Development of antinociceptive tolerance and physical dependence following morphine i.c.v. infusion in mice Natalie R. Lenard ⁎ , Sandra C. Roerig Louisiana State University Health Sciences Center-Shreveport, Department of Pharmacology, Toxicology, and Neuroscience, United States Received 23 December 2004; received in revised form 5 October 2005; accepted 11 October 2005 Abstract The chronic i.c.v. infusion of morphine has been reported for rats but not for mice. In the current report, the antinociceptive tolerance to both i.c.v. morphine infusion and s.c. implantation of morphine pellets in mice was compared. Physical dependence after i.c.v. morphine infusion was also evaluated. Osmotic minipumps were filled with morphine (50 mM), connected to i.c.v. cannulae, and implanted s.c. to deliver 50 nmol/h for 3 days (i.e., 3.6 μmol total). Robust jumping precipitated by naloxone (1 mg/kg, s.c.) indicated the development of physical dependence. Tolerance to i.c.v., i.t., and i.v. morphine (6.3-, 2.0-, and 4.4-fold, respectively) was observed using the tail flick test. Mice implanted with pellets containing 75 mg morphine for 3 days (i.e., ∼260 μmol total) were also tolerant to morphine (6.5-, 7.5- and 18-fold, respectively). Thus, the tolerance developed using the two methods was not identical. These results allow comparison of morphine tested by 3 different routes (i.c.v., i.t., and i.v.) after chronic morphine treatment by two routes (i.c.v. and s.c.) in a single study. © 2005 Elsevier B.V. All rights reserved. Keywords: Intracerebroventricular; Osmotic minipump; Morphine pellet; Chronic; Naloxone; Withdrawal; Tolerance; (Mice) 1. Introduction The development of tolerance and physical dependence to morphine in both rats and mice are commonly achieved using morphine pellets implanted under the skin. There are also studies demonstrating that, in rats, both tolerance and physical dependence develop to morphine infused chronically using osmotic minipumps connected to i.c.v. cannulae (Huffman et al., 1985; Narita et al., 1994; Tokuyama et al., 2000). Mice, on the other hand, have been infused i.c.v. with neuropeptide Y (Mashiko et al., 2003), beta-amyloid (Permanne et al., 2002), melanin-concentrating hormone (Gomori et al., 2003), and other compounds, but there are no reports of morphine i.c.v. infusion in mice. Osmotic minipumps are useful for the continuous adminis- tration of drugs and/or hormones because stable drug concen- trations can be rapidly achieved and there is no stress associated with repeated injections. The use of osmotic minipumps is ideal for drugs that produce physical dependence (e.g., morphine) because continuously infused animals do not undergo periodic withdrawal during the times between injections. To eliminate the confounding factor of drug access to brain opioid receptors following systemic administration, the continuous i.c.v. infu- sion of morphine is a good model for the study of the devel- opment of tolerance and physical dependence. In the current studies, we demonstrated that mice infused with morphine i.c.v. for 3 days developed antinociceptive tol- erance to i.c.v., i.t., or i.v. administered morphine, as well as physical dependence. In studies comparing chronic i.c.v. infu- sion to chronic s.c. administration (i.e., s.c. morphine pellets), the tolerance i.c.v. morphine produced by morphine pellet implantation was similar to that produced by i.c.v. infusion of morphine. However, animals implanted with morphine pellets developed a greater degree of tolerance to i.t. and i.v. morphine than animals that had been infused with i.c.v. morphine. These data will likely be useful for other investigators because 2 different routes of chronic administration and 3 different routes of acute administration are compared using the same experi- mental conditions. European Journal of Pharmacology 527 (2005) 71 – 76 www.elsevier.com/locate/ejphar ⁎ Corresponding author. Pennington Biomedical Research Center, 6400 Per- kins Road, Baton Rouge, LA 70809. Tel.: +1 2257633164; fax: +1 2257630274. E-mail address: lenardnr@pbrc.edu (N.R. Lenard). 0014-2999/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ejphar.2005.10.031