The 5-HT1B serotonin receptor regulates methylphenidate-induced gene expression in the striatum: Differential effects on immediate-early genes

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

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.

Chronic Methylphenidate Effects on Brain Gene Expression: An Exploratory Review

Methylphenidate (MP) is a psychostimulant commonly prescribed for individuals with attention deficit hyperactivity disorder (ADHD) but it is also taken with and without a prescription for performance enhancement. Prior research has characterized the effects of MP on behavior, cognition, and neurochemistry. This exploratory review covers the uses of MP and examined the effects of MP on gene expression in the brain following exposure. Overall, MP causes a wide-spread potentiation of genes, in a region-specific manner; consequently, inducing neuronal alterations, such as synaptic plasticity and transmission, resulting in observed behaviors and affects. Monoamine neurotransmitters and post-synaptic density protein genes generally had a potentiating effect in gene expression after exposure to MP.

Fluoxetine potentiates methylphenidate-induced behavioral responses: Enhanced locomotion or stereotypies and facilitated acquisition of cocaine self-administration

The medical psychostimulant methylphenidate (MP) is used to treat attention-deficit hyperactivity disorder and recreationally as a "cognitive enhancer". MP is a dopamine reuptake inhibitor, but does not affect serotonin. Serotonin contributes to addiction-related gene regulation and behavior. Previously, we showed that enhancing serotonin action by adding a selective serotonin reuptake inhibitor, fluoxetine (FLX), to MP potentiates MP-induced gene regulation in striatum and nucleus accumbens, mimicking cocaine effects. Here, we investigated the behavioral consequences of MP+FLX treatment. Young adult male rats received MP (5 mg/kg, i.p.) or MP+FLX (5 mg/kg each) daily for 6-8 days. Behavioral effects were assessed in an open-field test during the repeated treatment. Two weeks later the motor response to a cocaine challenge (25 mg/kg) and the rate of acquisition of cocaine self-administration behavior were determined. Our results demonstrate that FLX potentiates effects of MP on open-field behavior. However, we found differential behavioral responses to MP+FLX treatment, as approximately half of the rats developed high rates of focal stereotypies (termed "MP+FLX/high reactivity" group), whereas the other half did not, and only showed increased locomotion ("MP+FLX/low reactivity" group). Two weeks later, cocaine-induced locomotion and stereotypies were positively correlated with MP+FLX-induced behavior seen at the end of the repeated MP+FLX treatment. Moreover, the MP+FLX/high reactivity group, but not the low reactivity group, showed facilitated acquisition of cocaine self-administration. These results demonstrate that repeated MP+FLX treatment can facilitate subsequent cocaine taking behavior in a subpopulation of rats. These findings suggest that MP+FLX exposure in some individuals may increase the risk for psychostimulant use later in life.

Effects of the serotonin 5-HT1B receptor agonist CP 94253 on the locomotor activity and body temperature of preweanling and adult male and female rats

Serotonin 5-HT_1A receptor agonists increase locomotor activity of both preweanling and adult rodents. The part played by the 5-HT_1B receptor in locomotion is less certain, with preliminary evidence suggesting that the actions of 5-HT_1B receptor agonists are not uniform across ontogeny. To more fully examine the role of 5-HT_1B receptors, locomotor activity and axillary temperatures of preweanling and adult male and female rats was assessed. In the first experiment, adult (PD 70) and preweanling (PD 10 and PD 15) male and female rats were injected with the 5-HT_1B agonist CP 94253 (2.5-10 mg/kg) immediately before locomotor activity testing and 60 min before axillary temperatures were recorded. In the second experiment, specificity of drug action was determined in PD 10 rats by administering saline, WAY 100635 (a 5-HT_1A antagonist), or GR 127935 (a 5-HT_1B antagonist) 30 min before CP 94253 (10 mg/kg) treatment. CP 94253 significantly increased the locomotor activity of preweanling rats on PD 10, an effect that was fully attenuated by GR 127935. Conversely, CP 94253 significantly decreased the locomotor activity of male and female adult rats, while CP 94253 did not affect the locomotor activity of PD 15 rats. Regardless of age, CP 94253 (2.5-10 mg/kg) significantly reduced the axillary temperatures of preweanling and adult rats. When considered together, these results show that 5-HT_1B receptor stimulation activates motor circuits in PD 10 rats; whereas, 5-HT_1B receptor agonism reduces the overall locomotor activity of adult rats, perhaps by blunting exploratory tendencies.

Effects of repeated treatment with the 5-HT1A and 5-HT1B agonists (R)-( +)-8-hydroxy-DPAT and CP-94253 on the locomotor activity and axillary temperatures of preweanling rats: evidence of tolerance and behavioral sensitization

RATIONALE

Drugs that stimulate 5-HT_1A/1B receptors produce both tolerance and behavioral sensitization in adult rats and mice, yet it is unknown whether the same types of plasticity are evident during earlier ontogenetic periods.

OBJECTIVE

The purpose of this study was to determine whether repeated treatment with selective 5-HT_1A and/or 5-HT_1B agonists cause tolerance and/or sensitization in preweanling rats.

METHODS

In Experiments 1 and 2, male and female preweanling rats were given a single pretreatment injection of saline, the 5-HT_1A agonist (R)-( +)-8-hydroxy-DPAT (8-OH-DPAT), or the 5-HT_1B agonist CP-94253 on PD 20. After 48 h, rats received a challenge injection of 8-OH-DPAT or CP-94253, respectively. In Experiment 3, rats were pretreated with saline or DPAT + CP on PD 20 and challenged with the same drug cocktail on PD 22. In Experiment 4, the tolerance- or sensitization-inducing properties of 8-OH-DPAT, CP-94253, or DPAT + CP were tested after a 4-day pretreatment regimen on PD 17-20.

RESULTS

On the first pretreatment day, 8-OH-DPAT, CP-94253, and DPAT + CP increased locomotion and caused hypothermia. Repeated treatment with 8-OH-DPAT (2 or 8 mg/kg) or DPAT + CP caused locomotor sensitization in preweanling rats. In contrast, tolerance to the hypothermic effects of 8-OH-DPAT (8 mg/kg), CP-94253 (5-20 mg/kg), or DPAT + CP was evident after repeated drug treatment.

CONCLUSIONS

During the preweanling period, a single injection of a selective 5-HT_1A or 5-HT_1B agonist is capable of producing drug-induced plasticity. A pretreatment administration of 8-OH-DPAT causes both tolerance (hypothermia) and behavioral sensitization (locomotor activity) in preweanling rats, whereas repeated CP-94253 treatment results in tolerance.

Fluoxetine Potentiates Oral Methylphenidate-Induced Gene Regulation in the Rat Striatum

Methylphenidate (MP) is combined with selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine (FLX) to treat various disorders. MP, a dopamine reuptake inhibitor, helps manage attention-deficit hyperactivity disorder (ADHD) and is abused as a cognitive enhancer; it has a reduced addiction liability. We showed that combining FLX (serotonin) with MP potentiates MP-induced gene regulation in the striatum. These studies used intraperitoneal drug administration, which is relevant for MP abuse. Clinically, MP and FLX are taken orally (slower bioavailability). Here, we investigated whether chronic oral administration of MP and FLX also altered striatal gene regulation. MP (30/60 mg/kg/day), FLX (20 mg/kg/day), and MP + FLX were administered in rats' drinking water for 8 h/day over 4 weeks. We assessed the expression of dynorphin and substance P (both markers for striatal direct pathway neurons) and enkephalin (indirect pathway) by in situ hybridization histochemistry. Chronic oral MP alone produced a tendency for increased dynorphin and substance P expression and no changes in enkephalin expression. Oral FLX alone did not increase gene expression. In contrast, when given together, FLX greatly enhanced MP-induced expression of dynorphin and substance P and to a lesser degree enkephalin. Thus, FLX potentiated oral MP-induced gene regulation predominantly in direct pathway neurons, mimicking cocaine effects. The three functional domains of the striatum were differentially affected. MP + SSRI concomitant therapies are indicated in ADHD/depression comorbidity and co-exposure occurs with MP misuse as a cognitive enhancer by patients on SSRIs. Our findings indicate that MP + SSRI combinations, even given orally, may enhance addiction-related gene regulation.

Ayahuasca blocks the reinstatement of methylphenidate-induced conditioned place preference in mice: behavioral and brain Fos expression evaluations

RATIONALE

Accumulating evidence suggests that ayahuasca, a hallucinogenic beverage used in traditional Amazonian communities for ritualistic and curative purposes, has been associated with reduced rates of substance use disorders. However, the brain mechanisms underlying the therapeutic effects of ayahuasca have not yet been fully elucidated.

OBJECTIVES

The aim of the present study was to investigate the effects of treatment with ayahuasca on the rewarding properties of the psychostimulant methylphenidate.

METHODS

The rewarding properties of ayahuasca (100 mg/kg, orally) and methylphenidate (10 mg/kg, i.p.) were investigated using the conditioned place preference (CPP) model. Furthermore, we evaluated the effects of repeated treatment with ayahuasca on the reinstatement of methylphenidate-induced CPP. Fos expression was evaluated in different limbic structures (cingulate cortex-area 1, prelimbic cortex, infralimbic cortex, orbitofrontal cortex-lateral orbital area, nucleus accumbens core and shell, ventral tegmental area, dorsal striatum, and basolateral amygdala) upon each experimental phase.

RESULTS

Both ayahuasca and methylphenidate induced CPP in mice. However, ayahuasca had limited effects on Fos expression, while methylphenidate altered Fos expression in several brain regions associated with the behavioral effects of drugs of abuse. Treatment with ayahuasca after conditioning with methylphenidate blocked the reinstatement of methylphenidate-induced CPP. Those behavioral effects were accompanied by changes in Fos expression patterns, with ayahuasca generally blocking the changes in Fos expression induced by conditioning with methylphenidate and/or reexposure to methylphenidate.

CONCLUSIONS

Our findings suggest that ayahuasca restored normal brain function in areas associated with the long-term expression of drug wanting/seeking in animals conditioned to methylphenidate.

Selective Regulation of 5-HT1B Serotonin Receptor Expression in the Striatum by Dopamine Depletion and Repeated L-DOPA Treatment: Relationship to L-DOPA-Induced Dyskinesias

Dopamine and serotonin in the basal ganglia interact in a bidirectional manner. On the one hand, serotonin (5-HT) receptors regulate the effects of dopamine agonists on several levels, ranging from molecular signaling to behavior. These interactions include 5-HT receptor-mediated facilitation of dopamine receptor-induced gene regulation in striatal output pathways, which involves the 5-HT1B receptor and others. Conversely, there is evidence that dopamine action by psychostimulants regulates 5-HT1B receptor expression in the striatum. To further investigate the effects of dopamine and agonists on 5-HT receptors, we assessed the expression of 5-HT1B and other serotonin receptor subtypes in the striatum after unilateral dopamine depletion by 6-OHDA and subsequent treatment with L-DOPA (5 mg/kg; 4 weeks). Neither dopamine depletion nor L-DOPA treatment produced significant changes in 5-HT2C, 5-HT4, or 5-HT6 receptor expression in the striatum. In contrast, the 6-OHDA lesion caused a (modest) increase in 5-HT1B mRNA levels throughout the striatum. Moreover, repeated L-DOPA treatment markedly further elevated 5-HT1B expression in the dopamine-depleted striatum, an effect that was most robust in the sensorimotor striatum. A minor L-DOPA-induced increase in 5-HT1B expression was also seen in the intact striatum. These changes in 5-HT1B expression mimicked changes in the expression of neuropeptide markers (dynorphin, enkephalin mRNA) in striatal projection neurons. After repeated L-DOPA treatment, the severity of L-DOPA-induced dyskinesias and turning behavior was positively correlated with the increase in 5-HT1B expression in the associative, but not sensorimotor, striatum ipsilateral to the lesion, suggesting that associative striatal 5-HT1B receptors may play a role in L-DOPA-induced behavioral abnormalities.