Fig. 3. Low levels of β-endorphin are not compensated by other endogenous opioids.

(A to C) Plasma levels of dynorphin, enkephalin, and endomorphin are unchanged in red versus black mice. (D and E) Loss of the dynorphin gene (Dyn^−/−) has no effect on nociceptive thresholds in red or black mice. Graphs show means ± SEM; numbers in parentheses = n per group.