Effects of opioid receptor ligands in rats trained to discriminate 22 from 2 hours of food deprivation suggest a lack of opioid involvement in eating for hunger.

It is well accepted that opioids promote feeding for reward. Some studies suggest a potential involvement in hunger-driven intake, but they suffer from the scarcity of methodologies differentiating between factors that intersect eating for pleasure versus energy. Here, we used a unique food deprivation discrimination paradigm to test a hypothesis that, since opioids appear to control feeding reward, injection of opioid agonists would not produce effects akin to 22 h of food deprivation. We trained rats to discriminate between 22 h and 2 h food deprivation in a two-lever, operant discrimination procedure. We tested whether opioid agonists at orexigenic doses produce discriminative stimulus effects similar to 22 h deprivation. We injected DAMGO, DSLET, or orphanin FQ in the paraventricular hypothalamic nucleus (PVN), a site regulating hunger/satiety, and butorphanol subcutaneously (to produce maximum consumption). We assessed the ability of the opioid antagonist, naltrexone, to reduce the discriminative stimulus effects of 22 h deprivation and of the 22 h deprivation-like discriminative stimulus effects of PVN-injected hunger mediator, neuropeptide Y (NPY). In contrast to PVN NPY, centrally or peripherally injected opioid agonists failed to induce discriminative stimuli similar to those of 22 h deprivation. In line with that, naltrexone did not reduce the hunger discriminative stimuli induced by either 22 h deprivation or NPY administration in 2 h food-restricted subjects, even though doses used therein were sufficient to decrease deprivation-induced feeding in a non-operant setting in animals familiar with consequences of 2 h and 22 h deprivation. We conclude that opioids promote feeding for reward rather than in order to replenish lacking energy.

Chronic sugar intake dampens feeding-related activity of neurons synthesizing a satiety mediator, oxytocin.

Increased tone of orexigens mediating reward occurs upon repeated consumption of sweet foods. Interestingly, some of these reward orexigens, such as opioids, diminish activity of neurons synthesizing oxytocin, a nonapeptide that promotes satiety and feeding termination. It is not known, however, whether consumption-related activity of the central oxytocin system is modified under chronic sugar feeding reward itself. Therefore, we examined how chronic consumption of a rewarding high-sucrose (HS) vs. bland cornstarch (CS) diet affected the activity of oxytocin cells in the hypothalamus at the time of meal termination. Schedule-fed (2h/day) rats received either a HS or CS powdered diet for 20 days. On the 21st day, they were given the same or the opposite diet, and food was removed after the main consummatory activity was completed. Animals were perfused 60 min after feeding termination and brains were immunostained for oxytocin and the marker of neuronal activity, c-Fos. The percentage of c-Fos-positive oxytocin cells in the hypothalamic paraventricular nucleus was significantly lower in rats chronically exposed to the HS than to the CS diet, regardless of which diet they received on the final day. A similar pattern was observed in the supraoptic nucleus. We conclude that the chronic rather than acute sucrose intake reduces activity of the anorexigenic oxytocin system. These findings indicate that chronic consumption of sugar blunts activity of pathways that mediate satiety. We speculate that a reduction in central satiety signaling precipitated by regular intake of foods high in sugar may lead to generalized overeating.

Central nociceptin/orphanin FQ system elevates food consumption by both increasing energy intake and reducing aversive responsiveness.

Nociceptin/orphanin FQ (N/OFQ), the nociceptin opioid peptide (NOP) receptor ligand, increases feeding when injected centrally. Initial data suggest that N/OFQ blocks the development of a conditioned taste aversion (CTA). The current project further characterized the involvement of N/OFQ in the regulation of hunger vs. aversive responses in rats by employing behavioral, immunohistochemical, and real-time PCR methodology. We determined that the same low dose of the NOP antagonist [Nphe(1)]N/OFQ(1-13)NH(2) delivered via the lateral ventricle diminishes both N/OFQ- and deprivation-induced feeding. This anorexigenic effect did not stem from aversive consequences, as the antagonist did not cause the development of a CTA. When [Nphe(1)]N/OFQ(1-13)NH(2) was administered with LiCl, it moderately delayed extinction of the LiCl-induced CTA. Injection of LiCl + antagonist compared with LiCl alone generated an increase in c-Fos immunoreactivity in the central nucleus of the amygdala. The antagonist alone elevated Fos immunoreactivity in the paraventricular nucleus of the hypothalamus, nucleus of the solitary tract, and central nucleus of the amygdala. Hypothalamic NOP mRNA levels were decreased during energy intake restriction induced by aversion, as well as in non-CTA rats food-restricted to match CTA-reduced consumption. Brain stem NOP was upregulated only in aversion. Prepro-N/OFQ mRNA showed a trend toward upregulation in restricted rats (P = 0.068). We conclude that the N/OFQ system promotes feeding by affecting the need to replenish lacking calories and by reducing aversive responsiveness. It may belong to mechanisms that shift a balance between the drive to ingest energy and avoidance of potentially tainted food.

Complexity of neural mechanisms underlying overconsumption of sugar in scheduled feeding: involvement of opioids, orexin, oxytocin and NPY.

A regular daily meal regimen, as opposed to ad libitum consumption, enforces eating at a predefined time and within a short timeframe. Hence, it is important to study food intake regulation in animal feeding models that somewhat reflect this pattern. We investigated the effect of scheduled feeding on the intake of a palatable, high-sugar diet in rats and attempted to define central mechanisms - especially those related to opioid signaling--responsible for overeating sweet foods under such conditions. We found that scheduled access to food, even as challenging as 20 min per day, does not prevent overconsumption of a high-sucrose diet compared to a standard one. An opioid receptor antagonist, naloxone, at 0.3-1 mg/kg b. wt., decreased the intake of the sweet diet, whereas higher doses were required to reduce bland food consumption. Real-time PCR analysis revealed that expression of hypothalamic and brainstem genes encoding opioid peptides and receptors did not differ in sucrose versus regular diet-fed rats, which suggests that scheduled intake of sweet food produces only a transient change in the opioid tone. Intake of sugar was also associated with upregulation of orexin and oxytocin genes in the hypothalamus and NPY in the brainstem. We conclude that scheduled consumption of sugar diets is associated with activity of a complex network of neuroregulators involving opioids, orexin, oxytocin and NPY.

Alpha-melanocyte stimulating hormone and ghrelin: central interaction in feeding control.

Alpha-melanocyte stimulating hormone (alpha-MSH) and ghrelin play significant yet opposite roles in the regulation of feeding: alpha-MSH inhibits, whereas ghrelin stimulates consumption. The two peptidergic systems may interact in the process of food intake control. A single report published thus far has shown that a synthetic agonist of the melanocortin receptors, MTII, injected in the hypothalamic paraventricular nucleus (PVN) decreases feeding generated by ghrelin. We found that very low doses of alpha-MSH and MTII administered ICV significantly reduced ghrelin-dependent hyperphagia. However, an endogenous molecule, alpha-MSH, infused in the PVN did not exert an inhibitory effect on ghrelin-induced consumption, whereas the effective dose of PVN MTII exceeded that necessary to decrease short-term deprivation-induced feeding. We conclude that it is likely that in feeding regulation alpha-MSH and ghrelin "interact" at the central nervous system level, but the involvement of the PVN in this interaction appears questionable.

Intraventricular ghrelin activates oxytocin neurons: implications in feeding behavior.

Ghrelin affects behavioral and physiological responses, such as feeding or the activity of the HPA axis. Distribution of its receptor in central sites involved in neuroendocrine control, including the hypothalamic paraventricular nucleus, indicates that interplay with multiple neuropeptidergic systems underlies ghrelin's actions. We report that intracerebroventricular ghrelin increases c-Fos immunoreactivity of oxytocin neurons in magno and parvocellular portions of the paraventricular nucleus. The orexigenic response to ghrelin administered at the dose that activates oxytocin neurons can be further elevated by pretreatment with a selective oxytocin receptor antagonist. Our data suggest that oxytocin may be responsible for the mediation of some effects induced by ghrelin. Modifications in the activity of the oxytocin system may alter some of these effects.

Central ghrelin induces feeding driven by energy needs not by reward.

Centrally administered ghrelin, the endogenous agonist of the growth hormone secretegogue receptor, powerfully stimulates food intake. Although the orexigenic action of this peptide has been well established, it remains unclear whether ghrelin-induced hyperphagia is driven by energy needs or by reward. In our study ghrelin was injected into the lateral cerebral ventricle or the hypothalamic paraventricular nucleus of rats given a choice between a palatable yet calorie-dilute sucrose solution and a calorically dense chow. As a result of intraventricular and hypothalamic paraventricular ghrelin injections, animals increased intake of chow but not sucrose. When the sucrose solution was offered as the only source of calories, rats treated with ghrelin infused in the ventricle and site-specifically increased sucrose consumption. These results suggest that the primary effect of ghrelin is to stimulate food intake to satisfy energy needs.

Paraventricular opioids alter intake of high-fat but not high-sucrose diet depending on diet preference in a binge model of feeding.

Previous work from our laboratory indicates that when rats are given a choice between a high-fat and a high-sucrose diet, opioid blockade with naltrexone (NTX) in a reward-related site (central amygdala) inhibits intake of the preferred diet only, whereas NTX injected into a homeostasis-related site, such as the hypothalamic paraventricular nucleus (PVN), inhibits intake of both diets. However, other work suggests that opioids increase intake of fat specifically. The present study further investigates the role of PVN opioids in food choices made by calorically-replete animals. We used a binge model with chow-maintained rats given 3-h access to a choice of a high-fat or high-sucrose diet 3 days a week. We hypothesized that intra-PVN injection of the mu-opioid agonist, DAMGO (0, 0.025, 0.25, and 2.5 nmol) would enhance, and NTX (0, 10, 30, and 100 nmol) would inhibit intake of both diets to an equal extent. We found that when animals were divided into groups according to sucrose or fat preference, DAMGO increased fat intake in fat-consuming animals, while having no effect on intake of either diet in sucrose-consuming animals. NTX, however, inhibited fat intake in both groups. Intra-PVN NTX did not inhibit intake of sucrose when presented in the absence of a fat choice, but did so when injected peripherally. Furthermore, intra-PVN and systemic NTX inhibited intake of chow by 24-h-food-deprived animals. These results indicate a complex role for PVN opioids in food intake with preference, nutrient type, and energy state affecting the ability of these compounds to change behavior.

Effect of opioid receptor ligands injected into the rostral lateral hypothalamus on c-fos and feeding behavior.

The lateral hypothalamic area (LHa) is an important brain site for the regulation of food intake. Central injection of opioids increases food intake, and the LHa contains mu and kappa opioid receptors, both of which are involved in feeding behavior. It is unclear whether opioids impact feeding when injected directly into the rostral portion of the LHa (rLHa) in rats. We performed a series of studies in which free-feeding rLHa-cannulated rats were injected with opioid agonists (DAMGO, morphine, dynorphin, U-50488H) followed by the measurement of food intake at 1, 2, and 4 h postinjection. To determine whether opioid receptor ligands administered into the rLHa affect neuronal activation in this brain site, we studied cFos immunoreactivity (cFos IR) in response to rLHa stimulation with naltrexone. We found that the only compound that stimulated feeding behavior was morphine. The other agonists had no effect on food consumption. Naltrexone injection into the rLHa increased neural activation in the LHa, indicating the presence of functional opioid receptors in this region. These data suggest that although neuronal activity is affected by opioid agents acting in the rLHa, administration of selective mu and kappa opioid ligands in this subdivision of the LHa does not have a reliable effect on feeding behavior.

Intraparaventricular neuropeptide Y and ghrelin induce learned behaviors that report food deprivation in rats.

Rats were trained to discriminate between 2 and 22-h food deprivation in a choice paradigm. During tests, 20 min of food consumption eliminated internal stimuli associated with 22-h food deprivation. In other tests, rats food-restricted for 2 h were given neuropeptide Y or ghrelin by administration into the paraventricular nucleus of the hypothalamus. Both neurochemicals induced effects similar to those following 22-h food restriction (increased behavior appropriate for 22-h deprivation). These findings suggest that internal stimuli produced by 22-h food deprivation are altered by food consumption and mimicked by feeding-inducing neurochemicals administered into a brain area associated with feeding regulation. Thus, hunger discrimination is a useful model to examine neurochemical and dietary factors that alter internal states associated with eating.

Functional interaction between nociceptin/orphanin FQ and alpha-melanocyte-stimulating hormone in the regulation of feeding.

Nociceptin/orphanin FQ (N/OFQ), an endogenous agonist of the opioid N/OFQ (NOP) receptor, increases food intake when administered centrally. As N/OFQ is part of a larger neural network that governs consummatory behavior, presumably its orexigenic properties stem from interplay with other neuropeptidergic components of the feeding-related circuitry. One such peptide may be the ligand of the melanocortin-3 and -4 receptors, alpha-melanocyte-stimulating hormone (alpha-MSH), which is known to inhibit food intake. The aim of the present study was to establish whether there is a functional "interaction" between N/OFQ and alpha-MSH in the regulation of feeding. By using double immunostaining for c-Fos and alpha-MSH, we found that intracerebroventricular (i.c.v.) injection of N/OFQ at a 10nmol dose that moderately prolongs deprivation-induced food intake in rats, decreases activation of alpha-MSH neurons involved in feeding termination. However, i.c.v. injections of alpha-MSH at doses previously established to reduce deprivation-induced feeding, do not decrease hyperphagia generated by N/OFQ in ad libitum-fed animals. Our results suggest that while alpha-MSH does not appear to modify the orexigenic response to N/OFQ in sated rats, the NOP receptor ligand promotes a decrease in activation of neurons synthesizing the anorexigenic peptide, alpha-MSH, at the time of re-feeding. Thus, to some degree, the stimulatory effect of N/OFQ on consumption may arise from this peptide's inhibitory influence on activity of anorexigenic pathways containing alpha-MSH.

Ghrelin induces feeding in the mesolimbic reward pathway between the ventral tegmental area and the nucleus accumbens.

Ghrelin, a powerful orexigenic peptide released from the gut, stimulates feeding when injected centrally and has thus far been implicated in regulation of metabolic, rather than hedonic, feeding behavior. Although ghrelin's effects are partially mediated at the hypothalamic arcuate nucleus, via activation of neurons that co-express neuropeptide Y and agouti-related protein (NPY/Agrp neurons), the ghrelin receptor is expressed also in other brain sites. One of these is the ventral tegmental area (VTA), a primary node of the mesolimbic reward pathway, which sends dopaminergic projections to the nucleus accumbens (Acb), among other sites. We injected saline or three doses of ghrelin (0, 0.003, 0.03, or 0.3 nmol) into the VTA or Acb of rats. We found a robust feeding response with VTA injection of ghrelin, and a more moderate response with Acb injection. Because opioids modulate feeding in the VTA and Acb, we hypothesized that ghrelin's effects in one site were dependent on opioid signaling in the opposite site. The general opioid antagonist, naltrexone (NTX), injected into the Acb did not affect feeding elicited by ghrelin injection into the VTA, and NTX in the VTA did not affect feeding elicited by ghrelin injected into the Acb. These results suggest interaction of a metabolic factor with the reward system in feeding behavior, indicating that hedonic responses can be modulated by homeostatic factors.

Chronic sucrose ingestion enhances mu-opioid discriminative stimulus effects.

Sucrose affects a variety of opioid-related behaviors. We hypothesized that, if sucrose ingestion alters opioidergic circuitry, opioid-induced discriminative stimulus effects would be enhanced following sucrose intake. In the present study, rats were trained to discriminate nalbuphine (3.2 mg/kg, s.c.) from saline in an operant choice procedure. After acquiring the discrimination, subjects were injected with a single nalbuphine dose (0.1-3.2 mg/kg) and given 30-min access to 30% sucrose or water. Sucrose consumption did not alter nalbuphine's discriminative stimulus effects under these conditions. During subsequent tests, training was suspended, and rats received continuous access to sucrose (9 days) or water (8 days). Chronic sucrose consumption increased the potency of nalbuphine to produce its discriminative stimulus effects by 3-fold. These findings suggest chronic sucrose consumption results in changes in opioid-system function that modulates the effects of exogenously administered opioids.

Injection of neuropeptide W into paraventricular nucleus of hypothalamus increases food intake.

Neuropeptide W (NPW) is an endogenous ligand for G protein-coupled receptor 7 (GPR7). There are two forms of the peptide, designated as neuropeptide W-23 (NPW23) and neuropeptide W-30 (NPW30). In the current study we found that intracerebroventricular administration of NPW23 increased c-Fos immunoreactivity (IR) in a variety of brain sites, many of which are involved in the regulation of feeding. In particular, we noted that c-Fos IR levels were increased in hypocretin-expressing neurons in the perifornical region of the lateral hypothalamus (LH). We then studied whether injection of NPW23 into the paraventricular nucleus of the hypothalamus (PVN) and the LH increased food intake over a 24-h time period. Intra-PVN injection of NPW23 at doses ranging from 0.1 to 3 nmol increased feeding for up to 4 h, and doses ranging from 0.3 to 3 nmol increased feeding for up to 24 h. In contrast, only the 3-nmol dose of NPW23 increased feeding after administration into the LH. Together, these data suggest a modulatory role for NPW in the control of food intake.

Intra-amygdalar injection of DAMGO: effects on c-Fos levels in brain sites associated with feeding behavior.

It is well known that the mu opioid agonist, Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO), increases food intake in rats when injected into a variety of brain sites including the central nucleus of the amygdala (CeA). Immunohistochemical studies measuring c-Fos immunoreactivity (IR) suggest that the CeA contributes to opioid-related feeding. In the current study, we injected 2 nmol of DAMGO and measured food intake, c-Fos IR levels in various brain sites involved in feeding behavior, and mu opioid receptor internalization. We also studied the effect of CeA-injected DAMGO on LiCl-induced increases in c-Fos IR in the amygdala. As was expected, intra-CeA injection of DAMGO increased food intake of rats over a 4-h period. DAMGO injection into the CeA also resulted in mu opioid receptor internalization in the CeA, indicating activation of mu opioid receptor expressing neurons in this site. Administration of DAMGO into the CeA increased c-Fos IR levels in the shell of the nucleus accumbens (NAcc), but not in 17 other brain sites that were studied. We also found that intra-CeA injection of DAMGO, prior to LiCl injection, decreased c-Fos IR levels in the CeA compared to vehicle-injected rats. Thus, intra-CeA administration of DAMGO may increase feeding, in part, by activating neurons in the shell of the nucleus accumbens and by inhibiting activity of selected neurons in the CeA.

Hypothalamic paraventricular injections of ghrelin: effect on feeding and c-Fos immunoreactivity.

The paraventricular hypothalamic nucleus (PVN) appears to integrate orexigenic properties of a novel peptide, ghrelin. Thus, we examined central mechanisms underlying feeding generated by intra-PVN ghrelin. We established that 0.03 nmol of PVN-injected ghrelin was the lowest dose increasing food consumption and it induced c-Fos immunoreactivity (a marker of neuronal activation) in the PVN itself, as well as in other feeding-related brain areas, including the hypothalamic arcuate and dorsomedial nuclei, central nucleus of the amygdala, and nucleus of the solitary tract. We conclude that the PVN, as part of larger central circuitry, mediates orexigenic properties of ghrelin.

Neural basis of orexigenic effects of ghrelin acting within lateral hypothalamus.

Ghrelin stimulates feeding when administered centrally and peripherally. The lateral hypothalamus (LH) is thought to mediate ghrelin-induced hyperphagia. Thus, we examined central mechanisms underlying feeding generated by LH ghrelin. We determined that 0.3nmol of LH-injected ghrelin was the lowest dose increasing food consumption and it induced Fos immunoreactivity (IR; a marker of neuronal activation) in feeding-related brain areas, including the hypothalamic paraventricular, arcuate, and dorsomedial nuclei, amygdala, and nucleus of the solitary tract. Also, LH ghrelin induced Fos IR in LH orexin neurons. We conclude that the LH, as part of larger central circuitry, integrates orexigenic properties of ghrelin.

Effects of intracerebroventricular ethanol on ingestive behavior and induction of c-Fos immunoreactivity in selected brain regions.

The early changes in the central nervous system (CNS) following drinking of ethanol (ETOH) are poorly understood. It is known that chronic intracerebroventricular (ICV) administration of ethanol to rats induces preference for imbibed alcohol solutions. These results suggest that ICV ethanol could alter taste preference. In the present study, we tested whether ETOH[ICV] could induce a conditioned taste preference (CTP) or aversion (CTA) and alter c-Fos immunoreactivity (c-Fos-IR) in brain regions associated with feeding, aversion, and/or reward. Acute ETOH[ICV], as tested in the ETOH-naïve rat, did not induce CTA nor affect the amount of water imbibed by treated rats. The effects of ETOH[ICV] on intake and preference were determined using a novel palatable (i.e. sweet) noncaloric 0.1% saccharin solution. A single dose of ETOH[ICV] in the ETOH-nai;ve animal induced a CTP for saccharin. ETOH[ICV] significantly increased c-Fos-IR in a number of brain sites associated with feeding and reward including the bed nucleus of the stria terminalis, lateral dorsal area (BSTLD); nucleus accumbens, shell area (AcbSh); hypothalamic paraventricular nucleus (PVN); and lateral septum, ventral area (LSV). Thus, ETOH induced a CTP, not CTA, via central mechanisms; it increased c-Fos-IR in specific sites associated with feeding and reward.

Peptides that regulate food intake: effect of peptide histidine isoleucine on consummatory behavior in rats.

Peptide histidine isoleucine (PHI) and VIP are derived from the same precursor. While central VIP decreases food intake, potential effects of PHI on feeding have not been studied. In the current study, we found that PHI administered intracerebroventricularly (ICV) or into the hypothalamic paraventricular nucleus (PVN) or central nucleus of the amygdala (CeA) decreased food consumption in overnight-deprived rats. The magnitude of an anorexigenic response to PHI differed depending on the injection route: ICV-infused peptide evoked the most potent effect. We determined that that only PVN- and CeA-injected PHI did not have aversive consequences. In addition, we infused anorexigenic doses of PHI via the same routes and assessed Fos immunoreactivity of PVN oxytocin (OT) and vasopressin (VP) neurons using double immunohistochemistry. OT and VP are thought to promote feeding termination. PHI increased the percentage of Fos-positive OT neurons regardless of the injection route. PVN- and ICV-infused PHI induced activation of VP cells. We conclude that central PHI has an inhibitory influence on food intake in rats. The PVN, with OT and VP neurons, and CeA may be involved in the mediation of anorexigenic effects of PHI.

Naltrexone infusion inhibits the development of preference for a high-sucrose diet.

We hypothesized that the opioid antagonist naltrexone would inhibit the redevelopment of a preference for a high-sucrose diet after an abstention period from this diet. Rats that chose between a starch or sucrose diet for 10 days preferred the sucrose diet. Rats were then given access to the starch diet alone for another 10-day period. A miniosmotic pump containing saline or naltrexone was then implanted (70 microg/h; 1.7 mg/day) for approximately 10 days. During the saline infusion, 77% of the total energy came from the sucrose diet, whereas during the naltrexone infusion, 33% of the total energy came from the sucrose diet. We repeated this study in another group of rats but did not restrict the sucrose diet. In this case naltrexone failed to decrease preference for the sucrose diet. Thus naltrexone infusion inhibited redevelopment of a preference for a sucrose diet after a period of restriction to a starch diet for 10 days but had no effect on preference if both diets were present throughout the study.