Attenuation of dynamin‐dependent internalization decreases antinociception during the expression of morphine tolerance

Abstract

Mu‐opioid receptor (MOP) internalization has been proposed as a cellular mechanism of morphine tolerance. In order to test this hypothesis in vivo, the fluorescent MOP agonist, DERM‐A594, was microinjected into the periaqueductal gray (vlPAG). Rats were pretreated with either saline (0.4 μl) or morphine (5 μg/0.4 μl) 2X/day for 2 days. On day 3, rats were given either a saline or morphine challenge 30 min prior to DERM‐A594 (300 ng/0.4 μl). Intracellular DERM‐A594 intensity was similar in all groups except for a significant decrease in DERM‐A594 label in morphine tolerant rats given a morphine challenge (F (3, 34) = 3.899, p<0.05). These results suggest that repeated morphine administration increases morphine‐induced internalization of MOPs. To further test this idea, a myr‐dynamin inhibitory peptide (Dyn+) and a scrambled peptide (Dyn−) were microinjected into the vlPAG prior to microinjection of DERM‐A594. Dyn+ reduced DERM‐A594 antinociception compared to Dyn− (15 ± 3 s vs. 33 ± 5 s). Dyn+ also reduced morphine antinociception in both saline (D50 = 8.1 μg vs. Dyn− = 3.8 μg) and morphine pretreated rats (D50 = 10.7 μg vs. Dyn− = 5.8 μg). A comparable shift in D50 values in saline and morphine pretreated rats suggests that Dyn+ attenuates antinociception similarly in both groups. Thus, these studies demonstrate that internalization is important for MOP signaling in the vlPAG. Supported by DA023318(TM), DA015498(MM).