Norepinephrine Blocks a Calcium Current of Adult Rat Sympathetic Neurons Via an Alpha 2-adrenoceptor

The effects of alpha-adrenoceptor agonist and antagonist drugs on the Ca2+ current of acutely isolated adult rat superior cervical ganglion (SCG) neurons were investigated to characterize the adrenoceptor which mediates a catecholamine-induced decrease of the Ca2+ current. Ca2+ currents were recorded using the whole-cell variant of the patch-clamp technique from neurons isolated enzymatically from adult rat SCG. Norepinephrine (1 microM) produced a rapid, reversible, and concentration-dependent decrease in Ca2+ current amplitude and slowed the rising phase of the Ca2+ current. These effects could be mimicked by clonidine (1 microM), an alpha 2-agonist but not by the alpha 1-agonist phenylephrine (1 microM). The norepinephrine-induced decrease in Ca2+ current amplitude was attenuated in the presence of idazoxan (1 microM), an alpha 2-antagonist, but was unaffected in the presence of the alpha 1-antagonist prazosin (1 microM). Neither antagonist displayed any Ca2+ current blocking activity. These results suggest that the alpha-receptor which mediates the norepinephrine-induced decrease of the Ca2+ current in adult rat SCG neurons is of the alpha 2-subtype.