Multiple intramembrane receptor-receptor interactions in the regulation of striatal dopamine D2 receptors.

Adenosine A2A, group I mGlu and neurotensin receptors have been previously found to modulate the binding characteristics of dopamine D2 receptors in membrane preparations from rat striatum. In the present study it is shown that stimulation of different combinations of striatal A2A, group I mGlu and neurotensin receptors induce different effects on the modulation of D2 receptor binding to those obtained when they are separately stimulated using maximal effective concentrations. In competitive inhibition experiments of dopamine versus the D2 receptor antagonist [3H]raclopride the addition of the A2A receptor agonist CGS 21680, the group I mGlu receptor agonist DHPG or neurotensin induced a decrease in the affinity of the high affinity state of the dopamine D2 receptors for dopamine. When added together CGS 21680 plus neurotensin induced the same effect as when administered alone, CGS 21680 plus DHPG induced a synergistic effect and DHPG plus neurotensin lost their modulating effect on D2 receptor binding. These results demonstrate the existence of multiple intramembrane receptor-receptor interactions in the regulation of striatal D2 receptors.