In morphinised rats SKF 38393 converts dopamine D2 receptor-mediated forward locomotion into backward walking

The behavioural responses to RU 24213 (D2 agonist) and SKF 38393 (D1 agonist) were studied in the morphine cataleptic rat. D2 stimulation evoked slow forward walking coupled with head-down sniffing that was blocked both by SCH 23390 (D1 antagonist) and metoclopramide (D2 antagonist). D1 stimulation was without effect by itself, but when administered together with RU 24213 it reversed the direction of walking and initiated licking and chewing. Backward walking was attenuated by metoclopramide and restored to forward locomotion by SCH 23390. These data further show that D1 receptors exert an important modulatory influence on motor behaviours mediated by the D2 site.