Norepinephrine inhibits a Ca 2+ current in rat sympathetic neurons via a G-protein

Abstract

The effects of norepinephrine on a Ca 2+ current from acutely isolated and short-term (24 h) cultured adult rat superior cervical ganglion neurons were studied using the whole-cell variant of the patch-clamp technique. Norepinephrine produced a rapid, reversible and concentration-dependent reduction of the Ca 2+ current. Accurately timed applications of norepinephrine (3 μM) showed that the development of Ca 2+ current inhibition was delayed by up to 11 s after application of norepinephrine. Internal 500 μM guanylyl-imidodiphosphate (Gpp(NH)p) or guanosine-5′-O-(3-thiotriphosphate) (GTP-γ-S) decreased the Ca 2+ current amplitude and induced a biphasic rising phase of the Ca 2+ current. Under these conditions, the reduction of Ca 2+ current amplitude by 3 μM norepinephrine was virtually abolished when compared with cells dialysed with GTP-containing internal solutions. Internal dialysis with solutions containing 2 mM guanosine-5′-O-(2-thiodiphosphate) (GDP-β-S) increased the Ca 2+ current amplitude and reduced the inhibition produced by 3 μM norepinephrine compared to cells dialysed with control internal solution. Treatment with 200 ng/ml pertussis toxin for 12–16 h greatly reduced the norepinephrine-induced Ca 2+ current inhibition. Internal dialysis with solutions containing 500 μM cyclic adenosine 3′,5′-monophosphate (cyclic AMP) and 3-isobutyl-1-methylxanthine had no significant effect on either the Ca 2+ current inhibition by norepinephrine or the Ca 2+ current amplitude. These results suggest that norepinephrine blocks a Ca 2+ current in adult rat superior cervical ganglion neurons via a pertussis toxin-sensitive G-protein which is independent of intracellular cyclic AMP.