G protein-coupled inward rectifier modulated by dopamine agonists in cultured substantia nigra neurons.

Dopamine release from dopaminergic neurons in the substantia nigra plays an important role in regulating their activity. In the present experiments the whole-cell clamp technique was used to elucidate the modulatory role of dopamine on K+ conductance in substantia nigra neurons in culture. In pars compacta neurons held near the resting potential, dopamine and quinpirole (a D2 receptor agonist) increased membrane conductance. This response was blocked by S(-)-sulpiride, a D2 receptor antagonist. The current-voltage relation of the quinpirole-induced current showed an inward rectification with a reversal potential that approximately equaled the K+ equilibrium potential. Intracellular application of a non-hydrolysable GTP analogue, guanosine 5'-O-(3-thiotriphosphate; 0.1-0.5 mM) into pars compacta neurons produced an initial spontaneous increase in an inwardly rectifying K+ conductance. Quinpirole markedly accelerated this conductance increase. In cells loaded with a poorly metabolized GDP analogue, guanosine 5'-O-(2-thiodiphosphate), the quinpirole response was diminished. Treatment of pars compacta neurons with pertussis toxin abolished the D2 response. Intracellular application of cyclic AMP (1 mM) neither mimicked nor occluded the D2 response. These results indicate that D2 receptor agonists produce neuronal inhibition by activating an inwardly rectifying K+ current, that this D2 agonist effect is mediated by a pertussis toxin-sensitive G protein, and that cyclic AMP does not mediate the D2 response. Unexpectedly, in cells loaded with guanosine 5'-O-(3-thiotriphosphate) after the inwardly rectifying K+ conductance spontaneously reached a maximum, it started to decline slowly. In cells loaded with guanosine 5'-O-(2-thiodiphosphate), the quinpirole response became diminished at first, but then the membrane conductance slowly increased, together with an increase in the quinpirole response. These results were explained by an interplay of two G proteins, one producing an increase and the other producing a decrease in the K+ conductance. In some pars reticulata neurons, R(+)-SKF-38393 (a D1 receptor agonist) reduced an inwardly rectifying K+ conductance; this would excite the neuron under current clamp conditions. The present study shows that cultured substantia nigra neurons produce functional responses to dopamine agonists. Using these neurons, the ionic mechanisms and signal transduction mechanisms of the responses were elucidated in a more critical manner than in previous studies.