaller doses of remifentanil. When 10 µg/kg/infusion midazolam was combined with 0.1 µg/kg/infusion remifentanil, monkeys chose the mixture over 0.32 µg/kg/infusion remifentanil alone. Thus, monkeys prefer some opioid/benzodiazepine mixtures to larger doses of the opioid alone, suggesting that opioid/benzodiazepine coabuse might be due to increased potency (and possibly effectiveness) of opioids to produce reinforcing effects.