Deletion of dopamine D1 and D3 receptors differentially affects spontaneous behaviour and cocaine-induced locomotor activity, reward and CREB phosphorylation

Co-localization of dopamine D1 and D3 receptors in striatal neurons suggests that these two receptors interact at a cellular level in mediating dopaminergic function including psychostimulant-induced behaviour. To study D1 and D3 receptor interactions in cocaine-mediated effects, cocaine-induced locomotion and reward in mice lacking either D1, D3 or both receptors were analysed. Spontaneous locomotor activity was increased in D1(-/-) and D1(-/-)D3(-/-) mice and D1(-/-)D3(-/-) mice did not exhibit habituation of spontaneous rearing activity. Cocaine (20 mg/kg) increased locomotor activity in wild-type and D3(-/-) mice, failed to stimulate activity in D1(-/-) mice and reduced activity in D1(-/-)D3(-/-) mice. In the conditioned place preference, all groups exhibited reward at 5, 10 and 20 mg/kg of cocaine. D1(-/-)D3(-/-) mice did not demonstrate preference at 2.5 mg/kg of cocaine although preference was observed in wild-type, D1(-/-) and D3(-/-) mice. The transcription factor cAMP-responsive element binding protein (CREB) is activated by phosphorylation in striatal regions following dopamine receptor activation. Striatal pCREB levels following acute cocaine were increased in wild-type and D3(-/-) mice and decreased in D1(-/-) and D1(-/-)D3(-/-) mice. After repeated administration of 2.5 mg/kg of cocaine, D1(-/-) mice had lower pCREB levels in caudate-putamen and nucleus accumbens. Our findings suggest that, although spontaneous and cocaine-induced horizontal activity depended mainly on the presence of the D1 receptor, there may be crosstalk between D1 and D3 receptors in rearing habituation and the perception of cocaine reward at low doses of the drug. Furthermore, alterations in pCREB levels were associated with changes in cocaine-induced locomotor activity but not reward.