Regulation of hind-limb tone by adenosine A2A receptor in rats

Adenosine A2A receptor agonists produce a hypokinetic state (catalepsy) that is believed to reflect antagonistic interaction of A2A and dopamine D2 receptors in the basal ganglia. In addition to catalepsy, pharmacological blockade of D2 receptors produces rigidity. However there are conflicting data about the effect of A2A agonists on muscle tone, with some reports indicating an increase, while other data suggest that A2A catalepsy is dominated by muscle hypotonia. We investigated the effect on resistance to imposed movements of systemic cataleptic doses of the selective A2A agonist CGS21680 (5 mg/kg), and compared it with the effect of the D2 antagonist raclopride (5 mg/kg), in rats. Total resistance is made up of elastic and viscous components. The elastic component is velocity independent, and is referred to as "stiffness," whereas viscosity, which dampens responses to imposed movements, is velocity dependent. Using a method for quantifying total joint resistance that enabled separate identification of stiffness and viscosity, we found that during catalepsy evoked by either drug there was a clear increase in joint rigidity. Both CGS21680 and raclopride significantly increased joint stiffness, the velocity independent component of rigidity that is most affected in Parkinsonism. In contrast, the effect of CGS21680 on the velocity-dependent viscosity component was less robust than for raclopride, and did not reach significance, possibly reflecting an interaction with sedative effects via extrastriatal receptors. The effect of CGS21680 and raclopride on joint stiffness is thus consistent with previous findings suggesting functional antagonism of A2A and D2 receptors in the basal ganglia.