A new twist in the brain-gut axis

Whey protein hydrolysates improve high-fat-diet-induced obesity by modulating the brain-peripheral axis of GLP-1 through inhibition of DPP-4 function in mice

PURPOSE

Obesity is a growing global health concern. Recent literature indicates a prominent role of glucagon-like peptide-1 (GLP-1) in glucose metabolism and food intake. The synergistic action of GLP-1 in the gut and brain is responsible for its satiety-inducing effect, suggesting that upregulation of active GLP-1 levels could be an alternative strategy to combat obesity. Dipeptidyl peptidase-4 (DPP-4) is an exopeptidase known to inactivate GLP-1, suggesting that its inhibition could be a crucial strategy for effectively extending the half-life of endogenous GLP-1. Peptides derived from partial hydrolysis of dietary proteins are gaining traction due to their inhibitory activity on DPP-4.

METHODS

Whey protein hydrolysate from bovine milk (bmWPH) was produced using simulated in situ digestion, purified using RP-HPLC, and characterized for DPP-4 inhibition. The antiadipogenic and antiobesity activity of bmWPH was then studied in 3T3-L1 preadipocytes and high-fat diet-induced obesity (HFD) mice model, respectively.

RESULTS

The dose-dependent inhibitory effect of bmWPH on the catalytic activity of DPP-4 was observed. Additionally, bmWPH suppressed adipogenic transcription factors and DPP-4 protein levels, leading to a negative effect on preadipocyte differentiation. In an HFD mice model, co-administration of WPH for 20 weeks downregulated adipogenic transcription factors, resulting in a concomitant reduction in whole body weight and adipose tissues. Mice fed with bmWPH also showed a marked reduction in DPP-4 levels in WAT, liver, and serum. Furthermore, HFD mice fed with bmWPH exhibited increased serum and brain GLP levels, which led to a significant decrease in food intake.

CONCLUSION

In conclusion, bmWPH reduces body weight in HFD mice by suppressing appetite through GLP-1, a satiety-inducing hormone, in both the brain and peripheral circulation. This effect is achieved through modulation of both the catalytic and non-catalytic activity of DPP-4.

Cholecystokinin-8s excites identified rat pancreatic-projecting vagal motoneurons

It is known that cholecystokinin (CCK) acts in a paracrine fashion to increase pancreatic exocrine secretion via vagal circuits. Recent evidence, however, suggests that CCK-8s actions are not restricted to afferent vagal fibers, but also affect brain stem structures directly. Within the brain stem, preganglionic neurons of the dorsal motor nucleus of the vagus (DMV) send efferent fibers to subdiaphragmatic viscera, including the pancreas. Our aims were to investigate whether DMV neurons responded to exogenously applied CCK-8s and, if so, the mechanism of action. Using whole cell patch-clamp recordings we show that perfusion with CCK-8s induced a concentration-dependent excitation in approximately 60% of identified pancreas-projecting DMV neurons. The depolarization was significantly reduced by tetrodotoxin, suggesting both direct (on the DMV membrane) and indirect (on local synaptic circuits) effects. Indeed, CCK-8s increased the frequency of miniature excitatory currents onto DMV neurons. The CCK-A antagonist, lorglumide, prevented the CCK-8s-mediated excitation whereas the CCK-B preferring agonist, CCK-nonsulfated, had no effect, suggesting the involvement of CCK-A receptors only. In voltage clamp, the CCK-8s-induced inward current reversed at -106 +/- 3 mV and the input resistance increased by 150 +/- 15%, suggesting an effect mediated by the closure of a potassium conductance. Indeed, CCK-8s reduced both the amplitude and the time constant of decay of a calcium-dependent potassium conductance. When tested with pancreatic polypeptide (which reduces pancreatic exocrine secretion), cells that responded to CCK-8s with an excitation were, instead, inhibited by pancreatic polypeptide. These data indicate that CCK-8s may control pancreas-exocrine secretion also via an effect on pancreas-projecting DMV neurons.

Relief of acute pain in chronic idiopathic gastroparesis with intravenous phentolamine

OBJECTIVE

To report a case in which complete relief of pain associated with gastroparesis, with promotion of gastric emptying, was achieved with administration of phentolamine.

CASE SUMMARY

A 37-year-old opiate-tolerant female with a history of recurrent abdominal pain, gastroparesis, cyclic vomiting syndrome, and migraine headaches was admitted to the emergency department (ED) with severe acute abdominal pain, nausea, and vomiting. The patient had been previously implanted with a permanent gastric electrical stimulator and she was adherent to her prokinetic, antiemetic, analgesic, and migraine prophylactic medications. Pain relief was achieved rapidly and completely in the ED with sympatholysis by administration of intravenous phentolamine 0.5 mg/kg over 60 minutes, with continuous cardiac monitoring. At a 2 month follow-up visit, the patient reported chronic pain relief, and a decrease in opiate doses was maintained by oral administration of clonidine 0.1 mg twice daily.

DISCUSSION

Gastroparesis represents a difficult treatment challenge because management of gastric dysmotility and the accompanying severe abdominal pain is often mutually exacerbating and ineffective. Sympatholysis by intravenous phentolamine provided profound and immediate relief of acute gastroparesis-related abdominal pain in our patient. The mechanism of phentolamine is believed to be receptor blockade at alpha-adrenergic receptors and, therefore, inhibition of the peripheral sensitizing effects of circulating norepinephrine. Although action at a peripheral nerve level is presumed, modulation of alpha-adrenoreceptors receptors is also possible at the dorsal root ganglion or at other central nervous system sites.

CONCLUSIONS

The dramatic relief of acute pain in gastroparesis by phentolamine observed in this case would warrant investigation of a larger, controlled case series. Patients who respond to intravenous sympatholysis may likewise be candidates for longer term sympathetic modulation with oral sympatholytics.

Gut peptides and the regulation of appetite

There is a growing worldwide epidemic of obesity. Obese people have a higher incidence of type 2 diabetes and cardiovascular disease, and hence present increasing social, financial and health burdens. Weight loss is always difficult to achieve through lifestyle changes alone, and currently licensed anti-obesity drug treatments, such as orlistat and sibutramine, if tolerated, only achieve modest weight loss. Therefore, there is a need to identify more potent pharmacological targets. In the last 10 years, discoveries of new hormones such as leptin and ghrelin, together with greater understanding of previously described hormones such as cholecystokinin (CCK), pancreatic polypeptide (PP), peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), have led to a rapid increase in our knowledge of the regulation of energy balance. Among the most important factors, controlling appetite and satiety are peptide hormones released from the gut. In this paper, we provide a full up-to-date overview of the current state of knowledge of this field, together with the potential of these peptides as drugs, or as other therapeutic targets, in the treatment of obesity. Finally, we propose an integrated model to describe the complex interplay of these hormones in the broader physiology of energy balance.

Glucagon-related peptide 1 (GLP-1): hormone and neurotransmitter

The interest in glucagon-like petide-1 (GLP-1) and other pre-proglucagon derived peptides has risen almost exponentially since seminal papers in the early 1990s proposed to use GLP-1 agonists as therapeutic agents for treatment of type 2 diabetes. A wealth of interesting studies covering both normal and pathophysiological role of GLP-1 have been published over the last two decades and our understanding of GLP-1 action has widened considerably. In the present review, we have tried to cover our current understanding of GLP-1 actions both as a peripheral hormone and as a central neurotransmitter. From an initial focus on glycaemic control, GLP-1 research has been diverted to study its role in energy homeostasis, neurodegeneration, cognitive functions, anxiety and many more functions. With the upcoming introduction of GLP-1 agonists on the pharmaceutical venue, we have witnessed an outstanding example of how initial ideas from basic science laboratories have paved their way to become a novel therapeutic strategy to fight diabetes.

Effects of pancreatic polypeptide on pancreas-projecting rat dorsal motor nucleus of the vagus neurons

We investigated the pre- and postsynaptic effects of pancreatic polypeptide (PP) on identified pancreas-projecting neurons of the rat dorsal motor nucleus of the vagus in thin brain stem slices. Perfusion with PP induced a TTX- and apamin-sensitive, concentration-dependent outward (22% of neurons) or inward current (21% of neurons) that was accompanied by a decrease in input resistance; PP was also found to affect the amplitude of the action potential afterhyperpolarization. The remaining 57% of neurons were unaffected. PP induced a concentration-dependent inhibition in amplitude of excitatory (n = 22 of 30 neurons) and inhibitory (n = 13 of 17 neurons) postsynaptic currents evoked by electrical stimulation of the adjacent nucleus of the solitary tract, with an estimated EC(50) of 30 nM for both. The inhibition was accompanied by an alteration in the paired pulse ratio, suggesting a presynaptic site of action. PP also decreased the frequency, but not amplitude, of spontaneous excitatory (n = 6 of 11 neurons) and inhibitory currents (n = 7 of 9 neurons). In five neurons, chemical stimulation of the area postrema (AP) induced a TTX-sensitive inward (n = 3) or biphasic (outward and inward) current (n = 2). Superfusion with PP reversibly reduced the amplitude of these chemically stimulated currents. Regardless of the PP-induced effect, the vast majority of responsive neurons had a multipolar somata morphology with dendrites projecting to areas other than the fourth ventricle or the central canal. These results suggest that pancreas-projecting rat dorsal motor nucleus of the vagus neurons are heterogeneous with respect to their response to PP, which may underlie functional differences in the vagal modulation of pancreatic functions.

The neurobiology of zinc in health and disease

The use of zinc in medicinal skin cream was mentioned in Egyptian papyri from 2000 BC (for example, the Smith Papyrus), and zinc has apparently been used fairly steadily throughout Roman and modern times (for example, as the American lotion named for its zinc ore, 'Calamine'). It is, therefore, somewhat ironic that zinc is a relatively late addition to the pantheon of signal ions in biology and medicine. However, the number of biological functions, health implications and pharmacological targets that are emerging for zinc indicate that it might turn out to be 'the calcium of the twenty-first century'.

Alcoholic pancreatitis

Without doubt, alcohol consumption is one of the most important considerations in adults with acute or chronic pancreatitis. Understanding chronic pancreatitis as a complex disorder in which complimentary factors are required for recurrent acute and late chronic pancreatitis to develop in subsets of patients is critical for the early diagnosis and management of these individuals. Recent pathophysiological and genetic findings represent the beginning of major diagnostic and treatment breakthroughs that are likely to continue for the foreseeable future. The information provided in this article should provide the physician with a fresh perspective and remind the clinician of the importance of an accurate and complete history, and the need to document the actual alcohol consumption, pattern of drinking, and raise appropriate concerns if signs of alcoholism are detected. If alcohol-associated pancreatitis is detected, then limitation of pancreatic damage, limitation of progression, or preventative intervention should become the major concern.

Biochemical aspects in autism spectrum disorders: updating the opioid-excess theory and presenting new opportunities for biomedical intervention

Autism is a lifelong condition usually described as affecting social, cognitive and imaginative abilities. For many years, parents and some professionals have observed that in concordance with the behavioural and psychological symptoms of the condition, there are a number of physiological and biochemical correlates which may also be of relevance to the syndrome. One area of interest that encompasses many of these observations is the opioid-excess theory of autism. The main premise of this theory is that autism is the result of a metabolic disorder. Peptides with opioid activity derived from dietary sources, in particular foods that contain gluten and casein, pass through an abnormally permeable intestinal membrane and enter the central nervous system (CNS) to exert an effect on neurotransmission, as well as producing other physiologically-based symptoms. Numerous parents and professionals worldwide have found that removal of these exogenously derived compounds through exclusion diets can produce some amelioration in autistic and related behaviours. There is a surprisingly long history of research accompanying these ideas. The aim of this paper is to review the accompanying evidence in support of this theory and present new directions of intervention as a result of it.

Roles of pancreatic polypeptide in regulation of food intake

Pancreatic polypeptide (PP) is produced in pancreatic islets of Langerhans and released into the circulation after ingestion of a meal. Peripherally administered PP suppresses food intake and gastric emptying. On the other hand, central administration of PP elicits food intake and gastric emptying. Therefore, PP actions on food intake may be, in part, attributable to gastric emptying. PP transgenic mice exhibit decreases in both food intake and gastric emptying rate that were clearly reversed by anti-PP antiserum. PP is an anorexigenic signal in the periphery and an orexigenic signal in the central nervous system.

Orexins in the brain-gut axis

Orexins (hypocretins) are a novel pair of neuropeptides implicated in the regulation of energy balances and arousal. Previous reports have indicated that orexins are produced only in the lateral hypothalamic area, although orexin-containing nerve fibers were observed throughout the neuroaxis. Recent evidence shows that orexins and functional orexin receptors are found in the periphery. Vagal and spinal primary afferent neurons, enteric neurons, and endocrine cells in both the gut and pancreas display orexin- and orexin receptor-like immunoreactivity. Orexins excite secretomotor neurons in the guinea pig gut and modulate gastric and intestinal motility and secretion. In addition, orexins modulate hormone release from pancreatic endocrine cells. Moreover, fasting up-regulates the phosphorylated form of cAMP response element binding protein in orexin-immunoreactive enteric neurons, indicating a functional response to food status in these cells. The purpose of this article is to summarize evidence for the existence of a brain-gut network of orexin-containing cells that appears to play a role in the acute regulation of energy homeostasis.

Pancreatic polypeptide receptors: affinity, sodium sensitivity and stability of agonist binding

Cloned rat, human and guinea-pig Y4 pancreatic polypeptide (PP) receptors expressed in Chinese hamster ovary (CHO) cells, as well as the rabbit Y4-like PP receptor, show a selective sensitivity to Na+ over K+ ion in PP attachment, but little sensitivity to Na+ in dissociation of bound PP peptides. Agonist binding to Y4 receptors of intact CHO cells also shows much greater sensitivity to Na+ over K+, and a tenacious attachment of the bound agonist. Binding sensitivity to K+ is greatly enhanced upon receptor solubilization. Pancreatic polypeptide sites also show large sensitivity to modulators of Na+ transport such as N5-substituted amilorides and to RFamides, as different from Y1 or Y2 receptors. Thus, PP binding is modulated by cation-induced changes in site environment (with selectivity for Na+) and ultimately results in a blocking attachment. This would support receptor operation in the presence of ion gradients, as well as prolonged agonist-delimited signaling activity (which can include partial antagonism). Also, this could point to an evolutionary adaptation enabling small numbers of PP receptors to perform extensive metabolic tasks in response to low agonist signals.

The area postrema lesions alter the inhibitory effects of peripherally infused pancreatic polypeptide on pancreatic secretion

Circulating PP binds to specific receptors in the DVC through the AP, but the mechanism through which these brain receptors affect pancreatic secretion is not clear. We hypothesize that the removal of the AP (APX) will alter the effects of PP on pancreatic secretion. APX or sham procedures were performed in anesthetized male Wistar rats. After a 1-month recovery, one group of rats were infused with either PP (30 or 100 pmol/kg per h) or vehicle under basal or 2-DG-stimulated (75 mg/kg, i.v. bolus) conditions for studying pancreatic exocrine secretion. A second parallel group was sacrificed for examination of PP receptor binding in the brain stem. A third group received an intraperitoneal injection of PP at the dose of 4.15x10(4) pmol/kg (200 microg/kg) and c-fos expression in the brain stem was examined. APX eliminated PP binding sites in the DVC as assessed by autoradiography. PP infusion caused a dose-dependent decrease in basal protein secretion. APX partially reversed PP inhibition of basal protein secretion when infused at 30 pmol/kg per h, and at 100 pmol/kg per h stimulated pancreatic fluid secretion and reversed the inhibition of protein secretion. During 2-DG stimulation the effects of PP and 2-DG on pancreatic fluid and protein secretion were parallel. PP dose-dependently inhibited 2-DG-stimulated secretion in sham rats. APX reduced the pancreatic fluid (54%) and protein (46%) secretory response to 2-DG. However, PP at 30 pmol/kg per h remained a potent inhibitor of 2-DG-stimulated pancreatic secretion in APX rats. This effect was blunted with PP at 100 pmol/kg per h in APX rats, possibly related to the stimulatory effect of high-dose PP in APX rats without 2-DG. Furthermore, i.p. PP induced significantly greater c-fos activation of NTS neurons in APX rats than sham rats, despite the apparent absence of PP binding sites in the DVC. We conclude that in awake rats, PP inhibits basal secretion, in part, through the AP. Furthermore, and unlike PYY, PP inhibits 2-DG-stimulated pancreatic secretion, and it does so through an AP-independent mechanism. The possibility that the mechanism may involve the DVC cannot be excluded since i.p. injection of PP activates c-fos expression in DVC neurons. Thus, PP and PYY may regulate different components of the pancreatic secretory control system through unique pathways.

Neural contribution to the effect of glucagon-like peptide-1-(7-36) amide on arterial blood pressure in rats

This study was designed to determine the contribution of the central nervous system (CNS) to the effects of glucagon-like peptide-1-(7-36) amide (tGLP-1) on arterial blood pressure and heart rate in rats. Accordingly, intracerebroventricular administration of the peptide produced an increase in cardiovascular parameters, which was blocked by previous administration of exendin-(9-39) through the same route, but not when it was intravenously injected. Intravenous administration of tGLP-1 produced a significant increase in arterial blood pressure and heart rate, which was blocked by the previous intracerebroventricular or intravenous administration of exendin-(9-39). Bilateral vagotomy blocked the stimulating effect of intracerebroventricular tGLP-1 administration on arterial blood pressure and heart rate. Also, bilateral vagotomy prevented the blocking effect of intracerebroventricular but not of intravenous exendin-(9-39) on cardiovascular parameters after intravenous administration of tGLP-1. These findings suggest that the action of tGLP-1 on cardiovascular parameters is under a dual control generated in the CNS and in peripheral structures and that the neural information emerging in the brain is transmitted to the periphery through the vagus nerve.

The neuropeptide Y (Y4) receptor is highly expressed in neurones of the rat dorsal vagal complex

Recently, the cDNA encoding the Y4 neuropeptide Y (NPY) receptor cDNA was cloned from a rat genomic library. The Y4 receptor is characterized by having a high affinity for pancreatic polypeptide (PP) and peptide YY (PYY). By using in situ hybridization histochemistry with 35S-labelled riboprobes, we have visualized the cellular expression of mRNA encoding the Y4 receptor protein in the rat dorsal vagal complex at the light microscopical level. High densities of silver grains were observed over neurones of the dorsal vagal motor nucleus, and over neurones of a subregion of the nucleus of the solitary tract known as the subnucleus gelatinosus. Furthermore, cells within the ventral margin of the area postrema expressed high levels of Y4 mRNA. These observations indicate that circulating PP and/or NPY/PYY via the blood-brain barrier-free area postrema and subpostremal area could influence neurones of the dorsal vagal complex with profound influence on numerous homeostatic mechanisms governed by this nuclear complex.

Distribution of pancreatic polypeptide receptors in the rat brain

Pancreatic polypeptide (PP) is a regulatory peptide that modulates gastrointestinal function. Previously we demonstrated PP receptors in the brainstem and interpeduncular nucleus, and the PP receptors in the brainstem appear to modulate gastric motility and pancreatic exocrine secretion. The purpose of this study is to extend our understanding of the distribution of PP receptors in the rat brain in order to determine the systems that are potentially modulated by PP. Rat brains were studied using 125I-PP receptor autoradiography on cryostat sections of the entire brain cut in three planes (horizontal, sagittal, and coronal). Brain regions exhibiting PP binding sites were confirmed when identified in all three planes of section. Saturable PP binding was identified in the hypothalamus (arcuate and paraventricular n), the rostral forebrain (medial preoptic area, anterior olfactory nucleus, islands of Calleja, the dorsal endopiriform n, piriform cortex, and the bed n of the stria terminalis), medial amygdaloid n; the thalamus (anteromedial thal. n; reuniens thal. n; and paraventricular thal n), the interpeduncular red nucleus, substantia nigra, parabrachial n; locus coeruleus, mesencephalic trigeminal n, dorsal motor n of the vagus, the n solitary tract, and the area postrema. We conclude that PP receptors are distributed widely throughout the rat brain. The distribution of many of these PP binding sites corresponds to brain regions regulating digestion and autonomic function. We speculate, based on the patterns of binding in the olfactory and limbic systems, that PP receptors might be involved in positive reinforcement of ingestion behavioral as well as modulation of gastrointestinal function.

Passage of peptides across the blood-brain barrier: pathophysiological perspectives

Blood-borne peptides are capable of affecting the central nervous system (CNS) despite being separated from the CNS by the blood-brain barrier (BBB), a monolayer comprised of brain endothelial and ependymal cells. Blood-borne peptides can directly affect the CNS after they cross the BBB by nonsaturable and saturable transport mechanisms. The ability of peptides to cross the BBB to a meaningful degree suggests that the BBB may act as a modulatory pathway in the exchange of informational molecules between the brain and the peripheral circulation. The permeability of the BBB to peptides is a regulatory process affected by developmental, physiological, and pathological events. This regulation sets the stage for the relation between peptides and the BBB to be involved in pathophysiological events. For example, some of the classic actions of melanocortins on the CNS are explained by their abilities to cross the BBB, whereas aspects of feeding and alcohol-related behaviors are associated with the passage of other specific peptides across the BBB. The BBB should no longer be considered a static barrier but should be recognized as a regulatory interface controlling the exchange of informational molecules, such as peptides, between the blood and CNS.

EM523L, a nonpeptide motilin agonist, stimulates gastric emptying and pancreatic polypeptide secretion

We investigated the efficacy and the mechanism of action of EM523L, a nonpeptide motilin agonist (motilide), on the stimulation of gastric emptying and on the release of gut peptides after ingestion of a solid meat in normal controls (n = 8) and in diabetic patients (n = 8) with signs of neuropathy. A dose of 2 mg EM523L was administered IV over 15 min just after ingestion of a solid meal (200 kcal Gastric emptying was measured by a radionuclide technique. EM523L accelerated gastric emptying and markedly augmented postprandial pancreatic polypeptide (PP) response in both normal control and diabetic patients. This may suggest the mediation of the Vagal-cholinergic pathway to accelerate gastric emptying. The present study offers a promising therapeutic potential of the motilide in gastrointestinal motility disorders like those observed in diabetics mellitus.

Distribution of GLP-1 binding sites in the rat brain: evidence that exendin-4 is a ligand of brain GLP-1 binding sites

The distribution and biochemical properties of glucagon-like peptide (GLP)-1(7-36) amide (GLP-1) binding sites in the rat brain were investigated. By receptor autoradiography of tissue sections, the highest densities of [125I]GLP-1 binding sites were identified in the lateral septum, the subfornical organ (SFO), the thalamus, the hypothalamus, the interpenduncular nucleus, the posterodorsal tegmental nucleus, the area postrema (AP), the inferior olive and the nucleus of the solitary tract (NTS). Binding studies with [125I][Tyr39] exendin-4, a GLP-1 receptor agonist, showed an identical distribution pattern of binding sites. Binding specificity and affinity was investigated using sections of the brainstem containing the NTS. Binding of [125I]GLP-1 to the NTS was inhibited concentration-dependently by unlabelled GLP-1 and [Tyr39]exendin-4 with KI values of 3.5 and 9.4 nM respectively. Cross-linking of hypothalamic membranes with [125I]GLP-1 or [125I][Tyr39]exendin-4 identified a single ligand-binding protein complex with a molecular mass of 63,000 Da. The fact that no GLP-1 binding sites were detected in the cortex but that they were detected in the phylogenetically oldest parts of the brain emphasizes that GLP-1 may be involved in the regulation of vital functions. In conclusion, the biochemical data support the idea that the central GLP-1 receptor resembles the peripheral GLP-1 receptor. Furthermore, the presence of GLP-1 binding sites in the circumventricular organs suggests that these may be receptors which act as the target for both peripheral blood-borne GLP-1 and GLP-1 in the nervous system.