Anorectic response to amino acid imbalance: a selective serotonin3 effect?

Effects of supplementing limiting amino acids on milk production in dairy cows consuming a corn grain and soybean meal-based diet

Limiting amino acids (AAs) for milk production in dairy cows fed on a concentrate diet of corn grain and soybean meal was evaluated in this study. Four lactating and multiparous Holstein cows (in third or fourth parities, with an average body weight of 633 ± 49.2 kg), 8 to 9 weeks into their lactation period, were used in a 4 × 4 Latin square design. The experiment comprised four dietary treatments: (1) no intravenous infusion (control); (2) control plus intravenous infusion of an AA mixture of 6 g/d methionine, 19.1 g/d lysine, 13.8 g/d isoleucine, and 15.4 g/d valine (4AA); (3) control plus intravenous infusion of the AA mixture without methionine (no-Met); and (4) control plus intravenous infusion of the AA mixture without lysine (no-Lys). All animals were fed on a controlled diet (1 kg/d alfalfa hay, 10 kg/d silage, 14 kg/d concentrate mixture, ad libitum timothy hay). The AA composition of the diet and blood were determined using an automatic AA analyzer. Milk composition (protein, fat, lactose, urea nitrogen, and somatic cell counts) was determined using a MilkoScan. The results showed that feed intake for milk production did not differ from that of intravenous infusion using a limiting AA mixture. The 4AA treatment numerically had the highest milk yield (32.4 kg/d), although there was no difference when compared with the control (31.2 kg/d), no-Met (31.3 kg/d), and no-Lys (31.7 kg/d) treatments. The concentration of AAs in blood plasma of cows in all treatments, mainly isoleucine and valine, increased significantly compared with that of control. The no-Met treatment increased (p < 0.05) the concentration of lysine in the blood relative to the control and no-Lys treatments, whereas the no-Lys treatment increased (p < 0.05) the concentration of methionine relative to the control and no-Met treatments. In conclusion, milk production increased when feeding 10 g/d methionine to the cows, together with their concentrate diet of corn grain and soybean meal.

Acute tryptophan depletion in humans: a review of theoretical, practical and ethical aspects

The acute tryptophan depletion (ATD) technique has been used extensively to study the effect of low serotonin in the human brain. This review assesses the validity of a number of published criticisms of the technique and a number of previously unpublished potential criticisms. The conclusion is that ATD can provide useful information when results are assessed in conjunction with results obtained using other techniques. The best-established conclusion is that low serotonin function after tryptophan depletion lowers mood in some people. However, this does not mean that other variables, altered after tryptophan depletion, are necessarily related to low serotonin. Each aspect of brain function has to be assessed separately. Furthermore, a negative tryptophan depletion study does not mean that low serotonin cannot influence the variable studied. This review suggests gaps in knowledge that need to be filled and guidelines for carrying out ATD studies.

5-HT(3) receptors: role in disease and target of drugs

Serotonin type 3 (5-HT(3)) receptors are pentameric ion channels belonging to the superfamily of Cys-loop receptors. Receptor activation either leads to fast excitatory responses or modulation of neurotransmitter release depending on their neuronal localisation. 5-HT(3) receptors are known to be expressed in the central nervous system in regions involved in the vomiting reflex, processing of pain, the reward system, cognition and anxiety control. In the periphery they are present on a variety of neurons and immune cells. 5-HT(3) receptors are known to be involved in emesis, pain disorders, drug addiction, psychiatric and GI disorders. Progress in molecular genetics gives direction to personalised medical strategies for treating complex diseases such as psychiatric and functional GI disorders and unravelling individual drug responses in pharmacogenetic approaches. Here we discuss the molecular basis of 5-HT(3) receptor diversity at the DNA and protein level, of which our knowledge has greatly extended in the last decade. We also evaluate their role in health and disease and describe specific case-control studies addressing the involvement of polymorphisms of 5-HT3 subunit genes in complex disorders and responses to drugs. Furthermore, we focus on the actual state of the pharmacological knowledge concerning not only classical 5-HT(3) antagonists--the setrons--but also compounds of various substance classes targeting 5-HT(3) receptors such as anaesthetics, opioids, cannabinoids, steroids, antidepressants and antipsychotics as well as natural compounds derived from plants. This shall point to alternative treatment options modulating the 5-HT(3) receptor system and open new possibilities for drug development in the future.

Functional variants of the serotonin receptor type 3A and B gene are associated with eating disorders

OBJECTIVE

As a key player in modulating both human physiological and behavioural functions including anxiety, perception and in particular appetite, serotonin (5-hydroxytryptamine, 5-HT) is likely to be involved in the aetiology of eating disorders. Studies showing serotonin receptor type 3 (5-HT3) receptors to mediate food intake depression (anorexic response) have triggered our interest in investigating the putative role of variants in the 5-HT3 receptor genes, HTR3A and HTR3B, in the susceptibility to anorexia nervosa (AN) and bulimia nervosa (BN).

METHODS

Two hundred and sixty-five patients with AN and 91 patients with BN as well as 191 healthy controls served as a pilot study group for mutational analysis by direct sequencing. Variants showing a significant association were subsequently genotyped in an independent Spanish cohort of 78 patients with AN and 119 patients with BN as well as 331 healthy controls for replication purposes.

RESULTS

In the pilot study, we found the coding HTR3B variant, p.Y129S, (rs1176744, P = 0.004, odds ratio = 2.06) to be associated with the restrictive subtype of AN. The association was confirmed in the Spanish study group (P = 0.034, odds ratio = 2.26).

CONCLUSION

Our study provides first evidence for an involvement of 5-HT3 variants in the aetiopathology of eating disorders in humans.

Aspectos psicobiológicos do comportamento alimentar

Esta revisão aborda alguns aspectos psicobiológicos ligados à manifestação do comportamento alimentar, e tem como objetivo evidenciar a relação entre os principais processos neuropsicológicos e a neurociência nutricional. Algumas estruturas neurais estão associadas ao controle alimentar por mecanismos distintos e correlatos que ocorrem no hipotálamo, hipocampo e em outras áreas como no cerebelo, bulbo olfatório, glândulas pituitária e pineal que exercem funções distintas, porém influênciam o comportamento alimentar, intermediadas geralmente por neurotransmissores comuns. Os precursores dos neuroquímicos apresentam funções específicas, sendo a influência na alimentação relevante no contexto comportamental da escolha de alimentos. Os processos sensoriais na alimentação como paladar, olfato, visão e audição interagem entre si e com outras estruturas e vias neurais, participando também do controle do apetite e da saciedade, que culminam na iniciação e no término da alimentação. A interação entre aspectos neurais no processo de consumo de alimento promove a manifestação do comportamento alimentar específico para cada espécie em seu ambiente.

Peptides that regulate food intake: somatostatin alters intake of amino acid-imbalanced diets and taste buds of tongue in rats

The present studies were designed to evaluate a potential dose-dependent effect of somatostatin (SRIF) administered peripherally on intake of either a low-protein basal diet or threonine-imbalanced diet (THR-IMB), on body weight gain (DeltaBW), gut motility, and on the histology of taste buds in rats. SRIF administration had a dual effect related to its concentration, increasing the intake of THR-IMB diet at low concentration and decreasing THR-IMB diet at high concentration. During the light phase, SRIF treatment increased the intake of THR-IMB diet, suggesting that the usual anorectic effect induced by intake of THR-IMB diet was attenuated. High-dosage SRIF decreases gastrointestinal motility, which, in turn, can decrease food intake and DeltaBW. The combination of THR-IMB diet regimen and SRIF treatment also induced significant modifications on the taste buds of the tongue. The feeding response to an amino acid-imbalanced diet includes a learned aversion to the diet, and animals may use taste in establishing that aversion. Modifications of taste buds of SRIF-treated rats eating THR-IMB diet might explain the increase of imbalanced diet intake if treated rats perceive this food as less aversive.

Dietary L-lysine deficiency increases stress-induced anxiety and fecal excretion in rats

Little is known about the psychobehavioral consequences of a dietary deficiency of the amino acid, L-lysine. This report demonstrates that a 4-d long L-lysine deficiency in rats interfered with the normal circadian release of the neurotransmitter serotonin, but not dopamine, measured by in vivo microdialysis in the central nucleus of the amygdala. L-Lysine deficiency was induced by feeding rats a L-lysine-deficient diet. Controls were pair-fed a L-lysine-sufficient diet. Footshock stress-induced anxiety, measured in an elevated plus-maze paradigm, and wrap-restraint stress-stimulated fecal excretion were significantly greater in the L-lysine-deficient rats than in the controls. We conclude that a severe deficiency of dietary L-lysine enhances serotonin release in the amygdala, with subsequent changes in psychobehavioral responses to stress.

Ondansetron attenuates CCK induced satiety and c-fos labeling in the dorsal medulla

Serotonin 5-HT(3) antagonists have been suggested for treatment of several disorders involving altered gastrointestinal (GI) function. CCK also has well documented GI actions on both food intake and vago-vagal reflexes. To evaluate potential interactions, the effect of a 5-HT(3) antagonist, ondansetron, on exogenous CCK induced satiety and c-fos activation was determined. Ondansetron reduced both actions of CCK by approximately 50%. The reduction in c-fos was localized to a specific subregion of the dorsal medulla, suggesting that a distinct subpopulation of CCK receptive fibers are modulated by 5-HT(3) ligands. Treatments using 5-HT(3) antagonists also may affect endogenous CCK functions.

Differential effects of selective vagotomy and tropisetron in aminoprivic feeding

Both total subdiaphragmatic vagotomy (TVAGX) and serotonin(3) receptor blockade with tropisetron or ondansetron attenuate amino acid-imbalanced diet (Imb) anorexia. Total vagotomy is less effective than tropisetron in reducing Imb-induced anorexia and also blunts the tropisetron effect. With the use of electrocautery at the subdiaphragmatic level of the vagus, we severed the ventral and dorsal trunks as well as the hepatic, ventral gastric, dorsal gastric, celiac, and accessory celiac branches separately or in combination to determine which vagal branches or associated structures may be involved in these responses. Rats were prefed a low-protein diet. On the first experimental day, tropisetron or saline was given intraperitoneally 1 h before presentation of Imb. Cuts including the ventral branch, i.e., TVAGX, ventral vagotomy (above the hepatic branch), and hepatic + gastric vagotomies (but not hepatic branch cuts alone) caused the highest (P < 0.05) Imb intake on day 1 with or without tropisetron. The responses to tropisetron were not affected significantly. On days 2-8, groups having vagotomies that included the hepatic branch recovered faster than sham-treated animals. Because the hepatic and gastric branches together account for most of the vagal innervation to the proximal duodenum, this area may be important in the initial responses, whereas structures served by the hepatic branch alone apparently act in the later adaptation to Imb.

Dorsomedial hypothalamic lesions alter intake of an imbalanced amino acid diet in rats

Within 3 h of ingesting an imbalanced amino acid diet (IAAD), rats show attenuated intake. The associated conditioned taste aversion can be ameliorated by giving the serotonin3 receptor blocker, tropisetron (TROP). A recent c-fos study indicated that the dorsomedial hypothalamic nucleus (DMN) may be activated 2-3 h after ingestion of IAAD. In Experiment 1, DMN-lesioned rats (DMNL) or sham-operated (SHAM) rats were injected with saline (SAL) or TROP just before introduction of IAAD. By 3 h, SAL-DMNL rats consumed more (P < 0.01) of the IAAD than did the SAL-SHAM rats. Thereafter, over the next 21 h, the intake of the SAL-DMNL group returned to control levels. TROP treatment enhanced the intake of the treated groups; the TROP and the lesion effect were additive (P < 0.01). By d 4 of receiving the IAAD, the DMNL groups were eating less than SHAM rats (P < 0.05). The data suggest that the DMN may be involved in the early detection of the amino acid deficiency induced by IAAD, is not involved in the TROP effect and is necessary for proper long-term adaptation to an IAAD.

Neuropeptide Y and somatostatin in the anterior piriform cortex alter intake of amino acid-deficient diets

Neuropeptides affect food intake via peripheral and brainstem mechanisms, but their roles in mediating feeding via the cerebral cortex have received little attention. The anterior piriform cortex (APC) appears to play a critical role in neuroperception of deficiencies of essential amino acids (AA) and the anorectic response to such deficiencies. The neural circuitry underlying the role of this paleocortex in these events is not understood. We have shown that neurons containing neuropeptide Y (NPY) and somatostatin (SOM) are cytoarchitecturally in positions to relate synaptically to the neurons of the APC which may mediate responses to AA. Thus, we hypothesized that NPY and SOM administered intracortically to the APC would directly affect food intake in a threonine-imbalanced model. We determined that NPY at 1-1.5 nmol decreased intake of the AA-deficient diet for 3 h, with a cumulative effect that extended through 6 h. SOM had a dual effect; at 1 pmol it increased intake of the AA-deficient diet for 3 h; at 2 nmol, SOM decreased intake of the AA-deficient diet for over 9 h, with a cumulative effect that persisted through 12 h. In the first 3 h, intake of animals receiving 1 pmol of SOM differed significantly from those receiving 2 nmol. These results suggest that NPY and SOM affect the cortical circuitry responsible for recognition of deficiencies in nutritionally essential AA, and that the timing of the cortical responses to the peptides may be related to the time course of the anorectic responses.