D1-like and D2-like dopamine receptors have synergistic and antagonistic effects on behavior. To understand the mechanisms underlying these effects, we studied dopamine signaling genetically in Caenorhabditis elegans. Knocking out a D2-like receptor, DOP-3, caused locomotion defects similar to those observed in animals lacking dopamine. Knocking out a D1-like receptor, DOP-1, reversed the defects of the DOP-3 knockout. DOP-3 and DOP-1 have their antagonistic effects on locomotion by acting in the same motor neurons, which coexpress the receptors and which are not postsynaptic to dopaminergic neurons. In a screen for mutants unable to respond to dopamine, we identified four genes that encode components of the antagonistic Go and Gq signaling pathways, including Go itself and two subunits of the regulator of G protein signaling (RGS) complex that inhibits Gq. Our results indicate that extrasynaptic dopamine regulates C. elegans locomotion through D1- and D2-like receptors that activate the antagonistic Gq and Go signaling pathways, respectively.
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