Effect of quipazine, a selective 5-HT3 agonist, on dietary self-selection of different macronutrient diets in male and female rats

Contrasting effects of 5-HT3 receptor stimulation of the nucleus accumbens or ventral tegmentum on food intake in the rat

Although serotonin (5-HT) signaling is known to regulate food intake and energy homeostasis, the roles of the 5-HT₃ receptor in feeding processes have been elusive. 5-HT₃ receptors are found throughout mesolimbic circuitry that promote feeding not only in response to hunger, but also to the palatable and rewarding properties of food. These experiments examined if stimulation or blockade of the 5-HT₃ receptor of the nucleus accumbens (NAcc) or ventral tegmentum affected food intake in the rat in response to hunger or the presence of a palatable diet. Rats (N=6-9/group) received bilateral injections of the 5-HT₃ agonist m-chlorophenylbiguanide hydrochloride (mCPBG; at 0.0, 10.0, or 20.0μg/0.5μl/side) or the 5-HT₃ antagonist ondansetron hydrochloride (at 0.0, 1.0, 2.0, or 5.0μg/0.5μl/side) into either the NAcc or the ventral tegmentum. NAcc 5-HT₃ receptor stimulation significantly increased 2-h food intake in food-deprived animals offered rat chow and in a separate group of unrestricted rats offered a sweetened fat diet. In contrast to the feeding increase seen with NAcc treatments, stimulation of 5-HT₃ receptors of the ventral tegmentum significantly reduced food and water intake in food-restricted animals; reductions of intake in non-restricted rats offered the palatable diet did not approach significance. Blockade of the 5-HT₃ receptor had no effect on feeding in either brain region. These data support a functional role for serotonergic signaling in the mesolimbic pathway on motivated behavior, and demonstrate that 5-HT₃ receptors differentially modulate food consumption in a region-dependent manner.

Effect of chronic treatment with clomipramine on food intake, macronutrient selection and body weight gain in rats

Long-term treatment with clomipramine (CMI), a tricyclic antidepressant, induces food craving and body weight gain in patients. The present study investigated the effects of chronic treatment with CMI on total food intake, macronutrient selection, and body weight gain in rats. Male Wistar rats were maintained on a dietary self-selection regime with separate sources of protein, fat and carbohydrate. Animals received i.p. injections of CMI (0, 3, 10, 30 mg/kg) during 27 consecutive days. Food consumption and body weight were recorded daily and results were calculated as average of three consecutive days, namely during pre-treatment (3 d before pharmacological treatment), treatment (7th-9th; 16th-18th and 25th-27th days), and post-treatment (28th-33rd days). Results showed that CMI (30 mg/kg) significantly decreased energy intake during all treatment period, an effect that was related to a decrease in both carbohydrate-rich diet intake and body weight gain. At dose of 3 mg/kg CMI increased the total energy intake in the 16th-18th days, suggesting an apparent biphasic effect of chronic treatment with CMI on caloric intake. Chronic administration with CMI (27 d) did not alter protein-rich or fat-rich diet consumption. The main result of this study indicated that chronic treatment with CMI decreases rather than increase food consumption and body weight gain in rats exposed to a macronutrient self-selection procedure.

Macronutrient selection in experimental animals

To address the question of how animals choose diets from an array of nutritionally different foods, researchers have designed experiments in which animals are provided with separate sources of the three macronutrients, protein, fat and carbohydrate. These experiments are useful for investigating the neuroanatomical and neurochemical mechanisms involved in food choice, as well as for assessing the effects of changes in physiological status (e.g., pregnancy; lactation and increased energy expenditure) or disease states (e.g., diabetes; and obesity) on nutrient intakes.

Human urocortin 2, a corticotropin-releasing factor (CRF)2 agonist, and ovine CRF, a CRF1 agonist, differentially alter feeding and motor activity

Two corticotropin-releasing factor (CRF) receptor families have been identified (CRF1 and CRF2). Whereas anxiogenic-like roles for the CRF1 receptor have been identified, behavioral functions of the CRF2 receptor remain obscure. Urocortin 2 (Ucn 2), a CRF-related peptide that selectively binds CRF2 receptors, was recently identified and recognized for its central anorectic properties. The present study tested the hypothesis that the anorexigenic mode of action of Ucn 2 differed from that of ovine CRF (oCRF), a preferential CRF1 receptor agonist. The behavioral effects of intracerebroventricular administration of Ucn 2 were compared with those of oCRF in nondeprived male Wistar rats (n=102). Ucn 2 reduced 6-h food and water intake at doses that did not induce visceral illness (0.1, 1, and 10 microg), as indicated by kaolin intake. Ucn 2 retained its potent anorectic activity in rats receiving a highly palatable cafeteria diet, preferentially reducing intake of carbohydrate (CHO)-rich items while sparing intake of mixed-fat/CHO items. In contrast to Ucn 2, oCRF (10 microg) suppressed 6-h intake of cafeteria diet-fed rats without regard to macronutrient composition. Rather, oCRF most potently suppressed intake of preferred food items. Whereas oCRF had short-onset motor-activating effects, Ucn 2 had nondose-dependent, delayed-onset motor-suppressing effects. Thus, central infusion of a CRF2 receptor agonist suppressed intake of both bland and palatable diets without inducing behavioral arousal or malaise, and the profile of anorexigenic effects qualitatively differed from those of a CRF1 receptor agonist. The results suggest the existence of distinct forms of CRF1- and CRF2-mediated anorexia.

Effect of sibutramine on macronutrient selection in male and female rats

Sibutramine, a serotonin-noradrenaline reuptake inhibitor (SNRI), has been shown to be a safe and effective weight-loss drug. The purpose of the present study was to examine whether sibutramine has an effect on macronutrient selection in both female and male rats in addition to total food intake. Wistar rats of both sexes were divided into three groups, and each group was offered a different set of three sensorily contrasting macronutrient-specific diets, each set including carbohydrate-, protein-, and fat-rich diets. Sibutramine (10 mg/kg) was shown to consistently decrease carbohydrate and fat intake at all data points regardless of gender and diet. Intake of carbohydrate differed between male and female rats at 2 h post administration with 2.5 and 5 mg/kg of sibutramine. The effect of sibutramine on protein intake was diet- and gender-specific. All doses of sibutramine decreased total food intake regardless of gender and diet group beginning at 6 h post administration. In conclusion, sibutramine affected macronutrient selection and emphasis on dietary recommendations, as well as appropriate dosage according to gender should be considered during therapy.

Effect of melatonin on total food intake and macronutrient choice in rats

Melatonin, a hormone secreted in a rhythmic manner over 24 h mainly by the pineal gland, is used to alleviate the symptoms of jetlag and treat sleeping problems. The objective of the present study was to examine the effects of a 7-h phase-shift from the natural peak of melatonin secretion on total food intake and macronutrient selection. Forty-eight adult Wistar rats of both sexes were divided in three dietary groups, each group offered a simultaneous and different choice of a carbohydrate- and a protein-rich diet. Macronutrient intakes following intraperitoneal administration of four doses of melatonin (3000, 6000, 10000 and 15000 pg/ml blood) at dark onset were examined. Melatonin increased short- (4 h postinjection) and long-term (12 h postinjection) nocturnal total food intake in both male and female rats, mainly with the two highest doses. This effect of melatonin was mainly due to a short-term increase of intake across all carbohydrate-rich diet preparations (dextrin/cornstarch, cornstarch, and sucrose/cornstarch) and across genders. This consistent effect of melatonin on the intake of carbohydrate-rich diets with contrasting sensory attributes rules out the possibility that melatonin acts on sensorymotor pathways, thus suggesting that melatonin's effect on food intake is controlled by the carbohydrate content of the diet. In contrast, melatonin could be affecting some sensory or motor processes peculiar to the ingestion of protein since it increased protein-rich diet intake inconsistently across the various preparations (casein, soy isolate, and egg protein) as well as genders. This evidence supports the view that melatonin acts as a time indicator, reinforcing the animals with a "night cue", and favors predominant carbohydrate intake normally occurring at the beginning of the activity period.

Chronic paroxetine infusion influences macronutrient selection in male Sprague-Dawley rats

Decreased intake and weight loss are among the side effects frequently reported with chronic selective serotonin reuptake inhibitor (SSRI) use in both humans and animals. In an earlier study, we documented that paroxetine administered for several weeks induced a weight loss of greater than 10% in some male Sprague-Dawley rats (Pharmacol. Biochem. Behav. 63 (1999) 435). As a follow-up to that work, we investigated in this study whether such treatment influenced dietary macronutrient selection. Animals were first habituated to foods containing principally either proteins, fats, or carbohydrates in a self-selection paradigm, after which they were implanted intraperitoneally with osmotic minipumps that delivered either paroxetine (7.5 mg/kg/day) or vehicle (50:50 ethanol:water) for 28 days; food intake and weight changes were documented during this period. No acute effects of the drug were apparent. By the fifth day of treatment, significant differences in weight gain between groups were observed and thereafter generally maintained for the remainder of the study, with animals receiving paroxetine showing about an 8% decrease in weight gain overall. Carbohydrate and fat intakes were significantly reduced, whereas preference was unchanged in fats and proteins and initially decreased in carbohydrates; in the latter, this pattern reversed and exceeded vehicle animals for the second half of the study. Several hypotheses are discussed with respect to specific and nonspecific effects of paroxetine on feeding and macronutrient selection.

5-HT1A receptor activity disrupts spontaneous alternation behavior in rats

Agonists selective for three different serotonin (5-HT) receptor subtypes were tested for the ability to disrupt spontaneous alternation behavior (SAB) in the CD strain of rats. Rats were scored for alternation or repetition in their choice of arms of a T-maze equally baited with chocolate milk. Compared with vehicle controls, the 5-HT(1A) agonist 8-hydroxy-dipropylaminotetraline (8-OH-DPAT; 2 mg/kg) significantly (P<.0001) increased repetitive choices (disrupted SAB). In contrast, intraperitoneal injections with the 5-HT(2) agonist R-(-)-dimethoxyiodophenylaminoethane (DOI; 1 mg/kg) or the 5-HT(3) agonist N-methyl quipazine (NMQ; 3 mg/kg) had no significant effect on SAB in CD rats. Onset of vicarious trial and error (VTE) behavior prolonged the time required for each rat to select an arm of the T-maze when injected with either 8-OH-DPAT (P<.0001) or buspirone (1-2 mg/kg), a 5-HT(1A) partial agonist. The disruption of SAB and the induction of VTE behavior were reversible with behavioral scores returning to preinjection levels within 48 h after injections. The disruption of SAB by 8-OH-DPAT was also seen with the Long-Evans rat strain. The results extend the use of the SAB model and point to a specific role of 5-HT(1A) receptors in the induction of repetitive behavioral patterns.

Pharmacological approaches for the treatment of obesity

The high incidence of obesity, its multifactorial nature, the complexity and lack of knowledge of the bodyweight control system, and the scarcity of adequate therapeutics have fuelled anti-obesity drug development during a considerable number of years. Irrespective of the efforts invested by researchers and companies, few products have reached a minimum level of effectiveness, and even fewer are available in medical practice. As a consequence of anti-obesity research, our knowledge of the bodyweight control system increased but, despite this, the pharmacological approaches to the treatment of obesity have not resulted yet in effective drugs. This review provides a panoramic of the multiple different approaches developed to obtain workable drugs. These approaches, however, rely in only four main lines of action: control of energy intake, mainly through modification of appetite;control of energy expenditure, essentially through the increase of thermogenesis;control of the availability of substrates to cells and tissues through hormonal and other metabolic factors controlling the fate of the available energy substrates; andcontrol of fat reserves through modulation of lipogenesis and lipolysis in white adipose tissue. A large proportion of current research is centred on neuropeptidic control of appetite, followed by the development of drugs controlling thermogenic mechanisms and analysis of the factors controlling adipocyte growth and fat storage. The adipocyte is also a fundamental source of metabolic signals, signals that can be intercepted, modulated and used to force the brain to adjust the mass of fat with the physiological means available. The large variety of different approaches used in the search for effective anti-obesity drugs show both the deep involvement of researchers on this field and the large amount of resources devoted to this problem by pharmaceutical companies. Future trends in anti-obesity drug research follow closely the approaches outlined; however, the increasing mass of information on the molecular basis of bodyweight control and obesity will in the end prevail in our search for effective and harmless anti-obesity drugs.

Studies on modulation of feeding behavior by atypical antipsychotics in female mice

The aim of this study was to examine the effects of different doses of typical antipsychotics, chlorpromazine (0.25-1 mg/kg) and haloperidol (0.25-1 mg/kg), and atypical antipsychotics, clozapine (0.5-2 mg/kg), olanzapine (0.25-1 mg/kg), risperidone (0.5-2 mg/kg), sulpiride (10-40 mg/kg) and dopamine D1 antagonist, SCH 23390 (0.25-1 mg/kg) on feeding behavior at different time intervals after acute administration. The study further investigated the central dopamine and serotonergic receptor involvement in clozapine-induced hyperphagia using SKF 38393, quinpirole and quipazine. Then, the authors also examined the effect of subchronic treatment for 21 days with fluoxetine on clozapine-induced hyperphagia and modulation of body weight and fat pad weights. The feeding behavior was assessed in nondeprived mice by presenting the palatable chow to different groups of mice in glass petri dishes and recording the food consumed at different time intervals. After acute administration, significant (P<.05) increase in food intake was observed at different time intervals with different doses of both typical and atypical antipsychotics. Further, clozapine-induced hyperphagia was significantly (P<.05) reversed after treatment with SKF 38393 (dopamine D1 agonist), quinpirole (dopamine D2 agonist) and quipazine (5-HT1B, 5-HT2 and 5-HT3 agonist). In subchronic study, treatment with fluoxetine (10 mg/kg) significantly (P<.05) antagonized the increase in body weight and food intake induced by clozapine (2 mg/kg). The current investigations underscore the reported increases in food intake and body weight gain observed with antipsychotics. The study further confirms the involvement of dopamine D1, D2 and serotonergic receptor involvement in clozapine-mediated hyperphagia. Further, the serotonergic agents may prove useful to counteract antipsychotic-induced obesity.