Food intake is inversely correlated with central nervous system histamine receptor (H1) concentrations in male Sprague-Dawley rats fed normal, low protein, low energy or poor quality protein diets.

The reported studies were designed to examine relationships between whole-brain histamine receptors (H1) and food intake in male Sprague-Dawley rats. Three different experiments were conducted. In each experiment, control rats were fed normal protein (25 g casein/100 g food) and normal metabolizable energy (16.21 kJ/100 g food) diets. Feeding low protein diets (1 g casein/100 g food) elevated central H1 receptor concentrations (P < 0.0027) and reduced voluntary food intake (P < 0.007) compared with normal diets. Feeding low energy diets lowered H1 receptor concentrations (P < 0.0089) and increased voluntary food intake (P < 0.0012). Low quality protein diets also affected the central nervous histaminergic system. Whole-brain H1 receptor concentrations were significantly higher for rats fed low quality protein (25 g gelatin/100 g food) compared with rats fed casein (P < 0.0001). Rats fed medium quality protein (25 g wheat gluten/100 g food) or low quality protein ate significantly less food (P < 0.0001). In all experiments, dietary manipulation affected central histamine receptors. Elevated concentrations of H1 receptors were associated with a decrease in food intake whereas lowered concentrations of H1 receptors were associated with an increase in food intake (P < 0.001). The results of these experiments support the hypothesis that central histamine H1 receptor concentrations in male rats are inversely correlated with voluntary food intake and affected by dietary composition.

Gender affects rats' central nervous system histaminergic responses to dietary manipulation.

The histaminergic system (histamine and its H1-receptor) of the central nervous system has been implicated in control of food intake. The reported studies were designed to examine the effects of food restriction and very low (1%) protein diets on central nervous system H1-receptors in male and female rats. In a series of experiments, groups of rats were freely fed a 25% protein diet, a 1% protein diet, or fed the 25% protein diet at 4 g/100 g body weight for 14-20 d. When freely fed 25% protein diets, females had higher whole-brain H1-receptor binding than males on d 1 (female 122.36 +/- 4.53 and male 65.78 +/- 3.82 pmol/g protein; P < 0.001). Changing diets affected central H1-receptor binding in both males and females (P < 0.003). When rats were fed both restricted levels of food and 1% protein diets, the receptor binding of males increased by d 5 whereas that of females decreased by d 5 (P < 0.001). When fed 1% protein diets, females had decreased H1-receptor binding (98.4 +/- 2.38 pmol/g protein) and that in males increased to 119.81 +/- 5.09 pmol/g protein. After 15 d, females had eaten significantly more food than males: females 166 +/- 4.9 g, males 124 +/- 1.9 g (P< 0.0007). Males had a significantly greater weight loss than females: males -28.8 +/- 2.6 g, females -17.08 +/- 0.97 g (P < 0.0007). When fed restricted diets, females had decreased H1-receptor binding (93.81 +/- 5.58 pmol/g) whereas binding in males increased to 111.27 +/- 8.55 pmol/g. Preliminary saturation binding studies indicated that restricted food intake lowered receptor density (females consuming 25% protein: 715 +/- 30 pmol/g protein; female restricted: 467 +/- 28 pmol/g protein, P < 0.05), while 1% protein increased receptor sensitivity, i.e., lowered KD (males consuming 25% protein: 15.3 +/- 1.8 nmol; males fed low protein: 2.8 +/- 0.27 nmol). This study suggests that dietary manipulation affects central H1-receptor binding in a gender-specific manner, thereby modulating central histaminergic activity during food or protein deficit.

Dietary induced anorexia: a review of involvement of the histaminergic system.

This review examines possible relationships between anorexia, dietary intake and central nervous system histaminergic activity. The hypothesis being reviewed is that one component of normal or pathophysiological neuroregulation of food intake involves histaminergic activity in the central nervous system, as influenced by concentrations and bioperiodicities of histamine and/or histamine receptors. Changes in concentrations of receptors are gender specific. Low protein quality or quantity diets elevate both central histamine and histamine receptors (H1) in rats while significantly decreasing their food intake. When injected with histaminergic antagonists, rats fed low protein diets increase food intake and have improved efficiency of weight gain. This review supports a dual hypotheses: central histaminergic activity is involved in the regulation of food intake, but food intake patterns (including dietary composition or energy content) can modify central histaminergic activity. This review also suggests that modified histamine and/or H1 receptor concentrations are potential mechanisms for elevated central histaminergic activity in food intake-related pathophysiological states. Thus, dietary interventions (clinically- or self-imposed) which modify food intake or diet composition have the potential of affecting the histaminergic system. Also, drugs with antihistaminergic properties have the potential of affecting food intake/weight gain patterns by interfering with normal neurochemical signals.

Growth requirements of endothelial cells in culture: variations in serum and amino acid concentrations.

Endothelial cell growth in vitro is limited to the availability of nutrients from commercially available media and added serum. Nutrients, such as amino acids, are chiefly derived from the cell culture medium, rather than from added serum, and optimal endothelial cell growth may be dependent on amino acid levels in the culture media. To test this hypothesis, porcine pulmonary artery-derived endothelial cells were exposed to culture medium 199 (M199), amino acid-deficient M199 (dM199), as well as dM199 supplemented with amino acids. Cell protein was similar in cells cultured for 3 d in M199 supplemented with 1, 3, 5 or 10% bovine calf serum, respectively. Addition of amino acid solutions (L-amino acids [Laa], DL-amino acids [DLaa], 2Laa, or Laa+glutamine) to dM199 demonstrated a cell dependence for optimal growth on the type of amino acids as well as on the total available nitrogen in the media. Compared with M199, dM199 supplemented with Laa only partially supported long-term growth of endothelial cells in culture. On the other hand, dM199 supplemented with either 2Laa, DLaa, or Laa+ glutamine was superior over M199 with regard to endothelial cell growth. The addition of Laa+glutamine to dM199 was most growth-supporting, with an increase of over 2.6-fold in total cell protein compared with cells cultured with M199. These results suggest that, in addition to the presence of essential amino acids, total available nitrogen in culture media may be a critical factor for optimal endothelial cell growth.

Manipulation of central nervous system histamine or histaminergic receptors (H1) affects food intake in rats.

The reported studies were designed to examine relationships between central nervous system histamine, histaminergic receptors (H1) and food intake in rats. The hypothesis being tested was as follows: "One component of the neuroregulation of food intake involves histaminergic activity in the hypothalamus as influenced by variation of histamine levels and/or H1 receptor concentrations." We performed combinations of dietary, surgical and pharmacological treatments on male or female rats. We fed groups of male or female rats diets containing either 4 g casein/100 g diet (low protein diet) or 25 q casein/100 g diet (normal protein). Rats with surgical ablation of the paraventricular nucleus did not decrease food intake when fed the low protein diet, whereas adrenalectomized rats did. Increasing central histamine levels decreased food intake, whereas decreasing central histamine increased food intake. Rats injected with histaminergic (H1) antagonists lost the ability to detect low protein diet in short-term experiments and had improved efficiency of weight gain. Rats that were fed the low protein diet or pair-fed the normal protein diet had greater H1 receptor concentrations in whole brain preparations when compared with rats fed the normal protein diet. No differences were noted due to gender. Thus, manipulation of histamine levels affected food intake as hypothesized, i.e., increasing central histamine decreased food in rats fed the normal protein diet, whereas decreasing central histamine or blockade of H1 receptors increased food intake in rats fed the low protein diet.

Weanling rats display bioperiodicity of growth and food intake rates.

Bioperiodicities in biological phenomena have long been studied to gain insight into the dynamics of living organisms. However, apparent periodicities in rates of growth and food intake have often been either ignored or attributed to random error. We have found that such periodicities are not random but take the form of infradian rhythms (period > 28 h), with multiple periods including circasemiseptan (period = 3.5 d). In the reported experiments, we examine the existence of periodic, oscillatory phenomena in growth and eating patterns of rats. The cosinor model of Halberg was used to establish the occurrence of rhythms and quantify rhythm characteristics of components identified in least squares spectra. The four parameters of the rhythm were found to be altered by concentrations of the limiting dietary nutrient in that they had different spectral signatures. Existence of such oscillations in growth rates and food intake rates could have important consequences in designing feeding and growth experiments for achieving optimal physiological responses as well as providing possible insights into eating disorders.

Determination of nutritional requirements in rats: variation with time of weight gain responses to indispensable amino acids.

The Saturation Kinetics Model (SKM) can be used to describe physiological responses as functions of a limiting dietary nutrient. Physiological responses also vary with time, and, by graphing each parameter of the SKM as a function of time, the model equation can be used to produce a three-dimensional response surface, allowing the investigator to predict requirement as a function of both dietary nutrient concentration and time. To test this hypothesis, rats were fed diets containing graded levels of indispensable amino acids (0-10 g/100 g). The inhibition form of the SKM was able to predict the complete response range of weight gain, food intake, weight-specific weight gain and weight-specific food intake for each amino acid on a day-by-day basis. With a complete response range established, nutrient requirements (defined as maximum responses) could be determined explicitly by the equation: Requirement = (K0.5.KS)0.5, where K0.5 and KS are parameters of the SKM. No arbitrary decisions (such as break points, slopes, percentages of maximum, etc.) were necessary to accomplish this. This approach allows one to set nutritional requirements and optimize responses in a dynamic system without causing inhibiting and/or toxic responses. In each case (valine, methionine, threonine and histidine), the dietary amino acid concentration required for maximal weight gain exceeded current NRC recommendations, which may be low because they fall in the more time-dependent portion of the response curve whereas maximum responses do not.

The determination of nutritional requirements: mathematical modeling of nutrient-response curves.

The Saturation Kinetics Model (SKM) is useful in describing physiological responses as functions of a limiting dietary nutrient. We have recently expanded the SKM to predict the inhibited portions of the nutrient-response curve. By using the SKM, nutrient requirements can be predicted analytically by, requirement = (K0.5 x KS)0.5. It is also possible to set an upper and lower dietary nutrient concentration which encompasses the 100% response range for each response, thereby giving an inhibition or toxicity index. This index allows one to set nutritional requirement levels precisely, optimizing responses without moving into inhibiting or toxic ranges of nutrients. The model equation can also be converted to a three-dimensional representation by graphing each parameter as a function of time. This allows one to visualize a three-dimensional response surface, showing response as a function of time and dietary nutrient concentration.

Histidine, histamine, and the neuroregulation of food intake: a review and hypothesis.

Feeding, a behavior regulated by the central nervous system (CNS), includes the acquisition of specific essential nutrients and the maintenance of energy balance. Modulation of feeding behavior is a normal part of survival, but certain pathological conditions interrupt or modify regulatory aspects of feeding, thereby leading to inappropriate intake. This review examines aspects of metabolism associated with the anorexia seen in animals suffering from protein-energy malnutrition (PEM). The main focus is the indispensable amino acid histidine (His), the biosynthetic precursor of the neurotransmitter histamine (HA). In kwashiorkor-like PEM, His is elevated in plasma and brain, whereas all other indispensable amino acids are decreased. The elevation of His in the brain is to concentrations five times normal. Because the rate of HA synthesis in the brain is a function of the His concentration, His elevation raises the possibility of a profound direct effect of CNS function. In children, PEM consistently produces the symptoms of depressed food intake, edema, growth failure, and psychomotor changes. One known central effect of HA is the stimulation of ACTH and corticosteroid release. Based on these observations, the hypothesis being examined is as follows: one component of the pathophysiological neuroregulation of food intake involved the His-induced variation of HA concentration in the hypothalamus and the subsequently altered neurochemical activity at the corticotropin-releasing factor (CRF) neurons o the paraventricular nucleus (PVN).

The determination of nutritional requirements in rats: mathematical modeling of sigmoidal, inhibited nutrient-response curves.

The Saturation Kinetics Model (SKM) is useful in describing many physiological responses as functions of a limiting dietary nutrient. However, as nutrients are fed at higher dietary concentrations, responses become inhibited and diminish from their usual plateaus. By adding an inhibition constant (Ks) to the SKM in a manner consistent with substrate inhibition (based on enzyme kinetics), it becomes possible to predict the inhibited portions of the nutrient-response curve. To test this, rats were fed diets of graded levels of casein (0-75%) or lysine (0-6.2%), and weight gains and food intakes were measured daily for up to 2 wk. The inhibition form of the SKM was able to predict the complete response range of each experiment, producing a Ks (weight gain) at a dietary level of 50.60% for casein and 7.56% for lysine. It was also possible to set up an upper and lower dietary nutrient concentration that encompassed the 100% response range for each response, thereby giving an inhibition or toxicity index of 2.02 for casein and 4.98 for lysine. This index allows one to set nutritional requirement levels precisely, optimizing responses without moving into inhibiting or toxic ranges of nutrients. Based on growth response curves, requirements were 25.61% for casein and 1.97% for lysine.

Brain histidine and food intake in rats fed diets deficient in single amino acids.

Histidine (His) is elevated in plasma and brain during protein deficiency as well as in several pathological conditions, leading to the possibility of a direct effect on central nervous system (CNS) function. In this study, groups of weanling rats were fed diets containing graded levels of casein or a single indispensable amino acid (IAA: Leu, Val, Ile, Phe, Trp, Thr, Met or Lys) in order to produce nutritionally-deficient states. Body weight gains and food intakes were recorded daily for 2 wk. Whole brain and serum samples were obtained and analyzed for amino acid (AA) content. All weight gain and food intake responses could be predicted by the Saturation Kinetics Model. The only consistent pattern observed in AA profiles which could be correlated with food intake was an increase in brain His concentrations. Limiting dietary casein or IAA elevated brain His above controls 2.5- and 1.5-fold, respectively. Food intake was generally depressed by 50% at brain His concentrations above 105 nmol/g. Since His is the precursor of the depressant neurotransmitter histamine (HA), systemic increases may be significant in that HA could be a possible cause of the anorexia observed in protein and IAA deficiency.

New method for formulation of amino acid concentrations and ratios in diets of rats.

Parameters derived from the application of the saturation kinetics model (SKM) to rat growth experiments were used to formulate a complete dietary amino acid mix for weanling rats. Dietary ratios and concentrations for indispensable amino acids (IAA) and arginine were calculated using the parameter K0.5. A curve shift technique was used to determine dietary concentrations for conditionally indispensable amino acids (CAA) and dispensable amino acids (DAA). The model was also used to determine a dietary ratio of (IAA + Arg)/(CAA + DAA). Using the dietary amino acid concentrations suggested by the model and an (IAA + Arg)/(CAA + DAA) ratio of 1, a growth response curve was constructed and compared to a similar curve using the amino acid mix of Rogers and Harper. The modeling approach produced a 10-15% improvement in growth over the Rogers and Harper mix. The SKM is discussed in terms of calculating an ideal nutrient ratio and choosing a desired response level. It is demonstrated that the model can rapidly produce accurate estimates for dietary amino acid levels, while minimizing required numbers of laboratory animals.

Analysis of BP response and Ca2+ metabolism using the saturation kinetics model.

The provision of supplemental dietary calcium (dCa) lowers blood pressure (BP) in the spontaneously hypertensive rat (SHR). Whether calcium's antihypertensive effects can be expressed in the presence of potentially hypertensinogenic nutrients is not known. Furthermore, the amount of dCa required to attenuate hypertension in the SHR remains undetermined. Along with establishing the effects of dCa on BP under conditions of a high Na+ intake, we sought to define the lowest dose of dCa associated with the greatest attenuation in arterial pressure in the young SHR. Thirty-five 6-wk-old SHR were fed one of five diets containing either 0.1, 0.25, 0.5, 1.0, or 2.0% dCa. All diets contained 1.0% Na+. The rates of change (delta) in body weight, BP, and serum ionized calcium were determined between 6 and 20 wk of age. Bone density (BD) was measured only at 20 wk of age. The data were analyzed using the saturation kinetics model. Results indicate that the half-maximal dose (K50) of dCa needed to lower pressure is 0.67 +/- 0.18%, which is higher than the K50 for weight (0.23 +/- 0.18) and BD (0.36 +/- 0.22). It is concluded that supplemental dCa lowers BP despite a high Na+ intake. Furthermore, a dose of approximately 1.5 dCa is sufficient to attenuate the rate of hypertension in the young growing SHR.

Prediction of brain and serum free amino acid profiles in rats fed graded levels of protein.

Daily weight gains and food intakes were measured in male, 120-g rats fed graded levels of dietary casein. After 14 d, serum and brain amino acid concentrations were measured. All physiological responses were tested for a functional relationship to dietary casein concentration. Food intake, weight gain and many serum amino acid profiles were shown to be saturable functions of percent casein in the diet. In general, essential amino acids increased in serum with increasing dietary casein concentration while nonessential amino acids decreased with increasing dietary casein concentration. Brain amino acid concentrations were shown to be linear functions of serum levels with the exceptions of phenylalanine and the acidic amino acids. Most amino acids showed a smaller range of values in brain than in serum. The exceptions were the levels of threonine, glutamine, serine and histidine, which were three times greater in brain than in serum. Brain levels of the neutral amino acids tryptophan and tyrosine were highly correlated with the amino acid/neutral amino acid ratios in serum, whereas leucine was negatively correlated. Brain histidine, which was inversely correlated with dietary casein, was found to correlate with specific food intake patterns. The four-parameter mathematical model for physiological responses was able to predict all the observed saturation type responses in the experiment.

A new protein quality evaluation index based on growth responses of rats.

The protein efficiency ratio (PER) is the official method for protein quality evaluation in the United States and Canada. Two other widely used indices of evaluation are the slope ratio and net protein ratio (NPR) methods. Each of these methods has problems associated with its calculation and interpretation. In this paper, a new index, actual protein utilization ( APU ) is discussed, and its relationship to the other indices is examined. Each of the indices is given a theoretical basis by relating it to the four parameter mathematical model for physiological responses. To compare the indices, growth bioassays were carried out using male rats (40, 60, 115 g) consuming three different proteins (casein, lactalbumin and soy) and an amino acid mixture (Rogers and Harper). Dose-response curves were generated for each diet, and the indices PER, NPR, Slope and APU were calculated. APU was shown to have certain characteristics that make it superior to the other methods of protein quality evaluation, i.e., it closely approximates the growth response curve and it incorporates a term for the protein intake required for maintenance.

Control of physiological response in the rat by dietary nutrient concentration.

In three separate experiments, growing, male Sprague-Dawley rats were fed diets which contained: 1) graded levels of fiber 0-70%, 2) graded levels of pyridoxine 1-10 mg/kg diet, and 3) graded levels of casein 0-30%. The following physiological responses were measured in each respective experiment: 1) food intake, weight gain, serum triglycerides, 2) food intake, weight gain, SGPT levels, and 3) weight specific food intake, weight gain, relative testes weight. Diets were fed as a single source, and in each case, physiological response could be predicted as a function of dietary nutrient concentration. When self-selection is prevented, rats establish new steady-state response profiles, which are sigmoidal in shape and dependent on the interaction of the rats' genetic potential with the environmental configuration.