Membranes with dopamine receptors coupled to G proteins: Purification from theLymnaea central nervous system

Abstract

A membrane fraction, which contained dopamine receptors and heterotrimeric G proteins, was purified from homogenate of molluscan (Lymnaea) CNS tissues. Radioligand binding analysis with the use of [7.8-3H] dopamine detected the presence of a high-affinity binding site in this fraction. [7.8-3H] Dopamine was displaced in a dose-dependent manner by dopamine antagonists, S(-)-sulpiride, (±)-SKF83566, and fluphenazine. Radioligand binding analysis of purified membranes with the use of labelled GDP showed the presence of a high affinity binding site withBmax=92±5 pmol/mg of protein andKd=64±10 nM. GDP, in contrast to GTP, markedly increased [7.8-3H] dopamine binding in the absence of metal cations (the maximum increase was 2.5-fold). Added separately, Na and Mg ions decreased the stimulatory influence of GDP. Jointly, these ions completely abolished this GDP influence on the [7.8-3H] dopamine binding. In the membrane fraction, GTPase activity in the presence of dopamine increased during an initial period and then decreased below the basal level. Therefore, we have demonstrated that in our experiments dopamine receptors in the purified membrane fraction are functionally coupled with heterotrimeric G proteins, but their interaction displays some specific features.