Serotonin metabolism and other biochemical parameters in infantile autism. A controlled study of 22 autistic children

The serotonin metabolism was extensively studied in 22 couples of autistic children and age- and sex-matched controls. Histamine, calcium, and uric acid were also measured in urine and whole blood or plasma. Autistics and controls did not differ in histamine, and only minor changes were noticed in calcium content. According to previous reports, serotonin levels were often, but not always, elevated in the blood of autistic children. Based on data including urinary serotonin and 5-hydroxyindoleacetic acid, platelet serotonin uptake and efflux, platelet monoamine oxidase and glutathione peroxidase activities, and uric acid and plasma tryptophan, the origin(s) of such hyperserotonemia in autism appear(s) to be of metabolic origin, i.e., a decreased catabolism and/or an increased biosynthesis of serotonin.     

Disorders of catecholamine metabolism in infantile autism. Comparative study of 22 autistic children

In a group of 22 autistic children aged 5 to 16 y., and a group of normal controls matched for age and sex, catecholamines metabolism has been investigated in plasma, platelets and urine. This investigation was part of a research project in which several biological parameters (including serotonin) were simultaneously explored in the same children. In the autistic group, epinephrine and norepinephrine and dopamine were significantly lower in isolated platelets, and no significant difference was found between the two groups for the urinary excretion of epinephrine, norepinephrine, dopamine, DOPAC and MHPG. Other differences between the two groups in the statistical correlations of several biochemical parameters (plasma norepinephrine and dopamine with platelet MAO activity, platelet norepinephrine with platelet dopamine, platelet dopamine, platelet dopamine with platelet serotonin) also suggest abnormalities of bioamine metabolism in the platelets of autistic children.     

Low platelet-poor plasma levels of serotonin in adult autistic patients

BACKGROUND: Hyperserotonemia has been reported in about a third of autistic patients. However, most studies have examined whole blood levels of serotonin (5-HT), the vast majority of which is found in platelets. The aim of this study was to determine 5-HT levels in platelet-poor plasma (PPP) in a group of adult patients with autism. METHODS: Levels of PPP 5-HT were compared between 10 adult drug-free autistic patients and 12 healthy controls. The Ritvo-Freeman Real-Life Rating Scale and the Overt Aggression Scale (OAS) were administered to the autistic group as a measure of symptom severity. RESULTS: Significantly lower PPP 5-HT levels were observed in the autistic group as compared to the controls (p = 0.03). In addition, PPP 5-HT levels were inversely correlated with OAS scores among subjects with autism (r = -0.64, p < 0.05). CONCLUSION: PPP 5-HT ('free') levels appear to be low in autistic patients and may play a role in the pathophysiology and symptomatology of the disorder.     

Serotonin transporter gene promoter variants do not explain the hyperserotoninemia in autistic children

Autism is a biologically-heterogeneous disease. Distinct subgroups of autistic patients may be marked by intermediate phenotypes, such as elevated serotonin (5-HT) blood levels, potentially associated with different underlying disease mechanisms. This could lead to inconsistent genetic association results, such as those of prior studies on serotonin transporter (5-HTT) gene promoter variants and autistic disorder. Contributions of 5-HTT gene promoter alleles to 5-HT blood levels were thus investigated in 134 autistic patients and 291 first-degree relatives. Mean 5-HT blood levels are 11% higher in autistic patients carrying the L/L genotype, compared to patients with the S/S or S/L genotype; this trend is not observed in first-degree relatives. The probability of inheriting L or S alleles is significantly enhanced in patients with 5-HT blood levels above or below the mean, respectively (P < 0.05), but quantitative TDT analyses yield a non-significant trend (P = 0.10), as this polymorphism explains only 2.5% of the variance in 5-HT blood levels of autistic patients. In conclusion, 5-HTT gene promoter variants seemingly exert a small effect on 5-HT blood levels in autistic children, which largely does not account for hyperserotoninemia. Nonetheless, the inconsistent outcome of prior association studies could partly stem from a selection bias of hyper- or hypo-serotoninemic probands.     

5-HT2 receptor distribution shown by [18F] setoperone PET in high-functioning autistic adults

The serotonergic system is implicated in disordered emotional behavior. Autism is characterized by impaired processing of emotional information. The serotonergic (5-HT) system is also critically involved in brain development, and abnormal brain synthesis of serotonin is observed in autism. Furthermore, whole blood and platelet serotonin have been reported to be elevated in autism. The authors examined the CNS serotonin system in autism in vivo. 5-HT2 receptors were visualized by PET imaging of [18F]setoperone-binding in this pilot study of 6 high-functioning autistic adults and 10 matched-control participants. Autism subjects had less thalamic [18F]setoperone binding than controls, when covaried for age, but no difference reached significance in other areas. A negative relationship between thalamic binding and history of language impairment was also observed. Further studies will be needed to gain a clearer picture of the role of the 5-HT system in autism.     

Catecholamines metabolism in infantile autism: A controlled study of 22 autistic children

In a group of 22 autistic children aged 5 to 16 years and a group of normal controls matched for age and sex, catecholamines metabolism was investigated in plasma, platelets, and urine. This investigation was part of a research project in which several biological parameters (including serotonin) were explored simultaneously in the same children. In the autistic group, epinephrine and norepinephrine were significantly elevated in plasma, while epinephrine, norepinephrine, and dopamine were significantly lower in isolated platelets. No significant difference was found between the two groups for the urinary excretion of epinephrine, norepinephrine, dopamine, DOPAC, and MHPG. Other differences between the two groups in the statistical correlations of several biochemical parameters also suggest abnormalities of bioamine metabolism in the platelets of autistic children.This work was supported by a grant of the Institut National de la Santé et de la Recherche Médicale (INSERM-ATP n 73.79.105). We are indebted to Prof. M. Da Prada, Pharmaceutical Research Department, Hoffman-La Roche, Basel, for his continuous help throughout this work, and to Ms. D. Pasques, Hôpital Saint-Louis, Paris, for her excellent technical assistance.     

Serotonergic responsivity in male young adults with autistic disorder. Results of a pilot study

Altered serotonergic function has been postulated to exist in autistic disorder. Central serotonergic responsivity was assessed with a neuroendocrine challenge test in seven male young adults with autistic disorder and in seven age- and gender-matched healthy controls. Binding indexes and physiologic responsivity of the platelet serotonin-2 (5-HT2) receptor complex were also measured, as was whole-blood serotonin content. Compared with controls, autistic subjects had substantially blunted prolactin release in response to a 60-mg oral dose of fenfluramine hydrochloride, an indirect serotonin agonist [corrected]. Furthermore, the magnitude of serotonin-amplified platelet aggregation, mediated by the platelet 5-HT2 receptor complex, was reduced in the autistic group, as was the mean number of platelet 5-HT2 receptor sites. Among autistic subjects, fenfluramine-induced prolactin release correlated positively with the serotonin-amplified platelet aggregation response and negatively with whole-blood serotonin content. The results of the present study are compatible with the hypothesis that central serotonergic responsivity is decreased in male autistic young adults. Correlations between central and peripheral serotonergic measures in autistic subjects suggest that systemic alterations in serotonergic function may occur in autism.     

Serotonin and measured intelligence

Blood serotonin (5HT) has been shown to be elevated in 30% of autistic children and 50% of severely mentally retarded children. Ninety-eight normal adult subjects were studied to determined if there was an inverse relationship between whole blood 5HT in normal adults of average and above-average intelligence. There was a trend toward a negative correlation between whole blood 5HT and Vocabulary scores that would not account for hyperserotonemia in autistic or mentally retarded individuals. Female subjects had significantly greater whole blood 5HT than male subjects. There was no difference in whole blood 5HT collected before and after volume depletion of 450 ml, providing further evidence of the intraindividual stability of whole blood serotonin levels. There was no relationship between age and whole blood 5HT in a group of normal adult subjects.This study was completed with the assistance of the Blood Bank staff at the University of Chicago Medical Center. In addition, the study was supported by the Freedman-Falk Academic Psychiatry Fund, the Harris Center for Developmental Studies, and NICCHD postdoctoral training grant #HDMC 5T32 HD07307-02 HCB.     

Williams syndrome: Serotonin's association with developmental disabilities

Reiss et al. (1985) described two autistic children with the Williams syndrome, a dysmorphic developmental syndrome of unknown cause. Both children also showed elevated blood serotonin levels. The present report describes two prepubescent females with the characteristic features of Williams syndrome, who are not autistic and who have blood serotonin levels within the normal range. These findings suggest that further study of developmental disorders that coexist with autism may help clarify the relationship between autism and putative biological markers such as hyperserotonemia.The authors thank Arthur Yuwiler, for his support and assistance in this study. We also thank the patients and their families for their cooperation.     

Seizure disorders in autism

Several reports have suggested that autistic individuals are at greater risk for developing seizure disorders, particularly in adolescence. In this study the frequency of seizures in a series of 192 autistic individuals was examined; 21% of cases had exhibited a seizure disorder. Seizure disorders were more common among individuals with lower IQ. Age specific incidence revealed a 3- to 22-fold increase in risk for seizure relative to the normal population. In contrast to previous studies, risk for developing seizures was highest during early childhood although it was also elevated during early adolescence.     

Developmental changes in brain serotonin synthesis capacity in autistic and nonautistic children

Serotonin content, serotonin uptake sites, and serotonin receptor binding measured in animal studies are all higher in the developing brain, compared with adult values, and decline before puberty. Furthermore, a disruption of synaptic connectivity in sensory cortical regions can result from experimental increase or decrease of brain serotonin before puberty. The purpose of the present study was to determine whether brain serotonin synthesis capacity is higher in children than in adults and whether there are differences in serotonin synthesis capacity between autistic and nonautistic children. Serotonin synthesis capacity was measured in autistic and nonautistic children at different ages, using alpha[11C]methyl-L-tryptophan and positron emission tomography. Global brain values for serotonin synthesis capacity (K complex) were obtained for autistic children (n = 30), their nonautistic siblings (n = 8), and epileptic children without autism (n = 16). K-complex values were plotted according to age and fitted to linear and five-parameter functions, to determine developmental changes and differences in serotonin synthesis between groups. For nonautistic children, serotonin synthesis capacity was more than 200% of adult values until the age of 5 years and then declined toward adult values. Serotonin synthesis capacity values declined at an earlier age in girls than in boys. In autistic children, serotonin synthesis capacity increased gradually between the ages of 2 years and 15 years to values 1.5 times adult normal values and showed no sex difference. Significant differences were detected between the autistic and epileptic groups and between the autistic and sibling groups for the change with age in the serotonin synthesis capacity. These data suggest that humans undergo a period of high brain serotonin synthesis capacity during childhood, and that this developmental process is disrupted in autistic children.     

Urinary 5-hydroxyindoleacetic acid and whole blood serotonin and tryptophan in autistic and normal subjects

Urinary 5-hydroxyindoleacetic acid (5-HIAA) excretion in two consecutive collection periods (5:00 PM-11:00 PM and 11:00 PM-8:00 AM) and whole blood serotonin (5-HT) and tryptophan (TRP) were measured in groups of unmedicated autistics (n = 16), medicated autistics (n = 20), and normal controls (n = 27). Whole blood 5-HT values were significantly higher in unmedicated autistics compared to normal controls. No significant differences were found in 5-HIAA excretion (microgram/mg creatinine, mean +/- SD) between unmedicated autistics (4.07 +/- 1.52) and normal controls (3.50 +/- 1.07), or between medicated (5.35 +/- 2.93) and drug-free autistic individuals. No correlations were found between 5-HT values and urinary 5-HIAA excretion. Urinary 5-HIAA (microgram/mg creatinine, mean +/- SD) was significantly greater in hyperserotonemic autistic subjects (4.88 +/- 0.87) compared to normal controls (3.50 +/- 1.07, total collection period; p = 0.002). The relevance of these findings to the possibility that increased gut production of 5-HT might cause the elevated whole blood 5-HT levels seen in autism is discussed.     

Obstetric complications in individuals diagnosed with autism and in healthy controls

The aim of the present study was to investigate birth complications in Israeli autistic probands. We interviewed 206 mothers of autistic probands and 152 healthy control mothers with a structured tool encompassing prenatal, perinatal, and neonatal complications. Analysis of obstetric suboptimality, derived by summing all positive items of each of the 3 categories and dividing them by the number of patients analyzed, revealed no prenatal between-group difference. The controls had a somewhat elevated perinatal suboptimality score, whereas the autistic probands had a significantly greater neonatal suboptimality score. These differences in obstetric suboptimality were retained after controlling for the demographic parameters found different between the 2 groups (sex of participants and mothers' years of schooling). Our findings suggest that the presence of nonspecific neonatal factors, rather than the specific influence of individual severe insults, may account for the elevated neonatal suboptimality found in probands diagnosed with autism compared with healthy controls.     

Platelet imipramine binding in autistic subjects

Previous reports of elevated platelet serotonin (5-HT) concentrations in autistic subjects suggest that platelet 5-HT uptake might be altered in autism. Parameters of 3H-imipramine (IMI) binding were measured in 11 drug-free autistic subjects and 10 normal volunteers. Similar means (+/- SD) for Bmax (autistics, 1350 +/- fmole/mg protein; normals, 1590 +/- 206 fmole/mg protein) and Kd (autistics, 0.98 +/- 0.10 nM; normals, 0.94 +/- 0.13 nM) were found in the two groups. The normal number (Bmax) and affinity (Kd) of the IMI binding site in autistic subjects suggest that the regulation of 5-HT uptake is not different in autism.     

Platelet serotonin concentrations in autistic children and members of their families

Children with early infantile autism have often high platelet serotonin (5 HT) levels. According to this fact, we wondered if a genetic abnormality of 5 HT metabolism could be detected in family members of autistic children. We has thus determined platelet 5 HT levels in these children's mothers, fathers and siblings. It is noteworthy to find out that: autistic children have a mean platelet 5 HT level significantly higher than their siblings' (p less than 0.01); fathers' levels are significantly lower (-29%; p less than 0.001) than those of other family members' and of control group. Furthermore, for future purposes, we studied platelet 5 HT levels at different times during pregnancy in controls and in a woman already mother of an autistic child. We observed a raise (+31%; p less than 0.01) in platelet 5 HT during the first trimester of the pregnancy with a lowering towards normal levels during the eight month, in the woman with an autistic child as well as in the controls.     

Platelet 5-HT2 serotonin receptor binding sites in autistic children and their first-degree relatives

We examined platelet serotonin2 [5-hydroxytryptamine2 (5-HT2)] receptor binding sites, whole blood serotonin (5-HT), and plasma norepinephrine (NE) in male autistic children and their first-degree relatives. Saturation studies utilizing 125I-spiroperidol labeled the 5-HT2 sites with an affinity of 224.6 +/- 84.4 pmol/L (Kd). No group differences, i.e., autistic (n = 12), siblings (n = 6), parents (n = 22), control (adult; n = 7: child; n = 10), were seen for either the Kd or the total number of sites (Bmax: 14.3 +/- 10.9 fmol/mg protein). No correlations were found in any group between binding parameters (Kd or Bmax) and whole blood 5-HT. For the parental group, inverse correlations were found between NE and Bmax (standing NE, rs = -0.67, n = 21, p = 0.001; supine NE, rs = -0.49, n = 22, p = 0.021). In the autistic group, no correlation was seen between plasma NE and Bmax. A correlation between the autistic boys' Bmax and their fathers' Bmax was observed (rs = 0.79, n = 11, p = 0.004). These findings suggest (1) circulating NE may be involved in heterologous regulation of 5-HT2 receptors in the platelet and (2) genetic (paternal-filial) factors may play a role in the expression of 5-HT2 binding sites in the platelet. These preliminary findings are discussed in relation to heterologous receptor regulation. The relationships between these findings and either the pathophysiology of autism or hyperserotonemia in autism are unknown.     

Developmental regression and mitochondrial dysfunction in a child with autism

Autistic spectrum disorders can be associated with mitochondrial dysfunction. We present a singleton case of developmental regression and oxidative phosphorylation disorder in a 19-month-old girl. Subtle abnormalities in the serum creatine kinase level, aspartate aminotransferase, and serum bicarbonate led us to perform a muscle biopsy, which showed type I myofiber atrophy, increased lipid content, and reduced cytochrome c oxidase activity. There were marked reductions in enzymatic activities for complex I and III. Complex IV (cytochrome c oxidase) activity was near the 5% confidence level. To determine the frequency of routine laboratory abnormalities in similar patients, we performed a retrospective study including 159 patients with autism (Diagnostic and Statistical Manual of Mental Disorders-IV and Childhood Autism Rating Scale) not previously diagnosed with metabolic disorders and 94 age-matched controls with other neurologic disorders. Aspartate aminotransferase was elevated in 38% of patients with autism compared with 15% of controls (P <.0001). The serum creatine kinase level also was abnormally elevated in 22 (47%) of 47 patients with autism. These data suggest that further metabolic evaluation is indicated in autistic patients and that defects of oxidative phosphorylation might be prevalent.     

Altered serotonin synthesis in the dentatothalamocortical pathway in autistic boys

Based on reports of increased platelet serotonin in 30 to 50% of autistic subjects, abnormal serotonergic neurotransmission may be important in the pathogenesis of autism. However, serotonin metabolite measurements in cerebrospinal fluid of autistic subjects have failed to demonstrate consistent abnormalities. Using α-[¹¹C]methyl-L ?tryptophan as a tracer for serotonin synthesis with positron emission tomography, we now report unilateral alterations of serotonin synthesis in the dentatothalamocortical pathway in autistic boys. Asymmetries of serotonin synthesis were found in frontal cortex, thalamus, and dentate nucleus of the cerebellum in all 7 boys, but not in the 1 autistic girl studied. Decreased serotonin synthesis was found in the left frontal cortex and thalamus in 5 of the 7 boys and in the right frontal cortex and thalamus in the 2 remaining autistic boys. In all 7 cases, elevated serotonin synthesis in the contralateral dentate nucleus was observed. Statistically significant differences between autistic boys and their nonautistic siblings (n = 5) were obtained when comparing asymmetry indices for frontal cortex, thalamus, and dentate nucleus combined as well as individually for frontal cortex and thalamus. These serotonergic abnormalities in a brain pathway, important for language production and sensory integration, may represent one mechanism underlying the pathophysiology of autism.     

Platelet and lymphocyte free intracellular calcium in affective disorders

Summary Many studies have demonstrated pharmacologic similarities between platelet and brain 5-HT₂ binding sites. Therefore it may be possible to use platelets as a model for the central serotonergic neuron. Accordingly, a previcus report (Kusumi et al. 1991b) about elevated [Ca²⁺]_i after serotonin stimulation in platelets of depressed patients was interpreted as further evidence for enhanced serotonergic sensitivity in depression. However, a very recent study showed an enhanced thrombin-induced platelet Ca²⁺ response, rather suggesting abnormalities of intracellular Ca²⁺ regulation in affective disorders. In the present study we have determined 5-HT₂-and thrombin-induced Ca²⁺ responses in platelets and additionally phytohemagglutin (PHA)-induced Ca²⁺ increase in lymphocytes of medicated depressed patients (8 mono- and 2 bipolar, HRSD>17) and of ten sex- and age-matched controls. The results showed no significant difference in basal calcium levels between the two groups and no significant difference in the Ca²⁺ response to thrombin although the response was higher in the patients. The Ca²⁺ increase after serotonin stimulation in depressed patients was significantly (P<0.05) higher than in healthy controls. By contrast, the Ca²⁺ response to PHA in lymphocytes was significantly decreased in the patients. Our data confirm elevated Ca²⁺ responses after 5-HT₂ receptor activation even in medicated depressed patients. However, Ca²⁺ responses in lymphocytes were decreased. Together with the observations of an enhanced Ca²⁺ response in platelets after thrombin stimulation, we speculate that the findings rather suggest alterations of [Ca²]_i regulation in depression than specific changes of serotonergic sensitivity.     

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_2Ar) 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_2Ar in autistic subjects.