Fig. 4. Moxonidine and morphine synergize to alleviate mechanical hyperalgesia. Dose–response curves for moxonidine, morphine, and moxonidine–morphine administered intrathecally separately and in combination. (A ) Dose–response curves of the spinal antihyperalgesic effect of moxonidine (open circles, solid lines, ED50: 14 pmol, 4.1–50), morphine (open triangles, dashed lines, ED50: 64 pmol, 30–135), moxonidine in the presence of morphine (closed circles, solid lines, ED50: 0.3 pmol, 0.17–0.43), and morphine in the presence of moxonidine (closed triangles, dashed lines, ED50: 1.2 pmol, 0.7–1.7). (B ) Isobolographic representation of the antihyperalgesic (percent inhibition) effect of the combination of moxonidine–morphine in nerve-injured mice. Drug interactions may be illustrated through construction of such isobolograms. The ED50values of clonidine or moxonidine and morphine are respectively plotted as the y - and x -axis intercepts. The thicker lines directed from each ED50value toward zero represent the respective lower confidence limits of each ED50value. The straight line connecting these two points is the theoretical additive line. The open circle that lies on or near the theoretical additive line represents the calculated theoretical ED50value of the combination where the interaction is additive. The closed circle represents the experimentally observed ED50value of the combination of clonidine–morphine. If the interaction is synergistic, the closed circle will be plotted significantly below the theoretical additive line and outside the lower confidence limits of ED50values of clonidine and morphine. In this isobologram, the ED50value of the combination of clonidine–morphine is significantly lower than that of the theoretical additive ED50value and is synergistic.