Discriminative Stimulus Effects of Benzodiazepine (BZ)₁ Receptor-Selective Ligands in Rhesus Monkeys

Abstract

Drug discrimination was used to examine the effects of benzodiazepine (BZ) 1 receptor-selective ligands in rhesus monkeys. In diazepam-treated (5.6 mg/kg, p.o.) monkeys discriminating the nonselective BZ antagonist flumazenil (0.32 mg/kg, s.c.), the BZ 1 -selective antagonist ␤-carboline-3-carboxylate-t-butyl ester (␤-CCt) substituted for flumazenil. The onset of action of ␤-CCt was delayed with a dose of 5.6 mg/kg ␤-CCt substituting for flumazenil 2 h after injection. In monkeys discriminating the nonselective BZ agonist midazolam (0.56 mg/kg, s.c.), the BZ 1 -selective agonists zaleplon (ED 50 ϭ 0.78 mg/kg) and zolpidem (ED 50 ϭ 1.73 mg/kg) substituted for midazolam. The discriminative stimulus effects of midazolam, zaleplon, and zolpidem were antagonized by ␤-CCt (1.0 -5.6 mg/kg, s.c.), and the effects of zaleplon and zolpidem were also antagonized by flumazenil (0.01-0.32 mg/kg, s.c.). Schild analyses supported the notion of a simple, competitive interaction between ␤-CCt and midazolam (slope ϭ Ϫ1.08; apparent pA 2 ϭ 5.41) or zaleplon (slope ϭ Ϫ1.57; apparent pA 2 ϭ 5.49) and not between ␤-CCt and zolpidem. Schild analyses also were consistent with a simple, competitive interaction between flumazenil and zaleplon (slope ϭ Ϫ1.03; apparent pA 2 ϭ 7.45) or zolpidem (slope ϭ Ϫ1.11; apparent pA 2 ϭ 7.63). These results suggest that the same BZ receptor subtype(s) mediate(s) the effects of midazolam, zolpidem, and zaleplon under these conditions and that selective binding of BZ ligands does not necessarily confer selective effects in vivo.Drugs facilitating GABA A -mediated chloride flux (e.g., agonists; positive GABA A modulators) at benzodiazepine (BZ) receptors elicit sedative-hypnotic, anxiolytic, muscle relaxant, and anticonvulsant effects with an improved margin of safety compared with other positive GABA A modulators (e.g., barbiturates; for review, see Woods et al., 1992). Different BZ receptors comprise at least one of six different protein subunits designated ␣ 1-6 and are divided into two subtypes: BZ 1 receptors containing ␣ 1 -subunits and BZ 2 receptors containing ␣ 2 -, ␣ 3 -, or ␣ 5 -subunits (Sanger et al., 1994 for review). A more recent nomenclature adopted by the International Union of Pharmacology proposes an alternative designation for BZ receptors (Barnard et al., 1998). It has been postulated that the various effects elicited by BZ site ligands are mediated by different BZ receptor subtypes. This notion has been supported by transgenic mouse studies showing that mutations targeted at ␣ 2 subunits decrease the anxiolytic effects of BZs, whereas mutations targeted at ␣ 1 subunits decrease the sedative-hypnotic effects of BZs (Rudolph et al., 1999;McKernan et al., 2000).Most BZ receptor ligands have relatively little selectivity for particular BZ receptor subtypes, making it difficult to differentiate the functional significance of BZ receptor heterogeneity. BZs such as diazepam and midazolam are nonselective (Huang et al., 2000), whereas non-BZ compounds such as zaleplon and zolpidem ...

Characterization of the Interaction of Indiplon, a Novel Pyrazolopyrimidine Sedative-Hypnotic, with the GABAAReceptor

“…This was apparent from the inhibition of [ 3 H]Ro 15-1788 binding to rat cerebral cortex and cerebellar membranes (K i values of 0.55 and 0.45 nM, respectively), from the binding of [ 3 H]indiplon itself in rat cerebral cortex and cerebellar membranes (K D ϭ 1.01 and 0.53 nM, respectively) and from the potentiation of GABA-evoked chloride currents in cultured rat neurons (EC 50 ϭ 11.6 nM). It has long been documented that compounds such as triazolam, zolpidem, and zaleplon all exhibit full agonist activity at the benzodiazepine site using a variety of methods, including potentiation of GABA currents (Im et al, 1993), discriminative stimulus effects in rats and rhesus monkeys (Sanger et al, 1999;McMahon et al, 2002), and positron emission tomographic quantitation (Abadie et al, 1996). Indiplon also exhibited the characteristics of a benzodiazepine site agonist in GABA shift experiments and showed full agonist efficacy in the patch-clamp experiments.…”

Discriminative Stimulus Effects of Positive GABAAModulators and Other Anxiolytics, Sedatives, and Anticonvulsants in Untreated and Diazepam-Treated Monkeys

“…Thus, although these discrimination assays seem to be selective for positive GABA A modulation and not other mechanisms underlying anxiolytic, sedative or anticonvulsant activity, some drugs (e.g., valproic acid) have in common with positive GABA A modulators the ability to modulate benzodiazepine dependence and withdrawal. Compounds that modulate GABA with high efficacy at benzodiazepine receptors have been shown to substitute for midazolam and to attenuate flumazenil in diazepam-treated monkeys Lelas et al, 1999;McMahon et al, , 2002). In the current study, flunitrazepam substituted for midazolam and attenuated flumazenil; both results are consistent with flunitrazepam having high affinity at benzodiazepine receptor subtypes comprising ␣ 1 -, ␣ 2 -, ␣ 3 -, and ␣ 5 -subunits and having high efficacy in positively modulating GABA at these subtypes in vitro (Smith et al, 2001).…”

“…A variety of approaches have been used to study benzodiazepine tolerance and dependence (e.g., observational procedures; Lukas and Griffiths, 1982), including drug discrimination. For example, the benzodiazepine antagonist flumazenil has been established as a discriminative stimulus in rhesus monkeys treated daily with the benzodiazepine diazepam McMahon et al, , 2002. The consequences of chronic benzodiazepine treatment (e.g., tolerance) have been examined by comparing the discriminative stimulus effects of various compounds in diazepam-treated monkeys discriminating flumazenil to the discriminative stimulus effects of the same compounds in untreated monkeys discriminating midazolam (Lelas et al, 1999;McMahon et al, , 2002.…”

“…For example, the benzodiazepine antagonist flumazenil has been established as a discriminative stimulus in rhesus monkeys treated daily with the benzodiazepine diazepam McMahon et al, , 2002. The consequences of chronic benzodiazepine treatment (e.g., tolerance) have been examined by comparing the discriminative stimulus effects of various compounds in diazepam-treated monkeys discriminating flumazenil to the discriminative stimulus effects of the same compounds in untreated monkeys discriminating midazolam (Lelas et al, 1999;McMahon et al, , 2002. Such comparisons might be relevant to benzodiazepine dependence and withdrawal because the flumazenil discriminative stimulus in diazepam-treated monkeys is qualitatively similar to discriminative stimulus effects that emerge when diazepam treatment is temporarily discontinued .…”

Quantitative Analyses of Antagonism: Combinations of Midazolam and Either Flunitrazepam or Pregnanolone in Rhesus Monkeys Discriminating Midazolam

“…Flumazenil might also be expected to attenuate the combined effects of a benzodiazepine and a neuroactive steroid. Flumazenil antagonizes benzodiazepines in monkeys discriminating midazolam, shifting doseeffect curves rightward (Lelas et al, 1999(Lelas et al, , 2000McMahon et al, 2002); however, it enhances the effects of neuroactive steroids, shifting dose-effect curves leftward (McMahon and France, 2005;Gerak and France, 2011), which reflects the positive efficacy of flumazenil (Dantzer and Pério, 1982). Thus, flumazenil could enhance or attenuate the combined effects of benzodiazepines and neuroactive steroids, depending on the proportion of each drug in the mixture.…”