A withdrawal hyperalgesia test for physical dependence: evaluation of mu and mixed-partial opioid agonists

The physical dependence-inducing properties of analgesics are commonly assessed by the appearance of typical narcotic withdrawal signs (weight loss, jumping, etc.) upon cessation of administration or treatment with a narcotic antagonist. We sought to develop a simple model of physical dependence based upon withdrawal hyperalgesia. The test compound was injected seven times over a period of three days. At various times after the last injection, 6% aqueous NaCl was injected i.p. and the mice were observed for stereotyped writhing. To assess the magnitude of hyperalgesia, the animals were also challenged with several concentrations of saline administered at the time of peak sensitivity and a saline EC50 was thereby derived. In this manner, it was demonstrated that morphine causes a marked, dose-related physical dependence. Other mixed opioids, pentazocine, ethylketocyclazocine, nalbuphine, and butorphanol (in decreasing order of effectiveness) were also active. In nonpretreated mice, naloxone caused a dose-related appearance of saline-induced writhing, suggestive of a blockade of endogenous analgesic mechanisms. Writhing in response to low concentrations of i.p. saline provides a sensitive test of analgesic physical dependence.