Lobelane Inhibits Methamphetamine-Evoked Dopamine Release via Inhibition of the Vesicular Monoamine Transporter-2

Abstract

Lobeline is currently being evaluated in clinical trials as a methamphetamine abuse treatment. Lobeline interacts with nicotinic receptor subtypes, dopamine transporters (DATs), and vesicular monoamine transporters (VMAT2s). Methamphetamine inhibits VMAT2 and promotes dopamine (DA) release from synaptic vesicles, resulting ultimately in increased extracellular DA. The present study generated structure-activity relationships by defunctionalizing the lobeline molecule and determining effects on [ H]dihydrotetrabenazine binding, inhibition of [3 H]DA uptake into striatal synaptic vesicles and synaptosomes, the mechanism of VMAT2 inhibition, and inhibition of methamphetamine-evoked DA release. Compared with lobeline, the analogs exhibited greater potency inhibiting DA transporter (DAT) function. Saturated analogs, lobelane and nor-lobelane, exhibited high potency (K i ϭ 45 nM) inhibiting vesicular [ 3 H]DA uptake, and lobelane competitively inhibited VMAT2 function. Lobeline and lobelane exhibited 67-and 35-fold greater potency, respectively, in inhibiting VMAT2 function compared to DAT function. Lobelane potently decreased (IC 50 ϭ 0.65 M; I max ϭ 73%) methamphetamine-evoked DA overflow, and with a greater maximal effect compared with lobeline (IC 50 ϭ 0.42 M, I max ϭ 56.1%). These results provide support for VMAT2 as a target for inhibition of methamphetamine effects. Both transisomers and demethylated analogs of lobelane had reduced or unaltered potency inhibiting VMAT2 function and lower maximal inhibition of methamphetamine-evoked DA release compared with lobelane. Thus, defunctionalization, cis-stereochemistry of the side chains, and presence of the piperidino N-methyl are structural features that afford greatest inhibition of methamphetamine-evoked DA release and enhancement of selectivity for VMAT2. The current results reveal that lobelane, a selective VMAT2 inhibitor, inhibits methamphetamine-evoked DA release and is a promising lead for the development of a pharmacotherapeutic for methamphetamine abuse.

Novel N-1,2-Dihydroxypropyl Analogs of Lobelane Inhibit Vesicular Monoamine Transporter-2 Function and Methamphetamine-Evoked Dopamine Release

“…2. K i values for GBR 12909, cocaine, lobeline, and lobelane (Table 1) are consistent with previously reported findings (Reith et al, 1994;Han and Gu, 2006;Nickell et al, 2010). Replacement of the N-methyl in lobelane with a N-1,2-diol moiety generally afforded analogs that were 1-to 10-fold less potent (K i ϭ 1.43-9.5 M) at DAT compared with lobelane.…”

“…Through an interaction with VMAT2, lobeline inhibits the neurochemical and behavioral effects of methamphetamine (Teng et al, 1997(Teng et al, , 1998Harrod et al, 2001;Miller et al, 2001;Dwoskin and Crooks, 2002;Nickell et al, 2010). Lobelane, a lobeline analog with greater selectivity for VMAT2, decreased both methamphetamineevoked DA release (IC 50 ϭ 0.65 M; I max ϭ 73.2%; same experimental conditions as the current work) and methamphetamine self-administration (Zheng et al, 2005a;Neugebauer et al, 2007;Beckmann et al, 2010;Nickell et al, 2010Nickell et al, , 2011. Unfortunately, further development of lobelane as an effective pharmacotherapy was hindered by unacceptable drug-likeness properties.…”

“…1), the principal alkaloid in Lobelia inflata, inhibits the neurochemical and behavioral effects of methamphetamine through its interaction with VMAT2 (Teng et al, 1997(Teng et al, , 1998Harrod et al, 2001;Miller et al, 2001;Dwoskin and Crooks, 2002;Nickell et al, 2010 Teng et al, 1997Teng et al, , 1998, and methamphetamine-evoked DA release (IC 50 ϭ 0.42 M), supporting the tenet that VMAT2 is a viable therapeutic target for the development of treatments for methamphetamine abuse. In further support of this hypothesis, lobeline decreases methamphetamine selfadministration in rats (Harrod et al, 2001).…”

“…1), a chemically defunctionalized, saturated analog of lobeline, competitively inhibited DA uptake at VMAT2 and exhibited increased affinity and selectivity for VMAT2 compared with lobeline (Miller et al, 2001;. Lobelane inhibited methamphetamine-evoked DA release and decreased methamphetamine self-administration; however, tolerance developed to the latter behavior effects (Neugebauer et al, 2007;Nickell et al, 2010). Unfortunately, lobelane exhibits decreased water solubility and diminished drug-likeness properties caused by its decreased polarity resulting from removal of the keto and hydroxyl functionalities of lobeline.…”

“…Taking into account VMAT2 as a component of the mechanism of action of methamphetamine, our research focus has been the discovery of novel therapeutic agents that target VMAT2. Structure-activity relationships (SARs) have been generated to elucidate novel pharmacophores that modify VMAT2 function with the aim of developing effective treatments for methamphetamine abuse (Zheng et al, 2005a,b;Crooks et al, 2010;Nickell et al, 2010Nickell et al, , 2011Horton et al, 2011).…”