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Comparative Cue Generalisation Profiles of L-838,417, SL651498

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Comparative Cue Generalisation Profiles of L-838,417, SL651498 JPET Fast Forward. Published on December 8, 2005 as DOI: 10.1124/jpet.105.094003 JPET FastThis Forward.article has not Published been copyedited on and December formatted. The 8, final 2005 version as DOI:10.1124/jpet.105.094003may differ from this version. JPET #94003 Title Page Comparative cue generalisation profiles of L-838,417, SL651498, zolpidem, CL218,872, ocinaplon, bretazenil, zopiclone and various benzodiazepines in chlordiazepoxide and zolpidem drug Downloaded from discrimination jpet.aspetjournals.org Mirza NR, Rodgers RJ and Mathiasen LS NeuroSearch A/S, 93 Pederstrupvej, Ballerup, DK-2750, Denmark (NRM, LSM); Behavioural Neuroscience Laboratory, Institute of Psychological at ASPET Journals on October 1, 2021 Sciences, University of Leeds, Leeds LS2 9JT, England (JRR, LSM) 1 Copyright 2005 by the American Society for Pharmacology and Experimental Therapeutics. JPET Fast Forward. Published on December 8, 2005 as DOI: 10.1124/jpet.105.094003 This article has not been copyedited and formatted. The final version may differ from this version. JPET #94003 Running Title Page: Subtype-selective GABAA receptor modulators in drug discrimination Contact and address for correspondence: Naheed R. Mirza Department of In-vivo Pharmacology NeuroSearch A/S 93 Pederstrupvej Downloaded from DK-2750 Ballerup Denmark Tele: +45 44 60 82 71 jpet.aspetjournals.org FAX: +45 44 60 80 80 E-mail: [email protected] at ASPET Journals on October 1, 2021 Number of text pages: 19 Number of tables: 0 Number of figures: 4 Number of references: 35 Number of words in Abstract: 252 Number of words in Introduction: 1028 Number of words in Discussion: 1598 Non-standard abbreviations: None Preferred section assignment: Behavioural Pharmacology 2 JPET Fast Forward. Published on December 8, 2005 as DOI: 10.1124/jpet.105.094003 This article has not been copyedited and formatted. The final version may differ from this version. JPET #94003 Abstract The zolpidem discriminative cue is mediated by GABAA-α1 receptors, whereas the chlordiazepoxide cue may be mediated via non-α1 GABAA receptors, since compounds with selective affinity for GABAA-α1 receptors fully generalise to the former cue. We predicted that L- 838,417, a partial agonist at non-α1 GABAA receptors and an antagonist at GABAA-α1 receptors, would generalise to the chlordiazepoxide but not the zolpidem discriminative cue. SL651498 is a full agonist at GABAA-α2 receptors, with lower efficacy at GABAA-α3 receptors, and least efficacy Downloaded from at GABAA-α1 and GABAA-α5 receptors. Since SL651498 has efficacy at GABAA-α1 receptors, we anticipated it would generalise to both discriminative cues. Rats were trained to discriminate either zolpidem (3 mg/kg) or chlordiazepoxide (5 mg/kg) from vehicle using a two lever operant jpet.aspetjournals.org procedure. The generalisation profiles of L-838,417 and SL651498 were compared with non- selective full agonists, GABAA-α1-selective ligands zolpidem and CL218,872, the non-selective partial agonist bretazenil, and the novel anxioselective drug ocinaplon. A non-selective partial at ASPET Journals on October 1, 2021 agonist was included since L-838,417 and SL651498 are partial agonists at some GABAA receptors, and this property may influence their generalisation profiles. All non-selective full agonists and ocinaplon fully generalised to both cues. CL218,872 and zolpidem generalised to zolpidem only, whereas L-838,417 fully generalised to chlordiazepoxide only. SL651498 fully generalised to chlordiazepoxide and occasioned significant zolpidem-appropriate responding. Bretazenil was similar to SL651498. In conclusion, at this training dose, the chlordiazepoxide discriminative stimulus is mediated primarily via non-α1 GABAA receptors, and the generalisation profiles of the ligands tested appears to correspond with their in vitro profiles at GABAA receptor subtypes. 3 JPET Fast Forward. Published on December 8, 2005 as DOI: 10.1124/jpet.105.094003 This article has not been copyedited and formatted. The final version may differ from this version. JPET #94003 Introduction The majority of GABAA receptors in the mammalian brain contain at least one α, β, and γ subunit (Barnard et al., 1998). The likely stoichiometry is two α, two β and one γ subunit arranged pentamerically around a central pore through which Cl- ions flow when the receptor is activated. Classical benzodiazepines modulate GABAA neurotransmission by binding to a site that traverses an α and γ subunit of α1, α2, α3 or α5 containing GABAA receptors (Sieghart, 1995). Mice with a point mutation of either α1 (Rudolph et al., 1999; McKernan et al., 2000) or α2 (Low et al., 2000) Downloaded from subunits are insensitive to the motor-impairing and anxiolytic-like effects of diazepam, respectively, suggesting different functional roles for GABAA- α1 and α2 receptors. jpet.aspetjournals.org Pharmacological evidence has also alluded to different functional roles for GABAA receptor subtypes (e.g., Griebel et al., 1999a, Chen et al., 1996). For example, the triazolopyridazine CL- 218,872 was found to bind with different affinity to two populations of benzodiazepine receptor, at ASPET Journals on October 1, 2021 originally designated type I (α1 containing GABAA receptors) and II (α2, α3 or α5 containing GABAA receptors) receptors (Klepner et al., 1979; Squires et al., 1979). Furthermore, the benzodiazepine receptor antagonist β-carboline-3-carboxylate t-butyl ester (β-CCT) selectively antagonised the anticonvulsant and anxiolytic, but not the myorelaxant, effects of diazepam, whereas flumazenil blocked all three effects (Shannon et al., 1984). Further studies confirmed β- CCT was more selective than flumazenil in antagonising the behavioural effects of benzodiazepine site ligands in vivo (Griebel et al., 1999b; Paronis et al., 2001; Rowlett et al., 2005b). In vitro studies showed that β-CCT had selective affinity for GABAA-α1 receptors (Huang et al., 1999), and was more potent at antagonising zolpidem potentiation of GABA’s effect at GABAA-α1 receptors than diazepam potentiation of GABA at GABAA-α3 or GABAA-α5 receptors (Griebel et al., 1999b). 4 JPET Fast Forward. Published on December 8, 2005 as DOI: 10.1124/jpet.105.094003 This article has not been copyedited and formatted. The final version may differ from this version. JPET #94003 The discriminative stimulus effects of benzodiazepines have been widely described, and this tool has been employed to determine receptor selectivity and efficacy of benzodiazepine site ligands (see review by Lelas et al., 2000). For example, in squirrel monkeys trained to discriminate zolpidem at low (1 mg/kg) and high doses (>3 mg/kg), generalisation to non-selective benzodiazepines occurred in the former but not latter set of animals (Rowlett et al., 1999; Rowlett et al., 2000). Moreover, in animals discriminating high doses of zolpidem, generalisation to the GABAA-α1 selective drug zaleplon occured. Thus, at certain training doses, the zolpidem Downloaded from discriminative stimulus is mediated via GABAA-α1 receptors. In rats the zolpidem discriminative stimulus might also be mediated via GABAA-α1 receptors, whereas the chlordiazepoxide α discriminative stimulus might be mediated by non- 1 GABAA receptors (Sanger and Benavides, jpet.aspetjournals.org 1993; Sanger et al., 1999). This is based on the finding that drugs with affinity-selectivity for GABAA-α1 receptors (e.g., zolpidem, CL218,872) fully generalised to the zolpidem but not the chlordiazepoxide cue. at ASPET Journals on October 1, 2021 In this study we tested the hypothesis that the zolpidem and chlordiazepoxide discriminative stimuli in rats are mediated predominantly via GABAA-α1 and non-α1 GABAA receptors, respectively, by testing novel compounds not previously available as well as well-characterised pharmacological tools. The compounds tested fell in to various categories on the basis of affinity and efficacy: (1) non-selective full agonists, modulators which potentiate the effect of GABA at GABAA- α1, α2, α3 and α5 receptors equivalently (Sieghart, 1995; Hadingham et al., 1993; Faure-Halley et al., 1993). This category included benzodiazepines and zopiclone, largely overlapping with compounds tested previously (Sanger and Benavides, 1993; Sanger et al., 1999); (2) selective positive benzodiazepine site modulators, which were subdivided in to two groups: (2a) compounds with selective affinity, including the imidazopyridine zolpidem and triazolopyridazine CL218,872, ligands that selectively bind to GABAA-α1 receptors. These compounds selectively potentiate the effect of GABA at 5 JPET Fast Forward. Published on December 8, 2005 as DOI: 10.1124/jpet.105.094003 This article has not been copyedited and formatted. The final version may differ from this version. JPET #94003 GABAA-α1 receptors, although this is concentration-dependent; i.e., they only show 10-20 fold affinity selectivity for GABAA-α1 receptors over GABAA-α2 or α3 receptors, despite >1000-fold selectivity over GABAA-α5 receptors (Sieghart, 1995; Hadingham et al., 1992; 1993; Faure-Halley et al., 1993). To add to the complexity, zolpidem has high intrinsic efficacy at GABAA-α1 receptors, whereas CL218,872 displays reduced efficacy at this receptor, i.e., CL218,872 is a partial agonist at GABAA-α1 receptors. (2b) Functionally (or efficacy) selective compounds include L-838,417 (McKernan et al., 2000) and SL651,498 (Griebel et al., 2001). These compounds bind with similar Downloaded from affinity to α1, α2, α3 or α5 containing GABAA receptors, but are able
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