Norepinephrine mediated uncoupling of alpha1-adrenergic receptors from phosphoinositide hydrolysis is rapid and separate from the down-regulation of the receptors

The authors have studied the regulation of the coupling of alpha1 receptor binding sites to inositide hydrolysis in primary neuronal cultures prepared from the brain stem and hypothalamus of neonatal Wistar-Kyoto rats. Cultures were exposed to norepinephrine (NE) and the NE stimulated phosphatidylinositol (PI) response or binding of [125I]HEAT to alpha1 recognition sites was determined. Exposure of neurons to 10 μM NE for 2 hr decreases stimulated PI hydrolysis by 70-80%, whereas alpha1 receptor number and affinity are unchanged at this time. Further stimulation of the cultures results in a down-regulation of receptor number with a maximal loss of 50% binding sites after a 24 hr pretreatment. The uncoupled NE stimulated PI response after a 2 hr pretreatment with NE returns to 80-90% of control 4 hr after removal of the NE. Down-regulated alpha1receptor binding sites after 24 hr of pretreatment with NE return after a 72 hr washout period, and the effect is blocked by cycloheximide (3.6 μM). These results suggest that stimulation of alpha1 receptors causes a rapid uncoupling of the receptor from PI hydrolysis and that the response returns to normal in a matter of hours. In contrast, the return of receptors following down-regulation requires much longer times and appears to involve other mechanisms