Lobeline displaces [3H]dihydrotetrabenazine binding and releases [3H]dopamine from rat striatal synaptic vesicles: comparison with d-amphetamine.
Lobeline, an alkaloid from Indian tobacco (Lobelia 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 KD of 1.67 nM and Bmax of 8.68 pmol/mg of protein. Lobeline inhibited [3H]DTBZ binding with an IC50 of 0.90 microM, consistent with its previously reported IC50 of 0.88 microM 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 microM, 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 [3H]DA release from [3H]DA-preloaded synaptic vesicles in the absence of drug revealed a t1/2 of 2.12 min. Lobeline and d-amphetamine evoked [3H]DA release with EC50 values of 25.3 and 2.22 microM, respectively. At a concentration 10 times the EC50, lobeline and d-amphetamine significantly decreased the t1/2 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.
Teng
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College of Pharmacy and Graduate Center for Toxicology, University of Kentucky, Lexington 40536-0082, USA.
Crooks
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Dwoskin
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D000818:Animals; D001667:Binding, Competitive / Q000502:physiology; D001692:Biological Transport / Q000187:drug effects; D003342:Corpus Striatum / Q000737:chemistry; D003913:Dextroamphetamine / Q000494:pharmacology*; D004298:Dopamine / Q000493:pharmacokinetics; D015259:Dopamine Agents / Q000494:pharmacology*; D008120:Lobeline / Q000494:pharmacology*; D008297:Male; D008562:Membrane Glycoproteins / Q000378:metabolism; D026901:Membrane Transport Proteins; D009479:Neuropeptides; D018377:Neurotransmitter Agents / Q000378:metabolism; D018722:Nicotinic Agonists / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013572:Synaptic Vesicles / Q000737:chemistry / Q000378:metabolism*; D013747:Tetrabenazine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D014316:Tritium; D050492:Vesicular Biogenic Amine Transport Proteins; D050493:Vesicular Monoamine Transport Proteins
D015259:Dopamine Agents; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009479:Neuropeptides; D018377:Neurotransmitter Agents; D018722:Nicotinic Agonists; C493454:Slc18a2 protein, rat; D050492:Vesicular Biogenic Amine Transport Proteins; D050493:Vesicular Monoamine Transport Proteins; D014316:Tritium; C032811:dihydrotetrabenazine; D008120:Lobeline; D003913:Dextroamphetamine; D004298:Dopamine; D013747:Tetrabenazine