Eating induced rise in LHA-dopamine correlates with meal size in normal and bulbectomized rats

Ventromedial Nucleus of Hypothalamus is Related to the Development of Cancer-Induced Anorexia: In Vivo Microdialysis Study

Based on reports that increased hypothalamic ventromedial nucleus (VMN) - serotonin (5-HT) is associated with cancer anorexia and recent findings in our laboratory that low levels of dopamine (DA) in the VMN are associated with prolonged inter meal intervals thus decreased food intake, and reports that setting up satiation is concomitant with descending levels of DA in the rostromedial hypothalamus, we hypothesized that an elevated 5-HT to low DA ratio in the VMN modulates food intake in cancer anorexia. Methods: In Expt 1: A microdialysis cannula guide was placed stereotactically into the VMN of methylcholanthrene (MCA) sarcoma tumor-bearing (TB) Fischer rats and in non-tumor-bearing (NTB) and pair-fed (PF) controls. When TB rats manifested anorexia by a decrease in food intake, VMN-5-HT, its metabolite 5-hydroxyindolacetic acid (5-HIAA), and DA with its metabolite 3,4,-dihydroxyphenylacetic acid (DOPAC) were measured by in vivo microdialysis using HPLC during baseline, in response to food, and after feeding. In Expt 2: TB rats had tumor removed and VMN microdialysis performed 7 days later. Results: Increased 5-HT release and turnover, and significantly reduced DA release with increased DOPAC occured in TB vs NTB or PF rats. When food was offered, intake in TB rats was significantly lower than in NTB control rats. During eating, VMN-5-HT rose and peaked significantly earlier in TB vs NTB rats, while DA release was significantly reduced. With eating, the 5-HT and DA metabolism became reduced in all rats. Seven days after surgical removal of the tumor, 24h food intake had increased to the level of controls; and when food was offered during microdialysis, intake in TB rats increased (ns relative to control), but was not yet normal. VMN microdialysis showed that 5-HT was normal at baseline, as well as during and after eating, while DA remained depressed. The metabolic turnover of 5-HT and DA was significantly lower in TB-r and PF vs NTB rats. We conclude that increased 5-HT/DA ratio is related to the development of cancer-induced anorexia.

Systemic Nicotine Administration Suppresses Food Intake Via Reduced Meal Sizes in Both Male and Female Rats

The appetite suppressing effect of tobacco products, via the main pharmacological agent nicotine, is a major reason for its usage both by woman and man. Food intake (FI) could be changed by altering either meal size (MZ) or meal number (MN), which are regulated dependently in a reciprocal manner. The present study investigated the effect of systemic nicotine administration on the rat feeding pattern. Because of gender differences in the effects of nicotine, both male and female rats were studied. Alzet mini-osmotic pumps (Model 2001) and the automated rat eatometer were used to evaluate the feeding pattern of male and female Fischer 344 rats during seven days of systemic nicotine infusion (6 mg/kg b.w. s.c.). The main findings are: 1) systemic nicotine infusion decreased food intake in both sexes; 2) the decreased food intake was due to significantly reduced meal sizes while meal numbers were not altered significantly in either males or females; 3) the cyclical pattern of vaginal smears, food intake, meal number and meal size of female rats was not affected by nicotine administration. We conclude that the feeding suppressive effect of nicotine, which is due to reduced meal sizes and thus satiation, is not sex-hormones related.

Hypothalamic dopamine signaling regulates brown fat thermogenesis

Dopamine signaling is a crucial part of the brain reward system and can affect feeding behavior. Dopamine receptors are also expressed in the hypothalamus, which is known to control energy metabolism in peripheral tissues. Here we show that pharmacological or chemogenetic stimulation of dopamine receptor 2 (D2R) expressing cells in the lateral hypothalamic area (LHA) and the zona incerta (ZI) decreases body weight and stimulates brown fat activity in rodents in a feeding-independent manner. LHA/ZI D2R stimulation requires an intact sympathetic nervous system and orexin system to exert its action and involves inhibition of PI3K in the LHA/ZI. We further demonstrate that, as early as 3 months after onset of treatment, patients treated with the D2R agonist cabergoline experience an increase in energy expenditure that persists for one year, leading to total body weight and fat loss through a prolactin-independent mechanism. Our results may provide a mechanistic explanation for how clinically used D2R agonists act in the CNS to regulate energy balance.

Role of eating disorders-related polymorphisms in obesity pathophysiology

Human biological system provides innumerable neuroendocrine inputs for food intake control, with effects on appetite's modulation and the satiety signs. Its regulation is very complex, engaging several molecular interactions with many tissues, hormones, and neural circuits. Thus, signaling molecules that control food intake are critical for normal energy homeostasis and a deregulation of these pathways can lead to eating disorders and obesity. In line of this, genetic factors have a significantly influence of the regulation of neural circuits controlling the appetite and satiety pathways, as well as the regulation of brain reward systems. Single Nucleotide Polymorphisms (SNPs) in genes related to hypothalamic appetite and satiety mechanisms, further in multiple neurotransmitter systems may contribute to the development of major Eating Disorders (EDs) related to obesity, among them Binge Eating Disorder (BED) and Bulimia Nervosa (BN), which are discussed in this review.

Dopamine modulates excitatory transmission to orexin neurons in a receptor subtype-specific manner

Dopamine (DA) can promote or inhibit consummatory and reward-related behaviors by activating different receptor subtypes in the lateral hypothalamus and perifornical area (LH/PF). Because orexin neurons are involved in reward and localized in the LH/PF, DA may modulate these neurons to influence reward-related behaviors. To determine the cellular mechanism underlying dopaminergic modulation of orexin neurons, the effect of DA on excitatory transmission to these neurons was investigated using in vitro electrophysiology on rat brain slices. We found that low concentrations (0.1-1 µM) of DA increased evoked excitatory postsynaptic current amplitude while decreasing paired-pulse ratio. In contrast, high concentrations (10-100 µM) of DA did the opposite. The excitatory effect of low DA was blocked by the D₁ receptor antagonist SCH-23390, whereas the inhibitory effect of high DA was blocked by the D₂ receptor antagonist sulpiride. These results indicate distinct roles of D₁ and D₂ receptors in bidirectional presynaptic modulation of excitatory transmission. DA had stronger effects on isolated synaptic activity than repetitive ones, suggesting that sensitivity to dopaminergic modulation depends on the level of network activity. In orexin neurons from high-fat diet-fed rats, a high concentration of DA was less effective in suppressing repetitive synaptic activity compared with chow controls. Therefore, in diet-induced obesity, intense synaptic inputs may preferentially reach orexin neurons while intermittent signals are inhibited by high DA levels. In summary, our study provides a cellular mechanism by which DA may exert opposite behavioral effects in the LH/PF through bidirectional modulation of orexin neurons via different DA receptors.

Dietary Slowly Digestible Starch Triggers the Gut-Brain Axis in Obese Rats with Accompanied Reduced Food Intake

SCOPE

Slowly digestible starch (SDS), as a functional carbohydrate providing a slow and sustained glucose release, may be able to modulate food intake through activation of the gut-brain axis.

METHODS AND RESULTS

Diet-induced obese rats were used to test the effect on feeding behavior of high-fat (HF) diets containing an SDS, fabricated to digest into the ileum, as compared to rapidly digestible starch (RDS). Ingestion of the HF-SDS diet over an 11-week period reduced daily food intake, through smaller meal size, to the same level as a lean body control group, while the group consuming the HF-RDS diet remained at a high food intake. Expression levels (mRNA) of the hypothalamic orexigenic neuropeptide Y (NPY) and Agouti-related peptide (AgRP) were significantly reduced, and the anorexigenic corticotropin-releasing hormone (CRH) was increased, in the HF-SDS fed group compared to the HF-RDS group, and to the level of the lean control group.

CONCLUSION

SDS with digestion into the ileum reduced daily food intake and paralleled suppressed expression of appetite-stimulating neuropeptide genes associated with the gut-brain axis. This novel finding suggests further exploration involving a clinical study and potential development of SDS-based functional foods as an approach to obesity control.

Caffeine Modulates Food Intake Depending on the Context That Gives Access to Food: Comparison With Dopamine Depletion

Caffeine is a methylxanthine consumed in different contexts to potentiate alertness and reduce fatigue. However, caffeine can induce anxiety at high doses. Caffeine is also a minor psychostimulant that seems to act as an appetite suppressant, but there are also reports indicating that it could stimulate appetite. Dopamine also is involved in food motivation and in behavioral activation. In the present series of experiments, we evaluated the effects of acute administration of caffeine on food consumption under different access conditions. CD1 male adult mice had access to highly palatable food (50% sucrose) in a restricted but habitual context, under continuous or intermittent access as well as under anxiogenic, or effortful conditions. Caffeine (2.5-20.0 mg/kg) increased intake at the highest dose under familiar continuous and intermittent access. However, this high dose reduced food intake in the dark-light paradigm. In contrast, a dopamine-depleting agent, tetrabenazine (TBZ; 1.0-8.0 mg/kg) did not affect food intake in any of those experimental conditions. In the T-maze-barrier task that evaluates seeking and taking of food under effortful conditions, caffeine (10.0 mg/kg) decreased latency to reach the food, but did not affect selection of the high-food density arm that required more effort, or the total amount of food consumed. In contrast, TBZ (4.0 mg/kg) reduced selection of the high food density arm with the barrier, thus affecting amount of food consumed. Interestingly, a small dose of caffeine (5.0 mg/kg) was able to reverse the anergia-inducing effects produced by TBZ in the T-maze. These results suggest that caffeine can potentiate or suppress food consumption depending on the context. Moreover, caffeine did not change appetite, and did not impair orientation toward food under effortful conditions, but it rather helped to achieve the goal by improving speed and by reversing performance to normal levels when fatigue was induced by dopamine depletion.

[Hunger and satiety factors in the regulation of pleasure associated with feeding behavior]

Feeding is an instinctive behavior accompanied by rewarding feeling of pleasure during obtaining and ingesting food, corresponding to the preparatory and consummatory phases of motivated behavior, respectively. Perception of this emotional state together with alternating feelings of hunger and satiety drives the feeding behavior. Because alterations of feeding behavior including either overeating or anorexia may lead to obesity and cachexia, respectively, understanding the neurochemical mechanisms of regulation of feeding pleasure may help to develop new therapies of these diseases. The dopamine (DA) system of the mesolimbic projections plays a key role in behavioral reward in general and is also involved in regulating feeding-associated pleasure in the forebrain including the nucleus accumbens (NAc) and the lateral hypothalamic area (LHA). It suggests that this DA system can be selectively activated by factors specific to different types of motivated behavior including hunger- and satiety- related hormones. Indeed, central administrations of either orexigenic ghrelin or anorexigenic α-melanocyte-stimulating hormone (α-MSH) increase DA release in the NAc. However, DA has also been shown to inhibit food intake when injected into the LHA, historically known as a « hunger center », indicating DA functional involvement in regulation of both appetite and feeding pleasure. Although both NAc and LHA contain neurons expressing melanocortin receptors, only the LHA receives the α-MSH containing nerve terminals from the α-MSH producing neurons of the hypothalamic arcuate nucleus, the main relay of the peripheral hunger and satiety signals to the brain. A recent study showed that α-MSH in the LHA enhances satiety and inhibits feeding pleasure while potently stimulating DA release in this area during both preparatory and consummatory phases of feeding. It suggests that altered signaling by α-MSH to the DA system in the LHA may be involved in the pathophysiology of obesity and anorexia and the possible underlying mechanisms are discussed.

Concentration-dependent activation of dopamine receptors differentially modulates GABA release onto orexin neurons

Dopamine (DA) and orexin neurons play important roles in reward and food intake. There are anatomical and functional connections between these two cell groups: orexin peptides stimulate DA neurons in the ventral tegmental area and DA inhibits orexin neurons in the hypothalamus. However, the cellular mechanisms underlying the action of DA on orexin neurons remain incompletely understood. Therefore, the effect of DA on inhibitory transmission to orexin neurons was investigated in rat brain slices using the whole-cell patch-clamp technique. We found that DA modulated the frequency of spontaneous and miniature IPSCs (mIPSCs) in a concentration-dependent bidirectional manner. Low (1 μM) and high (100 μM) concentrations of DA decreased and increased IPSC frequency, respectively. These effects did not accompany a change in mIPSC amplitude and persisted in the presence of G-protein signaling inhibitor GDPβS in the pipette, suggesting that DA acts presynaptically. The decrease in mIPSC frequency was mediated by D2 receptors whereas the increase required co-activation of D1 and D2 receptors and subsequent activation of phospholipase C. In summary, our results suggest that DA has complex effects on GABAergic transmission to orexin neurons, involving cooperation of multiple receptor subtypes. The direction of dopaminergic influence on orexin neurons is dependent on the level of DA in the hypothalamus. At low levels DA disinhibits orexin neurons whereas at high levels it facilitates GABA release, which may act as negative feedback to curb the excitatory orexinergic output to DA neurons. These mechanisms may have implications for consummatory and motivated behaviours.

Dopamine release in the lateral hypothalamus is stimulated by α-MSH in both the anticipatory and consummatory phases of feeding

α-Melanocyte-stimulating hormone (α-MSH), is a hypothalamic neuropeptide signaling satiation, but it is not known if α-MSH may stimulate dopamine release in a feeding control brain region of the lateral hypothalamic area (LHA), during the anticipatory and consummatory phases of feeding behavior. To address these questions, dynamics of dopamine release were measured in 15 min microdialysis samples simultaneously from the LHA and the nucleus accumbens (NAc) during consecutive exposure and provision of food and 1% sucrose in Wistar rats after overnight food deprivation. α-MSH was infused via the microdialysis probe either into the LHA or NAc starting before food exposure. Food, sucrose and water intakes were automatically monitored and analyzed concomitantly with microdialysis samples. We found that LHA-α-MSH-infused rats stopped eating earlier and consumed less food and sucrose as compared to control and NAc-α-MSH-infused rats. Exposure to food produced a peak of LHA dopamine in both LHA-α-MSH and NAc-α-MSH-infused rats but not in the controls. During food provision, LHA dopamine levels were strongly elevated in LHA-α-MSH infused rats, while delivery of α-MSH into the NAc induced a less intense increase of dopamine in both NAc and LHA. In all rats, LHA dopamine levels correlated inversely with sucrose intake. In conclusion, our study showed that α-MSH stimulates dopamine release in the LHA during both the anticipatory and consummatory phases of feeding, decreases food intake and inhibits sucrose intake. These data suggest that LHA dopamine release can be involved in α-MSH anorexigenic effects.

Pharmacological manipulations in animal models of anorexia and binge eating in relation to humans

Eating disorders, such as anorexia nervosa (AN), bulimia nervosa (BN) and binge eating disorders (BED), are described as abnormal eating habits that usually involve insufficient or excessive food intake. Animal models have been developed that provide insight into certain aspects of eating disorders. Several drugs have been found efficacious in these animal models and some of them have eventually proven useful in the treatment of eating disorders. This review will cover the role of monoaminergic neurotransmitters in eating disorders and their pharmacological manipulations in animal models and humans. Dopamine, 5-HT (serotonin) and noradrenaline in hypothalamic and striatal regions regulate food intake by affecting hunger and satiety and by affecting rewarding and motivational aspects of feeding. Reduced neurotransmission by dopamine, 5-HT and noradrenaline and compensatory changes, at least in dopamine D2 and 5-HT(2C/2A) receptors, have been related to the pathophysiology of AN in humans and animal models. Also, in disorders and animal models of BN and BED, monoaminergic neurotransmission is down-regulated but receptor level changes are different from those seen in AN. A hypofunctional dopamine system or overactive α2-adrenoceptors may contribute to an attenuated response to (palatable) food and result in hedonic binge eating. Evidence for the efficacy of monoaminergic treatments for AN is limited, while more support exists for the treatment of BN or BED with monoaminergic drugs.

Hypothalamic food intake regulation in a cancer-cachectic mouse model

BACKGROUND

Appetite is frequently affected in cancer patients leading to anorexia and consequently insufficient food intake. In this study, we report on hypothalamic gene expression profile of a cancer-cachectic mouse model with increased food intake. In this model, mice bearing C26 tumour have an increased food intake subsequently to the loss of body weight. We hypothesise that in this model, appetite-regulating systems in the hypothalamus, which apparently fail in anorexia, are still able to adapt adequately to changes in energy balance. Therefore, studying changes that occur on appetite regulators in the hypothalamus might reveal targets for treatment of cancer-induced eating disorders. By applying transcriptomics, many appetite-regulating systems in the hypothalamus could be taken into account, providing an overview of changes that occur in the hypothalamus during tumour growth.

METHODS

C26-colon adenocarcinoma cells were subcutaneously inoculated in 6 weeks old male CDF1 mice. Body weight and food intake were measured three times a week. On day 20, hypothalamus was dissected and used for transcriptomics using Affymetrix chips.

RESULTS

Food intake increased significantly in cachectic tumour-bearing mice (TB), synchronously to the loss of body weight. Hypothalamic gene expression of orexigenic neuropeptides NPY and AgRP was higher, whereas expression of anorexigenic genes CCK and POMC were lower in TB compared to controls. In addition, serotonin and dopamine signalling pathways were found to be significantly altered in TB mice. Serotonin levels in brain showed to be lower in TB mice compared to control mice, while dopamine levels did not change. Moreover, serotonin levels inversely correlated with food intake.

CONCLUSIONS

Transcriptomic analysis of the hypothalamus of cachectic TB mice with an increased food intake showed changes in NPY, AgRP and serotonin signalling. Serotonin levels in the brain showed to correlate with changes in food intake. Further research has to reveal whether targeting these systems will be a good strategy to avoid the development of cancer-induced eating disorders.

Dopamine signaling in the amygdala, increased by food ingestion and GLP-1, regulates feeding behavior

Mesolimbic dopamine plays a critical role in food-related reward processing and learning. The literature focuses primarily on the nucleus accumbens as the key dopaminergic target in which enhanced dopamine signaling is associated with reward. Here, we demonstrate a novel neurobiological mechanism by which dopamine transmission in the amygdala regulates food intake and reward. We show that food intake was associated with increased dopamine turnover in the amygdala. Next, we assess the impact of direct intra-amygdala D1 and D2 receptor activation on food intake and sucrose-driven progressive ratio operant conditioning in rats. Amygdala D2 receptor activation reduced food intake and operant behavior for sucrose, whereas D2 receptor blockade increased food intake but surprisingly reduced operant behavior. In contrast, D1 receptor stimulation or blockade did not alter feeding or operant conditioning for food. The glucagon-like peptide 1 (GLP-1) system, a target for type 2 diabetes treatment, in addition to regulating glucose homeostasis, also reduces food intake. We found that central GLP-1R receptor activation is associated with elevated dopamine turnover in the amygdala, and that part of the anorexic effect of GLP-1 is mediated by D2 receptor signaling in the amygdala. Our findings indicate that amygdala dopamine signaling is activated by both food intake and the anorexic brain-gut peptide GLP-1 and that amygdala D2 receptor activation is necessary and sufficient to change feeding behavior. Collectively these studies indicate a novel mechanism by which the dopamine system affects feeding-oriented behavior at the level of the amygdala.

Differential role of D1 and D2 receptors in the perifornical lateral hypothalamus in controlling ethanol drinking and food intake: possible interaction with local orexin neurons

BACKGROUND

The neurotransmitter dopamine (DA), acting in various mesolimbic brain regions, is well known for its role in promoting motivated behaviors, including ethanol (EtOH) drinking. Indirect evidence, however, suggests that DA in the perifornical lateral hypothalamus (PF/LH) has differential effects on EtOH consumption, depending on whether it acts on the DA 1 (D1) or DA 2 (D2) receptor subtype, and that these effects are mediated in part by local peptide systems, such as orexin/hypocretin (OX) and melanin-concentrating hormone (MCH), known to stimulate the consumption of EtOH.

METHODS

The present study in brain-cannulated Sprague-Dawley rats measured the effects of dopaminergic compounds in the PF/LH on drinking behavior in animals trained to consume 7% EtOH and also on local peptide mRNA expression using digoxigenin-labeled in situ hybridization in EtOH-naïve animals.

RESULTS

Experiments 1 and 2 showed that the D1 agonist SKF81297 (10.8 nmol/side) in the PF/LH significantly increased food intake, while tending to increase EtOH intake, and the D1 antagonist SCH23390 significantly decreased EtOH intake without affecting food intake. In contrast, the D2 agonist quinelorane (6.2 nmol/side) in the PF/LH significantly reduced EtOH consumption, while the D2 antagonist sulpiride increased it. Experiments 3 and 4 revealed differential effects of PF/LH injection of the DA agonists on local OX mRNA, which was increased by the D1 agonist and decreased by the D2 agonist. These DA agonists had no impact on MCH expression.

CONCLUSIONS

These results support a stimulatory role of the PF/LH D1 receptor in promoting the consumption of both EtOH and food, in contrast to a suppressive effect of the D2 receptor on EtOH drinking. They further suggest that these receptors affect consumption, in part, through local OX-expressing neurons. These findings provide new evidence for the function of PF/LH DA receptor subtypes in controlling EtOH and food intake.

Effect of subcutaneous injection of estradiol on feeding and drinking behaviors and body weight in basolateral amygdaloid lesioned rats

BACKGROUND

Estradiol is known to inhibit food intake (FI), water intake (WI) and body weight (BW) across the species including women and it is most evident in rats. Ovariectomy in rats and menopause in women produce hyperphagia and obesity. Estradiol substitution in ovariectomized (OVX) rats and hormone replacement in women reverses these changes suggesting that lack of estradiol causes eating related disorders. However, the neurobiological target/s for estradiol mediating effects remains largely unknown. While lesions of basolateral amygdala (BLA) also produce hyperphagia, polydipsia and obesity in female rats suggesting BLA normally inhibits these behaviors.

PURPOSE

Since ovariectomy is a useful model to study postmenopausal obesity in women, we have investigated the role of BLA in ovariectomy induced ingestive behaviors.

METHODS

Ovariectomy and stereotaxic lesions in experimental group (n = 6) whereas sham operations in control group (n = 6) were carried out in female rats. Estradiol was injected subcutaneously (s.c) before and after lesions in experimental group and vehicle was injected in control group.

RESULTS

Data from the present study shows that there was an additional increase in FI, WI and BW in OVX animals following BLA lesions, but this additive effect was small compared to sham operated controls. Conversely, OVX rats with lesions have shown small but significant reductions in FI, WI, and lost less BW, following s.c injection of estradiol compared to rats with intact BLA.

CONCLUSION

These findings suggest that ovariectomy and estradiol induced changes on ingestive behaviors and body weight are partly mediated via BLA.

Obesogenic phenotype of offspring of dams fed a high multivitamin diet is prevented by a post-weaning high multivitamin or high folate diet

Background

High multivitamin (10-fold AIN-93G, HV) diets fed during pregnancy to Wistar rats increase characteristics of metabolic syndrome in offspring when weaned to the recommended vitamin (RV) diet.

Objective

To determine if the effects of HV gestational diets on obesogenic phenotypes in the offspring arise as a consequence of altered hypothalamic control of feeding behavior and if their increased food intake could be prevented by feeding them HV or high folate (10-fold folate, HFol) diets.

Methods

Male offspring of dams fed HV diet during pregnancy weaned to RV, HV or HFol diets were compared to those born to RV dams and weaned to RV diet for 29 weeks. Food intake over 72 hours and body weight were measured bi-weekly and weekly, respectively. Glucose response to a glucose load was measured at 18 weeks post-weaning. Hypothalamic gene expression of feeding-related neuropeptides including neuropeptide Y, pro-opiomelanocortin (POMC), insulin receptor, leptin receptor, brain-derived neurotrophic factor (BDNF), receptors for dopamine (DopaR1/2/5) and serotonin (SeroR1A/2A/2C), as well as global DNA methylation and brain and plasma folate concentrations were measured at 29 weeks post-weaning.

Results

HV or HFol pup diets increased brain and plasma folate concentrations and prevented the increase in food intake (5%, P=0.03), body weight (8%, P=0.0006) and glucose response to a glucose load (36%, P=0.02) found in those fed the RV diet. Expression of anorexigenic POMC (P=0.004) and BDNF (P=0.02) was higher, and DopaR1 was lower (P=0.06) in pups fed the HV diet. The HFol pup diet partially brought BDNF to the control level (P=0.02) and lowered SeroR2A (P=0.008). Expression of other genes was unaffected. Global DNA methylation was similar among the diet groups.

Conclusion

The obesogenic phenotype in offspring from HV fed dams is prevented by feeding HV or HFol pup diets, possibly due to post-weaning modulation of food intake regulatory mechanisms.

The CCKB antagonist CI988 reduces food intake in fasted rats via a dopamine mediated pathway

Studies have shown a reduction of food intake following peripheral and brain injection of CCK. However, it remains to be established whether endogenous central CCK is involved in the regulation of food intake. We investigated the role of central CCK in the regulation of food intake by pharmacological manipulation of the CCK(B) (CCK(2)) receptor system. Intracerebroventricularly (ICV) cannulated male Sprague Dawley rats were fasted for 24h and received an ICV injection of the CCK(B) receptor antagonist CI988 at a dose of 10 nmol or 49 nmol or vehicle. Another group received two consecutive ICV injections consisting of the corticotropin-releasing factor (CRF) receptor-1 (CRF(1)) antagonist, CP376395 (3 nmol) or the CRF(2) receptor antagonist, K41498 (2 nmol) alone, or followed by CI988 (49 nmol). Lastly, another group of rats received an intraperitoneal (IP) injection of the dopamine antagonist, flupentixol (~197 and ~493nmol/kg) alone, or followed by CI988 (49 nmol, ICV). Cumulative food intake was assessed for 11h. Vehicle injected rats showed a robust feeding response. CI988 at 49 nmol reduced food intake by 30% starting at 2h post injection. CP376395 and K41498 had no effect on food intake. Flupentixol injected IP at a dose of 197 and 493 nmol/kg alone did not modulate food intake whereas the higher dose blocked the CI988-induced reduction of feeding. During the dark phase, CI988 had no effect on food intake in unfasted rats. In summary, CCK(B) signaling is involved in the regulation of food intake after a fast likely by downstream dopamine signaling.

Dopamine acts as a partial agonist for α2A adrenoceptor in melanin-concentrating hormone neurons

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that promotes positive energy balance and anxiety. Since dopamine (DA) is also closely implicated in these functions, the present study investigated the effect of DA on MCH neurons. Using whole-cell patch-clamp recordings in rat brain slices, we found that DA hyperpolarizes MCH neurons by activating G-protein-activated inwardly rectifying K(+) (GIRK) channels. Pharmacological study indicated that the effect was mediated by α2A adrenoceptors, not DA receptors. DA-induced outward current was also observed in the presence of tetrodotoxin or the dopamine β-hydroxylase inhibitor fusaric acid, suggesting that DA directly binds to α2A receptors on MCH neurons, rather than acting presynaptically or being transformed into norepinephrine (NE) in the slice preparation. The effects of NE and DA were concentration-dependent with EC(50) of 5.9 and 23.7 μm, respectively, and a maximal effect of 106.6 and 57.2 pA, respectively, suggesting that DA functions as a partial agonist. Prolonged (5 min) activation of α2A receptors by either DA or NE attenuated the subsequent response to DA or NE, while 5 s applications were not sufficient to induce desensitization. Therefore, a history of α2A receptor activation by DA or NE can have a lasting inhibitory effect on the catecholaminergic transmission to MCH neurons. Our study suggests that α2A receptors expressed by MCH neurons may be one of the pathways by which DA and NE can interact and modulate mood and energy homeostasis, and this cross talk may have functional implications in mood disorders and obesity.

Feeding behavior as seen through the prism of brain microdialysis

The knowledge of feeding behavior mechanisms gained through brain microdialysis is reviewed. Most of the chemical changes so far reported concern to the limbic system in rodents. A picture showing increases and decreases of extracellular neurotransmitters correlating to different aspects of feeding behavior is gradually emerging. Depending on the region, the same neurotransmitter may signal opposite aspects of feeding. Dopamine (DA) in the nucleus accumbens (NAC) correlates with food reward, stimulus saliency, and goal directed hyperlocomotion but in the ventromedial hypothalamus DA correlates with satiety and hypolocomotion. The findings accumulated in the last 25 years suggest that the control of a particular function relies on the interaction of several neurotransmitters rather than on a single neurotransmitter. The poor sensitivity of most analytical techniques hinders time and spatial resolution of microdialysis. Therefore, neurochemical correlates of short lasting behaviors are hard to figure out. As new and more sensitive analytical techniques are applied, new neurochemical correlates of feeding show up. Sometimes the proper analytical techniques are simply not available. As a consequence, critical signals such as neuropeptides are not yet completely placed in the puzzle. Despite such limitations, brain microdialysis has yielded a great deal of knowledge on the neurochemical basis of feeding.

Dopamine depresses melanin concentrating hormone neuronal activity through multiple effects on α2-noradrenergic, D1 and D2-like dopaminergic receptors

Two neuronal populations of the lateral hypothalamus that, respectively, produce melanin-concentrating hormone (MCH) and orexin peptides are crucially involved in control of metabolism, feeding and related goal-oriented behaviors. In contrast to orexin neurons, mainly involved in short-term regulation of feeding, MCH neurons participate in long-term control of energy storage and body weight. Beyond its effect on feeding, MCH has also been shown to be involved in regulation of seeking behavior and addiction through modulation of dopamine (DA) metabolism. This regulation is essential for reinforcement-associated behaviors. Moreover, drugs of abuse, which increase extracellular DA levels, are known to decrease food intake. Consistent with this observation, DA has been shown to modulate orexin neurons of the lateral hypothalamus. However, no study is available concerning the effects of DA on MCH neurons. Whole-cell patch-clamp recordings were done in hypothalamic mouse brain slices. MCH neurons were identified by Tau-Cyan-GFP labeling using a transgenic mouse model (MCH-GFP). First, we show that DA (10-200 μM) induces an outward current in MCH neurons. However, this current is not due to activation of DA receptors, but mediated through activation of α2-noradrenergic receptors and subsequent opening of G-protein activated inward rectifier K+ (GIRK) channels. Current-clamp experiments revealed that this GIRK-activation leads to hyperpolarization, thus decreasing excitability of MCH neurons. Furthermore, we confirm that MCH neurons receive mainly GABAergic inputs rather than glutamatergic ones. We show that DA modulates these inputs in a complex manner: at low concentrations, DA activates D1-like receptors, promoting presynaptic activity, whereas, at higher concentrations (100 μM), D2-like receptor activation inhibits presynaptic activity. Overall, DA should lead to a decrease in MCH neuron excitability, likely resulting in down-regulation of MCH release and feeding behavior.

Central dopaminergic circuitry controlling food intake and reward: implications for the regulation of obesity

Prevalence of obesity in the general population has increased in the past 15 years from 15% to 35%. With increasing obesity, the coincident medical and social consequences are becoming more alarming. Control over food intake is crucial for the maintenance of body weight and represents an important target for the treatment of obesity. Central nervous system mechanisms responsible for control of food intake have evolved to sense the nutrient and energy levels in the organism and to coordinate appropriate responses to adjust energy intake and expenditure. This homeostatic system is crucial for maintenance of stable body weight over long periods of time of uneven energy availability. However, not only the caloric and nutritional value of food but also hedonic and emotional aspects of feeding affect food intake. In modern society, the increased availability of highly palatable and rewarding (fat, sweet) food can significantly affect homeostatic balance, resulting in dysregulated food intake. This review will focus on the role of hypothalamic and mesolimbic/mesocortical dopaminergic (DA) circuitry in coding homeostatic and hedonic signals for the regulation of food intake and maintenance of caloric balance. The interaction of dopamine with peripheral and central indices of nutritional status (e.g., leptin, ghrelin, neuropeptide Y), and the susceptibility of the dopamine system to prenatal insults will be discussed. Additionally, the importance of alterations in dopamine signaling that occur coincidently with obesity will be addressed.

A history of caloric restriction induces neurochemical and behavioral changes in rats consistent with models of depression

A history of dieting is common in individuals suffering from eating disorders for which depression and mood disturbances are also comorbid. We investigated the effect of a history of caloric restriction (HCR) in rats that involved cyclic food restriction and refeeding with varying levels of access to palatable food (PF) on: 1) responses to the SSRI, fluoxetine; 2) monoamine levels in brain regions central to the control of feeding, reward, and mood regulation; and 3) behavioral tests of anxiety and depression. HCR coupled with intermittent but not daily access to PF exaggerated rats' anorectic response to fluoxetine (p<0.05); was associated with a significant 71% and 58% reduction of 5-HT and dopamine, respectively, in the medial prefrontal cortex; and induced behaviors consistent with models of depression. HCR, irrespective of access to PF, abolished the strong association between 5-HT and dopamine turnover in the nucleus accumbens in control rats (r=0.71 vs. -0.06, p<0.01). Access to PF, irrespective of HCR, reduced hypothalamic dopamine. Together, these findings suggest that a history of frequent food restriction-induced weight fluctuation imposes neurochemical changes that negatively impact feeding and mood regulation.

Bidirectional dopaminergic modulation of excitatory synaptic transmission in orexin neurons

Orexin neurons in the lateral hypothalamus (LH)/perifornical area (PFA) are known to promote food intake as well as provide excitatory influence on the dopaminergic reward pathway. Dopamine (DA), in turn, inhibits the reward pathway and food intake through its action in the LH/PFA. However, the cellular mechanism by which DA modulates orexin neurons remains largely unknown. Therefore, we examined the effect of DA on the excitatory neurotransmission to orexin neurons. Whole-cell patch-clamp recordings were performed using acute rat hypothalamic slices, and orexin neurons were identified by their electrophysiological and immunohistochemical characteristics. Pharmacologically isolated action potential-independent miniature EPSCs (mEPSCs) were monitored. Bath application of DA induced a bidirectional effect on the excitatory synaptic transmission dose dependently. A low dose of DA (1 microM) increased mEPSC frequency, which was blocked by the D1-like receptor antagonist SCH 23390, and mimicked by the D1-like receptor agonist SKF 81297. In contrast, higher doses of DA (10-100 microM) decreased mEPSC frequency, which could be blocked with the D2-like receptor antagonist, sulpiride. Quinpirole, the D2-like receptor agonist, also reduced mEPSC frequency. None of these compounds affected the mEPSCs amplitude, suggesting the locus of action was presynaptic. Furthermore, DA (1 microM) induced an increase in the action potential firing, whereas DA (100 microM) hyperpolarized and ceased the firing of orexin neurons, indicating the effect of DA on excitatory synaptic transmission may influence the activity of the postsynaptic cell. In conclusion, our results suggest that D1- and D2-like receptors have opposing effects on the excitatory presynaptic terminals impinging onto orexin neurons.

Differential autonomic nervous system response in obese and anorexic chickens (Gallus gallus)

Effect of reserpine on body weight (BW), feed intake (FI), brain and plasma catecholamine and indoleamine concentrations in high- (HWS) and low- (LWS) weight selected lines of chickens was investigated. Chicks from each line were assigned to three treatment groups and injected intraperitoneally with 0, 1.25, or 2.50 mg/kg of reserpine at hatch, and again at 5 weeks-of-age. Chick BW and FI were determined weekly. At 7 weeks-of-age, 12 males and females from each group were sacrificed for neurotransmitter analysis. In the HWS line there was a dose-dependent decrease in BW through 7 weeks-of-age, whereas in the LWS line BW decreased only through the first 2 weeks-of-age. In the LWS line, norepinephrine (NE), epinephrine, and 3,4-dihydroxyphenylacetate concentrations decreased in the brain in a linear and quadratic manner in response to reserpine, but not in the HWS line. Both lines showed linear decreases in dopamine levels in response to reserpine; however, serotonin was not affected by reserpine. Chickens in the HWS line had greater plasma NE, and lower 5-hydroxyindoleacetic acid than those in the LWS line. In conclusion, it appears that chickens from the HWS line were more sensitive to the BW reducing effects of reserpine than those from the LWS line, with the latter appearing to have greater sympathetic nervous system activity.

Hypothalamic integration of immune function and metabolism

The immune and neuroendocrine systems are closely involved in the regulation of metabolism at peripheral and central hypothalamic levels. In both physiological (meals) and pathological (infections, traumas and tumors) conditions immune cells are activated responding with the release of cytokines and other immune mediators (afferent signals). In the hypothalamus (central integration), cytokines influence metabolism by acting on nucleus involved in feeding and homeostasis regulation leading to the acute phase response (efferent signals) aimed to maintain the body integrity. Peripheral administration of cytokines, inoculation of tumor and induction of infection alter, by means of cytokine action, the normal pattern of food intake affecting meal size and meal number suggesting that cytokines acted differentially on specific hypothalamic neurons. The effect of cytokines-related cancer anorexia is also exerted peripherally. Increase plasma concentrations of insulin and free tryptophan and decrease gastric emptying and d-xylose absorption. In addition, in obesity an increase in interleukin (IL)-1 and IL-6 occurs in mesenteric fat tissue, which together with an increase in corticosterone, is associated with hyperglycemia, dyslipidemias and insulin resistance of obesity-related metabolic syndrome. These changes in circulating nutrients and hormones are sensed by hypothalamic neurons that influence food intake and metabolism. In anorectic tumor-bearing rats, we detected upregulation of IL-1beta and IL-1 receptor mRNA levels in the hypothalamus, a negative correlation between IL-1 concentration in cerebro-spinal fluid and food intake and high levels of hypothalamic serotonin, and these differences disappeared after tumor removal. Moreover, there is an interaction between serotonin and IL-1 in the development of cancer anorexia as well as an increase in hypothalamic dopamine and serotonin production. Immunohistochemical studies have shown a decrease in neuropeptide Y (NPY) and dopamine (DA) and an increase in serotonin concentration in tumor-bearing rats, in first- and second-order hypothalamic nuclei, while tumor resection reverted these changes and normalized food intake, suggesting negative regulation of NPY and DA systems by cytokines during anorexia, probably mediated by serotonin that appears to play a pivotal role in the regulation of food intake in cancer. Among the different forms of therapy, nutritional manipulation of diet in tumor-bearing state has been investigated. Supplementation of tumor bearing rats with omega-3 fatty acid vs. control diet delayed the appearance of tumor, reduced tumor-growth rate and volume, negated onset of anorexia, increased body weight, decreased cytokines production and increased expression of NPY and decreased alpha-melanocyte-stimulating hormone (alpha-MSH) in hypothalamic nuclei. These data suggest that omega-3 fatty acid suppressed pro-inflammatory cytokines production and improved food intake by normalizing hypothalamic food intake-related peptides and point to the possibility of a therapeutic use of these fatty acids. The sum of these data support the concept that immune cell-derived cytokines are closely related with the regulation of metabolism and have both central and peripheral actions, inducing anorexia via hypothalamic anorectic factors, including serotonin and dopamine, and inhibiting NPY leading to a reduction in food intake and body weight, emphasizing the interconnection of the immune and neuroendocrine systems in regulating metabolism during infectious process, cachexia and obesity.

A role for dopamine in feeding responses produced by orexigenic agents

Dopamine-deficient (DD) mice become hypophagic and die of starvation by 3 to 4 weeks of age unless dopamine is restored by daily treatment with l-3-4-dihydroxyphenylalanine (l-dopa). We demonstrate here that DD mice mount qualitatively normal counter-regulatory blood glucose responses to insulin and 2-deoxy-d-glucose (2-DG). However, unlike control mice, DD mice fail to eat in response to acute glucoprivation induced by insulin or 2-DG. They also have a severely blunted response to central administration of peptide YY (PYY). Viral-mediated restoration of dopamine synthesis to the central caudate putamen (CPu) of DD mice rescues feeding and survival. However, this treatment fails to restore insulin- and 2-DG-induced feeding despite normalizing feeding in response to food deprivation and PYY. Since dopamine signaling in the CPu is not sufficient for glucoprivation-induced feeding, we propose that this feeding behavior may be mediated by dopamine in an anatomically distinct brain region.

Changes in hypothalamic neuropeptide Y and monoaminergic system in tumor-bearing rats: Pre- and post-tumor resection and at death

BACKGROUND

Cancer anorexia is influenced by the neuropeptidergic and monoaminergic systems. We hypothesize that serotonin (5-HT), dopamine (DA) and neuropeptide Y (NPY) concentrations in paraventricular (PVN), ventromedial (VMN), and lateral hypothalamus (LHA) areas are abnormal in tumor-bearing rats.

METHODS

Fifty-five Fischer rats (240-280 g) were divided into MCA tumor-bearing (TB), nontumor-bearing (NTB), pair-fed (PF), TB sacrificed at the end of experiment (TB-Terminal), TB resection (TB-Resection), NTB sham-operated (NTB-Sham) and pair-fed sham-operated (PF-Sham) groups. Rats were sacrificed at onset of anorexia (TB, NTB, and PF) and 9 days after tumor resection (TB-Resection, NTB-Sham, PF-Sham, and TB-Terminal). Bilateral PVN, VMN, and LHA were harvested for NPY, 5-HT, and DA analyses.

RESULTS

Food intake decreased in TB versus NTB (P < .05). In TB versus NTB, an increase of 5-HT in PVN and VMN occurred with a concomitant decrease in DA. NPY in PVN, VMN, and LHA decreased (P < .05). In TB-Resection versus NTB-Sham, 5-HT, DA, NPY, and FI normalized after tumor resection.

CONCLUSIONS

Cancer anorexia is associated with abnormal serotonin, dopamine, and NPY concentrations, expressed by an increase in 5-HT and a decrease in DA and NPY. After tumor resection, these alterations normalized, providing evidence that the levels of these substances change with anorexia in tumor-bearing rats.

Sibutramine induces potential-dependent exocytotic release but not carrier-mediated release of dopamine and 5-hydroxytryptamine

In order to clarify the mechanism underlying the anti-obesity effects of sibutramine, we examined the effects of sibutramine on extracellular levels of dopamine and 5-hydroxytryptamine (5-HT) through microdialysis in the striatum in unanesthetized and freely moving rats. Sibutramine (5 mg/kg, oral administration (p.o.)) increased extracellular dopamine and 5-HT levels in rat striatum. The tricyclic antidepressant dosulepin (80 mg/kg, p.o. or 1 microM perfusion through the striatal probe) increased 5-HT levels only. Sibutramine-induced dopamine release was antagonized by perfusion of tetrodotoxin (1 microM) through the microdialysis probe in the striatum. However, sibutramine-induced dopamine release was not inhibited by prazosin (1 mg/kg, intraperitoneal injection (i.p.)), a suppressor of serotonergic activity in the striatum via blockade of alpha(1)-adrenoceptors, or perfusion with nomifensine (1 microM), an inhibitor of dopamine re-uptake. These results suggest that sibutramine increases dopamine levels in the striatum by exocytotic release and not by a carrier-mediated mechanism.

Altered action of dopamine and cholecystokinin on lateral hypothalamic neurons in rats raised under different feeding conditions

Single-unit activity was recorded in the lateral hypothalamus (LH) of adult Wistar rats anaesthetized with urethane. The rats were differently nourished till weaning by raising in small (SL), control (CL) or large litters (LL). They gained significantly different body weight leading to overweight in SL (mean: 428.4 g on day 90) and underweight in LL rats (mean 399.5 g) compared to CLs (414.5 g). The mean basal firing rate of LH neurons differed, it was lowest in SL and highest in LL rats. The proportion of neurons changing their firing rate by more than 30% in response to iontophoretically administered dopamine (DA) was significantly greater in SL (76%) than LL rats (54%). Effects of DA were significantly more often blocked by a D1 receptor antagonist in LL than CLs. The responsiveness to cholecystokinin (CCK) alone and coadministered with DA was also greater in SL than LL. Furthermore, the proportion of neurons inhibited by DA alone and in the presence of CCK was significantly greater in SL than LL rats. In conclusion, litter size and difference in nourishment during early postnatal development of rats seem to determine LH basal firing rate. The increased neuronal responsiveness to exogenous DA and CCK in neonatally overfed SL rats may indicate a decreased activity of these endogenous signals which normally contribute to limitation of energy intake.

VMN/LHA functional inhibition in tumor-bearing rats suggests hypothalamic involvement in cancer anorexia

Food intake is mainly controlled in the hypothalamus via a series of functionally related nuclei, including the ventromedial nucleus of hypothalamus (VMN) and the lateral hypothalamic area (LHA). Since food intake is the product of meal number and meal size, we investigated the role of the VMN and LHA in influencing these feeding indices and in mediating cancer anorexia in tumor-bearing (TB) rats, via temporarily inhibiting VMN or LHA. Adult male Fischer-344 rats (n = 23) inoculated with 106 MCA sarcoma cells were studied. When anorexia developed, rats were randomly assigned to stereotaxically located bilateral intra-VMN or intra-LHA microinjections of the neuronal blocker colchicine (CX; n = 6 each group) or saline (n = 6 and n = 5, respectively). Non TB rats (NTB; n = 7) served as controls. Food intake and feeding indices were recorded by a computerized device. At onset of anorexia, a reduction of meal number occurred, leading to reduced food intake. After inhibition of VMN activity by CX, meal number significantly increased, so that food intake increased and almost normalized. In contrast, intra-LHA microinjection of either CX or saline resulted in reduction of meal size, leading to reduced food intake and death. Findings suggest that VMN and LHA influence meal number and meal size, respectively. Since cancer anorexia mainly results from an initial reduction of meal number and the inhibition of VMN led to an increase in meal number, the early effect of tumor growth on VMN activity may be an early step leading to reduced food intake.

Expression of dopaminergic receptors in the hypothalamus of lean and obese Zucker rats and food intake

As revealed by previous microdialysis studies, basal and food intake-accompanied dopamine release significantly differs in the hypothalamus of obese vs. lean Zucker rats. In the present study, we determined whether dopaminergic receptors are also compromised in obesity. Dopaminergic D(1) and D(2) receptor mRNA expression was studied in the ventromedial hypothalamus (VMH), lateral hypothalamic area (LHA), and the adenohypophysis (AH) of obese and lean Zucker rats using RT-PCR technique. In obese Zucker rats, we found an upregulation of D(1) receptor mRNA in the VMH and AH and a downregulation in the LHA, whereas D(2) receptor mRNA was downregulated in both the VMH and LHA, but not changed in the AH, compared with lean rats. Also, an increase of D(1) receptor staining was seen in the paraventricular nucleus of obese rats by immunohistochemistry. We selected the VMH to test if the observed changes in the dopamine receptor expression of obese rats induce behavioral sensitization to dopamine as expressed by hyperphagia. The overnight food-deprived rats received a single VMH injection (10 nmol) of sulpiride (D(2) receptor antagonist) or saline as control, then food was provided and 1-h food intake was measured. Food intake after sulpiride vs. saline injection was greater in obese rats but was not different in lean rats. Our data suggest that downregulation of D(2) receptor in the hypothalamus at least in the VMH induces behavior sensitization for having large meals. Low D(2) receptor expression may be causal for an exaggerated dopamine release observed in obese rats during food ingestion and for reduced satiety feedback effect of dopamine. High level of D(1) receptor expression in the VMH and low in the LHA may also contribute to the specific feeding pattern in obese rats represented by large meal size and low meal number.

Hypothalamic dopaminergic receptor expressions in anorexia of tumor-bearing rats

Our past microdialysis studies in ventromedial hypothalamic nucleus (VMN) and lateral hypothalamic area (LHA) of changes in dopamine concentrations in response to changes in food intake [characterized as feeding pattern (changes in meal number and size)] in anorexia of cancer show abnormal presynaptic dopaminergic neurotransmission. To determine postsynaptic receptor status, studies were done in tumor-bearing (TB) and non-tumor-bearing (NTB) free-feeding control rats while continuously measuring their food intake via a rat eater meter. When TB rats developed anorexia, TB and control rats were killed, and postsynaptic D(1)- and D(2)-receptor mRNA expression in LHA and VMN were measured via RT-PCR. At anorexia, food intake decreased initially by a decrease in meal number, whereas a concurrent increase in meal size occurred for 24 h in an attempt to maintain food intake constant. Then meal size also decreased. At this time, D(1)- and D(2)-receptor mRNA expressions in LHA and VMN of TB vs. controls were significantly upregulated. Verification of D(1)- or D(2)-receptor changes to changes in meal number and size at anorexia was made by injection of intra-VMN or -LHA dopaminergic receptor antagonists. Intra-VMN D(1)-receptor antagonist (SCH-23390) in TB rats decreased food intake mainly via a decrease in meal size. Intra-VMN D(2)-receptor antagonist (sulpiride) in TB rats increased food intake via an increase in meal number and in NTB free-feeding rats by an increase in meal size. Intra-LHA D(1)-receptor antagonist in TB rats had no effect on food intake or feeding pattern. Intra-LHA D(2)-receptor antagonist in TB and in NTB free-feeding rats increased food intake via an increase in meal number. Our data provide evidence that postsynaptic dopaminergic receptor subtypes in the hypothalamus are involved in the regulation of meal size, meal number, and thus food intake in anorectic TB rats.

Nicotine alters the usual reciprocity between meal size and meal number in female rat

Tobacco smoking reduces appetite and body weight (BW). Cessation of smoking leads to hyperphagia and weight gain. Daily food intake (FI) is a function of meal number (MN) and meal size (MZ), i.e., FI=MNxMZ. Under normal conditions, the female Fischer rat has a periodic reciprocal fluctuation between MZ and MN corresponding to phase of estrous cycle. Wide fluctuations between MZ and MN compensate each other to keep FI constant. Nicotine (5 mg/kg BW/day) was infused via osmotic minipump for 7 days. Controls received saline. FI, MZ, and MN were measured by an Automated Computerized Rat Eater Meter. Nicotine significantly decreased BW and FI via a decrease in MZ without compensatory increase of MN. Nicotine cessation led to hyperphagia, normalizing BW loss via an increase in MZ, which exceeded a compensatory decrease in MN. Nicotine significantly prolonged the estrous cycle by an extension of proestrous phase. Nicotine significantly lengthened the intermeal interval (IMI), delaying the start of the next meal and simultaneously decreasing subsequent MZ. Stopping nicotine led to normalization of IMI and MZ. Data show that nicotine alters the usual reciprocal regulation between MZ and MN and leads to a prolongation of the estrous cycle.

Altered dopaminergic transmission in the anorexic anx/anx mouse striatum

We demonstrate abnormal dopaminergic neurotransmission in anorexic mice, homozygous for a recessive mutation (anx) causing starvation and motor disturbances. Isolated neurons from anx/anx striatum displayed a markedly increased activity of the Na+,K+-ATPase compared with normal littermates. Dopamine down-regulates Na+,K+-ATPase activity in striatal medium spiny neurons in rat, mouse and guinea pig. However, addition of dopamine in vitro failed to suppress the increased activity in anx/anx striatal neurons. Striatal dopamine and its metabolites, but not norepinephrine, were slightly but significantly lower in anx/anx mice than in normal littermates. We suggest that abnormal dopaminergic transmission may contribute to the anx phenotype.

Intra-supraoptic nucleus sulpiride improves anorexia in tumor-bearing rats

Previous studies suggest that the dopaminergic system in the supraoptic nucleus (SON) is involved not only in the water balance control but also in the food intake regulation. Since we reported that an injection of the D2 receptor antagonist, sulpiride, into specific hypothalamic nuclei (e.g. the LHA, or the VMN) increases food intake in anorectic tumor-bearing rats, as well as in normal rats, we hypothesized that an injection of sulpiride into the SON would also improve cancer anorexia. Sulpiride injection (4 microg/0.5 microl) into bilateral SON of anorectic tumor-bearing male rats significantly improved food intake via increases in both meal size and meal number. These data suggest that pharmacological manipulation of the hypothalamic dopaminergic system is feasible in amelioration of cancer anorexia.

Role of hypothalamic monoamines in nicotine-induced anorexia in menopausal rats

BACKGROUND

Nicotine reduces body weight by reducing appetite. Estradiol modulates food intake. Menopause or ovariectomy (Ovx) increases food intake and body weight. Nicotine and estradiol individually influence hypothalamic dopamine (DA) and serotonin (5-HT), whose interaction influences food intake and body weight. We investigated whether lower weight gain in menopausal smokers is mediated via changes in hypothalamic DA/5-HT.

METHODS

Ovx or sham-operated female rats had 2 microdialysis guide cannulas simultaneously implanted in ipsilateral ventromedial nucleus of hypothalamus (VMN) and contralateral lateral hypothalamic area (LHA). Rats were divided into 4 groups and received a continuous subcutaneous infusion of nicotine or saline Ovx and sham. DA and 5-HT in LHA and VMN were measured by in vivo microdialysis.

RESULTS

Nicotine infusion decreased food intake and body weight in Ovx and sham groups. Increase in LHA-DA and VMN-5-HT in sham group occurred with nicotine, whereas an increase in VMN-DA in Ovx groups with and without nicotine and VMN-5-HT in Ovx group with nicotine was observed.

CONCLUSIONS

In the presence of estradiol (ovary intact sham-operated rats), nicotine lowers food intake and body weight via increased LHA-DA and VMN-5-HT. In menopause (Ovx rats), nicotine lowers food intake and body weight only via increased VMN-DA and 5-HT. Data show that lower weight gain is mediated via changes in hypothalamic monoamines, primarily via ventromedial hypothalamus.

Feeding behavior during sialodacryoadenitis viral infection in rats

Sialodacryoadenitis (SDA) is a highly contagious common viral infection in rats, akin to mumps in humans. Anorexia occurs during such viral infection. But the pattern of the decrease in food intake (a decrease in either meal size and meal number or both) during spontaneous viral infection has not been previously characterized. We observed the onset of anorexia and an abnormal feeding pattern during an opportunistic SDA viral infection in our rat colony. We thus studied seven male rats. Before the viral infection there was a positive association between food intake and meal number (P<.05). After infection food intake decreased by 68%. This occurred via a significant decrease in meal size (by 69%) (P<.05); and a nonsignificant decrease in meal number (P=.71). This pattern of decreased food intake is similar to that occurring during indomethacin-induced ulcerative ileitis, where we previously measured an increase in plasma tumor-necrosis factor (TNF)-alpha. Anorexia in response to bacterial lipopolysaccharide administration, which is also linked to plasma TNF-alpha, is however, caused only via a decrease in meal number. The differences in the decrease in the feeding pattern between the SDA viral and a bacterial infection suggest that factors other than TNF-alpha alone play a significant role in the mechanism of anorexia during a viral infection.

Synchronized release of dopamine and serotonin in the medial and lateral hypothalamus of rats

A positive linear correlation between dopamine and serotonin release was found in the ventromedial hypothalamus and in the lateral hypothalamic area in fasting rats and in fed rats during intermeal intervals. Dopamine release in the ventromedial hypothalamus positively correlated with dopamine and serotonin release in the lateral hypothalamic area, which occurred only during intermeal intervals and was non-significant during the meal consumption periods or during fasting. Meal size correlated significantly only with a decrease in serotonin release in the lateral hypothalamic area. The study was designed to evaluate the relationship between dopamine and serotonin release in these hypothalamic areas and their dependence on feeding status. Microdialysis was performed simultaneously via two probes, one in the ventromedial hypothalamus and the other in the contralateral lateral hypothalamic area, of freely moving male lean Zucker rats over 24h with preserved light and dark phase, either with ad libitum access to food and water, or when no food was available. Dopamine and serotonin concentrations were measured by high-performance liquid chromatography with electrochemical detection in 20-min dialysis samples. Time-series analysis was applied to determine linear correlations between monoamines and in relation to food intake. Data showed that release of dopamine and serotonin is synchronized within the ventromedial hypothalamus and lateral hypothalamic area, particularly in the dark phase and when no food was ingested. However, synchronized release of monoamines between these nuclei occurred only during intermeal intervals: the periods of satiety. These findings suggest a tight relationship between dopaminergic and serotonergic systems of the lateral hypothalamic area and ventromedial hypothalamus, which is influenced by the feeding state and which may be involved in maintaining the balance within and between the centers of the parasympathetic and sympathetic nervous systems. The data also illustate that food intake is coupled unequivocally to the release of dopamine and serotonin in the hypothalamus, suggesting it as a mechanism of activation of postsynaptic neurons associated with new metabolic status.

Hypothalamic dopamine and serotonin in the regulation of food intake

Because daily food intake is the product of the size of a meal and the frequency of meals ingested, the characteristic of meal size to meal number during a 24-h light-dark cycle constitutes an identifiable pattern specific to normal states and obesity and that occurs during early cancer anorexia. An understanding of simultaneous changes in meal size and meal number (constituting a change in feeding patterns) as opposed to an understanding of only food intake provides a more insightful dynamic picture reflecting integrated behavior. We have correlated this to simultaneous changes in dopamine and serotonin concentrations and to their postsynaptic receptors, focusing simultaneously on two discrete hypothalamic food-intake-related nuclei, in response to the ingestion of food. The relation between concentrations of dopamine and serotonin limited to the lateral hypothalamic area (LHA) and the ventromedial nucleus (VMN) as they relate to the influence of meal size and meal number during the hyperphagia of obesity and anorexia of cancer as measured in our experiments are discussed. Based on these data, conceptual models are proposed concerning: 1) an "afferent-efferent neurotransmitter unit," with facilitatory or inhibitory neuropeptide properties to generate an appropriate neuroendocrine and neuronal response that ultimately modifies food intake; 2) initiation and termination of a meal, thereby determining the number and size of a meal under normal conditions; and 3) a schema integrating the onset mechanism of cancer anorexia. Nicotine is used as a tool to further explore the relation of meal size to meal number, with a focus on simultaneous changes in dopamine and serotonin concentrations in the LHA and VMN with the onset of acute anorexia of nicotine infusion and acute hyperphagia of nicotine cessation. Data concerning the role of sex-related hormones on dopamine and serotonin with regard to the LHA and VMN in relation to the modulation of food intake are also presented.

Dopamine and serotonin VMN release is related to feeding status in obese and lean Zucker rats

Study of neurotransmitter role in food intake regulation in a leptin signaling deficient model, such as the Zucker rat, would benefit in the understanding of mechanisms of human obesity, in which leptin resistance is a common syndrome. We studied dopamine (DA) and serotonin (5-HT) concentrations in vivo in the ventromedial nucleus (VMN) of the hypothalamus, as they relate to eating after food deprivation in obese and lean 9-week-old male Zucker rats. DA and 5-HT concentrations were measured by HPLC via microdialysis before and during refeeding in 24-h food-deprived rats. Before food was provided, mean baseline DA and 5-HT levels were lower in obese than in lean rats (9.2 +/- 0.9 vs 15.1 +/- 1.9 pg/10 microl, p < 0.01, and 0.68 +/- 0.05 vs 1.17 +/- 0.02 pg/10 microl, p < 0.001, respectively). Food intake was accompanied by a decrease in DA levels in both obese and lean rats to 64% (p < 0.01) and 65% (p < 0.02) of their baseline levels respectively. 5-HT levels were significantly increased during eating by 41% in obese and 35% in lean rats (p < 0.01) from the baseline levels. Thus in obese rats with altered leptin signaling we found an unaltered pattern of DA and 5-HT release associated with food deprivation and refeeding, but with presence of their low levels. This points to an impaired postsynaptic monoaminergic action to produce an adequate metabolic response in obese Zucker rats in response to feeding state.

VMN dopaminergic graft and feeding pattern in obese Zucker rats

OBJECTIVE

To study the role of dopamine in the ventromedial hypothalamus (VMN) in the regulation of meal size and meal number during obesity.

METHODS

Embryonic mesencephalic cells rich in dopaminergic neurons from lean rats were grafted into the VMN of obese Zucker rats. Since food intake is the product of meal size and number, these variables were measured using a rat 'eater meter'. Dopamine and serotonin concentrations in the VMN were assayed in grafted and control rats via in vivo microdialysis and HPLC two months after transplantation.

RESULTS

Food intake increased in grafted rats due to an increase of both meal size and meal number 2 weeks after implantation and to an increase of meal size with insufficient compensatory decrease of meal number 2 months after transplantation. Grafted rats showed higher absolute dopamine and lower serotonin concentrations in the VMN.

CONCLUSION

It would appear that an increase of dopamine and a decrease of serotonin in the VMN of grafted obese rats may correlate with increase in meal number and meal size, respectively. Since obese Zucker rats usually display an enlarged meal size, we deduce from the data that chronically elevated VMN dopamine and low serotonin are involved in producing the large meal size observed during obesity.

Infusion of nicotine into the LHA enhances dopamine and 5-HT release and suppresses food intake

Nicotine administration induces hypophagia. Because of the involvement of hypothalamic neurotransmitters in food intake control, we hypothesized that increased activity of the lateral hypothalamic dopamine (LHA-DA) and/or serotonin (LHA-5-HT) may be responsible for nicotine-induced hypophagia. Either 4 mM nicotine or vehicle was administered via reverse microdialysis technique into the LHA of overnight food-deprived rats for 60 min; then food was provided for 40 min. The LHA-DA, 5-HT and their intermediate metabolites, DOPAC and 5-HIAA, were continuously measured during 20-min intervals before, during, and after nicotine administration. Continuous nicotine administration for 60 min increased LHA-DA and DOPAC concentrations during the first 40 min, and induced a long-lasting increase in LHA-5-HT release, until 120 min after the start nicotine administration, even when nicotine administration was stopped. The food intake during the 40-min refeeding period was significantly lower when rats received nicotine. Eating induced a significant and short-lasting increase in the LHA-DA and a long-lasting increase in the LHA-5-HT. These findings indicate that nicotine enhances dopaminergic and serotonergic activity in the LHA, and that the enhanced LHA-5-HT activity may contribute to nicotine-induced hypophagia.

Effect of operative stress on food intake and feeding pattern in female rats

Effects of operative stress on food intake, meal size, and meal number were measured in 15 female rats before and after jugular vein catheterization. All rats had 5-d estrous cycles which correlated with cyclical feeding patterns that were most prominent during dark phase eating. In proestrous, meal number peaked (30.3+/-1.32), and meal size reached a nadir (0.33+/-0.02 g) with some corresponding change in food intake (9.8+/-0.38 g). Following operation on day 11, the cyclical variation of food intake, meal number, and meal size with estrous cycle was lost for the first 3 d, as was the diurnal rhythm in food intake. Eight rats recovered their dark phase feeding pattern by day 17 (recovered group), while 7 had not done so even by day 24 (non-recovered group). Food intake decreased to 40% of baseline in the recovered group and to 25% in the non-recovered group on day 11, increasing to 70% by day 14 in both groups and matching preoperative levels by day 17. Similar postoperative decreases were observed in meal number and meal size. Light phase feeding was increased, the ratio of day to night food intake being three times preoperative levels even at day 24. Preoperatively, non-recovered rats were similar to the recovered rats in all feeding indexes and continued to have estrous cycling in vaginal smears postoperatively. In the non-recovered rats, meal size more than doubled and meal number was depressed by 47% of preoperative levels and remained low until the end of the study. We conclude that operative stress disrupted cyclical and diurnal rhythms in food intake. In female rats, meal size is the first index to recover, increasing temporarily to maintain food intake.

Use of orchiectomy and testosterone replacement to explore meal number-to-meal size relationship in male rats

Because food intake is a function of meal number and meal size and because gender-related hormones are involved in feeding regulation, we explored effects of orchiectomy and testosterone replacement on the relationship between meal number and size and changes in resulting feeding patterns in adult male rats, randomized into orchiectomy and sham-operation groups. A rat eater meter measured feeding indexes for 1 wk before and 2 wk after castration and during 8 days of testosterone replacement. Orchiectomy leads to an immediate change in the meal number-to-size relationship, resulting in 1) change in pattern of feeding; 2) a significant decrease in dark-phase meal number; 3) a significant increase in dark-phase meal size, but insufficient to offset decrease in meal number, so total food intake significantly decreased during dark phase; 4) no significant change in light-phase meal number; and 5) an increase in meal size leading to an increased food intake during light phase, which offset decreased food intake in dark cycle and resulted in no net significant change in food intake after orchiectomy. Testosterone replacement acutely reversed effects of orchiectomy on meal number-to-meal size relationship, restoring feeding pattern. Data suggest that androgens immediately influence the meal number-to-meal size relationship. The speed of onset seen after orchiectomy suggests that the influence of testosterone on food intake may also occur partially via a nongenomic effect.