Effects of mu-CPP and mesulergine on dietary choices in deprived rats: possible mechanisms of their action

Although it has been well established that compounds that stimulate 5-HT(2C) and/or 5-HT(1B) receptors induce hypophagia by promoting satiety process, the relative role of these receptor subtypes in dietary choices remains to be fully determined. m-CPP is considered a useful probe of 5-HT(2C) receptor function in vivo and its administration reduces food intake and appetite in humans and rats. Conversely, the non-selective 5-HT(2C) receptor antagonist mesulergine elicits feeding in rats. Food intake and dietary choices were measured in a food-deprivation experimental protocol employing male Wistar rats. Animals were given access for a 4-h period to a pair of isocaloric diets. These two diets were enriched in protein or carbohydrate proportions, respectively, but fat content was held constant. The mixed 5-HT(2C/1B) receptor agonist, m-CPP, led to a dose-dependent hypophagia, due to substantial reduction in carbohydrate consumption while protein intake was spared (0.62, 1.25 and 2.50 mg/kg i.p., respectively). The non-selective 5-HT(2C) receptor antagonist and also D2 agonist, mesulergine, on its own produced a significant dose-dependent increase in both protein and carbohydrate diets (1.0 and 3.0 mg/kg i.p., respectively). Combined treatment with m-CPP, at its maximum effective dose, and mesulergine dose-dependently reversed m-CPP-induced hypophagia, during the 4-h test period. In order to clarify the effects of mesulergine on dietary choices since it is simultaneously a dopamine agonist besides its antiserotonergic properties, the D2 agonist apomorphine was also used. Apomorphine caused a dose-dependent increase in protein intake while carbohydrate and total food intake remained nearly unchanged (0.5 and 1.0 mg/kg i.p., respectively). It is concluded that the mesulergine-induced hyperphagic response on both diets is the expression of a dual mode of action, due to its 5-HT(2C) antagonist activity together with D2 agonist properties. The results further indicate that the activation of hypothalamic 5-HT(2C) receptors may be involved in both protein sparing and carbohydrate suppressing effects of 5-HT (m-CPP-like effect), whereas an important role in increase of protein consumption seems to have the dopaminergic system probably through D2 receptors (apomorphine-like and mesulergine-like effects, respectively).     

Centrally administered galanin-like peptide modifies food intake in the rat: a comparison with galanin

Galanin-like peptide (GALP) is a recently identified neuropeptide that shares sequence homology with the orexigenic neuropeptide, galanin. In contrast to galanin, GALP is reported to bind preferentially to the galanin receptor 2 subtype (GalR2) compared to GalR1. The aim of this study was to determine the effect of GALP on feeding, body weight and core body temperature after central administration in rats compared to the effects of galanin. Intracerebroventricular (i.c.v.) injection of GALP (1 micro g-10 micro g) significantly stimulated feeding at 1 h in both satiated and fasted Sprague-Dawley rats. However, 24 h after GALP injection, body weight gain was significantly reduced and food intake was also usually decreased. In addition, i.c.v. GALP caused a dose-related increase in core body temperature, which lasted until 6-8 h after injection, and was reduced by peripheral administration of the cyclooxygenase inhibitor, flurbiprofen (1 mg/kg). Similar to GALP, i.c.v. injection of galanin (5 micro g) significantly increased feeding at 1 h in satiated rats. However, there was no difference in food intake and body weight at 24 h, and galanin only caused a transient rise in body temperature. Thus, similar to galanin, GALP has an acute orexigenic effect on feeding. However, GALP also has an anorectic action, which is apparent at a later time. Therefore, GALP has complex opposing actions on energy homeostasis.     

Relationship of increased food intake and plasma ACTH levels to 5-HT1A receptor activation in rats

Various putative agonists of the 5-HT_1A receptor subtype induce feeding in rats, probably by activating raphé somatodendritic 5-HT autoreceptors. These drugs also produce a marked increase in plasma concentrations of corticotropin (ACTH). In the present experiment we attempted to localize the site of action of 5-HT_1A agonists on the secretion of ACTH and examined the relationship between 5-HT_1A agonist-induced feeding and ACTH secretion. Rats were injected with either the high affinity 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (0.016-1.0 mg/kg, s.c.) or the novel anxiolytics buspirone, gepirone or ipsapirone (2.0–16.0 mg/k/g, s.c.), and either had their food intake measured 2 hr post injection or were sacrificed 30–40 min post injection for measurement of plasma ACTH. Plasma ACTH also was measured in rats pretreated with the serotonin synthesis inhibitor, para-chlorophenylalanine (PCPA) for three days (150 mg/kg, i.p. per day) and subsequently injected with 8-OH-DPAT (0.3 mg/kg, s.c.).

As previously reported, the 5-HT_1A agonists increased both food agonists increased both food intake and plasma ACTH concentrations. After 8-OH-DPAT, ipsapirone and gepirone the amount of food consumed was positively correlated with the concentration of plasma ACTH. No such correlation was evident following buspirone. PCPA pretreatment resulted in near total depletion of brain 5-HT content but had no effect on the ACTH rise induced by 8-OH-DPAT. Therefore, in contrast to the presynaptic site previously proposed for 5-HT_1A agonist-induced feeding, the present results suggest a agonist-induced feeding, the present results suggest a postsynaptic location for the 5-HT_1A receptor mediating ACTH release.     

GABAergic mechanisms of the lateral parabrachial nucleus on sodium appetite

GABAergic activation in the lateral parabrachial nucleus (LPBN) induces sodium and water intake in satiated and normovolemic rats. In the present study we investigated the effects of GABAA receptor activation in the LPBN on 0.3M NaCl, water, 2% sucrose and food intake in rats submitted to sodium depletion (treatment with the diuretic furosemide subcutaneously+sodium deficient food for 24h), 24h food deprivation or 24 h water deprivation. Male Holtzman rats with bilateral stainless steel cannulas implanted into the LPBN were used. In sodium depleted rats, muscimol (GABAA receptor agonist, 0.5 nmol/0.2 microl), bilaterally injected into the LPBN, produced an inconsistent increase of water intake and two opposite effects on 0.3M NaCl intake: an early inhibition (4.3+/-2.7 versus saline: 14.4+/-1.0 ml/15 min) and a late facilitation (37.6+/-2.7 versus saline: 21.1+/-0.9 ml/180 min). The pretreatment of the LPBN with bicuculline (GABAA receptor antagonist, 1.6 nmol) abolished these effects of muscimol. Muscimol into the LPBN also reduced food deprivation-induced food intake in the first 30 min of test (1.7+/-0.6g versus saline: 4.1+/-0.6g), without changing water deprivation-induced water intake or 2% sucrose intake in sodium depleted rats. Therefore, although GABAA receptors in the LPBN are not tonically involved in the control of sodium depletion-induced sodium intake, GABAA receptor activation in the LPBN produces an early inhibition and a late facilitation of sodium depletion-induced sodium intake. GABAA activation in the LPBN also inhibits food intake, while it consistently increases only sodium intake and not water, food or sucrose intake.     

Dopamine receptor sensitive effect of dizocilpine on feeding behaviour

The effect of intracerebroventricular administration of dizocilpine on feeding behaviour and adrenal corticrotropic hormone (ACTH)-induced anorexia in elevated plus maze was examined. Dizocilpine (10, 20 and 40 nmol/rat, i.c.v.) showed a dose-dependent increase in food intake in 16 h food deprived rats. Dopamine receptor antagonists such as SCH 23390 (0.25 and 0.5 mg/kg, i.p.), pimozide (0.5 and 1 mg/kg, i.p.) and haloperidol (0.25 and 0.5 mg/kg, i.p.) dose-dependently blocked dizocilpine (40 nmol)-induced potentiation of food intake. Brain dopamine depletion by pretreatment with reserpine (5 mg/kg, i.p.) and alpha-methyl-p-tyrosine (200 mg/kg, i.p.) decreased food intake in rats. Similarly, pretreatment with reserpine and alpha-methyl-p-tyrosine (AMPT) reversed the hyperphagic effect of dizocilpine (20 and 40 nmol). Intracerebroventricular administration of ACTH (5 μg/rat) produced significant diminution of feeding duration and increased tasting latency and feeding latency in elevated plus maze which was reversed by dizocilpine (40 nmol). SCH 23390 (0.25 mg/kg), pimozide (0.5 mg/kg) and haloperidol (0.25 mg/kg) reversed the effect of dizocilpine on ACTH-induced behaviours in elevated plus maze. The present observations support and extend the hypothesis that endogenous excitatory aminoacids (EAAs) play a role in the control of food intake. Further, dizocilpine-induced hyperphagia and dizocilpine-induced reversal of ACTH effect on feeding behaviour in elevated plus maze involve DAergic mediation.     

Comparative effects of the antipsychotics sulpiride and risperidone in female rats on energy balance, body composition, fat morphology and macronutrient selection

Previous studies showed that the antipsychotic drugs (APDs) sulpiride (SUL) and risperidone (RIS) induced body weight gain (BWG), hyperphagia, and increased serum levels of leptin, prolactin and corticosterone in female rats. Neither SUL nor RIS increased BWG or food intake (FI) in male rats. To further develop the animal model of APD-induced obesity, SUL (20 mg/kg/sc), RIS (0.5 mg/kg/sc) or vehicle (1 cm³/kg/sc) were administered to female Wistar rats for 10 or 12 days. Body composition, fat tissue morphology, energy expenditure and food efficiency were assessed in animals fed a high-fat diet. In another experiment, macronutrient selection was evaluated in animals fed with pure diets. SUL and RIS significantly increased BWG and FI, with a stronger effect of SUL. Both drugs increased fat gain and food efficiency, and did not modify energy expenditure. Obesity was due to adipocyte hyperplasia. SUL-treated rats significantly decreased fat intake (p=0.039), showed a tendency to increase protein intake and did not modify carbohydrate consumption. No differences were observed between the RIS and the vehicle group. The macronutrient selection pattern differs from that observed in obese people undergoing APD treatment and in most animal models of obesity. Those findings suggest that SUL administration does not properly model APD treatment in humans. Results on macronutient selection in RIS-treated rats must be considered as preliminary, since in this experiment the animals did not gain weight significantly. Other diet protocols and lower APD doses must be tested to further characterize the RIS model.     

A novel neurotensin peptide analog given extracranially decreases food intake and weight in rodents

Neurotensin decreases food intake in the rat when injected into the cerebral ventricles. We tested the effect of a novel neurotensin analog (NT69L), injected intra-peritoneally (i.p.), on weight gain and food intake in rats. Sprague-Dawley rats (270 g) were injected i. p. with either saline or NT69L at 0.001 or 0.010 mg/kg. In further experiments, larger rats at a more steady state on the growth curve (400 g) were injected with either saline or 0.010 or 1 mg/kg NT69L. Food intake, water consumption and body weight were recorded daily. Weight gain was significantly reduced in the smaller rats injected with 0.001 or 0.010 mg/kg, showing only a 8.5 and 9.0% increase in original weight, respectively, as compared to a 29% increase for the controls. The larger rats injected with 1 mg/kg, had a significant reduction in body weight with a 3.0% decrease in original body weight as compared to a 2.4% increase for the controls. Food intake was significantly reduced suggesting that the weight loss observed after injection of NT69L was attributable in part to a reduction in food intake. The genetically obese Zucker rats injected with NT69L (1 mg/kg) had a significant reduction in weight gain and food intake. NT69L significantly increased blood glucose and corticosterone levels and decreased TSH and T4 in Sprague-Dawley and Zucker rats, an effect that was only transitory. NT69L also caused a decrease in norepinephrine in both the hypothalamus and nucleus accumbens, and an increase in dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and serotonin. In this study, NT69L exhibited a consistent and dramatic effect on body weight and food intake in Sprague-Dawley and obese Zucker rats, and enabled us to study the role that NT plays in weight control and the functional interactions of NT with brain amines, and metabolic and endocrinological parameters.     

Relationship of increased food intake and plasma ACTH levels to 5-HT1A receptor activation in rats

Various putative agonists of the 5-HT_1A receptor subtype induce feeding in rats, probably by activating raphé somatodendritic 5-HT autoreceptors. These drugs also produce a marked increase in plasma concentrations of corticotropin (ACTH). In the present experiment we attempted to localize the site of action of 5-HT_1A agonists on the secretion of ACTH and examined the relationship between 5-HT_1A agonist-induced feeding and ACTH secretion. Rats were injected with either the high affinity 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (0.016-1.0 mg/kg, s.c.) or the novel anxiolytics buspirone, gepirone or ipsapirone (2.0–16.0 mg/k/g, s.c.), and either had their food intake measured 2 hr post injection or were sacrificed 30–40 min post injection for measurement of plasma ACTH. Plasma ACTH also was measured in rats pretreated with the serotonin synthesis inhibitor, para-chlorophenylalanine (PCPA) for three days (150 mg/kg, i.p. per day) and subsequently injected with 8-OH-DPAT (0.3 mg/kg, s.c.).As previously reported, the 5-HT_1A agonists increased both food agonists increased both food intake and plasma ACTH concentrations. After 8-OH-DPAT, ipsapirone and gepirone the amount of food consumed was positively correlated with the concentration of plasma ACTH. No such correlation was evident following buspirone. PCPA pretreatment resulted in near total depletion of brain 5-HT content but had no effect on the ACTH rise induced by 8-OH-DPAT. Therefore, in contrast to the presynaptic site previously proposed for 5-HT_1A agonist-induced feeding, the present results suggest a agonist-induced feeding, the present results suggest a postsynaptic location for the 5-HT_1A receptor mediating ACTH release.     

Intermittent, chronic fenfluramine administration to rats repeatedly suppresses food intake despite substantial brain serotonin reductions

The mechanisms by which fenfluramine suppresses food intake and body weight have been linked to its ability to enhance transmission across serotonin synapses in brain. This drug initially lowers body weight and suppresses food intake, yet after repeated administration food intake soon returns to normal and body weight no longer decreases. Fenfluramine also causes rapid and prolonged reductions in brain serotonin concentrations, which might account for its loss of appetite suppression. This possibility has been evaluated in rats by assessing if intermittent, chronic fenfluramine administration could suppress food intake during each treatment period, and if so, whether such an effect occurs in the presence of reduced brain serotonin levels. Rats were injected once daily with 10 mg/kg D,L-fenfluramine for 5 days, and then received no injections for the next 5 days. Control rats received only vehicle injections. This 10-day sequence was repeated five more times. During each period of fenfluramine administration, daily food intake dropped markedly the first 1-2 days of treatment, but returned to pretreatment values by day 5. Daily food intake was normal or slightly above normal during non-injection periods. Body weight dropped modestly during each period of fenfluramine administration, and rose during each subsequent period when injections had ceased. Serotonin concentrations and synthesis rates in several brain regions were markedly reduced at early, middle, and late periods of the experiment. Despite the long-term reduction in brain serotonin pools produced by fenfluramine, the drug continues to reduce food intake and body weight. Several possible interpretations of these findings are considered, based on the multiple mechanisms through which this drug has been proposed to modify synaptic serotonin transmission.     

Noradrenergic innervation of the paraventricular nucleus: Specific role in control of carbohydrate ingestion

Acute injections of norepinephrine (NE) into the hypothalamic paraventricular nucleus (PVN) have been shown to elicit eating in satiated rats. The present report examines the effects of acute and chronic PVN infusion of NE on intake of various liquid and mixed solid diets and on selection of the pure macronutrients, carbohydrate, protein, and fat. The results demonstrate that noradrenergic stimulation of the PVN (40 nmoles of NE) reliably enhances ingestion of pure sucrose diets (liquid and solid) and also of sweet and non-sweet milk solutions. Saccharin intake, in contrast, is unaffected. In preference tests, rats injected with NE show a greater increase in consumption of sucrose cubes compared with lab chow pellets, but exhibit an equivalent preference for sweet and non-sweet carbohydrate-rich diets. Tests with the three pure macronutrients simultaneously available reveal, after NE injection, a strong and selective increase in consumption of carbohydrate, with little or no change in intake of fat and, in some cases, a suppression of protein intake. This clear preference for carbohydrate can be seen with chronic NE infusion (5 nmoles every 30 min over a 14-day period), as well as after acute NE injection (40 nmoles), and also with long-term (24-hr) as well as short-term (1-hr) food intake measurements. This and other evidence suggests that the PVN noradrenergic system may play a specific and unique role in the control of carbohydrate ingestion.