RATIONALE: The extended ternary complex theory of receptor function states a ligand can be classified as an inverse agonist, agonist, or neutral antagonist based on its ability to preferentially stabilize the inactive conformation, the active conformation, or to have no preference for conformational state, respectively. OBJECTIVES: While serotonin(2C) (5-HT(2C)) receptor ligands are classified accordingly in vitro, whether the phenomenon of inverse agonism manifests itself and/or is physiologically relevant in vivo is unknown. METHODS: Therefore, we tested a range of proposed agonists, neutral antagonists, and inverse agonists with activity at 5-HT(2C) receptors in three groups of pigeons trained to discriminate saline from: 1.0 mg/kg MK212, an agonist; 0.1 mg/kg methysergide, a proposed neutral antagonist; or, 10 mg/kg mianserin, a proposed inverse agonist. RESULTS: Based on the patterns of substitution, the discriminative stimulus effects of MK212 appear to be mediated through agonist actions and the stimulus effects of methysergide and mianserin appear to be mediated through antagonist actions. Selective 5-HT(2B/2C) inverse agonist SB206,553 (1 mg/kg) blocked the MK212 discriminative stimulus cue and substituted (0.32-10 mg/kg) in both methysergide- and mianserin-trained pigeons, confirming a 5-HT(2C) receptor role in mediating these discriminative stimuli. Inverse agonists and neutral antagonists fully substituted for methysergide. In addition to SB206,553, methysergide (0.032-1.0 mg/kg) and 5-HT(1A) agonist 8-OH-DPAT (0.32-1.0 mg/kg) substituted completely for mianserin suggesting a complex discriminative stimulus profile for this proposed inverse agonist. CONCLUSIONS: These data and the subsequent analyses suggest that the discriminative cues of MK212, methysergide, and mianserin are different and that the drug discrimination paradigm is a useful functional assay to examine intrinsic efficacy in vivo.
RATIONALE: The extended ternary complex theory of receptor function states a ligand can be classified as an inverse agonist, agonist, or neutral antagonist based on its ability to preferentially stabilize the inactive conformation, the active conformation, or to have no preference for conformational state, respectively. OBJECTIVES: While serotonin(2C) (5-HT(2C)) receptor ligands are classified accordingly in vitro, whether the phenomenon of inverse agonism manifests itself and/or is physiologically relevant in vivo is unknown. METHODS: Therefore, we tested a range of proposed agonists, neutral antagonists, and inverse agonists with activity at 5-HT(2C) receptors in three groups of pigeons trained to discriminate saline from: 1.0 mg/kg MK212, an agonist; 0.1 mg/kg methysergide, a proposed neutral antagonist; or, 10 mg/kg mianserin, a proposed inverse agonist. RESULTS: Based on the patterns of substitution, the discriminative stimulus effects of MK212 appear to be mediated through agonist actions and the stimulus effects of methysergide and mianserin appear to be mediated through antagonist actions. Selective 5-HT(2B/2C) inverse agonist SB206,553 (1 mg/kg) blocked the MK212 discriminative stimulus cue and substituted (0.32-10 mg/kg) in both methysergide- and mianserin-trained pigeons, confirming a 5-HT(2C) receptor role in mediating these discriminative stimuli. Inverse agonists and neutral antagonists fully substituted for methysergide. In addition to SB206,553, methysergide (0.032-1.0 mg/kg) and 5-HT(1A) agonist 8-OH-DPAT (0.32-1.0 mg/kg) substituted completely for mianserin suggesting a complex discriminative stimulus profile for this proposed inverse agonist. CONCLUSIONS: These data and the subsequent analyses suggest that the discriminative cues of MK212, methysergide, and mianserin are different and that the drug discrimination paradigm is a useful functional assay to examine intrinsic efficacy in vivo.
Authors: D W Bonhaus; K K Weinhardt; M Taylor; A DeSouza; P M McNeeley; K Szczepanski; D J Fontana; J Trinh; C L Rocha; M W Dawson; L A Flippin; R M Eglen Journal: Neuropharmacology Date: 1997 Apr-May Impact factor: 5.250
Authors: Elena Castro; Alvaro Díaz; Antonio Rodriguez-Gaztelumendi; Elena Del Olmo; Angel Pazos Journal: Neuropharmacology Date: 2008-09-17 Impact factor: 5.250
Authors: Mark G Devlin; Nicola J Smith; Olivia M Ryan; Elizabeth Guida; Patrick M Sexton; Arthur Christopoulos Journal: Eur J Pharmacol Date: 2004-09-13 Impact factor: 4.432
Authors: Ying-Chu Chen; Rachel M Hartley; Noelle C Anastasio; Kathryn A Cunningham; Scott R Gilbertson Journal: ACS Chem Neurosci Date: 2017-04-19 Impact factor: 4.418