Assessment of benzodiazepine receptor heterogeneity in vivo: apparent pA 2 and pK B analyses from behavioral studies

Abstract

The purpose of this review was to establish in vivo apparent pA2 and pKB values for antagonism of the discriminative stimulus effects of benzodiazepine ligands, and to compare these values to those obtained from other behavioral procedures. Articles were chosen from the Medline data base from January 1976 to August 1995. This literature consisted of studies with flumazenil (Ro 15-1788) as the antagonist, as well as other benzodiazepine ligands (beta-carbolines, CGS 9896, CGS 8216). The dose which occasioned 50% of the maximal response (ED50) was obtained from values estimated from the graphs presented in each article. These ED50 values were used to conduct apparent pKB and apparent pA2 analyses. Apparent pA2 values for antagonism of the discriminative stimulus effects of diazepam in rats were the following (antagonist, pA2, slope): flumazenil, 4.7, -1.5; beta-CCE, 4.0, -3.0; beta-CCtB, 5.0, -2.2. The apparent pA2 value for CGS 8216 antagonism of the discriminative stimulus effects of diazepam in rats was 5.74, -2.22 (reported in Shannon and Davis 1984). The mean apparent pA2 value for flumazenil antagonism of the discriminative stimulus effects of diazepam in rhesus monkeys was 6.55, with a mean slope of -1.42. Analysis of baboon data from Ator and Griffiths (1986) revealed apparent pKB values for flumazenil antagonism of the discriminative stimulus effects of lorazepam that were lower than the pKB values for either zopiclone or CL 218,872. Analyses of the pKB data also revealed the following: no effect of route of administration (rat, PO versus IP; baboon, PO versus IM), no effect of pretreatment time (grouped into two categories: 10-20 min and 40-70 min, in rats and non-human primates), and a species effect (pKB values for rats were reliably lower than either non-human primates or pigeons, rhesus monkeys were lower than baboons). The apparent pA2 and pKB values obtained in the present review were similar across behavioral assays, except that, in squirrel monkeys, flumazenil pKB values for antagonism of benzodiazepine-induced decreases in schedule-controlled behavior were lower than pKB values obtained from drug discrimination studies. This review provides apparent pA2 values for antagonism of the discriminative stimulus effects of benzodiazepine ligands and provides evidence from pKB analyses consistent with functional heterogeneity of benzodiazepine receptors in vivo.

Triazolam discrimination in squirrel monkeys distinguishes high-efficacy agonists from other benzodiazepines and non-benzodiazepine drugs

“…Another possibility is that the difference from -1.0 in the slope of the Schild plot obtained in the current study is due to behavioral effects of flumazenil which have been demonstrated in some procedures (Haefely 1988;Rowlett and Woolverton 1996). However, it might be expected that the intrinsic efficacy of flumazenil would similarly affect antagonism of both zolpidem and triazolam.…”

“…However, despite orderly dose-dependent shifts to the right in the triazolam dose-effect curves, the slope of the Schild plot was different from unity. At least three possibilities could account for this apparent lack of competitive interaction: non-equilibrium conditions, intrinsic efficacy of flumazenil, or multiple BZ receptors mediating the triazolam DS (Rowlett and Woolverton 1996;Kenakin 1997). In the present study, flumazenil was administered at the same pretreatment time and by the same route as in the zolpidem study, in which the slope of the Schild plot was not different from unity (Rowlett et al 1999).…”

Antagonism of the Ethanol-Like Discriminative Stimulus Effects of Ethanol, Pentobarbital, and Midazolam in Cynomolgus Monkeys Reveals Involvement of Specific GABAAReceptor Subtypes

“…Apparent pK B quantifies shifts in each dose-response function after Ro15-4513 or flumazenil pretreatment and estimates the affinity of Ro15-4513 and flumazenil for the receptor population mediating the effects of the test drug (e.g., Rowlett and Woolverton, 1996). When three or more shifts in the dose-response function were obtained, apparent pA 2 was calculated to more accurately assess Ro15-4513 and flumazenil affinity.…”

Sex Differences in Cannabinoid 1 vs. Cannabinoid 2 Receptor-Selective Antagonism of Antinociception Produced by Δ9-Tetrahydrocannabinol and CP55,940 in the Rat

“…Potency ratios were calculated to determine whether there were sex differences in the degree to which rimonabant shifted agonist dose-effect curves (Pharm/PCS, version 4.2). Finally, apparent pK B values were determined to provide an estimate of rimonabant affinity in females versus males: pK B ϭ Ϫlog [B/(dose ratio Ϫ 1)], where B is the antagonist dose in moles/kilogram and dose ratio is the ED 50 antagonist ϩ agonist/ED 50 vehicle ϩ agonist (Negus et al, 1993;Rowlett and Woolverton, 1996).…”

“…To investigate a pharmacodynamic mechanism underlying sex differences in agonist/antagonist interaction in the present study, complete agonist dose-effect curves were obtained, alone and in combination with the most effective doses of each antagonist. In vivo apparent pK B values (Negus et al, 1993;Rowlett and Woolverton, 1996) were calculated from these curves to test the hypothesis that sex differences in behavioral effects of cannabinoid drugs are caused by sex differences in the affinity of cannabinoid drugs for cannabinoid receptors. The time course of antagonist effect as well as plasma levels of antagonist were examined to determine whether sex differences in antagonism of cannabinoid agonistinduced behaviors could be attributed to sex differences in antagonist duration of action or absorption.…”