Dopamine D1 and D2 antagonists block L-dopa-elicited air-stepping in neonatal rats

When administered to rat pups that have been suspended in air, L-3,4-dihydroxyphenylalanine (L-DOPA) induces a highly stereotypic locomotor response referred to as air-stepping. In order to determine the respective roles of dopamine D1 and D2 receptors in the expression of air-stepping, the abilities of the selective D1 antagonist SCH 23390 and the selective D2 antagonist spiperone to block L-DOPA-induced air-stepping in 5-day-old rat pups were assessed. Both antagonists increased latency to onset, and both decreased total duration of air-stepping by decreasing the number and length of air-stepping episodes. Neither SCH 23390 nor spiperone altered the topography of the air-stepping response; however, subjects receiving lower doses of spiperone spent significantly more time with the body dorsiflexed and limbs extended (extension/tremor activity) than did control subjects. The results suggest that both the D1 and D2 receptor subtypes are involved in the production of L-DOPA-induced locomotor activity.