Effects of threonine injections in the lateral hypothalamus on intake of amino acid imbalanced diets in rats

Brain Signaling of Indispensable Amino Acid Deficiency

Our health requires continual protein synthesis for maintaining and repairing tissues. For protein synthesis to function, all the essential (indispensable) amino acids (IAAs) must be available in the diet, along with those AAs that the cells can synthesize (the dispensable amino acids). Here we review studies that have shown the location of the detector for IAA deficiency in the brain, specifically for recognition of IAA deficient diets (IAAD diets) in the anterior piriform cortex (APC), with subsequent responses in downstream brain areas. The APC is highly excitable, which makes is uniquely suited to serve as an alarm for reductions in IAAs. With a balanced diet, these neurons are kept from over-excitation by GABAergic inhibitory neurons. Because several transporters and receptors on the GABAergic neurons have rapid turnover times, they rely on intact protein synthesis to function. When an IAA is missing, its unique tRNA cannot be charged. This activates the enzyme General Control Nonderepressible 2 (GCN2) that is important in the initiation phase of protein synthesis. Without the inhibitory control supplied by GABAergic neurons, excitation in the circuitry is free to signal an urgent alarm. Studies in rodents have shown rapid recognition of IAA deficiency by quick rejection of the IAAD diet.

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 brain's response to an essential amino acid-deficient diet and the circuitous route to a better meal

The essential (indispensable) amino acids (IAA) are neither synthesized nor stored in metazoans, yet they are the building blocks of protein. Survival depends on availability of these protein precursors, which must be obtained in the diet; it follows that food selection is critical for IAA homeostasis. If even one of the IAA is depleted, its tRNA becomes quickly deacylated and the levels of charged tRNA fall, leading to disruption of global protein synthesis. As they have priority in the diet, second only to energy, the missing IAA must be restored promptly or protein catabolism ensues. Animals detect and reject an IAA-deficient meal in 20 min, but how? Here, we review the molecular basis for sensing IAA depletion and repletion in the brain's IAA chemosensor, the anterior piriform cortex (APC). As animals stop eating an IAA-deficient meal, they display foraging and altered choice behaviors, to improve their chances of encountering a better food. Within 2 h, sensory cues are associated with IAA depletion or repletion, leading to learned aversions and preferences that support better food selection. We show neural projections from the APC to appetitive and consummatory motor control centers, and to hedonic, motivational brain areas that reinforce these adaptive behaviors.

Bile duct obstruction is associated with early postoperative upregulation of liver uncoupling protein-2 and reduced circulating glucose concentration in the rat

OBJECTIVE

To evaluate whether upregulation of liver and muscle uncoupling protein 2 (UCP-2) is an acute phenomenon in obstructive jaundice and associated with secondary metabolic effects.

METHODS

Male Sprague-Dawley rats were divided into four groups: bile duct ligated (BDL) and sham-operated pair-fed (PF), ad libitum fed (AL), and controls. BDL, PF, and AL rats were further divided into subgroups according to the interval postoperatively when they were reanesthetized and sampled for tissue and blood: 2, 4, and 8 d, respectively. Bilirubin, liver enzymes, glucose, free fatty acids, and insulin in blood plasma were analyzed. Liver and muscle tissue were sampled for UCP-2 and adenosine triphosphate analysis.

RESULTS

The BDL rats showed an increase of the liver UCP-2 expression compared with PF and AL rats (P<0.05) 4 d postoperatively. Liver adenosine triphosphate in BDL rats showed a decrease compared with sham-operated controls at all intervals (P<0.05). Plasma glucose concentration in BDL rats was decreased compared with the other groups. Free fatty acids showed an initial increase 2 d postoperatively compared with sham-operated controls and PF and AL rats (P<0.05) at the corresponding time point.

CONCLUSION

Obstructive jaundice is associated with an early upregulation of liver UCP-2, reduced liver adenosine triphosphate content, and decreased plasma glucose concentration, supporting the hypothesis that obstructive jaundice results in impaired energy homeostasis in the liver, which might cause decreased glucose output and hypoglycemia as a consequence.

Forebrain melanocortin signaling enhances the hindbrain satiety response to CCK-8

Melanocortin 4 receptors (MC4R) are hypothesized to mediate the central nervous system actions of leptin to enhance the satiety effects of cholecystokinin (CCK). To further elucidate this mechanism, we confirmed that peripheral administration of CCK-8 is less effective in producing this effect in MC4R-deficient mice (MC4R(-/-)). Whereas intraperitoneal (ip) CCK-8 at 0.75 nmol/kg lean body mass (lbm) suppressed food intake in wild-type mice, CCK-8 doses of 7.5 nmol/kg lbm were required to attenuate food intake in MC4R(-/-) mice. To determine whether melanocortin signaling in the hypothalamic paraventricular nucleus (PVN) participates in regulating this CCK satiety response, we administered the MC3/MC4R antagonist, SHU9119, into the PVN of rats before ip CCK-8 administration. PVN administration of SHU9119 attenuated the ability of CCK-8 to reduce 30-min food intake by 20%. To determine whether MC4R are expressed by PVN neurons that project directly to hindbrain nuclei involved in the satiety response to ip CCK-8, the retrograde tracer fluorescent cholera toxin subunit B was injected into the nucleus tractus solitarius (NTS) of the hindbrain. After 4 days, labeled PVN neurons were collected by laser capture microdissection and found to express MC4R mRNA by quantitative RT-PCR analysis. These data provide evidence for a neuroanatomical link between hypothalamic melanocortin signaling in the PVN and NTS neurons that regulate food intake. These findings highlight the contribution of melanocortin signaling in the PVN toward regulating the satiety effects of CCK-8 while acknowledging that melanocortin-dependent pathways in other brain regions and/or melanocortin-independent mechanisms are also important in this mechanism.

Distinct effects of anterior pyriform cortex and the lateral hypothalamus lesions on protein intake in rats

Several specific locations in brain, including pyriform cortex and hypothalamus, are associated with regulation of food intake. Although lesions of these locations significantly alter food intake, their involvement in the selection of macronutrients is not well characterized. In this study, we examined distinct effects of anterior pyriform cortex (APC) and lateral hypothalamus (LH) lesions on protein intake in rats. The APC or LH of male adult rats were lesioned by treatment with kainic acid, and the rats were then given free access to two kinds of casein diets containing high (60%) and low (5%) protein. Total energy content of these diets was kept constant by changing the carbohydrate content. Following the APC lesions, body weight and food intake decreased, but returned to control levels on day 13 and day 4, respectively. APC lesions did not change the ratio of protein intake. In contrast, LH lesions disturbed body weight gain and the selection of a high protein diet for at least two weeks, although food intake returned to control levels by day 2. Our results suggest that LH, but not APC, may play an important role in the selection of protein intake in rats.

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.

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.

Phosphorylation of Ca2+/calmodulin-dependent protein kinase type ii and the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (ampa) receptor in response to a threonine-devoid diet

The anterior piriform cortex (APC) functions as a chemosensor for indispensable amino acid deficiency and responds to this deficiency with increased activity, as indicated by observations including averaged evoked-potentials and c-fos expression in the APC. Little is known of the intracellular signaling mechanisms that mediate this deficiency-related increase in neuronal excitability, but previous studies have shown effects on intracellular Ca2+ in deficient APC slices in vitro. In the present study we hypothesized that indispensable amino acid deficiency increases intraneuronal Ca2+, resulting in autophosphorylation of calcium/calmodulin-dependent protein kinase type II (CaMKII) in vivo. Results demonstrated that phosphorylation levels of CaMKII (pCaMKII) in APC neurons increased at 20 and 40 min after a single meal of threonine-devoid diet. Phosphorylation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunit (GluR1) at the serine 831 (S831) site was modestly increased in the APC in response to a threonine-devoid meal. The GluR1 subunit also showed increased phosphorylation at the 845 (S845) site, suggesting additional signaling mechanisms. Although phosphorylation of CaMKII was sustained, phosphorylation of the GluR1 subunit returned to control levels by 40 min. These effects of amino acid deficiency did not occur throughout the brain as neither CaMKII nor GluR1 showed increased phosphorylation in the neocortex. These findings support the notion that calcium and glutamate signaling in the APC, but not throughout the brain, are triggered during early responses to amino acid deficiency. They also suggest that longer-term changes in APC neurons in response to such a deficiency may be mediated at least in part by CaMKII.

Evidence that paraventricular nucleus oxytocin neurons link hypothalamic leptin action to caudal brain stem nuclei controlling meal size

Hindbrain projections of oxytocin neurons in the parvocellular paraventricular nucleus (pPVN) are hypothesized to transmit leptin signaling from the hypothalamus to the nucleus of the solitary tract (NTS), where satiety signals from the gastrointestinal tract are received. Using immunocytochemistry, we found that an anorectic dose of leptin administered into the third ventricle (3V) increased twofold the number of pPVN oxytocin neurons that expressed Fos. Injections of fluorescent cholera toxin B into the NTS labeled a subset of pPVN oxytocin neurons that expressed Fos in response to 3V leptin. Moreover, 3V administration of an oxytocin receptor antagonist, [d-(CH2)5,Tyr(Me)2,Orn8]-vasotocin (OVT), attenuated the effect of leptin on food intake over a 0.5- to 4-h period (P < 0.05). Furthermore, to determine whether oxytocin contributes to leptin's potentiation of Fos activation within NTS neurons in response to CCK, we counted the number of Fos-positive neurons in the medial NTS (mNTS) after 3V administration of OVT before 3V leptin and intraperitoneal CCK-8 administration. OVT resulted in a significant 37% decrease (P < 0.05) in the potentiating effect of leptin on CCK activation of mNTS neuronal Fos expression. Furthermore, 4V OVT stimulated 2-h food intake by 43% (P < 0.01), whereas 3V OVT at the same dose was ineffective. These findings suggest that release of oxytocin from a descending pPVN-to-NTS pathway contributes to leptin's attenuation of food intake by a mechanism that involves the activation of pPVN oxytocin neurons by leptin, resulting in increased sensitivity of NTS neurons to satiety signals.

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.

The rapid anorectic response to a threonine imbalanced diet is decreased by injection of threonine into the anterior piriform cortex of rats

Rats quickly recognize and reject diets deficient in an essential amino acid. The purpose of this study was to determine whether the anterior piriform cortex (APC), the site traditionally recognized as the amino acid chemosensor, plays a role in this early behavior. Rats had cannulae implanted bilaterally into the APC, and were injected with either saline vehicle or 2 nmoles of threonine (n = 6 per group). All rats were then fed a diet imbalanced with respect to threonine. The threonine-injected group had first meals of longer duration and consumed more food. These data conformed to expectations derived from earlier studies of responses to the first meal of an amino acid imbalanced diet. We conclude that the concentration of the dietary limiting amino acid in the APC regulates acceptance and rejection of amino acid deficient diets.

Effect of lysine on afferent activity of the hepatic branch of the vagus nerve in normal and L-lysine-deficient rats

Amino acid deficiency was modeled by feeding rats a diet deficient in the essential L-amino acid, L-lysine (L-lys). There is a rapid anorectic response to such a diet, and a strong preference for L-lys develops during the deficiency. While the brain appears to trigger this preference, the peripheral pathways that inform the brain about the deficiency are not well understood. One possible information pathway may utilize an "amino acid sensor" in the hepatoportal region. In the present study, we measured in vivo neural activity in normal and L-lys-deficient rat. Compared to the normally fed controls, we found an approximately 100-fold increase in the firing sensitivity of the L-lys sensors in vagal afferent fibers from the hepatoportal region of the L-lys-deficient rats. Injection of 10 mM L-lys into the hepatoportal circulation, but not D-lysine (D-lys), evoked an increase in afferent activity. While L-lys deficiency enhanced the sensitivity of the L-lys sensors, the sensitivity due to other small amino acid sensors remained unchanged. Finally, we observed a time-dependent response of the lysine sensors to lysine deficiency. It required 3-4 days of maintenance on the lysine-deficient diet for the sensitivity of the L-lys sensors to change. Taken together, these results provide additional data to support the existence of putative L-amino acid sensors in the hepatoportal circulation. Additionally, they describe several characteristics of the L-lys sensors and show that these sensors may contribute to the adaptation to dietary L-lys deficiency and to maintenance of L-amino acid homeostasis.

Effects of threonine injections in the lateral hypothalamus on intake of amino acid imbalanced diets in rats

Previous work from this laboratory suggests that animals decrease their intake of an amino acid imbalanced diet (IMB), due in part to a drop in the concentration of the dietary limiting amino (DLAA) in the anterior piriform cortex (APC). Administration of the DLAA, but not of a non-limiting amino acid into the APC, blocks the anorectic response to IMB. To our knowledge, the effects of DLAA injections on intake of a diet devoid of the DLAA (DEV), have not been examined in areas outside the APC. We hypothesized that the LH is a potential chemosensory area for DLAA. Our objectives were: (1) to determine whether injections of the DLAA threonine into the lateral hypothalamus (LH) alter intake of a threonine-devoid diet (DEV); and (2) to examine the dose-response effects of threonine injections into the LH on intake of threonine-corrected diet (COR). Administration of threonine into the LH stimulated DEV intake during the first 6 h at the 0.25 and 1-nmol doses by approximately 26 and 24%, respectively. Threonine (0.25, 2.5 nmol) did not alter COR intake at any time during the first 12 h. Our results suggest that: (1) the LH, along with the APC, likely acts as a chemosensory brain area for indispensable amino acids; and (2) both the APC and LH are part of a circuit that is involved in the short term anorectic response to amino acid imbalanced diets.