Fig. 3.

A fixed dose analysis (A) for the interaction between intrathecal gabapentin (300 µg) and MK 801 during phase 1 and isobologram (B) for the interaction between gabapentin and MK 801 during phase 2 in the formalin test. Data for a fixed dose analysis are presented as the sum of flinches. At a fixed dose analysis, addition of gabapentin to MK 801 was significantly different from that of MK 801 alone, which means that gabapentin increased the antinociception of MK 801. At an isobologram, the ED₅₀ values for each agent are plotted on the x- and y-axes, respectively, and the thick lines represent the S.E.M. of the ED₅₀. The straight line connecting each ED₅₀ value is the theoretical additive line and the point on this line is the theoretical additive ED₅₀. The experimental ED₅₀ point was significantly different from the theoretical ED₅₀ point, indicating a synergistic interaction. Each line represents the mean±SEM of 5-8 rats. Compared with MK 801, ^*p<0.01, ^†p<0.001.