Figure 4. Dose-effect relations and regression lines for clonidine, morphine, [D-Pen², D-Pen⁵] enkephalin (DPDPE), clonidine plus morphine, and clonidine plus DPDPE 15 min after administration (deemed as the peak effect times for drugs used alone). Each point and bar represent the mean value and SEM. Doses are plotted in log scale; for combinations, combined total doses are plotted on the x-axis. By least-squares regression analysis, doses for 50% maximum possible effect were calculated as follows: 6.2 micro gram for clonidine alone, 2.3 micro gram for morphine alone, 16.4 for DPDPE alone, 0.9 micro gram (0.6 micro gram clonidine plus 0.3 micro gram morphine) for the clonidine-morphine combination, and 6.8 micro gram (1.9 clonidine plus 4.9 DPDPE) for the clonidine-DPDPE combination.