Lobeline displaces [³H] dihydrotetrabenazine binding and releases [³H]dopamine from rat striatal synaptic vesicles: Comparison with d- amphetamine

Lobeline, an alkaloid from Indian tobacco (Lobella inflata), is classified as a nicotinic agonist and is currently used as a smoking cessation agent. However, our previous in vitro studies demonstrate that lobeline does not act as a nicotinic agonist but alters presynaptic dopamine (DA) storage by potently inhibiting DA uptake into synaptic vesicles. Recently, d-amphetamine has been reported to act at the level of the synaptic vesicle to alter presynaptic function. The present in vitro studies further elucidate the mechanism of lobeline's action and compare its effects with those of d-amphetamine. [³H]Dihydrotetrabenazine ([³H ] DTBZ), used routinely to probe a high-affinity binding site on the vesicular monoamine transporter (VMAT2), bound to vesicle membranes from rat striatum with a K(D) of 1.67 nM and B(max) of 8.68 pmol/mg of protein. Lobeline inhibited [³H]DTBZ binding with an IC₅₀ of 0.90 μM, consistent with its previously reported IC₅₀ of 0.88 μM for inhibition of [³H]DA uptake into vesicles. These results suggest that lobeline specifically interacts with DTBZ sites on VMAT2 to inhibit DA uptake into synaptic vesicles. Interestingly, d- amphetamine inhibited [³H]DTBz binding to vesicle membranes with an IC₅₀ of 39.4 μM, a concentration 20 times greater than reported for inhibition of VMAT2 function, suggesting that d-amphetamine interacts with a different site than lobeline on VMAT2 to inhibit monoamine uptake. Kinetic analysis of [all] DA release from [³H] DA-preloaded synaptic vesicles in the absence of drug revealed a t(1/2) of 2.12 min. Lobeline and d-amphetamine evoked [³H]DA release with EC⁵⁰ values of 25.3 and 2.22 μM, respectively. At a concentration 10 times the EC₅₀^, lobeline and d-amphetamine significantly decreased the t(1/2) of [³H]DA release to 1.58 and 1.48 min, respectively. Thus, in contrast to d-amphetamine, which is equipotent in inhibiting DA uptake and promoting release from the synaptic vesicles, lobeline more potently (28-fold) inhibits DA uptake (via an interaction with the DTBZ site on VMAT2) than it evokes DA release to redistribute presynaptic DA storage.