Figure 2. Log dose-response curves for reduction in tactile hyperesthesia produced by intrathecal injection of clonidine alone, MK-801 alone, and clonidine combined with a fixed dose (1 micro gram) of MK-801 (CLON*). Doses above 20 micro gram MK were not carried out because of motor weakness. The left shift of ED⁵⁰for CLON* than the ED⁵⁰for CLON, even though added the effect for 1 micro gram MK-801, indicates a supra-additive interaction. Each point represents the mean %MPE with SEM in five or six rats.