Stimulant-conditioned locomotion is not affected by blockade of D1 and/or D2 dopamine receptors during conditioning

A series of experiments were conducted to investigate the role of dopamine (DA) D₁ and D₂ receptor subtypes in stimulant-conditioned locomotion in rats. Expt. 1 demonstrated that locomotion could be induced by a testing situation when that situation was previously paired with (+)-amphetamine (1.5 mg/kg, s.c.) or a D₂ receptor selective agonist (PHNO, 15 or 30 μg/kg, s.c.), but not when the drug treatments were given 3 h after exposure to the situation. The selective D₂ receptor antagonist, haloperidol (50 μg/kg, i.p.), and the D₁ receptor antagonist, SCH 23390 (20 μg/kg, s.c.), blocked amphetamine-induced locomotion during the pairing process, but failed to block amphetamine-conditioned locomotion as assessed during a drug-free test in Expt. 2. This was true when the antagonists were given separately or together. The results of Expts. 3 and 4 showed that doses of the D₁ (20 μg/kg, s.c.) and D₂ antagonist (250 μg/kg, i.p.) that blocked the unconditioned locomotor effects of PHNO failed to block its conditioned locomotion. It is concluded that neither D₁ nor D₂ DA receptors are essential for the development of stimulant-conditioned locomotion.