Cholera Toxin Induces Cyclic AMP‐Independent Down‐Regulation of Gsα and Sensitization of α2‐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 nM. In neurons treated with 100 ng/ml cholera toxin for 24 h, the half-maximal concentration was lowered to 3.2 ± 1.4 nM without changes in the maximal effect of UK 14,304. The pretreatment with cholera toxin also increased the inhibitory action of 10 nM 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 μM forskolin or 100 μM 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 trans-location 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.