Single-dose tolerance induced by morphine: influence of the interval between doses. Insert: Left panel, the mice were icv-injected with 10 nmol morphine and after the analgesic effect had ceased, desensitization was evaluated by icv-injection of a second and identical dose of this opioid at different time intervals. Right panel, the animals received several icv-injections of 10 nmol morphine spaced 6 h, 24 h, 48 h and 72 h from the first, and antinociception was determined in the tail-flick test at its peak effect 30 min after each injection. Values shown are the mean ± SEM from groups of 8–12 mice. *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. The internalization and Ser375 phosphorylation of the MORs was studied after administration of a second dose of 10 nmol morphine 6 h after the first. For further details see Fig. 1 and Results.