Abstract
Adenosine A 2A receptors are preferentially expressed in rat striatum, where they are concentrated in dendritic spines of striatopallidal medium spiny neurons and exist in a heteromeric complex with D 2 dopamine (DA) receptors. Behavioral and biochemical studies indicate an antagonistic relationship between A 2A and D 2 receptors. Previous studies have demonstrated that food-restricted (FR) rats display behavioral and striatal cellular hypersensitivity to D 1 and D 2 DA receptor stimulation. These alterations may underlie adaptive, as well as maladaptive, behaviors characteristic of the FR rat. The present study examined whether FR rats are hypersensitive to the A 2A receptor agonist, CGS-21680. In Experiment 1, spontaneous horizontal motor activity did not differ between FR and ad libitum fed (AL) rats, while vertical activity was greater in the former. Intracerebroventricular (i.c.v.) administration of CGS-21680 (0.25 and 1.0 nmol) decreased both types of motor activity in FR rats, and returned vertical activity levels to those observed in AL rats. In Experiment 2, FR rats given access to a running wheel for a brief period outside of the home cage rapidly acquired wheel running while AL rats did not. Pretreatment with CGS-21680 (1.0 nmol) blocked the acquisition of wheel running. When administered to FR subjects that had previously acquired wheel running, CGS-21680 suppressed the behavior. In Experiment 3, CGS-21680 (1.0 nmol) activated both ERK 1/2 and CREB in caudate-putamen with no difference between feeding groups. However, in nucleus accumbens (NAc), CGS-21680 failed to activate ERK 1/2 and selectively activated CREB in FR rats. These results indicate that FR subjects are hypersensitive to several effects of an adenosine A 2A agonist, and suggest the involvement of an upregulated A 2A receptor-linked signaling pathway in NAc. Medications targeting the A 2A receptor may have utility in the treatment of maladaptive behaviors associated with FR, including substance abuse and compulsive exercise.
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