Central infusion of glucagon-like peptide-1-(7-36) amide (GLP-1) receptor antagonist attenuates lithium chloride-induced c-Fos induction in rat brainstem

Plasma hormone levels and central c-Fos expression in ferrets after systemic administration of cholecystokinin

Posterior pituitary hormone secretion and central neural expression of the immediate-early gene product c-Fos was examined in adult ferrets after intravenous administration of CCK octapeptide. Pharmacological doses of CCK (1, 5, 10, or 50 microg/kg) did not induce emesis, but elicited behavioral signs of nausea and dose-related increases in plasma vasopressin (AVP) levels without significant increases in plasma oxytocin (OT) levels. CCK activated neuronal c-Fos expression in several brain stem viscerosensory regions, including a dose-related activation of neurons in the dorsal vagal complex (DVC). Activated brain stem neurons included catecholaminergic and glucagon-like peptide-1-positive cells in the DVC and ventrolateral medulla. In the forebrain, activated neurons were prevalent in the paraventricular and supraoptic nuclei of the hypothalamus and also were observed in the central nucleus of the amygdala and bed nucleus of the stria terminalis. Activated hypothalamic neurons included cells that were immunoreactive for AVP, OT, and corticotropin-releasing factor. Comparable patterns of brain stem and forebrain c-Fos activation were observed in ferrets after intraperitoneal injection of lithium chloride (LiCl; 86 mg/kg), a classic emetic agent. However, LiCl activated more neurons in the area postrema and fewer neurons in the nucleus of the solitary tract compared with CCK. Together with results from previous studies in rodents, our findings support the view that nauseogenic treatments activate similar central neural circuits in emetic and nonemetic species, despite differences in treatment-induced emesis and pituitary hormone secretion.

Glucagon-like peptide (GLP)-2 action in the murine central nervous system is enhanced by elimination of GLP-1 receptor signaling

Glucagon-like peptide-2 (GLP-2) regulates energy homeostasis via effects on nutrient absorption and maintenance of gut mucosal epithelial integrity. The biological actions of GLP-2 in the central nervous system (CNS) remain poorly understood. We studied the sites of endogenous GLP-2 receptor (GLP-2R) expression, the localization of transgenic LacZ expression under the control of the mouse GLP-2R promoter, and the actions of GLP-2 in the murine CNS. GLP-2R expression was detected in multiple extrahypothalamic regions of the mouse and rat CNS, including cell groups in the cerebellum, medulla, amygdala, hippocampus, dentate gyrus, pons, cerebral cortex, and pituitary. A 1.5-kilobase fragment of the mouse GLP-2R promoter directed LacZ expression to the gastrointestinal tract and CNS regions in the mouse that exhibited endogenous GLP-2R expression, including the cerebellum, amygdala, hippocampus, and dentate gyrus. Intracerebroventricular injection of GLP-2 significantly inhibited food intake during dark-phase feeding in wild-type mice. Disruption of glucagon-like peptide-1 receptor (GLP-1R) signaling with the antagonist exendin-(9-39) in wild-type mice or genetically in GLP-1R(-)/- mice significantly potentiated the anorectic actions of GLP-2. These findings illustrate that CNS GLP-2R expression is not restricted to hypothalamic nuclei and demonstrate that the anorectic effects of GLP-2 are transient and modulated by the presence or absence of GLP-1R signaling in vivo.

Minireview: the glucagon-like peptides

The glucagon-like peptides GLP-1 and GLP-2 are produced in enteroendocrine L cells of the small and large intestine and secreted in a nutrient-dependent manner. GLP-1 regulates nutrient assimilation via inhibition of gastric emptying and food intake. GLP-1 controls blood glucose following nutrient absorption via stimulation of glucose-dependent insulin secretion, insulin biosynthesis, islet proliferation, and neogenesis and inhibition of glucagon secretion. Experiments using GLP-1 antagonists and GLP-1 receptor-/- mice indicate that the glucoregulatory actions of GLP-1 are essential for glucose homeostasis. In the central nervous system, GLP-1 regulates hypothalamic-pituitary function and GLP-1-activated circuits mediate the CNS response to aversive stimulation. GLP-2 maintains the integrity of the intestinal mucosal epithelium via effects on gastric motility and nutrient absorption, crypt cell proliferation and apoptosis, and intestinal permeability. Both GLP-1 and GLP-2 are rapidly inactivated in the circulation as a consequence of amino-terminal cleavage by the enzyme dipeptidyl peptidase IV (DP IV). The actions of these peptides on nutrient absorption and energy homeostasis and the efficacy of GLP-1 and GLP-2 in animal models of diabetes and intestinal diseases, respectively, suggest that analogs of these peptides may be clinically useful for the treatment of human disease.

Antagonism of central glucagon-like peptide-1 receptors enhances lipopolysaccharide-induced fever

Lipopolysaccharide (LPS; a model of systemic bacterial infection) causes fever and activates glucagon-like peptide-1 (GLP-1) neurons in the caudal brainstem. The present study examined whether central GLP-1 receptor signaling plays a functional role in LPS-induced fever. Adult male Sprague-Dawley rats were injected i.p. with LPS (0 or 100 microg/kg), then were infused intracerebroventricularly with GLP-1 receptor antagonist (0 or 10 microg) delivered 2.5 h after injection of LPS or vehicle. Core body temperature was measured at 30-min intervals for 6.5 h after LPS treatment. Consistent with previous reports, body temperature was significantly elevated within 90 min and remained elevated for the remainder of the monitoring period. The pyrogenic effect of LPS was amplified in rats that received central infusion of GLP-1 receptor antagonist, although the antagonist by itself did not alter body temperature. These findings suggest that endogenous GLP-1 acts at central receptors to limit the fever response in rats after i.p. administration of LPS.

The human nucleus of the solitary tract: visceral pathways revealed with an "in vitro" postmortem tracing method

Visceral relay neurons in the nucleus of the solitary tract (NTS) regulate behavior and autonomic reflex functions. NTS projections have been extensively characterized in animal studies but not in humans. For the first time, NTS fiber trajectories in the human medulla oblongata were revealed with an "in vitro" postmortem tracing method. Local intramedullary pathways were labeled by direct pressure injections of free horseradish peroxidase centered on the medial subnucleus at a level adjacent to true obex. Labeled elements were resolved by peroxidase histochemistry as a dark brown intracellular reaction product. A prominent transtegmental system of axons emerged from the NTS injection sites and entered the intermediate reticular zone, a region corresponding to an autonomic reflex center in other mammals. A medial system of axons arched across the dorsomedial reticular formation toward the dorsal medullary raphe and projected ventrally toward the nucleus gigantocellularis. A small lateral fiber trajectory coursed towards the dorsomedial zone of spinal trigeminal nucleus caudalis. Presumptive terminals appeared as dustings of fine punctate processes within the NTS, dorsomotor nucleus and reticular formation. NTS projections in humans resemble those identified in other mammals including primates. Axonal tracing studies predict that visceral impulses in humans may transmit over evolutionarily conserved pathways involved in autonomic feedback control and stress adaptation.

The role of CNS glucagon-like peptide-1 (7-36) amide receptors in mediating the visceral illness effects of lithium chloride

Peripheral administration of large doses of lithium chloride (LiCl) to rats causes a spectrum of effects that are consistent with visceral illness. LiCl reduces food intake, decreases salt ingestion after sodium depletion, induces pica, and produces robust conditioned taste aversions. Because some of the effects of peripheral LiCl are mimicked by centrally administered glucagon-like peptide-1 (7-36) amide (GLP-1), we hypothesized that this peptide is involved in the neural pathways by which LiCl causes visceral illness. To test this hypothesis, we pretreated rats with a selective and potent GLP-1 receptor antagonist given directly into the third ventricle via an indwelling cannula before administration of peripheral LiCl. The GLP-1 receptor antagonist completely blocked the effect of LiCl to reduce food intake, induce pica, and produce a conditioned taste aversion. The same dose of GLP-1 receptor antagonist did not reverse the LiCl-induced reduction in NaCl intake. The data indicate a role for GLP-1 receptors in the CNS pathway that mediates some of the effects of visceral illness.

Neuroendocrine function and response to stress in mice with complete disruption of glucagon-like peptide-1 receptor signaling

Glucagon-like peptide-1 (GLP-1), a potent regulator of glucose homeostasis, is also produced in the central nervous system, where GLP-1 has been implicated in the neuroendocrine control of hypothalamic-pituitary function, food intake, and the response to stress. The finding that intracerebroventricular GLP-1 stimulates LH, TSH, corticosterone, and vasopressin secretion in rats prompted us to assess the neuroendocrine consequences of disrupting GLP-1 signaling in mice in vivo. Male GLP-1 receptor knockout (GLP-1R-/-) mice exhibit reduced gonadal weights, and females exhibit a slight delay in the onset of puberty; however, male and female GLP-1R-/- animals reproduce successfully and respond appropriately to fluid restriction. Although adrenal weights are reduced in GLP-1R-/- mice, hypothalamic CRH gene expression and circulating levels of corticosterone, thyroid hormone, testosterone, estradiol, and progesterone are normal in the absence of GLP-1R-/- signaling. Intriguingly, GLP-1R-/- mice exhibit paradoxically increased corticosterone responses to stress as well as abnormal responses to acoustic startle that are corrected by glucocorticoid treatment. These findings suggest that although GLP-1R signaling is not essential for development and basal function of the murine hypothalamic-pituitary-adrenal axis, abrogation of GLP-1 signaling is associated with impairment of the behavioral and neuroendocrine responses to stress.

A functional role for central glucagon-like peptide-1 receptors in lithium chloride-induced anorexia

The present study sought to determine whether central glucagon-like peptide-1 (GLP-1)-receptor signalling contributes to the anorexigenic effects of systemically administered lithium chloride (LiCl). Male Sprague-Dawley rats with chronic intracerebroventricular (ICV) cannulas were acclimated to a feeding schedule that included daily 30-min access to palatable mash. In the first experiment, ICV infusion of a GLP-1-receptor antagonist [exendin-4-(3-39)] significantly attenuated (10 microgram dose) or completely blocked (20 microgram dose) the inhibition of food intake produced by subsequent ICV infusion of GLP-1-(7-36) amide (5 microgram). In the second experiment, rats were infused with 0, 10, or 20 microgram of the GLP-1-receptor antagonist ICV, followed by injection of 0.15 M LiCl (50 mg/kg ip) or the same volume of 0.15 M NaCl. The ability of LiCl treatment to suppress food intake was significantly attenuated in rats that were pretreated with the GLP-1-receptor antagonist. These results support the view that central mechanisms underlying LiCl-induced anorexia include a prominent role for endogenous GLP-1 neural pathways.

Glucagon-like peptide-1 (7-36) amide: a central regulator of satiety and interoceptive stress

Glucagon-like peptide-1 (7-36) amide (GLP-1) is processed from proglucagon in the distal ileum as well as in the CNS. In the periphery, GLP-1 acts as an incretin factor and profoundly inhibits upper gastrointestinal motility ('ileal brake'), the latter presumably involving the CNS. Within the CNS, GLP-1 has a satiating effect, since administration of GLP-1 into the third cerebral ventricle reduces short-term food intake (and meal size), while administration of GLP-1 antagonists have the opposite effect. In addition, activation of GLP-1 receptors in certain brain regions elicits strong taste aversions. Similarities between toxin- and GLP-1-induced neuronal activity in the CNS (brain stem) suggest a role for central GLP-1 receptors in relaying interoceptive stress. Thus, regionally distinct GLP-1 receptor populations in the CNS may be involved in satiety or malaise. It is argued that the satiating and aversive aspects of GLP-1 serve homeostatic and nonhomeostatic functions with respect to maintenance of nutrient balance.

Interoceptive stress activates glucagon-like peptide-1 neurons that project to the hypothalamus

This study tested the hypothesis that systemic stressors in rats activate glucagon-like peptide-1 (GLP-1)-immunoreactive neurons in the caudal brain stem, including those that project to the paraventricular nucleus of the hypothalamus (PVN). Neural tracer was microinjected into the PVN to retrogradely label brain stem neurons. Seven to ten days later, rats were injected with lithium chloride (LiCl; 50 mg/kg). Additional non-tracer-injected rats were treated with lipopolysaccharide (LPS; 100 microgram/kg) or CCK (100 microgram/kg) or were allowed to consume a very large meal. Rats were killed 90-120 min after drug treatment or 30 min after the meal. Brains were processed for immunocytochemical localization of c-Fos (a marker of neuronal activation), GLP-1, and, when appropriate, neural tracer. The majority of GLP-1 neurons were activated to express c-Fos after LiCl, LPS, or CCK treatment, including (in LiCl-treated rats) those projecting to the PVN. In contrast, GLP-1 neurons rarely expressed c-Fos after ingestion of a large meal, despite prominent activation of other brain stem neurons. These results suggest that GLP-1 neurons are uniquely activated in situations of interoceptive stress, and may participate in adaptive hypothalamic stress responses.