alpha 2-Adrenoreceptor-mediated inhibition of acetylcholine-induced noradrenaline release from rat sympathetic neurons: an action at voltage-gated Ca2+ channels.

[3H]Noradrenaline release was studied in cultured sympathetic neurons derived from superior cervical ganglia of neonatal rats. Acetylcholine elicited a concentration- and time-dependent increase in 3H outflow which was half-maximal at about 300 microM and within 5 s. The overflow induced by 10 s exposure to 300 micro A acetylcholine was reduced by the nicotinic antagonist hexamethonium, but increased by the muscarinic antagonist atropine. Cd2+ (300 microM) prevented the overflow evoked by electrical field stimulation, but reduced acetylcholine-induced overflow by less than 50%. Removal of extracellular Ca2+ abolished stimulation-evoked tritium overflow irrespective of the stimulus. The selective alpha2-adrenoceptor agonist UK 14,304 inhibited acetylcholine-evoked overflow to a significantly smaller extent (approximately 25% maximal inhibition) than electrically induced overflow ( > or = 45% maximal inhibition). These inhibitory effects were antagonized by the alpha2-adrenoceptor antagonist yohimbine. Noradrenaline (0.1 microM) reduced acetylcholine-evoked overflow to the same extent as did UK 14,304 (0.1 microM). UK 14,304 had no effect when 3H overflow was evoked by acetylcholine in the presence of 300 microM Cd2+. Currents through nicotinic acetylcholine receptors and voltage-activated Ca2+ currents were studied with the whole-cell variant of teh patch-clamp technique. UK 14,304 reduced nicotinic acetylcholine receptor currents and voltage-activated Ca2+ currents with similar potency and efficacy. Yohimbine, however, antagonized only the inhibition of voltage-activated Ca2+ currents, but not the effects of UK 14,304 on nicotinic receptor currents. Furthermore, yohimbine per se reduced currents through nicotinic receptors. Noradrenaline (10 microM) inhibited voltage-dependent Ca2+ currents just as did UK 14,304 (10 microM), but failed to reduce currents through nicotinic acetylcholine receptor channels. Cd2+ (300 microM) abolished voltage-activated Ca2+ currents and reduced nicotinic acetylcholine receptor currents by 65%. These results indicate that acetylcholine evokes noradrenaline release from rat sympathetic neurons by activation of nicotinic receptors and restricts this release via muscarinic receptors. The acetylcholine-induced transmitter release is based on two mechanisms, one involving and the other one bypassing voltage-dependent Ca2+ channels. alpha2-Adrenoceptor activation reduces voltage-activated Ca2+ currents and effects exclusively the component of acetylcholine-induced release which involves voltage-dependent Ca2+ channels. These results support the hypothesis that voltage-activated Ca2+ channels are the sole site of autoinhibitory alpha2-adrenergic effects on transmitter release from rat sympathetic neurons. The inhibitory effects of alpha2-adrenoceptor agonists and antagonists on currents through nicotinic acetylcholine receptors are not mediated by an alpha2-adrenoceptor.