Satiated Rats Research Articles

Exaggerated feeding response to central galanin-like peptide administration in diet-induced obese rats

Galanin-like peptide (GALP) is a newly identified neuropeptide implicated in the regulation of metabolism and reproduction. GALP gene expression is decreased in the hypothalamus of genetically obese rodents, such as fa/fa rats and ob/ob mice, and central administration of GALP increases feeding in satiated rats. The effect of dietary obesity on GALP-induced feeding is unknown, so this study characterized the effects of central administration of GALP on feeding in a rat model of diet-induced obesity. Male Sprague-Dawley rats (n = 21) were randomly assigned to receive standard laboratory chow (12% fat as kcal) or high-fat cafeteria diet (35% fat) for 12 weeks before intracerebroventricular (icv) cannulae were implanted. Seven days later, rats received 0,0.2 or 0.3 nmol doses of GALP in randomized order at least 48 h apart. Food intake was measured at 0.5,1,2, 4 and 24 h post administration and body weight was measured at 24 h. Rats were maintained on their respective diets throughout the entire feeding experiment. Implementation of the high-fat diet led to significantly greater caloric intake (230%) and body weight (28%) compared to chow-fed control rats. GALP-induced feeding was rapid and maximal in both dietary groups at 30 min post injection. The 0.3 nmol dose of GALP led to significantly larger increases in caloric intake in high-fat fed rats than in chow-fed controls (35.4 +/- 3.7 and 22.1 +/- 1.3 kcal, respectively, at 30 min). It is not known if diet-induced obesity alters endogenous GALP levels, but our data suggest that adaptive responses in GALP signaling might occur during chronic overfeeding. One possible explanation is an increased sensitivity and/or number of specific GALP receptors, although actions of exogenous GALP may also represent pharmacological actions at galanin receptors.

Ejaculation induces long-lasting behavioural changes in male rats in the forced swimming test: evidence for an increased sensitivity to the antidepressant desipramine

Copulation to exhaustion induces a sexual inhibitory state featured by a decreased motivation. Since diminished motivation is a key symptom of depression, we analysed if sexually exhausted animals showed increased levels of depressive-like behaviour in the forced swimming test (FST). Besides, sexual activity has been reported to have reinforcing properties. Thus, we analysed whether different levels of sexual activity modified the development of the behavioural despair in the FST. Finally, the effect of a sub-threshold dose of desipramine (DMI, 2.5 mg/kg) was evaluated in animals with different sexual conditions. Male adult rats were divided into: (a) naïve rats and (b) animals executing one intromission (1-INTR) or (c) one ejaculation (1-EJ) and (d) sexually satiated rats, classified as sexually responsive (R) and non-responsive (NR). No differences were found in immobility behaviour between sexually exhausted and naïve rats. In the pre-test sessions of the FST males attaining ejaculation (1-EJ, R and NR) had lower levels of immobility or showed a tendency towards such a diminution, while animals with sexual activity not involving ejaculation did not. Data suggest a “protective” effect of ejaculation against the development of depressive-like behaviour. Finally, the sub-effective dose of DMI produced an antidepressant-like action in all animals that ejaculated. In conclusion, sexual experience involving ejaculation modifies the behavioural expression of rats in the FST and their sensitivity to antidepressant drugs like DMI. Both responses reflect brain plastic changes induced by ejaculation.

GABA A receptor activation in the lateral parabrachial nucleus induces water and hypertonic NaCl intake

Inhibitory serotonergic and cholecystokinergic mechanisms in the lateral parabrachial nucleus and central GABAergic mechanisms are involved in the regulation of water and NaCl intake. In the present study we investigated if the GABA A receptors in the lateral parabrachial nucleus are involved in the control of water, NaCl and food intake in rats. Male Holtzman rats with stainless steel cannulas implanted bilaterally into the lateral parabrachial nucleus were used. Bilateral injections of muscimol (0.2nmol/0.2μl) into the lateral parabrachial nucleus strongly increased 0.3M NaCl (20.3±7.2 vs. saline: 2.6±0.9ml/180 min) without changing water intake induced by the treatment with the diuretic furosemide combined with low dose of the angiotensin converting enzyme inhibitor captopril s.c. In euhydrated and satiated rats, bilateral lateral parabrachial nucleus injections of muscimol (0.2 and 0.5nmol/0.2μl) induced 0.3M NaCl intake (12.1±6.5 and 32.5±7.3ml/180 min, respectively, vs. saline: 0.4±0.2ml/180 min) and water intake (5.2±2.0 and 7.6±2.8ml/180 min, respectively, vs. saline: 0.8±0.4ml/180 min), but no food intake (2±0.4g/240 min vs. saline: 1±0.3g/240 min). Bilateral lateral parabrachial nucleus injections of the GABA A antagonist bicuculline (1.6nmol/0.2μl) abolished the effects of muscimol (0.5nmol/0.2μl) on 0.3M NaCl and water intake. Muscimol (0.5nmol/0.2μl) into the lateral parabrachial nucleus also induced a slight ingestion of water (4.2±1.6ml/240 min vs. saline: 1.1±0.3ml/240 min) when only water was available, a long lasting (for at least 2 h) increase on mean arterial pressure (14±4 mm Hg, vs. saline: −1±1 mm Hg) and only a tendency to increase urinary volume and Na + and K + renal excretion. Therefore the activation of GABA A receptors in the lateral parabrachial nucleus induces strong NaCl intake, a small ingestion of water and pressor responses, without changes on food intake.

P.8.083 Neuroprotective effects of EGIS-11229, a novel AMPA receptor antagonist with improved metabolic stability

Glutamate and its receptor agonists, NMDA, AMPA, and KA, elicit feeding when microinjected into the lateral hypothalamus (LH) of satiated rats. However, determining the relative contributions of AMPA receptors (AMPARs) and KA receptors (KARs) to LH feeding mechanisms has been difficult due to a lack of receptor selective agonists and antagonists. Furthermore, LH injection of KA produces behavioral hyperactivity, questioning a role for KARs in feeding selective stimulation. In the present study, we used the KAR agonist, (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA), which selectively binds the GluR5 subunit of KARs, to stimulate feeding, presumably via KAR activation. Using ATPA, we tested whether: (1) LH injection of ATPA elicits feeding, (2) prior treatment with the non-selective AMPA/KAR antagonist, CNQX, suppresses ATPA-elicited feeding, and (3) LH injection of ATPA elicits behavioral patterns specific for feeding. We found that injection of ATPA (0.1 and 1 nmol) elicited an intense feeding response (e.g., 4.8 ± 1.6 g) that was blocked by LH pretreatment with CNQX, but was unaffected by pretreatment with the AMPAR selective antagonist, GYKI 52466. Furthermore, minute-by-minute behavioral analysis revealed that LH injection of ATPA increased time spent feeding to 55% of the initial test period with little or no effects on other behaviors at any time. In contrast, LH injection of KA similarly increased feeding but also produced intense locomotor activity. These data suggest that selective activation of LH KARs containing GluR5 subunit(s) is sufficient to elicit feeding.

NPY mediates the feeding elicited by muscimol injections into the nucleus accumbens shell

Injections of muscimol into the nucleus accumbens shell (AcbSh) induce large increases in food intake in satiated rats and also activate neurons in a number of feeding-related brain regions, including NPY-containing neurons in the arcuate hypothalamic nucleus and cells in the paraventricular hypothalamic nucleus. This suggests that the NPY system may participate in the expression of AcbSh-mediated feeding behavior. Therefore, we examined the effects of intraventricular administration of the Y1 receptor antagonist 1229U91 or the Y5 receptor antagonist L-152,804 on AcbSh-mediated food intake. Intra-AcbSh muscimol elicited a large increase in food intake which was potently suppressed by blocking either central Y1 or Y5 receptors. Our results suggest that the AcbSh influences food intake, in part, through the release of NPY.

Anorexia and weight loss in male rats 24 h following single dose treatment with orexin-1 receptor antagonist SB-334867

Acute systemic treatment with the selective orexin-1 receptor antagonist SB-334867 (30 mg/kg, i.p.) has been reported not only to inhibit food intake and to accelerate behavioural satiety in rats, but also to produce a significant loss of bodyweight over the 24 h period post-dosing. The present studies were designed to test the hypothesis that the inhibition of weight gain following acute treatment with SB-334867 is due to a persistent anorectic action of the compound. In Experiment 1, the acute effects of SB-334867 (30 mg/kg, i.p.) on food intake and behaviour in a 1 h test with palatable mash were assessed as a function of injection-test interval. Results confirmed that, when administered 30 min prior to testing, SB-334867 significantly suppressed mash intake and accelerated behavioural satiety. More importantly, significant anorexia and behavioural change were also observed when animals were tested 24 h, but not 48 h, post-dosing. As previously reported, all animals treated with the orexin-1 receptor antagonist lost bodyweight over the 24 h period following acute treatment. The generality of these findings was confirmed in Experiment 2, where acute treatment with SB-334867 (30 mg/kg, i.p.) significantly suppressed home cage chow consumption over the 24 h period post-dosing, an effect also accompanied by a significant loss of bodyweight. The results of Experiment 3 showed that, following i.p. administration of 30 mg/kg, SB-334867 has good CNS penetration, reaches peak plasma and brain concentrations at 30 min, and maintains good exposure over 4 h post-dosing. Overall, current data support the hypothesis that a persistent anorectic action contributes to the significant loss of bodyweight observed 24 h following acute dosing with SB-334867. As the compound is virtually undetectable in plasma or brain beyond 8 h post-dosing, and since nothing is known about potentially active metabolites, we consider the possibility that single dose treatment with SB-334867 results in enduring alterations to the orexin-1 receptor and/or downstream signalling pathways.

Evidence for time?place learning in the Morris water maze without food restriction but with increased response cost

Time–place learning is the ability to distinguish between resources that vary in location at different times of day. Only one previous report has demonstrated successful time–place learning without using food as reward. In this experiment, satiated rats failed to form time–place discriminations in a Morris water maze while food deprived rats did, leading to the conclusion that food system activation is necessary for time–place learning. However, in addition to food system activation, response cost was also increased, which previously has been demonstrated to be effective in allowing the formation of time–place discriminations. The purpose of these two experiments is to test whether food system activation or heightened response cost allowed for time–place learning in the Morris water maze. In the first experiment, we replicate the failure to find time–place discriminations in the Morris water maze without food restriction and without increased response cost. In the second experiment, we found that increased response cost without food restriction was effective in allowing the formation of a time–place discrimination. The implications of this result are discussed in light of the timing mechanism used for time–place discriminations, the nature of the response cost, and the event–time–place tripartite association.

A potent and selective NPY Y5 antagonist reduces food intake but not through blockade of the NPY Y5 receptor.

These studies were performed to test the hypothesis that endogenous neuropeptide Y (NPY) acting on the NPY Y(5) receptor subtype contributes to the control of food intake. The hypothesis was tested using S 25585-a newly synthesized NPY Y(5) receptor antagonist. S 25585 was shown to be a high-affinity antagonist of the NPY Y(5) receptor subtype (IC(50) 5 nM) with no significant affinity toward other NPY receptor subtypes and over 40 other receptors, channels or uptake systems. S 25585 (7.5 mg/kg, i.p.) did not induce a conditioned taste aversion, significantly alter need-induced sodium appetite or induce pica, suggesting that at this dose the compound did not induce illness or malaise. In satiated rats, S 25585 (5.0 and 7.5 mg/kg, i.p.) significantly decreased the overfeeding induced by i.c.v. injection of NPY (1 microg) and the highly selective NPY Y(5) receptor agonist [hPP(1-17), Ala(31), Aib(32)]NPY (0.7 microg). In rats fasted for 4 h immediately before the dark phase, analysis of the microstructure of feeding behavior revealed that S 25585 significantly increased latency to eat and significantly decreased the duration and size of the meals without altering the meal number or eating rate. Analysis of the behavioral satiety sequence at this time revealed that the animals passed through the normal pattern of feeding, grooming and resting. Although S 25585 appeared to be influencing a physiological system controlling appetite, this does not involve the NPY Y(5) receptor since the antagonist also markedly reduced food intake in the NPY Y(5) knockout mouse. The results presented do not support a role for the NPY Y(5) receptor in the control of food intake. The results further illustrate that it is imperative that the activity of any new NPY Y(5) antagonist be assessed in the NPY Y(5) knockout mouse before assuming that its effect on food intake is due to blockade of this receptor.

Effects of neuromedin U on the pulsatile LH secretion in ovariectomized rats in association with feeding conditions

We examined the effects of intracerebroventricular injection of neuromedin U (NMU), at a dose that is reported to induce satiety in rats, on the pulsatile luteinizing hormone (LH) secretion in adult ovariectomized (OVX) rats under a normal feeding or a 48-h fasted condition. In OVX rats under the normal feeding condition, injection of NMU (1nmol/3μl) significantly decreased the mean LH concentration without affecting the frequency or amplitude of LH pulses, but under the 48-h fasted condition, it significantly decreased the mean LH concentration and the frequency of LH pulses without affecting the amplitude. The interpulse interval was significantly lengthened by NMU injection under the normal and the 48-h fasted condition, but the effect under the 48-h fasted condition was greater than under the normal feeding condition. We also confirmed that the 48-h fasted condition per se did not affect the pulsatile LH secretion in OVX rats. We suggest that NMU and fasting synergistically inhibit the pulsatile LH secretion, even though NMU has been said to act as a satiety factor.

Dual roles in feeding for AMPA/kainate receptors: receptor activation or inactivation within distinct hypothalamic regions elicits feeding behavior

We have previously shown that hypothalamic injections of glutamate, or agonists of its ionotropic receptors (iGluRs), elicit intense feeding responses in satiated rats [Brain Res. 613 (1993) 88, Brain Res. 630 (1993) 41]. While attempting to clarify the role of the AMPA and kainate (KA) receptor subtypes in glutamatergic feeding systems, we discovered that lateral hypothalamic (LH) injection of high doses of the competitive AMPA/KA receptor antagonist, NBQX (10 and 30 nmol), elicited a pronounced feeding response. We questioned whether this effect was due to inactivation of AMPA or possibly KA receptors. To determine whether other AMPA/KA antagonists can also elicit feeding, we tested whether injection of CNQX, another AMPA/KA receptor antagonist, also stimulates eating and whether these feeding stimulatory effects were due to antagonists' actions in the LH or in other hypothalamic sites. Here we report that NBQX and CNQX elicit feeding in a dose dependent manner and are most effective when injected into the perifornical hypothalamus (PFH), or into the paraventricular nucleus (PVN) and, to a lesser extent, into the LH of satiated rats. In contrast, AMPA was most effective in stimulating feeding when injected into the LH, confirming previous reports. These data suggest that either activation or inactivation of AMPA/KA receptors in distinct but overlapping hypothalamic sites may be sufficient to induce feeding behavior, indicating a broadened role for glutamate in hypothalamic feeding mechanisms.

Identification of hypothalamic nuclei involved in the orexigenic effect of melanin-concentrating hormone.

The hypothalamic neuropeptide melanin-concentrating hormone (MCH) increases feeding when injected intracerebroventricularly in rats. To identify the hypothalamic nuclei responsible for the orexigenic effect, we injected the peptide into discrete hypothalamic nuclei known to express the MCH receptor, MCH1R. MCH (0.6 nmol) elicited a rapid and significant increase in feeding in satiated rats following injection into the arcuate nucleus (0-1 h: 421 +/- 60%; P < 0.01). An elevation in feeding was also observed following injection into the paraventricular nucleus, which was sustained up to 4 h post injection (0-4 h: 218 +/- 29%; P < 0.01). A significant increase in feeding during this time period was also observed following injection into the dorsomedial nucleus (0-4 h: 155 +/- 12%; P < 0.05). No significant alteration in feeding was observed following injection into the supraoptic nucleus, lateral hypothalamic area, medial preoptic area, anterior hypothalamic area, or ventromedial nucleus of the hypothalamus. To identify the neurotransmitters that may be potentially involved in this effect, we examined their release from hypothalamic explants in vitro following exogenous MCH administration. MCH (1 micro M) increased the release of the orexigenic neurotransmitters neuropeptide Y (37.8 +/- 6.0 fmol/explant vs. basal 30.2 +/- 4.3 fmol/explant; P < 0.05) and agouti-related peptide (4.1 +/- 0.6 fmol/explant vs. basal 2.4 +/- 0.2 fmol/explant; P < 0.05) and decreased the release of the anorectic neurotransmitters alpha-MSH (41.7 +/- 6.8 fmol/explant vs. basal 65.9 +/- 11.0 fmol/explant; P < 0.01) and cocaine- and amphetamine-regulated transcript (112.3 +/- 12.4 fmol/explant vs. basal 167.4 +/- 13.0 fmol/explant; P < 0.001). These studies suggest that the orexigenic effect of MCH may be mediated via activation or inhibition of these feeding circuits within the arcuate nucleus and paraventricular nucleus of the hypothalamus.

Acquisition of a palatable-food-sustained appetitive behavior in satiated rats is dependent on the dopaminergic response to this food in limbic areas

Rats exposed to repeated unavoidable stress show decreased dopamine output in the nucleus accumbens shell (NAcS) and do not acquire vanilla sugar (VS)-sustained appetitive behavior (VAB). Rats treated with lithium for 3 weeks also show decreased NAcS dopamine output, yet they acquire VAB. Feeding a novel palatable food increases extraneuronal dopamine levels in the NAcS and medial prefrontal cortex (mPFC) in rats. In order to investigate the role of food-induced dopamine release in VAB acquisition, we studied by microdialysis the dopaminergic response in the NAcS and mPFC to the presentation and consumption of VS in satiated control rats, and in satiated rats exposed to repeated stress or lithium treatment. The dopaminergic response to VS was also studied in rats familiar with VS, or that had acquired VAB. In control rats, VS feeding was accompanied by increased dopamine output in the NAcS and mPFC, and one-trial habituation to this effect developed in the NAcS. Rats exposed to a 7-day stress showed reduced interest in VS pellets, and when fed VS they did not show a dopaminergic response in the NAcS and mPFC. Lithium-treated rats rapidly ate VS pellets and showed increased dopamine output in the NAcS and mPFC, with no habituation in the NAcS response. Rats familiar with VS and rats that had already learned VAB ate VS pellets. The first group showed a lower dopaminergic response to VS consumption than the control group, but the latter showed no dopaminergic response in the NAcS and mPFC. We propose that the limbic dopaminergic response to a novel palatable food plays a role in associative learning and that it is predictive of the competence to learn an appetitive behavior. Moreover, in our experimental conditions a phasic increase in mesolimbic dopamine no longer signals the VS stimulus once it has become a reinforcer in an appetitive task.

Chronic MCH-1 receptor modulation alters appetite, body weight and adiposity in rats

Central administration of the neuropeptide melanin-concentrating hormone (MCH) stimulates feeding in rodents. We studied the effects of intracerebroventricular (i.c.v.) administration of an MCH-1 receptor agonist (Compound A) and an MCH-1 receptor antagonist (Compound B) on feeding in satiated rats. Compound B (10 μg, i.c.v.) blocked the acute orexigenic effect of Compound A (5 μg, i.c.v.). In an experiment designed to either stimulate or inhibit MCH-1 receptor signaling over an extended period, rats received continuous i.c.v. infusions of vehicle (saline), Compound A (30 μg/day), Compound B (30 or 48 μg/day) or neuropeptide Y (24 μg/day, as positive control) via implantable infusion pumps. Continuous MCH-1 receptor activation recapitulated the obese phenotype of MCH-over-expressor mice, manifest as enhanced feeding (+23%, P<0.001), caloric efficiency and body weight gain (+38%, P<0.005) over the 14-day period relative to controls. Chronic MCH-1 receptor activation also elevated plasma insulin and leptin levels significantly. Conversely, continuous MCH-1 receptor antagonism led to sustained reductions in food intake (−16%, P<0.001), body weight gain (−35%, P<0.01), and body fat gain relative to controls, without an effect on lean mass. Antagonism of the MCH-1 receptor may be an effective approach for the treatment of obesity.

Effects of systematic variation in presatiation and fasting on the behavioural satiety sequence in male rats

Preclinical research on the neurobiology of appetite regulation is increasingly employing detailed behavioural assessment in addition to measures of food intake. One of the most widely used approaches examines treatment effects on the behavioural satiety sequence (BSS), a concept describing the predictable pattern of behavioural change (feeding⇒grooming⇒resting) associated with the process of satiation in rats, mice and other mammals. Surprisingly, however, comparatively little published detail is available concerning the impact of more natural appetite modulators on the BSS. In two separate experiments, a continuous monitoring technique was used to calibrate the effects of prefeeding (3, 6 and 9 min) and prior fasting (3, 6 and 12 h) on the microstructure of rat behaviour during a 1 h test with palatable mash. Prefeeding significantly increased eating latencies and reduced both food intake and total duration (but not frequency) of feeding behaviour. The reduction in time spent eating was most evident during the first 15 min of the test when feeding is normally at peak levels. Although behavioural structure was fully preserved in all test conditions, the two larger preloads resulted in shifts to the left (i.e., an acceleration) in the BSS. In contrast, fasting for 6 and 12 h (but not 3 h) increased food intake and duration (but not frequency) of feeding behaviour in the early part of the test. All fasting conditions (including 3 h) produced considerable shifts to the right (i.e., a delay) in the BSS, confirming the greater sensitivity of this measure relative to food intake per se. The potential utility of these reference profiles is discussed in relation to drug-induced changes in food intake and the BSS.

Inhibitory Effect of Apelin-12 on Nocturnal Food Intake in the Rat

Apelin, the endogenous peptide ligand of the APJ receptor, is expressed in brain regions implicated in food and water intake. This study reports for the first time, the effect of apelin-12, one of the most potent apelin peptides, on spontaneous (nocturnal) feeding. Randomised intracerebroventricular injection of 1, 3 and 10 nmol apelin-12 or saline vehicle, 10 min prior to lights out, led to dose-dependent reductions in food intake 2-4 h after injection (n=7; p<0.05). This suggests that apelin-12 exerts a delayed inhibitory effect on nocturnal feeding. Relative to saline, no effect was observed on total 24-h food intake post injection. In contrast, day-time administration of 10 nmol apelin-12 to satiated rats stimulated feeding (n=5-11; p<0.05); lower doses had no effect. No changes in water intake were observed after apelin-12. These results suggest that apelin is involved in the central control of feeding.

Effects of Galanin-Like Peptide on Food Intake and the Hypothalamo-Pituitary-Thyroid Axis

Galanin-like peptide (GALP) is a novel hypothalamic peptide synthesised in neurons in the arcuate nucleus which project to the paraventricular nucleus (PVN). GALP has recently been identified as an orexigenic peptide. In this study we aimed to further characterise the hypothalamic action of this peptide in energy homeostasis. Firstly, we investigated the orexigenic effect of GALP in the PVN and compared its effects with galanin and galanin 2–29. Secondly, we examined the effect of PVN administration of GALP and galanin on circulating thyroid-stimulating hormone (TSH). PVN administration of GALP significantly increased the food intake of satiated rats 1 h after administration at doses of 0.3, 1 and 3 nmol. In comparison with paraventricular administration of galanin, GALP was a more potent orexigen, whereas galanin 2–29, the relatively selective GAL R2 agonist, had no effect on food intake. Both GALP and galanin administration (1 nmol) into the PVN significantly decreased the level of circulating TSH. To investigate the mechanism of these effects, we examined the effect of GALP and galanin application on neuropeptide release from hypothalamic explants in vitro. GALP peptide (100 nM) stimulated the release of the orexigenic peptide neuropeptide Y from hypothalamic explants and decreased the release of the anorectic peptide cocaine-and-amphetamine-regulated transcript, whereas galanin (100 nM) peptide had no significant effect on the release of either peptide. Both GALP (100 nM) and galanin (100 nM) inhibited the release thyrotrophin-releasing hormone. These data suggest that in the PVN, GALP may play a role in energy homeostasis by stimulating food intake and suppressing TSH release.

Peptides that regulate food intake: appetite-inducing accumbens manipulation activates hypothalamic orexin neurons and inhibits POMC neurons.

Corticolimbic circuits involving the prefrontal cortex, amygdala, and ventral striatum determine the reward value of food and might play a role in environmentally induced obesity. Chemical manipulation of the nucleus accumbens shell (AcbSh) has been shown to elicit robust feeding and Fos expression in the hypothalamus and other brain areas of satiated rats. To determine the neurochemical phenotype of hypothalamic neurons receiving input from the AcbSh, we carried out c-Fos/peptide double-labeling immunohistochemistry in various hypothalamic areas known to contain feeding peptides, from rats that exhibited a significant feeding response after AcbSh microinjection of the GABA(A) agonist muscimol. In the perifornical area, a significantly higher percentage of orexin neurons expressed Fos after muscimol compared with saline injection. In contrast, Fos expression was not induced in melanin-concentrating hormone and cocaine-amphetamine-related transcript (CART) neurons. In the arcuate nucleus, Fos activation was significantly lower in neurons coexpressing CART and proopiomelanocortin, and there was a tendency for higher Fos expression in neuropeptide Y neurons. In the paraventricular nucleus, no significant activation of oxytocin and CART neurons was found. Thus AcbSh manipulation may elicit food intake through coordinated stimulation of hypothalamic neurons expressing orexigenic peptides and suppression of neurons expressing anorexigenic peptides. However, activation of many neurons not expressing these peptides suggests that additional peptides/transmitters in the lateral hypothalamus and accumbens projections to other brain areas might also be involved.

Orphanin FQ-induced hyperphagia is mediated by corticosterone and central glucocorticoid receptors

Orphanin FQ (Nociceptin) has been reported to stimulate food intake in satiated rats and to stimulate corticosterone release. A large body of evidence exists to link central feeding systems with the regulation of corticosterone. In this study, we sought to determine whether or not circulating corticosterone is necessary for the induction of food intake by Orphanin FQ. We found that intracerebroventricular injection of Orphanin FQ (0.64–5 nmoles) dose dependently stimulated food intake and plasma corticosterone within 30 min of injection. Removal of corticosterone, by adrenalectomy, abolished the hyperphagic effect of Orphanin FQ. The stimulatory effect of Orphanin FQ on food intake was still negated following a low dose of corticosterone replacement (corresponding to a plasma corticosterone concentration of 1.86±0.99 μg/dl). However, following a larger dose of corticosterone replacement (corresponding to a plasma corticosterone concentration of 8.92±0.55 μg/dl) the feeding effect was fully restored. We concluded this study by testing the glucocorticoid receptor antagonist, RU486 (Mifepristone, 80 μg/2 μl) on Orphanin FQ-induced feeding. Central injection of RU486, 30 min prior to injection of Orphanin FQ, significantly reduced Orphanin FQ-induced food intake in comparison to vehicle-treated controls. Overall, these data demonstrate the necessity for circulating corticosterone in the mediation of Orphanin-FQ-induced feeding and suggest that the mechanism through which the hyperphagic effect is obtained involves activation of central glucocorticoid receptors.

Acute and chronic administration of melanin-concentrating hormone enhances food intake and body weight in Wistar and Sprague-Dawley rats.

Although melanin-concentrating hormone (MCH) is believed to be an important regulator of feeding behavior, both its acute and chronic effects on food intake as well as its interaction with other brain peptides involved in the control of appetite remain unclear. Therefore, the acute effects of MCH on food intake and the chronic effect of MCH on food intake and the gene expression of various hypothalamic peptides involved in the control of appetite were studied in rats. Either the acute or the continuous intraventricular infusion of MCH for 12 days stimulated feeding in both Wistar or Sprague-Dawley rats. Removal of the hypothalamus at the end of the chronic infusion studies allowed measurement of the expression of mRNAs encoding for MCH, neuropeptide Y (NPY), orexin, agouti gene-related peptide, cocaine and amphetamine-related transcript and neurotensin-neuropeptides involved in the control of appetite. Chronic intraventricular infusion of MCH activated only NPY mRNA synthesis in Sprague-Dawley rats. The increase in food intake in response to MCH in Sprague-Dawley rats did not appear to be due to the release of NPY since combination studies demonstrated consistently additive effects of the two peptides on food intake at maximum or near maximum doses. These results strongly suggest that MCH is an orexigenic peptide involved in the control of both short- and long term food intake in satiated rats and further indicate that the MCH pathway is a possible target for the control of food intake and obesity.

Endocannabinoid levels in rat limbic forebrain and hypothalamus in relation to fasting, feeding and satiation: stimulation of eating by 2-arachidonoyl glycerol.

Endocannabinoids are implicated in appetite and body weight regulation. In rodents, anandamide stimulates eating by actions at central CB1 receptors, and hypothalamic endocannabinoids may be under the negative control of leptin. However, changes to brain endocannabinoid levels in direct relation to feeding or changing nutritional status have not been investigated. We measured anandamide and 2-arachidonoyl glycerol (2-AG) levels in feeding-associated brain regions of rats, during fasting, feeding of a palatable food, or after satiation. Endocannabinoid levels were compared to those in rats fed ad libitum, at a point in their daily cycle when motivation to eat was absent. Fasting increased levels of anandamide and 2-AG in the limbic forebrain and, to a lesser extent, of 2-AG in the hypothalamus. By contrast, hypothalamic 2-AG declined as animals ate. No changes were detected in satiated rats. Endocannabinoid levels in the cerebellum, a control region not directly involved in the control of food intake, were unaffected by any manipulation. As 2-AG was most sensitive to variation during feeding, and to leptin regulation in a previous study, we examined the behavioural effects of 2-AG when injected into the nucleus accumbens shell, a limbic forebrain area strongly linked to eating motivation. 2-AG potently, and dose-dependently, stimulated feeding. This effect was attenuated by the CB1 receptor antagonist SR141716. These findings provide the first direct evidence of altered brain levels of endocannabinoids, and of 2-AG in particular, during fasting and feeding. The nature of these effects supports a role for endocannabinoids in the control of appetitive motivation.