Vilazodone, a Selective Serotonin Reuptake Inhibitor with Diminished Impact on Methylphenidate-Induced Gene Regulation in the Striatum: Role of 5-HT1A Receptor

Abstract

Selective serotonin reuptake inhibitors (SSRIs), including fluoxetine, are frequently combined with medical psychostimulants such as methylphenidate (Ritalin), for example, in the treatment of attention-deficit hyperactivity disorder/depression comorbidity. Co-exposure to these medications also occurs with misuse of methylphenidate as a recreational drug by patients on SSRIs. Methylphenidate, a dopamine reuptake blocker, produces moderate addiction-related gene regulation. Findings show that SSRIs such as fluoxetine given in conjunction with methylphenidate potentiate methylphenidate-induced gene regulation in the striatum in rats, consistent with a facilitatory action of serotonin on addiction-related processes. These SSRIs may thus increase methylphenidate's addiction liability. Here, we investigated the effects of a novel SSRI, vilazodone, on methylphenidate-induced gene regulation. Vilazodone differs from prototypical SSRIs in that, in addition to blocking serotonin reuptake, it acts as a partial agonist at the 5-HT1A serotonin receptor subtype. Studies showed that stimulation of the 5-HT1A receptor tempers serotonin input to the striatum. We compared the effects of acute treatment with vilazodone (10-20 mg/kg) with those of fluoxetine (5 mg/kg) on striatal gene regulation (zif268, substance P, enkephalin) induced by methylphenidate (5 mg/kg), by in situ hybridization histochemistry combined with autoradiography. We also assessed the impact of blocking 5-HT1A receptors by the selective antagonist WAY-100635 (0.5 mg/kg) on these responses. Behavioral effects of these drug treatments were examined in parallel in an open-field test. Our results show that, in contrast to fluoxetine, vilazodone did not potentiate gene regulation induced by methylphenidate in the striatum, while vilazodone enhanced methylphenidate-induced locomotor activity. However, blocking 5-HT1A receptors by WAY-100635 unmasked a potentiating effect of vilazodone on methylphenidate-induced gene regulation, thus confirming an inhibitory role for 5-HT1A receptors. Our findings suggest that vilazodone may serve as an adjunct SSRI with diminished addiction facilitating properties and identify the 5-HT1A receptor as a potential therapeutic target to treat addiction.

Keywords: Fluoxetine; Gene expression; Methylphenidate; Psychostimulant; SSRI; Striatum; Vilazodone; zif268.

Figure 1.

Effects of methylphenidate plus vilazodone vs. methylphenidate plus fluoxetine treatment on zif268 and substance P expression in the striatum and behavior. (A, B) Mean density values (mean±SEM, in % of MP) measured in the six sectors (zif268; A) or the three dorsal sectors (substance P; B) of the middle striatum (see diagram, lower right) are given for the groups that received an injection of vehicle (Veh), methylphenidate (5 mg/kg) (MP), methylphenidate plus fluoxetine (5 mg/kg) (MP+FLX), methylphenidate plus vilazodone (10 mg/kg) (MP+VIL) or vilazodone alone (VIL) (n=7–11). (C) Ambulation counts (mean±SEM, in % of MP) in the 40-min open-field test are shown for these treatment groups. Sector abbreviations: m, medial; d, dorsal; dl, dorsolateral; vl, ventrolateral; c, central; v, ventral. *** P<0.001, ** P<0.01, * P<0.05, vs. MP; ^### P<0.001, ^## P<0.01, ^# P<0.05, as indicated.

Figure 2.

Effects of methylphenidate plus vilazodone vs. methylphenidate plus fluoxetine treatment, as well as effects of the 5-HT1A antagonist WAY-100635, on zif268 expression in the striatum. (A) Illustrations of film autoradiograms show zif268 expression in the mid-level striatum in rats that received (from upper left) vehicle (Veh), methylphenidate (5 mg/kg) (MP), methylphenidate plus fluoxetine (5 mg/kg) (MP+FLX), vilazodone (10 mg/kg) (VIL), methylphenidate plus vilazodone (MP+VIL), WAY-100635 (0.5 mg/kg) (WAY), methylphenidate plus WAY-100635 (MP+WAY), or methylphenidate plus vilazodone plus WAY-100635 (MP+VIL+WAY). The maximal hybridization signal is black. (B) Maps depict the regional distribution of increases in zif268 expression across the 23 sectors on the rostral, middle and caudal striatal levels. Shown are the increases (vs. the MP group) in the MP+FLX or MP+VIL groups (potentiation) (experiment 1, left) and the increases (vs. the MP group) in the MP+VIL or MP+VIL+WAY groups (experiment 2, right). The data are expressed relative to the maximal increase observed in each experiment (% of max.). Sectors with a statistically significant difference vs. MP controls (P<0.05) are coded as indicated. Sectors without significant effect are in white.

Figure 3.

Effects of blocking the 5-HT1A receptor by the selective 5-HT1A antagonist WAY-100635 on methylphenidate plus vilazodone-induced zif268 expression in the striatum and behavior. (A) Mean density values (mean±SEM, in % of MP) measured in four sectors of the middle striatum (see diagram, Fig. 1) are shown for the groups that received an injection of vehicle (Veh), methylphenidate (5 mg/kg) (MP), methylphenidate plus vilazodone (10 mg/kg) (MP+VIL), methylphenidate plus vilazodone plus WAY-100635 (0.5 mg/kg) (MP+VIL+WAY), methylphenidate plus WAY-100635 (MP+WAY), or WAY-100635 (WAY) (n=5–8). (B) Ambulation (left) and stereotypy counts (right) (mean±SEM, in % of MP) in the open-field test are given for these treatment groups. Abbreviations: m, medial; d, dorsal; dl, dorsolateral; c, central. *** P<0.001, ** P<0.01, * P<0.05, vs. MP; ^### P<0.001, ^## P<0.01, ^# P<0.05, as indicated.

Figure 4.

Effects of blocking the 5-HT1A receptor by the selective 5-HT1A antagonist WAY-100635 on methylphenidate plus vilazodone-induced substance P (A) and enkephalin (B) expression in the striatum. Mean density values (mean±SEM, in % of MP) measured in four sectors of the middle striatum are given for the groups that received vehicle (Veh), methylphenidate (MP), methylphenidate plus vilazodone (MP+VIL), methylphenidate plus vilazodone plus WAY-100635 (MP+VIL+WAY), methylphenidate plus WAY-100635 (MP+WAY), or WAY-100635 only (WAY). Abbreviations: m, medial; d, dorsal; dl, dorsolateral; c, central. * P<0.05, vs. MP; ^### P<0.001, ^## P<0.01, ^# P<0.05, as indicated.