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

ENX-101, a GABAA receptor α2,3,5-selective positive allosteric modulator, displays antiseizure effects in rodent seizure and epilepsy models

OBJECTIVE

γ-Aminobutyric acid type A (GABAA) receptor positive allosteric modulators (PAMs) that lack α-subunit selectivity, including benzodiazepines such as diazepam, exhibit antiseizure actions in animal models and in humans. ENX-101 is a deuterated analog of the ⍺2,3,5-selective GABAA receptor PAM L-838,417. The purpose of this study was to characterize the α-subunit selectivity of ENX-101 and evaluate its antiseizure potential in preclinical seizure and epilepsy models.

METHODS

ENX-101 potentiation of GABA chloride current responses in cells expressing recombinant GABAA receptors were evaluated using an automated patch clamp assay. Antiseizure effects of ENX-101 were examined in the mouse 6 Hz test at 32 and 44 mA, amygdala kindled rats, and Genetic Absence Epilepsy Rat from Strasbourg (GAERS).

RESULTS

ENX-101 displayed partial PAM activity with respect to diazepam at GABAA receptors containing α2, α3, or α5 subunits but did not enhance GABA responses of GABAA receptors containing α1 subunits. ENX-101 (30, 100, and 300 mg/kg, i.p.) and diazepam protected most animals in the 6 Hz model at 32 mA but was less effective at 44 mA. In amygdala kindled rats, ENX-101 (1-100 mg/kg, p.o.) reduced behavioral seizure severity and afterdischarge duration in a dose-dependent manner. ENX-101 (0.075-100 mg/kg, p.o.) caused dose-dependent, persistent (>130 min) inhibition of spontaneous spike-and-wave discharges (SWDs) in GAERS, whereas diazepam transiently inhibited discharges. ENX-101 did not cause motor impairment, as measured by performance in the rotarod assay.

SIGNIFICANCE

ENX-101 is an α2,α3,α5-selective GABAA receptor PAM that has high potency and partial efficacy. The drug is highly effective in rodent seizure and epilepsy models. ENX-101 is most potent in the GAERS model of absence epilepsy, and active in the 6 Hz model and amygdala kindled rats. These results demonstrate that a partial, subtype-selective GABAA receptor PAM has activity in translationally validated preclinical epilepsy screening models. Clinical evaluation of ENX-101 as a treatment for focal and generalized epilepsies is warranted.

Self-Administration of Fentanyl-Alprazolam Combinations by Rhesus Monkeys Responding under a Progressive-Ratio Schedule

This study evaluated the reinforcing effects of fentanyl, alone or in combination with the benzodiazepine alprazolam, in rhesus monkeys (3 females, 3 males). Subjects were trained to self-administer the opioid remifentanil (0.3 µg/kg/injection) under a progressive-ratio schedule of reinforcement. The reinforcing effects of fentanyl (0.1-10 µg/kg/injection) or alprazolam (1.0-100 µg/kg/injection) alone, or in combinations of fixed proportions (1:1, 1:3, and 3:1 fentanyl:alprazolam, with 1:1 based on the potencies of drugs alone) were evaluated in single-day test sessions (with double determinations). Dose-equivalence analysis was used to determine the extent to which fentanyl and alprazolam combinations differed from additivity. Fentanyl functioned as a positive reinforcer in all monkeys, while alprazolam was a reinforcer in 3 of 6 monkeys only. Therefore, drug combination data were grouped as "alprazolam-taking" and "non-alprazolam-taking" monkeys. For alprazolam-taking monkeys, we observed additive effects for the 3:1 and 1:3 combinations, and a significant supra-additive interaction for the 1:1 combination of fentanyl and alprazolam. For 2 of the 3 non-alprazolam-taking monkeys, the combination of fentanyl and alprazolam resulted in enhanced reinforcing effects relative to either drug alone. However, the one monkey showed primarily inhibitory, or suppressive effects, with the 3:1 dose combination resulting in a relatively modest rightward shift in the fentanyl dose-response function. In summary, our findings show that combining fentanyl and alprazolam generally result in proportion-dependent additive or supra-additive enhancements. These data raise the possibility that the prevalence of opioid-benzodiazepine polydrug abuse may reflect a unique enhancement of these drugs' reinforcing effects, although individual differences may exist. SIGNIFICANCE STATEMENT: Addressing the critical question of the degree to which benzodiazepines can modulate the abuse-related effects of opioids may provide improved pathways to treatment of this common form of polydrug addiction. In the present study, we show that combinations of the opioid fentanyl and the benzodiazepine alprazolam can be more reinforcing than either drug alone in a rhesus monkey model, suggesting that enhancement of reinforcement processes may underlie this prevalent form of polydrug use disorder.

Designer Benzodiazepines' Activity on Opioid Receptors: A Docking Study

BACKGROUND

Previous studies have reported that benzodiazepines (BZDs) seem to enhance euphoric and reinforcing properties of opioids in opioid users so that a direct effect on opioid receptors has been postulated, together with a possible synergistic induction of severe side effects due to co use of BDZs and opioids. This is particularly worrisome given the appearance on the market of designer benzodiazepines (DBZDs), whose activity/toxicity profiles are scarcely known.

OBJECTIVES

This study aimed to evaluate, through computational studies, the binding affinity (or lack thereof) of 101 DBZDs identified online on the kappa, mu, and delta opioid receptors (K, M, DOR); and to assess whether their mechanism of action could include activation of the latter.

METHODS

MOE® was used for the computational studies. Pharmacophore mapping based on strong opioids agonist binders' 3D chemical features was used to filter the DBZDs. Resultant DBZDs were docked into the crystallised 3D active conformation of KOR (PDB6B73), DOR (PDB6PT3) and MOR (PDB5C1M). Co-crystallised ligands and four strong agonists were used as reference compounds. A score (S, Kcal/mol) representative of the predicted binding affinity, and a description of ligand interactions were obtained from MOE®.

RESULTS

The docking results, filtered for S < -8.0 and the interaction with the Asp residue, identified five DBZDs as putative binders of the three ORs : ciclotizolam, fluloprazolam, JQ1, Ro 48-6791, and Ro 48-8684.

CONCLUSION

It may be inferred that at least some DBZDs may have the potential to activate opioid receptors. This could mediate/increase their anxiolytic, analgesic, and addiction potentials, as well as worsen the side effects associated with opioid co-use.

Examining the effects of psychoactive drugs on complex behavioral processes in laboratory animals

Behavioral pharmacology has been aided significantly by the development of innovative cognitive tasks designed to examine complex behavioral processes in laboratory animals. Performance outcomes under these conditions have provided key metrics of drug action which serve to supplement traditional in vivo assays of physiologic and behavioral effects of psychoactive drugs. This chapter provides a primer of cognitive tasks designed to assay different aspects of complex behavior, including learning, cognitive flexibility, memory, attention, motivation, and impulsivity. Both capstone studies and recent publications are highlighted throughout to illustrate task value for two distinct but often interconnected translational strategies. First, task performance in laboratory animals can be utilized to elucidate how drugs of abuse affect complex behavioral processes. Here, the expectation is that adverse effects on such processes will have predictive relevance to consequences that will be experienced by humans. Second, these same task outcomes can be used to evaluate candidate therapeutics. In this case, the extent to which drug doses with medicinal value perturb task performance can contribute critical information for a more complete safety profile appraisal and advance the process of medications development. Methodological and theoretical considerations are discussed and include an emphasis on determining selectivity in drug action on complex behavioral processes.

Benzodiazepines: Their Use either as Essential Medicines or as Toxics Substances

This review highlights the nature, characteristics, properties, pharmacological differences between different types of benzodiazepines, the mechanism of action in the central nervous system, and the degradation of benzodiazepines. In the end, the efforts to reduce the benzodiazepines' adverse effects are shown and a reflection is made on the responsible uses of these medications.

The relationship between exposure to general anesthetic agents and the risk of developing an impulse control disorder

Most studies examining the effect of extended exposure to general anesthetic agents (GAAs) have demonstrated that extended exposure induces both structural and functional changes in the central nervous system. These changes are frequently accompanied by neurobehavioral changes that include impulse control disorders that are generally characterized by deficits in behavioral inhibition and executive function. In this review, we will.