Simultaneous alterations of brain and plasma serotonin concentrations and liver cytochrome P450 in rats fed on a tryptophan-free diet

Impacts of essential amino acids on energy balance

BACKGROUND

Obesity develops due to an imbalance in energy homeostasis, wherein energy intake exceeds energy expenditure. Accumulating evidence shows that manipulations of dietary protein and their component amino acids affect the energy balance, resulting in changes in fat mass and body weight. Amino acids are not only the building blocks of proteins but also serve as signals regulating multiple biological pathways.

SCOPE OF REVIEW

We present the currently available evidence regarding the effects of dietary alterations of a single essential amino acid (EAA) on energy balance and relevant signaling mechanisms at both central and peripheral levels. We summarize the association between EAAs and obesity in humans and the clinical use of modifying the dietary EAA composition for therapeutic intervention in obesity. Finally, similar mechanisms underlying diets varying in protein levels and diets altered of a single EAA are described. The current review would expand our understanding of the contribution of protein and amino acids to energy balance control, thus helping discover novel therapeutic approaches for obesity and related diseases.

MAJOR CONCLUSIONS

Changes in circulating EAA levels, particularly increased branched-chain amino acids (BCAAs), have been reported in obese human and animal models. Alterations in dietary EAA intake result in improvements in fat and weight loss in rodents, and each has its distinct mechanism. For example, leucine deprivation increases energy expenditure, reduces food intake and fat mass, primarily through regulation of the general control nonderepressible 2 (GCN2) and mammalian target of rapamycin (mTOR) signaling. Methionine restriction by 80% decreases fat mass and body weight while developing hyperphagia, primarily through fibroblast growth factor 21 (FGF-21) signaling. Some effects of diets with different protein levels on energy homeostasis are mediated by similar mechanisms. However, reports on the effects and underlying mechanisms of dietary EAA imbalances on human body weight are few, and more investigations are needed in future.

The regulation of liver cytochrome P450 expression and activity by the brain serotonergic system in different experimental models

Introduction: Cytochrome P450 (CYP) metabolizes vital endogenous (steroids, vitamins) and exogenous (drugs, toxins) substrates. Studies of the last decade have revealed that the brain dopaminergic and noradrenergic systems are involved in the regulation of CYP. Recent research indicates that the brain serotonergic system is also engaged in its regulation.Areas covered: This review focuses on the role of the brain serotonergic system in the regulation of liver CYP expression. It shows the effect of lesion and activation of the serotonergic system after peripheral or intracerebral injections of neurotoxins, serotonin precursor, or serotonin (5-HT) receptor agonists. An opposite role of the hypothalamic paraventricular and arcuate nuclei and 5-HT receptors present therein in the regulation of CYP is described. The engagement of those nuclei in the neuroendocrine regulation of CYP by hypothalamic releasing or inhibiting hormones, pituitary hormones, and peripheral gland hormones are shown.Expert opinion: In general, the brain serotonergic system negatively regulates liver cytochrome P450. However, the effects of serotonergic agents on the enzyme expression depend on their mechanism of action, the route of administration (intracerebral/peripheral), as well as on local intracerebral site of injection and 5-HT receptor-subtypes present therein.

Albumin is a secret factor involved in multidirectional interactions among the serotoninergic, immune and endocrine systems that supervises the mechanism of CYP1A and CYP3A regulation in the liver

This review focuses on albumin, which is involved in multidirectional interactions among the immune, endocrine and serotoninergic systems and supervises the regulation of cytochrome P450 (CYP) isoforms under conditions of both normal liver function and liver insufficiency. Special attention is paid to albumin, thyroid hormones, testosterone and tryptophan hydroxylase in these interactions as well as their potential roles in liver regeneration. The association of these factors with inflammation and the modification of the mechanism of hepatic drug-metabolizing CYP isoform regulation are also presented because changes in the expression of CYP isoforms in the liver may result in subsequent changes to a marker substance used for testing CYP activity, thus providing a simple way to control the liver regeneration process or the dangerous stimulation of hepatocarcinogenesis.

Excess dietary tryptophan mitigates aflatoxicosis in growing quails

A biological assay was carried out to evaluate the impact of dietary tryptophan (TRP) in aflatoxin B₁ -contaminated diets (AFB₁ -D) on performance, blood parameters, immunity, meat quality and microbial populations of intestine in Japanese quails. Six experimental diets were formulated to include two levels of dietary TRP; 2.9 (moderate high: MH-TRP) and 4.9 g/kg (excess: Ex-TRP); and three levels of AFB₁ (0.0, 2.5, and 5.0 mg/kg). Each experimental diet was fed to the one of the six groups of birds from 7 to 35 days of age in a completely randomized design with 2 × 3 factorial arrangement. Decrease in feed intake, body weight gain and gain:feed in birds fed 5.0 mg/kg AFB₁ -D was restored to the control level by 4.9 g TRP/kg of the diet. The hepatic enzymes in blood were elevated in quails fed on AFB₁ -D but attenuated by 4.9 g TRP/kg of the diet (Ex-TRP; p ≤ .01). High serum uric acid in birds challenged with AFB₁ significantly decreased by Ex-TRP (p ≤ .01). The skin thickness to 2,4-dinitro-1-chlorobenzene challenge suppressed by AFB₁ but increased by Ex-TRP diet (p ≤ .02). The AFB₁ increased the malondialdehyde in meat, whereas TRP efficiently diminished malondialdehyde production (p ≤ .01). The greatest drip loss and pH in meat were observed in the birds fed 5.0 mg/kg AFB₁ -D but Ex-TRP augmented the adverse effects of AFB₁ (p ≤ .01). The Ex-TRP reduced the total microbial and Escherichia coli counts (p ≤ .01). The adverse effect of AFB₁ on ileal Lactic acid bacteria was completely prevented by Ex-TRP (p ≤ .03). This study showed that tryptophan supplementation could be considered as a powerful nutritional tool to ameliorate the adverse effects of AFB₁ in growing quails.

Altered cytokine profile under control of the serotonergic system determines the regulation of CYP2C11 and CYP3A isoforms

The aim of this study is to assess a potential mechanism by which the serotonergic system can control the expression and activity of cytochrome (CYP) 2C11 and CYP3A isoforms during liver insufficiency. A rat model of diethylnitrosamine (DEN)-induced liver insufficiency was developed by administering 50 mg/kg of DEN twice a week for 7 weeks. Dysfunction of the serotonergic system was evoked by feeding the rats with a tryptophan-free diet for three weeks. Dysfunction of the serotonergic system during liver insufficiency decreased the level of proinflammatory cytokines (TGF-β and IL-1β) and increased the level of an anti-inflammatory cytokine (IL-4). Simultaneously, activation of the repressive mechanism IL-4/JAK1/STAT6/SOCS1 of the JAK2/STAT5b-mediated signal transduction pathway and the pERK1/2/GR/STAT6 signal transduction pathway resulted in the suppression of the CYP2C11 and CYP3A isoforms. Moreover, dysfunction of the serotonergic system during liver insufficiency equalized the level of testosterone to the basal level, did not change the steady state of the corticosterone level and significantly enhanced the reduced level of growth hormone. An altered cytokine profile under control of the serotonergic system determines the regulation of CYP2C11 and CYP3A isoforms during liver insufficiency through mechanisms based on posttranscriptional and posttranslational processes.

The Effect of Chronic Treatment with Lurasidone on Rat Liver Cytochrome P450 Expression and Activity in the Chronic Mild Stress Model of Depression

Recent studies indicated an important role of the monoaminergic nervous systems (dopaminergic, noradrenergic, and serotonergic systems) and stress in the regulation of cytochrome P450 (CYP) expression and activity in the liver. The aim of our present research was to determine the effect of the novel atypical neuroleptic drug with antidepressant properties lurasidone, on the expression (mRNA and protein level) and activity of liver CYP isoforms involved in the metabolism of drugs and endogenous steroids, in the chronic mild stress (CMS) model of depression. Male Wistar rats were subjected to CMS for 7 weeks. Lurasidone (3 mg/kg per os per day) was administered to nonstressed or stressed animals for 5 weeks (weeks 3-7 of CMS). It has been found that 1) CMS moderately affects CYP (CYP2B, CYP2C11, and CYP3A), and its effects are different from those observed after other kinds of psychologic stress, such as repeated restraint stress or early-life maternal deprivation; 2) chronic lurasidone influences the expression and/or activity of CYP2B, CYP2C11, and CYP3A isoforms; and 3) CMS modifies the action of lurasidone on CYP expression and function, leading to different effects of the neuroleptic in nonstressed and stressed rats. Based on the obtained results, it can be suggested that the metabolism of endogenous substrates (e.g., steroids) and drugs, catalyzed by the isoforms CYP2B, CYP2C11, or CYP3A, may proceed at a different rate in the two groups of animals (nonstressed and stressed) in the rat CMS model.

Damage to the Brain Serotonergic System Increases the Expression of Liver Cytochrome P450

Genes coding for cytochrome P450 are regulated by endogenous hormones such as the growth hormone, corticosteroids, thyroid, and sex hormones. Secretion of these hormones is regulated by the respective hypothalamus-pituitary-secretory organ axes. Since the brain sends its serotonergic projections from the raphe nuclei to the hypothalamus, we have assumed that damage to these nuclei may affect the neuroendocrine regulation of cytochrome P450 expression in the liver. Thereby, 5,7-dihydroxytryptamine (5,7-DHT), a serotonergic neurotoxin, was injected into the dorsal and median raphe nuclei of male Wistar rats. Ten days after the neurotoxin injections, the brain concentrations of neurotransmitters, serum hormone, and cytokine levels, as well as the expression of cytochrome P450 in the liver were measured. Injection of 5,7-DHT decreased serotonin concentration in the brain followed by a significant rise in the levels of the growth hormone, corticosterone, and testosterone, and a drop in triiodothyronine concentration in the serum. No changes in interleukin (IL) levels (IL-2 and IL-6) were observed. Simultaneously, the activity and protein level of liver CYP1A, CYP3A1, and CYP2C11 rose (the activity of CYP2A/2B/2C6/2D was not significantly changed). Similarly, the mRNA levels of CYP1A1, CYP1A2, CYP2C11, and CYP3A1 were elevated. This is the first report demonstrating the effect of intracerebral administration of serotonergic neurotoxin on liver cytochrome P450. The obtained results indicate involvement of the brain serotonergic system in the neuroendocrine regulation of liver cytochrome P450 expression. The physiologic and pharmacological significance of the findings is discussed.

A study of blood serotonin and serotonin transporter promoter variant (5-HTTLPR) polymorphism in Egyptian autistic children

Background:

Autism spectrum disorder (ASD) is a complex, heterogeneous neurodevelopmental disorder with onset during early childhood. Most studies have reported an elevation in platelet serotonin in persons with autism. The serotonin (5-hydroxytryptamine; 5-HT) transporter in the brain uptakes 5-HT from extracellular spaces. It is also present in platelets, where it takes up 5-HT from plasma. Polymorphisms in serotonin transporter gene (SLC6A4) were frequently studied in many neuropsychiatric disorders.

Materials and Methods:

We have measured the plasma 5-HT levels in 20 autistic male children and 20 control male children by the enzyme-linked immunosorbent assay (ELISA) method. In addition, the SLC6A4 promoter region (5-HTTLPR) insertion/deletion (I/D) polymorphism was studied, using whole genomic DNA.

Results:

Plasma serotonin was significantly low in autistic children compared to control (P = 0.001), although correlation to severity of autism was not significant. The frequency of short (S) allele in autism cases was 10% and in the control group it was absent.

Conclusion:

Our study demonstrated an increased prevalence of 5-HTTLPR S allele in autism subjects. Significantly decreased plasma serotonin was detected in autism subjects, with no significant relationship between 5-HTTLPR genotype and plasma 5-HT being evident.

5-HT1A/1B receptors as targets for optimizing pigmentary responses in C57BL/6 mouse skin to stress

Stress has been reported to induce alterations of skin pigmentary response. Acute stress is associated with increased turnover of serotonin (5-hydroxytryptamine; 5-HT) whereas chronic stress causes a decrease. 5-HT receptors have been detected in pigment cells, indicating their role in skin pigmentation. To ascertain the precise role of 5-HT in stress-induced pigmentary responses, C57BL/6 mice were subjected to chronic restraint stress and chronic unpredictable mild stress (CRS and CUMS, two models of chronic stress) for 21 days, finally resulting in abnormal pigmentary responses. Subsequently, stressed mice were characterized by the absence of a black pigment in dorsal coat. The down-regulation of tyrosinase (TYR) and tyrosinase-related proteins (TRP1 and TRP2) expression in stressed skin was accompanied by reduced levels of 5-HT and decreased expression of 5-HT receptor (5-HTR) system. In both murine B16F10 melanoma cells and normal human melanocytes (NHMCs), 5-HT had a stimulatory effect on melanin production, dendricity and migration. When treated with 5-HT in cultured hair follicles (HFs), the increased expression of melanogenesis-related genes and the activation of 5-HT1A, 1B and 7 receptors also occurred. The serum obtained from stressed mice showed significantly decreased tyrosinase activity in NHMCs compared to that from nonstressed mice. The decrease in tyrosinase activity was further augmented in the presence of 5-HTR1A, 1B and 7 antagonists, WAY100635, SB216641 and SB269970. In vivo, stressed mice received 5-HT precursor 5-hydroxy-l-tryptophan (5-HTP), a member of the class of selective serotonin reuptake inhibitors (fluoxetine; FX) and 5-HTR1A/1B agonists (8-OH-DPAT/CP94253), finally contributing to the normalization of pigmentary responses. Taken together, these data strongly suggest that the serotoninergic system plays an important role in the regulation of stress-induced depigmentation, which can be mediated by 5-HT1A/1B receptors. 5-HT and 5-HTR1A/1B may constitute novel targets for therapy of skin hypopigmentation disorders, especially those worsened with stress.

The antidepressant-like action of mGlu5 receptor antagonist, MTEP, in the tail suspension test in mice is serotonin dependent

RATIONALE

Numerous studies indicate the potential antidepressant actions of several mGlu5 receptor antagonists, including 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP). The explanation for the mechanism of these effects might be a key step in finding new antidepressant drugs (AD).

OBJECTIVES

The aim of the present study was to investigate the possible role of the serotonergic system in the antidepressant-like activity of MTEP in the tail suspension test (TST) in C57BL/6J mice, using selected antagonists of serotonergic receptors and by applying two different methods of serotonin (5-HT) depletion.

RESULTS

The results of our studies showed that the mGlu5 receptor antagonist, MTEP, similar to the fluoxetine used as reference AD, did not induce antidepressant-like effects in mice pretreated with tryptophan hydroxylase inhibitor, parachlorophenylalanine. On the other hand, MTEP worked as a potential AD in the TST in mice fed on a tryptophan-free (TRP-free) diet for 3 weeks. However, fluoxetine, which was used as a reference control was also active in this experiment, suggesting that a TRP-free diet was not sufficiently effective in reducing the 5-HT level. Furthermore, we showed that the 5HT2A/2C antagonist, ritanserin, yet not the 5-HT1A antagonist, WAY100635, 5HT1B antagonist, SB224289 or 5HT4 antagonist, GR125487, reversed the antidepressant-like effects of MTEP in the TST. Finally, a sub-effective dose ofMTEP coadministered with a sub-effective dose of citalopram induced an antidepressant-like effect in the TST in mice.

CONCLUSION

The results of our studies suggest the involvement of serotonergic system activation in the antidepressant-like effects of the mGlu5 antagonist, MTEP, in the TST in mice.

Pharmacological potential of biogenic amine-polyamine interactions beyond neurotransmission

Histamine, serotonin and dopamine are biogenic amines involved in intercellular communication with multiple effects on human pathophysiology. They are products of two highly homologous enzymes, histidine decarboxylase and l-aromatic amino acid decarboxylase, and transmit their signals through different receptors and signal transduction mechanisms. Polyamines derived from ornithine (putrescine, spermidine and spermine) are mainly involved in intracellular effects related to cell proliferation and death mechanisms. This review summarizes structural and functional evidence for interactions between components of all these amine metabolic and signalling networks (decarboxylases, transporters, oxidases, receptors etc.) at cellular and tissue levels, distinct from nervous and neuroendocrine systems, where the crosstalk among these amine-related components can also have important pathophysiological consequences. The discussion highlights aspects that could help to predict and discuss the effects of intervention strategies.