The effects of the 5-HT(2C) receptor antagonist SB242084 on locomotor activity induced by selective, or mixed, indirect serotonergic and dopaminergic agonists

Interactions of serotonin (5-HT)2 receptor-targeting ligands and nicotine: locomotor activity studies in rats

Male Wistar rats were used to verify the hypothesis that serotonin (5-HT)(2A) or 5-HT(2C) receptors may control the locomotor effects evoked by nicotine (0.4 mg/kg). The 5-HT(2A) receptor antagonist (M100,907), the 5-HT(2A) receptor agonist (DOI), the 5-HT(2C) receptor antagonist (SB 242,084), and the 5-HT(2C) receptor agonists (Ro 60-0175 and WAY 163,909) were used. M100,907 (0.5-2mg/kg) did not alter, while DOI (1 mg/kg) enhanced the nicotine-induced hyperlocomotion. The effect of DOI was antagonized by M100,907 (1 mg/kg). SB 242,084 (0.25-1 mg/kg) augmented, while Ro 60-0175 (1 and 3 mg/kg) and WAY 163,909 (1.5 mg/kg) decreased the overall effect of acute nicotine; effects of Ro 60-0175 and WAY 163,909 were attenuated by SB 242,084 (0.125 mg/kg). In another set of experiments, M100,907 (2 mg/kg) on Day 10 attenuated, while DOI (0.1-1 mg/kg) enhanced the nicotine-evoked conditioned hyperlocomotion in rats repeatedly (Days 1-5) treated with nicotine in experimental chambers. SB 242,084 (0.125 or 1 mg/kg) did not change, while Ro 60-0175 (1 mg/kg) or WAY 163,909 (1.5 mg/kg) decreased the expression of nicotine-induced conditioned hyperactivity. Only DOI (0.3 and 1 mg/kg) and SB 242,084 (1 mg/kg) enhanced the basal locomotion. The present data indicate that 5-HT(2A) receptors are significant for the expression of nicotine-evoked conditioned hyperactivity. Conversely, 5-HT(2C) receptors play a pivotal role in the acute effects of nicotine. Pharmacological stimulation of 5-HT(2A) receptors enhances the conditioned hyperlocomotion, while activation of 5-HT(2C) receptors decreases both the response to acute nicotine and conditioned hyperactivity.

Nicotinic receptors differentially modulate the induction and expression of behavioral sensitization to methylphenidate in rats

RATIONALE

Nicotinic acetylcholine receptors (nAChRs) regulate sensitization to stimulant drugs such as d-amphetamine and cocaine.

OBJECTIVES

The current study determined if nAChRs modulate the induction and/or expression of behavioral sensitization to high methylphenidate doses.

METHODS

In experiment 1, rats received saline or mecamylamine (3 mg/kg, sc), followed by saline or methylphenidate (5.6 or 10 mg/kg, sc) during 10 daily sessions; the effect of methylphenidate (1-17 mg/kg, sc) alone was determined 14 days later. In experiment 2, rats received saline or dihydro-beta-erythroidine (DHbetaE; 3 mg/kg, sc), followed by saline or 5.6 mg/kg of methylphenidate. In experiment 3, rats received saline or methylphenidate (5.6 or 10 mg/kg, sc) alone for 10 days; the effect of acute mecamylamine (3 mg/kg, sc) on the response to methylphenidate (1-17 mg/kg, sc) was determined 14 days later. Locomotor activity, sniffing, rearing, grooming, and stereotypy ratings were dependent measures.

RESULTS

Methylphenidate produced dose-dependent increases in locomotor activity, sniffing, and stereotypy on day 1 and these effects were enhanced on day 10, indicative of sensitization. Mecamylamine attenuated methylphenidate-induced stereotypy only on day 1, but reduced locomotor activity, sniffing, rearing, and stereotypy on day 10 and during the methylphenidate challenge phase; similar results were obtained with DHbetaE. However, acute mecamylamine did not alter the effects of the methylphenidate challenge following the induction of sensitization to methylphenidate alone.

CONCLUSIONS

Although nAChRs do not appear to regulate the expression of methylphenidate-induced behavioral sensitization, inhibition of high-affinity beta2 subunit nAChRs attenuates the induction of behavioral sensitization to high doses of methylphenidate.

Chronic reductions in serotonin transporter function prevent 5-HT1B-induced behavioral effects in mice

BACKGROUND

Obsessive-compulsive disorder (OCD) is characterized by intrusive thoughts, images, or impulses and/or repetitive stereotypical behavior. Obsessive-compulsive disorder patients exhibit reduced prepulse inhibition (PPI) and symptom exacerbation after challenge with 5-HT1B receptor agonists. Recently, gain-of-function alleles of the serotonin transporter (5-HTT) have been associated with OCD. We tested the hypothesis that reducing 5-HTT function chronically, either genetically or via serotonin reuptake inhibitor (SRI) treatment, attenuates PPI deficits and perseverative hyperlocomotion induced by 5-HT1B agonists in mice.

METHODS

Mice received subchronic or chronic pretreatment with the SRI fluoxetine and acute treatment with RU24969 (5-HT1A/1B agonist) or 8-OH-DPAT (5-HT1A agonist) and were assessed for PPI, locomotor activity, and spatial patterns of locomotion. The same measures were evaluated in 5-HTT wild-type (WT), heterozygous (HT), and knockout (KO) mice after RU24969 treatment. The effects of WAY100635 (5-HTA antagonist) or GR127935 (5-HT1B/D antagonist) pretreatment on RU24969-induced effects were evaluated. Finally, 5-HT1B binding and functional coupling were assessed in 5-HTT-WT, -HT, and -KO mice, and normal fluoxetine-treated mice.

RESULTS

Chronic, but not subchronic, fluoxetine treatment prevented RU24969-induced PPI deficits and perseverative hyperlocomotion. These RU24969-induced effects were mediated via 5-HT1B and not 5-HT1A receptors. 5-HTT-KO mice showed no effects of RU24969, and 5-HTT-HT mice exhibited intermediate phenotypes. 5-HT1B binding and functional coupling were reduced in the globus pallidus and substantia nigra of 5-HTT-KO mice.

CONCLUSIONS

Our results demonstrate that chronic, but not subchronic, fluoxetine treatment and 5-HTT knockout robustly attenuate 5-HT1B agonist-induced PPI deficits and perseverative hyperlocomotion. These results may have implications for the etiology and treatment of OCD.

Serotonin2C receptors in the medial prefrontal cortex facilitate cocaine-induced dopamine release in the rat nucleus accumbens

A functional balance between excitatory and inhibitory control over dopamine (DA)-dependent behavioral and neurochemical effects of cocaine is afforded by the serotonin(2C) receptor (5-HT(2C)R) located within the ventral tegmental area and the nucleus accumbens (NAc). The 5-HT(2C)R located in the medial prefrontal cortex (mPFC) has also been shown to inhibit cocaine-induced behaviors perhaps through inhibition of DA function in the NAc. Using in vivo microdialysis in halothane-anesthetized rats, we tested this hypothesis by assessing the influence of mPFC 5-HT(2C)Rs on cocaine-induced DA outflow in the NAc shell. Intra-mPFC injection of the 5-HT(2C)R agonist Ro 60-0175 at 5 microg/0.2 microl, but not 1 microg/0.2 microl, potentiated the increase in accumbal DA outflow induced by the intraperitoneal administration of 10 mg/kg of cocaine. Conversely, cocaine-induced accumbal DA outflow was significantly reduced by the intra-mPFC injection of the selective 5-HT(2C)R antagonist SB 242084 (0.5 microg/0.2 microl) or SB 243213 (0.5 and 1 microg/0.2 microl). These results show that mPFC 5-HT(2C)Rs exert a positive control over cocaine-induced accumbal DA outflow. Observations further support the idea that the overall action of central 5-HT(2C)Rs on accumbal DA output is dependent on the functional balance among different 5-HT(2C)R populations located within the mesocorticoaccumbens system, and that 5-HT(2C)Rs can modulate DA-dependent behaviors independently of changes of accumbal DA release itself.

Repeated antipsychotic treatment progressively potentiates inhibition on phencyclidine-induced hyperlocomotion, but attenuates inhibition on amphetamine-induced hyperlocomotion: relevance to animal models of antipsychotic drugs

Clinical observations indicate that antipsychotic action starts early and increases in magnitude with repeated treatment. Animal models that faithfully capture this time course of action are few. Inhibition of hyperlocomotion induced by amphetamine or phencyclidine has been widely used as a screening tool for the antipsychotic activity of a drug. We thus investigated whether repeated antipsychotic treatment could produce an early-onset and progressively increased antagonistic effect on amphetamine or phencyclidine-induced hyperlocomotion as a way of assessing the validity of such models in capturing time course of antipsychotic action. On each of the five consecutive test days, different groups of rats (n=6-7/group) received an initial injection of either haloperidol (0.01-0.10 mg/kg, sc), clozapine (5-20.0 mg/kg, sc), olanzapine (1.0 mg/kg, sc), chlordiazepoxide (10.0 mg/kg, ip) or vehicle (sterile water, sc) 30 min prior to a second injection of either amphetamine (1.5 mg/kg, sc) or phencyclidine (3.2 mg/kg, sc). Motor activity was subsequently monitored for 60 min after amphetamine or phencyclidine treatment. Repeated treatment of haloperidol, clozapine, or olanzapine progressively potentiated inhibition on repeated phencyclidine-induced hyperlocomotion and prolonged this action over the five consecutive days. In contrast, antipsychotic inhibition on repeated amphetamine-induced hyperlocomotion was gradually attenuated and shortened. Repeated treatment of chlordiazepoxide, a benzodiazepine anxiolytic, retained its inhibition on amphetamine-induced hyperlocomotion, but had no effect on phencyclidine-induced one. These results suggest that repeated phencyclidine-induced hyperlocomotion model based on repeated antipsychotic treatment regimen is capable of capturing the progressive increase pattern of antipsychotic treatment seen in the clinic and differentiating antipsychotics from anxiolytics; thus it may serve as a better model for the investigation of the neurobiological mechanisms of action of antipsychotic drugs and delineating the pathophysiology of schizophrenia.

Locomotor stimulation produced by 3,4-methylenedioxymethamphetamine (MDMA) is correlated with dialysate levels of serotonin and dopamine in rat brain

(+/-)-3,4-Methylenedioxymethamphetamine (MDMA, or Ecstasy) is an illicit drug that evokes transporter-mediated release of monoamines, including serotonin (5-HT) and dopamine (DA). Here we monitored the effects of MDMA on neurochemistry and motor activity in rats, as a means to evaluate relationships between 5-HT, DA, and behavior. Male rats undergoing in vivo microdialysis were housed in chambers equipped with photobeams for measurement of ambulation (i.e., forward locomotion) and stereotypy (i.e., head weaving and forepaw treading). Microdialysis probes were placed into the n. accumbens, striatum or prefrontal cortex in separate groups of rats. Dialysate samples were assayed for 5-HT and DA by microbore HPLC-ECD. Rats received two i.v. injections of MDMA, 1 mg/kg followed by 3 mg/kg 60 min later; neurochemical and locomotor parameters were measured concurrently. MDMA produced dose-related elevations in extracellular 5-HT and DA in all regions, with the magnitude of 5-HT release always exceeding that of DA release. MDMA-induced ambulation was positively correlated with dialysate DA levels in all regions (P<0.05-0.0001) and with dialysate 5-HT in striatum and cortex (P<0.001-0.0001). Stereotypy was strongly correlated with dialysate 5-HT in all areas (P<0.001-0.0001) and with dialysate DA in accumbens and striatum (P<0.001-0.0001). These data support previous work and suggest the complex spectrum of behaviors produced by MDMA involves 5-HT and DA in a region- and modality-specific manner.

Gene profiling the response to repeated cocaine self-administration in dorsal striatum: a focus on circadian genes

Alterations in gene expression in the dorsal striatum caused by chronic cocaine exposure have been implicated in the long-term behavioral changes associated with cocaine addiction. To gain further insight into the molecular alterations that occur as a result of cocaine self-administration, we conducted a microarray analysis of gene expression followed by bioinformatic gene network analysis that allowed us to identify adaptations at the level of gene expression as well as into interconnected networks. Changes in gene expression were examined in the dorsal striatum of rats 1 day after they had self-administered cocaine for 7 days under a 24-h access, discrete trial paradigm (averaging 98 mg/kg/day). Here we report the regulation of the circadian genes Clock, Bmal1, Cryptochrome1, Period2, as well as several genes that are regulated by/associated with the circadian system (i.e., early growth response 1, dynorphin). We also observed regulation of other relevant genes (i.e., Nur77, beta catenin). These changes were then linked to curated pathways and formulated networks which identified circadian rhythm processes as affected by cocaine self-administration. These data strongly suggest involvement of circadian-associated genes in the brain's response to cocaine and may contribute to an understanding of addictive behavior including disruptions in sleep and circadian rhythmicity.

Serotonin, inhibition, and negative mood

Pavlovian predictions of future aversive outcomes lead to behavioral inhibition, suppression, and withdrawal. There is considerable evidence for the involvement of serotonin in both the learning of these predictions and the inhibitory consequences that ensue, although less for a causal relationship between the two. In the context of a highly simplified model of chains of affectively charged thoughts, we interpret the combined effects of serotonin in terms of pruning a tree of possible decisions, (i.e., eliminating those choices that have low or negative expected outcomes). We show how a drop in behavioral inhibition, putatively resulting from an experimentally or psychiatrically influenced drop in serotonin, could result in unexpectedly large negative prediction errors and a significant aversive shift in reinforcement statistics. We suggest an interpretation of this finding that helps dissolve the apparent contradiction between the fact that inhibition of serotonin reuptake is the first-line treatment of depression, although serotonin itself is most strongly linked with aversive rather than appetitive outcomes and predictions.

Serotonin is an evolutionarily ancient neuromodulator probably best known for its role in psychiatric disorders. However, that role has long appeared contradictory to its role in normal function, and indeed its various roles in normal affective behaviors have been hard to reconcile. Here, we model two predominant functions of normal serotonin function in a highly simplified reinforcement learning model and show how these may explain some of its complex roles in depression and anxiety.

Interactions between ethanol and cocaine, amphetamine, or MDMA in the rat: thermoregulatory and locomotor effects

RATIONALE

(+/-)-3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is often taken recreationally with ethanol (EtOH). In rats, EtOH may potentiate MDMA-induced hyperactivity, but attenuate hyperthermia.

OBJECTIVE

Experiment 1 compared the interactions between EtOH (1.5 g/kg) and MDMA (6.6 mg/kg) with EtOH + cocaine (COCA; 10 mg/kg) and EtOH + amphetamine (AMPH; 1 mg/kg) on locomotor activity and thermoregulation. Experiment 2 used a weaker dose of MDMA (3.3 mg/kg) and larger doses of COCA (20 mg/kg) and AMPH (2 mg/kg).

MATERIALS AND METHODS

Drug treatments were administered on four occasions (2, 5, and 2 days apart, respectively; experiment 1) or two (2 days apart; experiment 2).

RESULTS

All psychostimulants increased activity, and EtOH markedly increased the effect of MDMA. AMPH alone-related hyperactivity showed modest sensitization across treatment days, while MDMA + EtOH activity showed marked sensitization. AMPH, COCA, and MDMA induced hyperthermia of comparable amplitude (+1 to +1.5 degrees C). Co-treatment with EtOH and AMPH (1 mg/kg) or COCA (10 mg/kg) produced hypothermia greater than that produced by EtOH alone. Conversely, EtOH attenuated MDMA-related hyperthermia, an effect increasing across treatment days. These results demonstrate that the interaction between MDMA and EtOH may be different from the interaction between EtOH and AMPH or COCA.

CONCLUSION

Because of potential health-related consequences of such polydrug misuse, it is worth identifying the mechanisms underlying these interactions, especially between EtOH and MDMA. Given the different affinity profiles of the three drugs for serotonin, dopamine, and norepinephrine transporters, our results appear compatible with the possibility of an important role of serotonin in at least the EtOH-induced potentiation of MDMA-induced hyperlocomotion.

5-HT 1B receptor-mediated serotoninergic modulation of methylphenidate-induced locomotor activation in rats

Previous studies have shown that the dopamine (DA) uptake blocker methylphenidate, a psychostimulant widely used for the treatment of attention-deficit hyperactivity disorder (ADHD), prevents the neurotoxic effects of the highly abused DA releaser methamphetamine. However, there is a lack of information about the pharmacological interactions of these two drugs at the behavioral level. When systemically administered within an interval of 2 h, previous administration of methylphenidate (10 mg/kg, intraperitoneal (i.p.)) did not modify locomotor activation induced by methamphetamine. On the other hand, previous administration of methamphetamine (1 mg/kg, i.p.) markedly potentiated methylphenidate-induced motor activation. With in vivo microdialysis experiments, methamphetamine and methylphenidate were found to increase DA extracellular levels in the nucleus accumbens (NAs). Methamphetamine, but not methylphenidate, significantly increased the extracellular levels of serotonin (5-HT) in the NAs. Methamphetamine-induced 5-HT release remained significantly elevated for more than 2 h after its administration, suggesting that the increased 5-HT could be responsible for the potentiation of methylphenidate-induced locomotor activation. In fact, previous administration of the 5-HT uptake blocker fluoxetine (10 mg/kg, i.p.) also potentiated the motor activation induced by methylphenidate. A selective 5-HT 1B receptor antagonist (GR 55562; 1 mg/kg), but not a 5-HT2 receptor antagonist (ritanserin; 2 mg/kg, i.p.), counteracted the effects of methamphetamine and fluoxetine on the motor activation induced by methylphenidate. Furthermore, a 5-HT 1B receptor agonist (CP 94253; 1-10 mg/kg, i.p.) strongly and dose-dependently potentiated methylphenidate-induced locomotor activation. The 5-HT 1B receptor-mediated modulation of methylphenidate-induced locomotor activation in rat could have implications for the treatment of ADHD.

Differential regulation of the mesoaccumbens dopamine circuit by serotonin2C receptors in the ventral tegmental area and the nucleus accumbens: an in vivo microdialysis study with cocaine

Stimulation of central serotonin2C receptor (5-HT(2C)R) inhibits dopamine (DA)-dependent neurochemical and behavioral effects of cocaine, while 5-HT(2C)Rs locally expressed into the ventral tegmental area (VTA) and the nucleus accumbens (NAc) exert opposite functional control over cocaine-induced behavioral effects. Using in vivo microdialysis in halothane-anesthetized rats, we tested the hypothesis that this functionally opposite regulation of the mesoaccumbens DA pathway relies on the ability of 5-HT(2C)Rs in the VTA and the NAc to inhibit and enhance respectively cocaine-induced accumbal DA outflow. Intra-VTA injection of the 5-HT(2C)R agonist Ro 60-0175 at 5 microg/0.2 microl, but not 1 microg/0.2 microl, attenuated the increase in accumbal DA outflow induced by the systemic administration of 10 mg/kg of cocaine. Intra-VTA injection of the 5-HT(2C)R antagonist SB 242084 at either dose (0.1 or 0.5 microg/0.2 microl) did not modify the effects of cocaine. Intra-NAc application of Ro 60-0175 dose-dependently excited (0.1 microM) and inhibited (1 microM) cocaine-induced DA outflow. In contrast, intra-NAc application of SB 242084 resulted in diametrically opposite effects when applied at these concentrations. These results further support the idea that the overall action of central 5-HT(2C)Rs on accumbal DA output is dependent, at least in part, on the functional balance between different 5-HT(2C)R populations within the NAc and within the mesoaccumbens DA pathway (VTA vs NAc).

Effects of early methylphenidate exposure on morphine- and sucrose-reinforced behaviors in adult rats: relationship to dopamine D2 receptors

Methylphenidate is commonly used to treat Attention Deficit Hyperactivity Disorder (ADHD) in school-aged children, and there is an increasing trend to prescribe methylphenidate to younger preschool-aged children. While the efficacy of methylphenidate is not in question, there is evidence that early methylphenidate treatment may have long-term effects on later drug responsiveness. The goal of this study was to determine whether early exposure to methylphenidate would alter morphine-induced conditioned place preference (CPP) and sucrose-reinforced lever-pressing in young adult rats. We also assessed whether early methylphenidate exposure would impact dopamine D(2) binding sites. Sprague-Dawley rats were treated with methylphenidate (0, 2, or 5 mg/kg) once a day from PD 11-PD 20. On PD 60, morphine-induced CPP or sucrose-reinforced lever-pressing was assessed. A 10-day CPP procedure was used, which included 1 preconditioning day, 8 conditioning days, and 1 test day. After CPP testing, D(2) receptor binding was determined in striatal and accumbal tissue samples. In the sucrose experiment, rats were trained to lever-press on a progressive ratio schedule for one sucrose pellet. Results showed that early exposure to methylphenidate (5 mg/kg) increased the magnitude of morphine-induced CPP. Exposure to methylphenidate did not alter the number of D(2) binding sites, however, there were positive correlations between the number of D(2) binding sites and the strength of the CPP. In the sucrose-reinforced lever-press experiment, rats exposed to methylphenidate (2 and 5 mg/kg) had higher break points than saline controls. These results suggest that early exposure to methylphenidate alters reward system functioning, thereby making these systems more sensitive to appetitive stimuli.