Absorption of Morphine from a Slow-Release Emulsion Used to Induce Morphine Dependence in Rats Abdallah Salem and Wendy Hope Department of Clinical and Experimental Pharmacology, The University of Adelaide (A.S.), South Australia, Adelaide 5005; and Department of Pharmaceutical Biology and Pharmacology, Victorian College of Pharmacy, Monash University (W.H.), Parkville Victoria 3052, Australia This study was performed to measure absorption of morphine from the injection site following treatment of rats with slow-release emulsions formulated with morphine hydrochloride and morphine base. Samples of emulsion were collected from the injection site of halothane anesthetized animals at 24 and 48 h following emulsion treatment and concentrations of morphine remaining in the emulsion were analyzed using high-performance liquid chroma- tography (HPLC). In another group of morphine-treated rats, at times equivalent to collecting samples of emulsion, the intensity of naloxone-precipitated withdrawal behaviors was moni- tored. Both morphine base- and hydrochloride-containing emulsions induced a high degree of physical dependence in animals treated over 48 h. Release of morphine from emulsions containing morphine base was slower than that from the hydrochloride formulations. In the 24-h morphine base-treated animals, approximately 45% was absorbed from the injection site as opposed to 99% in the 24-h morphine hydrochloride-treated animals. These results suggest that morphine base containing emulsions provide a more sustained exposure to the opioid. © 1999 Elsevier Science Inc. Key Words: Morphine emulsions and development of morphine dependence; Rats Introduction In experiments aimed at inducing morphine depen- dence in animals, various techniques have been devel- oped to achieve the sustained plasma levels of morphine required to induce dependence. Techniques described for induction of morphine dependence include multiple intraperitoneal injections over the treatment period (Dougherty et al., 1987), subcutaneously implanted mor- phine-containing pellets (Way et al., 1969; Blasig et al., 1973; Cerletti et al., 1976), osmotic minipumps (Wei and Loh, 1976), and oil in water emulsions from which morphine is released slowly (Collier et. al., 1972; War- hurst et al., 1984). Although subcutaneous implantation of morphine pellets is a commonly used technique that reliably induces morphine dependence, animals must be anes- thetized while the pellets are implanted. In addition, it is important to maintain tight controls over pellet formu- lations, as changes in formulation (e.g., coating of pel- lets) can result in marked differences in the rate of release of morphine from the pellets and, therefore, differences in the magnitude of the physical dependence developed (Meyer and Sparber, 1976). A further com- plication following the use of subcutaneous pellet prep- arations is that it has frequently been found necessary to remove the pellet from the implantation site before withdrawal behavior can be induced (Bhargava, 1977). Depot preparations that do not have the drawback of requiring surgical implantation under anesthesia can be obtained by formulating the drug into an appropriate vehicle. Collier et al. (1972) have described a method of formulating morphine into an oil-in-water emulsion that can be injected subcutaneously to provide a depot from which the drug is released slowly. These authors dem- onstrated that when laboratory rats were given a single injection of an emulsion containing 25 mg/mL morphine base, dependence was present 24 h later. The technique therefore has marked advantages over the subcutaneous Address reprint requests to Dr. Abdallah Salem, Department of Clinical and Experimental Pharmacology, The University of Adelaide, South Australia, Adelaide 5005, Australia. E-mail: asalem@medicine. adelaide.edu.au. Received November 2, 1998; revised and accepted December 1, 1998. Journal of Pharmacological and Toxicological Methods 40, 159 –164 (1998) © 1999 Elsevier Science Inc. All rights reserved. 1056-8719/98/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S1056-8719(98)00050-1