Rat strain differences in the opiate abstinence syndrome

Abstract

Albino rats from two outbred and three inbred strains received subcutaneous implants of either morphine or placebo pellets, followed by subcutaneous injections of naloxone hydrochloride, 0.4 mg/kg. The naloxone-precipitated opiate abstinence syndrome was observed 1, 2, or 3 days after implantation of the pellet. Fourteen behavioral and physiological responses were monitored before and after precipitated withdrawal. The strains differed in activity rate and in the number of escape attempts and wet-dog shakes which occurred prior to naloxone-precipitated withdrawal. The strains differed in activity rate and in the number of escape attempts and wet-dog shakes which occurred prior to naloxone-precipitated withdrawal. Further strain differences were evident following naloxone-precipitated withdrawal in spite of statistical correction for prenaloxone differences. The differences were most evident in the frequency of escape attempts, wet-dog shakes, teeth-chattering episodes, and activity counts. After naloxone injection, responding changed depending upon the length of exposure to the implanted morphine pellet in four of the strains observed; length of exposure did not affect the response profile of Holtzman rats significantly. This study confirms the assumption that differences between strains are significant in terms of their responses to opiates, and emphasizes the need to investigate these differences in further detail.