5-HT2A/2C receptor signaling via phospholipase A2 and arachidonic acid is attenuated in mice lacking the serotonin reuptake transporter

Serotonin 2C receptors are also important in head-twitch responses in male mice

RATIONALE

Serotonergic psychedelics exert their effects via their high affinity for serotonin (5-HT) receptors, particularly through activating 5-HT2A receptors (5-HT2AR), employing the frontal cortex-dependent head-twitch response (HTR). Although universally believed to be so, studies have not yet fully ascertained whether 5-HT2AR activation is the sole initiator of these psychedelic effects. This is because not all 5-HT2AR agonists exhibit similar pharmacologic properties.

OBJECTIVE

This study aims to identify and discriminate the roles of 5-HT2AR and 5-HT2CR in the HTR induced by Methallylescaline (MAL) and 4-Methyl-2,5,β-trimethoxyphenethylamine (BOD) in male mice. Also, an analysis of their potential neurotoxic properties was evaluated.

METHODS

Male mice treated with MAL and BOD were evaluated in different behavioral paradigms targeting HTR and neurotoxicity effects. Drug affinity, pharmacological blocking, and molecular analysis were also conducted to support the behavioral findings. The HTR induced by DOI has been extensively characterized in male mice, making it a good positive control for this study, specifically for comparing the pharmacological effects of our test compounds.

RESULTS

The activation of 5-HT2CR, alone or in concert with 5-HT2AR, produces a comparable degree of HTRs (at a dose of 1 mg·kg-1), with divergent 5-HT2CR- and 5-HT2AR-Gqα11-mediated signaling and enhanced neurotoxic properties (at a dose of 30 mg·kg-1) coupled with activated pro-inflammatory cytokines. These findings show these compounds' potential psychedelic and neurotoxic effects in male mice.

CONCLUSION

These findings showed that while 5-HT2AR is the main initiator of HTR, the 5-HT2CR also has a distinct property that renders it effective in inducing HTR in male mice.

Agomelatine inhibits platelet aggregation through melatonin receptor-dependent and independent mechanisms

AIMS

Melatonin is known to inhibit platelet aggregation induced by arachidonic acid (AA). In the present study we investigated whether agomelatine (Ago), an antidepressant with agonist activity at melatonin receptor 1 (MT1) and MT2 could reduce platelets aggregation and adhesion.

MAIN METHODS

Human platelets from healthy donors were used to test the in vitro effects of Ago in the presence of different platelet activators. We performed aggregation and adhesion assays, thromboxane B₂ (TxB₂), cAMP and cGMP measurements, intra-platelet calcium registration and flow cytometry assays.

KEY FINDINGS

Our data revealed that different concentrations of Ago reduced AA- and collagen-induced human platelet aggregation in vitro. Ago also reduced AA-induced increase in thromboxane B₂ (TxB₂) production, intracellular calcium levels and P-selectin expression at plasma membrane. The effects of Ago in AA-activated platelets were likely dependent on MT1 as they were blocked by luzindole (a MT1/MT2 antagonist) and mimicked by the MT1 agonist UCM871 in a luzindole-sensitive manner. The MT2 agonist UCM924 was also able to inhibit platelet aggregation, but this response was not affected by luzindole. On the other hand, although UCM871 and UCM924 reduced collagen-induced platelet aggregation and adhesion, inhibition of collagen-induced platelet aggregation by Ago was not mediated by melatonin receptors because it was not affected by luzindole.

SIGNIFICANCE

The present data show that Ago suppresses human platelet aggregation and suggest that this antidepressant may have the potential to prevent atherothrombotic ischemic events by reducing thrombus formation and vessel occlusion.

Serotonin-induced vascular permeability is mediated by transient receptor potential vanilloid 4 in the airways and upper gastrointestinal tract of mice

Endothelial and epithelial cells form physical barriers that modulate the exchange of fluid and molecules. The integrity of these barriers can be influenced by signaling through G protein-coupled receptors (GPCRs) and ion channels. Serotonin (5-HT) is an important vasoactive mediator of tissue edema and inflammation. However, the mechanisms that drive 5-HT-induced plasma extravasation are poorly defined. The Transient Receptor Potential Vanilloid 4 (TRPV4) ion channel is an established enhancer of signaling by GPCRs that promote inflammation and endothelial barrier disruption. Here, we investigated the role of TRPV4 in 5-HT-induced plasma extravasation using pharmacological and genetic approaches. Activation of either TRPV4 or 5-HT receptors promoted significant plasma extravasation in the airway and upper gastrointestinal tract of mice. 5-HT-mediated extravasation was significantly reduced by pharmacological inhibition of the 5-HT2A receptor subtype, or with antagonism or deletion of TRPV4, consistent with functional interaction between 5-HT receptors and TRPV4. Inhibition of receptors for the neuropeptides substance P (SP) or calcitonin gene-related peptide (CGRP) diminished 5-HT-induced plasma extravasation. Supporting studies assessing treatment of HUVEC with 5-HT, CGRP, or SP was associated with ERK phosphorylation. Exposure to the TRPV4 activator GSK1016790A, but not 5-HT, increased intracellular Ca2+ in these cells. However, 5-HT pre-treatment enhanced GSK1016790A-mediated Ca2+ signaling, consistent with sensitization of TRPV4. The functional interaction was further characterized in HEK293 cells expressing 5-HT2A to reveal that TRPV4 enhances the duration of 5-HT-evoked Ca2+ signaling through a PLA2 and PKC-dependent mechanism. In summary, this study demonstrates that TRPV4 contributes to 5-HT2A-induced plasma extravasation in the airways and upper GI tract, with evidence supporting a mechanism of action involving SP and CGRP release.

Fear Extinction and Predictive Trait-Like Inter-Individual Differences in Rats Lacking the Serotonin Transporter

Anxiety disorders are associated with a failure to sufficiently extinguish fear memories. The serotonergic system (5-hydroxytryptamine, 5-HT) with the 5-HT transporter (5-HTT, SERT) is strongly implicated in the regulation of anxiety and fear. In the present study, we examined the effects of SERT deficiency on fear extinction in a differential fear conditioning paradigm in male and female rats. Fear-related behavior displayed during acquisition, extinction, and recovery, was measured through quantification of immobility and alarm 22-kHz ultrasonic vocalizations (USV). Trait-like inter-individual differences in novelty-seeking, anxiety-related behavior, habituation learning, cognitive performance, and pain sensitivity were examined for their predictive value in forecasting fear extinction. Our results show that SERT deficiency strongly affected the emission of 22-kHz USV during differential fear conditioning. During acquisition, extinction, and recovery, SERT deficiency consistently led to a reduction in 22-kHz USV emission. While SERT deficiency did not affect immobility during acquisition, genotype differences started to emerge during extinction, and during recovery rats lacking SERT showed higher levels of immobility than wildtype littermate controls. Recovery was reflected in increased levels of immobility but not 22-kHz USV emission. Prominent sex differences were evident. Among several measures for trait-like inter-individual differences, anxiety-related behavior had the best predictive quality.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Natural Psychoplastogens As Antidepressant Agents

Increasing prevalence and burden of major depressive disorder presents an unavoidable problem for psychiatry. Existing antidepressants exert their effect only after several weeks of continuous treatment. In addition, their serious side effects and ineffectiveness in one-third of patients call for urgent action. Recent advances have given rise to the concept of psychoplastogens. These compounds are capable of fast structural and functional rearrangement of neural networks by targeting mechanisms previously implicated in the development of depression. Furthermore, evidence shows that they exert a potent acute and long-term positive effects, reaching beyond the treatment of psychiatric diseases. Several of them are naturally occurring compounds, such as psilocybin, N,N-dimethyltryptamine, and 7,8-dihydroxyflavone. Their pharmacology and effects in animal and human studies were discussed in this article.

Brain serotonin transporter binding, plasma arachidonic acid and depression severity: A positron emission tomography study of major depression

BACKGROUND

Serotonin transporter (5-HTT) binding and polyunsaturated fatty acids (PUFAs) are implicated in major depressive disorder (MDD). Links between the two systems in animal models have not been investigated in humans.

METHODS

Using positron emission tomography (PET) and [¹¹C]DASB, we studied relationships between 5-HTT binding potential and plasma levels of PUFAs docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (AA) in medication-free MDD patients (n = 21). PUFAs were quantified using transesterification and gas chromatography. Binding potential BP_P, and alternative outcome measures BP_F and BP_ND, were determined for [¹¹C]DASB in six a priori brain regions of interest (ROIs) using likelihood estimation in graphical analysis (LEGA) to calculate radioligand total distribution volume (V_T), and a validated hybrid deconvolution approach (HYDECA) that estimates radioligand non-displaceable distribution volume (V_ND) without a reference region. Linear mixed models used PUFA levels as predictors and binding potential measures as outcomes across the specified ROIs; age and sex as fixed effects; and subject as random effect to account for across-region binding correlations. As nonlinear relationships were observed, a quadratic term was added to final models.

RESULTS

AA predicted both 5-HTT BP_P and depression severity nonlinearly, described by an inverted U-shaped curve. 5-HTT binding potential mediated the relationship between AA and depression severity.

LIMITATIONS

Given the small sample and multiple comparisons, results require replication.

CONCLUSIONS

Our findings suggest that AA status may impact depression pathophysiology through effects on serotonin transport. Future studies should examine whether these relationships explain therapeutic effects of PUFAs in the treatment of MDD.

Do reproductive hormones control Gulf toadfish pulsatile urea excretion?

Gulf toadfish (Opsanus beta) can excrete the majority of their nitrogenous waste as urea in distinct pulses across their gill. Urea pulses are controlled by cortisol and serotonin (5-HT) and are believed to contain chemical signals that may communicate reproductive and/or social status. The objectives of this study were to determine if reproductive hormones are involved in controlling pulsatile urea excretion, and if toadfish respond to prostaglandins as a chemical signal. Specifically, 11-ketotestosterone (11-KT), estradiol (E2), and the teleost pheromone prostaglandin E₂ (PGE₂) were investigated. Castration during breeding season did not affect pulsatile urea excretion but serial injections of 11-KT outside of breeding season did result in a 48% reduction in urea pulse size in fish of both sexes. Injections of E2 and PGE₂, on the other hand, did not alter urea excretion patterns. Toadfish also did not pulse urea in response to waterborne exposure of PGE₂ suggesting that this compound does not serve as a toadfish pheromone alone. Toadfish have significantly higher plasma 5-HT during breeding season compared to the months following breeding season. Future research should focus on the composition of the chemical signal in toadfish and the potential importance of seasonal changes in plasma 5-HT in toadfish pulsatile urea excretion and teleost reproduction in general.

Alterations of Expression of the Serotonin 5-HT4 Receptor in Brain Disorders

The serotonin 4 receptor, 5-HT₄R, represents one of seven different serotonin receptor families and is implicated in a variety of physiological functions and their pathophysiological variants, such as mood and depression or anxiety, food intake and obesity or anorexia, or memory and memory loss in Alzheimer's disease. Its central nervous system expression pattern in the forebrain, in particular in caudate putamen, the hippocampus and to lesser extent in the cortex, predispose it for a role in executive function and reward-related actions. In rodents, regional overexpression or knockdown in the prefrontal cortex or the nucleus accumbens of 5-HT₄R was shown to impact mood and depression-like phenotypes, food intake and hypophagia; however, whether expression changes are causally involved in the etiology of such disorders is not clear. In this context, more data are emerging, especially based on PET technology and the use of ligand tracers that demonstrate altered 5-HT₄R expression in brain disorders in humans, confirming data stemming from post-mortem tissue and preclinical animal models. In this review, we would like to present the current knowledge of 5-HT₄R expression in brain regions relevant to mood/depression, reward and executive function with a focus on 5-HT₄R expression changes in brain disorders or caused by drug treatment, at both the transcript and protein levels.

Perturbed Developmental Serotonin Signaling Affects Prefrontal Catecholaminergic Innervation and Cortical Integrity

Proper development of the medial prefrontal cortex (mPFC), crucial for correct cognitive functioning, requires projections from, among others, the serotonergic (5-HT) and catecholaminergic systems, but it is unclear how these systems influence each other during development. Here, we describe the parallel development of the 5-HT and catecholaminergic prefrontal projection systems in rat and demonstrate a close engagement of both systems in the proximity of Cajal-Retzius cells. We further show that in the absence of the 5-HT transporter (5-HTT), not only the developing 5-HT but also the catecholaminergic system, including their projections towards the mPFC, are affected. In addition, the layer identity of the mPFC neurons and reelin-positive interneuron number and integration are altered in the absence of the 5-HTT. Together, our data demonstrate a functional interplay between the developing mPFC 5-HT and catecholaminergic systems, and call for a holistic approach in studying neurotransmitter systems-specific developmental consequences for adult behavior, to eventually allow the design of better treatment strategies for neuropsychiatric disorders.

Metabotropic Glutamate2 Receptors Play a Key Role in Modulating Head Twitches Induced by a Serotonergic Hallucinogen in Mice

There is substantial evidence that glutamate can modulate the effects of 5-hydroxytryptamine_2A (5-HT_2A) receptor activation through stimulation of metabotropic glutamate_2/3 (mGlu_2/3) receptors in the prefrontal cortex. Here we show that constitutive deletion of the mGlu₂ gene profoundly attenuates an effect of 5-HT_2A receptor activation using the mouse head twitch response (HTR). MGlu₂ and mGlu₃ receptor knockout (KO) as well as age-matched ICR (CD-1) wild type (WT) mice were administered (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and observed for head twitch activity. DOI failed to produce significant head twitches in mGlu₂ receptor KO mice at a dose 10-fold higher than the peak effective dose in WT or mGlu₃ receptor KO mice. In addition, the mGlu_2/3 receptor agonist LY379268, and the mGlu₂ receptor positive allosteric modulator (PAM) CBiPES, potently blocked the HTR to DOI in WT and mGlu₃ receptor KO mice. Conversely, the mGlu_2/3 receptor antagonist LY341495 (10 mg/kg) increased the HTR produced by DOI (3 mg/kg) in mGlu₃ receptor KO mice. Finally, the mGlu₂ receptor potentiator CBiPES was able to attenuate the increase in the HTR produced by LY341495 in mGlu₃ receptor KO mice. Taken together, all of these results are consistent with the hypothesis that that DOI-induced head twitches are modulated by mGlu₂ receptor activation. These results also are in keeping with a critical autoreceptor function for mGlu₂ receptors in the prefrontal cortex with differential effects of acute vs. chronic perturbation (e.g., constitutive mGlu₂ receptor KO mice). The robust attenuation of DOI-induced head twitches in the mGlu₂ receptor KO mice appears to reflect the critical role of glutamate in ongoing regulation of 5-HT_2A receptors in the prefrontal cortex. Future experiments with inducible knockouts for the mGlu₂ receptor and/or selective mGlu₃ receptor agonists/PAMs/antagonists could provide an important tools in understanding glutamatergic modulation of prefrontal cortical 5-HT_2A receptor function.

Psychedelics

Psychedelics (serotonergic hallucinogens) are powerful psychoactive substances that alter perception and mood and affect numerous cognitive processes. They are generally considered physiologically safe and do not lead to dependence or addiction. Their origin predates written history, and they were employed by early cultures in many sociocultural and ritual contexts. After the virtually contemporaneous discovery of (5R,8R)-(+)-lysergic acid-N,N-diethylamide (LSD)-25 and the identification of serotonin in the brain, early research focused intensively on the possibility that LSD and other psychedelics had a serotonergic basis for their action. Today there is a consensus that psychedelics are agonists or partial agonists at brain serotonin 5-hydroxytryptamine 2A receptors, with particular importance on those expressed on apical dendrites of neocortical pyramidal cells in layer V. Several useful rodent models have been developed over the years to help unravel the neurochemical correlates of serotonin 5-hydroxytryptamine 2A receptor activation in the brain, and a variety of imaging techniques have been employed to identify key brain areas that are directly affected by psychedelics. Recent and exciting developments in the field have occurred in clinical research, where several double-blind placebo-controlled phase 2 studies of psilocybin-assisted psychotherapy in patients with cancer-related psychosocial distress have demonstrated unprecedented positive relief of anxiety and depression. Two small pilot studies of psilocybin-assisted psychotherapy also have shown positive benefit in treating both alcohol and nicotine addiction. Recently, blood oxygen level-dependent functional magnetic resonance imaging and magnetoencephalography have been employed for in vivo brain imaging in humans after administration of a psychedelic, and results indicate that intravenously administered psilocybin and LSD produce decreases in oscillatory power in areas of the brain's default mode network.

Pretreatment levels of the fatty acid handling proteins H-FABP and CD36 predict response to olanzapine in recent-onset schizophrenia patients

Traditional schizophrenia pharmacotherapy remains a subjective trial and error process involving administration, titration and switching of drugs multiple times until an adequate response is achieved. Despite this time-consuming and costly process, not all patients show an adequate response to treatment. As a consequence, relapse is a common occurrence and early intervention is hampered. Here, we have attempted to identify candidate blood biomarkers associated with drug response in 121 initially antipsychotic-free recent-onset schizophrenia patients treated with widely-used antipsychotics, namely olanzapine (n=40), quetiapine (n=23), risperidone (n=30) and a mixture of these drugs (n=28). Patients were recruited and investigated as two separate cohorts to allow biomarker validation. Data analysis showed the most significant relationship between pre-treatment levels of heart-type fatty acid binding protein (H-FABP) and response to olanzapine (p=0.008, F=8.6, β=70.4 in the discovery cohort and p=0.003, F=15.2, β=24.4 in the validation cohort, adjusted for relevant confounding variables). In a functional follow-up analysis of this finding, we tested an independent cohort of 10 patients treated with olanzapine and found that baseline levels of plasma H-FABP and expression of the binding partner for H-FABP, fatty acid translocase (CD36), on monocytes predicted the reduction of psychotic symptoms (p=0.040, F=6.0, β=116.3 and p=0.012, F=11.9, β=-0.0054, respectively). We also identified a set of serum molecules changed after treatment with antipsychotic medication, in particular olanzapine. These molecules are predominantly involved in cellular development and metabolism. Taken together, our findings suggest an association between biomarkers involved in fatty acid metabolism and response to olanzapine, while other proteins may serve as surrogate markers associated with drug efficacy and side effects.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

Brain Arachidonic Acid Incorporation and Turnover are not Altered in the Flinders Sensitive Line Rat Model of Human Depression

Brain serotonergic signaling is coupled to arachidonic acid (AA)-releasing calcium-dependent phospholipase A2. Increased brain serotonin concentrations and disturbed serotonergic neurotransmission have been reported in the Flinders Sensitive Line (FSL) rat model of depression, suggesting that brain AA metabolism may be elevated. To test this hypothesis, (14)C-AA was intravenously infused to steady-state levels into control and FSL rats derived from the same Sprague-Dawley background strain, and labeled and unlabeled brain phospholipid and plasma fatty acid concentrations were measured to determine the rate of brain AA incorporation and turnover. Brain AA incorporation and turnover did not differ significantly between controls and FSL rats. Compared to controls, plasma unesterified docosahexaenoic acid was increased, and brain phosphatidylinositol AA and total lipid linoleic acid and n-3 and n-6 docosapentaenoic acid were significantly decreased in FSL rats. Several plasma esterified fatty acids differed significantly from controls. In summary, brain AA metabolism did not change in FSL rats despite reported increased levels of serotonin concentrations, suggesting possible post-synaptic dampening of serotonergic neurotransmission involving AA.

5-HT2C receptor desensitization moderates anxiety in 5-HTT deficient mice: from behavioral to cellular evidence

BACKGROUND

Desensitization and blockade of 5-HT2C receptors (5-HT2CR) have long been thought to be central in the therapeutic action of antidepressant drugs. However, besides behavioral pharmacology studies, there is little in vivo data documenting antidepressant-induced 5-HT2CR desensitization in specific brain areas.

METHODS

Mice lacking the 5-HT reuptake carrier (5-HTT(-/-)) were used to model the consequences of chronic 5-HT reuptake inhibition with antidepressant drugs. The effect of this mutation on 5-HT2CR was evaluated at the behavioral (social interaction, novelty-suppressed feeding, and 5-HT2CR-induced hypolocomotion tests), the neurochemical, and the cellular (RT-qPCR, mRNA editing, and c-fos-induced expression) levels.

RESULTS

Although 5-HTT(-/-) mice had an anxiogenic profile in the novelty-suppressed feeding test, they displayed less 5-HT2CR-mediated anxiety in response to the agonist m-chlorophenylpiperazine in the social interaction test. In addition, 5-HT2CR-mediated inhibition of a stress-induced increase in 5-HT turnover, measured in various brain areas, was markedly reduced in 5-HTT(-/-) mutants. These indices of tolerance to 5-HT2CR stimulation were associated neither with altered levels of 5-HT2CR protein and mRNA nor with changes in pre-mRNA editing in the frontal cortex. However, basal c-fos mRNA production in cells expressing 5-HT2CR was higher in 5-HTT(-/-) mutants, suggesting an altered basal activity of these cells following sustained 5-HT reuptake carrier inactivation. Furthermore, the increased c-fos mRNA expression in 5-HT2CR-like immune-positive cortical cells observed in wild-type mice treated acutely with the 5-HT2CR agonist RO-60,0175 was absent in 5-HTT(-/-) mutants.

CONCLUSIONS

Such blunted responsiveness of the 5-HT2CR system, observed at the cell signaling level, probably contributes to the moderation of the anxiety phenotype in 5-HTT(-/-) mice.

D2-like receptor activation does not initiate a brain docosahexaenoic acid signal in unanesthetized rats

BACKGROUND

The polyunsaturated fatty acid, docosahexaenoic acid (DHA), participates in neurotransmission involving activation of calcium-independent phospholipase A2 (iPLA2), which is coupled to muscarinic, cholinergic and serotonergic neuroreceptors. Drug induced activation of iPLA2 can be measured in vivo with quantitative autoradiography using 14C-DHA as a probe. The present study used this approach to address whether a DHA signal is produced following dompaminergic (D)2-like receptor activation with quinpirole in rat brain. Unanesthetized rats were infused intravenously with 14C-DHA one minute after saline or quinpirole infusion, and serial blood samples were collected over a 20-minute period to obtain plasma. The animals were euthanized with sodium pentobarbital and their brains excised, coronally dissected and subjected to quantitative autoradiography to derive the regional incorporation coefficient, k*, a marker of DHA signaling. Plasma labeled and unlabeled unesterified DHA concentrations were measured.

RESULTS

The incorporation coefficient (k*) for DHA did not differ significantly between quinpirole-treated and control rats in any of 81 identified brain regions. Plasma labeled DHA concentration over the 20-minute collection period (input function) and unlabeled unesterified DHA concentration did not differ significantly between the two groups.

CONCLUSION

These findings demonstrate that D2-like receptor initiated signaling does not involve DHA as a second messenger, and likely does not involve iPLA2 activation.

Gpm6b deficiency impairs sensorimotor gating and modulates the behavioral response to a 5-HT2A/C receptor agonist

The neuronal tetraspan proteins, M6A (Gpm6a) and M6B (Gpm6b), belong to the family of proteolipids that are widely expressed in the brain. We recently reported Gpm6a deficiency as a monogenetic cause of claustrophobia in mice. Its homolog proteolipid, Gpm6b, is ubiquitously expressed in neurons and oligodendrocytes. Gpm6b is involved in neuronal differentiation and myelination. It interacts with the N-terminal domain of the serotonin transporter (SERT) and decreases cell-surface expression of SERT. In the present study, we employed Gpm6b null mutant mice (Gpm6b(-/-)) to search for behavioral functions of Gpm6b. We studied male and female Gpm6b(-/-) mice and their wild-type (WT, Gpm6b(+/+)) littermates in an extensive behavioral test battery. Additionally, we investigated whether Gpm6b(-/-) mice exhibit changes in the behavioral response to a 5-HT2A/C receptor agonist. We found that Gpm6b(-/-) mice display completely normal sensory and motor functions, cognition, as well as social and emotionality-like (anxiety, depression) behaviors. On top of this inconspicuous behavioral profile, Gpm6b(-/-) mice of both genders exhibit a selective impairment in prepulse inhibition of the acoustic startle response. Furthermore, in contrast to WT mice that show the typical locomotion suppression and increase in grooming activity after intraperitoneal administration of DOI [(±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride], Gpm6b(-/-) mice demonstrate a blunted behavioral response to this 5-HT2A/C receptor agonist. To conclude, Gpm6b deficiency impairs sensorimotor gating and modulates the behavioral response to a serotonergic challenge.

PPP3CC gene: a putative modulator of antidepressant response through the B-cell receptor signaling pathway

Antidepressant pharmacogenetics represents a stimulating, but often discouraging field. The present study proposes a combination of several methodologies across three independent samples. Genes belonging to monoamine, neuroplasticity, circadian rhythm and transcription factor pathways were investigated in two samples (n=369 and 88) with diagnosis of major depression who were treated with antidepressants. Phenotypes were response, remission and treatment-resistant depression. Logistic regression including appropriate covariates was performed. Genes associated with outcomes were investigated in the STAR*D (Sequenced Treatment Alternatives to Relieve Depression) genome-wide study (n=1861). Top genes were further studied through a pathway analysis. In both original samples, markers associated with outcomes were concentrated in the PPP3CC gene. Other interesting findings were particularly in the HTR2A gene in one original sample and the STAR*D. The B-cell receptor signaling pathway proved to be the putative mediator of PPP3CC's effect on antidepressant response (P=0.03). Among innovative candidates, PPP3CC, involved in the regulation of immune system and synaptic plasticity, seems promising for further investigation.

Conservation of 5-HT1A receptor-mediated autoinhibition of serotonin (5-HT) neurons in mice with altered 5-HT homeostasis

Firing activity of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) is controlled by inhibitory somatodendritic 5-HT_1A autoreceptors. This autoinhibitory mechanism is implicated in the etiology of disorders of emotion regulation, such as anxiety disorders and depression, as well as in the mechanism of antidepressant action. Here, we investigated how persistent alterations in brain 5-HT availability affect autoinhibition in two genetically modified mouse models lacking critical mediators of serotonergic transmission: 5-HT transporter knockout (Sert-/-) and tryptophan hydroxylase-2 knockout (Tph2-/-) mice. The degree of autoinhibition was assessed by loose-seal cell-attached recording in DRN slices. First, application of the 5-HT_1A-selective agonist R(+)-8-hydroxy-2-(di-n-propylamino)tetralin showed mild sensitization and marked desensitization of 5-HT_1A receptors in Tph2-/- mice and Sert-/- mice, respectively. While 5-HT neurons from Tph2-/- mice did not display autoinhibition in response to L-tryptophan, autoinhibition of these neurons was unaltered in Sert-/- mice despite marked desensitization of their 5-HT_1A autoreceptors. When the Tph2-dependent 5-HT synthesis step was bypassed by application of 5-hydroxy-L-tryptophan (5-HTP), neurons from both Tph2-/- and Sert-/- mice decreased their firing rates at significantly lower concentrations of 5-HTP compared to wildtype controls. Our findings demonstrate that, as opposed to the prevalent view, sensitivity of somatodendritic 5-HT_1A receptors does not predict the magnitude of 5-HT neuron autoinhibition. Changes in 5-HT_1A receptor sensitivity may rather be seen as an adaptive mechanism to keep autoinhibition functioning in response to extremely altered levels of extracellular 5-HT resulting from targeted inactivation of mediators of serotonergic signaling.

Genetic and pharmacological manipulations of the serotonergic system in early life: neurodevelopmental underpinnings of autism-related behavior

Serotonin, in its function as neurotransmitter, is well-known for its role in depression, autism and other neuropsychiatric disorders, however, less known as a neurodevelopmental factor. The serotonergic system is one of the earliest to develop during embryogenesis and early changes in serotonin levels can have large consequences for the correct development of specific brain areas. The regulation and functioning of serotonin is influenced by genetic risk factors, such as the serotonin transporter polymorphism in humans. This polymorphism is associated with anxiety-related symptoms, changes in social behavior, and cortical gray and white matter changes also seen in patients suffering from autism spectrum disorders (ASD). The human polymorphism can be mimicked by the knockout of the serotonin transporter in rodents, which are as a model system therefore vital to explore the precise neurobiological mechanisms. Moreover, there are pharmacological challenges influencing serotonin in early life, like prenatal/neonatal exposure to selective serotonin reuptake inhibitors (SSRI) in depressed pregnant women. There is accumulating evidence that this dysregulation of serotonin during critical phases of brain development can lead to ASD-related symptoms in children, and reduced social behavior and increased anxiety in rodents. Furthermore, prenatal valproic acid (VPA) exposure, a mood stabilizing drug which is also thought to interfere with serotonin levels, has the potency to induce ASD-like symptoms and to affect the development of the serotonergic system. Here, we review and compare the neurodevelopmental and behavioral consequences of serotonin transporter gene variation, and prenatal SSRI and VPA exposure in the context of ASD.

Translational studies on regulation of brain docosahexaenoic acid (DHA) metabolism in vivo

One goal in the field of brain polyunsaturated fatty acid (PUFA) metabolism is to translate the many studies that have been conducted in vitro and in animal models to the clinical setting. Doing so should elucidate the role of PUFAs in the human brain, and effects of diet, drugs, disease and genetics on this role. This review discusses new in vivo radiotracer kinetic and neuroimaging techniques that allow us to do this, with a focus on docosahexaenoic acid (DHA). We illustrate how brain PUFA metabolism is influenced by graded reductions in dietary n-3 PUFA content in unanesthetized rats. We also show how kinetic tracer techniques in rodents have helped to identify mechanisms of action of mood stabilizers used in bipolar disorder, how DHA participates in neurotransmission, and how brain DHA metabolism is regulated by calcium-independent iPLA₂β. In humans, regional rates of brain DHA metabolism can be quantitatively imaged with positron emission tomography following intravenous injection of [1-¹¹C]DHA.

Phospholipase C mediates (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-, but not lysergic acid diethylamide (LSD)-elicited head bobs in rabbit medial prefrontal cortex

The phenethylamine and indoleamine classes of hallucinogens demonstrate distinct pharmacological properties, although they share a serotonin(2A) (5-HT(2A)) receptor mechanism of action (MOA). The 5-HT(2A) receptor signals through phosphatidylinositol (PI) hydrolysis, which is initiated upon activation of phospholipase C (PLC). The role of PI hydrolysis in the effects of hallucinogens remains unclear. In order to better understand the role of PI hydrolysis in the MOA of hallucinogens, the PLC inhibitor, 1-[6-((17β-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione (U73122), was used to study the effects of two hallucinogens, the phenethylamine, (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), and the indoleamine, lysergic acid diethylamide (LSD). PI hydrolysis was quantified through release of [3H]inositol-4-phosphate from living rabbit frontocortical tissue prisms. Head bobs were counted after hallucinogens were infused into the medial prefrontal cortex (mPFC) of rabbits. Both DOI and LSD stimulated PI hydrolysis in frontocortical tissue through activation of PLC. DOI-stimulated PI hydrolysis was blocked by 5-HT(2A/2C) receptor antagonist, ketanserin, whereas the LSD signal was blocked by 5-HT(2B/2C) receptor antagonist, SB206553. When infused into the mPFC, both DOI- and LSD-elicited head bobs. Pretreatment with U73122 blocked DOI-, but not LSD-elicited head bobs. The two hallucinogens investigated were distinct in their activation of the PI hydrolysis signaling pathway. The serotonergic receptors involved with DOI and LSD signals in frontocortical tissue were different. Furthermore, PLC activation in mPFC was necessary for DOI-elicited head bobs, whereas LSD-elicited head bobs were independent of this pathway. These novel findings urge closer investigation into the intracellular mechanism of action of these unique compounds.

Genetic variation in 5-hydroxytryptamine transporter expression causes adaptive changes in 5-HT₄ receptor levels

Genetic variation in 5-HT transporter (5-HTT) expression is a key risk factor for psychiatric disorder and has been linked to changes in the expression of certain 5-HT receptor subtypes. This study investigated the effect of variation in 5-HTT expression on 5-HT₄ receptor levels in both 5-HTT knockout (KO) and overexpressing (OE) mice using autoradiography with the selective 5-HT₄ receptor radioligand, [³H]SB207145. Compared to wild-type (5-HTT⁺/⁺) controls, homozygous 5-HTT KO mice (5-HTT⁻/⁻) had reduced 5-HT₄ receptor binding site density in all brain regions examined (35-65% of 5-HTT⁺/⁺). In contrast, the density of 5-HT₄ receptor binding sites was not significantly different between heterozygous 5-HTT KO mice (5-HTT⁻/⁺) and 5-HTT⁺/⁺ mice. The 5-HT synthesis inhibitor p-chlorophenylalanine (250 mg/kg twice daily for 3 d) abolished the difference in 5-HT₄ binding between 5-HTT⁻/⁻ and 5-HTT⁺/⁺ mice in all brain regions. Compared to wild-type (WT) littermate controls, 5-HTT OE mice had increased 5-HT₄ binding density across all brain regions, except amygdala (118-164% of WT) and this difference between genotypes was reduced by the 5-HTT inhibitor, fluoxetine (20 mg/kg twice daily, 3 d). Together, these findings suggest that variation in 5-HTT expression causes adaptive changes in 5-HT₄ receptor levels which are directly linked to alterations in 5-HT availability.

Serotonin2C receptors in the nucleus accumbens are involved in enhanced alcohol-drinking behavior

Dopamine and serotonin (5-HT) in the nucleus accumbens (ACC) and ventral tegmental area of the mesoaccumbens reward pathways have been implicated in the mechanisms underlying development of alcohol dependence. We used a C57BL/6J mouse model with increased voluntary alcohol-drinking behavior by exposing the mice to alcohol vapor for 20 consecutive days. In the alcohol-exposed mice, the expression of 5-HT(2C) receptor mRNA increased in the ACC, caudate nucleus and putamen, dorsal raphe nucleus (DRN), hippocampus and lateral hypothalamus, while the protein level of 5-HT(2C) receptor significantly increased in the ACC. The expression of 5-HT(7) receptor mRNA increased in the ACC and DRN. Contents of 5-HT decreased in the ACC shell (ACC(S) ) and DRN of the alcohol-exposed mice. The basal extracellular releases of dopamine (DA) and 5-HT in the ACC(S) increased more in the alcohol-exposed mice than in alcohol-naïve mice. The magnitude of the alcohol-induced ACC(S) DA and 5-HT release in the alcohol-exposed mice was increased compared with the control mice. Intraperitoneal (i.p.) administration or local injection into ACC(S) of the 5-HT(2C) receptor antagonist, SB-242084, suppressed voluntary alcohol-drinking behavior in the alcohol-exposed mice. But the i.p. administration of the 5-HT(7) receptor antagonist, SB-258719, did not have significant effects on alcohol-drinking behavior in the alcohol-exposed mice. The effects of the 5-HT(2C) receptor antagonist were not observed in the air-exposed control mice. These results suggest that adaptations of the 5-HT system, especially the upregulation of 5-HT(2C) receptors in the ACC(S) , are involved in the development of enhanced voluntary alcohol-drinking behavior.

5-HT2 ligands in the treatment of anxiety and depression

INTRODUCTION

One third of depressed patients do not respond adequately to conventional antidepressants including the selective serotonin reuptake inhibitors (SSRIs). Therefore, multi-target drugs or augmentation strategies have been developed for the management of SSRIs-resistant patients. In this context, the 5-HT(2) receptor subtypes represent promising targets but their precise roles have yet to be determined.

AREAS COVERED

The aim of this review is to shed some light on the preclinical evidence supporting the use of 5-HT(2A) and/or 5-HT(2C) receptor antagonists such as antipsychotics, as potential effective adjuncts in SSRIs-resistant depression. This review synthesizes the current literature about the behavioral, electrophysiological and neurochemical effects of 5-HT(2) receptors ligands on the monoaminergic systems but also on adult hippocampal neurogenesis.

EXPERT OPINION

Although studies support the hypothesis that the inactivation of 5-HT(2A) and/or 5-HT(2C) receptors might be of interest to reinforce different facets of the therapeutic activity of SSRIs, this pharmacological strategy remains debatable notably because of the lack of chronic data in relevant animal models. Conversely, emerging evidence suggests that the activation of 5-HT(2B) receptor is required for antidepressant-like activity, opening the way to new therapeutic approaches. However, the potential risks related to the enhancement of monoaminergic neurotransmissions could represent a major concern.

Head-twitch response in rodents induced by the hallucinogen 2,5-dimethoxy-4-iodoamphetamine: a comprehensive history, a re-evaluation of mechanisms, and its utility as a model

Two primary animal models persist for assessing hallucinogenic potential of novel compounds and for examining the pharmacological and neurobiological substrates underlying the actions of classical hallucinogens, the two-lever drug discrimination procedure and the drug-induced head-twitch response (HTR) in rodents. The substituted amphetamine hallucinogen, serotonin 2 (5-HT(2) ) receptor agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI) has emerged as the most popular pharmacological tool used in HTR studies of hallucinogens. Synthesizing classic, recent, and relatively overlooked findings, addressing ostensibly conflicting observations, and considering contemporary theories in receptor and behavioural pharmacology, this review provides an up-to-date and comprehensive synopsis of DOI and the HTR model, from neural mechanisms to utility for understanding psychiatric diseases. Also presented is support for the argument that, although both the two-lever drug discrimination and the HTR models in rodents are useful for uncovering receptors, interacting proteins, intracellular signalling pathways, and neurochemical processes affected by DOI and related classical hallucinogens, results from both models suggest they are not reporting hallucinogenic experiences in animals.

Long term impact of prenatal exposure to SSRIs on growth and body weight in childhood: evidence from animal and human studies

Prenatal exposure to SSRIs has the potential to alter fetal 5-HT signalling during critical periods of development: the long-term consequences of which have not been well studied. Of particular interest are the potential long-term effects of prenatal SSRI exposure on growth and body weight in later life, given the role of the serotonergic system in regulating food intake and body weight. Animal studies demonstrate that changes in 5-HT homeostasis during critical periods of fetal development can lead to sex-specific molecular and functional alterations in the serotonergic and HPA systems, leading to an increased risk of overweight in male, but not female, offspring in later life. This review highlights the evidence and the need for studies in humans to determine whether prenatal SSRI exposure is associated with alterations in child growth and body weight and the importance of delineating these effects from those of the underlying maternal illness.

Imaging brain signal transduction and metabolism via arachidonic and docosahexaenoic acid in animals and humans

The polyunsaturated fatty acids (PUFAs), arachidonic acid (AA, 20:4n-6) and docosahexaenoic acid (DHA, 22:6n-3), important second messengers in brain, are released from membrane phospholipid following receptor-mediated activation of specific phospholipase A(2) (PLA(2)) enzymes. We developed an in vivo method in rodents using quantitative autoradiography to image PUFA incorporation into brain from plasma, and showed that their incorporation rates equal their rates of metabolic consumption by brain. Thus, quantitative imaging of unesterified plasma AA or DHA incorporation into brain can be used as a biomarker of brain PUFA metabolism and neurotransmission. We have employed our method to image and quantify effects of mood stabilizers on brain AA/DHA incorporation during neurotransmission by muscarinic M(1,3,5), serotonergic 5-HT(2A/2C), dopaminergic D(2)-like (D(2), D(3), D(4)) or glutamatergic N-methyl-d-aspartic acid (NMDA) receptors, and effects of inhibition of acetylcholinesterase, of selective serotonin and dopamine reuptake transporter inhibitors, of neuroinflammation (HIV-1 and lipopolysaccharide) and excitotoxicity, and in genetically modified rodents. The method has been extended for the use with positron emission tomography (PET), and can be employed to determine how human brain AA/DHA signaling and consumption are influenced by diet, aging, disease and genetics.

Prenatal exposure to selective serotonin reuptake inhibitors and risk of childhood overweight

The objective was to investigate the association between prenatal selective serotonin reuptake inhibitor (SSRI) exposure and overweight in offspring at 4–5 years of age. We conducted a retrospective cohort study using linked records from the Women's and Children's Health Network in South Australia, Australia. Women were eligible to participate if they gave birth to singleton, live-born infants between September 2000 and December 2005. Women were excluded if they received a dispensing for an antidepressant other than SSRIs or an antipsychotic or an anti-epileptic or had a chronic medical condition. Of the 6560 eligible women, 71 received a dispensing for an SSRI (exposed), 204 had a reported psychiatric illness but did not receive a dispensing for any antidepressant (untreated psychiatric illness) and 6285 did not have a reported psychiatric illness and did not receive a dispensing for any antidepressant (unexposed). Childhood overweight was classified as a body mass index >85th percentile, based on age and sex. At 4–5 years of age, female offspring of exposed mothers were less likely to be overweight compared with female offspring of mothers with an untreated psychiatric illness [adjusted Prevalence Ratio (aPR) 0.23; 95% confidence interval (CI) 0.05–0.98] and female offspring of unexposed mothers (aPR 0.27; 0.07–0.99). No association with overweight was observed among male offspring of exposed mothers compared with male offspring of mothers with an untreated psychiatric illness (aPR 1.17; 0.54–2.51) and male offspring of unexposed mothers (aPR 0.93; 0.52–1.67). Further research is required to confirm these findings and examine the potential mechanisms behind the sex-specific differences.

Docosahexaenoic acid (DHA) incorporation into the brain from plasma, as an in vivo biomarker of brain DHA metabolism and neurotransmission

Docosahexaenoic acid (DHA) is critical for maintaining normal brain structure and function, and is considered neuroprotective. Its brain concentration depends on dietary DHA content and hepatic conversion from its dietary derived n-3 precursor, α-linolenic acid (α-LNA). We have developed an in vivo method in rats using quantitative autoradiography and intravenously injected radiolabeled DHA to image net incorporation into the brain of unesterified plasma DHA, and showed with this method that the incorporation rate of DHA equals the rate of brain metabolic DHA consumption. The method has been extended for use in humans with positron emission tomography (PET). Furthermore, imaging in unanesthetized rats using DHA incorporation as a biomarker in response to acute N-methyl-D-aspartate administration confirms that regional DHA signaling is independent of extracellular calcium, and likely mediated by a calcium-independent phospholipase A(2) (iPLA(2)). Studies in mice in which iPLA(2)-VIA (β) was knocked out confirmed that this enzyme is critical for baseline and muscarinic cholinergic signaling involving DHA. Thus, quantitative imaging of DHA incorporation from plasma into brain can be used as an in vivo biomarker of brain DHA metabolism and neurotransmission.

Animal models of depression and anxiety: What do they tell us about human condition?

While modern neurobiology methods are necessary they are not sufficient to elucidate etiology and pathophysiology of affective disorders and develop new treatments. Achievement of these goals is contingent on applying cutting edge methods on appropriate disease models. In this review, the authors present four rodent models with good face-, construct-, and predictive-validity: the Flinders Sensitive rat line (FSL); the genetically "anxious" High Anxiety-like Behavior (HAB) line; the serotonin transporter knockout 5-HTT(-/-) rat and mouse lines; and the post-traumatic stress disorder (PTSD) model induced by exposure to predator scent, that they have employed to investigate the nature of depression and anxiety.

Altered 5-HT2C receptor agonist-induced responses and 5-HT2C receptor RNA editing in the amygdala of serotonin transporter knockout mice

Background

The serotonin 5-HT_2C receptor (5-HT_2CR) is expressed in amygdala, a region involved in anxiety and fear responses and implicated in the pathogenesis of several psychiatric disorders such as acute anxiety and post traumatic stress disorder. In humans and in rodent models, there is evidence of both anxiogenic and anxiolytic actions of 5-HT_2C ligands. In this study, we determined the responsiveness of 5-HT_2CR in serotonin transporter (SERT) knockout (-/-) mice, a model characterized by increased anxiety-like and stress-responsive behaviors.

Results

In the three-chamber social interaction test, the 5-HT_2B/2C agonist mCPP decreased sociability and sniffing in SERT wildtype (+/+) mice, both indicative of the well-documented anxiogenic effect of mCPP. This 5-HT_2C-mediated response was absent in SERT -/- mice. Likewise, in the open field test, the selective 5-HT_2C agonist RO 60-0175 induced an anxiogenic response in SERT +/+ mice, but not in SERT -/- mice. Since 5-HT_2CR pre-mRNA is adenosine-to-inosine (A-to-I) edited, we also evaluated the 5-HT_2CR RNA editing profiles of SERT +/+ and SERT -/- mice in amygdala. Compared to SERT +/+ mice, SERT-/- mice showed a decrease in less edited, highly functional 5-HT_2C isoforms, and an increase in more edited isoforms with reduced signaling efficiency.

Conclusions

These results indicate that the 5-HT_2CR in the amygdala of SERT -/- mice has increased RNA editing, which could explain, at least in part, the decreased behavioral responses to 5-HT_2C agonists in SERT -/- mice. These alterations in 5-HT_2CR in amygdala may be relevant to humans with SERT polymorphisms that alter SERT expression, function, and emotional behaviors.

Serotonin transporter knockout and repeated social defeat stress: impact on neuronal morphology and plasticity in limbic brain areas

Low expression of the human serotonin transporter (5-HTT) gene presumably interacts with stressful life events enhancing susceptibility for affective disorders. 5-Htt knockout (KO) mice display an anxious phenotype, and behavioural differences compared to wild-type (WT) mice are exacerbated after repeated loser experience in a resident-intruder stress paradigm. To assess whether genotype-dependent and stress-induced behavioural differences are reflected in alterations of neuronal morphology in limbic areas, we studied dendritic length and complexity of pyramidal neurons in the anterior cingulate and infralimbic cortices (CG, IL), hippocampus CA1 region, and of pyramidal neurons and interneurons in the lateral (La) and basolateral (BL) amygdaloid nuclei in Golgi-Cox-stained brains of male WT and 5-Htt KO control and loser mice. Spine density was analysed for IL apical and amygdaloid apical and basal pyramidal neuron dendrites. While group differences were absent for parameters analysed in CG, CA1 and amygdaloid interneurons, pyramidal neurons in the IL displayed tendencies to shorter and less spinous distal apical dendrites in 5-Htt KO controls, and to extended proximal dendrites in WT losers compared to WT controls. In contrast, spine density of several dendritic compartments of amygdaloid pyramids was significantly higher in 5-Htt KO mice compared to WT controls. While a tendency to increased spine density was observed in the same dendritic compartments in WT after stress, changes were lacking in stressed compared to control 5-Htt KO mice. Our findings indicate that disturbed 5-HT homeostasis results in alterations of limbic neuronal morphology, especially in higher spinogenesis in amygdaloid pyramidal neurons. Social stress leads to similar but less pronounced changes in the WT, and neuroplasticity upon stress is reduced in 5-Htt KO mice.

Cerebral metabolic responses to 5-HT2A/C receptor activation in mice with genetically modified serotonin transporter (SERT) expression

Variation in the human serotonin transporter gene (hSERT; 5-HTT) resulting in a life-long alteration in SERT function influences anxiety and the risk of developing affective disorders. The mechanisms underlying the influence of the hSERT gene on these phenotypes remain unclear but may involve altered 5-HT receptor function. Here we characterise the cerebral metabolic response to 5-HT(2A/C) receptor activation in two transgenic mouse models of altered SERT function, SERT knock-out (SERT KO) and hSERT over-expressing (hSERT OE) mice, to test the hypothesis that genetically mediated variability in SERT expression alters 5-HT(2A/C) function. We found that a constitutive increase in SERT expression (hSERT OE) enhanced, whereas a constitutive decrease in SERT expression (SERT KO) attenuated, 5-HT(2A/C) function. Therefore, altered 5-HT(2A/C) receptor functioning in response to hSERT gene variation may contribute to its influence on affective phenotypes.

A defect in Δ6 and Δ5 desaturases may be a factor in the initiation and progression of insulin resistance, the metabolic syndrome and ischemic heart disease in South Asians

The high incidence of insulin resistance and the metabolic syndrome in South Asians remains unexplained. I propose that a defect in the activity of Δ⁶ and Δ⁵ desaturases and consequent low plasma and tissue concentrations of polyunsaturated fatty acids such as γ-linolenic acid (GLA), dihomo-γ-linolenic acid (DGLA), arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and formation of their anti-inflammatory products prostaglandin E₁ (PGE₁), prostacyclin (PGI₂), PGI₃, lipoxins, resolvins, protectins, maresins and nitrolipids could be responsible for the high incidence of insulin resistance, the metabolic syndrome and ischemic heart disease (IHD) in South Asians. This proposal is supported by the observation that South Asian Indians have lower plasma and tissue concentrations of GLA, DGLA, AA, EPA and DHA, the precursors of PGE₁, PGI₂, PGI₃, lipoxins, resolvins, protectins, and nitrolipids, the endogenous molecules that prevent platelet aggregation, vasoconstriction, thrombus formation, leukocyte activation and possess anti-inflammatory action and thus, are capable of preventing the development of insulin resistance, atherosclerosis, hypertension, type 2 diabetes mellitus and premature ischemic heart disease. Genetic predisposition, high carbohydrate intake, lack of exercise, tobacco use and low birth weight due to maternal malnutrition suppress the activity of Δ⁶ and Δ⁵ desaturases that leads to low plasma and tissue concentrations of polyunsaturated fatty acids and their products. This implies that adequate provision of polyunsaturated fatty acids and co-factors needed for their metabolism, and efforts to enhance the formation of their beneficial metabolites PGE₁, PGI₂, PGI₃, lipoxins, resolvins, protectins, maresins and nitrolipids could form a novel approach in the prevention and management of these diseases in this high-risk population.

The serotonin 5-HT(2A) receptor agonist TCB-2: a behavioral and neurophysiological analysis

RATIONALE

There are few reports on the high-affinity 5-HT(2A) agonist (4-Bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2).

OBJECTIVES

Here we provide the first behavioral and neurophysiological profile of TCB-2 in C57BL/6J mice, with direct comparisons to the 5-HT(2A/2C) agonist (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), in addition to determinations of 5-HT(2A) mediation via pretreatment with the selective 5-HT(2A) antagonist MDL 11,939.

RESULTS

In a dose-dependent manner, TCB-2 induced head twitches, decreased food consumption in food-deprived mice, induced hypothermia, and increased corticosterone levels, with no effects on locomotor activity or anxiety-like behaviors in the open field. Similar effects were observed in side-by-side dose-response comparisons with DOI; although at the highest dose tested (5.0 mg/kg), TCB-2 induced significantly fewer head twitches, and a significantly enhanced hypothermic response, versus DOI. Pretreatment with MDL 11,939 blocked head twitches and temperature change following TCB-2 and DOI, confirming 5-HT(2A) mediation of these responses. Although MDL 11,939 pretreatment blocked DOI-induced suppression of feeding, MDL 11,939 had no effect on TCB-2-induced suppression of feeding. Previous studies show that 5-HT(2A) function is altered by changes in serotonin transporter (SERT) expression and function. In SERT knockout (-/-) mice, TCB-2-induced head twitches and hypothermia were greatly diminished compared to SERT wild-type (+/+) mice.

CONCLUSIONS

The current studies are important, as they are the first to assess the effects of TCB-2 in mice, and are among the first to report the behavioral and neurophysiological effects of this conformationally restricted phenethylamine analog compound, which has 65-fold greater effects on signaling via the phosphoinositide versus arachidonic acid pathways.

Chronic imipramine but not bupropion increases arachidonic acid signaling in rat brain: is this related to 'switching' in bipolar disorder?

Agents effective against mania in bipolar disorder are reported to decrease turnover of arachidonic acid (AA) in phospholipids and expression of calcium-dependent AA-selective cytosolic phospholipase A(2) (cPLA(2)) in rat brain. In contrast, fluoxetine, an antidepressant that is reported to switch bipolar depressed patients to mania, increases cPLA(2) expression and AA turnover in rat brain. We therefore hypothesized that antidepressants that increase switching to mania generally increase cPLA(2) and AA turnover in brain. To test this hypothesis, adult male CDF-344 rats were administered imipramine and bupropion, with reported high and low switching rates, respectively, at daily doses of 10 and 30 mg kg(-1) i.p., respectively, or i.p. saline (control) for 21 days. Frontal cortex expression of different PLA(2) enzymes and AA turnover rates in brain when the rats were unanesthetized were measured. Compared with chronic saline, chronic imipramine but not bupropion significantly increased cortex cPLA(2) mRNA activity, protein and phosphorylation, expression of the cPLA(2) transcription factor, activator protein-2alpha (AP-2alpha) and AA turnover in phospholipids. Protein levels of secretory phospholipase A(2), calcium-independent phospholipase A(2), cyclooxygenase (COX)-1 and COX-2 were unchanged, and prostaglandin E(2) was unaffected. These results, taken with prior data on chronic fluoxetine in rats, suggest that antidepressants that increase the switching tendency of bipolar depressed patients to mania do so by increasing AA recycling and metabolism in brain. Mania in bipolar disorder thus may involve upregulated brain AA metabolism.

A nonlinear relationship between cerebral serotonin transporter and 5-HT(2A) receptor binding: an in vivo molecular imaging study in humans

Serotonergic neurotransmission is involved in the regulation of physiological functions such as mood, sleep, memory, and appetite. Within the serotonin transmitter system, both the postsynaptically located serotonin 2A (5-HT(2A)) receptor and the presynaptic serotonin transporter (SERT) are sensitive to chronic changes in cerebral 5-HT levels. Additionally, experimental studies suggest that alterations in either the 5-HT(2A) receptor or SERT level can affect the protein level of the counterpart. The aim of this study was to explore the covariation between cerebral 5-HT(2A) receptor and SERT in vivo in the same healthy human subjects. Fifty-six healthy human subjects with a mean age of 36 +/- 19 years were investigated. The SERT binding was imaged with [(11)C]3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile (DASB) and 5-HT(2A) receptor binding with [(18)F]altanserin using positron emission tomography. Within each individual, a regional intercorrelation for the various brain regions was seen with both markers, most notably for 5-HT(2A) receptor binding. An inverted U-shaped relationship between the 5-HT(2A) receptor and the SERT binding was identified. The observed regional intercorrelation for both the 5-HT(2A) receptor and the SERT cerebral binding suggests that, within the single individual, each marker has a set point adjusted through a common regulator. A quadratic relationship between the two markers is consistent with data from experimental studies of the effect on SERT and 5-HT(2A) receptor binding of chronic changes in 5-HT levels. That is, the observed association between the 5-HT(2A) receptor and SERT binding could be driven by the projection output from the raphe nuclei, but other explanations are also at hand.

Brain arachidonic acid uptake and turnover: implications for signaling and bipolar disorder

PURPOSE OF REVIEW

Arachidonic acid was first detected in the brain in 1922. Although earlier work examined the role of arachidonic acid in growth and development, more recent advancements have elucidated roles for arachidonic acid in brain health and disease.

RECENT FINDINGS

In this review, we summarize evidence demonstrating that unesterified arachidonic acid in the plasma pool, which is supplied in part from adipose, is readily taken up and incorporated into brain phospholipids. By labeling plasma unesterified arachidonic acid, it is possible to trace the subsequent release of arachidonic acid from brain phospholipids upon neuroreceptor-mediated release by phospholipase A2 in response to drugs and neuroinflammation in rodents. With the synthesis of 11C labeled fatty acids, brain arachidonic acid signaling can now be measured in humans with position emission tomography. Arachidonic acid signals are known to regulate important biological functions, including neuroinflammation and excitotoxicity, and we focus on how the brain arachidonic acid cascade is a common target of drugs used to treat bipolar disorder (e.g. lithium, carbamazepine and valproate).

SUMMARY

A better understanding of the regulation of arachidonic acid uptake into the brain and the brain arachidonic acid cascade could lead to new imaging techniques and the identification of novel therapeutic targets in excitotoxicity, neuroinflammation and bipolar disorder.

Functional interactions between 5-HT2A and presynaptic 5-HT1A receptor-based responses in mice genetically deficient in the serotonin 5-HT transporter (SERT)

BACKGROUND AND PURPOSE

Despite decreased presynaptic 5-HT(1A) and altered 5-HT(2A) receptor function in genetically-deficient serotonin (5-HT) transporter (SERT) mice, the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY 100635) still induced head twitches in these mice, a well-established 5-HT(2A) receptor-mediated response.

EXPERIMENTAL APPROACH

Interactions between 5-HT(1A) and 5-HT(2A) receptors were assessed using the head-twitch response following 5-HT(1A) and 5-HT(2A) receptor agonists and antagonists in SERT wild-type (+/+), heterozygous (+/-), and knockout (-/-) mice. The role of brain 5-HT availability in WAY 100635 induced head twitches was also examined.

KEY RESULTS

WAY 100635 induced head twitches in a SERT gene-dose dependent manner, inducing 5-fold more head twitches in SERT -/- versus SERT +/+ mice. In SERT -/- mice, inhibition of 5-HT synthesis with p-chlorophenylalanine (PCPA) markedly depleted tissue 5-HT in all five brain areas examined and abolished WAY 100635 induced head twitches. Further, the selective 5-HT reuptake inhibitor fluvoxamine increased WAY 100635 induced head twitches in SERT +/+ and +/- mice. Head twitches following the 5-HT(2A) receptor agonist (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI) were robust in SERT +/+ and +/- mice but much reduced in SERT -/- mice. DOI-induced head twitches were decreased by the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in SERT +/+ and +/- mice. All drug-induced head twitches were blocked by the 5-HT(2A) receptor antagonist a-Phenyl-1-(2-phenylethyl)-4-piperidinemethanol (MDL 11,939).

CONCLUSIONS AND IMPLICATIONS

These data show that indirect activation of 5-HT(2A) receptors via blockade of presynaptic 5-HT(1A) receptors potentiated head-twitch responses, suggesting functional interactions between these receptors, interactions affected by altered 5-HT availability. Our findings strongly support the correlation of WAY 100635 induced head twitches with increased 5-HT availability, induced genetically or pharmacologically.

Lack of the serotonin transporter in mice reduces locomotor activity and leads to gender-dependent late onset obesity

OBJECTIVE

Mice deficient of the serotonin transporter (5-HTT ko) mice have a reduced brain serotonin content and develop late-onset obesity. To elucidate the pathophysiology of this obesity, we analyzed the expression of the interrelated weight-regulatory molecules: brain-derived neurotrophic factor (BDNF) and leptin receptor (LR) in brain areas associated with nutrition and activity.

RESEARCH DESIGN AND METHODS

We investigated feeding behavior, physical activity and metabolic parameters of 5-HTT ko and wild-type mice and measured the expression of BDNF and LR in brain areas associated with nutrition and activity using quantitative real-time PCR. The influence of age, gender and fasting was analyzed.

RESULTS

Male 5-HTT ko mice developed obesity without hyperphagia from the age of 5 months. Physical activity was reduced in old male, but not old female, 5-HTT ko mice. The BDNF gene expression in frontal cortex was elevated in young, but reduced in old 5-HTT ko mice. Fasting failed to increase the BDNF gene expression in frontal cortex of young 5 HTT ko mice and in the hypothalamus in old 5-HTT ko mice. The fasting-induced hypothalamic increase of LR was absent in both young and old 5-HTT ko mice.

CONCLUSIONS

We propose that low brain serotonin level due to the 5-HTT ko genotype leads to reduced physical activity and low BDNF, which together with the lack of fasting-induced hypothalamic BDNF and LR production results in late-onset obesity. Although lack of the 5-HTT is a genetic vulnerability factor for obesity, female gender is protective.

Serotonin transporter deficiency increases abdominal fat in female, but not male rats

Depression and abdominal obesity often co-occur, predominantly in women, and are associated with an increased risk for the development of glucose intolerance and subsequently type 2 diabetes. The underlying mechanisms are poorly understood. We found that female, but not male, depression-prone serotonin transporter knockout (SERT(-/-)) rats had a strong increase (54%) in abdominal fat, whereas no increases in plasma concentrations of glucose and insulin were observed. Surprisingly, application of a high-fat, high-sucrose (HFHS)-choice diet, which results in increased abdominal fat deposition and increased plasma glucose levels in wild-type rats, did not result in elevated plasma glucose levels in female SERT(-/-) rats. Our results show that serotonin transporter deficiency affects abdominal fat deposition in a sex-dependent way, but protects against rises in glucose levels, and thereby potentially glucose intolerance. The increased abdominal fat formation could result from serotonin-mediated developmental changes and provides heuristic value for understanding the effects of the depression-associated serotonin transporter promoter polymorphism in humans.

Tramadol and another atypical opioid meperidine have exaggerated serotonin syndrome behavioural effects, but decreased analgesic effects, in genetically deficient serotonin transporter (SERT) mice

The serotonin syndrome is a potential side-effect of serotonin-enhancing drugs, including antidepressants such as selective serotonin reuptake inhibitors (SSRIs) and monoamine oxidase inhibitors (MAOIs). We recently reported a genetic mouse model for the serotonin syndrome, as serotonin transporter (SERT)-deficient mice have exaggerated serotonin syndrome behavioural responses to the MAOI tranylcypromine and the serotonin precursor 5-hydroxy-l-tryptophan (5-HTP). As numerous case reports implicate the atypical opioids tramadol and meperidine in the development of the human serotonin syndrome, we examined tramadol and meperidine as possible causative drugs in the rodent model of the serotonin syndrome in SERT wild-type (+/+), heterozygous (+/-) and knockout (-/-) mice. Comparisons were made with SERT mice treated with either vehicle or morphine, an opioid not implicated in the serotonin syndrome in humans. Here we show that tramadol and meperidine, but not morphine, induce serotonin syndrome-like behaviours in mice, and we show that this response is exaggerated in mice lacking one or two copies of SERT. The exaggerated response to tramadol in SERT-/- mice was blocked by pretreatment with the 5-HT1A antagonist WAY 100635. Further, we show that morphine-, meperidine- and tramadol-induced analgesia is markedly decreased in SERT-/- mice. These studies suggest that caution seems warranted in prescribing or not warning patients receiving SSRIs or MAOIs that dangerous side-effects may occur during concurrent use of tramadol and similar agents. These findings suggest that it is conceivable that there might be increased vulnerability in individuals with SERT polymorphisms that may reduce SERT by more than 50%, the level in SERT+/- mice.

Conserved role for the serotonin transporter gene in rat and mouse neurobehavioral endophenotypes

The serotonin transporter knockout (SERT(-/-)) mouse, generated in 1998, was followed by the SERT(-/-) rat, developed in 2006. The availability of SERT(-/-) rodents creates the unique possibility to study the conservation of gene function across species. Here we summarize SERT(-/-) mouse and rat data, and discuss species (dis)similarities in neurobehavioral endophenotypes. Both SERT(-/-) rodent models show a disturbed serotonergic system, altered nociception, higher anxiety, decreased social behavior, as well as increased negative emotionality, behavioral inhibition and decision making. Used to model a wide range of psychiatric disorders, SERT(-/-) rodents may be particularly valuable in research on neurodevelopmental disorders such as depression, anxiety, and possibly autism. We conclude that SERT function is conserved across mice and rats and that their behavioral profile arises from common neurodevelopmental alterations. Because mice and rats have species-specific characteristics that confer differential research advantages, a comparison of the two models has heuristic value in understanding the mechanisms and behavioral outcome of SERT genetic variation in humans.

Imaging elevated brain arachidonic acid signaling in unanesthetized serotonin transporter (5-HTT)-deficient mice

Certain polymorphisms reduce serotonin (5-HT) reuptake transporter (5-HTT) function and increase susceptibility to psychiatric disorders. Heterozygous (5-HTT(+/-))-deficient mice, models for humans with these polymorphisms, have elevated brain 5-HT concentrations and behavioral abnormalities. As postsynaptic 5-HT(2A/2C) receptors are coupled to cytosolic phospholipase A(2) (cPLA(2)), which releases arachidonic acid (AA) from membrane phospholipid, 5-HTT-deficient mice may have altered brain AA signaling and metabolism. To test this hypothesis, signaling was imaged as an AA incorporation coefficient k(*) in unanesthetized homozygous knockout (5-HTT(-/-)), 5-HTT(+/-) and wild-type (5-HTT(+/+)), mice following saline (baseline) or 1.5 mg/kg s.c. DOI, a partial 5-HT(2A/2C) receptor agonist. Enzyme activities, metabolite concentrations, and head-twitch responses to DOI were also measured. Baseline k(*) was widely elevated by 20-70% in brains of 5-HTT(+/-) and 5-HTT(-/-) compared to 5-HTT(+/+) mice. DOI increased k(*) in 5-HTT(+/+) mice, but decreased k(*) in 5-HTT-deficient mice. Brain cPLA(2) activity was elevated in 5-HTT-deficient mice; cyclooxygenase activity and prostaglandin E(2) and F(2alpha) and thromboxane B(2) concentrations were reduced. Head-twitch responses to DOI, although robust in 5-HTT(+/+) and 5-HTT(+/-) mice, were markedly fewer in 5-HTT(-/-) mice. Pretreatment with para-chlorophenylalanine, a 5-HT synthesis inhibitor, restored head twitches in 5-HTT(-/-) mice to levels in 5-HTT(+/+) mice. We propose that increased baseline values of k(*) in 5-HTT-deficient mice reflect tonic cPLA(2) stimulation through 5-HT(2A/2C) receptors occupied by excess 5-HT, and that reduced k(*) and head-twitch responses to DOI reflected displacement of receptor-bound 5-HT by DOI with a lower affinity. Increased baseline AA signaling in humans having polymorphisms with reduced 5-HTT function might be identified using positron emission tomography.

Modeling rare gene variation to gain insight into the oldest biomarker in autism: construction of the serotonin transporter Gly56Ala knock-in mouse

Alterations in peripheral and central indices of serotonin (5-hydroxytryptamine, 5-HT) production, storage and signaling have long been associated with autism. The 5-HT transporter gene (HTT, SERT, SLC6A4) has received considerable attention as a potential risk locus for autism-spectrum disorders, as well as disorders with overlapping symptoms, including obsessive-compulsive disorder (OCD). Here, we review our efforts to characterize rare, nonsynonymous polymorphisms in SERT derived from multiplex pedigrees carrying diagnoses of autism and OCD and present the initial stages of our effort to model one of these variants, Gly56Ala, in vivo. We generated a targeting vector to produce the Gly56Ala substitution in the Slc6a4 locus by homologous recombination. Following removal of a neomycin resistance selection cassette, animals exhibiting germline transmission of the Ala56 variant were bred to establish a breeding colony on a 129S6 background, suitable for initial evaluation of biochemical, physiological and behavioral alterations relative to SERT Gly56 (wild-type) animals. SERT Ala56 mice were achieved and exhibit a normal pattern of transmission. The initial growth and gross morphology of these animals is comparable to wildtype littermate controls. The SERT Ala56 variant can be propagated in 129S6 mice without apparent disruption of fertility and growth. We discuss both the opportunities and challenges that await the physiological/behavioral analysis of Gly56Ala transgenic mice, with particular reference to modeling autism-associated traits.