Table 1: Morphine treatment schedule.

Group 1: Naïve (without any treatment)
Group 2: Saline single injection or 1 injection each day for 8 days, s.c. (data combined as there was no difference between the two groups)
Group 3: MorX1 (morphine10 mg/kg, one injection, s.c.)
Group 4: Sal X8 (saline, 8 injections, 1 injection each day, for 8 days, s.c.)
Group 5: MorX8 (morphine10 mg/kg, 1 injection each day × 8 days, s.c.)
Group 6: Esc, morphine escalating dose (10, 20, 20, 40, 40, 80, 160 mg/kg, s.c.)
Group 7: PP24h, acute implantation of placebo pellet, sacrificed 24 h later
Group 8: PM24h, acute implantation of morphine pellet (25 mg), sacrificed 24 h later
Group 9: PP7d, chronic implantation of placebo pellets (on day 1 and day 4) and sacrificed on day 7
Group 10: PM7d, chronic implantation of morphine pellets (25 mg) (on day 1 and day 4) and sacrificed on day 7