title: The 5-HT3B subunit affects high-potency inhibition of 5-HT3 receptors by morphine.
abstract: OBJECTIVE
  Morphine is an antagonist at 5-HT(3) A receptors. 5-HT(3) and opioid receptors are expressed in many of the same neuronal pathways where they modulate gut motility, pain and reinforcement. There is increasing interest in the 5-HT3B subunit, which confers altered pharmacology to 5-HT(3) receptors. We investigated the mechanisms of inhibition by morphine of 5-HT(3) receptors and the influence of the 5-HT3B subunit.
  
  
  METHODS
  5-HT-evoked currents were recorded from voltage-clamped HEK293 cells expressing human 5-HT3A subunits alone or in combination with 5-HT3B subunits. The affinity of morphine for the orthosteric site of 5-HT(3) A or 5-HT(3) AB receptors was assessed using radioligand binding with the antagonist [(3) H]GR65630.
  
  
  RESULTS
  When pre-applied, morphine potently inhibited 5-HT-evoked currents mediated by 5-HT(3) A receptors. The 5-HT3B subunit reduced the potency of morphine fourfold and increased the rates of inhibition and recovery. Inhibition by pre-applied morphine was insurmountable by 5-HT, was voltage-independent and occurred through a site outside the second membrane-spanning domain. When applied simultaneously with 5-HT, morphine caused a lower potency, surmountable inhibition of 5-HT(3) A and 5-HT(3) AB receptors. Morphine also fully displaced [(3) H]GR65630 from 5-HT(3) A and 5-HT(3) AB receptors with similar potency.
  
  
  CONCLUSIONS
  These findings suggest that morphine has two sites of action, a low-affinity, competitive site and a high-affinity, non-competitive site that is not available when the channel is activated. The affinity of morphine for the latter is reduced by the 5-HT3B subunit. Our results reveal that morphine causes a high-affinity, insurmountable and subunit-dependent inhibition of human 5-HT(3) receptors.
authors: The Institute of Academic Anaesthesia, Centre for Neuroscience, University of Dundee, Dundee, UK.
authors: Daniel T
authors: DT
authors: Baptista-Hon
authors: Tarek Z
authors: TZ
authors: Deeb
authors: Nidaa A
authors: NA
authors: Othman
authors: Douglas
authors: D
authors: Sharp
authors: Tim G
authors: TG
authors: Hales
mesh_terms: D001667:Binding, Competitive; D057809:HEK293 Cells; D006801:Humans; D007093:Imidazoles / Q000494:pharmacology; D007211:Indoles / Q000494:pharmacology; D009020:Morphine / Q000494:pharmacology*; D021122:Protein Subunits / Q000502:physiology*; D011869:Radioligand Assay; D044406:Receptors, Serotonin, 5-HT3 / Q000502:physiology*; D012701:Serotonin / Q000494:pharmacology; D058831:Serotonin 5-HT3 Receptor Antagonists / Q000494:pharmacology*; D017366:Serotonin Receptor Agonists / Q000494:pharmacology
chemical_list: D007093:Imidazoles; D007211:Indoles; D021122:Protein Subunits; D044406:Receptors, Serotonin, 5-HT3; D058831:Serotonin 5-HT3 Receptor Antagonists; D017366:Serotonin Receptor Agonists; C059068:GR 65630; D012701:Serotonin; D009020:Morphine