Tetrabenazine inhibition of monoamine uptake and methamphetamine behavioral effects: locomotor activity, drug discrimination and self-administration

Abstract

Tetrabenazine (TBZ), a benzoquinolizine derivative, binds with high affinity to the vesicular monoamine transporter-2 (VMAT2), inhibiting uptake of cytosolic monoamines. The current study aimed to provide preclinical evidence supporting the potential use of TBZ as a treatment for methamphetamine abuse. Effects of TBZ on function of the dopamine transporter (DAT) and serotonin transporter (SERT) in striatal and hippocampal synaptosomes, respectively, and on VMAT2 function in isolated striatal synaptic vesicles were determined. Effect of TBZ (acute, 0.1-3.0 mg/kg, s.c.; repeated, 1.0 mg/kg for 7 days) on locomotor activity in methamphetamine-sensitized rats was assessed. Ability of TBZ (0.1-3.0 mg/kg; s.c.) or vehicle to decrease the discriminative effect of methamphetamine also was determined. Ability of TBZ (acute, 0.1-1.0 mg/kg, s.c.; repeated, 0.1 or 1.0 mg/kg for 7 days) to specifically decrease methamphetamine self-administration was determined; for comparison, a separate group of rats was assessed for effects of TBZ on food-maintained responding. Results show that TBZ was 11-fold more potent inhibiting DAT than SERT, and 2.5-fold more potent inhibiting VMAT2 than DAT. Results from behavioral studies showed that the lowest dose of TBZ transiently increased methamphetamine self-administration, whereas higher TBZ doses decreased methamphetamine self-administration. Also, TBZ at high doses decreased methamphetamine locomotor sensitization and discriminative stimulus effects, as well as food-maintained responding. Thus, despite acting as a potent VMAT2 inhibitor, these preclinical results indicate that TBZ lacks behavioral specificity as an inhibitor of methamphetamine-induced reinforcement, diminishing its viability as a suitable treatment for methamphetamine abuse.

Figure 1

Panel A: TBZ (1 nM – 100 μM) inhibits [³H]DA (○)and [³H]5-HT (◆)uptake into rat striatal and hippocampal synaptosomes and [³H]DA (▪) uptake into rat striatal synaptic vesicle preparations. Nonspecific [³H]DA and [³H]5-HT uptake at DAT and SERT, respectively, were determined in the presence of 10 μM nomifensine and 10 μM fluoxetine, respectively. Nonspecific uptake at VMAT2 was determined in the presence of Ro4-1284 (10 μM). Data are presented as pmol/min/mg protein (mean ± SEM) specific [³H]DA and [³H]5-HT uptake as a percentage of control (CON; in the absence of TBZ). Control values for [³H]DA (DAT), [³H]5-HT (SERT), and [³H]DA (VMAT2) uptake were 24.2 ± 1.08, 0.63 ± 0.11, and 49.3 + 3.81 pmol/min/mg protein, respectively. * p< 0.05 different from Ki value for VMAT2; n = 4-9 rats/assay. Panel B: Kinetic analysis of TBZ inhibition of [³H]DA uptake into rat striatal synaptic vesicles. The concentration of TBZ utilized for the kinetic analysis was the Ki concentration from the inhibition curve illustrated in Panel A. Data are presented as pmol/min/mg (mean ± SEM) [³H]DA uptake. Control represents [³H]DA uptake in the absence of TBZ. Nonspecific uptake was determined in the presence of Ro4-1284 (10 μM). K_m and V_max values (mean ± SEM) are provided in the inset. * p < 0.05 different from control; n = 4-7 rats/group.

Figure 2

Acute and repeated TBZ decreases locomotor activity in methamphetamine-sensitized rats. Data are presented as total distance traveled (cm; mean ± SEM) for the first 15 min of the 90-min locomotor session. Panel A: TBZ (0.1-3.0 mg/kg) or vehicle (VEH; 0) was administered 15 min prior to methamphetamine (METH, 1.0 mg/kg) or saline (SAL). # represents difference between TBZ dose and VEH (0), when collapsed across METH and SAL treatment, p < 0.05; n = 6 rats/group. Panel B: TBZ (1.0 mg/kg) or VEH was administered 15 min prior to methamphetamine (1.0 mg/kg) for 7 consecutive sessions. Prior to the 8^th session, TBZ was not administered prior to methamphetamine (No PT). * represents a difference from VEH, p < 0.05; n = 6 rats/group.

Figure 3

TBZ (3.0 mg/kg) decreases the discriminative stimulus effects of methamphetamine in rats trained to discriminate methamphetamine (1.0 mg/kg) from saline. Panel A: Percent of responses on the methamphetamine-appropriate lever following methamphetamine (0.1 – 1.0 or 1.7 mg/kg) in combination with TBZ (0.1 - 3.0 mg/kg, open symbols) or vehicle (closed circles). n = 6 rats/group. Panel B: Response rate following methamphetamine (0.1 – 1.0 or 1.7 mg/kg) in combination with TBZ (0.1 - 3.0 mg/kg, open symbols) or vehicle (closed circles). n = 6 rats/group.

Figure 4

TBZ produces a biphasic dose effect on methamphetamine (METH) self-administration, with a low dose (0.1 mg/kg) increasing responding acutely and a high dose (1.0 mg/kg) decreasing responding across repeated pretreatments. Panel A: Number of METH infusions during the first 15 min of the session following TBZ (0.1-1.0 mg/kg) or vehicle (VEH), expressed as a percentage of VEH control (7.0 ± 0.8 infusions; dashed line). * indicates a difference from VEH control, p < 0.05; n = 5-8 rats/group. Panel B: Number of METH infusions during the first 15 min of the 7 consecutive sessions following repeated pretreatment with TBZ (0.1 or 1.0 mg/kg), expressed as a percentage of VEH control (7.0 ± 1.0 infusions; dashed line). Prior to the 8^th session, TBZ was not administered as a pretreatment (No PT). * indicates a difference from VEH control, p < 0.05; n = 5-8 rats/group.

Figure 5

TBZ produces a dose-dependent decrease in food-maintained responding, both acutely and repeatedly. Panel A: Number of food pellets earned during the first 15 min of the session following TBZ (0.1-1.0 mg/kg) or vehicle (VEH), expressed as a percentage of VEH control (30.5 ± 2.4 food pellets; dashed line). * indicates a difference from VEH control, p < 0.05; n = 6 rats/group. Panel B: Number of food pellets earned during the first 15 min of the 7 consecutive sessions following repeated pretreatment with TBZ (0.1 or 1.0 mg/kg), expressed as a percentage of VEH control (25.8 ± 2.4 food pellets; dashed line). Prior to the 8^th session, TBZ was not administered as a pretreatment (No PT). * indicates a difference from VEH control, p < 0.05; n = 6 rats/group.