This study compares the regulation of basal signaling of MOR, delta-(DOR), and kappa-(KOR) opioid receptors after pretreatment with morphine or receptor-selective agonists, in transfected human embryonic kidney 293 cell membranes. Moreover, agonist pretreatment converts the neutral antagonists naloxone and naltrexone into inverse agonists, suppressing basal signaling, whereas analogs with reduced C6-position, e.g., 6beta-naltrexol, remain neutral antagonists at MOR under any condition. Whereas agonist pretreatment desensitizes receptors to subsequent agonist stimulation, basal signaling of mu-opioid receptor (MOR) was shown to increase.
Opioid receptors display basal signaling (constitutive, agonist-independent activity), which seems to be regulated by agonist exposure.
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