Fig. 2. Dose–effect relation of cervicomedullary intrathecal morphine during phase 1 (closed triangle) and phase 2 (closed square). Data are presented as the mean ± SEM for 6–8 animals. Values statistically different from vehicle are indicated for P < 0.05 (*). Dashed lines were calculated by least-squares linear regression. Data on the y-axis were converted to percentage reduction of the saline response by the following formula:[1 − (drug response/saline response)]× 100.