Strains exhibit divergent morphine-dependent analgesic and allodynic/hyperalgesic profiles with corresponding changes in <i>MOR-1K</i> gene expression.

<p>(A) A timeline of chronic morphine administration and the assessments of pain behavior and gene expression are shown. (B-E) Compared to C57BL/6J, 129S6 mice exhibit increased paw withdrawal thresholds and fail to develop allodynia or hyperalgesia. In contrast, CXB7/ByJ exhibit decreased paw withdrawal thresholds and increased allodynia and hyperalgesia. (F) 129S6 mice exhibit decreased <i>MOR-1K</i> gene expression levels on days 1 and 4, corresponding to their analgesic behavioral profile. In contrast, CXB7/ByJ mice exhibit increased <i>MOR-1K</i> gene expression levels on day 4, corresponding to their hyperalgesic behavioral profile. For behavioral graphs, blue background indicates a decrease in pain sensitivity (analgesia), and red background indicates an increase in pain sensitivity (allodynia/hyperalgesia). Panels B-E: <i>N</i> = 15-16/group. Data expressed as mean ± SEM. <i>***p<</i>0.001, <i>**p<</i>0.01, *<i>p</i><0.05 different from C57BL/6J. ^###<i>p<</i>0.001, ^##<i>p</i><0.01, ^#<i>p</i><0.05 different from 129S6. Panel F: <i>N</i> = 7/group. Data expressed as means ± SEM. *<i>p</i><0.05 different from baseline.