Figure 3. Drug injection effects on pain.

A) Blue circles represent sites where local injection of opioid agonists reduces pain in rodents in at least one study, as measured by either reduction in pain-related behaviors or enhanced latency to escape from a painful stimulus. Gray circles represent regions that are involved in pain modulation where we failed to find studies testing the effect of local microinjections of opioid agonists on analgesia. B) Green circles represent sites where local injection of cannabinoid agonists reduces pain in rodents in at least one study, as measured by either reduction in pain-related behaviors or enhanced latency to escape from a painful stimulus. Gray circles represent regions that are involved in pain modulation where we failed to find studies testing the effect of local microinjections of cannabinoid agonists on analgesia. Black circles indicate that every reviewed study found no effect of local injection of cannabinoid agonists within this region on analgesia.