Cholera Toxin Induces Cyclic AMP‐Independent Down‐Regulation of G sα and Sensitization of α 2 ‐Autoreceptors in Chick Sympathetic Neurons

The role of the stimulatory GTP‐binding protein (G S ) in the α 2 ‐autoinhibitory modulation of noradrenaline release was investigated in cultured chick sympathetic neurons. The α 2 ‐adrenoceptor agonist UK 14,304 caused a concentration‐dependent reduction of electrically evoked [ 3 H]noradrenaline release with half‐maximal effects at 14.0 ± 5.5 n M . In neurons treated with 100 ng/ml cholera toxin for 24 h, the half‐maximal concentration was lowered to 3.2 ± 1.4 n M without changes in the maximal effect of UK 14,304. The pretreatment with cholera toxin also increased the inhibitory action of 10 n M UK 14,304 when compared with the inhibition of noradrenaline release in untreated cultures derived from the same cell population. In cultures treated with either 10 µ M forskolin or 100 µ M 8‐bromo‐cyclic AMP, neither the half‐maximal concentration nor the maximal effect of UK 14,304 was altered. Cholera toxin, forskolin, and 8‐bromo‐cyclic AMP all induced an increase in spontaneous outflow and a reduction in electrically evoked overflow, effects not observed after a pretreatment with dideoxyforskolin. Exposure of neurons to cholera toxin, but not to forskolin or 8‐bromo‐cyclic AMP, induced a translocation of α‐subunits of G s (G sα ) from particulate to soluble fractions and led ultimately to a complete loss of G sα from the neurons. In contrast, no effect was seen on the distribution of either α‐subunits of G i ‐ or G o ‐type G proteins or of β‐subunits. These results indicate that cholera toxin causes a selective, cyclic AMP‐independent down‐regulation of G sα . This down‐regulation of G sα is associated with the sensitization of α 2 ‐autoreceptors.