5-Hydroxytryptophan rescues serotonin response to stress in prefrontal cortex of hyperphenylalaninaemic mice

New findings about neuropathological outcomes in the PKU mouse throughout lifespan

Phenylketonuria (PKU, OMIM 261600) is a genetic disorder caused by a deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH). If left untreated, PKU leads to systemic phenylalanine (Phe) accumulation, which can result in irreversible brain damage and intellectual disabilities. In the last 60 years, early and strict dietary restriction of phenylalanine (Phe) intake proved to prevent the severe clinical phenotype of untreated PKU. While the specific mechanisms through which phenylalanine causes brain damage are still poorly understood, preclinical models have been deeply explored to characterize the neurotoxic effect of Phe on neurodevelopmental processes. At the same time, that on the aging brain still needs to be explored. In the brain of untreated PAHEnu2(-/-) mouse, we previously reported a reduction of myelin basic protein (MBP) during postnatal development up to 60 PND. Later in the diseased mouse's life, a spontaneous and persistent restoration of MBP was detected. In this present longitudinal study, ranging from 14 to 540 post-natal days (PND) of untreated PAHEnu2(-/-) mice, we further investigated: a) the long-life consistency of two Phe-related brain metabolic alterations, such as large neutral amino acids (LNAA) and biogenic amine neurotransmitters' depletion; b) the outcome of locomotor functions during the same life span; c) the integrity of myelin as assessed ex vivo by central (hippocampus) and peripheral (extensor digitorum longus-sciatic nerve) action potential conduction velocities. In contrast with the results of other studies, brain Leu, Ile, and Val concentrations were not significantly altered in the brain PAHEnu2(-/-) mouse. On the other hand, 3-O-Methyldopa (3-OMD, a biomarker of L-DOPA), serotonin, and its associated metabolites were reduced throughout most of the considered time points, with consistent reductions observed prevalently from 14 to 60 PND. Normal saltatory conduction was restored after 60 PND and remained normal at the last examination at 360 PND, resulting nonetheless in a persistent locomotor impairment throughout a lifetime. These new findings contribute to laying the foundations for the preclinical characterization of aging in PKU, confirming neurotransmitter defects as consistent metabolic traits. LNAAs have a minor role, if any, in brain damage pathogenesis. Transient myelin synthesis failure may impact brain connectivity during postnatal development but not nervous signal conduction.

Behavioural and physiological responses to stressors in sheep with temperament classified by genotype or phenotype

The single nucleotide polymorphism (SNP) rs107856856, located in the tryptophan hydroxylase-2 gene, is associated with the behavioural phenotype for sheep temperament measured at weaning. Here, we tested the association between that SNP and physiological and behavioural responses to stressors in adult sheep. Two groups of adult sheep, one with genotype A/A (calm genotype) and the other with G/G (nervous genotype) in rs107856856, were selected from 160 sheep and were exposed, twice, to an open-field arena and an isolation box test (IBT). During each repeat, the behaviour and physiological responses (cortisol, prolactin, dehydroepiandrosterone [DHEA], brain derived neurotrophic factor [BDNF], characteristics of the response of body temperature, and oxidative stress) were measured. The behavioural and physiological responses of the sheep were compared between genotypes and also between groups classified on their phenotype as assessed by their initial isolation box score ("low responders" and "high responders"). The SNP rs107856856 had some effects on the behavioural phenotype (IBT score) but no effects on the physiological response to stress (cortisol, prolactin, DHEA, BDNF, oxidative stress or changes in body temperature) in the adult sheep, probably because the sheep were exposed, and therefore had adapted, to human contact during their life.

Phenylketonuria

Phenylketonuria (PKU; also known as phenylalanine hydroxylase (PAH) deficiency) is an autosomal recessive disorder of phenylalanine metabolism, in which especially high phenylalanine concentrations cause brain dysfunction. If untreated, this brain dysfunction results in severe intellectual disability, epilepsy and behavioural problems. The prevalence varies worldwide, with an average of about 1:10,000 newborns. Early diagnosis is based on newborn screening, and if treatment is started early and continued, intelligence is within normal limits with, on average, some suboptimal neurocognitive function. Dietary restriction of phenylalanine has been the mainstay of treatment for over 60 years and has been highly successful, although outcomes are still suboptimal and patients can find the treatment difficult to adhere to. Pharmacological treatments are available, such as tetrahydrobiopterin, which is effective in only a minority of patients (usually those with milder PKU), and pegylated phenylalanine ammonia lyase, which requires daily subcutaneous injections and causes adverse immune responses. Given the drawbacks of these approaches, other treatments are in development, such as mRNA and gene therapy. Even though PAH deficiency is the most common defect of amino acid metabolism in humans, brain dysfunction in individuals with PKU is still not well understood and further research is needed to facilitate development of pathophysiology-driven treatments.

Tetrahydrobiopterin treatment in phenylketonuria: A repurposing approach

In phenylketonuria (PKU) patients, early diagnosis by neonatal screening and immediate institution of a phenylalanine-restricted diet can prevent severe intellectual impairment. Nevertheless, outcome remains suboptimal in some patients asking for additional treatment strategies. Tetrahydrobiopterin (BH4 ) could be one of those treatment options, as it may not only increase residual phenylalanine hydroxylase activity in BH4 -responsive PKU patients, but possibly also directly improves neurocognitive functioning in both BH4 -responsive and BH4 -unresponsive PKU patients. In the present review, we aim to further define the theoretical working mechanisms by which BH4 might directly influence neurocognitive functioning in PKU having passed the blood-brain barrier. Further research should investigate which of these mechanisms are actually involved, and should contribute to the development of an optimal BH4 treatment regimen to directly improve neurocognitive functioning in PKU. Such possible repurposing approach of BH4 treatment in PKU may improve neuropsychological outcome and mental health in both BH4 -responsive and BH4 -unresponsive PKU patients.

Tetrahydrobiopterin treatment reduces brain L-Phe but only partially improves serotonin in hyperphenylalaninemic ENU1/2 mice

Hyperphenylalaninemia (HPA) caused by hepatic phenylalanine hydroxylase (PAH) deficiency has severe consequences on brain monoamine neurotransmitter metabolism. We have studied monoamine neurotransmitter status and the effect of tetrahydrobiopterin (BH4) treatment in Pahenu1/enu2 (ENU1/2) mice, a model of partial PAH deficiency. These mice exhibit elevated blood L-phenylalanine (L-Phe) concentrations similar to that of mild hyperphenylalaninemia (HPA), but brain levels of L-Phe are still ~5-fold elevated compared to wild-type. We found that brain L-tyrosine, L-tryptophan, BH4 cofactor and catecholamine concentrations, and brain tyrosine hydroxylase (TH) activity were normal in these mice but that brain serotonin, 5-hydroxyindolacetic acid (5HIAA) and 3-methoxy-4-hydroxyphenylglycol (MHPG) content, and brain TH protein, as well as tryptophan hydroxylase type 2 (TPH2) protein levels and activity were reduced in comparison to wild-type mice. Parenteral L-Phe loading conditions did not lead to significant changes in brain neurometabolite concentrations. Remarkably, enteral BH4 treatment, which normalized brain L-Phe levels in ENU1/2 mice, lead to only partial recovery of brain serotonin and 5HIAA concentrations. Furthermore, indirect evidence indicated that the GTP cyclohydrolase I (GTPCH) feedback regulatory protein (GFRP) complex may be a sensor for brain L-Phe elevation to ameliorate the toxic effects of HPA. We conclude that BH4 treatment of HPA toward systemic L-Phe lowering reverses elevated brain L-Phe content but the recovery of TPH2 protein and activity as well as serotonin levels is suboptimal, indicating that patients with mild HPA and mood problems (depression or anxiety) treated with the current diet may benefit from supplementation with BH4 and 5-OH-tryptophan.

A new therapy prevents intellectual disability in mouse with phenylketonuria

Untreated phenylketonuria (PKU) results in severe neurodevelopmental disorders, which can be partially prevented by an early and rigorous limitation of phenylalanine (Phe) intake. Enzyme substitution therapy with recombinant Anabaena variabilis Phe Ammonia Lyase (rAvPAL) proved to be effective in reducing blood Phe levels in preclinical and clinical studies of adults with PKU. Aims of present study were: a) to gather proofs of clinical efficacy of rAvPAL treatment in preventing neurological impairment in an early treated murine model of PKU; b) to test the advantages of an alternative delivering system for rAvPAL such as autologous erythrocytes. BTBR-Pah^enu2-/- mice were treated from 15 to 64 post-natal days with weekly infusions of erythrocytes loaded with rAvPAL. Behavioral, neurochemical, and brain histological markers denoting untreated PKU were examined in early treated adult mice in comparison with untreated and wild type animals. rAvPAL therapy normalized blood and brain Phe; prevented cognitive developmental failure, brain depletion of serotonin, dendritic spine abnormalities, and myelin basic protein reduction. No adverse events or inactivating immune reaction were observed. In conclusion present study testifies the clinical efficacy of rAvPAL treatment in a preclinical model of PKU and the advantages of erythrocytes as carrier of the enzyme in term of frequency of the administrations and prevention of immunological reactions.

Targeting mGlu5 Metabotropic Glutamate Receptors in the Treatment of Cognitive Dysfunction in a Mouse Model of Phenylketonuria

We studied group-I metabotropic glutamate (mGlu) receptors in Pah^enu2 (ENU2) mice, which mimic the genetics and neurobiology of human phenylketonuria (PKU), a metabolic disorder characterized, if untreated, by autism, and intellectual disability (ID). Male ENU2 mice showed increased mGlu5 receptor protein levels in the hippocampus and corpus striatum (but not in the prefrontal cortex) whereas the transcript of the mGlu5 receptor was unchanged. No changes in mGlu1 receptor mRNA and protein levels were found in any of the three brain regions of ENU2 mice. We extended the analysis to Homer proteins, which act as scaffolds by linking mGlu1 and mGlu5 receptors to effector proteins. Expression of the long isoforms of Homer was significantly reduced in the hippocampus of ENU2 mice, whereas levels of the short Homer isoform (Homer 1a) were unchanged. mGlu5 receptors were less associated to immunoprecipitated Homer in the hippocampus of ENU2 mice. The lack of mGlu5 receptor-mediated long-term depression (LTD) in wild-type mice (of BTBR strain) precluded the analysis of hippocampal synaptic plasticity in ENU2 mice. We therefore performed a behavioral analysis to examine whether pharmacological blockade of mGlu5 receptors could correct behavioral abnormalities in ENU2 mice. Using the same apparatus we sequentially assessed locomotor activity, object exploration, and spatial object recognition (spatial novelty test) after displacing some of the objects from their original position in the arena. Systemic treatment with the mGlu5 receptor antagonist, MPEP (20 mg/kg, i.p.), had a striking effect in the spatial novelty test by substantially increasing the time spent in exploring the displaced objects in ENU2 mice (but not in wild-type mice). These suggest a role for mGlu5 receptors in the pathophysiology of ID in PKU and suggest that, also in adult untreated animals, cognitive dysfunction may be improved by targeting these receptors with an appropriate therapy.

Unstable Maternal Environment Affects Stress Response in Adult Mice in a Genotype-Dependent Manner

Early postnatal events exert powerful effects on development, inducing persistent functional alterations in different brain network, such as the catecholamine prefrontal-accumbal system, and increasing the risk of developing psychiatric disorders later in life. However, a vast body of literature shows that the interaction between genetic factors and early environmental conditions is crucial for expression of psychopathologies in adulthood. We evaluated the long-lasting effects of a repeated cross-fostering (RCF) procedure in 2 inbred strains of mice (C57BL/6J, DBA/2), known to show a different susceptibility to the development and expression of stress-induced psychopathologies. Coping behavior (forced swimming test) and preference for a natural reinforcing stimulus (saccharine preference test) were assessed in adult female mice of both genotypes. Moreover, c-Fos stress-induced activity was assessed in different brain regions involved in stress response. In addition, we evaluated the enduring effects of RCF on catecholamine prefrontal-accumbal response to acute stress (restraint) using, for the first time, a new "dual probes" in vivo microdialysis procedure in mouse. RCF experience affects behavioral and neurochemical responses to acute stress in adulthood in opposite direction in the 2 genotypes, leading DBA mice toward an "anhedonic-like" phenotype and C57 mice toward an increased sensitivity for a natural reinforcing stimulus.

Early-onset behavioral and neurochemical deficits in the genetic mouse model of phenylketonuria

Phenylketonuria (PKU) is one of the most common human inborn errors of metabolism, caused by phenylalanine hydroxylase deficiency, leading to high phenylalanine and low tyrosine levels in blood and brain causing profound cognitive disability, if untreated. Since 1960, population is screened for hyperphenylalaninemia shortly after birth and submitted to early treatment in order to prevent the major manifestations of the disease. However, the dietetic regimen (phenylalanine free diet) is difficult to maintain, and despite the recommendation to a strict and lifelong compliance, up to 60% of adolescents partially or totally abandons the treatment. The development and the study of new treatments continue to be sought, taking advantage of preclinical models, the most used of which is the PAH^enu2 (BTBR ENU2), the genetic murine model of PKU. To date, adult behavioral and neurochemical alterations have been mainly investigated in ENU2 mice, whereas there are no clear indications about the onset of these deficiencies. Here we investigated and report, for the first time, a comprehensive behavioral and neurochemical assay of the developing ENU2 mice. Overall, our findings demonstrate that ENU2 mice are significantly smaller than WT until pnd 24, present a significant delay in the acquisition of tested developmental reflexes, impaired communicative, motor and social skills, and have early reduced biogenic amine levels in several brain areas. Our results extend the understanding of behavioral and cerebral abnormalities in PKU mice, providing instruments to an early preclinical evaluation of the effects of new treatments.

Short-term supplementation of isocaloric meals with L-tryptophan affects pig growth

L-Tryptophan (Trp) and some of its metabolites regulate the circadian rhythm in mammals. We aimed to investigate the effects of short-term supplementation of Trp in isocaloric meals on growth performance using the parameters of multiple blood biomarkers and free amino acids in growing pigs. A total of 32 Landrace × Yorkshire barrows with a mean body weight of 8.64 (±1.13) kg were randomly assigned to four groups and then fed with various concentrations of Trp diets daily. Our results showed that sequential supplementation of different concentrations of Trp in isocaloric meals decreased the feed:gain (F:G) ratio (P = 0.079) and plasma urea and albumin (P = 0.019), whereas the level of total protein did not. Among the essential and conditionally essential amino acids, the concentrations of histidine, isoleucine, proline, threonine, arginine, and valine in the plasma decreased (P < 0.05), whereas the concentrations of Trp, glycine, serine, and methionine increased (P < 0.01). In addition, concentrations of branched chain amino acids also significantly decreased (P = 0.004), while the rate of conversion of Trp to branched chain amino acids increased (P < 0.001). Taken together, we show that administration of a high concentration of Trp in breakfast with decreasing concentrations of Trp in lunch and dinner positively affected feed utilization and improved feed efficiency, at least in part, through the optimization of amino acid interconversions and nitrogen utilization.

Unbalance between Excitation and Inhibition in Phenylketonuria, a Genetic Metabolic Disease Associated with Autism

Phenylketonuria (PKU) is the most common genetic metabolic disease with a well-documented association with autism spectrum disorders. It is characterized by the deficiency of the phenylalanine hydroxylase activity, causing plasmatic hyperphenylalaninemia and variable neurological and cognitive impairments. Among the potential pathophysiological mechanisms implicated in autism spectrum disorders is the excitation/inhibition (E/I) imbalance which might result from alterations in excitatory/inhibitory synapse development, synaptic transmission and plasticity, downstream signalling pathways, and intrinsic neuronal excitability. Here, we investigated functional and molecular alterations in the prefrontal cortex (pFC) of BTBR-Pah^enu2 (ENU2) mice, the animal model of PKU. Our data show higher frequency of inhibitory transmissions and significant reduced frequency of excitatory transmissions in the PKU-affected mice in comparison to wild type. Moreover, in the pFC of ENU2 mice, we reported higher levels of the post-synaptic cell-adhesion proteins neuroligin1 and 2. Altogether, our data point toward an imbalance in the E/I neurotransmission favouring inhibition in the pFC of ENU2 mice, along with alterations of the molecular components involved in the organization of cortical synapse. In addition to being the first evidence of E/I imbalance within cortical areas of a mouse model of PKU, our study provides further evidence of E/I imbalance in animal models of pathology associated with autism spectrum disorders.

Impaired Neurotransmission in Early-treated Phenylketonuria Patients

Cerebral neurotransmitter (NT) deficiency has been suggested as a contributing factor in the pathophysiology of brain dysfunction in phenylketonuria (PKU), even in early-treated phenylketonuric patients. The study aimed to review dopamine and serotonin status in PKU, and the effect of the impaired neurotransmission. Several mechanisms are involved in the pathophysiology of PKU, primarily characterized by impaired dopamine and serotonin synthesis. These deficits are related to executive dysfunctions and social-emotional problems, respectively, in early treated patients. Blood phenylalanine is the main biomarker for treatment compliance follow-up, but further investigations and validation of peripheral biomarkers may be performed to monitor NT status. The development of new therapies is needed not only for decreasing blood and brain phenylalanine levels but also to improve NT syntheses.

5-Hydroxytryptophan, a precursor for serotonin synthesis, reduces seizure-induced respiratory arrest

OBJECTIVE

The DBA/1 mouse is a relevant animal model of sudden unexpected death in epilepsy (SUDEP), as it exhibits seizure-induced respiratory arrest (S-IRA) evoked by acoustic stimulation, followed by cardiac arrhythmia and death. Defects in serotonergic neurotransmission may contribute to S-IRA. The tryptophan hydroxylase-2 (TPH2) enzyme converts L-tryptophan to 5-hydroxytryptophan (5-HTP), a precursor for central nervous system (CNS) serotonin (5-HT) synthesis; and DBA/1 mice have a polymorphism that decreases TPH2 activity. We, therefore, hypothesized that supplementation with 5-HTP may bypass TPH2 and suppress S-IRA in DBA/1 mice.

METHODS

TPH2 expression was examined by Western blot in the brainstem of DBA/1 and C57BL/6J mice both with and without acoustic stimulation. Changes in breathing and cardiac electrical activity in DBA/1 and C57BL/6J mice that incurred sudden death during generalized seizures evoked by pentylenetetrazole (PTZ) were studied by plethysmography and electrocardiography. The effect of 5-HTP administration on seizure-induced mortality evoked by acoustic stimulation or by PTZ was investigated in DBA/1 mice.

RESULTS

Repetitive acoustic stimulation resulted in reduced TPH2 protein in the brainstem of DBA/1 mice as compared with C57BL/6J mice. S-IRA evoked by acoustic stimulation in DBA/1 mice was significantly reduced by 5-HTP. Following S-IRA, cardiac electrical activity could be detected for minutes before terminal asystole and death in both DBA/1 and C57BL/6J mice after PTZ treatment. The incidence of S-IRA by PTZ administration was greater in DBA/1 than in C57BL/6J mice, and administration of 5-HTP also significantly reduced S-IRA by PTZ in DBA/1 mice.

SIGNIFICANCE

Our data suggest that S-IRA is the primary event leading to death incurred in most DBA/1 and some C57BL/6J mice during PTZ-evoked seizures. Suppression of S-IRA by 5-HTP suggests that 5-HT transmission contributes to the pathophysiology of S-IRA, and that 5-HTP, an over-the-counter supplement available for human consumption, may be clinically useful in preventing SUDEP.

Gene Therapy for the Treatment of Neurological Disorders: Metabolic Disorders

Metabolic disorders comprise a large group of heterogeneous diseases ranging from very prevalent diseases such as diabetes mellitus to rare genetic disorders like Canavan Disease. Whether either of these diseases is amendable by gene therapy depends to a large degree on the knowledge of their pathomechanism, availability of the therapeutic gene, vector selection, and availability of suitable animal models. In this book chapter, we review three metabolic disorders of the central nervous system (CNS; Canavan Disease, Niemann-Pick disease and Phenylketonuria) to give examples for primary and secondary metabolic disorders of the brain and the attempts that have been made to use adeno-associated virus (AAV) based gene therapy for treatment. Finally, we highlight commonalities and obstacles in the development of gene therapy for metabolic disorders of the CNS exemplified by those three diseases.

High dose sapropterin dihydrochloride therapy improves monoamine neurotransmitter turnover in murine phenylketonuria (PKU)

Central nervous system (CNS) deficiencies of the monoamine neurotransmitters, dopamine and serotonin, have been implicated in the pathophysiology of neuropsychiatric dysfunction in phenylketonuria (PKU). Increased brain phenylalanine concentration likely competitively inhibits the activities of tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH), the rate limiting steps in dopamine and serotonin synthesis respectively. Tetrahydrobiopterin (BH4) is a required cofactor for TH and TPH activity. Our hypothesis was that treatment of hyperphenylalaninemic Pah(enu2/enu2) mice, a model of human PKU, with sapropterin dihydrochloride, a synthetic form of BH4, would stimulate TH and TPH activities leading to improved dopamine and serotonin synthesis despite persistently elevated brain phenylalanine. Sapropterin (20, 40, or 100mg/kg body weight in 1% ascorbic acid) was administered daily for 4 days by oral gavage to Pah(enu2/enu2) mice followed by measurement of brain biopterin, phenylalanine, tyrosine, tryptophan and monoamine neurotransmitter content. A significant increase in brain biopterin content was detected only in mice that had received the highest sapropterin dose, 100mg/kg. Blood and brain phenylalanine concentrations were unchanged by sapropterin therapy. Sapropterin therapy also did not alter the absolute amounts of dopamine and serotonin in brain but was associated with increased homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA), dopamine and serotonin metabolites respectively, in both wild type and Pah(enu2/enu2) mice. Oral sapropterin therapy likely does not directly affect central nervous system monoamine synthesis in either wild type or hyperphenylalaninemic mice but may stimulate synaptic neurotransmitter release and subsequent metabolism.

Large Neutral Amino Acid Supplementation Exerts Its Effect through Three Synergistic Mechanisms: Proof of Principle in Phenylketonuria Mice

Background

Phenylketonuria (PKU) was the first disorder in which severe neurocognitive dysfunction could be prevented by dietary treatment. However, despite this effect, neuropsychological outcome in PKU still remains suboptimal and the phenylalanine-restricted diet is very demanding. To improve neuropsychological outcome and relieve the dietary restrictions for PKU patients, supplementation of large neutral amino acids (LNAA) is suggested as alternative treatment strategy that might correct all brain biochemical disturbances caused by high blood phenylalanine, and thereby improve neurocognitive functioning.

Objective

As a proof-of-principle, this study aimed to investigate all hypothesized biochemical treatment objectives of LNAA supplementation (normalizing brain phenylalanine, non-phenylalanine LNAA, and monoaminergic neurotransmitter concentrations) in PKU mice.

Methods

C57Bl/6 Pah-enu2 (PKU) mice and wild-type mice received a LNAA supplemented diet, an isonitrogenic/isocaloric high-protein control diet, or normal chow. After six weeks of dietary treatment, blood and brain amino acid and monoaminergic neurotransmitter concentrations were assessed.

Results

In PKU mice, the investigated LNAA supplementation regimen significantly reduced blood and brain phenylalanine concentrations by 33% and 26%, respectively, compared to normal chow (p<0.01), while alleviating brain deficiencies of some but not all supplemented LNAA. Moreover, LNAA supplementation in PKU mice significantly increased brain serotonin and norepinephrine concentrations from 35% to 71% and from 57% to 86% of wild-type concentrations (p<0.01), respectively, but not brain dopamine concentrations (p = 0.307).

Conclusions

This study shows that LNAA supplementation without dietary phenylalanine restriction in PKU mice improves brain biochemistry through all three hypothesized biochemical mechanisms. Thereby, these data provide proof-of-concept for LNAA supplementation as a valuable alternative dietary treatment strategy in PKU. Based on these results, LNAA treatment should be further optimized for clinical application with regard to the composition and dose of the LNAA supplement, taking into account all three working mechanisms of LNAA treatment.

Strain-dependent variations in stress coping behavior are mediated by a 5-HT/GABA interaction within the prefrontal corticolimbic system

BACKGROUND

Serotonin and γ-aminobutyric acid (GABA) transmission is crucial in coping strategies.

METHODS

Here, using mice from 2 inbred strains widely exploited in behavioral neurochemistry, we investigated whether serotonin transmission in medial prefrontal cortex and GABA in basolateral amygdala determine strain-dependent liability to stress response and differences in coping.

RESULTS

C57BL/6J mice displayed greater immobility in the forced swimming test, higher serotonin outflow in medial prefrontal cortex, higher GABA outflow in basolateral amygdala induced by stress, and higher serotonin 1A receptor levels in medial prefrontal cortex accompanied by lower GABAb receptor levels in basolateral amygdala than DBA/2J mice. In assessing whether serotonin in medial prefrontal cortex determines GABA functioning in response to stress and passive coping behavior in C57BL/6J and DBA/2J mice, we observed that selective prefrontal serotonin depletion in C57BL/6J and DBA/2J reduced stress-induced GABA outflow in basolateral amygdala and immobility in the forced swimming test.

CONCLUSIONS

These results show that strain-dependent prefrontal corticolimbic serotonin/GABA regulation determines the strain differences in stress-coping behavior in the forced swimming test and point to a role of a specific neuronal system in genetic susceptibility to stress that opens up new prospects for innovative therapies for stress disorders.

Pharmacologic inhibition of L-tyrosine degradation ameliorates cerebral dopamine deficiency in murine phenylketonuria (PKU)

Monoamine neurotransmitter deficiency has been implicated in the etiology of neuropsychiatric symptoms associated with chronic hyperphenylalaninemia in phenylketonuria (PKU). Two proposed explanations for neurotransmitter deficiency in PKU include first, that chronically elevated blood L-phenylalanine (Phe) inhibits the transport of L-tyrosine (Tyr) and L-tryptophan (Trp), the substrates for dopamine and serotonin synthesis respectively, into brain. In the second hypothesis, elevated Phe competitively inhibits brain tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) activities, the rate limiting steps in dopamine and serotonin synthesis. Dietary supplementation with large neutral amino acids (LNAA) including Tyr and Trp has been recommended for individuals with chronically elevated blood Phe in an attempt to restore amino acid and monoamine homeostasis in brain. As a potential alternative treatment approach, we demonstrate that pharmacologic inhibition of Tyr degradation through oral administration of nitisinone (NTBC) yielded sustained increases in blood and brain Tyr, decreased blood and brain Phe, and consequently increased dopamine synthesis in a murine model of PKU. Our results suggest that Phe-mediated inhibition of TH activity is the likely mechanism of impaired dopamine synthesis in PKU. Pharmacologic inhibition of Tyr degradation may be a promising adjunct therapy for CNS monoamine neurotransmitter deficiency in hyperphenylalaninemic individuals with PKU.

Chronic Effect of Aspartame on Ionic Homeostasis and Monoamine Neurotransmitters in the Rat Brain

Aspartame is one of the most widely used artificial sweeteners globally. Data concerning acute neurotoxicity of aspartame is controversial, and knowledge on its chronic effect is limited. In the current study, we investigated the chronic effects of aspartame on ionic homeostasis and regional monoamine neurotransmitter concentrations in the brain. Our results showed that aspartame at high dose caused a disturbance in ionic homeostasis and induced apoptosis in the brain. We also investigated the effects of aspartame on brain regional monoamine synthesis, and the results revealed that there was a significant decrease of dopamine in corpus striatum and cerebral cortex and of serotonin in corpus striatum. Moreover, aspartame treatment significantly alters the tyrosine hydroxylase activity and amino acids levels in the brain. Our data suggest that chronic use of aspartame may affect electrolyte homeostasis and monoamine neurotransmitter synthesis dose dependently, and this might have a possible effect on cognitive functions.

Impact of stress on prefrontal glutamatergic, monoaminergic and cannabinoid systems

Stress has been shown to have marked and divergent effects on learning and memory which involves specific brain regions, such as spatial and declarative memory involving the hippocampus, memory of emotional arousing experiences and fear involving the amygdala, and executive functions and fear extinction involving the prefrontal cortex or the PFC. Response to stress involves a coordinated activation of a constellation of physiological systems including the activation of the hypothalamic-pituitary-adrenal (HPA) axis and other modulatory neurotransmitters and signaling systems. This paper presents a concise review of the effects of stress and glucocorticoids on the glutamatergic and monoaminergic (including noradrenergic, dopaminergic, and serotonergic systems) neurotransmitter systems as well as endocannabinoid signaling. Because of the breadth of the scope of this topic, the review is limited to the effects of stress on these brain systems on the prefrontal cortex, and where relevant, the hippocampus and the amygdala.

Behavioral and neurochemical characterization of new mouse model of hyperphenylalaninemia

Hyperphenylalaninemia (HPA) refers to all clinical conditions characterized by increased amounts of phenylalanine (PHE) in blood and other tissues. According to their blood PHE concentrations under a free diet, hyperphenylalaninemic patients are commonly classified into phenotypic subtypes: classical phenylketonuria (PKU) (PHE > 1200 µM/L), mild PKU (PHE 600-1200 µM/L) and persistent HPA (PHE 120-600 µM/L) (normal blood PHE < 120 µM/L). The current treatment for hyperphenylalaninemic patients is aimed to keep blood PHE levels within the safe range of 120-360 µM/L through a PHE-restricted diet, difficult to achieve. If untreated, classical PKU presents variable neurological and mental impairment. However, even mildly elevated blood PHE levels, due to a bad compliance to dietary treatment, produce cognitive deficits involving the prefrontal cortical areas, extremely sensible to PHE-induced disturbances. The development of animal models of different degrees of HPA is a useful tool for identifying the metabolic mechanisms underlying cognitive deficits induced by PHE. In this paper we analyzed the behavioral and biochemical phenotypes of different forms of HPA (control, mild-HPA, mild-PKU and classic-PKU), developed on the base of plasma PHE concentrations. Our results demonstrated that mice with different forms of HPA present different phenotypes, characterized by increasing severity of behavioral symptoms and brain aminergic deficits moving from mild HPA to classical PKU forms. In addition, our data identify preFrontal cortex and amygdala as the most affected brain areas and confirm the highest susceptibility of brain serotonin metabolism to mildly elevated blood PHE.

Prefrontal/amygdalar system determines stress coping behavior through 5-HT/GABA connection

Coping is defined as the behavioral and physiological effort made to master stressful situations. The ability to cope with stress leads either to healthy or to pathogenic outcomes. The medial prefrontal cortex (mpFC) and amygdala are acknowledged as having a major role in stress-related behaviors, and mpFC has a critical role in the regulation of amygdala-mediated arousal in response to emotionally salient stimuli. Prefrontal cortical serotonin (5-hydroxytryptamine (5-HT)) is involved in corticolimbic circuitry, and GABA has a major role in amygdala functioning. Here, using mice, it was assessed whether amygdalar GABA regulation by prefrontal 5-HT is involved in processing stressful experiences and in determining coping outcomes. First (experiment 1), bilateral selective 5-HT depletion in mpFC of mice reduced GABA release induced by stress in basolateral amygdala (BLA) and passive coping in the Forced Swimming Test (FST) (experiment 2). Moreover, prefrontal-amygdala disconnection procedure that combined a selective unilateral 5-HT depletion of mpFC and infusion of an inhibitor of GABA synthesis into the contralateral BLA, thereby to disrupt prefrontal-amygdalar serial connectivity bilaterally, showed that disconnection selectively decreases immobility in the FST. These results point to prefrontal/amygdala connectivity mediated by 5-HT and GABA transmission as a critical neural mechanism in stress-induced behavior.

The effects of sapropterin on urinary monoamine metabolites in phenylketonuria

BACKGROUND

Sapropterin dihydrochloride (BH4, tetrahydrobiopterin) can lower plasma phenylalanine (Phe) concentrations for a subset of patients with phenylketonuria (PKU), an inborn error of metabolism. Studies suggest that monoamine neurotransmitter concentrations are low in PKU patients. Sapropterin functions as a cofactor for hydroxylases specific to Phe, tyrosine, and tryptophan metabolism, pathways essential for catecholamine and serotonin synthesis.

OBJECTIVE

The objective of this study is to determine the impact of sapropterin on monoamine neurotransmitter status in patients with PKU.

DESIGN

58 PKU subjects were provided 20 mg/kg of sapropterin for 1 month. Those who responded with at least a 15% decrease in plasma Phe received sapropterin for 1 year, while Non-responders discontinued it. After an additional 3 months, Responders who demonstrated increased Phe tolerance and decreased medical food dependence were classified as Definitive, whereas Responders unable to liberalize their diet without compromising plasma Phe control were identified as Provisional. At study visits, patients provided blood for plasma amino acids, 3-day diet records, and 12-hour urine samples analyzed for epinephrine (E), dopamine (DA), dihydroxyphenylacetate (DOPAC), homovanillic acid (HVA), 3-methoxytyramine (3MT), serotonin (5HT), and 5-hydroxyindole acetic acid (5HIAA) using HPLC with electrochemical detection.

RESULTS

Compared with healthy non-PKU controls, subjects with PKU had significantly lower baseline concentrations of DA, HVA, 3MT, 5HT, and 5HIAA (p < 0.001 for all). Medical food protein intake had a direct association with DA, HVA, 5HT, and 5HIAA during the study (p < 0.05 for all), while plasma Phe had an inverse association with these markers (p < 0.01 for all). DOPAC was also associated with plasma Phe throughout the year (p = 0.035), although not at baseline. Patients with PKU had a significant increase in HVA (p = 0.015) after 1 month of sapropterin. When stratifying by Responder and Non-Responder status, significance of HVA increase in Non-responders (p = 0.041) was confirmed, but not in Responders (p = 0.081). A declining trend in urinary 5HIAA, significant only after controlling for plasma Phe (p = 0.019), occurred for Definitive Responders during the 1-year study.

CONCLUSION

Urinary monoamine concentrations are low in patients with PKU and are influenced by oral sapropterin and medical food intake, highlighting the importance of these therapies to neurotransmitter metabolism in phenylketonuria.

In vivo catecholaminergic metabolism in the medial prefrontal cortex of ENU2 mice: an investigation of the cortical dopamine deficit in phenylketonuria

OBJECTIVE

Phenylketonuria (PKU) is an inherited metabolic disease characterized by plasma hyperphenylalaninemia and several neurological symptoms that can be controlled by rigorous dietetic treatment. The cellular mechanisms underlying impaired brain functions are still unclear. It has been proposed, however, that phenylalanine interference in cognitive functions depends on impaired dopamine (DA) transmission in the prefrontal cortical area due to reduced availability of the precursor tyrosine. Here, using Pah(enu2) (ENU2) mice, the genetic murine model of PKU, we investigated all metabolic steps of catecholamine neurotransmission within the medial preFrontal Cortex (mpFC), availability of the precursor tyrosine, synthesis and release, to find an easy way to reinstate normal cortical DA neurotransmission.

METHODS AND RESULTS

Analysis of blood and brain levels of tyrosine showed reduced plasma and cerebral levels of tyrosine in ENU2 mice. Western blot analysis demonstrated deficient tyrosine hydroxylase (TH) protein levels in mpFC of ENU2 mice. Cortical TH activity, determined in vivo by measuring the accumulation of l-3,4-dihydroxyphenylalanine (L-DOPA) in mpFC after inhibition of L-aromatic acid decarboxylase with NSD-1015, was reduced in ENU2 mice. Finally, a very low dose of L-DOPA, which bypasses the phenylalanine-inhibited metabolic steps, restored DA prefrontal transmission to levels found in healthy mice.

CONCLUSION

The data suggests that a strategy of using tyrosine supplementation to treat PKU is unlikely to be effective, whereas small dose L-DOPA administration is likely to have a positive therapeutic effect.

Postnatal aversive experience impairs sensitivity to natural rewards and increases susceptibility to negative events in adult life

Evidence shows that maternal care and postnatal traumatic events can exert powerful effects on brain circuitry development but little is known about the impact of early postnatal experiences on processing of rewarding and aversive stimuli related to the medial prefrontal cortex (mpFC) function in adult life. In this study, the unstable maternal environment induced by repeated cross-fostering (RCF) impaired palatable food conditioned place preference and disrupted the natural preference for sweetened fluids in the saccharin preference test. By contrast, RCF increased sensitivity to conditioned place aversion (CPA) and enhanced immobility in the forced swimming test. Intracerebral microdialysis data showed that the RCF prevents mpFC dopamine (DA) outflow regardless of exposure to rewarding or aversive stimuli, whereas it induces a strong and sustained prefrontal norepinephrine (NE) release in response to different aversive experiences. Moreover, the selective mpFC NE depletion abolished CPA, thus indicating that prefrontal NE is required for motivational salience attribution to aversion-related stimuli. These findings demonstrate that an unstable maternal environment impairs the natural propensity to seek pleasurable sources of reward, enhances sensitivity to negative events in adult life, blunts prefrontal DA outflow, and modulates NE release in the reverse manner depending on the exposure to rewarding or aversive stimuli.

5-Hydroxytryptophan during critical postnatal period improves cognitive performances and promotes dendritic spine maturation in genetic mouse model of phenylketonuria

Although phenylketonuria (PKU) is the most common genetic cause of mental retardation, the cellular mechanisms underlying impaired brain function are still unclear. Using PAHenu2 mice (ENU2), the genetic mouse model of PKU, we previously demonstrated that high phenylalanine levels interfere with brain tryptophan hydroxylase activity by reducing the availability of serotonin (5-hydroxytryptamine, 5-HT), crucial for maturation of neuronal connectivity in the prefrontal cortex (PFC), around the third postnatal week, a critical period for cortical maturation. 5-Hydroxytryptophan (5-HTP), the product of tryptophan hydroxylation, is known to be a better treatment to increase brain 5-HT levels. In this study we investigated the role of 5-HT during the early postnatal period in cognitive disturbances and in cortical dendritic alterations of PKU subjects by restoring temporarily (postnatal days 14-21) physiological brain levels of 5-HT in ENU2 through 5-HTP treatment. In adult ENU2 mice early 5-HTP treatment reverses cognitive deficits in spatial and object recognition tests accompanied by an increase in spine maturation of pyramidal neurons in layer V of the prelimbic/infralimbic area of the PFC, although locomotor deficits are not recovered by treatment. Taken together, our results support the hypothesis that mental retardation in PKU depends on reduced availability of brain 5-HT during critical developmental periods that interferes with cortical maturation and point to 5-HTP supplementation as a highly promising additional tool to heal PKU patients.

Large neutral amino acids in the treatment of PKU: from theory to practice

Notwithstanding the success of the traditional dietary phenylalanine restriction treatment in phenylketonuria (PKU), the use of large neutral amino acid (LNAA) supplementation rather than phenylalanine restriction has been suggested. This treatment modality deserves attention as it might improve cognitive outcome and quality of life in patients with PKU. Following various theories about the pathogenesis of cognitive dysfunction in PKU, LNAA supplementation may have multiple treatment targets: a specific reduction in brain phenylalanine concentrations, a reduction in blood (and consequently brain) phenylalanine concentrations, an increase in brain neurotransmitter concentrations, and an increase in brain essential amino acid concentrations. These treatment targets imply different treatment regimes. This review summarizes the treatment targets and the treatment regimens of LNAA supplementation and discusses the differences in LNAA intake between the classical dietary phenylalanine-restricted diet and several LNAA treatment forms.