Central neuropeptide Y induces proximal stomach relaxation via Y1 receptors in the dorsal vagal complex of the rat

Oxytocin increased intragastric pressure in the forestomach of rats via the dorsal vagal complex

We previously reported that appetite-enhancing peptides facilitated phasic contractions of the distal stomach and relaxed the forestomach via the dorsal vagal complex (DVC). The present study investigated the effects of anorectic substances on gastric reservoir function. The effects of oxytocin on the motility of the forestomach were examined in rats anesthetized with urethane-chloralose. Gastric motor responses were measured using an intragastric balloon. The fourth ventricular administration of oxytocin (0.1 - 1.0 nmol) increased intragastric pressure (IGP) in the forestomach in a dose-dependent manner. Conversely, the administration of oxytocin (0.3 nmol) suppressed phasic contractions of the distal stomach. These responses were opposite to those of appetite-enhancing peptides in previous studies. The oxytocin response in the forestomach was not observed after bilateral cervical vagotomy. The effects of oxytocin on forestomach motility were examined in animals that underwent ablation of the area postrema (AP) to clarify its involvement. Although the magnitude of the response to the fourth ventricular administration of oxytocin decreased, a significant response was still observed. A microinjection of oxytocin (3 pmol) into the AP, the left medial nucleus of the nucleus tractus solitarius (mNTS), the left commissural part of the NTS, or the left dorsal motor nucleus of the vagus was performed. The oxytocin injection into the AP and/or mNTS induced a rapid and large increase in IGP in the forestomach. Prior injection of L-368,899, an oxytocin receptor antagonist, into both the AP and mNTS attenuated the oxytocin response of the forestomach induced by fourth ventricular administration of oxytocin. These results indicate that oxytocin acts on the AP and/or mNTS to increase IGP in the forestomach via vagal preganglionic neurons.

Ghrelin increases vagally mediated gastric activity by central sites of action

BACKGROUND

Vagally dependent gastric reflexes are mediated through vagal afferent fibers synapsing upon neurons of the nucleus tractus solitarius (NTS) which, in turn modulate the preganglionic parasympathetic dorsal motor nucleus of the vagus (DMV) neurons within the medullary dorsal vagal complex (DVC). The expression and transport of ghrelin receptors has been documented for the afferent vagus nerve, and functional studies have confirmed that vagal pathways are integral to ghrelin-induced stimulation of gastric motility. However, the central actions of ghrelin within the DVC have not been explored fully.

METHODS

We assessed the responses to ghrelin in fasted rats using: (i) in vivo measurements of gastric tone and motility following IVth ventricle application or unilateral microinjection of ghrelin into the DVC and (ii) whole cell recordings from gastric-projecting neurons of the DMV.

KEY RESULTS

(i) IVth ventricle application or unilateral microinjection of ghrelin into the DVC-elicited contractions of the gastric corpus via excitation of a vagal cholinergic efferent pathway and (ii) ghrelin facilitates excitatory, but not inhibitory, presynaptic transmission to DMV neurons.

CONCLUSIONS & INFERENCES

Our data indicate that ghrelin acts centrally by activating excitatory synaptic inputs onto DMV neurons, resulting in increased cholinergic drive by way of vagal motor innervation to the stomach.

Centrally administered ghrelin activates cardiac vagal nerve in anesthetized rabbits

Although central ghrelin has cardioprotective effect through inhibiting sympathetic nerve activity, the effects of central ghrelin on cardiac vagal nerve remain unknown. We investigated the effects of centrally administered ghrelin on cardiac autonomic nerve activities using microdialysis technique. A microdialysis probe was implanted in the right atrial wall adjacent to the sinoatrial node of an anesthetized rabbit and was perfused with Ringer's solution containing a cholinesterase inhibitor, eserine. After injection of ghrelin (1 nmol) into the right lateral cerebral ventricle, norepinephrine (NE) and acetylcholine (ACh) concentrations in the dialysate samples were measured as indices of NE and ACh release from nerve endings to the sinoatrial node using high-performance liquid chromatography. Heart rate was 270±4 bpm at baseline and decreased gradually after ghrelin injection to 234±9 bpm (P<0.01) at 60-80 min, followed by gradual recovery. Dialysate ACh concentration was 5.5±0.8 nM at baseline and increased gradually after ghrelin injection to 8.8±1.2 nM (P<0.01) at 60-80 min; the concentration started to decrease gradually from 100 to 120 min after injection reaching 5.6±0.8 nM at 160-180 min. Central ghrelin did not change mean arterial pressure or dialysate NE concentration. The elevated dialysate ACh concentration declined rapidly after transection of cervical vagal nerves. These results indicate that centrally administered ghrelin activates cardiac vagal nerve.

Modulation of inhibitory neurotransmission in brainstem vagal circuits by NPY and PYY is controlled by cAMP levels

Pancreatic polypeptides such as neuropeptide Y (NPY) and peptide YY (PYY) exert profound, vagally mediated effects on gastrointestinal (GI) motility. Vagal efferent outflow to the GI tract is determined principally by tonic GABAergic synaptic inputs onto dorsal motor nucleus of the vagus (DMV) neurons, yet neither peptide modulates GABAergic transmission. We showed recently that opioid peptides appear similarly ineffective because of the low resting cAMP levels. Using whole cell recordings from identified DMV neurons, we aimed to correlate the influence of brainstem cAMP levels with the ability of pancreatic polypeptides to modulate GABAergic synaptic transmission. Neither NPY, PYY, nor the Y1 or Y2 receptor selective agonists [Leu,Pro]NPY or NPY(3-36) respectively, inhibited evoked inhibitory postsynaptic current (eIPSC) amplitude unless cAMP levels were elevated by forskolin or 8-bromo-cAMP, by exposure to adenylate cyclase-coupled modulators such as cholecystokinin octapeptide (sulfated) (CCK-8s) or thyrotropin releasing hormone (TRH), or by vagal deafferentation. The inhibition of eIPSC amplitude by [Leu,Pro]NPY or NPY(3-36) was stable for approximately 30 min following the initial increase in cAMP levels. Thereafter, the inhibition declined gradually until the agonists were again ineffective after 60 min. Analysis of spontaneous and miniature currents revealed that such inhibitory effects were due to actions at presynaptic Y1 and Y2 receptors. These results suggest that, similar to opioid peptides, the effects of pancreatic polypeptides on GABAergic transmission depend upon the levels of cAMP within gastric inhibitory vagal circuits.

Fourth ventricular administration of ghrelin induces relaxation of the proximal stomach in the rat

The effects of fourth ventricular administration of ghrelin on motility of the proximal stomach were examined in anesthetized rats. Intragastric pressure (IGP) was measured using a balloon situated in the proximal part of the stomach. Administration of ghrelin into the fourth ventricle induced relaxation of the proximal stomach in a dose-dependent manner. Significant reduction of IGP was observed at doses of 3, 10, or 30 pmol. The administration of ghrelin (10 or 30 pmol) with growth hormone secretagogue receptor (GHS-R) antagonist ([d-Lys3] GHRP-6; 1 nmol) into the fourth ventricle did not induce a significant change in IGP. The sole administration of [d-Lys3] GHRP-6 also did not induce a significant change in IGP. Bilateral sectioning of the vagi at the cervical level abolished the relaxation induced by the administration of ghrelin (10 or 30 pmol) into the fourth ventricle, suggesting that relaxation induced by ghrelin is mediated by vagal preganglionic neurons. Microinjections of ghrelin (200 fmol) into the caudal part of the dorsal vagal complex (DVC) induced obvious relaxation of the proximal stomach. Similar injections into the intermediate part of the DVC did not induce significant change. Dose-response analyses revealed that the microinjection of 2 fmol of ghrelin into the caudal DVC significantly reduced IGP. These results revealed that ghrelin induced relaxation in the proximal stomach via GHS-R situated in the caudal DVC.

Central mechanisms of roles of taste in reward and eating

Taste is unique among sensory systems in its innate association with mechanisms of reward and aversion in addition to its recognition of quality, e.g., sucrose is sweet and preferable, and quinine is bitter and aversive. Taste information is sent to the reward system and feeding center via the prefrontal cortices such as the mediodorsal and ventrolateral prefrontal cortices in rodents and the orbitofrontal cortex in primates. The amygdala, which receives taste inputs, also influences reward and feeding. In terms of neuroactive substances, palatability is closely related to benzodiazepine derivatives and beta-endorphin, both of which facilitate consumption of food and fluid. The reward system contains the ventral tegmental area, nucleus accumbens and ventral pallidum and finally sends information to the lateral hypothalamic area, the feeding center. The dopaminergic system originating from the ventral tegmental area mediates the motivation to consume palatable food. The actual ingestive behavior is promoted by the orexigenic neuropeptides from the hypothalamus. Even palatable food can become aversive and avoided as a consequence of a postingestional unpleasant experience such as malaise. The neural mechanisms of this conditioned taste aversion will also be elucidated.

Characterization of brainstem peptide YY (PYY) neurons

Peptide YY (PYY), a member of the NPY superfamily of peptides, is predominantly synthesized by the colon and is thought to act on both the gut and brain to modulate energy homeostasis. Although neurons expressing PYY mRNA have also been reported in the brainstem, little is known about their physiological role and study of their projections has been problematic due to crossreactivity of PYY antibodies with NPY. In the present study we examined the localization of central PYY cell bodies in the mouse, rat, and monkey. In addition, efferent projections and afferent inputs of central PYY neurons were examined in rodents. Central PYY projections were examined by immunohistochemistry in the NPY knockout mouse, or with an NPY-preabsorbed PYY antibody in the rat to avoid any crossreactivity with NPY. In all species investigated PYY-immunoreactive (ir) cell bodies were localized exclusively to the gigantocellular reticular nucleus (Gi) of the rostral medulla. The highest density of PYY fibers was present within the solitary tract nucleus, specifically within the dorsal and lateral aspects. PYY fibers were also concentrated within the dorsal motor nucleus of the vagus and the hypoglossal nucleus. In addition, both orexin and melanin-concentrating hormone fibers made numerous close appositions with PYY cell bodies in the Gi. Collectively, the projection pattern and association with orexigenic neuropeptides suggest that brainstem PYY neurons may play a role in energy homeostasis through a coordinated effect on visceral, motor, and sympathetic output targets.

Effects of hindbrain melanin-concentrating hormone and neuropeptide Y administration on licking for water, saccharin, and sucrose solutions

Melanin-concentrating hormone (MCH) and neuropeptide Y (NPY) are orexigenic peptides found in hypothalamic neurons that project throughout the forebrain and hindbrain. The effects of fourth ventricle (4V) infusions of NPY (5 microg) and MCH (5 microg) on licking for water, 4 mM saccharin, and sucrose (0.1 and 1.0 M) solutions were compared to identify the contributions of each peptide to hindbrain-stimulated feeding. NPY increased mean meal size only for the sucrose solutions, suggesting that caloric feedback or taste quality is pertinent to the orexigenic effect; MCH infusions under identical testing conditions failed to produce increases for any tastant. A second experiment also observed no intake or licking effects after MCH doses up to 15 microg, supporting the conclusion that MCH-induced orexigenic responses require forebrain stimulation. A third experiment compared the 4V NPY results with those obtained after NPY infusions (5 microg) into the third ventricle (3V). In contrast to the effects observed after the 3V NPY injections and previously reported forebrain intracerebroventricular (ICV) NPY infusion studies, 4V NPY failed to increase meal frequency for any taste solution or ingestion rate in the early phases of the sucrose meals. Overall, 4V NPY responses were limited to intrameal behavioral processes, whereas forebrain ICV NPY stimulation elicited both consummatory and appetitive responses. The dissociation between MCH and NPY effects observed for 4V injections is consistent with reports that forebrain ICV injections of MCH and NPY produced nearly dichotomous effects on the pattern of licking microstructure, and, collectively, the results indicate that the two peptides have separate sites of feeding action in the brain.

Neuropeptide y receptor selective ligands in the treatment of obesity

Obesity is a serious public health problem throughout the world, affecting both developed societies and developing countries. The central nervous system has developed a meticulously interconnected circuitry in order to keep us fed and in an adequate nutritional state. One of these consequences is that an energy-dense environment favors the development of obesity. Neuropeptide Y (NPY) is one of the most abundant and widely distributed peptides in the central nervous system of both rodents and humans and has been implicated in a variety of physiological actions. Within the hypothalamus, NPY plays an essential role in the control of food intake and body weight. Centrally administered NPY causes robust increases in food intake and body weight and, with chronic administration, can eventually produce obesity. NPY activates a population of at least six G protein-coupled Y receptors. NPY analogs exhibit varying degrees of affinity and specificity for these Y receptors. There has been renewed speculation that ligands for Y receptors may be of benefit for the treatment of obesity. This review highlights the therapeutic potential of Y(1), Y(2), Y(4), and Y(5) receptor agonists and antagonists as additional intervention to treat human obesity.

Neural substrates for the processing of cognitive and affective aspects of taste in the brain

Taste is unique among the sensory systems in that, besides its recognition of quality, it is innately associated with hedonic aspects of reward and aversion. This review of the literature will show how taste information is conveyed through the central gustatory pathways to the cortical gustatory area and is processed in terms of qualitative and quantitative aspects. Taste information is also sent to the reward system and feeding center via several brain sites including the prefrontal cortex, insular cortex, and amygdala. The reward system contains the ventral tegmental area, nucleus accumbens, and ventral pallidum; it finally sends information to the lateral hypothalamic area, the feeding center. The dopamine system originating from the ventral tegmental area mediates the motivation to consume palatable food. The actual ingestive behavior is promoted by the orexigenic neuropeptides from the hypothalamus. In the last section, the neural substrate of learning and memory of taste is introduced and the biological mechanisms are elucidated.

Involvement of specific orexigenic neuropeptides in sweetener-induced overconsumption in rats

Palatability is one of the factors that regulates food and fluid intake and contributes to overconsumption in turn contributing to obesity. To elucidate the brain mechanisms of the palatability-induced ingestion, we explored the roles of six hypothalamic orexigenic neuropeptides, orexin, melanin-concentrating hormone (MCH), neuropeptide Y (NPY), agouti-related protein (AgRP), ghrelin and dynorphin, in the intake of a palatable solution, saccharin. Of the six peptides, intracerebroventricular (i.c.v.) administrations of orexin, MCH and NPY increased the intake of saccharin. Drinking of saccharin in turn elevated the mRNA levels of orexin and NPY, but not MCH. Pre-treatments of naloxone, an opioid antagonist, blocked the orexigenic effects of orexin and NPY. Specific gastric motor responses induced by central orexin-A and NPY are well known, however, MCH did not induce such responses. The i.c.v. administration of orexin-A facilitated gastric emptying. These results suggest that the overconsumption promoted by sweet and palatable tastes is attributed to the activation of orexigenic neuropeptides, such as orexin and NPY, and a downstream opioid system together with enhanced digestive functions.

Effects of neuropeptide Y on feeding microstructure: Dissociation of appetitive and consummatory actions

The effects of intracerebroventricular application of Neuropeptide Y (NPY) on licking microstructure for sucrose, saccharin, and water solutions were evaluated. In Experiment 1, NPY increased meal size for three sucrose concentrations (0.03 M, 0.3 M, and 1.0 M) by increasing licking burst number but not size and by extending meals more than four-fold in duration with a slow, sustained rate of ingestion in late phases of the meal. Results are consistent with the interpretation that NPY suppressed inhibitory postingestive feedback. Experiment 2 supported this conclusion. NPY significantly increased the number of meals initiated for water, 0.1% saccharin, and 1.0 M sucrose solutions, but meal size was only increased for 1.0 M sucrose. Therefore, NPY also increased appetitive feeding behaviors, but its consummatory effects were limited to caloric solutions. The results are discussed with regard to their potential to explain current discrepancies in the literature.

Effects of melanin-concentrating hormone on licking microstructure and brief-access taste responses

The effects of intracerebroventricular application of melanin-concentrating hormone (MCH) on licking for sucrose, quinine hydrochloride (QHCl), and water solutions were evaluated in two experiments. In experiment 1, rats received 90-min access to sucrose and water solutions after MCH or vehicle microinjection to the third ventricle (3V). MCH increased intake largely through increases in the rate of licking early in the meal and in the mean duration of lick bursts, suggesting an effect on gustatory evaluation. Therefore, in experiment 2, brief access tests were used with a series of sucrose and QHCl concentrations to behaviorally isolate the effects of intracerebroventricular MCH on gustatory evaluation. MCH uniformly increased licking for all sucrose solutions, water, and weak concentrations of QHCl; however, it had no effect on licking for the strongest concentrations of QHCl, which were generally avoided under control conditions. Thus MCH did not produce nonspecific increases in oromotor activity, nor did it change the perceived intensity of the tastants. We conclude that MCH enhanced the gain of responses to normally accepted stimuli at a phase of processing after initial gustatory detection and after the decision to accept or reject the taste stimulus. A comparison of 3V NPY and MCH effects on licking microstructure indicated that these two peptides increased intake via dichotomous behavioral processes; although NPY suppressed measures associated with inhibitory feedback from the gut, MCH appeared instead to enhance measures associated with hedonic taste evaluation.