The lack of A2A adenosine receptors diminishes the reinforcing efficacy of cocaine

Adenosine is an endogenous purine nucleoside, which acts as a neuromodulator in the central nervous system. A(2A) adenosine and D-2 dopamine receptors are colocalized in the same neurons in discrete brain areas, and the dopaminergic transmission plays a crucial role in the addictive properties of drugs of abuse, such as cocaine. In the present study, we have investigated the specific role of A(2A) adenosine receptors in cocaine-induced behavioral responses related to its addictive properties. For this purpose, we have evaluated the acute locomotor effects produced by cocaine and the development of locomotor sensitization by repeated cocaine administration. In addition, we have also examined cocaine acute rewarding properties using the conditioned place preference. Finally, we used the intravenous drug self-administration paradigm to investigate the acquisition of an operant response maintained by cocaine self-administration and the reinforcing efficacy of the drug in these knockout animals. Acute cocaine induced a similar increase of locomotor activity in mice lacking A(2A) adenosine receptors and wild-type littermates. Cocaine-induced locomotor sensitization and conditioned place preference were also maintained in A(2A) knockout mice. Nevertheless, these knockout mice showed a lower rate of cocaine self-administration than wildtype mice in both fixed ratio 1 and 3 schedules of reinforcement. Moreover, a reduction in the maximal effort to obtain a cocaine infusion was found in A(2A) knockout mice under a progressive ratio schedule. In addition, a vertical shift of the cocaine dose-response curve was observed in mice lacking A(2A) adenosine receptors in comparison with wild-type littermates. Our study demonstrates that A(2A) adenosine receptors play an important role in cocaine addictive properties, and these receptors seem to be required to develop the addictive effects of this drug.