Route of delivery of phenylalanine influences its effect on short-term food intake in adult male rats

l-phenylalanine modulates gut hormone release and glucose tolerance, and suppresses food intake through the calcium-sensing receptor in rodents

OBJECTIVE

High-protein diets (HPDs) are associated with greater satiety and weight loss than diets rich in other macronutrients. The exact mechanisms by which HPDs exert their effects are unclear. However, evidence suggests that the sensing of amino acids produced as a result of protein digestion may have a role in appetite regulation and satiety. We investigated the effects of l-phenylalanine (L-Phe) on food intake and glucose homeostasis in rodents.

METHODS

We investigated the effects of the aromatic amino-acid and calcium-sensing receptor (CaSR) agonist l-phenylalanine (L-Phe) on food intake and the release of the gastrointestinal (GI) hormones peptide YY (PYY), glucagon-like peptide-1 (GLP-1) and ghrelin in rodents, and the role of the CaSR in mediating these effects in vitro and in vivo. We also examined the effect of oral l-Phe administration on glucose tolerance in rats.

RESULTS

Oral administration of l-Phe acutely reduced food intake in rats and mice, and chronically reduced food intake and body weight in diet-induced obese mice. Ileal l-Phe also reduced food intake in rats. l-Phe stimulated GLP-1 and PYY release, and reduced plasma ghrelin, and also stimulated insulin release and improved glucose tolerance in rats. Pharmacological blockade of the CaSR attenuated the anorectic effect of intra-ileal l-Phe in rats, and l-Phe-induced GLP-1 release from STC-1 and primary L cells was attenuated by CaSR blockade.

CONCLUSIONS

l-Phe reduced food intake, stimulated GLP-1 and PYY release, and reduced plasma ghrelin in rodents. Our data provide evidence that the anorectic effects of l-Phe are mediated via the CaSR, and suggest that l-Phe and the CaSR system in the GI tract may have therapeutic utility in the treatment of obesity and diabetes. Further work is required to determine the physiological role of the CaSR in protein sensing in the gut, and the role of this system in humans.

Specific amino acids inhibit food intake via the area postrema or vagal afferents

To maintain nutrient homeostasis the central nervous system integrates signals that promote or inhibit eating. The supply of vital amino acids is tuned by adjusting food intake according to its dietary protein content. We hypothesized that this effect is based on the sensing of individual amino acids as a signal to control food intake. Here, we show that food intake was most potently reduced by oral L-arginine (Arg), L-lysine (Lys) and L-glutamic acid (Glu) compared to all other 17 proteogenic amino acids in rats. These three amino acids induced neuronal activity in the area postrema and the nucleus of the solitary tract. Surgical lesion of the area postrema abolished the anorectic response to Arg and Glu, whereas vagal afferent lesion prevented the response to Lys. These three amino acids also provoked gastric distension by differentially altering gastric secretion and/or emptying. Importantly, these peripheral mechanical vagal stimuli were dissociated from the amino acids' effect on food intake. Thus, Arg, Lys and Glu had a selective impact on food processing and intake suggesting them as direct sensory input to assess dietary protein content and quality in vivo. Overall, this study reveals novel amino acid-specific mechanisms for the control of food intake and of gastrointestinal function.

An intragastric amino acid mixture influences extracellular amino acid profiles in the lateral hypothalamic area of freely moving rats

We tested the effect of equicaloric loads of glucose (0.89 g) or a balanced amino acid mixture (0.85 g) on extracellular amino acid concentrations in the brains of freely moving rats. At 15:30 hours, the microdialysis probe was inserted into the lateral hypothalamic area of ambulatory rats, and food and water were removed. Dialysates were collected every 20 min from 1 h prior to gavage (18:00 hours) and until 3 h after the gavage. Amino acid concentrations in the dialysate were determined by reverse-phase HPLC. Following the amino acid gavage, extracellular amino acid concentrations significantly increased from baseline for alanine, isoleucine, leucine, methionine, threonine, tyrosine, and valine. Those elevations occurred within 20-40 min following the amino acid load, and lasted up to 100 min. After the glucose and water treatments, amino acid concentrations were either not affected or gradually diminished from baseline. We conclude that extracellular amino acid concentration in the lateral hypothalamus is influenced by the composition of food consumed.Key words: blood-brain barrier, food intake, glucose, microdialysis, protein.

Evidence for histamine involvement in the effect of histidine loads on food and water intake in rats

We examined the hypothesis that histidine is a regulator of short-term food and water intake in rats and that this control is through histidine's action as a precursor for histamine. The primary objectives were to measure food and water intake after histidine monohydrochloride monohydrate (His-HCl) given by intragastric (IG) and intraperitoneal (IP) routes of administration and to measure feeding and drinking responses to histidine when given after blockade of the histaminergic pathway by chlorpheniramine (CPA) and alpha-fluoromethylhistidine (FMH). Eight experiments were conducted using a back-to-back design. Rats were given treatment by IP or IG administration, and food and water intake was measured during time periods of 0-1, 1-2, 2-3 and 3-14 h. Histidine consistently reduced food intake with the sensitivity to IP much greater than to the IG route. The effect of histidine given by IP or IG on water intake was similar, generally causing an increase at least in the first hour. Histidine's action was not accounted for by its energy, pH or nitrogen content. Because FMH, which blocks the enzyme converting histidine to histamine, partially reversed the effect of histidine on food and water intake, these results support the hypothesis that histidine regulates food and water intake, at least in part, through its precursor control of histamine.

Buspirone-induced carbohydrate feeding is not influenced by route of administration and nutritional status

Rats were habituated to ad lib food intake from two isoenergetic diets that differed in carbohydrate and protein content. To examine the route of administration effect, buspirone (0.6, 1.0, and 1.4 mg/kg) was injected into satiated rats either subcutaneously or intraperitoneally. Overall, no route of administration effect was observed; however, when results of the lowest dose were analyzed separately, the subcutaneous route was more effective than the intraperitoneal route. Regardless of route of administration, buspirone increased food intake over the first 2 h of food presentation in a dose-dependent manner. Moreover, the increase was entirely attributed to increases in intake from the high carbohydrate diet. In the subsequent experiment, the effect of buspirone (0.6 mg/kg) was examined in both satiated (early light period) and nonsatiated rats (early dark period). Both groups responded to buspirone with an increase in carbohydrate intake. Despite differences in baseline intake, the absolute increase was similar between satiated and nonsatiated rats. These data suggest that both sensitivity and selectively of buspirone-induced feeding are neither influenced by route of administration nor nutritional status of rats.