The brain serotonin system in autism.

Autism spectrum disorders (ASDs) are among the most common neurodevelopmental diseases. These disorders are characterized by lack of social interaction, by repetitive behavior, and often anxiety and learning disabilities. The brain serotonin (5-HT) system is known to be crucially implicated in a wide range of physiological functions and in the control of different kinds of normal and pathological behavior. A growing number of studies indicate the involvement of the brain 5-HT system in the mechanisms underlying both ASD development and ASD-related behavioral disorders. There are some review papers describing the role of separate key players of the 5-HT system in an ASD and/or autistic-like behavior. In this review, we summarize existing data on the participation of all members of the brain 5-HT system, namely, 5-HT transporter, tryptophan hydroxylase 2, MAOA, and 5-HT receptors, in autism in human and various animal models. Additionally, we describe the most recent studies involving modern techniques for in vivo regulation of gene expression that are aimed at identifying exact roles of 5-HT receptors, MAOA, and 5-HT transporter in the mechanisms underlying autistic-like behavior. Altogether, results of multiple research articles show that the brain 5-HT system intimately partakes in the control of some types of ASD-related behavior, and that specific changes in a function of a certain 5-HT receptor, transporter, and/or enzyme may normalize this aberrant behavior. These data give hope that some of clinically used 5-HT-related drugs have potential for ASD treatment.

In Vitro and In Vivo Chaperone Effect of (R)-2-amino-6-(1R, 2S)-1,2-dihydroxypropyl)-5,6,7,8-tetrahydropterin-4(3H)-one on the C1473G Mutant Tryptophan Hydroxylase 2.

Tryptophan hydroxylase 2 (TPH2) is the key and rate-limiting enzyme of serotonin (5-HT) synthesis in the mammalian brain. The 1473G mutation in the Tph2 gene decreases TPH2 activity in the mouse brain by twofold. (R)-2-amino-6-(1R, 2S)-1,2-dihydroxypropyl)-5,6,7,8-tetrahydropterin-4(3H)-one (BH4) is a pharmacological chaperone for aromatic amino acid hydroxylases. In the present study, chaperone effects of BH4 on the mutant C1473G TPH2 were investigated in vitro and in vivo. In vitro BH4 increased the thermal stability (T50 value) of mutant and wild-type TPH2 molecules. At the same time, neither chronic (twice per day for 7 days) intraperitoneal injection of 48.3 mg/kg of BH4 nor a single intraventricular administration of 60 µg of the drug altered the mutant TPH2 activity in the brain of Balb/c mice. This result indicates that although BH4 shows a chaperone effect in vitro, it is unable to increase the activity of mutant TPH2 in vivo.

Effect of Hippocampal Overexpression of Dopamine Neurotrophic Factor (CDNF) on Behavior of Mice with Genetic Predisposition to Depressive-Like Behavior.

Cerebral dopamine neurotrophic factor (CDNF) is a promising agent for Parkinson's disease treatment. However, its role in regulation of non-motor behavior including various psychopathologies remains unclear. In this regard, the aim of the present work was to study effect of CDNF overexpression in hippocampus on behavior of the ASC mice (Antidepressant Sensitive Cataleptics) with genetic predisposition to depressive-like behavior. CDNF overexpression in the mouse hippocampal neurons was induced using an adeno-associated viral vector. Four weeks after stereotaxic injection of the AAV-CDNF construct into the dorsal hippocampus home cage activity, exploratory, anxious and depressive-like types of behavior, as well as spatial and associative learning were assessed. We found significant improvements in the dynamics of spatial learning in the Morris water maze in the CDNF-overexpressing animals. At the same time, no effect of CDNF was found on other types of behavior under study. Behavior of the experimental animals under home cage conditions did not differ from that in the control group, except for the decrease in the total amount of food eaten and slight increase in the number of sleep episodes during the light phase of the day. In the present study we also attempted to determine molecular basis for the above-mentioned changes through assessment of the gene expression pattern. We did not find significant changes in the mRNA level of key kinases genes involved in neuroplasticity and neuronal survival, as well as genes encoding receptors for the main neurotransmitter systems. However, the CDNF-overexpressing animals showed increased level of the spliced Xbp indicating activation of the Ire1α/Xbp-1 pathway traditionally associated with ER stress. Immunohistochemical analysis showed that CDNF was co-localized with the ER marker calreticulin. Thus, the effects of endogenous CDNF on behavior that we have found could be mediated by a specific molecular cascade, which emphasizes its difference from the classical neurotrophic factors.

A Truncated Receptor TrkB Isoform (TrkB.T1) in Mechanisms of Genetically Determined Depressive-like Behavior of Mice.

Depression is a mental disorder that significantly reduces quality of life, and the discovery of new drug targets is an urgent problem for modern neuroscience. Brain-derived neurotrophic factor (BDNF) and its receptors have been found to participate in mechanisms of depression and antidepressant drugs' action. In this study, we focused on a less-studied truncated isoform of receptor TrkB: TrkB.T1. Initially, we noticed that the level of TrkB.T1 is low in the hippocampus of Antidepressant-Sensitive Cataleptics (ASC) mice, which are characterized by genetically determined depressive-like behavior in contrast to "normal" C57BL/6J mice. Next, overexpression of TrkB.T1 receptor in hippocampal neurons of ACS mice was induced to clarify the role of this receptor in mechanisms of depressive-like behavior. TrkB.T1 overexpression lowered BDNF protein concentration in the hippocampus. On the behavioral level, TrkB.T1 overexpression severely decreased aggression and enhanced social behavior. Additionally, this excess of receptor TrkB.T1 slightly promoted anxiety and depressive-like behavioral traits without affecting learning and memory. Our results show that this TrkB isoform participates in the control of aggression, anxiety, and depressive-like behavior and in the regulation of BDNF system functioning in ASC mice (genetically predisposed to depressive-like behavior). Considering our findings, we believe that hippocampal receptor TrkB.T1 can be a drug target for the correction of behavioral pathologies.

Effects of Chronic Combined Treatment with Ketanserin and Fluoxetine in B6.CBA-D13Mit76C Recombinant Mice with Abnormal 5-HT1A Receptor Functional Activity.

The recombinant B6.CBA-D13Mit76C mouse strain is characterized by an altered sensitivity of 5-HT1A receptors and upregulated 5-HT1A gene transcription. Recently, we found that in B6.CBA-D13Mit76C mice, chronic fluoxetine treatment produced the pro-depressive effect in a forced swim test. Since 5-HT2A receptor blockade may be beneficial in treatment-resistant depression, we investigated the influence of chronic treatment (14 days, intraperitoneally) with selective 5-HT2A antagonist ketanserin (0.5 mg/kg), fluoxetine (20 mg/kg), or fluoxetine + ketanserin on the behavior, functional activity of 5-HT1A and 5-HT2A receptors, serotonin turnover, and transcription of principal genes of the serotonin system in the brain of B6.CBA-D13Mit76C mice. Ketanserin did not reverse the pro-depressive effect of fluoxetine, while fluoxetine, ketanserin, and fluoxetine + ketanserin decreased the functional activity of 5-HT1A receptors and Htr1a gene transcription in the midbrain and hippocampus. All tested drug regimens decreased the mRNA levels of Slc6a4 and Maoa in the midbrain. These changes were not accompanied by a significant shift in the levels of serotonin and its metabolite 5-HIAA. Notably, ketanserin upregulated enzymatic activity of tryptophan hydroxylase 2 (TPH2). Thus, despite some benefits (reduced Htr1a, Slc6a4, and Maoa transcription and increased TPH2 activity), prolonged blockade of 5-HT2A receptors failed to ameliorate the adverse effect of fluoxetine in the case of abnormal functioning of 5-HT1A receptors.

Amisulpride Decreases Tau Protein Hyperphosphorylation in the Brain of OXYS Rats.

AIM: In this study, OXYS rats of three ages (1, 3, and 6 months), a proven model of Alzheimer's disease (AD), at various stages of disease progression were used to thoroughly study the effects of amisulpride on behavior and tau protein phosphorylation. BACKGROUND: With the growing number of patients with AD, the problem of finding a cure is very acute. Neurodegeneration in AD has various causes, one of which is hyperphosphorylation of tau protein. OBJECTIVE: This study aimed to investigate whether amisulpride would affect pathological tau phosphorylation in AD. METHODS: We assessed the influence of chronic administration of amisulpride (3 weeks, 3 mg/kg per day, intraperitoneally)-a 5-HT7 receptor inverse agonist-on behavior and tau hyperphosphorylation in OXYS rats (at ages of 1, 3, and 6 months). RESULTS: Chronic administration of amisulpride dramatically decreased tau phosphorylation in the frontal cortex and hippocampus of 3-month-old OXYS rats. Additionally, in 1- and 3-month-old rats' hippocampi, amisulpride diminished the mRNA level of the Cdk5 gene encoding one of the main tau kinases involved in the 5-HT7 receptor-induced effect on tau phosphorylation. CONCLUSION: Thus, We found that chronic administration of amisulpride could reduce pathological tau hyperphosphorylation while reducing anxiety. We propose amisulpride to have therapeutic potential against AD and that it can be the most effective in the early stages of the disease.

Brain-Derived Neurotrophic Factor (BDNF) in Mechanisms of Autistic-like Behavior in BTBR Mice: Crosstalk with the Dopaminergic Brain System.

Disturbances in neuroplasticity undoubtedly play an important role in the development of autism spectrum disorders (ASDs). Brain neurotransmitters and brain-derived neurotrophic factor (BDNF) are known as crucial players in cerebral and behavioral plasticity. Such an important neurotransmitter as dopamine (DA) is involved in the behavioral inflexibility of ASD. Additionally, much evidence from human and animal studies implicates BDNF in ASD pathogenesis. Nonetheless, crosstalk between BDNF and the DA system has not been studied in the context of an autistic-like phenotype. For this reason, the aim of our study was to compare the effects of either the acute intracerebroventricular administration of a recombinant BDNF protein or hippocampal adeno-associated-virus-mediated BDNF overexpression on autistic-like behavior and expression of key DA-related and BDNF-related genes in BTBR mice (a widely recognized model of autism). The BDNF administration failed to affect autistic-like behavior but downregulated Comt mRNA in the frontal cortex and hippocampus; however, COMT protein downregulation in the hippocampus and upregulation in the striatum were insignificant. BDNF administration also reduced the receptor TrkB level in the frontal cortex and midbrain and the BDNF/proBDNF ratio in the striatum. In contrast, hippocampal BDNF overexpression significantly diminished stereotypical behavior and anxiety; these alterations were accompanied only by higher hippocampal DA receptor D1 mRNA levels. The results indicate an important role of BDNF in mechanisms underlying anxiety and repetitive behavior in ASDs and implicates BDNF-DA crosstalk in the autistic-like phenotype of BTBR mice.

The Role of C1473G Polymorphism in Mouse Triptophan Hydroxylase 2 Gene in the Acute Effects of Ethanol on the c-fos Gene Expression and Metabolism of Biogenic Amines in the Brain.

Tryptophan hydroxylase 2 is a key enzyme in the synthesis of the neurotransmitter serotonin, which plays an important role in the regulation of behavior and various physiological functions. We studied the effect of acute ethanol administration on the expression of the early response c-fos gene and metabolism of serotonin and catecholamines in the brain structures of B6-1473C and B6-1473G congenic mouse strains differing in the single-nucleotide substitution C1473G in the Tph2 gene and activity of the encoded enzyme. Acute alcoholization led to a significant upregulation of the c-fos gene expression in the frontal cortex and striatum of B6-1473G mice and in the hippocampus of B6-1473C mice and caused a decrease in the index of serotonin metabolism in the nucleus accumbens in B6-1473C mice and in the hippocampus and striatum of B6-1473G mice, as well as to the decrease in the norepinephrine level in the hypothalamus of B6-1473C mice. Therefore, the C1473G polymorphism in the Tph2 gene has a significant effect of acute ethanol administration on the c-fos expression pattern and metabolism of biogenic amines in the mouse brain.

On Associations between Fear-Induced Aggression, Bdnf Transcripts, and Serotonin Receptors in the Brains of Norway Rats: An Influence of Antiaggressive Drug TC-2153.

The Bdnf (brain-derived neurotrophic factor) gene contains eight regulatory exons (I-VIII) alternatively spliced to the protein-coding exon IX. Only exons I, II, IV, and VI are relatively well studied. The BDNF system and brain serotonergic system are tightly interconnected and associated with aggression. The benzopentathiepine TC-2153 affects both systems and exerts antiaggressive action. Our aim was to evaluate the effects of TC-2153 on the Bdnf exons I-IX's expressions and serotonin receptors' mRNA levels in the brain of rats featuring high aggression toward humans (aggressive) or its absence (tame). Aggressive and tame adult male rats were treated once with vehicle or 10 or 20 mg/kg of TC-2153. mRNA was quantified in the cortex, hippocampus, hypothalamus, and midbrain with real-time PCR. Selective breeding for high aggression or its absence affected the serotonin receptors' and Bdnf exons' transcripts differentially, depending on the genotype (strain) and brain region. TC-2153 had comprehensive effects on the Bdnf exons' expressions. The main trend was downregulation in the hypothalamus and midbrain. TC-2153 increased 5-HT1B receptor hypothalamusc mRNA expression. For the first time, an influence of TC-2153 on the expressions of Bdnf regulatory exons and the 5-HT1B receptor was shown, as was an association between Bdnf regulatory exons and fear-induced aggression involving genetic predisposition.

On the role of serotonin 5-HT1A receptor in autistic-like behavior: сross talk of 5-HT and BDNF systems.

Autism spectrum disorders (ASDs) are some of the most common neurodevelopmental disorders; however, the mechanisms underlying ASDs are still poorly understood. Serotonin (5-HT) and brain-derived neurotrophic factor (BDNF) are known as key players in brain and behavioral plasticity and interact with each other. 5-HT1A receptor is a principal regulator of the brain 5-HT system, which modulates normal and pathological behavior. Here we investigated effects of adeno-associated-virus-based 5-HT1A receptor overexpression in the hippocampus of BTBR mice (which are a model of autism) on various types of behavior and on the expression of 5-HT7 receptor, proBDNF, mature BDNF, and BDNF receptors (TrkB and p75NTR). The 5-HT1A receptor overexpression in BTBR mice reduced stereotyped behavior in the marble-burying test and extended the time spent in the center in the open field test. Meanwhile, this overexpression failed to affect social behavior in the three-chambered test, immobility time in the tail suspension test, locomotor activity in the open field test, and associative learning within the "operant wall" paradigm. The 5-HT1A receptor overexpression in the hippocampus raised hippocampal 5-HT7 receptor mRNA and protein levels. Additionally, the 5-HT1A receptor overexpression lowered both mRNA and protein levels of TrkB receptor but failed to affect proBDNF, mature BDNF, and p75NTR receptor expression in the hippocampus of BTBR mice. Thus, obtained results suggest the involvement of the 5-HT and BDNF systems' interaction mediated by 5-HT1A and TrkB receptors in the mechanisms underlying autistic-like behavior in BTBR mice.

Serotonin Receptors as a Potential Target in the Treatment of Alzheimer's Disease.

Alzheimer's disease (AD) is the most common cause of dementia worldwide that has an increasing impact on aging societies. Besides its critical role in the control of various physiological functions and behavior, brain serotonin (5-HT) system is involved in the regulation of migration, proliferation, differentiation, maturation, and programmed death of neurons. At the same time, a growing body of evidence indicates the involvement of 5-HT neurotransmission in the formation of insoluble aggregates of ß-amyloid and tau protein, the main histopathological signs of AD. The review describes the role of various 5-HT receptors and intracellular signaling cascades induced by them in the pathological processes leading to the development of AD, first of all, in protein aggregation. Changes in the functioning of certain types of 5-HT receptors or associated intracellular signaling mediators prevent accumulation of ß-amyloid plaques and tau protein neurofibrillary tangles. Based on the experimental data, it can be suggested that the use of 5-HT receptors as new drug targets will not only improve cognitive performance in AD, but will be also important in treating the causes of AD-related dementia.

Effects of the Cc2d1a/Freud-1 Knockdown in the Hippocampus of BTBR Mice on the Autistic-Like Behavior, Expression of Serotonin 5-HT1A and D2 Dopamine Receptors, and CREB and NF-kB Intracellular Signaling.

The mechanisms of autism are of extreme interest due to the high prevalence of this disorder in the human population. In this regard, special attention is given to the transcription factor Freud-1 (encoded by the Cc2d1a gene), which regulates numerous intracellular signaling pathways and acts as a silencer for 5-HT1A serotonin and D2 dopamine receptors. Disruption of the Freud-1 functions leads to the development of various psychopathologies. In this study, we found an increase in the expression of the Cc2d1a/Freud-1 gene in the hippocampus of BTBR mice (model of autistic-like behavior) in comparison with C57Bl/6J mice and examined how restoration of the Cc2d1a/Freud-1 expression in the hippocampus of BTBR mice affects their behavior, expression of 5-HT1A serotonin and D2 dopamine receptors, and CREB and NF-κB intracellular signaling pathways in these animals. Five weeks after administration of the adeno-associated viral vector (AAV) carrying the pAAV_H1-2_shRNA-Freud-1_Syn_EGFP plasmid encoding a small hairpin RNA (shRNA) that suppressed expression of the Cc2d1a/Freud-1 gene, we observed an elevation in the anxiety levels, as well as the increase in the escape latency and path length to the platform in the Morris water maze test, which was probably associated with a strengthening of the active stress avoidance strategy. However, the Cc2d1a/Freud-1 knockdown did not affect the spatial memory and phosphorylation of the CREB transcription factor, although such effect was found in C57Bl/6J mice in our previous study. These results suggest the impairments in the CREB-dependent effector pathway in BTBR mice, which may play an important role in the development of the autistic-like phenotype. The knockdown of Cc2d1a/Freud-1 in the hippocampus of BTBR mice did not affect expression of the 5-HT1A serotonin and D2 dopamine receptors and key NF-κB signaling genes (Nfkb1 and Rela). Our data suggest that the transcription factor Freud-1 plays a significant role in the pathogenesis of anxiety and active stress avoidance in autism.

Serotonin Receptor 5-HT2A Regulates TrkB Receptor Function in Heteroreceptor Complexes.

Serotonin receptor 5-HT2A and tropomyosin receptor kinase B (TrkB) strongly contribute to neuroplasticity regulation and are implicated in numerous neuronal disorders. Here, we demonstrate a physical interaction between 5-HT2A and TrkB in vitro and in vivo using co-immunoprecipitation and biophysical and biochemical approaches. Heterodimerization decreased TrkB autophosphorylation, preventing its activation with agonist 7,8-DHF, even with low 5-HT2A receptor expression. A blockade of 5-HT2A receptor with the preferential antagonist ketanserin prevented the receptor-mediated downregulation of TrkB phosphorylation without restoring the TrkB response to its agonist 7,8-DHF in vitro. In adult mice, intraperitoneal ketanserin injection increased basal TrkB phosphorylation in the frontal cortex and hippocampus, which is in accordance with our findings demonstrating the prevalence of 5-HT2A-TrkB heteroreceptor complexes in these brain regions. An expression analysis revealed strong developmental regulation of 5-HT2A and TrkB expressions in the cortex, hippocampus, and especially the striatum, demonstrating that the balance between TrkB and 5-HT2A may shift in certain brain regions during postnatal development. Our data reveal the functional role of 5-HT2A-TrkB receptor heterodimerization and suggest that the regulated expression of 5-HT2A and TrkB is a molecular mechanism for the brain-region-specific modulation of TrkB functions during development and under pathophysiological conditions.

The Implication of 5-HT Receptor Family Members in Aggression, Depression and Suicide: Similarity and Difference.

Being different multifactorial forms of psychopathology, aggression, depression and suicidal behavior, which is considered to be violent aggression directed against the self, have principal neurobiological links: preclinical and clinical evidence associates depression, aggression and suicidal behavior with dysregulation in central serotonergic (5-HT) neurotransmission. The implication of different types of 5-HT receptors in the genetic and epigenetic mechanisms of aggression, depression and suicidality has been well recognized. In this review, we consider and compare the orchestra of 5-HT receptors involved in these severe psychopathologies. Specifically, it concentrates on the role of 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT3 and 5-HT7 receptors in the mechanisms underlying the predisposition to aggression, depression and suicidal behavior. The review provides converging lines of evidence that: (1) depression-related 5-HT receptors include those receptors with pro-depressive properties (5-HT2A, 5-HT3 and 5-HT7) as well as those providing an antidepressant effect (5-HT1A, 5-HT1B, 5-HT2C subtypes). (2) Aggression-related 5-HT receptors are identical to depression-related 5-HT receptors with the exception of 5-HT7 receptors. Activation of 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C receptors attenuate aggressiveness, whereas agonists of 5-HT3 intensify aggressive behavior.

Serotonin 5-HT7 receptor overexpression in the raphe nuclei area produces antidepressive effect and affects brain serotonin system in male mice.

Heterodimerization between 5-HT7 and 5-HT1A receptors seems to play an important role in the mechanism of depression and antidepressant drug action. It was suggested that the shift of the ratio between 5-HT1A /5-HT7 hetero- and 5-HT1A /5-HT1A homodimers in presynaptic neurons toward 5-HT1A /5-HT1A homodimers is one of the reasons of depression. Consequently, the artificial elevation of 5-HT7 receptor number in presynaptic terminals might restore physiological homo-/heterodimer ratio resulting in antidepressive effect. Here we showed that adeno-associated virus (AAV)-based 5-HT7 receptor overexpression in the midbrain raphe nuclei area produced antidepressive effect in male mice of both C57Bl/6J and genetically predisposed to depressive-like behavior ASC (antidepressant sensitive cataleptics) strains. These changes were accompanied by the elevation of 5-HT7 receptor mRNA level in the frontal cortex of C57Bl/6J and its reduction in the hippocampus of ASC mice. The presence of engineered 5-HT7 receptor in the midbrain of both mouse strains was further demonstrated. Importantly that 5-HT7 receptor overexpression resulted in the reduction of 5-HT1A receptor level in the membrane protein fraction from the midbrain samples of C57Bl/6J, but not ASC, mice. 5-HT7 receptor overexpression caused an increase of 5-HIAA/5-HT ratio in the midbrain and the frontal cortex of C57Bl/6J and in all investigated brain structures of ASC mice. Thus, 5-HT7 receptor overexpression in the raphe nuclei area affects brain 5-HT system and causes antidepressive effect both in C57Bl/6J and in "depressive" ASC male mice. Obtained results indicate the involvement of 5-HT7 receptor in the mechanisms underlying depressive behavior.

The neuropeptide cycloprolylglycine produces antidepressant-like effect and enhances BDNF gene expression in the mice cortex.

BACKGROUND: Cycloprolylglycine (CPG) is an endogenous dipeptide with a wide range of psychotropic activity and putative therapeutic potential for depression. A small but growing body of data suggests that antidepressant-like effect of CPG is associated with neuroplastic changes in the brain or 5-HT system modulation. However, the mechanisms of the dipeptide action remain elusive. AIMS: Here, we characterize the effects of chronic CPG administration on behavior and genes expression of antidepressants sensitive catalepsy (ASC) mice strain, characterized by depressive-like behavior. METHODS: ASC mice were injected with saline, fluoxetine (10 mg/kg/day), or CPG (1 and 2 mg/kg/day) during 2 weeks. Behavior was studied using the open field test, novel object test, elevated plus maze test, forced swim test, and tail suspension test (TST). The expressions of genes coding BDNF, CREB, 5-HT1A and 5-HT2A receptors, TPH2, and SERT in the brain were measured with quantitative real-time reverse transcription polymerase chain reaction (RT-PCR). RESULTS: Chronic intraperitoneal administration of 1 and 2 mg/kg of CPG revealed the significant antidepressant-like effect by decreasing immobility time in the TST. At the same time, CPG did not negatively affect locomotor activity, cognition, or anxiety. In the real-time quantitative polymerase chain reaction (PCR) assay, chronic CPG treatment (2 mg/kg for 14 days) increased Bdnf mRNA level in the frontal cortex. CONCLUSIONS: Our findings extend the evidence for the effectiveness of CPG to reduce depressive-like behaviors. The antidepressant-like effect of CPG is mediated, as least in part, by BDNF-dependent mechanism. The exact mechanism remains to be elucidated, and further studies are warranted.

On an association between fear-induced aggression and striatal-enriched protein tyrosine phosphatase (STEP) in the brain of Norway rats.

Striatal-enriched protein tyrosine phosphatase (STEP) is a signal transduction protein involved in the pathogenesis of neuropathologies. A STEP inhibitor (TC-2153) has antipsychotic and antidepressant effects. Here, we evaluated the role of STEP in fear-induced aggression using Norway rats selectively bred for 90 generations for either high aggression toward humans (aggressive rats) or its absence (tame rats). We studied the effects of acute administration of TC-2153 on behavior and STEP expression in the brain of these animals and the influence of chronic treatment with TC-2153 on the behavior and STEP expression in aggressive rats in comparison with classic antidepressant fluoxetine, which is known to exert antiaggressive action. Acute TC-2153 administration decreased the aggressive reaction to humans in aggressive rats, while having no impact on the friendly behavior of tame rats. Moreover, in the elevated plus-maze test, the drug had an anxiolytic effect on both aggressive and tame rats. Aggressive rats demonstrated elevated levels of a STEP isoform (STEP46) as compared to tame animals, whereas acute TC-2153 administration significantly reduced STEP46 protein concentration in the brain of aggressive rats. Chronic treatment of aggressive rats with either TC-2153 or fluoxetine attenuated fear-induced aggression. Chronic administration of fluoxetine enhanced the exploratory activity in the elevated plus-maze test and decreased the STEP46 protein level in aggressive rats' hippocampus, whereas chronic TC-2153 administration did not affect these parameters. Thus, STEP46 can play an important role in the mechanisms of aggression and may mediate antiaggressive effects of TC-2153 and fluoxetine.

Effects of a Cc2d1a/Freud-1 Knockdown in the Hippocampus on Behavior, the Serotonin System, and BDNF.

The serotonin 5-HT1A receptor is one of the most abundant and widely distributed brain serotonin (5-HT) receptors that play a major role in the modulation of emotions and behavior. The 5-HT1A receptor gene (Htr1a) is under the control of transcription factor Freud-1 (also known as Cc2d1a/Freud-1). Here, using adeno-associated virus (AAV) constructs in vivo, we investigated effects of a Cc2d1a/Freud-1 knockdown in the hippocampus of C57BL/6J mice on behavior, the brain 5-HT system, and brain-derived neurotrophic factor (BDNF). AAV particles carrying the pAAV_H1-2_shRNA-Freud-1_Syn_EGFP plasmid encoding a short-hairpin RNA targeting mouse Cc2d1a/Freud-1 mRNA had an antidepressant effect in the forced swim test 5 weeks after virus injection. The knockdown impaired spatiotemporal memory as assessed in the Morris water maze. pAAV_H1-2_shRNA-Freud-1_Syn_EGFP decreased Cc2d1a/Freud-1 mRNA and protein levels. Furthermore, the Cc2d1a/Freud-1 knockdown upregulated 5-HT and its metabolite 5-hydroxyindoleacetic acid but not their ratio. The Cc2d1a/Freud-1 knockdown failed to increase mRNA and protein levels of Htr1a but diminished a 5-HT1A receptor functional response. Meanwhile, the Cc2d1a/Freud-1 knockdown reduced Creb mRNA expression and CREB phosphorylation and upregulated cFos mRNA. The knockdown enhanced the expression of a BDNF precursor (proBDNF protein), which is known to play a crucial part in neuroplasticity. Our data indicate that transcription factor Cc2d1a/Freud-1 is implicated in the pathogenesis of depressive disorders not only via the 5-HT1A receptor and transcription factor CREB but also through an influence on BDNF.

Effect of Central Administration of Brain-Derived Neurotrophic Factor (BDNF) on Behavior and Brain Monoamine Metabolism in New Recombinant Mouse Lines Differing by 5-HT1A Receptor Functionality.

Experiments were carried out on recombinant B6.CBA-D13Mit76C (B6-M76C) and B6.CBA-D13Mit76B (B6-M76B) mouse lines created by transferring a 102.73-118.83 Mbp fragment of chromosome 13, containing the 5-HT1A receptor gene, from CBA or C57BL/6 strains to a C57BL/6 genetic background, correspondingly. We have recently shown different levels of 5-HT1A receptor functionality in these mouse lines. The administration of BDNF (300 ng/mouse, i.c.v.) increased the levels of exploratory activity and intermale aggression only in B6-M76B mice, without affecting depressive-like behavior in both lines. In B6-M76B mice the behavioral alterations were accompanied by a decrease in the 5-HT2A receptor functional activity and the augmentation of levels of serotonin and its main metabolite, 5-HIAA (5-hydroxyindoleacetic acid), in the midbrain. Moreover, the levels of dopamine and its main metabolites, HVA (homovanillic acid) and DOPAC (3,4-dihydroxyphenylacetic acid), were also elevated in the striatum of B6-M76B mice after BDNF treatment. In B6-M76C mice, central BDNF administration led only to a reduction in the functional activity of the 5-HT1A receptor and a rise in DOPAC levels in the midbrain. The obtained data suggest the importance of the 102.73-118.83 Mbp fragment of mouse chromosome 13, which contains the 5-HT1A receptor gene, for BDNF-induced alterations in behavior and the brain monoamine system.

The 5-HT4 receptor interacts with adhesion molecule L1 to modulate morphogenic signaling in neurons.

Morphological remodeling of dendritic spines is critically involved in memory formation and depends on adhesion molecules. Serotonin receptors are also implicated in this remodeling, though the underlying mechanisms remain enigmatic. Here, we uncovered a signaling pathway involving the adhesion molecule L1CAM (L1) and serotonin receptor 5-HT4 (5-HT4R, encoded by HTR4). Using Förster resonance energy transfer (FRET) imaging, we demonstrated a physical interaction between 5-HT4R and L1, and found that 5-HT4R-L1 heterodimerization facilitates mitogen-activated protein kinase activation in a Gs-dependent manner. We also found that 5-HT4R-L1-mediated signaling is involved in G13-dependent modulation of cofilin-1 activity. In hippocampal neurons in vitro, the 5-HT4R-L1 pathway triggers maturation of dendritic spines. Thus, the 5-HT4R-L1 signaling module represents a previously unknown molecular pathway regulating synaptic remodeling.