Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Differential effect of sterols on dipole potential in hippocampal membranes: implications for receptor function

Dipole potential is the potential difference within the membrane bilayer, which originates due to the nonrandom arrangement of lipid dipoles and water molecules at the membrane interface. In this work, we have explored the possible correlation between functional modulation of a G protein-coupled receptor (the serotonin(1A) receptor) and membrane dipole potential, under conditions of altered membrane sterol composition. We have previously shown that the ligand binding activity of the hippocampal serotonin(1A) receptor is reduced upon cholesterol depletion and could be restored upon replenishment with cholesterol. Interestingly, when the replenishment was carried out with an immediate biosynthetic precursor of cholesterol (7-DHC), differing with cholesterol merely in a double bond, the ligand binding activity of the receptor was not restored. In order to understand the mechanistic framework of receptor-cholesterol interaction, we carried out dipole potential measurements of hippocampal membranes under these conditions, by the dual wavelength ratiometric approach using an electrochromic probe (di-8-ANEPPS). We show here that dipole potential of hippocampal membranes is reduced upon progressive depletion of cholesterol and is restored upon replenishment with cholesterol, but not with 7-DHC. Our results show that the recovery of ligand binding activity of the serotonin(1A) receptor upon replenishment with cholesterol (but not with 7-DHC) could be related to the differential ability of these closely related sterols to modulate membrane dipole potential. We conclude that subtle changes in membrane dipole potential could be crucial in understanding the complex interplay between membrane lipids and proteins in the cellular milieu.

Serotonin: from top to bottom

Serotonin is a monoamine neurotransmitter, which is phylogenetically conserved in a wide range of species from nematodes to humans. In mammals, age-related changes in serotonin systems are known risk factors of age-related diseases, such as diabetes, faecal incontinence and cardiovascular diseases. A decline in serotonin function with aging would be consistent with observations of age-related changes in behaviours, such as sleep, sexual behaviour and mood all of which are linked to serotonergic function. Despite this little is known about serotonin in relation to aging. This review aims to give a comprehensive analysis of the distribution, function and interactions of serotonin in the brain; gastrointestinal tract; skeletal; vascular and immune systems. It also aims to demonstrate how the function of serotonin is linked to aging and disease pathology in these systems. The regulation of serotonin via microRNAs is also discussed, as are possible applications of serotonergic drugs in aging research and age-related diseases. Furthermore, this review demonstrates that serotonin is potentially involved in whole organism aging through its links with multiple organs, the immune system and microRNA regulation. Methods to investigate these links are discussed.

Behavioral and Antinociceptive Effects of Different Psychostimulant Drugs in Prenatally Methamphetamine-Exposed Rats

Prenatal exposure to methamphetamine (METH) increases nociceptive sensitivity in adult rats. As the strong analgesics have high abuse potential and drugs of abuse are known to have analgesic properties, the aim was to study analgesic effect of different psychostimulants in control and prenatally METH-exposed rats. Latencies of withdrawal reflexes of hind limbs and the tail on thermal nociceptive stimuli were repeatedly measured in 15-min intervals after the application of 5 mg/kg s.c. of amphetamine (AMPH), methamphetamine (METH), cocaine (COC), 3,4-methylenedioxymethamphetamine (MDMA) or morphine (MOR). In all groups, AMPH induced on hind limbs stronger analgesia than METH and MDMA whereas COC and MOR were practically without any effect. On the tail, effect of AMPH did not differ from that of MOR. All psychostimulants increased defecation in comparison with MOR and in all groups the number of defecation boluses positively correlated with analgesia of the hind limbs. We did not confirm that prenatal exposure to METH makes adult rats more sensitive either to same drug or to other psychostimulants. The different analgesic potencies of psychostimulants and MOR at different body sites indicate the possible existence of a somatotopic organization of pain inhibition, which is controlled by different mechanisms.

Enhancing genotyping of MAOA-LPR and 5-HTT-LPR in rhesus macaques (Macaca mulatta)

BACKGROUND

Genetic variation in monoamine oxidase A (MAOA) and serotonin transporter (5-HTT)-linked polymorphic regions (LPR) is associated with neuropsychiatric behavior.

METHODS

We genotyped 37 macaques using conventional PCR product gel fractionation and by capillary electrophoresis of multiplexed amplicons and compared the data.

RESULTS

Genotype concordance was 97% and 95% for MAOA-LPR and 5-HTT-LPR, respectively. Capillary electrophoresis was more sensitive and cost-effective.

CONCLUSIONS

Multiplexing MAOA-LPR and 5-HTT-LPR will enhance the genotyping of large sample sets.

Normative database of the serotonergic system in healthy subjects using multi-tracer PET

The highly diverse serotonergic system with at least 16 different receptor subtypes is implicated in the pathophysiology of most neuropsychiatric disorders including affective and anxiety disorders, obsessive compulsive disorder, post-traumatic stress disorder, eating disorders, sleep disturbance, attention deficit/hyperactivity disorder, drug addiction, suicidal behavior, schizophrenia, Alzheimer, etc. Alterations of the interplay between various pre- and postsynaptic receptor subtypes might be involved in the pathogenesis of these disorders. However, there is a lack of comprehensive in vivo values using standardized procedures. In the current PET study we quantified 3 receptor subtypes, including the major inhibitory (5-HT(1A) and 5-HT(1B)) and excitatory (5-HT(2A)) receptors, and the transporter (5-HTT) in the brain of healthy human subjects to provide a database of standard values. PET scans were performed on 95 healthy subjects (age=28.0 ± 6.9 years; 59% males) using the selective radioligands [carbonyl-(11)C]WAY-100635, [(11)C]P943, [(18)F]altanserin and [(11)C]DASB, respectively. A standard template in MNI stereotactic space served for region of interest delineation. This template follows two anatomical parcellation schemes: 1) Brodmann areas including 41 regions and 2) AAL (automated anatomical labeling) including 52 regions. Standard values (mean, SD, and range) for each receptor and region are presented. Mean cortical and subcortical binding potential (BP) values were in good agreement with previously published human in vivo and post-mortem data. By means of linear equations, PET binding potentials were translated to post-mortem binding (provided in pmol/g), yielding 5.89 pmol/g (5-HT(1A)), 23.5 pmol/g (5-HT(1B)), 31.44 pmol/g (5-HT(2A)), and 11.33 pmol/g (5-HTT) being equivalent to the BP of 1, respectively. Furthermore, we computed individual voxel-wise maps with BP values and generated average tracer-specific whole-brain binding maps. This knowledge might improve our interpretation of the alterations taking place in the serotonergic system during neuropsychiatric disorders.

Adolescent amphetamine exposure elicits dose-specific effects on monoaminergic neurotransmission and behaviour in adulthood

Despite the growing non-medical consumption of amphetamine (Amph) during adolescence, its long-term neurobiological and behavioural effects have remained largely unexplored. The present research sought to characterize the behavioural profile and electrophysiological properties of midbrain monoaminergic neurons in adult rodents after Amph exposure during adolescence. Adolescent rats were administered vehicle, 0.5, 1.5, or 5.0 mg/kg.d Amph from postnatal day (PND) 30-50. At adulthood (PND 70), rats were tested in an open-field test (OFT) and elevated plus maze (EPM), paralleled by in-vivo extracellular recordings of serotonin (5-HT), dopamine (DA) and norepinephrine (NE) neurons from the dorsal raphe nucleus, ventral tegmental area, and locus coeruleus, respectively. 5-HT firing in adulthood was increased in rats that had received Amph (1.5 mg/kg.d) during adolescence. At this regimen, DA firing activity was increased, but not NE firing. Conversely, the highest Amph dose regimen (5.0 mg/kg.d) enhanced NE firing, but not DA or 5-HT firing rates. In the OFT, Amph (1.5 mg/kg.d) significantly increased the total distance travelled, while the other doses were ineffective. In the EPM, all three Amph doses increased time spent in the open arms and central platform, as well as the number of stretch-attend postures made. Repeated adolescent exposure to Amph differentially augments monoaminergic neuronal firing in a dose-specific fashion in adulthood, with corresponding alterations in locomotion, risk assessment (stretch-attend postures and central platform occupancy) and risk-taking behaviours (open-arm exploration). Thus, adolescent Amph exposure induces long-lasting neurophysiological alterations that may have implications for drug-seeking behaviour in the future.

Evidence for a role of a dopamine/5-HT6 receptor interaction in cocaine reinforcement

The putative 5-HT6 receptor agonist ST1936 has been shown to increase extracellular dopamine (DA) in the n.accumbens (NAc) shell and in the medial prefrontal cortex (PFCX). These observations suggest that 5-HT6 receptors modulate DA transmission in mesolimbic and mesocortical terminal DA areas. To investigate the behavioral counterpart of this interaction we studied in rats 1) the ability of ST1936 to maintain i.v. self-administration in fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement; 2) the effect of 5-HT6 receptor blockade on cocaine stimulated overflow of DA in dialysates from the PFCX and from the NAc shell and on cocaine i.v. self-administration. ST1936 was i.v. self-administered at unitary doses of 0.5-1 mg/kg on an FR1 and PR schedule of reinforcement, with breaking point of about 4. Pretreatment with the 5-HT6 antagonist SB271046 reduced by about 80% responding for ST1936. SB271046 also reduced cocaine-induced increase of dialysate DA in the NAc shell but not in the PFCX and impaired i.v. cocaine self-administration. These observations indicate that ST1936 behaves as a weak reinforcer and suggest that 5-HT6 receptors play a role in cocaine reinforcement via their facilitatory interaction with DA projections to the NAc shell. This novel 5-HT/DA interaction might provide the basis for a new pharmacotherapeutic strategy of cocaine addiction.

Neurocircuitry for modeling drug effects

The identification and functional understanding of the neurocircuitry that mediates alcohol and drug effects that are relevant for the development of addictive behavior is a fundamental challenge in addiction research. Here we introduce an assumption-free construction of a neurocircuitry that mediates acute and chronic drug effects on neurotransmitter dynamics that is solely based on rodent neuroanatomy. Two types of data were considered for constructing the neurocircuitry: (1) information on the cytoarchitecture and neurochemical connectivity of each brain region of interest obtained from different neuroanatomical techniques; (2) information on the functional relevance of each region of interest with respect to alcohol and drug effects. We used mathematical data mining and hierarchical clustering methods to achieve the highest standards in the preprocessing of these data. Using this approach, a dynamical network of high molecular and spatial resolution containing 19 brain regions and seven neurotransmitter systems was obtained. Further graph theoretical analysis suggests that the neurocircuitry is connected and cannot be separated into further components. Our analysis also reveals the existence of a principal core subcircuit comprised of nine brain regions: the prefrontal cortex, insular cortex, nucleus accumbens, hypothalamus, amygdala, thalamus, substantia nigra, ventral tegmental area and raphe nuclei. Finally, by means of algebraic criteria for synchronizability of the neurocircuitry, the suitability for in silico modeling of acute and chronic drug effects is indicated. Indeed, we introduced as an example a dynamical system for modeling the effects of acute ethanol administration in rats and obtained an increase in dopamine release in the nucleus accumbens-a hallmark of drug reinforcement-to an extent similar to that seen in numerous microdialysis studies. We conclude that the present neurocircuitry provides a structural and dynamical framework for large-scale mathematical models and will help to predict chronic drug effects on brain function.

Medication discovery for addiction: translating the dopamine D3 receptor hypothesis

The dopamine D3 receptor (D3R) has been investigated as a potential target for medication development to treat substance use disorders (SUDs) with a particular focus on cocaine and methamphetamine. Currently, there are no approved medications to treat cocaine and methamphetamine addiction and thus developing pharmacotherapeutics to complement existing behavioral strategies is a fundamental goal. Novel compounds with high affinity and D3R selectivity have been evaluated in numerous animal models of drug abuse and favorable outcomes in nonhuman primate models of self-administration and relapse have provided compelling evidence to advance these agents into the clinic. One approach is to repurpose drugs that share the D3R mechanism and already have clinical utility, and to this end buspirone has been identified as a viable candidate for clinical trials. A second, but substantially more resource intensive and risky approach involves the development of compounds that exclusively target D3R, such as GSK598809 and PG 619. Clinical investigation of these drugs or other novel D3R-selective agents will provide a better understanding of the role D3R plays in addiction and whether or not antagonists or partial agonists that are D3R selective are effective in achieving abstinence in this patient population.

Cholesterol depletion modulates detergent resistant fraction of human serotonin(1A) receptors

Insolubility of membrane components in non-ionic detergents such as Triton X-100 at low temperature is a widely used biochemical criterion to identify, isolate and characterize membrane domains. In this work, we monitored the detergent insolubility of the serotonin(1A) receptor in CHO cell membranes and its modulation by membrane cholesterol. The serotonin(1A) receptor is an important member of the G-protein coupled receptor family. It is implicated in the generation and modulation of various cognitive, behavioral and developmental functions and serves as a drug target. Our results show that a significant fraction (∼28%) of the serotonin(1A) receptor resides in detergent-resistant membranes (DRMs). Interestingly, the fraction of the serotonin(1A) receptor in DRMs exhibits a reduction upon membrane cholesterol depletion. In addition, we show that contents of DRM markers such as flotillin-1, caveolin-1 and GM₁ are altered in DRMs upon cholesterol depletion. These results assume significance since the function of the serotonin(1A) receptor has previously been shown to be affected by membrane lipids, specifically cholesterol. Our results are relevant in the context of membrane organization of the serotonin(1A) receptor in particular, and G-protein coupled receptors in general.

Intracellular Loop 2 Peptides of the Human 5HT1a Receptor are Differential Activators of Gi

Peptide mimics of intracellular loop 2 (ic2) of the human 5HT1a receptor have been studied with respect to their ability to inhibit agonist binding via interference with receptor-G-protein coupling. These peptides give shallow concentration-effect relationships. Additionally, these peptides have been studied with respect to their ability to trigger the signal transduction system of this Gi-coupled receptor. Two signaling parameters have been quantified: concentration of intracellular cAMP and changes in incorporation into the G protein of a stable analog of GTP. In both cases, peptide mimics near midloop of ic2 actually show agonist activity with efficacy falling off toward both loop termini near TM 3 and TM 4. Previous results have suggested that the loop region near the TM3/ic2 interface is primarily responsible for receptor-G-protein coupling, while the current result emphasizes the mid-ic2 loop region's ability to activate the G protein following initial coupling. A limited number of peptides from the receptor's TM5/ic3 loop vicinity were also studied regarding agonist inhibition and G-protein activation. These peptides provide additional evidence that the human 5HT1a receptor, TM5/ic3 loop region, is involved in both coupling and activation actions. Overall, these results provide further information about potential pharmacological intervention and drug development with respect to the human 5HT1a receptor/G-protein system. Finally, the structural evidence generated here provides testable models pending crystallization and X-ray analysis of the receptor.

Benzo[α]pyrene-induced anti-depressive-like behaviour in adult female mice: role of monoaminergic systems

Benzo[α]pyrene (B[α]P) is a ubiquitous environmental pollutant exhibiting adverse effects on cognitive function and behaviour. In this study, depressive or antidepressive effects of B[α]P were investigated. Here, we report that a subacute B[α]P oral exposure (0.02-0.2 mg/kg) increases mobility behaviour in female adult mice in the tail suspension test, but not in the forced swimming test, without altering locomotion, suggesting that the tail suspension test was a more sensitive indicator of B[α]P-induced neurobehavioural disturbance. This might be because of differences in neurochemical substrates and pathways, mediating the performance in these behavioural models of depression. The effect of B[α]P on female adult mice in the tail suspension test was similar to that obtained with subacute treatment of the antidepressant reference drug imipramine (10 mg/kg). Therefore, B[α]P at 0.02 mg/kg and 0.2 mg/kg induces an antidepressant-like effect in mice, suggesting a neurobehavioural disturbance after oral exposure to this environmental compound. Furthermore, oral exposure to B[α]P at 0.02 mg/kg significantly increased gene expression levels of the brain receptors 5-hydroxytryptamine (serotonin) 1A (5HT(1A)) and alpha-1D adrenergic (ADRA(1D)). In summary, the presented findings suggest that subacute oral exposure to B[α]P results in behavioural changes in female adult mice, possibly caused by alterations in the serotoninergic and adrenergic systems.

Potential serotonin 5-HT(1A) and dopamine D(4) receptor modulation of the discriminative stimulus effects of amphetamine in rats

Activation of the dopaminergic system underlies the behavioral effects of (+)-amphetamine, and plays a major role in its discriminative stimulus properties. Although serotonin receptors modulate dopamine levels in the brain, and 5-HT(1A) and 5-HT(2) receptor agonists do not mimic (+)-amphetamine, pretreatment with 5-HT(2A/2C) agonists significantly potentiates the (+)-amphetamine cue. Further, 5-HT(2) antagonists do not modify the discriminative stimulus effect of (+)-amphetamine, but 5-HT(1A) antagonists have never been tested in (+)-amphetamine-trained rats. This study sought to characterize the effects of the 5-HT(1A) antagonist WAY 100635 on (+)-amphetamine-induced discriminative stimulus effects. Male Sprague-Dawley rats were trained in a two-lever, fixed ratio 50, food-reinforced task with (+)-amphetamine sulfate (1.0 mg/kg, i.p., 30 min pretreatment time) as the discriminative stimulus. Substitution and combination tests with WAY 100635 were then performed. WAY 100635 did not produce substitution in amphetamine-trained rats, but significantly increased behavioral disruption. In combination tests 0.4 and 5.4 mg/kg doses of WAY 100635 potentiated the amphetamine cue. We suggest that low doses of WAY 100635 potentiated the (+)-amphetamine cue by blockade of 5-HT(1A) receptors, but stimulation of the dopamine D(4) receptor by higher doses of WAY 100635 may be responsible for potentiation of amphetamine-induced behavioral sensitization. The high percentage of behavioral disruption in substitution tests might suggest that rats trained to discriminate (+)-amphetamine from saline show behavioral sensitization that is not detectable by the drug discrimination assay but may be expressed as hyperactivity and stereotypic behavior that disrupts operant behavior.

Prenatal Cocaine Disrupts Serotonin Signaling-Dependent Behaviors: Implications for Sex Differences, Early Stress and Prenatal SSRI Exposure

Prenatal cocaine (PC) exposure negatively impacts the developing nervous system, including numerous changes in serotonergic signaling. Cocaine, a competitive antagonist of the serotonin transporter, similar to selective serotonin reuptake inhibitors (SSRIs), also blocks dopamine and norepinephrine transporters, leaving the direct mechanism through which cocaine disrupts the developing serotonin system unclear. In order to understand the role of the serotonin transporter in cocaine's effect on the serotonergic system, we compare reports concerning PC and prenatal antidepressant exposure and conclude that PC exposure affects many facets of serotonergic signaling (serotonin levels, receptors, transporters) and that these effects differ significantly from what is observed following prenatal SSRI exposure. Alterations in serotonergic signaling are dependent on timing of exposure, test regimens, and sex. Following PC exposure, behavioral disturbances are observed in attention, emotional behavior and stress response, aggression, social behavior, communication, and like changes in serotonergic signaling, these effects depend on sex, age and developmental exposure. Vulnerability to the effects of PC exposure can be mediated by several factors, including allelic variance in serotonergic signaling genes, being male (although fewer studies have investigated female offspring), and experiencing the adverse early environments that are commonly coincident with maternal drug use. Early environmental stress results in disruptions in serotonergic signaling analogous to those observed with PC exposure and these may interact to produce greater behavioral effects observed in children of drug-abusing mothers. We conclude that based on past evidence, future studies should put a greater emphasis on including females and monitoring environmental factors when studying the impact of PC exposure.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Desmosterol replaces cholesterol for ligand binding function of the serotonin(1A) receptor in solubilized hippocampal membranes: support for nonannular binding sites for cholesterol?

The serotonin(1A) receptor is an important member of the G-protein coupled receptor family, and is involved in the generation and modulation of a variety of cognitive and behavioral functions. Solubilization of the hippocampal serotonin(1A) receptor by CHAPS is accompanied by loss of cholesterol that results in a reduction in specific agonist binding activity. Replenishment of cholesterol to solubilized membranes restores membrane cholesterol content and significantly recovers specific agonist binding. In order to test the stringency of cholesterol requirement, we solubilized native hippocampal membranes followed by replenishment with desmosterol. Desmosterol is the immediate biosynthetic precursor of cholesterol in the Bloch pathway differing only in a double bond at the 24th position. Our results show that replenishment with desmosterol restores ligand binding of serotonin(1A) receptors. This is consistent with earlier results showing that desmosterol can replace cholesterol in a large number of cases. However, these results appear to be contradictory to our earlier findings, performed by sterol manipulation utilizing methyl-β-cyclodextrin, in which we observed that replacing cholesterol with desmosterol is unable to restore specific ligand binding of the hippocampal serotonin(1A) receptor. We discuss the possible molecular mechanism, in terms of nonannular lipid binding sites around the receptor, giving rise to these differences.

MDMA-induced c-Fos expression in oxytocin-containing neurons is blocked by pretreatment with the 5-HT-1A receptor antagonist WAY 100635

The popular party drug MDMA (3,4-methylenedioxymethamphetamine, "Ecstasy") increases sociability in both humans and laboratory animals. Recent research suggests that these prosocial effects may involve serotonin (5-HT)-stimulated hypothalamic release of the neuropeptide oxytocin. WAY 100635, a 5-HT(1A) receptor antagonist, prevents MDMA-induced increases in plasma oxytocin and also reduces MDMA-mediated increases in social interaction in rats. The present study used c-Fos immunohistochemistry to determine the possible role of 5-HT(1A) receptors in MDMA-mediated activation of oxytocin synthesizing neurons. Male Wistar rats (n=8/group) were administered MDMA (10 mg/kg, i.p.) with or without WAY 100635 (1 mg/kg, i.p.) pre-treatment and c-Fos expression was then assessed throughout the brain. MDMA significantly increased locomotor activity and this effect was partly prevented by WAY 100635, in agreement with previous studies. WAY 100635 significantly reduced MDMA-induced c-Fos expression in a subset of brain regions examined. A particularly prominent reduction was seen in the oxytocin-positive neurons of the supraoptic nucleus and paraventricular hypothalamus, with more modest reductions in the Islands of Calleja, median preoptic nucleus, somatosensory cortex and nucleus of the solitary tract. WAY 100635 did not alter MDMA-induced c-Fos expression in the striatum, thalamus, or central amygdala. These results indicate that MDMA's action on oxytocin producing cells in the hypothalamus is mediated through 5-HT(1A) receptors and that certain specific cortical, limbic and brainstem sites are also activated by MDMA via these receptors.

Contributions of serotonin in addiction vulnerability

The serotonin (5-hydroxytryptamine; 5-HT) system has long been associated with mood and its dysregulation implicated in the pathophysiology of mood and anxiety disorders. While modulation of 5-HT neurotransmission by drugs of abuse is also recognized, its role in drug addiction and vulnerability to drug relapse is a more recent focus of investigation. First, we review preclinical data supporting the serotonergic raphe nuclei and their forebrain projections as targets of drugs of abuse, with emphasis on the effects of psychostimulants, opioids and ethanol. Next, we examine the role of 5-HT receptors in impulsivity, a core behavior that contributes to the vulnerability to addiction and relapse. Finally, we discuss evidence for serotonergic dysregulation in comorbid mood and addictive disorders and suggest novel serotonergic targets for the treatment of addiction and the prevention of drug relapse.

5-HT receptors and reward-related behaviour: a review

The brain's serotonin (5-HT) system is key in the regulation of reward-related behaviours, from eating and drinking to sexual activity. The complexity of studying this system is due, in part, to the fact that 5-HT acts at many receptor subtypes throughout the brain. The recent development of drugs with greater selectivity for individual receptor subtypes has allowed for rapid advancements in our understanding of this system. Use of these drugs in combination with animal models entailing selective reward measures (i.e. intracranial self-stimulation, drug self-administration, conditioned place preference) have resulted in a greater understanding of the pharmacology of reward-related processing and behaviour (particularly regarding drugs of abuse). The putative roles of each 5-HT receptor subtype in the pharmacology of reward are outlined and discussed here. It is concluded that the actions of 5-HT in reward are receptor subtype-dependent (and thus should not be generalized) and that all studied subtypes appear to have a unique profile which is determined by content (e.g. receptor function, localization - both throughout the brain and within the synapse) and context (e.g. type of behavioural paradigm, type of drug). Given evidence of altered reward-related processing and serotonergic function in numerous neuropsychiatric disorders, such as depression, schizophrenia, and addiction, a clearer understanding of the role of 5-HT receptor subtypes in this context may lead to improved drug development and therapeutic approaches.

Mirtazapine alters cue-associated methamphetamine seeking in rats

BACKGROUND

Methamphetamine (METH) is a potent psychostimulant, repeated use of which can result in a substance abuse disorder. Withdrawn individuals are highly prone to relapse, which may be driven, at least in part, by a hyperresponsivity to METH-associated cues that can prompt METH-seeking. Clinically efficacious pharmacotherapies for METH abuse are critically needed. Mirtazapine (Remeron) is an atypical antidepressant that antagonizes activated norepinephrine(α)₂, histamine₁ serotonin (5-HT)₂(A/C), and 5-HT₃ receptors. This pharmacologic profile prompted our interest in its potential for preventing relapse to METH-taking. This study tested the hypothesis that mirtazapine would attenuate METH-seeking in rats trained to self-administer METH.

METHODS

Rats were trained to self-administer METH in a lever-pressing operant task. The effect of mirtazapine on METH-seeking was determined by evaluating lever pressing in the presence of cues previously associated with METH, but in the absence of METH reinforcement. Two paradigms were used: cue reactivity, wherein rats do not undergo extinction training, and a cue-induced reinstatement paradigm after extinction.

RESULTS

Mirtazapine (5.0 mg/kg) pretreatment reduced METH-seeking by ∼ 50% in the first 15 min of cue reactivity and cue-induced reinstatement testing. This mirtazapine dose did not significantly affect motor performance.

CONCLUSIONS

This study revealed the overlapping nature of cue reactivity and cue-induced reinstatement procedures and provided preclinical evidence that mirtazapine can attenuate METH-seeking behavior.

Role of serotonin 5-HT2A and 5-HT2C receptors on brain stimulation reward and the reward-facilitating effect of cocaine

RATIONALE

The serotonin 5-HT(2A) and 5-HT(2C) receptors, which are found in abundance in the mesolimbocortical dopaminergic system, appear to modulate the behavioral effects of cocaine.

OBJECTIVES

The present series of studies set out to investigate the role of 5-HT(2A) and 5-HT(2C) receptors on brain reward and on the reward-facilitating effect of cocaine and localize the neural substrates within the mesolimbocortical dopaminergic system that are responsible for these effects.

METHODS

Male Sprague-Dawley rats were implanted with stimulating electrodes and bilateral cannulae for the experiments involving microinjections and were trained to respond to electrical stimulation. In the first study, we examined the effects of systemic administration of selective 5-HT(2A) and 5-HT(2C) receptor agonists (TCB-2 and WAY-161503) and antagonists (R-96544 and SB-242084) on intracranial self-stimulation (ICSS). In the second study, we examined the effectiveness of TCB-2, WAY-161503, R-96544, and SB-242084 in blocking the reward-facilitating effect of cocaine. In the third study, we examined the effects of intra-medial prefrontal cortex (mPFC), intra-nucleus accumbens (NAC), and intra-ventral tegmental area (VTA) injection of WAY-161503 on the reward-facilitating effect of cocaine.

RESULTS

Acute systemic administration of TCB-2 and WAY-161503 increased ICSS threshold. Systemic WAY-161503 attenuated the reward-facilitating effect of cocaine. This effect was reversed by pretreatment with SB-242084. Intracranial microinjections of WAY-161503 into the mPFC and the NAC shell/core, but not the VTA, attenuated the reward-facilitating effect of cocaine.

CONCLUSION

These data indicate that 5-HT(2C) receptors within the mPFC and the NAC modulate the reinforcing effects of cocaine and provide evidence that 5-HT(2C) receptor agonists could be a possible drug discovery target for the treatment of psychostimulant addiction.

Attenuation of reinforcing and psychomotor stimulant effects of amphetamine by aripiprazole

Partial dopamine agonists are potential medications for the treatment of amphetamine addiction. They have been hypothesized to stabilize the dopamine system by acting as antagonists during high dopaminergic tone resulting from amphetamine use and as agonists during withdrawal. Aripiprazole is an atypical antipsychotic that acts as a partial D2 dopamine and a serotonin 5-HT₁(A) agonist and a serotonin 5-HT₂(A) antagonist. The aim of the present study was to examine the effects of aripiprazole on behaviors induced and maintained by d-amphetamine. To this end, intravenous d-amphetamine self-administration [fixed ratio 3 (FR3) schedule, 0.02 mg/infusion] and d-amphetamine-induced (0, 1.5 mg/kg intraperitoneally) locomotor activity, as well as spontaneous locomotor activity and sucrose pellet selfadministration (FR3 schedule) were studied in male Wistar rats after aripiprazole (0, 0.3, 1, 3 mg/kg i.p.) administration. Aripiprazole pre-treatment resulted in bidirectional effects on amphetamine self-administration. The 1 mg/kg dose increased, and the highest dose decreased the number of amphetamine infusions. In the locomotor activity experiments, aripiprazole attenuated amphetamine-induced activity dose-dependently and tended to suppress spontaneous activity. The highest aripiprazole doses decreased also sucrose pellet self-administration. The increase in amphetamine self-administration with the intermediate aripiprazole dose, as well as the decrease in amphetamine-induced locomotor activity, suggests that aripiprazole acted as a dopamine antagonist. Suppression of amphetamine and sucrose self-administration by the highest aripiprazole dose was probably caused by non-specific effects. Together, these results indicate that under conditions of dopaminergic stimulation, aripiprazole attenuates the reinforcing and psychomotor stimulant effects of d-amphetamine, but the dose range for this effect is rather limited.

Localization of 5-HT1A and 5-HT2A positive cells in the brainstems of control age-matched and Alzheimer individuals

Serotonin receptor 1A and 2A positive cells in postmortem brainstems were demonstrated via immunohistochemistry in eight control age-matched elderly individuals and eight Alzheimer patients. The 5-HT1A positive cells were found in substantia nigra, pontile nucleus, and vagal as well as dorsal raphe nucleus, while 5-HT2A receptor positive cells were found in motor, sensory and spinal trigeminal nuclei, pontile nucleus, substantia nigra, and nucleus solitarius. A comparison in density of positive cells per unit area was made between control age-matched and Alzheimer individuals. Statistically significant differences (p ≤ 0.01) in density were observed in 5-HT1A cells in pontile, dorsal raphe, and vagal nuclei between control age-matched and Alzheimer, and in 5-HT2A positive cells in the sensory trigeminal nucleus, between control and Alzheimer. This de novo study indicated the presence of 5-HT1A and 5-HT2A receptor positive cells in the above nuclei of human brainstem and revealed differences in density between control age-matched and Alzheimer, indicating possible functional derangements in Alzheimer patients in these areas. In addition, colocalization studies indicated that 5-HT1A receptors were in cholinergic cells and gamma-aminobutyric acid positive fibers were linked to 5-HT2A receptor positive cells. It is hoped that understanding these two important 5-HT receptors and their localization might lead to advances in future therapeutic development.

5-HT(1A)-like receptor activation inhibits abstinence-induced methamphetamine withdrawal in planarians

No pharmacological therapy is approved to treat methamphetamine physical dependence, but it has been hypothesized that serotonin (5-HT)-enhancing drugs might limit the severity of withdrawal symptoms. To test this hypothesis, we used a planarian model of physical dependence that quantifies withdrawal as a reduction in planarian movement. Planarians exposed to methamphetamine (10 μM) for 60 min, and then placed (tested) into drug-free water for 5 min, displayed less movement (i.e., withdrawal) than either methamphetamine-naïve planarians tested in water or methamphetamine-exposed planarians tested in methamphetamine. A concentration-related inhibition of withdrawal was observed when methamphetamine-exposed planarians were placed into a solution containing either methamphetamine and 5-HT (0.1-100 μM) or methamphetamine and the 5-HT(1A) receptor agonist 8-hydroxy-N,N-dipropyl-2-aminotetralin (8-OH-DPAT) (10, 20 μM). Planarians with prior methamphetamine exposure displayed enhanced withdrawal when tested in a solution of the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridyl)cyclohexanecarboxamide (WAY 100635) (1 μM). Methamphetamine-induced withdrawal was not affected by the 5-HT(2B/2C) receptor agonist meta-chlorophenylpiperazine (m-CPZ) (0.1-20 μM). These results provide pharmacological evidence that serotonin-enhancing drugs inhibit expression of methamphetamine physical dependence in an invertebrate model of withdrawal, possibly through a 5-HT(1A)-like receptor-dependent mechanism.

Cholesterol depletion mimics the effect of cytoskeletal destabilization on membrane dynamics of the serotonin1A receptor: A zFCS study

Single-point fluorescence correlation spectroscopy (FCS) of membrane-bound molecules suffers from a number of limitations leading to inaccurate estimation of diffusion parameters. To overcome such problems and with the overall goal of addressing membrane heterogeneities, we performed z-scan FCS (zFCS) of the serotonin(1A) receptor. We analyzed the results according to FCS diffusion laws that provide information on the organization of the diffusing species. Analysis of our results shows that the diffusion coefficients of the receptor and a fluorescently labeled phospholipid are similar when probed at length scales approximately 210 nm. We discuss the significance of the spatiotemporal evolution of dynamics of membrane-bound molecules in the overall context of membrane domains and heterogeneity. Importantly, our results show that the serotonin(1A) receptor exhibits confinement in cell membranes, possibly due to interaction with the actin cytoskeleton. Surprisingly, depletion of membrane cholesterol appears to reduce receptor confinement in a manner similar to that observed in the case of cytoskeletal destabilization, implying possible changes in the actin cytoskeleton induced upon cholesterol depletion. These results constitute the first report on G-protein-coupled receptor dynamics utilizing a combination of zFCS and the FCS diffusion laws, and present a convenient approach to explore cell membrane heterogeneity at the submicron level.

Involvement of the brain histaminergic system in addiction and addiction-related behaviors: a comprehensive review with emphasis on the potential therapeutic use of histaminergic compounds in drug dependence

Neurons that produce histamine are exclusively located in the tuberomamillary nucleus of the posterior hypothalamus and send widespread projections to almost all brain areas. Neuronal histamine is involved in many physiological and behavioral functions such as arousal, feeding behavior and learning. Although conflicting data have been published, several studies have also demonstrated a role of histamine in the psychomotor and rewarding effects of addictive drugs. Pharmacological and brain lesion experiments initially led to the proposition that the histaminergic system exerts an inhibitory influence on drug reward processes, opposed to that of the dopaminergic system. The purpose of this review is to summarize the relevant literature on this topic and to discuss whether the inhibitory function of histamine on drug reward is supported by current evidence from published results. Research conducted during the past decade demonstrated that the ability of many antihistaminic drugs to potentiate addiction-related behaviors essentially results from non-specific effects and does not constitute a valid argument in support of an inhibitory function of histamine on reward processes. The reviewed findings also indicate that histamine can either stimulate or inhibit the dopamine mesolimbic system through distinct neuronal mechanisms involving different histamine receptors. Finally, the hypothesis that the histaminergic system plays an inhibitory role on drug reward appears to be essentially supported by place conditioning studies that focused on morphine reward. The present review suggests that the development of drugs capable of activating the histaminergic system may offer promising therapeutic tools for the treatment of opioid dependence.

Behavioral evidence for the significance of serotoninergic (5-HT) receptors in cocaine addiction

Cocaine addiction has somatic, psychological, psychiatric, socio-economic and legal implications in the developed world. Presently, there is no medication approved for the treatment of cocaine addiction. In recent years, data from the literature (pre-clinical studies and clinical trials) have provided several lines of evidence that serotonin (5-HT) and 5-HT receptors play a modulatory role in the mechanisms of action of cocaine. Here we review the contribution of 5-HT receptor subtypes to cocaine sensitization, discrimination, conditioned place preference, self-administration, reinstatement of seeking behavior and withdrawal symptoms in laboratory animals. Additionally, the consequences of chronic cocaine exposure on particular 5-HT receptor-assigned functions in pre-clinical studies are presented.

Thermodynamics of peptide and non-peptide interactions with the human 5HT1a receptor

The human serotonin 1a receptor (H5HT1aR) is a highly studied member of the 7 transmembrane G protein-coupled receptors. This model receptor, negatively coupled to adenylyl cyclase via Gi, is linked to physiological processes such as cognition and mood regulation and to associated disorders like anxiety and depression. Gibb's free energies, enthalpies, and entropies were calculated for the agonist [(3)H]8-OH-DPAT in the presence of synthetic peptides derived from sequences of intracellular loops 2 and 3 of the H5HT1aR. For comparative purposes, the thermodynamic parameters were also determined in the presence of a limited number of ligand-binding site substances (the partial agonist dipropyltryptamine [DPT], and the full agonist [(3)H]8-OH-DPAT alone). All of these thermodynamic measurements were based on binding data accumulated over a range of temperatures (0-35 degrees C). Representative examples of binding constant experiments and van't Hoff plots are shown to establish the thermodynamic variables. Although differences exist between the peptides themselves and the non-peptide agonists, in all situations the binding events are highly entropy driven. Differences between this information and published data for rat 5HT1aR are discussed, as are relationships to other receptor systems. Overall, the conclusions should be useful in further defining a comprehensive model of 5HT1aR, and for future development of binding-site and non-binding-site directed agents for the receptor.

5HT-1A receptors and anxiety-like behaviours: studies in rats with constitutionally upregulated/downregulated serotonin transporter

Altered activity of brain serotonergic (5HT) system has been implicated in a wide range of behaviours and behavioural disorders, including anxiety. Functioning of 5HT-1A receptor has been suggested as a modulator of emotional balance in both, normal and pathological forms of anxiety. Here, we studied serotonergic modulation of anxiety-like behaviour using a genetic rat model with constitutional differences in 5HT homeostasis, named Wistar-Zagreb 5HT (WZ-5HT) rats. The model, consisting of high-5HT and low-5HT sublines, was developed by selective breeding of animals for extreme activities of peripheral (platelet) 5HT transporter, but selection process had affected also central 5HT homeostasis, as evidenced from neurochemical and behavioural studies. Anxiety-like behaviour in WZ-5HT rats was evaluated by two commonly used paradigms: open field and elevated-plus maze. The involvement of 5HT-1A receptors in behavioural response was assessed by measuring mRNA expression in cell bodies (raphe nuclei) and projection regions (frontal cortex, hippocampus) by use of RT-PCR and in situ hybridization, and by measuring functionality of cortical 5HT-1A receptors by use of [(3)H]8-OH-DPAT radioligand binding. Animals from the high-5HT subline exhibit increased anxiety-like behaviour and decreased exploratory activity when exposed to novel environment. No measurable differences in constitutional (baseline) functionality or expression of 5HT-1A receptors between sublines were found. The results support contribution of increased serotonergic functioning to the anxiety-like behaviour. They also validate the high-5HT subline of WZ-5HT rats as a potential model to study mechanisms of anxiety, especially of its nonpathological form, while the low-5HT subline may be useful to model sensation seeking phenotype.

Is ecstasy a drug of dependence?

This paper examines the evidence for an MDMA or "ecstasy" dependence syndrome. Animal evidence suggests that MDMA may be a less potent reinforcer than other drugs, but that it does have dependence potential. This suggests that (a) ecstasy dependence might be less likely than dependence upon other drugs; and (b) factors related to the behavioural and psychological aspects of reward and dependence may make a relatively greater contribution for ecstasy than for other drugs, where physically centred (and better understood) features of dependence may be more salient. Human evidence supports this proposition. Some people report problems with their use, but the literature suggests that physical features play a more limited role than psychological ones. Tolerance is apparent, and withdrawal is self-reported, but it is unclear whether these reports distinguish sub-acute effects of ecstasy intoxication from symptoms reflective of neuroadaptive processes underlying a "true" withdrawal syndrome. Studies examining the structure of dependence upon ecstasy suggest it may be different from drugs such as alcohol, methamphetamine and opioids. Consistent with studies of hallucinogens, a two-factor structure has been identified with factors suggestive of "compulsive use" and "escalating use". Regardless of the nature of any dependence syndrome, however, there is evidence to suggest that a minority of ecstasy users become concerned about their use and seek treatment. Further controlled studies are required to investigate this phenomenon.

Reward and the serotonergic system

Anhedonia, as a failure to experience rewarding stimuli, is a key characteristic of many psychiatric disorders including depression and schizophrenia. Investigations on the neurobiological correlates of reward and hedonia/anhedonia have been a growing subject of research demonstrating several neuromodulators to mediate different aspects of reward processing. Whereas the majority of research on reward mainly focused on the dopamine and opioid systems, a serotonergic mechanism has been neglected. However, recent promising results strengthen the pivotal role of serotonin in reward processing. Evidence includes electrophysical and pharmacological as well as genetic and imaging studies. Primate research using single-unit recording of neurons within the dorsal raphe nucleus argues for a serotonergic mediation of reward value, whereas studies using intracranial self-stimulation point to an important contribution of serotonin in modulating motivational aspects of rewarding brain stimulation. Pharmacological studies using agonists and antagonists of serotonergic receptor subtypes and approaches investigating an increase or decrease of the extracellular level of serotonin offer strong evidence for a serotonergic mediation, ranging from aversion to pleasure. This review provides an argument for serotonin as a fundamental mediator of emotional, motivational and cognitive aspects of reward representation, which makes it possibly as important as dopamine for reward processing.

5-HT1A receptors mediate detrimental effects of cocaine on long-term potentiation and expression of polysialylated neural cell adhesion molecule protein in rat dentate gyrus

The present study investigated the involvement of 5-HT(1A) receptors in the inhibitory effect of single administration of cocaine (COC, 15 mg/kg i.p.) on the induction of long-term potentiation (LTP) in slices of rat dentate gyrus (DG), prepared 30 min and 2 days after COC administration. These effects of COC were blocked by an antagonist of 5-HT(1A) receptors, WAY 100635 (0.4 mg/kg i.p.), which had been administered 20 min before COC. The detrimental effect of COC on LTP in slices prepared 30 min after COC administration could be prevented by blocking glucocorticoid receptors (GRs) using mifepristone (RU 38486, 10 mg/kg s.c. given 1 h before COC), similar as in slices obtained 2 days after COC as reported previously [Maćkowiak et al. (2008) Eur J Neurosci 27:2928-2937]. After a single administration of an agonist of 5-HT(1A) receptors, 8-OH-DPAT, (0.5 mg/kg i.p.), the level of LTP in slices prepared 2 days later was significantly decreased resembling the effect of COC. This effect of 8-OH-DPAT was antagonized by WAY 100635 (0.4 mg/kg i.p.), administered 20 min before 8-OH-DPAT and by RU 38486, given 1 h before 8-OH-DPAT. COC-induced inhibition of LTP could be blocked by the inhibitor of mitogen-activated protein kinase kinase 1/2 (MEK1/2), SL 327 (50 mg/kg i.p.), administered 1 h before COC, but not by the inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), LY 294002 (80 mg/kg i.p.). COC-induced reduction in the number of polysialylated neural cell adhesion molecule (PSA-NCAM)-positive neurons in rat dentate gyrus could also be prevented by WAY 100635, given 20 min before COC. These data indicate that the indirect 5-HT(1A) receptor activation by a single COC administration and subsequent stimulation of extracellular signal-regulated kinases (ERK 1/2) signaling pathway result in a decrease of the potential for long-term increase in synaptic efficacy in rat DG lasting at least two but less than 7 days, most likely via activation of the hypothalamic-pituitary-adrenal (HPA) axis.

Investigation of serotonin-1A receptor function in the human psychopharmacology of MDMA

Serotonin (5-HT) release is the primary pharmacological mechanism of 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy') action in the primate brain. Dopamine release and direct stimulation of dopamine D2 and serotonin 5-HT2A receptors also contributes to the overall action of MDMA. The role of 5-HT1A receptors in the human psychopharmacology of MDMA, however, has not yet been elucidated. In order to reveal the consequences of manipulation at the 5-HT1A receptor system on cognitive and subjective effects of MDMA, a receptor blocking study using the mixed beta-adrenoreceptor blocker/5-HT1A antagonist pindolol was performed. Using a double-blind, placebo-controlled within-subject design, 15 healthy male subjects were examined under placebo (PL), 20 mg pindolol (PIN), MDMA (1.6 mg/kg b.wt.), MDMA following pre-treatment with pindolol (PIN-MDMA). Tasks from the Cambridge Neuropsychological Test Automated Battery were used for the assessment of cognitive performance. Psychometric questionnaires were applied to measure effects of treatment on core dimensions of Altered States of Consciousness, mood and state anxiety. Compared with PL, MDMA significantly impaired sustained attention and visual-spatial memory, but did not affect executive functions. Pre-treatment with PIN did not significantly alter MDMA-induced impairment of cognitive performance and only exerted a minor modulating effect on two psychometric scales affected by MDMA treatment ('positive derealization' and 'dreaminess'). Our findings suggest that MDMA differentially affects higher cognitive functions, but does not support the hypothesis from animal studies, that some of the MDMA effects are causally mediated through action at the 5-HT1A receptor system.