Regulation of neural MOR phosphorylation and internalization by DAMGO. Insert: The mice were icv-injected with 200 pmol DAMGO and antinociception was determined by the warm water (52°C) tail-flick test at various time intervals post-injection. The desensitization produced by the priming dose of DAMGO was evaluated injecting a second and identical dose of this opioid at various intervals after the first dose. *Statistically significant with respect to the control (First dose) group; ANOVA, Student-Newman-Keuls test (SigmaStat, SPSS Science Software, Erkrath, Germany). Significance was set at P < 0.05. Details as in Figs. 1 & 2. The phosphorylation and internalization of MORs was also evaluated after administering the first dose of DAMGO. The internalized MORs co-precipitated β-arrestin-2 (β-ARR2) and C-Raf but not Gαi2 or Gβ1/2 proteins. For further details see Fig. 1 and Results.