Eighteen inbred strains were made physically dependent on morphine, an archetypical opioid narcotic, by administration over a 4-day period. Then, the jumping behavior precipitated by the administration of an opioid receptor antagonist (naloxone) to the dependent mice was measured. There were very large interstrain differences in the withdrawal-induced jumping behavior; SM/J mice averaged only one jump in the 15-min period following naloxone administration, whereas BUB/BnJ mice jumped over 100 times under the same conditions (Fig. 1). Other investigators [8] evaluated eight of these strains by a similar method, and a comparative Pearson's correlation analysis of the data for the eight common strains indicated that two studies yielded similar results (coefficient of 0.87, P = 0.05).
