Norepinephrine and amino acids in prepyriform cortex of rats fed imbalanced amino acid diets

Effects of rumen-encapsulated methionine and lysine supplementation and low dietary protein on nitrogen efficiency and lactation performance of dairy cows

Low crude protein (CP) diets might be fed to dairy cows without affecting productivity if the balance of absorbed AA were improved, which would decrease the environmental effect of dairy farms. The aim of this study was to investigate the effects of supplementing ruminally protected Lys (RPL) and Met (RPM) at 2 levels of dietary CP on nutrient intake, milk production, milk composition, milk N efficiency (MNE), and plasma concentrations of AA in lactating Holstein cows and to evaluate these effects against the predictions of the new NASEM (2021) model. Fifteen multiparous cows were used in a replicated 3 × 3 Latin square design with 21-d periods. The 3 treatments were (1) a high-protein (HP) basal diet containing 16.4% CP (metabolizable protein [MP] balance of -130 g/d; 95% of target values), (2) a medium-protein diet containing 15% CP plus RPL (60 g/cow per day) and RPM (25 g/cow per day; MPLM; MP balance of -314 g/d; 87% of target values), and (3) a low-protein diet containing 13.6% CP plus RPL (60 g/cow per day) and RPM (25 g/cow per day; LPLM; MP balance of -479 g/d; 80% of target values). Dry matter intake was less for cows fed MPLM and LPLM diets compared with those fed the HP diet. Compared with the HP diet, the intake of CP, neutral detergent fiber, acid detergent fiber, and organic matter, but not starch, was lower for cows fed MPLM and LPLM diets. Milk production and composition were not affected by MPLM or LPLM diets relative to the HP diet. Milk urea N concentrations were reduced for the MPLM and LPLM diets compared with the HP diet, indicating that providing a low-protein diet supplemented with rumen-protected AA led to greater N efficiency. There was no significant effect of treatment on plasma AA concentrations except for proline, which significantly increased for the MPLM treatment compared with the other 2 treatments. Overall, the results supported the concept that milk performance might be maintained when feeding lactating dairy cows with low CP diets if the absorbed AA balance is maintained through RPL and RPM feeding. Further investigations are needed to evaluate responses over a longer time period with consideration of all AA rather than on the more aggregated MP and the ratio between Lys and Met.

Feasibility of Supplying Ruminally Protected Lysine and Methionine to Periparturient Dairy Cows on the Efficiency of Subsequent Lactation

The objective of this study was to evaluate the effects of supplying ruminally protected Lys (RPL) and ruminally protected Met (RPM) to transition cows' diets on the efficiency of subsequent lactation. A total of 120 prepartum Holstein cows were assigned into four treatments blocked by the anticipated calving date, previous lactation milk yield, number of lactations, and body condition score and fed either RPL, RPM, or the combination (RPML) or control diet (CON) throughout the transition period (3 weeks before till 3 weeks after calving). From 22 to 150 days in milk (DIM), all animals (100 cows) were fed a combination of RPM and RPL (0.17% RPM and 0.41% RPL of DM; n = 25 cows/treatment) as follows; CON–RPML, RPM–RPML, RPL–RPML, and RPML–RPML. Milk production and dry matter intake (DMI) were measured daily; milk and blood samples were taken at 21, 30, 60, 90, 120, and 150 DIM. Supplemented amino acids (AA) were mixed with the premix and added to the total mixed ration during the experiment. DMI (p < 0.001) and energy-corrected milk (ECM, p = 0.04) were higher for cows that were fed RPML–RPML than other cows. Compared with CON–RPML, yields of milk total protein, lactose, and nitrogen efficiency were increased (p < 0.01), whereas milk urea nitrogen (MUN; p = 0.002) was decreased for other treatments. However, supplemental AA did not affect milk lactose percentage, fat yield, feed efficiency, or serum total protein concentration (p > 0.10). Transition cows that consumed AA had a greater peak of milk yield (p < 0.01), as well as quickly reached the peak of milk (p < 0.004). There were differences in β-hydroxybutyrate concentration during the early lactation, with a lower level for AA groups (p < 0.05), and the difference faded with the progression of lactation (p > 0.10). Fertility efficiency as measured by pregnancy rate was improved by supplemental AA during the perinatal period (p < 0.05). In conclusion, transition cows consumed RPM and RPL, increased post-calving DMI, milk production, milk protein yield, nitrogen efficiency, and improved fertility performance.

Supplementing Ruminally Protected Lysine, Methionine, or Combination Improved Milk Production in Transition Dairy Cows

The objectives of this study were to evaluate the effects of dietary supplementation of ruminally protected lysine (RPL), or methionine (RPM), and their combination (RPML) on the production efficiency of transition cows. A total of 120 pre-partum multiparous Holstein cows were assigned to four treatments based on previous lactation milk production, days (d) of pregnancy, lactation, and body condition score (BCS). Cows were fed a basal diet [pre-calving: 1.53 Mcal/kg dry matter (DM) and post-calving: 1.70 Mcal/kg DM] with or without supplemental ruminally protected amino acids (RPAA). Treatments were the basal diets without supplemental amino acids (CONTROL, n = 30), with supplemental methionine (RPM, pre-calving at 0.16% of DM and post-calving at 0.12% of DM, n = 30), with supplemental lysine (RPL, pre-calving at 0.33% of DM and post-calving at 0.24% DM, n = 30), and the combination (RPML, pre-calving at 0.16% RPM + 0.33% RPL of DM and post-calving at 0.12% RPM + 0.24 % RPL DM, n = 30). The dietary content of lysine was balanced to be within 6.157.2% metabolizable protein (MP)-lysine and that of methionine was balanced within 2.1-2.35% MP-methionine. Dry matter intake (DMI) was measured daily. Milk samples were taken on d 7, 14, and 21 days relative to calving (DRC), and milk yields were measured daily. Blood samples were taken on d -21, -14, -7 before expected calving and d 0, 7, 14, and 21 DRC. Data were analyzed using SAS software. There were significant Trt × time interactions (P < 0.01) for DMI pre- and post-calving period. The CON cows had lower DMI than RPM, RPL, and RPML, both pre-calving (P < 0.01) and post-calving periods (P < 0.01). Energy-corrected milk (P < 0.01), milk fat (P < 0.01), protein (P = 0.02), and lactose (P < 0.01) percentage levels were greater for RPM, RPL, and RPML cows compared to CON. Supplementing RPAA assisted in maintaining BCS post-calving than CON (P < 0.01). Blood concentrations of β-hydroxybutyrate decreased with RPM or RPL or the combination pre-calving (P < 0.01) and tended to decrease post-calving (P = 0.10). These results demonstrated that feeding RPL and RPM improved DMI and milk production efficiency, maintained BCS, and reduced β-hydroxybutyrate concentrations of transition cows.

Indispensable Amino Acid-Deficient Diets Induce Seizures in Ketogenic Diet-Fed Rodents, Demonstrating a Role for Amino Acid Balance in Dietary Treatments for Epilepsy

Background

Low protein amounts are used in ketogenic diets (KDs), where an essential (indispensable) amino acid (IAA) can become limiting. Because the chemically sensitive, seizurogenic, anterior piriform cortex (APC) is excited by IAA limitation, an imbalanced KD could exacerbate seizure activity.

Objective

We questioned whether dietary IAA depletion worsens seizure activity in rodents fed KDs.

Methods

In a series of 6 trials, male rats or gerbils of both sexes (6-8/group) were given either control diets (CDs) appropriate for each trial, a KD, or a threonine-devoid (ThrDev) diet for ≥7 d, and tested for seizures using various stimuli. Microchip analysis of rat APCs was also used to determine if changes in transcripts for structures relevant to seizurogenesis are affected by a ThrDev diet. Glutamate release was measured in microdialysis samples from APCs during the first meal after 7 d on a CD or a ThrDev diet.

Results

Adult rats showed increased susceptibility to seizures in both chemical (58%) and electroshock (doubled) testing after 7 d on a ThrDev diet compared with CD (each trial, P ≤ 0.05). Seizure-prone Mongolian gerbils had fewer seizures after receiving a KD, but exacerbated seizures (68%) after 1 meal of KD minus Thr (KD-T compared with CD, P < 0.05). In kindled rats fed KD-T, both counts (19%) and severities (77%) of seizures were significantly elevated (KD-T compared with CD, P < 0.05). Gene transcript changes were consistent with enhanced seizure susceptibility (7-21 net-fold increases, P = 0.045-0.001) and glutamate release into the APC was increased acutely (4-fold at 20 min, 2.6-fold at 60 min, P < 0.05) after 7 d on a ThrDev diet.

Conclusion

Seizure severity in rats and gerbils was reduced after KDs and exacerbated by ThrDev, both in KD- and CD-fed animals, consistent with the mechanistic studies. We suggest that a complete protein profile in KDs may improve IAA balance in the APC, thereby lowering the risk of seizures.

Central Amino Acid Sensing in the Control of Feeding Behavior

Dietary protein quantity and quality greatly impact metabolic health via evolutionary-conserved mechanisms that ensure avoidance of amino acid imbalanced food sources, promote hyperphagia when dietary protein density is low, and conversely produce satiety when dietary protein density is high. Growing evidence supports the emerging concept of protein homeostasis in mammals, where protein intake is maintained within a tight range independently of energy intake to reach a target protein intake. The behavioral and neuroendocrine mechanisms underlying these adaptations are unclear. While peripheral factors are able to signal amino acid deficiency and abundance to the brain, the brain itself is exposed to and can detect changes in amino acid concentrations, and subsequently engages acute and chronic responses modulating feeding behavior and food preferences. In this review, we will examine the literature describing the mechanisms by which the brain senses changes in amino acids concentrations, and how these changes modulate feeding behavior.

The anterior piriform cortex is sufficient for detecting depletion of an indispensable amino acid, showing independent cortical sensory function

Protein synthesis requires a continuous supply of all of the indispensable (essential) amino acids (IAAs). If any IAA is deficient, animals must obtain the limiting amino acid by diet selection. Sensing of IAA deficiency requires an intact anterior piriform cortex (APC), but does it act alone? Shortly after rats begin eating an IAA-deficient diet, the meal ends and EPSPs are activated in the APC; from there, neurons project to feeding circuits; the meal ends within 20 min. Within the APC in vivo, uncharged tRNA activates the general amino acid control non-derepressing 2 (GCN2) enzyme system increasing phosphorylation of eukaryotic initiation factor (P-eIF2α), which blocks general protein synthesis. If this paleocortex is sufficient for sensing IAA depletion, both neuronal activation and P-eIF2α should occur in an isolated APC slice. We used standard techniques for electrophysiology and immunohistochemistry. After rats ate IAA-devoid or -imbalanced diets, their depleted slices responded to different stimuli with increased EPSP amplitudes. Slices from rats fed a control diet were bathed in artificial CSF replete with all amino acids with or without the IAA, threonine, or a tRNA synthetase blocker, l-threoninol, or its inactive isomer, d-threoninol. Thr depletion in vitro increased both EPSP amplitudes and P-eIF2α. l (but not d)-threoninol also increased EPSP amplitudes relative to control. Thus, we show independent excitation of the APC with responses parallel to those known in vivo. These data suggest a novel idea: in addition to classical processing of peripheral sensory input, direct primary sensing may occur in mammalian cortex.

Uncharged tRNA and sensing of amino acid deficiency in mammalian piriform cortex

Recognizing a deficiency of indispensable amino acids (IAAs) for protein synthesis is vital for dietary selection in metazoans, including humans. Cells in the brain's anterior piriform cortex (APC) are sensitive to IAA deficiency, signaling diet rejection and foraging for complementary IAA sources, but the mechanism is unknown. Here we report that the mechanism for recognizing IAA-deficient foods follows the conserved general control (GC) system, wherein uncharged transfer RNA induces phosphorylation of eukaryotic initiation factor 2 (eIF2) via the GC nonderepressing 2 (GCN2) kinase. Thus, a basic mechanism of nutritional stress management functions in mammalian brain to guide food selection for survival.

Optical coherence tomography can measure axonal loss in patients with ethambutol-induced optic neuropathy

PURPOSE

To map and identify the pattern, in vivo, of axonal degeneration in ethambutol-induced optic neuropathy using optical coherence tomography (OCT). Ethambutol is an antimycobacterial agent often used to treat tuberculosis. A serious complication of ethambutol is an optic neuropathy that impairs visual acuity, contrast sensitivity, and color vision. However, early on, when the toxic optic neuropathy is mild and partly reversible, the funduscopic findings are often subtle and easy to miss.

METHODS

Three subjects with a history of ethambutol (EMB)-induced optic neuropathy of short-, intermediate-, and long-term visual deficits were administered a full neuro-ophthalmologic examination including visual acuity, color vision, contrast sensitivity, and fundus examination. In addition, OCT (OCT 3000, Humphrey-Zeiss, Dublin, CA) was performed on both eyes of each subject using the retinal nerve fiber layer (RNFL) analysis protocol. OCT interpolates data from 100 points around the optic nerve to effectively map out the RNFL.

RESULTS

The results were compared to the calculated average RNFL of normal eyes accumulated from four prior studies using OCT, n=661. In all subjects with history of EMB-induced optic neuropathy, there was a mean loss of 72% nerve fiber layer thickness in the temporal quadrant (patient A, with eventual recovery of visual acuity and fields, 58% loss; patient B, with intermediate visual deficits, 68% loss; patient C, with chronic visual deficits, 90% loss), with an average mean optic nerve thickness of 26+/-16 microm. There was a combined mean loss of 46% of fibers from the superior, inferior, and nasal quadrants in the (six) eyes of all three subjects (mean average thickness of 55+/-29 microm). In both sets (four) of eyes of the subjects with persistent visual deficits (patients B and C), there was an average loss of 79% of nerve fiber thickness in the temporal quadrant.

CONCLUSIONS

The OCT results in these patients with EMB-induced optic neuropathy show considerable loss especially of the temporal fibers. This is consistent with prior histopathological studies that show predominant loss of parvo-cellular axons (or small-caliber axons) within the papillo-macular bundle in toxic or hereditary optic neuropathies. OCT can be a valuable tool in the quantitative analysis of optic neuropathies. Additionally, in terms of management of EMB-induced optic neuropathy, it is important to properly manage ethambutol dosing in patients with renal impairment and to achieve proper transition to a maintenance dose once an appropriate loading dose has been reached.

NMDA receptor function within the anterior piriform cortex and lateral hypothalamus in rats on the control of intake of amino acid-deficient diets

Animals decrease intake of an indispensable amino acid (AA)-deficient or devoid diet, due in part to decreased dietary limiting AA (DLAA) concentrations within the anterior piriform cortex (APC), and to a recognition process that occurs as early as 20 min following exposure to AA deficiencies. Glutamate levels within the APC change in response to AA deficiencies. The APC projects to the lateral hypothalamus (LH), where glutamate acts to stimulate food intake. We hypothesize that the APC, through glutamatergic projections to the LH, inhibits the LH, which signals to reject the AA-deficient or devoid diet, and trigger aversions to the AA-deficient or devoid diet via an ascending pathway to the APC. We examined the effects of (1) bilateral APC and LH blockade of glutamate's NMDA receptors with the antagonist, D-AP5, (2) APC blockade of AMPA receptors with the antagonist, NBQX, to block glutamate transmission from the APC, and (3) direct injection of the agonist, NMDA, into the LH on intake of the AA-deficient, devoid, or corrected diet. Administration of D-AP5 into the APC increased intake of AA-deficient diet by 6 h, but D-AP5 in the LH decreased AA-devoid diet preferentially over AA corrected intake sooner. NBQX in the APC increased AA-deficient diet intake, also at 6 h. NMDA injection into the LH-stimulated intake of the AA corrected diet by 3 h, but did not affect AA-devoid diet intake. Thus, the glutamate receptors in the APC and LH are involved in the feeding responses to AA-deficient diet, albeit with regional differences. We suggest that glutamate mediates the anorectic responses to AA-deficient diets through recognition of AA-devoid diet with the glutamatergic output cells of the APC sending glutamate-based signals for changes in food intake within the LH and through learned avoidance of AA-deficient diet within the APC, as indicated through the more immediate and prolonged periods of activation within the LH and APC, respectively.

Diets deficient in indispensable amino acids rapidly decrease the concentration of the limiting amino acid in the anterior piriform cortex of rats

Diets deficient in an indispensable amino acid have long been known to suppress food intake in rats. Detection of dietary deficiency takes place in the anterior piriform cortex (APC). Recent studies showed that the response to amino acid deficiency takes as little as 15 min to develop, but few data exist to correlate the concentration of amino acids in the APC with this rapid response. The purpose of this study was to measure the concentration of amino acids in the APC in a behaviorally relevant time frame. Rats were preconditioned by consumption of a basal diet for 7-10 d, and then given a test diet with either a control or deficient amino acid profile. Both the threonine- and leucine-deficient diets reliably depleted threonine and leucine concentration in the APC within 30 min, respectively. The control diets and a diet lacking the dispensable amino acid glycine did not lead to amino acid depletion. In combination with previous studies, the present results show that the decrease in the concentration of indispensable amino acids in the APC may be the initial sensory signal for recognition of dietary amino acid deficiency.

Effects of axonal injury on ganglion cell survival and glutamate homeostasis

Axonal trauma leads to a series of pathologic events that can culminate in neuronal death. Optic nerve crush can be used to explore histologic and molecular changes in traumatic central nervous system malfunction. Although the precise mechanisms of retinal ganglion cell death after optic nerve crush have not been elucidated, glutamate antagonists can protect retinal ganglion cells after axotomy. We, therefore, evaluated the effect of optic nerve crush on levels of extracellular glutamate. Ganglion cell survival and extracellular glutamate levels were assessed from 1 to 28 days after optic nerve crush in Long-Evans rats. Optic nerve crush led to a rise in extracellular glutamate; this rise was blocked by treatment with memantine, riluzole, and nimodipine. Partial optic nerve crush leads to an increase in vitreal glutamate, perhaps through release of intracellular contents. This released glutamate can contribute to additional ganglion cell loss. Future work will help to additionally unravel the steps by which axotomy induces excitotoxic damage to ganglion cells, and perhaps indicate protective interventions.

Phosphorylation of eIF2alpha is involved in the signaling of indispensable amino acid deficiency in the anterior piriform cortex of the brain in rats

Sensing of indispensable amino acid (IAA) deficiency, an acute challenge to protein homeostasis, is demonstrated by rats as rejection of IAA-deficient diets within 20 min. The anterior piriform cortex (APC) of the brain in rats and birds is essential for this nutrient sensing, and is activated by IAA deficiency. Yet the mechanisms that sense and transduce IAA reduction to signaling in the APC, or indeed in any animal cells, are unknown. Because rejection of a deficient diet within 20 min is too rapid to be explained by transcription-derived signals, brain tissue was taken from rats after 20 min access to either a threonine-basal, -devoid, or -corrected diet and examined for proteins associated with early signaling of IAA deficiency in the yeast model. Western blots and immunohistochemistry showed that the phosphorylation of eukaryotic initiation factor 2-alpha (p-eIF2alpha[Ser51]) and translation of its downstream product, c-Jun, were increased (47%, P < 0.005, and 55%, P < 0.025, respectively) in APC from rats offered devoid, but not corrected diets, compared with those offered basal diets. This was not seen in other brain areas. In cells intensely labeled for cytoplasmic p-eIF2alpha, there was intense fluorescence for c-Jun in the nucleus. Thus, p-eIF2alpha, which is pivotal in the initiation of global protein translation, and its downstream product, the leucine zipper protein, c-Jun, are increased in the mammalian APC within the time frame necessary for the behavioral response. We suggest that p-eIF2alpha and c-Jun participate in signaling nutrient deficiency in the IAA-sensitive neurons of the APC.

Effects of amino acid deficiency on monoamines in the lateral hypothalamus (LH) in rats

Animals decrease intake of an indispensable amino acid deficient diet, due in part to decreased dietary limiting amino acid concentrations within the anterior piriform cortex (APC). In addition to studies supporting a primary role for the APC in this phenomenon, recent studies have shown that the lateral hypothalamus (LH), which receives projections from the APC, also mediates the anorectic response to amino acid deficiency. The neurochemical changes within the LH that accompany the anorexia to amino acid deficiency are unclear. We hypothesized that norepinephrine (NE), dopamine (DA) and serotonin, whose levels are altered in response to amino acid deficiency within the APC, also act within the LH to mediate amino acid deficiency-induced anorexia. We determined that ingestion of an amino acid devoid diet increased concentrations of NE and the serotonin metabolite, 5-hydroxyindoleacetic acid in the LH. The 5-hydroxytryptamine metabolite was increased overall, according to analysis by area under the curve. Individual points reached significance at 130 min; NE was elevated at 170 min. These results suggest that the sustained anorectic response following ingestion of an amino acid devoid diet may be associated with increased activity of the NE and 5-hydroxytryptamine systems in the LH.

Threonine deprivation rapidly activates the system A amino acid transporter in primary cultures of rat neurons from the essential amino acid sensor in the anterior piriform cortex

Omnivores show recognition of essential (indispensable) amino acid deficiency by changing their feeding behavior within 20 min, yet the cellular mechanisms of amino acid sensation in eukaryotes are poorly understood. The anterior piriform cortex (APC) of the brain in rats or its analog in birds likely houses the in vivo amino acid chemosensor. Because amino acid transporters adapt rapidly to essential amino acid deficiency in several cell models, we hypothesized that activation of electrogenic amino acid transport in APC neurons might contribute to the function of the amino acid sensor. We evaluated transport systems in primary cultures of neurons from the APC, hippocampus and cerebellum, or glia, incubated in complete or threonine-devoid (deficient) medium. After 10 min in deficient medium, uptake of threonine or a system A-selective substrate, methyl amino-isobutyric acid, was increased 60% in APC neurons only (P < 0.05). These results demonstrated upregulation of system A, an electrogenic amino acid-sodium symporter. This depletion-induced activation required sodium, intact intracellular trafficking, and phosphorylation of signal transduction-related kinases. Efflux studies showed that other transporter types were functional in the APC; they appeared to be altered dynamically in threonine-deficient cells in response to rapid increases in system A activity. The present data provided support for the chemical sensitivity of the APC and its role as the brain area housing the indispensable amino acid chemosensor. They also showed a region-specific, phosphorylation-dependent activation of the system A transporter in the brain in response to threonine deficiency.

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.

Transfer ribonucleic acid charging in rat brain after consumption of amino acid-imbalanced diets

Recognition of an amino acid-imbalanced diet (IMB) is thought to occur in the anterior piriform cortex (APC) of the brain in response to a decrease in the limiting amino acid. We hypothesized that tRNA charging is decreased after ingestion of IMB and that this is part of the mechanism by which a decrease in the limiting amino acid is recognized. We investigated this question by determining levels of charged and uncharged tRNA using the periodate oxidation method and also by using high performance liquid chromatography (HPLC) analysis of amino acids acylated to brain tRNA. Using the periodate method, we found that isoleucyl-tRNA in both whole brain and APC of rats fed an isoleucine-IMB was increased, rather than decreased, in comparison to the basal diet and the corrected diet. Using HPLC analysis, we found that the absolute amount of tRNA charged with the limiting amino acid was not altered by dietary treatment. These two experimental approaches measure different aspects of tRNA charging, but the results clearly indicate that a reduction in tRNA charging is unlikely to be the signal by which a limiting amino acid is recognized in the brain 2 h after ingestion of IMB.

GABA(A) and GABA(B) receptors in the anterior piriform cortex modulate feeding in rats

The effects of GABA(A) and GABA(B) receptors in the anterior piriform cortex (APC) on intake of an amino acid imbalanced diet and a basal diet were evaluated in rats. Administration of muscimol (GABA(A) receptor agonist) to the APC immediately suppressed ingestion of both amino acid imbalanced and basal diets. Central administration of bicuculline (a GABA(A) receptor antagonist) stimulated feeding of the amino acid imbalanced diet but had no effect on intake of the basal diet. The GABA(B) receptor antagonist phaclofen decreased consumption of the basal diet but did not affect consumption of the amino acid imbalanced diet. These findings demonstrate that manipulation of GABA-sensitive cells in the APC can have a pronounced effect on feeding behavior that is not selective to aminoprivic feeding. However, these data suggest that GABA(A) and GABA(B) receptors may function as regulators that are activated by monoaminergic systems and neuropeptides in response to amino acid imbalanced diet intake. Inhibitory effects of GABA(A) and GABA(B) receptors may modulate the pyramidal cells, contributing to the reduced feeding response to the amino acid imbalanced diet. Also, transcription of mRNA for both GABA receptors and the GABA reuptake transporter was affected by a threonine deficient but not a corrected diet, compared to the basal diet. Taken together, these results support the involvement of GABA receptors in the APC in feeding in general and the responses to amino acid deprivation in vivo.

A comparison of brain angiotensin II receptors during lactation and diestrus of the estrous cycle in the rat

During lactation there are many dramatic alterations in the hypothalamic-pituitary (HP) axis, as well as an increased demand for food and water. The renin-angiotensin system (RAS) is one of the major mediators of the HP axis. This study examined the receptors for ANG II in the rat brain during lactation and diestrus. Compared with diestrus, lactating rats had significant decreases in ANG II receptor binding in several forebrain regions, most notably in the arcuate nucleus/median eminence, dorsomedial hypothalamic nucleus (DMH), and lateral hypothalamic area (LHA). In contrast, there was an increase in ANG II receptor binding in the preoptic area during lactation. These significant changes in ANG II binding in the brain during lactation support the hypothesis that changes in the RAS may contribute to the dramatic changes in the HP axis during lactation. In addition, the significant reduction in ANG II binding in the DMH and LHA may be indicative of a role in the regulation of food intake, a function only recently associated with the RAS.

Essential amino acids affect interstitial dopamine metabolites in the anterior piriform cortex of rats

The anterior piriform cortex (APC) is essential for the anorectic reactions to an amino acid-imbalanced diet, and it also responds to repletion of the limiting amino acid. In the present study, we examine the dynamic changes of the interstitial dopamine metabolites in the APC following feeding of either an amino acid-corrected or -imbalanced diet. Microdialysates, collected from the APC, were analyzed using HPLC with electrochemical detection. The concentrations were 19.7 +/- 4.8 microg/L for 3, 4-dyhydroxyphenylacetic acid and 25.1 +/- 4.4 microg/L for homovanillic acid, respectively, in the baseline dialysates. After diet treatments, no significant changes occurred in 3, 4-dyhydroxyphenylacetic acid in the corrected (n = 7) or imbalanced (n = 9) groups vs. the basal group (n = 7). However, after feeding the threonine-corrected diet, the concentration of homovanillic acid was significantly less (P < 0.01) than after the basal and imbalanced diets. The homovanillic acid level in the corrected group was already significantly lower than in the basal group by 20 min (P < 0.05), and reached its lowest level at 70 min (P < 0.05). The concentrations of homovanillic acid in the corrected group remained at this low level until the end of the experiment. The present results introduce the idea that the dopaminergic system is involved in the feeding responses to essential amino acid repletion.

Increased intracellular calcium in rat anterior piriform cortex in response to threonine after threonine deprivation

The anterior piriform cortex (APC) may serve as the chemosensor for amino acid (AA) deficiency in rats. To investigate the mechanism by which the APC recognizes a limiting indispensable AA (IAA), we examined changes in intracellular calcium ([Ca2+]i) in APC slices after culture in medium with or without threonine (Thr) or lysine (Lys). The addition of 1 or 10 mM Thr to slices previously incubated in Thr-devoid medium resulted in a significant and sustained increase in [Ca2+]i compared to control slices; an effect not seen when isoleucine, another IAA, was added. Similar results were seen when lysine, but not threonine, was added to slices incubated in lysine-devoid medium. The rise in [Ca2+]i resulting from the addition of the limiting IAA to deficient slices may be linked to enhanced activity of the appropriate AA transporter. This is suggested by preliminary findings that serine, a small neutral AA that uses the same transporter as threonine, gave rise to an enhanced response in the Thr-deficient slice.

Inhibition of norepinephrine release in the rat ventromedial hypothalamic nucleus in essential amino acid deficiency

Effects of dietary amino acid deficiency on interstitial levels of norepinephrine (NE) were assessed in the ventromedial hypothalamic nucleus (VMH). Microdialysates, collected from the VMH, were analyzed using high pressure liquid chromatography with electrochemical detection (HPLC-EC). Ingestion of an amino acid imbalanced diet, which causes a rapid deficiency of the limiting amino acid, induced a significant decrease in the NE concentration from the VMH. The changes in the NE concentration appeared 60 min after diet ingestion and the lowest NE level was observed at 180 min. The present results suggest that ingestion of an amino acid imbalanced diet inhibits NE release in the VMH and support the hypothesis that the VMH plays a role in the integration of signals for the feeding responses to changes in essential amino acid availability.

Nitric oxide reduces hypophagia induced by threonine free diet in the rat

Food intake and concentrations of glutamic (GLU) and aspartic (ASP) acids in the nucleus accumbens were monitored in male Sprague-Dawley rats fed a threonine free diet. These variables were measured before and after an intracerebroventricular injection of 20 nmole nitroprusside (NP), a non-enzymatic nitric oxide donor. The same variables were also monitored in: (a) rats fed a threonine free diet and injected with saline; (b) animals fed a standard diet and injected with nitroprusside; (c) rats fed a standard diet and injected with saline. The results showed that the threonine-free diet reduced food intake and GLU and ASP concentrations in the accumbens. NP reduced the hypophagia, but it did not change GLU and ASP levels in rats fed the threonine-free diet. In animals fed the standard diet, NP increased GLU and ASP concentration, and food intake. No change was found in the animals with saline injection. These findings suggest that nitric oxide reduces the hypophagia in the rats fed a threonine-free diet. The lack of increase in GLU and ASP concentration in the nucleus accumbens of the hypophagic rats indicates that NP acts on hypophagia independently by GLU and ASP.

Dietary excess of vitamin B-6 affects the concentrations of amino acids in the caudate nucleus and serum and the binding properties of serotonin receptors in the brain cortex of rats

Vitamin B-6 is a cofactor in many reactions of nitrogen metabolism. Deficiency alters tissue amino acid concentrations but effects of excess vitamin B-6 have not been well described. We fed female rats (218 g, 7 per group) 1 (control), 10, 100, 175 or 250x) the National Research Council recommended level of pyridoxine HCl (7 mg/kg) for 10 wk and measured serum amino acids, amino acids and neurotransmitters in brain regions and the binding properties of serotonin receptors in the cerebral cortex using a ketanserin binding assay. Rats were decapitated, and unheparinized blood was obtained. In the caudate nucleus, concentrations of glutamate, threonine, taurine, methionine, gamma-amino-butyric acid and the sum of the essential amino acids in groups 10X and 100X were approximately 130 to 180% of control levels (P < 0.05); groups 1X, 175X and 250X were not different. A similar pattern was seen in the serum for serine, glycine, aspartate and ornithine; the latter two amino acids increased to over 200% of control in group 100X. In the ketanserin binding assay, both the antagonist binding affinity and the maximal number of binding sites were higher for group 100X than for 1X, 175X and 250X, and were higher for 10X than for 1X. Norepinephrine in the raphe nucleus followed a similar biphasic pattern. Excess dietary pyridoxine affected brain and serum concentrations of some amino acids and binding properties of cortical serotonin receptors in a biphasic pattern over the range of concentrations fed in this study.

Neurochemical changes after imbalanced diets suggest a brain circuit mediating anorectic responses to amino acid deficiency in rats

Amino acid-imbalanced (IMB) diets induce an acute amino acid deficiency and hypophagic responses in most animals. The neural circuits underlying these responses are unknown. To ascertain potential neural circuits involved in the recognition of IMB, we measured the concentrations of norepinephrine, dopamine, serotonin, their metabolites and 20 amino acids in 14 rat brain areas in three studies. Rats were prefed a basal diet with L-amino acids as the protein source for at least 1 wk. For the experiments, either threonine or isoleucine IMB diet was offered for 2.5 or 3.5 h. Brains were taken before (using a mildly IMB diet) or after (using moderately or severely IMB diet) food intake was significantly (P < 0.05) depressed. Brain areas were dissected and analyzed for monoamines, metabolites and amino acids. Only in the anterior piriform cortex (APC), a brain area that may contain the amino acid chemosensor, was the limiting amino acid lower in IMB groups than in controls across all of the experiments. Before the onset of the anorectic response to the IMB diets, monoaminergic activity was affected in areas that have recognized monosynaptic connections with the APC. We propose a circuit for the neural responses in the initial recognition of acute amino acid deprivation that begins with activation of the APC and includes areas in the hindbrain and hypothalamus. After a significant hypophagic response, serotonergic indicators were altered in areas of the taste pathway and the limbic system. These results suggest that different circuits mediate the initial recognition and secondary conditioned responses to IMB diets.

Preference for flavoured foods by lambs conditioned with intraruminal administration of nitrogen

We suggested that food preference depends on the interplay between flavour and post-ingestive effects, and we predicted that protein-restricted lambs would acquire preferences for foods paired with supplemental sources of N, including urea (Expts 1 and 2), casein (Expt 3), and gluten (Expt 4). In each experiment, twenty lambs, in two groups of ten, were conditioned as follows: on odd-numbered days, lambs in group 1 received wheat straw (Expts 1, 3, and 4) or ground barley (Expt 2) flavoured with a distinctive flavour, and lambs in group 2 received the same food but with a different flavour. On even-numbered days, flavours were switched and lambs received capsules containing different amounts of urea (ranging from 0.12 to 0.92 g N/d), casein (ranging from 0.23 to 0.69 g N/d), or gluten (ranging from 0.23 to 0.69 g N/d). After conditioning period of 8 d, lambs were given a two-choice test to determine preference for flavours paired with N. In Expts 1 and 2, lambs preferred the flavours conditioned with urea at lower doses (0.12 g N/d in Expt 1, 0.23 and 0.46 g N/d in Expt 2), but they avoided the flavour associated with urea at the highest dose (0.23 g N/d in Expt 1 and 0.92 g N/d in Expt 2). In Expts 3 and 4, lambs avoided the flavours associated with the lowest doses of casein or gluten (0.23 g N/d), but they preferred the flavours paired with casein or gluten at higher doses (0.46 and 0.69 g N/d). After conditioning, N administrations were suspended and lambs in Expts 3 and 4 were offered a choice of the two flavours at weekly intervals for 2 weeks (extinction); preferences persisted during extinction. Collectively, these results suggest that the post-ingestive effects of N in different forms and concentrations influenced the development of food preferences by lambs.

Small changes in essential amino acid concentrations alter diet selection in amino acid-deficient rats

The rat's sensitivity to changes in the dietary limiting amino acid concentration (LAA) was examined on the basis of dietary selection. Rats were adapted to purified low protein basal (Basal) diets in which threonine (Thr) was the LAA (0.188-0.212% wt/wt of diet). In Experiment 1, rats made a clear selection for their adaptation diet over a diet containing 0.012% less threonine after 2-3 d of choice. Rats made no clear dietary selection when given a choice between their adaptation diet and a diet containing 0.012% more threonine. Experiment 2 was conducted to examine the rat's sensitivity to small decreases in the LAA concentration. Rats adapted to a 0.200% Thr-Basal diet clearly responded to decreases as small as 0.009% in the concentration of threonine and selected against the more deficient diet when given a choice between it and the 0.200% Thr-Basal adaptation diet. Because plasma and brain amino acid concentrations are important for detection of other amino acid deficiencies, these variables were measured to determine whether they were affected by such small changes in dietary amino acid concentration. In Experiments 3 and 4, rats were adapted to the 0.200% Thr-Basal diet and then fed 0.188, 0.200 or 0.212% Thr-Basal diets for 6 h, or 0.188 and 0.212% Thr-Basal for 54 h. Amino acid concentrations in plasma, prepiriform cortex and anterior cingulate cortex were not significantly different among treatments. Norepinephrine concentration in the prepiriform cortex was not affected by dietary treatment. We conclude that small decreases in LAA concentration can cause selection against the more deficient diet, but that detection of such deficiencies does not require significant changes in plasma and brain amino acid concentrations.

Temporal-spatial pattern of c-fos expression in the rat brain in response to indispensable amino acid deficiency. I. The initial recognition phase

Rats reduce their food intake after ingestion of a small amount of an amino acid imbalanced (AA-IMB) diet that induces a pronounced amino acid deficiency. Two hours after ingesting a threonine-IMB diet, just when food intake is depressed significantly, the concentration of threonine is decreased in some but not all brain areas. Neural recognition of this decrease in the limiting amino acid is thought to be the first step in the anorectic responses to AA-IMB diets. To identify the regions of the brain that may be activated upon recognition of an AA-IMB diet, we examined the temporal-spatial distribution of Fos immunoreactive neurons at intervals after introduction of either threonine-IMB or control diets. We found that Fos immunoreactivity in the anterior piriform cortex and immediately surrounding areas, along with the infralimbic cortex, was increased selectively early (by 2 h) after introduction of the AA-IMB diet, and remained elevated through 3 h. The anterior piriform cortex is believed to function in neural recognition of amino acid deficiency. Fos immunoreactivity in the AA-IMB group increased over the control diet groups somewhat later in the dorsomedial nucleus of the hypothalamus. We hypothesize that these areas in the rostral forebrain may serve as neural relays in the early phases of the anorectic responses that occur upon recognition of amino acid deficiency.

Alpha 2 noradrenoceptors in the anterior piriform cortex decline with acute amino acid deficiency

The responses of the brain to the amino acid deficiency that occur after eating imbalanced amino acid diets (IMB) have been associated with decreased concentrations of norepinephrine (NE) and cAMP in the anterior piriform cortex (APC), an area essential for the initial feeding responses to amino acid deficiency. In addition, the anorectic responses to IMB were decreased after injections of the alpha 2 agonist, clonidine, and increased after injections of the alpha 2 antagonist, idazoxan, into the APC. Therefore, to study the role of the alpha 2-noradrenergic receptor further in this model, we measured alpha 2-noradrenergic receptor binding in the APC of rats fed two levels of threonine IMB or a low-protein basal control diet. After basal prefeeding for 10 days, rats were given either a mild IMB, a severe IMB, or the basal diet for 2.5 h. The APC, anterior cingulate cortex (AC), ventromedial hypothalamus (VMH), and lateral hypothalamus (LH) were assayed. Binding of [3H]p-aminoclonidine to alpha 2 receptors determined that alpha 2 binding was decreased the most in APC (P < 0.0003). Binding in APC was significantly correlated with food intake in the anorectic response to IMB (P < 0.001). In AC, binding was also significantly decreased, but less dramatically (P = 0.012), and was not correlated with food intake. There were no significant changes in LH or VMH, although alpha 2-noradrenergic binding in VMH tended to decrease with the severe IMB in a pattern similar to APC. Plasma glucose values did not differ after the same feeding protocol. These data support our hypothesis that NE activity in the APC plays a role in initiating the anorectic response to IMB, perhaps via the alpha 2-noradrenergic receptor.

Behavioral and neurochemical changes in folate-deficient mice

Weanling mice were fed an amino acid-based diet supplemented with 0 or 11.3 mumol folic acid/kg diet for approximately 38 days to study behavior and neurochemistry in folate deficiency. After approximately 5 wk, mice fed the unsupplemented diet weighted approximately 70% as much those fed the supplemented diet. After 2 wk, mice fed the unsupplemented diet consistently discarded (spilled) more food, and after approximately 5 wk, they had spilled 3 times more than mice fed the supplemented diet. Serum folate, brain folate and brain S-adenosylmethionine of mice fed the unsupplemented diet were 4, 53, and 60% as high, respectively, as those of mice fed the supplemented diet. Pathologic changes were not evident in brain, spinal cord, or skeletal muscle of folate-deficient mice. The hypothalamic 5-hydroxyindole acetic acid/serotonin ratio and caudate dopamine, homovanillic acid, and 3,4-dihydroxyphenylacetic acid concentrations were lower in deficient than control mice. Folate-deficient mice develop a behavioral activity, food spilling, which may have a neurochemical basis in the serotonin and dopamine systems.

Brain large neutral amino acids and catecholamines in parenterally nourished preterm rabbits

Total parenteral nutrition (TPN) has been adapted as a standard for providing nutrition to ill term and preterm infants. The availability of tyrosine in amino acid preparations utilized for TPN is limited and may potentiate a tyrosine-deficient state. Phenlyalanine hydroxylase activity, responsible for catalyzing tyrosine synthesis, has been suggested to be decreased in fetal and neonatal animals. Parenterally nourished premature rabbits (n = 16) and suckled rabbits (n = 19) were studied in order to compare growth parameters and amino acids in the plasma and brain, as well as whole brain catecholamine concentrations. Influx velocities into the brain of amino acids were also determined in these two groups. The preterm rabbit's average birth weight (42.6 +/- 6.0) was less than that of term rabbits (56.7 +/- 8.7, P < 0.005). Significantly lower concentrations of the catecholamine precursor tyrosine were found in both the plasma and brain of the parenterally nourished animals compared to the suckled animals. Tyrosine is reduced in the brain in TPN-supported animals reflecting both low tyrosine intake and increased plasma concentrations of large neutral amino acids that compete for uptake at the blood-brain barrier. However, no difference was observed between the two groups in their brain catecholamine concentrations. The seven-day parenterally nourished rabbit appears to be tyrosine-deficient but no evident effects on brain catecholamine concentrations were seen. The effects and impact of a tyrosine-deficient state might better be evaluated by regional evaluation of catecholaminergic areas of the brain or over a longer period of parenteral nutrition.

Disguised protein in lunch after low-protein breakfast conditions food-flavor preferences dependent on recent lack of protein intake

As in the conditioning of appetite for protein in the rat, human preference for and intake of a food at lunch was increased when the flavor of that food was paired with an adequate supply of protein, following a breakfast lacking in protein. Men and women rated their preferences for two flavors in tasted foods (soup and cornflour dessert) on test days before and after a day when one flavor was eaten in very low protein food and another day with a different flavor eaten in food containing protein, but with minimal sensory differences between these foods. Subjects given a low-protein drink preload preferred the protein-paired flavor, while those receiving a high-protein drink did not. In a second experiment, preferences were measured by intake as well as ratings, and the difference in amount of protein between high- and low-protein lunches was increased. By both measures, relative preference for high-protein-paired dessert flavors increased from before to after pairing. The increase in intake preference ratio for the protein-paired flavor was abolished by a high-protein preload. Thus, people have a learning mechanism whereby a lack in protein intake comes to cue the selection of protein-rich foods that are not known to be such, and/or loading with protein might trigger avoidance specifically of a high-protein diet.

Food intake adjustments of chicks: short term reactions to deficiencies in lysine, methionine and tryptophan

1. Two experiments were conducted to compare food intake responses of broiler chicks fed diets varying in lysine, methionine, and tryptophan. Diet D was formulated to create simultaneous deficiencies of lysine, methionine, and tryptophan. Diet A matched National Research Council (1984) recommendations for broilers, and diets B and C were, respectively, 2:1 and 1:2 mixes of diets A and D. 2. Short-term food intake can provide information on the sequences of adaptation of chicks to a diet deficient in essential amino acids. 3. Chicks consumed 26% less of diet D than A during the first 24 h posthatch. When chicks fed diet A or D to 7 d of age were then fed one of 4 diets singly, within 24 h intake was lowest for chicks fed diet D. Within 48 h, food intake of diet C was more than that of diet D and less than that of diet A, while for diet B intake was more than of diet D but not different from diet A. 4. In the second experiment, chicks were fed diet A to 8 d and then diets A or D alone or given a choice of diets A and D from 8 to 20 d of age. Within 4 to 8 h, food intake of chicks fed diet D alone decreased markedly followed by partial recovery within 24 h. In a choice setting, consistent preference of Diet A over Diet D was observed within 7 h followed by stabilisation at about 65% diet A to 35% diet D. 5. Chicks fed diet D alone from 8 to 20 d of age, then placed in the same choice situation preferred diet A to D with a delay of less than one h and stabilisation at about 85%. Chicks provided a choice of diets A and D from 8 to 20 d, and then diet D alone reduced their food intake more quickly than those not given a choice initially. 6. Broiler chicks appear to react to amino acid deficiencies within a short period (hours) by adjusting their feed intake and/or selection. The response is influenced by age and prior experience.

Timing and dose of amino acids injected into prepyriform cortex influence food intake

The effects of time before feeding and dose of dietary-limiting amino acids (DLAA) injected into the prepyriform cortex (PPC) on intake of amino acid-imbalanced diets were evaluated. Intake of imbalanced diet was increased from approximately 50% to approximately 75% of baseline when an optimal amount of DLAA (1 nmol L-isoleucine or 2 nmol L-threonine) was injected immediately prior to feeding. Injections made several hours prior to feeding were more effective, increasing intake of imbalanced diets to approximately 85% of baseline. Delivering two half-optimal doses of DLAA, several hours apart, increased intake of imbalanced diet only to the same level as a single injection of the optimal dose immediately prior to feeding. The increase in intake of a threonine-imbalanced diet after injecting 2 nmol threonine 6 h prior to feeding was abolished if an additional 2 nmol threonine was injected immediately prior to feeding. It appears that it is the sum of the changes in tissue DLAA concentrations in the PPC that are recognized and influence food intake when amino acid imbalanced diets are fed.

Liver denervation, 5-HT3 receptor antagonist, and intake of imbalanced amino acid diet

The serotonin3 receptor antagonist ICS 205-930 (ICS) may act peripherally to attenuate the anorectic response of rats given an imbalanced amino acid (IMB) diet. Rats were divided into four groups: SHAM+saline (sal); SHAM+ICS; total liver denervation (TLD) + sal; and TLD+ICS. Rats were then given a purified basal diet for 16 days. Next, the groups were injected with sal or 9 mg/kg BW of ICS at 0800 h and at 0900 h (lights out) an isoleucine IMB diet was presented. By 12 h postinjection, the food intake (FI) of TLD and SHAM rats receiving ICS was similarly higher (p < 0.02) than sal-injected counterparts whose FI was also similar; BW followed FI. By day 3, the SHAM groups had similar low FI, whereas the FI of the TLD groups was increasing. The above study was repeated with similar results. Liver innervation is not required for ICS attenuation of IMB diet-induced hypophagia. Also, while sal-injected TLD rats show a normal attenuation of consumption of the IMB diet on the first day of exposure, they subsequently consume more of the IMB diet than SHAM rats. The reason for this difference in TLD rats is not clear but may be related to metabolism of the IMB diet or possibly learning.

Dietary amino acid imbalance and neurochemical changes in three hypothalamic areas

The impact of feeding imbalanced amino acid diets on monoamine, metabolite and amino acid concentrations was measured in the ventromedial hypothalamus (VMH), lateral hypothalamus (LH) and paraventricular nucleus (PVN). After rats were fed either an isoleucine imbalanced diet, a threonine imbalanced diet, or the appropriate basal or corrected control diets, regional differences were found in neurochemical concentrations. Contrary to our expectations, the limiting amino acid was unchanged in the imbalanced groups, tending to be decreased only in the isoleucine imbalanced-diet group in the PVN. This is the first report that the limiting amino acid was not reduced uniformly in the brain after imbalanced amino acid feeding. In the VMH, norepinephrine (NE) was increased by 22% and 63% in the threonine and isoleucine imbalanced-diet groups, respectively. Since the concentration of NE was affected even before the decrease in feeding, both in the VMH, and, as previously reported, in the prepyriform cortex, the NE system may be involved in very early responses to imbalanced amino acid diets.

Some characteristics of threonine transport across the blood-brain barrier of the rat

Threonine entry into brain is altered by diet-induced changes in concentrations of plasma amino acids, especially the small neutrals. To study this finding further, we compared effects of various amino acids (large and small neutrals, analogues, and transport models) on transport of threonine and phenylalanine across the blood-brain barrier. Threonine transport was saturable and was usually depressed more by natural large than small neutrals. Norvaline and 2-amino-n-butyrate (AABA) were stronger competitors than norleucine. 2-Aminobicyclo[2.2.1]heptane-2-carboxylate (BCH), a model in other preparations for the large neutral (L) system, and cysteine, a proposed model for the ASC system only in certain preparations, reduced threonine transport; 2-(methylamino)isobutyrate (MeAIB; a model for the A system for small neutrals) did not. Phenylalanine transport was most depressed by cold phenylalanine and other large neutrals; threonine and other small neutrals had little effect. Norleucine, but not AABA, was a strong competitor; BCH was more competitive than cysteine or MeAIB. Absence of sodium did not affect phenylalanine transport, but decreased threonine uptake by 25% (p less than 0.001). Our results with natural, analogue, and model amino acids, and especially with sodium, suggest that threonine, but not phenylalanine, may enter the brain partly by the sodium-dependent ASC system.