Molecular genetics and animal models in autistic disorder

Are Autistic and Catatonic Regression Related? A Few Working Hypotheses Involving Gaba, Purkinje Cell Survival, Neurogenesis, and ECT

Autistic regression seems to occur in about a quarter of children with autism. Its cause is unknown. Late-onset autistic regression, that is, after 2 years of age, shares some features with catatonic regression. A working hypothesis is developed that some children with autistic regression suffer from early-onset catatonic regression. This hypothesis cannot be answered from current data and is difficult to address in clinical studies in the absence of definite markers of autistic and catatonic regression. Treatment implications are theoretical and involve the potential use of anticatatonic treatments for autistic regression. Focus is on electroconvulsive therapy (ECT)--an established but controversial treatment that is viewed by many, but not all, as the most effective treatment for severe, life-threatening catatonic regression. Clinical trials of ECT in early- or late-onset autistic regression in children have not been done yet. The effects of electroconvulsive seizures--the experimental analogue of ECT--should also be tested in gamma-aminobutyric acid-ergic animal models of autistic regression, autism, catatonia, and other neurodevelopmental disorders. Purkinje cell survival and neurogenesis are putative outcome measures in these models.

The genetic relationship between individual differences in social and nonsocial behaviours characteristic of autism

Two types of behaviours shown in children - those reflecting social impairment and nonsocial obsessive repetitive behaviours - are central to defining and diagnosing autism spectrum disorders (ASDs). Parent and teacher data on social and nonsocial behaviours were obtained from a community sample of >3000 7-year-old twin pairs. Social and nonsocial behaviours were only modestly correlated, and it was found that some individuals had extreme scores on either social or nonsocial scales but not both. Genetic model-fitting showed that social and nonsocial behaviours are both highly heritable, but their genetic overlap is modest, with most of the genetic influence being specific to either social or nonsocial behaviours. Considering these behaviours separately might help clarify gene-brain-behaviour pathways in future research.

Making and breaking serotonin neurons and autism

Dysfunction of brain serotonin system development is hypothesized to contribute to autistic behaviors. The testing of this hypothesis will likely depend on a better understanding of the genes and mechanisms involved in serotonin neuron cell fate specification. In this review we summarize the main features of vertebrate serotonin neuroanatomical development and recent studies that have revealed critical steps in the molecular genetic program that controls serotonin neuron phenotype. We then discuss the potential relevance of these findings to advances in autism research and to new molecular genetic tools under development that will impact future testing of the hypothesis.

Toward a developmental neurobiology of autism

Autism is a complex, behaviorally defined, developmental brain disorder with an estimated prevalence of 1 in 1,000. It is now clear that autism is not a disease, but a syndrome with a strong genetic component. The etiology of autism is poorly defined both at the cellular and the molecular levels. Based on the fact that seizure activity is frequently associated with autism and that abnormal evoked potentials have been observed in autistic individuals in response to tasks that require attention, several investigators have recently proposed that autism might be caused by an imbalance between excitation and inhibition in key neural systems including the cortex. Despite considerable ongoing effort toward the identification of chromosome regions affected in autism and the characterization of many potential gene candidates, only a few genes have been reproducibly shown to display specific mutations that segregate with autism, likely because of the complex polygenic nature of this syndrome. Among those, several candidate genes have been shown to control the early patterning and/or the late synaptic maturation of specific neuronal subpopulations controlling the balance between excitation and inhibition in the developing cortex and cerebellum. In the present article, we review our current understanding of the developmental mechanisms patterning the balance between excitation and inhibition in the context of the neurobiology of autism.

Serotonin transporter kinetics in rats selected for extreme values of platelet serotonin level

By selective breeding of Wistar rats for the extreme values of platelet serotonin (5HT) level (PSL), we have developed earlier two sublines of animals differing markedly in this parameter. Further studies, performed on the protein and mRNA levels, revealed platelet serotonin transporter (5HTt) as parameter underlying mentioned differences in PSL between sublines. In this work, we have performed full-kinetic analysis of platelet serotonin uptake (PSU) in animals from the genetically selected sublines. The results demonstrated marked differences in maximal velocity (V(max)) of the 5HT transporter, as contrasted to the lack of any difference in the Michaelis constant (K(m)). High correlation between PSL and V(max) of PSU was demonstrated, revealing that the number of membrane 5HT transporter sites is under genetic control and responsible for marked differences in PSL between high- and low-5HT sublines. These results enabled further selective breeding of animals for the extremes of V(max) of platelet 5HT transporter, and so the development of more specific model "Wistar-Zagreb 5HT rats".

What is Known About Autism

Autism is a neurodevelopmental disorder of genetic origins, with a heritability of about 90%. Autistic disorder is classed within the broad domain of pervasive developmental disorders (PDD) that also includes Rett syndrome, childhood disintegrative disorder, Asperger syndrome, and PDD not otherwise specified (PDD-NOS). Prevalence estimates suggest a rate of 0.1-0.2% for autism and 0.6% for the range of PDD disorders. There is considerable phenotypic heterogeneity within this class of disorders as well as continued debate regarding their clinical boundaries. Autism is the prototypical PDD, and is characterized by impairments in three core domains: social interaction, language development, and patterns of behavior (restricted and stereotyped). Clinical pattern and severity of impairment vary along these dimensions, and the level of cognitive functioning of individuals with autism spans the entire range, from profound mental retardation to superior intellect. There is no single biological or clinical marker for autism, nor is it expected that a single gene is responsible for its expression; as many as 15+ genes may be involved. However, environmental influences are also important, as concordance in monozygotic twins is less than 100% and the phenotypic expression of the disorder varies widely, even within monozygotic twins. Multiple susceptibility factors are being explored using varied methodologies, including genome-wide linkage studies, and family- and case-control candidate gene association studies. This paper reviews what is currently known about the genetic and environmental risk factors, neuropathology, and psychopharmacology of autism. Discussion of genetic factors focuses on the findings from linkage and association studies, the results of which have implicated the involvement of nearly every chromosome in the human genome. However, the most consistently replicated linkage findings have been on chromosome 7q, 2q, and 15q. The positive associations from candidate gene studies are largely unreplicated, with the possible exceptions of the GABRB3 and serotonin transporter genes. No single region of the brain or pathophysiological mechanism has yet been identified as being associated with autism. Postmortem findings, animal models, and neuroimaging studies have focused on the cerebellum, frontal cortex, hippocampus, and especially the amygdala. The cerebello-thalamo-cortical circuit may also be influential in autism. There is evidence that overall brain size is increased in some individuals with autism. Presently there are no drugs that produce major improvements in the core social or pragmatic language deficits in autism, although several have limited effects on associated behavioral features. The application of new techniques in autism research is being proposed, including the investigation of abnormal regulation of gene expression, proteomics, and the use of MRI and postmortem analysis of the brain.

Designing mouse behavioral tasks relevant to autistic-like behaviors

The importance of genetic factors in autism has prompted the development of mutant mouse models to advance our understanding of biological mechanisms underlying autistic behaviors. Mouse models of human neuropsychiatric diseases are designed to optimize (1) face validity, i.e., resemblance to the human symptoms; (2) construct validity, i.e., similarity to the underlying causes of the disease; and (3) predictive validity, i.e., expected responses to treatments that are effective in the human disease. There is a growing need for mouse behavioral tasks with all three types of validity for modeling the symptoms of autism. We are in the process of designing a set of tasks with face validity for the defining features of autism: deficits in appropriate reciprocal social interactions, deficits in verbal social communication, and high levels of ritualistic repetitive behaviors. Social approach is tested in an automated three-chambered apparatus that offers the subject a choice between a familiar environment, a novel environment, and a novel environment containing a stranger mouse. Preference for social novelty is tested in the same apparatus, with a choice between the start chamber, the chamber containing a familiar mouse, and the chamber containing a stranger mouse. Social communication is evaluated by measuring the ultrasonic distress vocalizations emitted by infant mouse pups and the parental response of retrieving the pup to the nest. Resistance to change in ritualistic repetitive behaviors is modeled by forcing a change in habit, including reversal of the spatial location of a reinforcer in a T-maze task and in the Morris water maze. Mouse behavioral tasks that may model additional features of autism are discussed, including tasks relevant to anxiety, seizures, sleep disturbances, and sensory hypersensitivity. Applications of these tests include (1) behavioral phenotyping of transgenic and knockout mice with mutations in genes relevant to autism, (2) characterization of mutant mice derived from random chemical mutagenesis, (3) DNA microarray analyses of genes in inbred strains of mice that differ in social interaction, social communication and resistance to change in habit, and (4) evaluation of proposed therapeutics for the treatment of autism.

A question of balance: a proposal for new mouse models of autism

Autism spectrum disorder (ASD) represents a major mental health problem with estimates of prevalence ranging from 1/500 to 1/2000. While generally recognized as developmental in origin, little to nothing is certain about its etiology. Currently, diagnosis is made on the basis of a variety of early developmental delays and/or regressions in behavior. There are no universally agreed upon changes in brain structure or cell composition. No biomarkers of any type are available to aid or confirm the clinical diagnosis. In addition, while estimates of the heritability of the condition range from 60 to 90%, as of this writing no disease gene has been unequivocally identified. The prevalence of autism is three- to four-fold higher in males than in females, but the reason for this sexual dimorphism is unknown. In light of all of these ambiguities, a proposal to discuss potential animal models may seem the heart of madness. However, parsing autism into its individual genetic, behavioral, and neurobiological components has already facilitated a 'conversation' between the human disease and the neuropathology and biochemistry underlying the disorder. Building on these results, it should be possible to not just replicate one aspect of autism but to connect the developmental abnormalities underlying the ultimate behavioral phenotype. A reciprocal conversation such as this, wherein the human disease informs on how to make a better animal model and the animal model teaches of the biology causal to autism, would be highly beneficial.

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.

Autism: a target of pharmacotherapies?

Autism is reaching epidemic proportions. The diagnosis can be made as early as 2 years of age, and autistic patients are expected to have a normal life span. Thus, in terms of the number of 'patient years', autism spectrum disorder (ASD) represents a market that is as large as that of the biggest neurological indication, Alzheimer's disease. However, despite the clear unmet medical need no effective treatment is yet available. This could be because the biology of ASD is not clearly understood and thus proper drug treatment has not been possible. However, significant advances are being made toward understanding the mechanisms of the disease. Here, we review the most recent preclinical advances in the hope that they will lead to a breakthrough in the near future.

Autism

Autism is a neurologic disorder with impairments in language, social communication, and behavior, which may improve over time, but which persist throughout the lifetime. The evaluation of autism requires a separation of clinical and research objectives and is done best in close cooperation with professionals in the fields of communication, education, and psychology. There are no biologic markers of autism. Regression in language and social communication is present in approximately 30% of children with autism and is most likely to occur between 18 and 24 months of age. Early deficits in social communication can be identified by the assessment of joint attention, affective reciprocity, and metacognition. Current evidence suggests that deficits in social cognition and communication in autism may be related to dysfunction in the amygdala, hippocampus, and related limbic and cortical structures. Other neuroanatomic structures, such as the cerebellum, also may form part of a distributed neuronal network responsible for social cognition and communication. Genetics play a major role in autism, but what is inherited and how broad the inheritable phenotype is remain unclear. At a neurochemical level, the principal neurotransmitter implicated in autism is serotonin. Seizures and epileptiform discharges are common in autism and are linked to cognitive dysfunction. The role of medication is to target specific symptoms and open windows of opportunity that allow implementation of a multimodal individualized educational plan.

Characterization of the various forms of the Reelin protein in the cerebrospinal fluid of normal subjects and in neurological diseases

Reelin is a large extracellular glycoprotein that is defective in reeler mutant mice and plays a well-established role during brain development in human as well as rodents. In the adult brain, Reelin is expressed in a subset of GABAergic interneurons. Its role in disease states is not clearly defined, although it is implicated in autism and psychoses such as schizophrenia. In this report, we show that Reelin immunoreactive proteins can be detected in the human cerebrospinal fluid (CSF) with monoclonal antibodies directed against the N- and C-terminal regions of the protein. In CSF, Reelin is present as different products due to processing at two main sites; preservation at -20 degrees C increases processing further. CSF Reelin originates from the brain tissue and not from plasma. The protein was detected in comparable concentrations in children and adults, and the signal varied largely from subject to subject with no obvious correlation with age or neurological disease state.

Expanded characterization of the social interaction abnormalities in mice lacking Dvl1

Dvl1 is one of three murine Dishevelled genes widely expressed in embryonic development and in the adult central nervous system. Dishevelled proteins are a necessary component of the Wnt and planar cell polarity developmental signaling pathways. We reported previously that mice deficient in Dvl1 exhibited abnormal social interaction and sensorimotor gating. To assess the validity of our earlier findings, we replicated the previous behavioral tests and included several new assays. The behaviors assessed included: social interaction, sensorimotor reflexes, motor activity, nociception, prepulse inhibition of acoustic startle (PPI) and learning and memory. Assessments with an explicit social component included: social dominance test, whisker trimming, nest building, home-cage huddling and ultrasonic vocalization rate analysis in pups. In addition, separate cohorts of wildtype and Dvl1-null mice were assessed for social recognition of a conspecific. Replicating the original report, Dvl1-null mice were impaired in several tasks containing an explicit social component. However, no impairment was observed in the social memory task. A previously observed deficit in PPI did not replicate in two institutions. In conclusion, we provide evidence that the social interaction phenotype of Dvl1-deficient mice has a strong genetic influence, but the sensorimotor gating deficit was subject to environmental influences. The specificity of observed social interaction deficits also suggests that lack of Dvl1 is associated with deficits in the recognition of social hierarchy and dominance.

Could ECT be effective in autism?

Autism is increasingly diagnosed, but therapeutic options are limited in many children. ECT is considered as a safe, effective, and life-saving treatment in people of all ages who suffer from affective disorders, acute psychosis, and, in particular, catatonia. There are recent speculations that certain types of autism may be the earliest expression of catatonia and that both disorders have identical risk factors. Therefore, ECT may improve autism and, if started early enough, may prevent further development of autistic symptoms in some children. The use of ECT in autism has never been systematically assessed. There have been two large ECT studies in children in the 1940s. Autism was not assessed in these studies because the autistic syndrome was just then being recognized as a separate entity. Findings from these studies add little to the hypothesis that ECT may be effective in autistic children, but attest to the safety and feasibility of ECT in children. Another limitation is the use of older ECT techniques. What may well be the greatest deterrent to use ECT in autism is widespread anti- ECT sentiment not only among the public but within the medical community as well. All child specialists--psychiatrists, neurologists, psychologists, and developmental pediatricians--should independently review the feasibility, potential, and risk of using ECT in autism. Unless anti-ECT prejudice can be overcome, it is unlikely that any ECT trial in autism is forthcoming. Research areas that may support the hypothesis that ECT is effective in autism should be pursued. First, any link between autism and catatonia should be further explored in clinical and biochemical studies. A GABA theory of autism and catatonia may be pivotal. Second, the role of abnormal GABAA receptor subunit genes in autism and catatonia should be further assessed. Candidate loci for autism and catatonia have been found on the long arm of chromosome 15 where three GABAA receptor subunits genes are located. The GABAA receptor beta 3 subunit gene (GABRB3) was the leading candidate gene for a subgroup of autism in two independent studies. Third, a novel genetic mouse model of autism should be tested. Mutant mice with a targeted deletion of the GABRB3 gene have a complete deficit of the beta 3 subunit of the GABAA receptor. This knockout mouse model seems promising to study developmental effects of altered GABAA receptor function as it relates to certain developmental disorders including autism.

Calcr, a brain-specific imprinted mouse calcitonin receptor gene in the imprinted cluster of the proximal region of chromosome 6

Expressed sequence tags (ESTs) in the human chromosome 7q21-q31 region were recently used to screen for allelic expression bias in monochromosomal hybrids retaining a paternal or maternal human chromosome 7. Six candidate imprinted genes were identified. In this study, we investigated parent-of-origin-specific expression profiles of their mouse homologues in the proximal region of chromosome 6. An imprinting analysis, using F1 mice from reciprocal crosses between the B6 and JF strains, demonstrated that the mouse calcitonin receptor gene ( Calcr) was expressed preferentially from the maternal allele in brain, whereas no allelic bias was detected in other tissues. Our results indicate that Calcr is imprinted in a tissue-specific manner, with a predominant expression from the maternal allele in the brain.

L’autisme : vers une nécessaire révolution culturelle

Autism is a pervasive developmental disorder of childhood characterised by disturbances in both social interactions and communication as well as stereotyped patterns of activities and behaviour. The increase in estimates of the prevalence of autism has raised the question of an "epidemic" of autism. More active case assessment and changes in diagnostic criteria probably account in large part for such increase. Investigators have attempted to define the neural pathophysiology of autism ever since the hypothesis of "refrigerator mother" as its cause was replaced by the view that it is a developmental disorder of the immature brain. However consensus is yet to be reached concerning the brain regions implicated. Psychoanalysis, cognitive psychology, neurophysiology, neuropharmacology, and genetics propose restricted view of the major issues leaving extensive areas unexplored. Therapeutic approaches induce only partial and uncertain results. There is no cure for autism but substantial evidence indicates that early, intensive, individualised education is beneficial for children. All modern intervention programs for autism affected children share a high degree of environmental structuring and predictability and an extensive individual approach. Autism being a behaviourally defined syndrome, it gave rise to a number of controversies concerning definition, classification, etiopathogenesis and therapeutics. In the 1990s a crisis has occurred in France with a loss of confidence between parents and psychiatrists with a problem concerning the means and ways of care of the autistic individual. The aim of this paper is to point out the different questions raised by autism in order to better understand this syndrome which touches upon essential behaviour-related aspects such as self consciousness, reality perception, the functioning of the thought and communication, as well as the role of hereditary and acquired influences in normal and pathological development.

Executive and social behaviors under nicotinic receptor regulation

Nicotine enhances several cognitive and psychomotor behaviors, and nicotinic antagonists cause impairments in tasks requiring cognitive effort. To explore the contribution of nicotinic receptors to complex cognitive functions, we developed an automated method to investigate sequential locomotor behavior in the mouse and an analysis of social behavior. We show that, in the beta2-/- mutant, the high-order spatiotemporal organization of locomotor behavior, together with conflict resolution and social interaction, is selectively dissociated from low-level, more automatic motor behaviors. Such deficits in executive functions resemble the rigid and asocial behavior found in some psychopathological disorders such as autism and attention deficit hyperactivity disorder.

Autism: a large unmet medical need and a complex research problem

Autism has been becoming the focus of attention as its apparently increasing prevalence is better appreciated. According to some estimates, the frequency of children with autistic spectrum disorder (ASD) can be as high as 1 in 150. The diagnosis can be made as early as 2 years of age, and autistic patients often have a normal life span. Thus, in terms of the number of "patient years," ASD represents a market that is as large as that of the biggest neurological indication, Alzheimer's disease. Despite the clear unmet medical need, no effective treatment is available. This may be because the mechanism of ASD is not understood. The aim of the present paper is to review recent advances in autism research and to discuss some of the most stressing problems mainly from a preclinical research standpoint. We hope to draw attention to the need to study this devastating disease that places an enormous burden on the society in general and the relatives and caregivers of autistic patients in particular.

Tianeptine: a novel strategy of psychopharmacological treatment of children with autistic disorder

OBJECTIVES

Many autistic children have problems of eye contact and expressive language that limit the effectiveness of educational and behavioural interventions. Few controlled psychopharmacological trials have been conducted in autistic children to determine which agents may be effective for these associated features.

METHODS

Twelve male children (7.3 +/- 3.3 years) with autistic disorder, diagnosed by ICD-10 criteria, completed a placebo-controlled, double-blind crossover trial of tianeptine, which lasted for 12 weeks. Subjects were included in the study if their eye contact and expressive language was inadaequate for their developmental level. Subjects had not tolerated or responded to other psychopharmacological treatments (neuroleptics, methylphenidate, clonidine or desipramine).

RESULTS

Teacher ratings on the aberrant behaviour checklist irritability, stereotypy and inappropriate speech factors were lower during treatment with tianeptine than during treatment with placebo. Clinician ratings (children's psychiatric rating scale autism, anger and speech deviance factors; children's global assessment scale; clinical global impressions efficacy) of videotaped sessions were not significantly different between tianeptine and placebo.

DISCUSSION

Tianeptine were modestly effective in the short-term treatment of irritability in some children with autistic disorder.

Genetics and genomics of behavioral and psychiatric disorders

Psychiatric conditions are to some degree under genetic influences. Despite the application of advanced genetic and molecular biological technologies, the genetic bases of the human behavioral traits and psychiatric diseases remains largely unresolved. Conventional genetic linkage approaches have not yielded definitive results, possibly because of the absence of objective diagnostic tests, the complex nature of human behavior or the incomplete penetrance of psychiatric traits. However, recent studies have revealed some genes of interest using multifaceted approaches to overcome these challenges. The approaches include using families in which specific behaviors segregate as a mendelian trait, utilization of endophenotypes as biological intermediate traits, identification of psychiatric disease phenotypes in genomic disorders, and the establishment of mouse models.

Activation of Wnt signaling rescues neurodegeneration and behavioral impairments induced by beta-amyloid fibrils

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, which is probably caused by the cytotoxic effect of the amyloid beta-peptide (Abeta). We report here molecular changes induced by Abeta, both in neuronal cells in culture and in rats injected in the dorsal hippocampus with preformed Abeta fibrils, as an in vivo model of the disease. Results indicate that in both systems, Abeta neurotoxicity resulted in the destabilization of endogenous levels of beta-catenin, a key transducer of the Wnt signaling pathway. Lithium chloride, which mimics Wnt signaling by inhibiting glycogen synthase kinase-3beta promoted the survival of post-mitotic neurons against Abeta neurotoxicity and recovered cytosolic beta-catenin to control levels. Moreover, the neurotoxic effect of Abeta fibrils was also modulated with protein kinase C agonists/inhibitors and reversed with conditioned medium containing the Wnt-3a ligand. We also examined the spatial memory performance of rats injected with preformed Abeta fibrils in the Morris water maze paradigm, and found that chronic lithium treatment protected neurodegeneration by rescuing beta-catenin levels and improved the deficit in spatial learning induced by Abeta. Our results are consistent with the idea that Abeta-dependent neurotoxicity induces a loss of function of Wnt signaling components and indicate that lithium or compounds that mimic this signaling cascade may be putative candidates for therapeutic intervention in Alzheimer's patients.

Animal models of mental retardation: from gene to cognitive function

About 2-3% of all children are affected by mental retardation, and genetic conditions rank among the leading causes of mental retardation. Alterations in the information encoded by genes that regulate critical steps of brain development can disrupt the normal course of development, and have profound consequences on mental processes. Genetically modified mouse models have helped to elucidate the contribution of specific gene alterations and gene-environment interactions to the phenotype of several forms of mental retardation. Mouse models of several neurodevelopmental pathologies, such as Down and Rett syndromes and X-linked forms of mental retardation, have been developed. Because behavior is the ultimate output of brain, behavioral phenotyping of these models provides functional information that may not be detectable using molecular, cellular or histological evaluations. In particular, the study of ontogeny of behavior is recommended in mouse models of disorders having a developmental onset. Identifying the role of specific genes in neuropathologies provides a framework in which to understand key stages of human brain development, and provides a target for potential therapeutic intervention.

Autism, macrocrania and epilepsy: how are they linked?

To evaluate the possible association of autistic disorder (AD), macrocrania and epilepsy, we performed a retrospective study comparing epileptic and non-epileptic AD patients with macrocrania, and AD patients with macrocrania to age- and sex-matched AD controls without macrocrania. We found macrocrania in 17.3% of 121 patients with AD. Epilepsy was not significantly more frequent in AD patients with macrocrania than in those without macrocrania. There were no significant differences in the other clinical characteristics studied except for epileptiform EEG abnormalities which were more often found in AD patients with epilepsy. AD with macrocrania and epilepsy is not a syndrome but may be a marker for a group of subjects with AD. A role for familial macrocrania needs further assessment.

Secretin as a neuropeptide

The role of secretin as a classical hormone in the gastrointestinal system is well-established. The recent debate on the use of secretin as a potential therapeutic treatment for autistic patients urges a better understanding of the neuroactive functions of secretin. Indeed, there is an increasing body of evidence pointing to the direction that, in addition to other peptides in the secretin/glucagon superfamily, secretin is also a neuropeptide. The purpose of this review is to discuss the recent data for supporting the neurocrine roles of secretin in rodents. By in situ hybridization and immunostaining, secretin was found to be expressed in distinct neuronal populations within the cerebellum and cerebral cortex, whereas the receptor transcript was found throughout the brain. In the rat cerebellum, secretin functions as a retrograde messenger to facilitate GABA transmission, indicating that it can modulate motor and other functions. In summary, the recent data support strongly the neuropeptide role of secretin, although the secretin-autism link remains to be clarified in the future.