CHOLERA-TOXIN INDUCES CYCLIC AMP-INDEPENDENT DOWN-REGULATION OF G(S-ALPHA) AND SENSITIZATION OF ALPHA(2)-AUTORECEPTORS IN CHICK SYMPATHETICNEURONS

The role of the stimulatory GTP-binding protein (G(s)) in the alpha(2) -autoinhibitory modulation of noradrenaline release was investigated i n cultured chick sympathetic neurons. The alpha(2)-adrenoceptor agonis t UK 14,304 caused a concentration-dependent reduction of electrically evoked [H-3]noradrenaline release with half-maximal effects at 14.0 /- 5.5 nM. In neurons treated with 100 ng/ml cholera toxin for 24 h, t he half-maximal concentration was lowered to 3.2 +/- 1.4 nm without ch anges in the maximal effect of UK 14,304. The pretreatment with choler a toxin also increased the inhibitory action of 10 nm UK 14,304 when c ompared with the inhibition of noradrenaline release in untreated cult ures derived from the same cell population. In cultures treated with e ither 10 mu M forskolin or 100 mu M 8-bromo-cyclic AMP, neither the ha lf-maximal concentration nor the maximal effect of UK 14,304 was alter ed. Cholera toxin, forskolin, and 8-bromo-cyclic AMP all induced an in crease in spontaneous outflow and a reduction in electrically evoked o verflow, effects not observed after a pretreatment with dideoxyforskol in. Exposure of neurons to cholera toxin, but not to forskolin or 8-br omo-cyclic AMP, induced a translocation of alpha-subunits of G(s) (G(s alpha)) from particulate to soluble fractions and led ultimately to a complete loss of G(s alpha) from the neurons. In contrast, no effect was seen on the distribution of either cr-subunits of G(i)- or G(o)-ty pe G proteins or of beta-subunits. These results indicate that cholera toxin causes a selective, cyclic AMP-independent downregulation of Gs alpha. This down-regulation of Gs alpha is associated with the sensit ization of alpha(2)-autoreceptors.