Preliminary studies on increased naloxone potency and its relationship to morphine tolerance and dependence in mice

1. Morphine pretreatment (8.0 mg/kg s.c.) induced no overt tolerance to its antinociceptive effect in mice 4 h later, but enhanced the antagonistic potency of naloxone. 2. A molecular sieve morphine pellet implanted for 24 h induced measurable tolerance, but the relative potency of naloxone was not significantly different from that observed after single-dose morphine pretreatment. The development of tolerance and increased naloxone potency do not, therefore, run parallel. 3. Naloxone-precipitated withdrawal symptoms were observed after single-dose morphine and after pellet implantation. However, molecular sieve morphine pellet implantation induced a higher degree of dependence as compared with single dose morphine pretreatment. 4. These results indicate that the rate of development of increased naloxone potency and of morphine tolerance and dependence do not run parallel. This implies that caution must be exercised in regarding increased naloxone potency as a sensitive indicator of the initiation and development of tolerance and dependence to morphine.