Agonist-dependent attenuation of GTPase activity of G proteins coupled to dopamine receptors in the molluscan nervous system

LowK _m GTPase activity was studied in the purified membranes from theLymnaea stagnalis CNS. Stimulation of GTPase activity of G proteins by dopamine in membrane preparations is transient; it is followed by reduction of the activity. Magnesium ions regulate activity of G proteins; thus, the value of Mg²⁺] _i is an important factor to be taken into account in investigations of functioning of G proteins. In order to determine the mean physiological level of the neuronal Mg²⁺] _i , neuronal elements of the molluscan CNS were separated from other cell types. The values of 1.16±0.19 and 1.49±0.08 mM were obtained for Mg²⁺] _i of the neuronal cells with Mag-fura-2 and Mag-fura-red, respectively. An inhibitory effect of dopamine was displayed within the range of free Mg²⁺ concentrations from 3 μM to 3 mM. GTPase activity was stimulated with Na⁺ in a dose-dependent manner. However, GTPase activity became independent of Na⁺ in the presence of dopamine. Replacing of ATP by 5′-adenylyl imidodiphosphate suppressed the inhibitory effect of dopamine, but did not completely prevent the dopamine influence. The effect of dopamine was dose-dependent and did not decrease after PTX treatment. The inhibitory influence of dopamine on the GTPase activity was reversed by a selective antagonist of the dopamine D₂ receptors, S(−)-sulpiride.