Opioid peptides and gastrointestinal symptoms in autism spectrum disorders

Effects of Different Cow-Milk Beta-Caseins on the Gut-Brain Axis: A Narrative Review of Preclinical, Animal, and Human Studies

The gut and brain communicate through bidirectional neural, endocrine, and immune signals to coordinate central nervous system activity with gastrointestinal function. Dysregulated inflammation can promote immune cell activation and increase entero-endocrine signaling and intestinal permeability; hence, a functional gut-brain axis is necessary for a healthy digestive system. The consumption of milk products can lead to gut discomfort via effects on gastrointestinal tract function and the inflammatory state, which, in turn, affect the brain. A1 β-casein and A2 β-casein are major components of bovine-milk protein, and their digestion may result in different physiological effects following the consumption of milk products. Peptides derived from A1 β-casein, such as β-casomorphins, may increase gut dysfunction and inflammation, thereby modulating the availability of bioactive metabolites in the bloodstream and contribute to changes in cognitive function. This narrative review examines the functional interrelationships between the consumption of cow-milk-derived β-caseins and their effect on the brain, immune system, and the gut, which together comprise the gut-brain axis.

The Role of Nutrition, Oxidative Stress, and Trace Elements in the Pathophysiology of Autism Spectrum Disorders

Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by deficits in social communication and interaction, alongside repetitive behaviors, and atypical sensory-motor patterns. The growing prevalence of ASD has driven substantial advancements in research aimed at understanding its etiology, preventing its onset, and mitigating its impact. This ongoing effort necessitates continuous updates to the body of knowledge and the identification of previously unexplored factors. The present study addresses this need by examining the roles of nutrition, oxidative stress, and trace elements in the pathophysiology of ASD. In this review, an overview is provided of the key dietary recommendations for individuals with ASD, including gluten-free and casein-free (GFCF) diets, ketogenic diets (KDs), and other nutritional interventions. Furthermore, it explores the involvement of oxidative stress in ASD and highlights the significance of trace elements in maintaining neuropsychiatric health. The impact of these factors on molecular and cellular mechanisms was discussed, alongside therapeutic strategies and their efficacy in managing ASD.

Autism in patients with eosinophilic gastrointestinal disease: A systematic review with meta-analysis

PURPOSE

Neurodevelopmental disorders, such as autism spectrum disorder (ASD), have been increasingly associated with eosinophilic gastrointestinal disorders (EGID). However, the relationship between these diseases remains unclear. We performed a systematic review with meta-analysis to address this issue.

METHODS

The search was performed according to the PRISMA guidelines using descriptors for ASD and EGIDs from the MEDLINE, Embase, PsycInfo, LILACS, and Web of Science databases. Observational studies with the prevalence of ASD in any EGID were included. The study protocol was registered on the PROSPERO platform under the number CRD42023455177.

RESULTS

The total dataset comprised 766,082 participants. The result of the single-arm meta-analysis showed an overall prevalence of ASD in the population with EGID of 21.59% (95% CI: 10.73-38.67). There was an association between EGID and ASD (OR: 3.44; 95% CI: 1.25-2.21), also significant when restricted only to EoE (OR: 3.70; 95% CI: 2.71-5.70).

DISCUSSION

Recent studies have implicated the influence of an inadequate epithelial barrier integrity in the pathogenesis of several diseases. The role of this mechanism can be extended to situations beyond allergic reactions, including other conditions with underlying immunological mechanisms. Several diseases are potentially related to the systemic effect of bacterial translocation in tissues with defective epithelial barriers.

CONCLUSION

Our meta-analysis provides evidence that supports the consideration of EGID in patients with ASD and ASD in patients with EGID. Despite its limitations, the results should also be validated by future studies, preferably using multicenter prospective designs in populations with low referral bias.

Progress in the study of intestinal microbiota involved in morphine tolerance

Morphine is a widely used opioid for treatment of pain. The attendant problems including morphine tolerance and morphine dependence pose a major public health challenge. In recent years, there has been increasing interest in the gastrointestinal microbiota in many physiological and pathophysiological processes. The connectivity network between the gut microbiota and the brain is involved in multiple biological systems, and bidirectional communication between them is critical in gastrointestinal tract homeostasis, the central nervous system, and the microbial system. Many research have previously shown that morphine has a variety of effects on the gastrointestinal tract, but none have determined the function of intestinal microbiota in morphine tolerance. This study reviewed the mechanisms of morphine tolerance from the perspective of dysregulation of microbiota-gut-brain axis homeostasis, by summarizing the possible mechanisms originating from the gut that may affect morphine tolerance and the improvement of morphine tolerance through the gut microbiota.

The Current View on the Paradox of Pain in Autism Spectrum Disorders

Autism spectrum disorder (ASD) is a neurodevelopmental disorder, which affects 1 in 44 children and may cause severe disabilities. Besides socio-communicational difficulties and repetitive behaviors, ASD also presents as atypical sensorimotor function and pain reactivity. While chronic pain is a frequent co-morbidity in autism, pain management in this population is often insufficient because of difficulties in pain evaluation, worsening their prognosis and perhaps driving higher mortality rates. Previous observations have tended to oversimplify the experience of pain in autism as being insensitive to painful stimuli. Various findings in the past 15 years have challenged and complicated this dogma. However, a relatively small number of studies investigates the physiological correlates of pain reactivity in ASD. We explore the possibility that atypical pain perception in people with ASD is mediated by alterations in pain perception, transmission, expression and modulation, and through interactions between these processes. These complex interactions may account for the great variability and sometimes contradictory findings from the studies. A growing body of evidence is challenging the idea of alterations in pain processing in ASD due to a single factor, and calls for an integrative view. We propose a model of the pain cycle that includes the interplay between the molecular and neurophysiological pathways of pain processing and it conscious appraisal that may interfere with pain reactivity and coping in autism. The role of social factors in pain-induced response is also discussed. Pain assessment in clinical care is mostly based on subjective rather than objective measures. This review clarifies the strong need for a consistent methodology, and describes innovative tools to cope with the heterogeneity of pain expression in ASD, enabling individualized assessment. Multiple measures, including self-reporting, informant reporting, clinician-assessed, and purely physiological metrics may provide more consistent results. An integrative view on the regulation of the pain cycle offers a more robust framework to characterize the experience of pain in autism.

Cerebral intestinal interaction in children with autism spectrum disorder

Background. Autism spectrum disorders (ASD) in children are associated with features of neuropsychological development, characterized by socio-communicative, emotional, and behavioral problems. The processes of interaction between the central and enteric nervous systems, taking into account the inherent RAS reactions of autonomic maladaptation, endogenous stress, eating behavior determine the pathophysiological mechanisms underlying the comorbid pathology of the digestive system. The study was aimed to analyze the cerebral intestinal interaction signs in children with ASD, taking into account the role of NSE and S-100 neurotransmitters. Materials and methods. Sixty-six children with ASD were examined, out of which 45 children had concomitant functional disorders of the digestive system (FDDS). Clinical manifestations of ASD were assessed by the CARS scale; FDDS was diagnosed based on the Rome IV criteria. Serum levels of NSE neurotransmitters and S-100 protein were measured immunochemically. The patients were examined after obtaining informed consent in compliance with the principles of bioethics. Results. According to the data obtained in children with ASD, the frequency of concomitant FDDS is 68.20 %. In this case, ASD in combination with functional disorders of the biliary tract (FDBT) is observed in 22.7 %, with irritable bowel syndrome (IBS) — in 27.30 %, with syntropic FDDS — in 18.20 % of patients. The study traced the association between severe ASD and concomitant FDDS, in particular ASD with isolated IBS (p = 0.004), ASD with isolated FDBT (p = 0.009), ASD with syntropic functional disorders (p = 0.041). Increased serum concentrations of the NSE and S-100 neurotransmitters have been observed in children with concomitant FDDS. The level of S-100 protein and clinical manifestations of FDDS reveal a correlation, the degree of which increases in the following sequence: IBS (r = 0.34), and syntropic FDDS (r = 0.48). Conclusions. Peculiarities of cerebral intestinal interactions in ASD determine the high frequency of FDDS, in the structure of which IBS dominated. The presence of concomitant pathology impacts the clinical manifestations of ASD, complicating its course to a greater extent in cases of syntropic FDDS. Involvement of enteric glial structures in ASD is accompanied by the NSE and S-100 neurotransmitters level increase on the background of concomitant FDDS. The diagnostic value of S-100 protein in IBS in children with ASD is shown.

Efficacy and Safety of Diet Therapies in Children With Autism Spectrum Disorder: A Systematic Literature Review and Meta-Analysis

Objective

Autism Spectrum Disorder is a neurodevelopmental disorder, with a rapid increase in recognition over the past decade. Interest in alternative therapies is growing annually, such as dietary therapies including gluten-free and/or casein-free diet, and the ketogenic diet. However, there is no consensus on the efficacy and safety of dietary therapy in children with ASD up to now. This study aimed to assess the efficacy and safety of these diet interventions for children with ASD based on a meta-analysis of global data.

Methods

Seven databases (Cochrane Library, PubMed, EMBASE, Web of Science, VIP, CNKI, and Wanfang) were searched according to the established inclusion criteria, from the inception of the databases to August 18, 2021. The Cochrane Bias risk assessment tool was intended to assess the quality of the included studies. Review Manager 5.4 software was used as an efficacy analysis tool of the included studies, taking the core autistic symptoms and scales of ASD as therapeutic efficacy evaluations.

Results

In total, 7 RCTs with 338 participants were finally obtained. All studies assessed the association between core autistic symptoms and therapeutic diet, showing a statistically significant effect (standard mean difference (SMD) of −0.51, 95% confidence interval (Cl): −0.81 to −0.21), in which two studies which followed the GFD diet reported significant reductions in social behaviors (SMD of−0.41, 95% Cl: −0.75 to −0.06), showing no correlation with the length of the interventions (P < 0.05). Two studies were performed in KD diet suggested a significant effect in core symptoms (SMD of −0.67, 95% Cl: −1.04 to −0.31). No statistically significant changes were observed in the GFCF diet, GFD diet, cognition, communication, and stereotypical behaviors subgroups (all P > 0.05).

Conclusion

The results of a meta-analysis suggest that diet therapies can significantly ameliorate core symptoms of ASD, and GFD diets are conducive to improving social behaviors. Although the results suggest the effectiveness of dietary therapy for ASD, limited by the small sample size of RCTs, more well-designed, and high-quality clinical trials are needed to validate the above conclusions.

Systematic Review Registration

https://www.crd.york.ac.uk/PROSPERO/, identifier: CRD42021277565.

Comparative Study on the Exacerbating Effects of Casein-Rich vs. Gluten-Rich Diets on Biochemical-Induced Features in Rodent Model of Autism

In relation to dietary intervention in individuals with autism spectrum disorder (ASD), certain food constituents especially gluten and casein are recognized to be challenging and should be restricted. In this study, levels of glutathione S-transferase, glutathione, lipid peroxides, serotonin (5-HT), interleukin-6 (IL-6), glutamate, and gamma aminobutyric acid (GABA) were measured in the brain homogenates of ASD rodent model. Rats were treated either with single dose clindamycin (30 mg/kg) or with propionic acid (PPA) (250 mg/kg) for 3 days and then fed a standard diet, casein-rich diet (CRD), or gluten-rich diet (GRD). The obtained data demonstrates that clindamycin and PPA induced oxidative stress, which was slightly affected by CRD. A marked increase in the proinflammatory cytokine (IL-6) concentration found in clindamycin- and PPA-treated groups was lower in CRD fed rats. Both CRDs and GRDs produced similar trends in glutamate levels. 5-HT levels were higher in the clindamycin- and PPA-treated groups and increased with a GRD but were less affected by a CRD. CRD could be less deleterious compared to GRD. Although the underlying cause of gastrointestinal symptoms in patients with ASD is not exactly known, the most widely accepted one is the opioid theory which is related to GRD and CRD.

Measures of gluten-related reactivity in children with autism spectrum disorders in the absence of overt gastrointestinal symptoms: a pilot study from the United Arab Emirates

Objectives

The aetiology of autism spectrum disorder (ASD) is multifactorial, sometimes genetic, and may be associated with abnormal immunological responses to peptides from proteins such as gluten. These peptides may cross the blood-brain barrier and affect neurotransmission, resulting in behavioural symptoms consistent with ASD. The aim of this study was to screen for markers of gluten-related immune reactivity in the absence of overt gastrointestinal symptoms in patients with ASD in the United Arab Emirates, a country associated with a high prevalence of ASD but lacking this type of research.

Methods

Patients diagnosed with ASD (using Diagnostic and Statistical Manual of Mental Disorders-IV-based criteria and Autism Diagnostic Observational Schedules) were compared with controls, regarding anti-tissue transglutaminase (tTG) immunoglobulin (Ig) A and anti-deamidated gliadin peptide (DGP) IgA levels.

Results

Sixty-six patients with ASD and 101 controls were included. Patients with ASD showed statistically significant lower anti-DGP IgA levels, but no significant difference in anti-tTG IgA levels, versus healthy controls. Correlations between immunological data and clinical symptoms were synergistic, but not statistically significant.

Conclusion

ASD may be associated with reduced levels of anti-DGP IgA.

Gastrointestinal Symptoms During Depressive Episodes in 3256 Patients with Major Depressive Disorders: Findings from the NSSD

BACKGROUND

Little is known how often depressive episodes are accompanied by gastrointestinal symptoms in major depressive disorders (MDD). The authors sought to determine the frequency and clinical correlates of gastrointestinal symptoms during episodes of depressive disorder.

METHODS

3,256 MDD patients from the National Survey on Symptomatology of Depression (NSSD), which was designed to investigate the magnitude of symptoms of current major depressive episodes in China, were enrolled and assessed for gastrointestinal symptoms in this study. Illness characteristics were compared in patients with a different frequency of gastrointestinal symptoms. Pearson correlation analysis and multiple linear regression analysis were employed to investigate the associations between gastrointestinal symptoms and psychological characteristics in the patients.

RESULTS

More than 70% of the subjects with depressive episodes had concomitant gastrointestinal symptoms. A higher frequency of gastrointestinal symptoms was associated with an increased risk of suicide ideation, suicide attempts, anxious mood, depressed mood, insomnia, feeling a failure, poor concentration, body pain, hopelessness, anger, and irritability. Pearson correlation analysis indicated moderate but significant associations between gastrointestinal symptoms and psychological characteristics (p<0.001). Multiple linear regression analysis showed that suicide ideation (β=0.161, p<0.001), anxiety mood (β=0.166, p = 0.006), insomnia (β =0.262, p<0.001), anger (β=0.144, p<0.001), feeling a failure (β =0.365, p<0.001), and body pain (β=0.581 p<0.001) were independently associated with gastrointestinal symptoms in MDD patients.

CONCLUSION

Gastrointestinal symptoms were one of the most prevalent clinical presentations of MDD. The associations between gastrointestinal symptoms and psychological characteristics may prove useful in expanding our understanding of how gastrointestinal symptoms contributes to MDD.

The Gut Microbiota and Oxidative Stress in Autism Spectrum Disorders (ASD)

Autism spectrum disorders (ASDs) are a kind of neurodevelopmental disorder with rapidly increasing morbidity. In recent years, many studies have proposed a possible link between ASD and multiple environmental as well as genetic risk factors; nevertheless, recent studies have still failed to identify the specific pathogenesis. An analysis of the literature showed that oxidative stress and redox imbalance caused by high levels of reactive oxygen species (ROS) are thought to be integral parts of ASD pathophysiology. On the one hand, this review aims to elucidate the communications between oxidative stress, as a risk factor, and ASD. As such, there is also evidence to suggest that early assessment and treatment of antioxidant status are likely to result in improved long-term prognosis by disturbing oxidative stress in the brain to avoid additional irreversible brain damage. Accordingly, we will also discuss the possibility of novel therapies regarding oxidative stress as a target according to recent literature. On the other hand, this review suggests a definite relationship between ASD and an unbalanced gastrointestinal tract (GIT) microbiota (i.e., GIT dysbiosis). A variety of studies have concluded that the intestinal microbiota influences many aspects of human health, including metabolism, the immune and nervous systems, and the mucosal barrier. Additionally, the oxidative stress and GIT dysfunction in autistic children have both been reported to be related to mitochondrial dysfunction. What is the connection between them? Moreover, specific changes in the GIT microbiota are clearly observed in most autistic children, and the related mechanisms and the connection among ASD, the GIT microbiota, and oxidative stress are also discussed, providing a theory and molecular strategies for clinical practice as well as further studies.

Nutritional interventions for autism spectrum disorder

Autism spectrum disorder (ASD) is an increasingly prevalent neurodevelopmental disorder with considerable clinical heterogeneity. With no cure for the disorder, treatments commonly center around speech and behavioral therapies to improve the characteristic social, behavioral, and communicative symptoms of ASD. Gastrointestinal disturbances are commonly encountered comorbidities that are thought to be not only another symptom of ASD but to also play an active role in modulating the expression of social and behavioral symptoms. Therefore, nutritional interventions are used by a majority of those with ASD both with and without clinical supervision to alleviate gastrointestinal and behavioral symptoms. Despite a considerable interest in dietary interventions, no consensus exists regarding optimal nutritional therapy. Thus, patients and physicians are left to choose from a myriad of dietary protocols. This review, summarizes the state of the current clinical and experimental literature on nutritional interventions for ASD, including gluten-free and casein-free, ketogenic, and specific carbohydrate diets, as well as probiotics, polyunsaturated fatty acids, and dietary supplements (vitamins A, C, B6, and B12; magnesium and folate).

Opioid system influences gut-brain axis: Dysbiosis and related alterations

Opioid drugs are widely used to treat chronic pain, but their misuse can lead to tolerance, dependence, and addiction and have created a significant public health problem. In addition, food-derived opioid peptides, known as exorphins, like gluten exorphins have been shown to have harmful effects in certain pathologies like celiac disease, for example. Several studies support the involvement of the opioid system in the development of disorders such as autism spectrum syndrome. Moreover, bidirectional communication between the intestine and brain has been shown to be altered in various neurodegenerative diseases including Alzheimer´s and Parkinson´s. The presence of opioid receptors in both the digestive tract and the central nervous system (CNS) suggests that opioid drugs and exorphins may modulate the gut-brain axis. Morphine, for example, has shown a dysbiotic effect on the bacterial microbiota in addition to inducing an increase in intestinal permeability facilitating bacterial translocation. Furthermore, certain components of bacteria can modify the expression of opioid receptors at the central level increasing sensitivity to pain. Strategies based on use of probiotics have resulted in improvements in symptoms of autism and Parkinson´s disease. In this manuscript, we review the role of the opioid system in disorders and CNS pathologies and the involvement of the gut-brain axis.

Tuning Down the Pain - An Overview of Allosteric Modulation of Opioid Receptors: Mechanisms of Modulation, Allosteric Sites, Modulator Syntheses

Opioid signaling plays a central role in pain perception. As such, it remains the main target in the development of antinociceptive agents, despite serious side effects involved. In recent years, hopes for improved opioid painkillers are rising, together with our understanding of allosterism and biased signaling mechanisms. In this review, we focus on recently discovered allosteric modulators of opioid receptors, insights into phenomena underlying their action, as well as on how they extend our understanding of mechanisms of previously known compounds. A brief overlook of their synthesis is also presented.

Occurrence, biological properties and potential effects on human health of β-casomorphin 7: Current knowledge and concerns

The genetic variant A1 of bovine β-casein (β-Cn) presents a His residue at a position 67 of the mature protein. This feature makes the Ile⁶⁶-His⁶⁷ bond more vulnerable to enzymatic cleavage, determining the release of the peptide β-Cn f(60-66), named β-casomorphin 7 (BCM7). BCM7 is an opioid-agonist for μ receptors, and it has been hypothesized to be involved in the development of different non-transmissible diseases in humans. In the last decade, studies have provided additional results on the potential health impact of β-Cn A1 and BCM7. These studies, here reviewed, highlighted a relation between the consumption of β-Cn A1 (and its derivative BCM7) and the increase of inflammatory response as well as discomfort at the gastrointestinal level. Conversely, the role of BCM7 and the effects of ingestion of β-Cn A1 on the onset or worsening of other non-transmissible diseases as caused or favored by still need proof of evidence. Overall, the reviewed literature demonstrates that the "β-Cn A1/BCM7 issue" remains an intriguing but not exhaustively explained topic in human nutrition. On this basis, policies in favor of breeding for β-Cn variants not releasing BCM7 and consumption of "A1-like" milk appear not yet sound for a healthier and safer nutrition.

Gut-brain axis in the executive function of austism spectrum disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired communication and social interactions, and repetitive behavioural patterns. These patterns are believed to be dysfunctional symptoms in executive processing, which impact other cognitive functions such as attention or cognitive flexibility. In recent years, several studies have shown that certain intestinal bacteria may play a role in shaping cognitive networks encompassing emotional and social domains. A microbiota-gut-brain axis is known to exist, establishing several mechanisms by which microbiota may modulate brain development, function and behaviour, including immune, endocrine and neural pathways. As the aetiology of ASD is largely unknown, some studies have shown that intestinal bacteria may be involved in its pathogenesis. The aim of this review was to focus on the role of the gut-brain axis in ASD and, specifically, on its role in executive functions. First, we summarize the relationship between the gastrointestinal and cognitive symptoms of ASD patients. In addition, we highlight the evidence that supports and emphasizes the involvement of gut microbiota, and the putative underlying mechanisms in this population. Finally, we present evidence from preclinical and clinical studies on the modulation of microbiota and their effects on cognitive symptoms, specifically in relation to executive function. In conclusion, manipulation of microbiota could be a positive intervention to improve ASD symptoms. However, more research evaluating the role of microbiota in the cognitive symptoms ASD is needed.

The Possible Role of the Microbiota-Gut-Brain-Axis in Autism Spectrum Disorder

New research points to a possible link between autism spectrum disorder (ASD) and the gut microbiota as many autistic children have co-occurring gastrointestinal problems. This review focuses on specific alterations of gut microbiota mostly observed in autistic patients. Particularly, the mechanisms through which such alterations may trigger the production of the bacterial metabolites, or leaky gut in autistic people are described. Various altered metabolite levels were observed in the blood and urine of autistic children, many of which were of bacterial origin such as short chain fatty acids (SCFAs), indoles and lipopolysaccharides (LPS). A less integrative gut-blood-barrier is abundant in autistic individuals. This explains the leakage of bacterial metabolites into the patients, triggering new body responses or an altered metabolism. Some other co-occurring symptoms such as mitochondrial dysfunction, oxidative stress in cells, altered tight junctions in the blood-brain barrier and structural changes in the cortex, hippocampus, amygdala and cerebellum were also detected. Moreover, this paper suggests that ASD is associated with an unbalanced gut microbiota (dysbiosis). Although the cause-effect relationship between ASD and gut microbiota is not yet well established, the consumption of specific probiotics may represent a side-effect free tool to re-establish gut homeostasis and promote gut health. The diagnostic and therapeutic value of bacterial-derived compounds as new possible biomarkers, associated with perturbation in the phenylalanine metabolism, as well as potential therapeutic strategies will be discussed.

Role of Milk-Derived Opioid Peptides and Proline Dipeptidyl Peptidase-4 in Autism Spectrum Disorders

Opioid peptides released during digestion of dietary proteins such as casein, were suggested to contribute to autism development, leading to the announcement of opioid excess hypothesis of autism. This paper examines role of enzyme proline dipeptidyl peptidase-4 (DPPIV; EC 3.4.14.5) and it is exogenous substrate, β-casomorphin-7 (BCM7) in autism etiology. Our study included measurements of DPPIV and BCM7 concentrations in serum and urine, which were analyzed with ELISA assays and activity of DPPIV was measured by colorimetric test. The effect of opioid peptides from hydrolysed bovine milk on DPPIV gene expression in peripheral blood mononuclear cells (PBMC) in autistic and healthy children was determined using the Real-Time PCR (Polymerase Chain Reaction) method. Our research included 51 healthy children and 86 children diagnosed with autism spectrum disorder (ASD, ICDF84). We determined that the concentration of BCM7 in serum was significantly, 1.6-fold, higher in the ASD group than in controls (p < 0.0001). Concentration of DPPIV was found to also be significantly higher in serum from ASD children compared to the control group (p < 0.01), while we did not notice significant difference in enzymatic activity of serum DPPIV between the two study groups. We confirmed correlation according to the gender between analyzed parameters. The inspiration for this study emanated from clinical experience of the daily diet role in relieving the symptoms of autism. Despite this, we have concluded that milk-derived opioid peptides and DPPIV are potentially factors in determining the pathogenesis of autism; conducted studies are still limited and require further research.

Role of food-derived opioid peptides in the central nervous and gastrointestinal systems

Opioid receptors are widely distributed in central nervous system and peripheral tissues. Endogenous opioid receptor ligands are involved in many physiological processes. Exogenous peptides, derived from food proteins with gastrointestinal proteases, also exert opioid-like activities, and they include gluten exorphins (wheat), casomorphins (milk), rubiscolins (spinach), and soymorphins (soybean). Milk-derived opioid peptides play both agonistic and antagonistic roles, and most of the opioid peptides exert regulatory functions in the central nervous system, related to nociception, emotion and memory after oral, intracerebroventricular, or intraperitoneal administration. This indicates that the peptides may have crossed the blood-brain barrier or acted peripherally. Furthermore, some food-derived opioid peptides influence gastrointestinal functions such as gut motility, hormone release, appetite, mucus production, and local immunity. In healthy states, food-derived opioid peptides could benefit both the nervous and digestive systems, whereas in pathological conditions, the gastrointestinal permeability change and opioid excess may contribute to pathogenesis of some disorders. PRACTICAL APPLICATIONS: Opioid receptors are important biological targets for the treatment of multiple diseases. Traditional opiate compounds, such as alkaloids, are demonstrated to exert numerous side effects, thereby limiting their clinical effectiveness. It is thought that food-derived opioid peptides may be safer than the alkaloids, and therefore can be applied in functional food development. In this review, we summarized the already discovered food opioid peptides from different sources, and elaborated their physiological functions on the central nervous and gastrointestinal systems. These effects support further exploration of the opioid peptides as therapeutic agents or as functional food ingredient for human health promotion.

Gut-Brain Psychology: Rethinking Psychology From the Microbiota-Gut-Brain Axis

Mental disorders and neurological diseases are becoming a rapidly increasing medical burden. Although extensive studies have been conducted, the progress in developing effective therapies for these diseases has still been slow. The current dilemma reminds us that the human being is a superorganism. Only when we take the human self and its partner microbiota into consideration at the same time, can we better understand these diseases. Over the last few centuries, the partner microbiota has experienced tremendous change, much more than human genes, because of the modern transformations in diet, lifestyle, medical care, and so on, parallel to the modern epidemiological transition. Existing research indicates that gut microbiota plays an important role in this transition. According to gut-brain psychology, the gut microbiota is a crucial part of the gut-brain network, and it communicates with the brain via the microbiota-gut-brain axis. The gut microbiota almost develops synchronously with the gut-brain, brain, and mind. The gut microbiota influences various normal mental processes and mental phenomena, and is involved in the pathophysiology of numerous mental and neurological diseases. Targeting the microbiota in therapy for these diseases is a promising approach that is supported by three theories: the gut microbiota hypothesis, the "old friend" hypothesis, and the leaky gut theory. The effects of gut microbiota on the brain and behavior are fulfilled by the microbiota-gut-brain axis, which is mainly composed of the nervous pathway, endocrine pathway, and immune pathway. Undoubtedly, gut-brain psychology will bring great enhancement to psychology, neuroscience, and psychiatry. Various microbiota-improving methods including fecal microbiota transplantation, probiotics, prebiotics, a healthy diet, and healthy lifestyle have shown the capability to promote the function of the gut-brain, microbiota-gut-brain axis, and brain. It will be possible to harness the gut microbiota to improve brain and mental health and prevent and treat related diseases in the future.