Effects of a Dopamine D3 receptor ligand, BP 897, on acquisition and expression of food-, morphine-, and cocaine-induced conditioned place preference, and food-seeking behavior in rats

The present study addressed the role of dopaminergic D-3 receptors (D3R) in motivational processes in rats. The effects of the selective D3R partial agonist, BP 897 (0.25-1 mg/kg, i.p.), on the establishment and the expression of conditioned place preference (CPP) supported by food, morphine (4 mg/kg, s.c.), or cocaine (2 mg/kg, s.c.) were investigated using an unbiased, one-compartment, place-conditioning procedure. When administered alone, BP 897 (0.05-2 mg/kg, i.p.) did not support CPP; on the contrary, conditioned place avoidance (CPA) was observed at 1 mg/kg, suggesting that this dose of BP 897 could be perceived as aversive. When given before each cocaine injection during the conditioning phase, BP 897 (1 mg/kg) prevented the establishment of CPP, and a single administration of BP 897 (0.5 and 1 mg/kg) before the test session impaired the expression of cocaine CPP. In contrast, neither the establishment nor the expression of food- and morphine-CPP were significantly altered by BP 897 (up to 1 mg/kg), whereas the full but less selective D-3/D2R agonists, 7-OH-DPAT (0.5-2 mug/kg, s.c.) and quinelorane (1 mug/kg, s.c.), prevented the acquisition of food CPP. In a within-session extinction schedule of lever pressing for food, BP 897 (0.06-2 mg/kg) was ineffective in potentiating response reinstatement induced by the noncontingent delivery of two food pellets, in contrast with quinelorane and 7-OH-DPAT where previous studies showed to be efficient in this respect (Duarte et al, 2003). These results indicate that BP 897 has no positive appetitive value on its own, and that a moderate degree of stimulation of D3R is not sufficient to modulate food-primed food-seeking behavior or alter incentive motivation for food, morphine, and/or their associated cues. However, D3R are likely involved in the perception of the rewarding value of cocaine and cocaine-paired cues. This suggests that the appetitive effects of cocaine are subserved by mechanisms different, at least in part, from those of morphine and food, and that D3R play a role only in the former.