Changes in the expression of G protein‐coupled receptor kinases and β‐arrestin 2 in rat brain during opioid tolerance and supersensitivity

We previously demonstrated that chronic treatment of rats with the µ‐opioid receptor agonist sufentanil induced pharmacological tolerance associated with µ‐opioid receptor desensitization and down‐regulation. Administration of the calcium channel blocker nimodipine during chronic treatment with sufentanil prevented µ‐opioid receptor down‐regulation, induced down‐stream supersensitization, and produced supersensitivity to the opioid effects. The focus of the present study was to determine a role for G protein‐coupled receptor kinases (GRKs) and β‐arrestin 2 in agonist‐induced µ‐opioid receptor signalling modulation during chronic opioid tolerance and supersensitivity. Tolerance was induced by 7‐day chronic infusion of sufentanil (2 µg/h). Supersensitivity was induced by concurrent infusion of sufentanil (2 µg/h) and nimodipine (1 µg/h) for 7 days. Antinociception was evaluated by the tail‐flick test. GRK2, GRK3, GRK6 and β‐arrestin 2 immunoreactivity levels were determined by western blot in brain cortices. Acute and chronic treatment with sufentanil induced analgesic tolerance, associated with up‐regulation of GRK2, GRK6, and β‐arrestin 2. GRK3 expression only was increased in the acutely treated group. When nimodipine was associated to the chronic opioid treatment, tolerance expression was prevented, and immunoreactivity levels of GRK2, GRK6 and β‐arrestin 2 recovered the control values. These data indicate that GRK2, GRK3, GRK6 and β‐arrestin 2 are involved in the short‐ and long‐term adaptive changes in µ‐opioid receptor activity, contributing to tolerance development in living animals. These observations also suggest that GRKs and β‐arrestin 2 could constitute pharmacological targets to prevent opioid tolerance development, and to improve the analgesic efficacy of opioid drugs.