Adenosine A1 receptors control dopamine D1-dependent [(3)H]GABA release in slices of substantia nigra pars reticulata and motor behavior in the rat.

Abnormalities in dopaminergic control of basal ganglia function play a key role in Parkinson's disease. Adenosine appears to modulate the dopaminergic control in striatum, where an inhibitory interaction between adenosine and dopamine receptors has been demonstrated. However the interaction has not been established in substantia nigra pars reticulata (SNr) where density of both receptors is high. Here we have explored the interaction between A1/D1 receptors in SNr. In SNr slices, SKF 38393, a selective D1 receptor agonist, produced a stimulation of depolarization-induced Ca(2+)-dependent [(3)H]GABA release that was inhibited by adenosine. The adenosine inhibition was abolished by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective adenosine A1 receptor antagonist. DPCPX per se enhanced GABA release, indicating inhibition of the release by endogenous adenosine. When D1 receptors were blocked with SCH 23390 or the slices were depleted of dopamine, the effect of DPCPX was suppressed, showing that activation of dopamine receptors was necessary for the adenosine inhibition. In normal slices, 2-chloro-n(6)-cyclopentyladenosine (CCPA), a selective A1 agonist, inhibited GABA release, but the inhibition was prevented by the blockade of D1 receptors with SCH 23390. Superperfusion with 8-bromo-cAMP produced a stimulation of GABA release that was not blocked by CCPA: this finding indicates that the blockade of D1 effects caused by activation of A1 receptors is specific. To see if these actions on GABA release were correlated with changes in motor behavior we studied the effect of unilateral intranigral injections of modifiers of adenosine A1 and dopamine D1 receptors in rats challenged with systemic methamphetamine. Both the A1 agonist CCPA and the D1 antagonist SCH 23390 produced ipsilateral turning whereas the A1 antagonist DPCPX caused contralateral turning. These motor effects are consistent with the findings on GABA release. The results indicate the presence of an inhibitory A1/D1 receptor interaction in SNr. The inhibition exerted by A1 adenosine receptors on GABAergic striatonigral transmission would be due exclusively to blockade of the facilitation resulting from activation of D1 dopamine receptors. The data permit to better understand the action of adenosine antagonists in the treatment of Parkinson's disease.