Abstract

The complex effect of lobeline on [³H]norepinephrine ([³H]NE) release was investigated in this study. Lobeline-induced release of [³H]NE from the vas deferens was strictly concentration-dependent. In contrast, electrical stimulation-evoked release was characterized by diverse effects of lobeline depending on the concentration used: at lower concentration (10 μM), it increased the release and at high concentration (100 and 300 μM), the evoked release of [³H]NE was abolished. The effect of lobeline on the basal release was [Ca²⁺]-independent, insensitive to mecamylamine, a nicotinic acetylcholine receptor antagonist, and to desipramine, a noradrenaline uptake inhibitor. However, lobeline-induced release was temperature-dependent: at low temperature (12°C), at which the membrane carrier proteins are inhibited, lobeline failed to increase the basal release. Lobeline dose dependently inhibited the uptake of [³H]NE into rat hippocampal synaptic vesicles and purified synaptosomes with IC₅₀ values of 1.19 ± 0.11 and 6.53 ± 1.37 μM, respectively. Lobeline also inhibited Ca²⁺ influx induced by KCl depolarization in sympathetic neurons measured with the Fura-2 technique. In addition, phenylephrine, an α₁-adrenoceptor agonist, contracted the smooth muscle of the vas deferens and enhanced stimulation-evoked contraction. Both effects were inhibited by lobeline. Our results can be best explained as a reversal of the monoamine uptake by lobeline that is facilitated by the increased intracellular NE level after lobeline blocks vesicular uptake. At high concentrations, lobeline acts as a nonselective Ca²⁺ channel antagonist blocking pre- and postjunctional Ca²⁺ channels serving as a counterbalance for the multiple transmitter releasing actions.