Selective breeding for high and low levels of opiate-induced analgesia in mice

Beginning with a genetically heterogeneous outbred stock of mice (Binghamton HET), selective breeding was conducted within two selected lines for a high analgesic (antinociceptive) response and a low analgesic response, respectively, to a narcotic analgesic (levorphanol tartrate) using the hot-plate test. A nonselected control line was also maintained concurrently. Four generations of predominantly mass selection have produced a marked divergence between the two oppositely selected lines, yielding a realized heritability (h²) of 0.32±0.05. The selection response was markedly asymmetrical, with a realized h² in the high direction of 0.45±0.10 and in the low direction of 0.12±0.07. These animals represent potentially useful subject material for research concerning the mechanisms and correlates of opiate-induced analgesia.