Lobeline and nicotine evoke [³H]overflow from rat striatal slices preloaded with [³H]dopamine: Differential inhibition of synaptosomal and vesicular [³H]dopamine uptake

Lobeline is currently being developed as a substitution therapy for tobacco smoking cessation. Activation of CNS dopamine (DA) systems results in the reinforcing properties of nicotine. The present study compared the effects of lobeline and nicotine on rat striatum. Both lobeline and nicotine evoked [³H]overflow from striatal slices superfused in the presence of pargyline and nomifensine in the buffer. Marked DA depletion (42-67%) and a concomitant 2-fold increase in dihydroxyphenylacetic acid (DOPAC) in spices superfused with high concentrations (30-100 μM) of lobeline were observed. The effect of nicotine (10μM) was inhibited in a concentration-dependent manner by mecamylamine (1-100 μM). However, lobeline (0.1-100 μM)evoked [³H]overflow was calcium-independent, and was not antagonized by mecamylamine (1-100 μM), suggesting a mechanism of action other than stimulation of nicotinic receptors. Lobeline inhibited [³H]DA uptake into synaptosomes (IC₅₀ = 80 ± 12 μM) and vesicles (IC₅₀ = 0.88 ± 0.001 μM), whereas nicotine (≤100 μM) did not inhibit synaptosomal or vesicular [³H]DA uptake. In the absence of pargyline and nomifensine in the buffer, endogenous DA was detected in superfusate only in those slices exposed to the highest concentration (100 μM) of lobeline. However, endogenous DOPAC concentration was increased in a concentration-dependent manner, indicating that lobeline exposure resulted in increased cytosolic DA which was rapidly metabolized to DOPAC. Under these conditions, lobeline (10-100 μM) also significantly depleted (66-85%) DA content; however, no change in DOPAC content was observed. The results suggest that, unlike nicotine, lobeline increases DA release by potent inhibition of DA uptake into synaptic vesicles, and a subsequent alteration in presynaptic DA storage.