Monoamine oxidases A and B gene polymorphisms in migraine patients

CURRENT PERSPECTIVES ON MITOCHONDRIAL DYSFUNCTION IN MIGRAINE

Mitochondria is an autonomous organelle that plays a crucial role in the metabolic aspects of a cell. Cortical Spreading Depression (CSD) and fluctuations in the cerebral blood flow have for long been mechanisms underlying migraine. It is a neurovascular disorder with a unilateral manifestation of disturbing, throbbing and pulsating head pain. Migraine affects 2.6 and 21.7% of the general population and is the major cause of partial disability in the age group 15-49. Higher mutation rates, imbalance in concentration of physiologically relevant molecules, oxidative stress biomarkers have been the main themes of discussion in determining the role of mitochondrial disability in migraine. The correlation of migraine with other disorders like hemiplegic migraine, MELAS, TTH, CVS, ischemic stroke and hypertension has helped in the assessment of the physiological and morphogenetic basis of migraine. Here, we have reviewed the different nuances of mitochondrial dysfunction and migraine. The different mtDNA polymorphisms that can affect the generation and transmission of nerve impulse has been highlighted and supported with research findings. In addition to this, the genetic basis of migraine pathogenesis as a consequence of mutations in nuclear DNA that can in turn affect the synthesis of defective mitochondrial proteins is discussed along with a brief overview of epigenetic profile. This review gives an overview of the pathophysiology of migraine and explores mitochondrial dysfunction as a potential underlying mechanism. Also, therapeutic supplements for managing migraine have been discussed at different junctures in this paper.

Computational Functional Genomics-Based AmpliSeq™ Panel for Next-Generation Sequencing of Key Genes of Pain

The genetic background of pain is becoming increasingly well understood, which opens up possibilities for predicting the individual risk of persistent pain and the use of tailored therapies adapted to the variant pattern of the patient's pain-relevant genes. The individual variant pattern of pain-relevant genes is accessible via next-generation sequencing, although the analysis of all "pain genes" would be expensive. Here, we report on the development of a cost-effective next generation sequencing-based pain-genotyping assay comprising the development of a customized AmpliSeq™ panel and bioinformatics approaches that condensate the genetic information of pain by identifying the most representative genes. The panel includes 29 key genes that have been shown to cover 70% of the biological functions exerted by a list of 540 so-called "pain genes" derived from transgenic mice experiments. These were supplemented by 43 additional genes that had been independently proposed as relevant for persistent pain. The functional genomics covered by the resulting 72 genes is particularly represented by mitogen-activated protein kinase of extracellular signal-regulated kinase and cytokine production and secretion. The present genotyping assay was established in 61 subjects of Caucasian ethnicity and investigates the functional role of the selected genes in the context of the known genetic architecture of pain without seeking functional associations for pain. The assay identified a total of 691 genetic variants, of which many have reports for a clinical relevance for pain or in another context. The assay is applicable for small to large-scale experimental setups at contemporary genotyping costs.

Effects of novel 17β-hydroxysteroid dehydrogenase type 10 inhibitors on mitochondrial respiration

Mitochondrial enzymes are targets of newly synthesized drugs being tested for the treatment of neurodegenerative disorders, such as Alzheimer's disease (AD). The enzyme 17β-hydroxysteroid dehydrogenase type 10 (HSD10) is a multifunctional mitochondrial protein that is thought to play a role in the pathophysiology of AD and is one of the targets of new potential AD drugs. The in vitro effects of frentizole, riluzole, AG18051, and 42 novel modulators of HSD10 (potential AD drugs) on citrate synthase (CS) activity, monoamine oxidase (MAO) activity, complex I- or complex II-linked mitochondrial respiratory rate, and complex I activity were measured in isolated pig brain mitochondria. Based on their minimal inhibitory effects on the respiratory rate of mitochondria and CS and complex I activity, six novel compounds were selected for further testing. Assuming that inhibition of MAO-B could be a desirable effect of AD drugs, only AG18051 and one new compound met the criteria for MAO-B inhibition with minimal drug-induced effects on mitochondrial respiration. In conclusion, our in vitro screening of mitochondrial effect of novel potential AD drugs has enabled the selection of the most promising molecules for further testing that are relatively safe in terms of drug-induced mitochondrial toxicity.

Drugs related to monoamine oxidase activity

Progress in understanding the role of monoamine neurotransmission in pathophysiology of neuropsychiatric disorders was made after the discovery of the mechanisms of action of psychoactive drugs, including monoamine oxidase (MAO) inhibitors. The increase in monoamine neurotransmitter availability, decrease in hydrogen peroxide production, and neuroprotective effects evoked by MAO inhibitors represent an important approach in the development of new drugs for the treatment of mental disorders and neurodegenerative diseases. New drugs are synthesized by acting as multitarget-directed ligands, with MAO, acetylcholinesterase, and iron chelation as targets. Basic information is summarized in this paper about the drug-induced regulation of monoaminergic systems in the brain, with a focus on MAO inhibition. Desirable effects of MAO inhibition include increased availability of monoamine neurotransmitters, decreased oxidative stress, decreased formation of neurotoxins, induction of pro-survival genes and antiapoptotic factors, and improved mitochondrial functions.

Serotonin and Migraine: Biology and Clinical Implications

Migraine is the most frequent neurological disorder in the adult population worldwide, affecting up to 12% of the general population and more frequent in women (~25%). It has a high impact on our society due to its disabling nature and, therein, reduced quality of life and increased absenteeism from work. Headache is the primary clinical manifestation and it has been associated with ‘a hereditary or predisposed sensitivity of neurovascular reactions to certain stimuli or to cyclic changes in the central nervous system’ (1). Amongst the many neurotransmitters in the brain, the serotonergic (serotonin, 5-HT) system from the brainstem raphe nucleus has been most convincingly implicated in migraine pathophysiology. The documented changes in 5-HT metabolism and in the processing of central 5-HT-mediated responses during and in between migraine attacks have led to the suggestion that migraine is a consequence of a central neurochemical imbalance that involves a low serotonergic disposition. Although the exact cascade of events that link abnormal serotonergic neurotransmission to the manifestation of head pain and the accompanying symptoms has yet to be fully understood, recent evidence suggests that a low 5-HT state facilitates activation of the trigeminovascular nociceptive pathway, as induced by cortical spreading depression. In this short review, we present and discuss the original and most recent findings that support a role for altered serotonergic neurotransmission in the manifestation of migraine headache.

Association Between Polymorphisms of DRD2, COMT, DBH, and MAO-A Genes and Migraine Susceptibility: A Meta-Analysis

Some epidemiological studies have investigated the relationship between genetic polymorphisms of DRD2, COMT, DBH, and MAO-A and migraine susceptibility, but the results are still inconsistent. Thus, our aim was to further assess the association through a meta-analysis.We examined 5 single nucleotide polymorphisms (SNPs) in 4 genes, including DRD2 rs1799732 and rs6275, DBH rs7239728, MAI-A-VNTR, and COMT rs4680, and performed a meta-analysis of 11 published case-control studies including 3138 cases and 4126 controls. Odd ratios (ORs) with 95% confidence intervals (95% CIs) were used to evaluate the association between the 5 genetic polymorphisms and migraine susceptibility.There was no significant relationship between migraine susceptibility and 4 genetic polymorphisms of DRD2 rs1799732 and rs6275, DBH rs7239728, and MAO-A-VNTR. Nevertheless, decreased risk of migraine was observed to be in association with COMT rs4680 polymorphism in overall analysis (AA vs. GG + GA: OR = 0.76, 95% CI = 0.60-0.97, PHet > 0.642, I = 0), and in Caucasian group after subgroup analysis (AA vs. GG + GA: OR = 0.75, 95% CI = 0.58-0.96, PHet > 0.433, I = 0).Studied polymorphisms of DRD2, DBH, and MAO-A genes may not be associated with migraine susceptibility. However, COMT rs4680 polymorphism may decrease the risk of migraine, especially in Caucasians. The failure to evaluate environmental influence and provide adjusted effect size estimates highlights the need for additional studies in a large number to take these factors into consideration, thus better elucidating the role of the genes tested in migraine.

An automated sequential injection spectrophotometric method for evaluation of tyramine oxidase inhibitory activity of some flavonoids

An automated sequential injection (SI) spectrophotometric system has been developed for evaluation of tyramine oxidase (TOD) inhibitory activity. The method is based on the inhibition of TOD that catalyzes the oxidation of tyramine substrate to produce aldehyde and hydrogen peroxide (H₂O₂). The produced H₂O₂ reacts with vanillic acid and 4-aminoantipyrine (4-AA) in the presence of peroxidase (POD) to form a quinoneimine dye, the absorbance of which is measured of absorbance at wavelength of 490 nm. The decrease of the quinoneimine dye is related to an increase of TOD inhibitory activity. Under the optimum conditions: 1.0 mM tyramine, 8 U mL(-1) TOD, 1.0 mM vanillic acid, 1.0 mM 4-AA and delay time of 10 s, some flavonoid compounds were examined for the TOD inhibitory activity expressed as IC₅₀ value. It was found that flavonols (quercetin and myricetin) and flavans (epicatechin gallate (ECG) and epigallocatechin (EGC)) showed higher TOD inhibitory activity than flavones and flavanones. The results of IC₅₀ values obtained from the proposed method and a batch-wise method were not significantly different from each other. Moreover, the SI system enabled automation of the analysis, leading to more convenient, more sensitive and faster analysis than the batch-wise method. A precise timing of the system also improves precision and accuracy of the assay, especially when the measurement of absorbance at non-steady state condition is involved.

Genetic variants of neurotransmitter-related genes and miRNAs in Egyptian autistic patients

Autism is a neurodevelopmental disorder with indisputable evidence for a genetic component. This work studied the association of autism with genetic variations in neurotransmitter-related genes, including MAOA uVNTR, MAOB rs1799836, and DRD2 TaqI A in 53 autistic patients and 30 healthy individuals. The study also analyzed sequence variations of miR-431 and miR-21. MAOA uVNTR was genotyped by PCR, MAOB and DRD2 polymorphisms were analyzed by PCR-based RFLP, and miR-431 and miR-21 were sequenced. Low expressing allele of MAOA uVNTR was frequently higher in female patients compared to that in controls (OR = 2.25). MAOB G allele frequency was more significantly increased in autistic patients than in controls (P < 0.001 for both males and females). DRD2 A1+ genotype increased autism risk (OR = 5.1). Severity of autism tends to be slightly affected by MAOA/B genotype. Plasma MAOB activity was significantly reduced in G than in A allele carrying males. There was no significant difference in patients and maternal plasma MAOA/B activity compared to controls. Neither mutations nor SNPs in miR-431 and miR-21 were found among studied patients. This study threw light on some neurotransmitter-related genes suggesting their potential role in Autism pathogenesis that warrants further studies and much consideration.

Combination of polymorphic variants in serotonin transporter and monoamine oxidase-A genes may influence the risk for early-onset alcoholism

The combinatory effect of polymorphisms in serotonin transporter and monoamine oxidase-A genes on the aetiopathogenesis of alcoholism was investigated in a sample of 714 individuals. Increased frequency of subjects having three 'suspected' genotypes (5-HTTLPR-LL, STin2-1010 and MAO-A 3-repeat allele) was found among type-2 alcoholic patients (P=0.0189). Results highlight serotonergic/genetic contribution to early-onset alcoholism.

Pharmacogenetics of pain and analgesia

Pain severity ratings and the analgesic dosing requirements of patients with apparently similar pain conditions may differ considerably between individuals. Contributing factors include those of genetic and environmental origin with epigenetic mechanisms that enable dynamic gene-environment interaction, more recently implicated in pain modulation. Insight into genetic factors underpinning inter-patient variability in pain sensitivity has come from rodent heritability studies as well as familial aggregation and twin studies in humans. Indeed, more than 350 candidate pain genes have been identified as potentially contributing to heritable differences in pain sensitivity. A large number of genetic association studies conducted in patients with a variety of clinical pain types or in humans exposed to experimentally induced pain stimuli in the laboratory setting, have examined the impact of single-nucleotide polymorphisms in various target genes on pain sensitivity and/or analgesic dosing requirements. However, the findings of such studies have generally failed to replicate or have been only partially replicated by independent investigators. Deficiencies in study conduct including use of small sample size, inappropriate statistical methods and inadequate attention to the possibility that between-study differences in environmental factors may alter pain phenotypes through epigenetic mechanisms, have been identified as being significant.

Association study of monoamine oxidase A/B genes and schizophrenia in Han Chinese

Background

Monoamine oxidases (MAOs) catalyze the metabolism of dopaminergic neurotransmitters. Polymorphisms of isoforms MAOA and MAOB have been implicated in the etiology of mental disorders such as schizophrenia. Association studies detected these polymorphisms in several populations, however the data have not been conclusive to date. Here, we investigated the association of MAOA and MAOB polymorphisms with schizophrenia in a Han Chinese population.

Methods

Two functional single nucleotide polymorphisms (SNPs), rs6323 of MAOA and rs1799836 of MAOB, were selected for association analysis in 537 unrelated schizophrenia patients and 536 healthy controls. Single-locus and Haplotype associations were calculated.

Results

No differences were found in the allelic distribution of rs6323. The G allele of rs1799836 was identified as a risk factor in the development of schizophrenia (P = 0.00001). The risk haplotype rs6323T-rs1799836G was associated with schizophrenia in female patients (P = 0.0002), but the frequency difference was not significant among male groups.

Conclusions

Our results suggest that MAOB is a susceptibility gene for schizophrenia. In contrast, no significant associations were observed for the MAOA functional polymorphism with schizophrenia in Han Chinese. These data support further investigation of the role of MAO genes in schizophrenia.

MAOA, DBH, and SLC6A4 variants in CHARGE: a case-control study of autism spectrum disorders

Genetic factors are established to contribute to the development of autism. We examined three loci, serotonin transporter (SLC6A4), dopamine β-hydroxylase (DBH), and the variable number of tandem repeat promoter of the monoamine oxidase A (MAOA) for association with autism in participants from the Childhood Autism Risks from Genetics and the Environment (CHARGE ) Study, the first large-scale population-based case-control investigation of both environmental and genetic contributions to autism risk. Among male children enrolled in the CHARGE study we tested associations between each of the three polymorphisms and autism (AU) (n = 119), or a combined group of autism and other autism spectrum disorders (AU+ASD, which includes an additional n = 53) as compared with typically developing controls (TD, n = 137). The case-control association analysis showed neither SLC6A4 nor DBH to be statistically significantly associated with AU or ASD. However, the male children carrying 4 tandem repeats in the promoter region of the MAOA gene showed a two-fold higher risk of AU (or AU+ASD) than those carrying allele 3, adjusted for confounders (OR = 2.02, 95% CI = 1.12, 3.65, P = 0.02 for AU vs. TD, and OR = 2.05, 95% CI = 1.19, 3.53, P = 0.01 for ASD vs. TD). In addition, children of mothers homozygous for the 4 tandem repeat allele showed at least a three-fold higher risk of AU (or AU+ASD) than those with mothers homozygous for allele 3 (OR = 3.07, 95% CI = 1.19, 7.91, P = 0.02 for AU vs. TD, and OR = 3.26, 95% CI = 1.35, 7.89, P = 0.009 for AU+ASD vs. TD). These results suggest a potential role of the functional MAOA promoter alleles in the male child, the mother, or both in ASD.

Epilepsy and serotonin (5HT): variations of 5HT-related genes in temporal lobe epilepsy

Several lines of evidence point to the role of serotonin (5HT) neurotransmission in the epileptogenesis. The present preliminary study investigated possible association of the temporal lobe epilepsy (TLE) with the polymorphisms in several 5HT-related genes, including serotonin transporter (5HTT), monoamine oxidase A (MAO-A) and serotonin receptors 5HT-1A, 5HT-1B and 5HT-2C. All participants (101 TLE patients and 170 healthy controls) were unrelated individuals of Croatian origin. 5HT-1B allele 861G was found to be slightly overrepresented in the patient group (p=0.0385). No significant differences between groups were observed for the other tested polymorphisms. Within the limitations imposed by the size of our sample, negative findings suggest that the respective loci do not make considerable contribution to the etiopathogenesis of TLE. Further examination of 5HT-1B gene, which yielded positive result at a trend level, is possibly warranted.

Frequencies of genetic polymorphisms related to triptans metabolism in chronic migraine

Chronic migraine (CM) prevalence ranges around 1–5%. Most of these patients usually treat their acute attacks with triptans, whose efficacy is extremely variable. A genetic basis for migraine is evident and many susceptibility genes have been described, as well as gene polymorphisms possibly implied in therapy response. Several factors could be involved in the evolution of episodic migraine into a chronic form, such as natural history, psychiatric comorbidity, and the individual’s response to therapy. During a study aimed at detecting connections between genotype and response to triptans administration, we characterized a CM population for polymorphisms in the genes coding for monoamine oxidase A, g-protein beta 3 and the cytochromes CYP3A4 and CYP1A2. Alleles and genotypes distributions were compared with known frequencies of healthy Caucasian populations. A significant association with CM was found for the long allele of monoamine oxidase A 30 bp VNTR and CYP1A2*1F variant. Such genomic analysis is part of an integrated platform able to evaluate different levels of metabolic pathways of drugs in CM and their influence in the chronicization process.

Association study of the serotoninergic system in migraine in the Spanish population

In order to evaluate the contribution of 19 serotonin-related genes to the susceptibility to migraine in a Spanish population we performed a case-control association study of 122 single nucleotide polymorphisms (SNPs), selected according to genetic coverage parameters, in 528 migraine patients -308 with migraine without aura (MO) and 220 with migraine with aura (MA)- and 528 sex-matched migraine-free controls. The single-marker analysis identified nominal associations with the migraine phenotype or with the MO or MA subtypes. The multiple-marker analysis revealed risk haplotypes in three genes that remained significantly associated with migraine after correction by permutations. Two-marker risk haplotypes were identified in the HTR2B (rs16827801T-rs10194776G) and MAOA (rs3027400G-rs2072743C) genes conferring susceptibility to MO, and a four-marker haplotype in DDC was specific of MA (rs2329340A-rs11974297C-rs2044859T-rs11761683G). The present study supports the involvement of HTR2B and MAOA genes in the genetic predisposition to MO, while DDC might confer susceptibility to MA. These results suggest a differential involvement of serotonin-related genes in the genetic background of MO and MA.

Monoamine oxidase (MAO) intron 13 polymorphism and platelet MAO-B activity in combat-related posttraumatic stress disorder

BACKGROUND

The neurobiology of posttraumatic stress disorder (PTSD) involves alterations in multiple neuroendocrine and neurotransmitter systems. Platelet monoamine oxidase (MAO-B) has been associated with susceptibility to various psychiatric disorders, personality traits and behaviors.

METHODS

Platelet MAO-B activity and MAO-B intron 13 polymorphism (a G/A substitution) were determined in male war veterans (n=106) with DSM-IV diagnosed current and chronic PTSD, divided into subgroups of PTSD patients with (n=28) or without (n=78) psychotic features, combat exposed veterans (n=41) who did not develop PTSD, and healthy control men (n=242).

RESULTS

Two-way ANOVAs revealed a significant effect of diagnosis and smoking, a significant effect of smoking, no significant effect of genotype, and no significant interaction between genotype, smoking or diagnosis, on platelet MAO-B activity. One-way ANOVAs showed significantly lower platelet MAO-B activity in smokers than in nonsmokers. After controlling for smoking, veterans with psychotic PTSD had significantly higher platelet MAO-B activity than veterans with or without PTSD, or healthy subjects.

LIMITATIONS

The results were obtained on peripheral biochemical marker, i.e. platelet MAO activity.

CONCLUSIONS

The MAO-B intron 13 polymorphism was not functional, and did not affect platelet MAO-B activity. The allele frequencies of the MAO-B genotype were similarly distributed among healthy controls and veterans with or without PTSD and/or psychotic symptoms. The results suggest that platelet MAO-B activity, controlled for smoking status, might be used as a peripheral marker of the psychotic symptoms in PTSD.

Migraine headaches are not associated with a unique depressive symptom profile: results from the Baltimore Epidemiologic Catchment Area Study

OBJECTIVE

There is a well-established link between migraine headaches and depression. However, it is unclear whether individuals with migraine experience a unique profile of depressive symptoms in comparison to individuals without migraine.

METHODS

This question was addressed using data from the Baltimore cohort of the Epidemiologic Catchment Area Study. The cross-sectional association between migraine headaches and each depressive symptom was calculated using logistic regression, and symptom profiles among those with migraine headaches (n=249) and those without (n=1480) were compared using generalized estimating equations.

RESULTS

Migraine headaches were associated with increased odds of reporting seven of nine depressive symptom groups by a factor of roughly 2. However, when the symptom profiles were compared, individuals with migraine headaches did not differ in their profile of symptoms.

CONCLUSION

These results suggest that individuals with migraine headaches are more likely to report depressive symptoms but do not display a unique profile of symptoms.

Genetic Contribution of Catechol-O-methyltransferase Polymorphism in Patients with Migraine without Aura

Background

Recent genetic association studies have investigated the possible genetic role of the dopaminergic system in migraine. Catechol-O-methyltransferase (COMT) is an enzyme that plays a crucial role in the metabolism of dopamine and its genetic polymorphism is associated with three- to fourfold variation of enzymatic activity.

Objectives

The objective of this study was to elucidate the role of the COMT polymorphism in the genetic susceptibility to migraine and its phenotypic expression in patients with migraine without aura (MWOA).

Methods

Ninety-seven patients with MWOA and 94 healthy volunteers were included in the study. After amplifying COMT genes by the polymerase chain reaction, we assessed their genotype frequencies and allele distributions by based on restriction fragment length polymorphisms. We classified all MWOA patients into two groups according to their COMT genotype: with the L allele (N = 43), and without this allele (N = 54).

Results

The genotype frequency and allele distribution of the COMT polymorphism did not differ between MWOA patients and the control group. During migraine attacks, MWOA patients with the L allele showed a higher pain intensity of headache (P = 0.001) and a higher incidence of the accompanying nausea/vomiting (94% vs 75%; P = 0.026) compared with MWOA patients without the L allele.

Conclusions

Although the COMT polymorphism does not appear to be involved in predisposition to the development of MWOA, this genetic factor could be involved in the phenotypic expression of MWOA.

The lack of association between monoamine oxidase (MAO) intron 13 polymorphism and platelet MAO-B activity among men

Monoamine oxidase (MAO), a mitochondrial flavine containing enzyme, exists in two isoenzymes, MAO-A and MAO-B. Platelets contain MAO-B subtype, proposed to be a biomarker for different personality characteristics and vulnerability for substance abuse. The most common polymorphism of MAO-B gene, a single base change (A or G) occurs in intron 13. It has been proposed to be a functional polymorphism, controlling the activity of MAO-B in platelets. The aim of the study was to determine the association between platelet MAO-B activity and MAO-B intron 13 polymorphism in 225 racially and ethnically uniform healthy Caucasian men of the Croatian origin. Our results showed that platelet MAO-B activity did not differ between subjects subdivided into those with <<A-allele>> or <<G-allele>>. This polymorphism of the MAO-B gene did not control the activity of the MAO-B in platelets. Platelet MAO-B activity was associated only with the smoking status, and it was significantly decreased in smokers when compared to nonsmokers. No significant association was found between MAO-B polymorphism and smoking status. In healthy individuals of the Croatian origin, the studied MAO-B polymorphism showed a lack of functional importance in regulating MAO-B activity in platelets. Since different populations may vary in the association between functional polymorphism and the MAO-B activity, and the genotype of transcription factor AP-2 beta was reported to be associated with altered platelet MAO-B activity, and with specific personality traits, further studies on different populations should be conducted to elucidate the molecular mechanism/s regulating platelet MAO-B activity.

Statistical distribution of blood serotonin as a predictor of early autistic brain abnormalities

BACKGROUND

A wide range of abnormalities has been reported in autistic brains, but these abnormalities may be the result of an earlier underlying developmental alteration that may no longer be evident by the time autism is diagnosed. The most consistent biological finding in autistic individuals has been their statistically elevated levels of 5-hydroxytryptamine (5-HT, serotonin) in blood platelets (platelet hyperserotonemia). The early developmental alteration of the autistic brain and the autistic platelet hyperserotonemia may be caused by the same biological factor expressed in the brain and outside the brain, respectively. Unlike the brain, blood platelets are short-lived and continue to be produced throughout the life span, suggesting that this factor may continue to operate outside the brain years after the brain is formed. The statistical distributions of the platelet 5-HT levels in normal and autistic groups have characteristic features and may contain information about the nature of this yet unidentified factor.

RESULTS

The identity of this factor was studied by using a novel, quantitative approach that was applied to published distributions of the platelet 5-HT levels in normal and autistic groups. It was shown that the published data are consistent with the hypothesis that a factor that interferes with brain development in autism may also regulate the release of 5-HT from gut enterochromaffin cells. Numerical analysis revealed that this factor may be non-functional in autistic individuals.

CONCLUSION

At least some biological factors, the abnormal function of which leads to the development of the autistic brain, may regulate the release of 5-HT from the gut years after birth. If the present model is correct, it will allow future efforts to be focused on a limited number of gene candidates, some of which have not been suspected to be involved in autism (such as the 5-HT4 receptor gene) based on currently available clinical and experimental studies.