Aim: KCNJ6 coding for potassium inwardly rectifying channels (Kir3.2, GIRK2) is important for opioid receptor transmission. The KCNJ6 rs2070995 AA genotype has been associated with increased opioid analgesic requirements in Japanese. We analyzed its consequences for other opioid effects.
Methods: Genotyping was done in 85 methadone-substituted former heroin addicts, 352 opioid-treated chronic pain patients, and in 51 healthy volunteers where miotic effects of levomethadone had been measured. Expression of Kir3.2 in the Edinger–Westphal nucleus of rat brains was analyzed by means of immunohistochemistry.
Results: Average daily methadone substitution doses during the first therapy year were larger in the AA genotype (n=4, 119.7±49.6 mg/day) than in other rs2070995 genotypes (77.5±26.2 mg/day, P=0.003) whereas AA carriers lacked opioid withdrawal symptoms. A similar tendency toward less opioid effectiveness was observed toward higher opioid dosing demands for analgesia in the AA genotype (n=17, opioid dose 2.03±0.45 log mg oral morphine equivalents per day, controls: 1.81±0.52 log mg oral morphine equivalents/day, P=0.093). In contrast, no pharmacogenetic effects were observed on miotic opioid effects. This could be traced back to the absence of Kir3.2 from the Edinger–Westphal nucleus in rat brains, a key cerebral structure governing pupil constriction.
Conclusion: The association of the KCNJ6 rs2070995 AA genotype with increased opioid requirements extends from analgesia to opiate substitution therapy. Opioid induced miosis is exempted for molecular histological reasons.
apharmazentrum frankfurt/ZAFES, Institute of Clinical Pharmacology, Goethe-University, Theodor Stern Kai 7, Frankfurt am Main
bDepartment of Neurology, Charité University Medicine Berlin, Charitéplatz
cInstitute for Integrative Neuroanatomy, Charité University Medicine Berlin, Philippstr., Berlin, Germany
Correspondence to Dr Jörn Lötsch, pharmazentrum frankfurt/ZAFES, Institute of Clinical Pharmacology, Goethe-University, Theodor Stern Kai 7, D-60590 Frankfurt am Main, Germany
Received 22 December 2009 Accepted 6 February 2010
Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Website (http://www.pharmacogeneticsandgenomics.com).