Coexisting lacertid lizard species Podarcis siculus and Podarcis melisellensis differ in dopamine brain concentrations.

In the eastern Adriatic, Podarcis siculus, an invasive species, competitively excludes the native Podarcis melisellensis. Monoamine neurotransmitters-serotonin (5HT), dopamine (DA), and noradrenaline (NA)-are implicated in social behavior, and could lie at the basis of the direct behavioral interference of P. siculus with P. melisellensis. To understand the relationship between social behavior and monoamines, as well as the differences in behavior between P. siculus and P. melisellensis, we developed a high-performance liquid chromatography (UV/VIS detection) method with which we were able to reliably measure concentrations of 5HT, DA, and NA in 32 brains of the two lizard species. We observed no statistically significant influence of species, sex, or their interaction on brain NA and 5HT concentrations. Statistically significant influence of species on dopamine levels were recorded, with P. siculus having twice as much dopamine in their brains. Taking into account that a significant aggressive relationship, with P. siculus dominating over P. melisellensis, has been previously observed, and that dopamine directly influences this behavior, the observed differences in dopamine levels could represent a trait in these species and may contribute to the competitive exclusion of P. melisellensis by P. siculus in the eastern Adriatic.

Beyond the Brain: Perinatal Exposure of Rats to Serotonin Enhancers Induces Long-Term Changes in the Jejunum and Liver.

Serotonin (5-hydroxytryptamine, 5HT) homeostasis is essential for many physiological processes in the central nervous system and peripheral tissues. Hyperserotonemia, a measurable sign of 5HT homeostasis disruption, can be caused by 5HT-directed treatment of psychiatric and gastrointestinal diseases. Its impact on the long-term balance and function of 5HT in the peripheral compartment remains unresolved and requires further research due to possible effects on human health. We explored the effects of perinatal 5HT imbalance on the peripheral organs responsible for serotonin metabolism-the jejunum, a synthesis site, and the liver, a catabolism site-in adult rats. Hyperserotonemia was induced by subchronic treatment with serotonin precursor 5-hydroxytryptophan (5HTP) or serotonin degradation inhibitor tranylcypromine (TCP). The jejunum and liver were collected on postnatal day 70 and analyzed histomorphometrically. Relative mRNA levels of 5HT-regulating proteins were determined using qRT-PCR. Compared to controls, 5HTP- and TCP-treated rats had a reduced number of 5HT-producing cells and expression of the 5HT-synthesising enzyme in the jejunum, and an increased expression of 5HT-transporter accompanied by karyomegaly in hepatocytes, with these differences being more pronounced in the TCP-treated animals. Here, we report that perinatal 5HT disbalance induced long-term cellular and molecular changes in organs regulating 5HT-metabolism, which may have a negative impact on 5HT availability and function in the periphery. Our rat model demonstrates a link between the developmental abnormalities of serotonin homeostasis and 5HT-related changes in adult life and may be suitable for exploring the neurobiological substrates of vulnerability to behavioral and metabolic disorders, as well as for modeling the adverse effects of the prenatal exposure to 5HT enhancers in the human population.

Methylation of serotonin regulating genes in cord blood cells: association with maternal metabolic parameters and correlation with methylation in peripheral blood cells during childhood and adolescence.

BACKGROUND: Serotonin (5-hydroxytryptamine, 5-HT) signaling is involved in neurodevelopment, mood regulation, energy metabolism, and other physiological processes. DNA methylation plays a significant role in modulating the expression of genes responsible for maintaining 5-HT balance, such as 5-HT transporter (SLC6A4), monoamine oxidase A (MAOA), and 5-HT receptor type 2A (HTR2A). Maternal metabolic health can influence long-term outcomes in offspring, with DNA methylation mediating these effects. We investigated associations between maternal metabolic parameters-pre-pregnancy body mass index (pBMI), gestational weight gain (GWG), and glucose tolerance status (GTS), i.e., gestational diabetes mellitus (GDM) versus normal glucose tolerance (NGT)-and cord blood methylation of SLC6A4, MAOA, and HTR2A in participants from our PlaNS birth cohort. CpG sites (15, 9, and 2 in each gene, respectively) were selected based on literature and in silico data. Methylation levels were quantified by bisulfite pyrosequencing. We also examined the stability of methylation patterns in these genes in circulating blood cells from birth to adolescence using longitudinal DNA methylation data from the ARIES database. RESULTS: None of the 203 PlaNS mothers included in this study had preexisting diabetes, 99 were diagnosed with GDM, and 104 had NGT; all neonates were born at full term by planned Cesarean section. Methylation at most CpG sites differed between male and female newborns. SLC6A4 methylation correlated inversely with maternal pBMI and GWG, while methylation at HTR2A site -1665 correlated positively with GWG. None of the maternal metabolic parameters statistically associated with MAOA methylation. DNA methylation data in cord blood and peripheral blood at ages 7 and 15 years were available for 808 participants from the ARIES database; 4 CpG sites (2 in SLC6A4 and 2 in HTR2A) overlapped between the PlaNS and ARIES cohorts. A positive correlation between methylation levels in cord blood and peripheral blood at 7 and 15 years of age was observed for both SLC6A4 and HTR2A CpG sites. CONCLUSIONS: Methylation of 5-HT regulating genes in cord blood cells is influenced by neonatal sex, with maternal metabolism playing an additional role. Inter-individual variations present in circulating blood cells at birth are still pronounced in childhood and adolescence.

Lasting mesothalamic dopamine imbalance and altered exploratory behavior in rats after a mild neonatal hypoxic event.

Introduction: Adversities during the perinatal period can decrease oxygen supply to the fetal brain, leading to various hypoxic brain injuries, which can compromise the regularity of brain development in different aspects. To examine the catecholaminergic contribution to the link between an early-life hypoxic insult and adolescent behavioral aberrations, we used a previously established rat model of perinatal hypoxia but altered the hypobaric to normobaric conditions. Methods: Exploratory and social behavior and learning abilities were tested in 70 rats of both sexes at adolescent age. Inherent vertical locomotion, sensory-motor functions and spatial learning abilities were explored in a subset of animals to clarify the background of altered exploratory behavior. Finally, the concentrations of dopamine (DA) and noradrenaline in midbrain and pons, and the relative expression of genes for DA receptors D1 and D2, and their down-stream targets (DA- and cAMP-regulated phosphoprotein, Mr 32 kDa, the regulatory subunit of protein kinase A, and inhibitor-5 of protein phosphatase 1) in the hippocampus and thalamus were investigated in 31 rats. Results: A lesser extent of alterations in exploratory and cognitive aspects of behavior in the present study suggests that normobaric conditions mitigate the hypoxic injury compared to the one obtained under hypobaric conditions. Increased exploratory rearing was the most prominent consequence, with impaired spatial learning in the background. In affected rats, increased midbrain/pons DA content, as well as mRNA levels for DA receptors and their down-stream elements in the thalamus, but not the hippocampus, were found. Conclusion: We can conclude that a mild hypoxic event induced long-lasting disbalances in mesothalamic DA signaling, contributing to the observed behavioral alterations. The thalamus was thereby indicated as another structure, besides the well-established striatum, involved in mediating hypoxic effects on behavior through DA signaling.

Maternal Metabolic State and Fetal Sex and Genotype Modulate Methylation of the Serotonin Receptor Type 2A Gene (HTR2A) in the Human Placenta.

The serotonin receptor 2A gene (HTR2A) is a strong candidate for the fetal programming of future behavior and metabolism. Maternal obesity and gestational diabetes mellitus (GDM) have been associated with an increased risk of metabolic and psychological problems in offspring. We tested the hypothesis that maternal metabolic status affects methylation of HTR2A in the placenta. The prospective study included 199 pairs of mothers and healthy full-term newborns. Genomic DNA was extracted from feto-placental samples and analyzed for genotypes of two polymorphisms (rs6311, rs6306) and methylation of four cytosine residues (-1665, -1439, -1421, -1224) in the HTR2A promoter region. Placental HTR2A promoter methylation was higher in male than female placentas and depended on both rs6311 and rs6306 genotypes. A higher maternal pre-gestational body mass index (pBMI) and, to a lesser extent, diagnosis of GDM were associated with reduced HTR2A promoter methylation in female but not male placentas. Higher pBMI was associated with reduced methylation both directly and indirectly through increased GDM incidence. Tobacco use during pregnancy was associated with reduced HTR2A promoter methylation in male but not female placentas. The obtained results suggest that HTR2A is a sexually dimorphic epigenetic target of intrauterine exposures. The findings may contribute to a better understanding of the early developmental origins of neurobehavioral and metabolic disorders associated with altered HTR2A function.

The Signature of Moderate Perinatal Hypoxia on Cortical Organization and Behavior: Altered PNN-Parvalbumin Interneuron Connectivity of the Cingulate Circuitries.

This study was designed in a rat model to determine the hallmarks of possible permanent behavioral and structural brain alterations after a single moderate hypoxic insult. Eighty-two Wistar Han (RccHan: WIST) rats were randomly subjected to hypoxia (pO2 73 mmHg/2 h) or normoxia at the first postnatal day. The substantially increased blood lactate, a significantly decreased cytochrome-C-oxygenase expression in the brain, and depleted subventricular zone suggested a high vulnerability of subset of cell populations to oxidative stress and consequent tissue response even after a single, moderate, hypoxic event. The results of behavioral tests (open-field, hole-board, social-choice, and T-maze) applied at the 30-45th and 70-85th postnatal days revealed significant hyperactivity and a slower pace of learning in rats subjected to perinatal hypoxia. At 3.5 months after hypoxic insult, the histochemical examination demonstrated a significantly increased number of specific extracellular matrix-perineuronal nets and increased parvalbumin expression in a subpopulation of interneurons in the medial and retrosplenial cingulate cortex of these animals. Conclusively, moderate perinatal hypoxia in rats causes a long-lasting reorganization of the connectivity in the cingulate cortex and consequent alterations of related behavioral and cognitive abilities. This non-invasive hypoxia model in the rat successfully and complementarily models the moderate perinatal hypoxic injury in fetuses and prematurely born human babies and may enhance future research into new diagnostic and therapeutic strategies for perinatal medicine.

Functional promoter polymorphism of the neuronal isoform of tryptophan hydroxylase (Tph2) in suicide.

The association between suicide and G-703T polymorphism of the tryptophan hydroxylase 2 (TPH2), the rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin, was studied in a sample of 291 suicide victims and 280 healthy subjects of Croatian origin. No significant differences were found between the groups. Obtained results do not support involvement of the investigated polymorphism in the susceptibility to suicide completion.

Population-based study of genetic variation in individuals with autism spectrum disorders from Croatia.

BACKGROUND: Genome-wide studies on autism spectrum disorders (ASDs) have mostly focused on large-scale population samples, but examination of rare variations in isolated populations may provide additional insights into the disease pathogenesis. METHODS: As a first step in the genetic analysis of ASD in Croatia, we characterized genetic variation in a sample of 103 subjects with ASD and 203 control individuals, who were genotyped using the Illumina HumanHap550 BeadChip. We analyzed the genetic diversity of the Croatian population and its relationship to other populations, the degree of relatedness via Runs of Homozygosity (ROHs), and the distribution of large (>500 Kb) copy number variations. RESULTS: Combining the Croatian cohort with several previously published populations in the FastME analysis (an alternative to Neighbor Joining) revealed that Croatian subjects cluster, as expected, with Southern Europeans; in addition, individuals from the same geographic region within Europe cluster together. Whereas Croatian subjects could be separated from a sample of healthy control subjects of European origin from North America, Croatian ASD cases and controls are well mixed. A comparison of runs of homozygosity indicated that the number and the median length of regions of homozygosity are higher for ASD subjects than for controls (p = 6 × 10(-3)). Furthermore, analysis of copy number variants found a higher frequency of large chromosomal rearrangements (>2 Mb) in ASD cases (5/103) than in ethnically matched control subjects (1/197, p = 0.019). CONCLUSIONS: Our findings illustrate the remarkable utility of high-density genotype data for subjects from a limited geographic area in dissecting genetic heterogeneity with respect to population and disease related variation.

5-HT2A receptor gene polymorphisms in Croatian subjects with autistic disorder.

Disturbances in the expression/function of the 5-HT2A receptor are implicated in autism. The association of the 5-HT2A receptor gene with autism was studied in the Croatian population. Distribution frequencies for alleles, genotypes and haplotypes of -1438 A/G and His452Tyr polymorphisms were compared in samples of 103 autistic and 214 control subjects. Significant overrepresentation of the G allele and the GG genotype of the -1438 A/G polymorphism was observed in group of autistic subjects, supporting the possible involvement of the 5-HT2A receptor in the development of autism.

Differences in cautiousness between mainland and island Podarcis siculus populations are paralleled by differences in brain noradrenaline/adrenaline concentrations.

Adaptive behavior is shaped by the type and intensity of selection pressures coming from the environment, such as predation risk and resource availability, and can be modulated by individual's neuroendocrine profile involving steroid hormones and the brain-stem monoaminergic circuits projecting to forebrain structures. Boldness when faced with a predator and exploration/activity when confronted with a new environment reflect the degree of cautiousness and/or "risk-taking" of an individual. In this study we have explored to which extent two populations of Podarcis siculus occupying different ecological niches: mainland (ML) and an islet (ISL) differ in the level of cautiousness and whether these differences are paralleled by differences in their monoaminergic profiles. Boldness was tested in the field as antipredator behavior, while novel space and object explorations were tested in a laboratory setting in an open field apparatus. Finally, serotonin, dopamine, noradrenaline (NA) and adrenaline (ADR) concentrations were measured in whole brain samples by ELISA. Lizards from ML population spent significantly more time hiding after a predator encounter in the field, displayed lower intensity of novel space exploration in a laboratory setting, and contained significantly higher whole-brain concentrations of NA and ADR than their ISL counterparts. Parallelism between the level of risk-taking behavior and concentrations of neurotransmitters mediating alertness and reaction to stress suggests that the differing environmental factors on ML and ISL may have shaped the degree of cautiousness in the residing lizard populations by affecting the activity of NA/ADR neural circuits.

Variable neurodevelopmental and morphological phenotypes of carriers with 12q12 duplications.

BACKGROUND: Variable size deletions affecting 12q12 have been found in individuals with neurodevelopmental disorders (NDDs) and distinct facial and physical features. For many genetic loci affected by deletions in individuals with NDDs, reciprocal duplications have been described. However, for the 12q12 region, there are no detailed descriptions of duplication cases in the literature. METHODS: We report a phenotypic description of a family with monozygotic twins diagnosed with NDDs, carrying a 9 Mb duplication at 12q12, and five other individuals with overlapping duplications ranging from 4.54 Mb up to 15.16 Mb. RESULTS: The duplication carriers had language delays, cognitive delays, and were diagnosed with autism spectrum disorder. Additionally, distinct facial features (e.g., high foreheads, deeply set eyes, short palpebral fissures, small ears, high nasal bridges, abnormalities of the nose tip, thin lips), large feet, and abnormalities in the digits were noted. We also describe incomplete penetrance of the NDD phenotypes among the individuals with 12q12 duplication. CONCLUSION: This case series expands our knowledge on this rare genetic aberration and suggests that large 12q12 duplications may increase the risk for developing NDDs.

Hyperserotonemia in autism: activity of 5HT-associated platelet proteins.

Disturbances in serotonin (5HT) neurotransmission have been indicated as biological substrates in several neuropsychiatric disorders including autism. Blood 5HT concentrations, elevated in about one-third of autistic subjects, are regulated through the action of peripheral 5HT-associated proteins. We have measured the activity of two platelet 5HT-associated proteins: 5HT transporter (5HTT) and monoamine oxidase B (MAOB), and indirectly studied the activity of 5HT(2A) receptor (5HT(2A)r) in 15 hyperserotonemic (HS) and 17 normoserotonemic (NS) autistic subjects, and 15 healthy controls (C). While mean velocities of 5HTT kinetics did not significantly differ among the groups, significant elevation in the mean velocity of MAOB kinetics was observed in NS subjects and was even more pronounced in HS subjects in comparison to controls. Also, a decrease in adenosine 5'-diphosphate-induced platelet aggregation of borderline significance was observed in NS subjects, compared to C subjects. The results suggest a possibility of upregulation of monoaminergic synthesis/degradation and, probably consequential, downregulation of 5HT(2A)r in autistic subjects.

Hyperserotonemia in autism: the potential role of 5HT-related gene variants.

Increased platelet serotonin level (PSL) has been consistently found in a portion of autistic patients. Suggested mechanisms for hyperserotonemia in autism have been increased synthesis of serotonin (5HT) by tryptophan hydroxylase (TPH), increased uptake into platelets through 5HT transporter (5HTt), diminished release from platelets through 5HT2A receptor (5HT2Ar) and decreased metabolism by monoamine oxydase (MAOA). The allelic influence of genes, encoding the mentioned 5HT elements, on PSL was investigated in 63 autistic subjects. Our study shows that 5HTt-LPR and -1438AG 5HT(2Ar) genotypes did not significantly affect PSL. However, significantly higher PSLs were observed in subjects with "cc" genotype of a218c TPH and subjects with "4" genotype of uVNTR MAOA. In addition, when TPH-cc and MAOA-4 were combined as "high 5HT" genotypes, a correlative increase in PSL was observed with the increase in the number of "high 5HT" genotypes. These results suggest a possible synergistic effect of genes regulating 5HT synthesis/degradation in dysregulation of the peripheral 5HT homeostasis of autistic patients.

Alcohol dependence and polymorphisms of serotonin-related genes: association studies.

Variations in 5HT-related genes contribute to the alterations of serotonergic neurotransmission, which is implicated in the etiopathology of alcoholism. In this preliminary study we have tested polymorphisms of genes involved in 5HT transport and turnover for their association with alcohol dependence. A case group of males with type 2 alcoholism (N=59) and a control group of healthy males (N=282), both of Croatian origin, were analyzed for the frequency distribution of polymorphisms in 5HT transporter (5HTT-VNTR2, 5HTT-LPR), monoamine oxidase A (MAOA-uVNTR) and B (MAOB-A/G) and tryptophan hydroxylase 1 (TPH1 A218C) and 2 (TPH2 G-703T) genes. An increase in the frequencies of 10-repeat allele (p = 0.010; OR = 1.73; 95% CI = 1.14-2.60) and 10/10 genotype (p = 0.006; OR = 2.57; 95% CI = 1.32-5.00) of the 5HTT-VNTR2 polymorphism was found in alcoholic patients. No differences between case and control groups were observed for the other tested polymorphisms. Present results support earlier studies implicating the role of 5HTT gene in alcoholism. The increase of sample size (in progress) is expected to enable search of more subtle differences, as well as re-evaluation of these preliminary findings.

Hyperserotonemia in adults with autistic disorder.

Hyperserotonemia is the most consistent serotonin-related finding in autism. The basis of this phenomenon, and its relationship to the central serotonergic dysfunction remains unclear. Platelet serotonin level (PSL) in 53 autistic adults and 45 healthy controls was measured. Mean PSL in autistic group (75.7 +/- 37.4 ng/microL) was significantly higher than the control sample (59.2 +/- 16.2 ng/microL) due to a presence of hyperserotonemic subjects which comprised 32% of the patients. PSL of autistic subjects did not correlate with the severity of symptoms, as measured by total CARS score, or the degree of mental retardation. However, significant negative relationship was observed between PSL and speech development, indicating the relationship between the peripheral 5HT concentrations and verbal abilities in autistic subjects.

Chronic postnatal monoamine oxidase inhibition affects affiliative behavior in rat pupso.

Monoamine neurotransmitters serotonin (5-HT), dopamine (DA), and noradrenaline (NA) act as important modulators of mammalian brain development and represent neurobiological substrates of affiliative behavior reflected in rat pups as a tendency to huddle or produce ultrasonic vocalizations (USV) when separated from the nest. Monoamines are metabolized through oxidative deamination catalyzed by the mitochondrial enzyme monoamine oxidase (MAO). In this study, we examined the consequences of postnatal MAO inhibition on affiliative behavior in rat pups. Pups received daily injections of either an irreversible non-selective MAO inhibitor tranylcypromine (TCP) or saline, from post-natal day (PND) 1 to PND 22. Quantitative and qualitative components of USV were analyzed on PNDs 10, 13 and 16 in order to determine the level of separation-induced anxiety and the modality of vocal communication. In comparison to control pups, TCP-treated pups displayed higher cortical 5-HT, DA and NA levels, higher peripheral 5-HT concentration, lower body mass throughout the pre-weaning period, higher isolation-induced drop in body temperature, and reduced total number of calls. Furthermore, they produced lower pitched calls of longer average duration without a preferable waveform. Our results demonstrate that chronic MAO inhibition by TCP primarily affects 5-HT concentrations, but also raises central catecholamine levels. They further indicate that disturbed monoaminergic homeostasis during early postnatal development leads to decreased weight-gain, compromised thermoregulation, and altered affiliative behavior in pre-weaning pups as reflected in reduced separation anxiety and inadequate vocal communication. Finally, they suggest a need for thorough examination of the potential effects of TCP and other monoamine inhibitors on the developing human brain.

Epigenetic adaptation of the placental serotonin transporter gene (SLC6A4) to gestational diabetes mellitus.

We tested the hypothesis that gestational diabetes mellitus (GDM) alters the DNA methylation pattern of the fetal serotonin transporter gene (SLC6A4), and examined the functional relevance of DNA methylation for regulation of the SLC6A4 expression in the human placenta. The study included 50 mother-infant pairs. Eighteen mothers were diagnosed with GDM and 32 had normal glucose tolerance (NGT). All neonates were of normal birth weight and born at term by planned Cesarean section. DNA and RNA were isolated from samples of tissue collected from the fetal side of the placenta immediately after delivery. DNA methylation was quantified at 7 CpG sites within the SLC6A4 distal promoter region using PCR amplification of bisulfite treated DNA and subsequent DNA sequencing. SLC6A4 mRNA levels were measured by reverse transcription-quantitative PCR (RT-qPCR). Functional SLC6A4 polymorphisms (5HTTLPR, STin2, rs25531) were genotyped using standard PCR-based procedures. Average DNA methylation across the 7 analyzed loci was decreased in the GDM as compared to the NGT group (by 27.1%, p = 0.037) and negatively correlated, before and after adjustment for potential confounder/s, with maternal plasma glucose levels at the 24th to 28th week of gestation (p<0.05). Placental SLC6A4 mRNA levels were inversely correlated with average DNA methylation (p = 0.010) while no statistically significant association was found with the SLC6A4 genotypes (p>0.05). The results suggest that DNA methylation of the fetal SLC6A4 gene is sensitive to the maternal metabolic state in pregnancy. They also indicate a predominant role of epigenetic over genetic mechanisms in the regulation of SLC6A4 expression in the human placenta. Longitudinal studies in larger cohorts are needed to verify these results and determine to which degree placental SLC6A4 changes may contribute to long-term outcomes of infants exposed to GDM.

DNA Methylation Analysis of HTR2A Regulatory Region in Leukocytes of Autistic Subjects.

Disturbed brain and peripheral serotonin homeostasis is often found in subjects with autism spectrum disorder (ASD). The role of the serotonin receptor 2A (HTR2A) in the regulation of central and peripheral serotonin homeostasis, as well as its altered expression in autistic subjects, have implicated the HTR2A gene as a major candidate for the serotonin disturbance seen in autism. Several studies, yielding so far inconclusive results, have attempted to associate autism with a functional SNP -1438 G/A (rs6311) in the HTR2A promoter region, while possible contribution of epigenetic mechanisms, such as DNA methylation, to HTR2A dysregulation in autism has not yet been investigated. In this study, we compared the mean DNA methylation within the regulatory region of the HTR2A gene between autistic and control subjects. DNA methylation was analysed in peripheral blood leukocytes using bisulfite conversion and sequencing of the HTR2A region containing rs6311 polymorphism. Autistic subjects of rs6311 AG genotype displayed higher mean methylation levels within the analysed region than the corresponding controls (P < 0.05), while there was no statistically significant difference for AA and GG carriers. Our study provides preliminary evidence for increased HTR2A promoter methylation in leukocytes of a portion of adult autistic subjects, indicating that epigenetic mechanisms might contribute to HTR2A dysregulation observed in individuals with ASD.

Rats with constitutionally upregulated/downregulated platelet 5HT transporter: differences in anxiety-related behavior.

Serotonin (5-hydroxytryptamine, 5HT) plays important roles in both embryonic development as a mediator of neurogenesis and in the mature brain as a neurotransmitter. Disturbances in serotonergic transmission have been indicated in several psychiatric disorders. In the search for the biological substrates of psychiatric diseases, studies using animal models represent complementary approaches to studies on human subjects. Wistar-Zagreb 5HT rats, with constitutionally upregulated/downregulated platelet 5HT transporter (termed high- and low-5HT rats, respectively), have been developed in our laboratory as a model for studying various aspects of 5HT function. In this work, we have searched for potential behavioral differences between Wistar-Zagreb 5HT rat sublines in three anxiety paradigms: hole-board, zero-maze, and social interaction test. In all three tests, significant differences in behavior between Wistar-Zagreb 5HT sublines have been observed, indicating higher levels of anxiety-related behavior in high-5HT rats. In the social interaction test, high-5HT animals spent less time in active contact with conspecifics and displayed a narrower spectrum of social behaviors than their low-5HT counterparts, while in the zero-maze and hole-board tasks, they showed a lower level of exploratory activity (head dips and nose pokes) in comparison to low-5HT rats. On the other hand, thigmotactic behavior (the percentage of time spent in open quadrants of zero-maze and the percentage of central holes visited in hole-board) did not differ between the sublines. The results suggest that as a result of selection process, a specific component of anxiety-related behavior (i.e. exploratory activity directed towards a novel environment and conspecifics) has been affected in Wistar-Zagreb 5HT rats.