Fig. 1. Effect of cervicomedullary intrathecal injection of morphine (inverted open triangle = 30 μg; closed triangle = 10 μg, open diamond = 6 μg, inverted closed triangle = 3 μg, open triangle = 1 μg) or vehicle (closed square) on formalin-induced behaviors. Data are presented as the mean ± SEM for 6–8 animals for morphine and 12 animals for vehicle and represent the duration of time rubbing the vibrissal pad versus  time (0–45 min) after formalin injection. Designation of statistical significance for data points (see Results) was omitted for illustrative clarity.