Morphology and distribution of efferent vagal innervation of rat pancreas as revealed with anterograde transport of Dil

Vagal pathways for systemic regulation of glucose metabolism

Maintaining blood glucose at an appropriate physiological level requires precise coordination of multiple organs and tissues. The vagus nerve bidirectionally connects the central nervous system with peripheral organs crucial to glucose mobilization, nutrient storage, and food absorption, thereby presenting a key pathway for the central control of blood glucose levels. However, the precise mechanisms by which vagal populations that target discrete tissues participate in glucoregulation are much less clear. Here we review recent advances unraveling the cellular identity, neuroanatomical organization, and functional contributions of both vagal efferents and vagal afferents in the control of systemic glucose metabolism. We focus on their involvement in relaying glucoregulatory cues from the brain to peripheral tissues, particularly the pancreatic islet, and by sensing and transmitting incoming signals from ingested food to the brain. These recent findings - largely driven by advances in viral approaches, RNA sequencing, and cell-type selective manipulations and tracings - have begun to clarify the precise vagal neuron populations involved in the central coordination of glucose levels, and raise interesting new possibilities for the treatment of glucose metabolism disorders such as diabetes.

Crosstalk of nervous and immune systems in pancreatic cancer

Pancreatic cancer is a highly malignant tumor known for its extremely low survival rate. The combination of genetic disorders within pancreatic cells and the tumor microenvironment contributes to the emergence and progression of this devastating disease. Extensive research has shed light on the nature of the microenvironmental cells surrounding the pancreatic cancer, including peripheral nerves and immune cells. Peripheral nerves release neuropeptides that directly target pancreatic cancer cells in a paracrine manner, while immune cells play a crucial role in eliminating cancer cells that have not evaded the immune response. Recent studies have revealed the intricate interplay between the nervous and immune systems in homeostatic condition as well as in cancer development. In this review, we aim to summarize the function of nerves in pancreatic cancer, emphasizing the significance to investigate the neural-immune crosstalk during the advancement of this malignant cancer.

The elusive cephalic phase insulin response: triggers, mechanisms, and functions

The cephalic phase insulin response (CPIR) is classically defined as a head receptor-induced early release of insulin during eating that precedes a postabsorptive rise in blood glucose. Here we discuss, first, the various stimuli that elicit the CPIR and the sensory signaling pathways (sensory limb) involved; second, the efferent pathways that control the various endocrine events associated with eating (motor limb); and third, what is known about the central integrative processes linking the sensory and motor limbs. Fourth, in doing so, we identify open questions and problems with respect to the CPIR in general. Specifically, we consider test conditions that allow, or may not allow, the stimulus to reach the potentially relevant taste receptors and to trigger a CPIR. The possible significance of sweetness and palatability as crucial stimulus features and whether conditioning plays a role in the CPIR are also discussed. Moreover, we ponder the utility of the strict classical CPIR definition based on what is known about the effects of vagal motor neuron activation and thereby acetylcholine on the β-cells, together with the difficulties of the accurate assessment of insulin release. Finally, we weigh the evidence of the physiological and clinical relevance of the cephalic contribution to the release of insulin that occurs during and after a meal. These points are critical for the interpretation of the existing data, and they support a sharper focus on the role of head receptors in the overall insulin response to eating rather than relying solely on the classical CPIR definition.

Functional anatomy of the vagus system: How does the polyvagal theory comply?

Due to its pivotal role in autonomic networks and interoception, the vagus attracts continued interest from both basic scientists and therapists of various clinical disciplines. In particular, the widespread use of heart rate variability as an index of autonomic cardiac control and a proposed central role of the vagus in biopsychological concepts, e.g., the polyvagal theory, provide a good opportunity to recall basic features of vagal anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents.

Emerging Roles of the Nervous System in Gastrointestinal Cancer Development

Simple Summary

Nerve–cancer cross-talk has increasingly become a focus of the oncology field, particularly in gastrointestinal (GI) cancers. The indispensable roles of the nervous system in GI tumorigenesis and malignancy have been dissected by epidemiological, experimental animal and mechanistic data. Herein, we review and integrate recent discoveries linking the nervous system to GI cancer initiation and progression, and focus on the molecular mechanisms by which nerves and neural receptor pathways drive GI malignancy.

Abstract

Our understanding of the fascinating connection between nervous system and gastrointestinal (GI) tumorigenesis has expanded greatly in recent years. Recent studies revealed that neurogenesis plays an active part in GI tumor initiation and progression. Tumor-driven neurogenesis, as well as neurite outgrowth of the pre-existing peripheral nervous system (PNS), may fuel GI tumor progression via facilitating cancer cell proliferation, chemoresistance, invasion and immune escape. Neurotransmitters and neuropeptides drive the activation of various oncogenic pathways downstream of neural receptors within cancer cells, underscoring the importance of neural signaling pathways in GI tumor malignancy. In addition, neural infiltration also plays an integral role in tumor microenvironments, and contributes to an environment in favor of tumor angiogenesis, immune evasion and invasion. Blockade of tumor innervation via denervation or pharmacological agents may serve as a promising therapeutic strategy against GI tumors. In this review, we summarize recent findings linking the nervous system to GI tumor progression, set the spotlight on the molecular mechanisms by which neural signaling fuels cancer aggressiveness, and highlight the importance of targeting neural mechanisms in GI tumor therapy.

Functional anatomy of the vagus system - Emphasis on the somato-visceral interface

Due to its pivotal role in autonomic networks, the vagus attracts continuous interest from both basic scientists and clinicians. In particular, recent advances in vagus nerve stimulation strategies and their application to pathological conditions beyond epilepsy provide a good opportunity to recall basic features of vagal peripheral and central anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents. Thus, the vagus system participates significantly in what may be defined as "somato-visceral interface".

Pancreas-Brain Crosstalk

The neural regulation of glucose homeostasis in normal and challenged conditions involves the modulation of pancreatic islet-cell function. Compromising the pancreas innervation causes islet autoimmunity in type 1 diabetes and islet cell dysfunction in type 2 diabetes. However, despite the richly innervated nature of the pancreas, islet innervation remains ill-defined. Here, we review the neuroanatomical and humoral basis of the cross-talk between the endocrine pancreas and autonomic and sensory neurons. Identifying the neurocircuitry and neurochemistry of the neuro-insular network would provide clues to neuromodulation-based approaches for the prevention and treatment of diabetes and obesity.

Role of hypothalamic de novo ceramides synthesis in obesity and associated metabolic disorders

Background

Sphingolipid-mediated signalling pathways are described as important players in the normal functioning of neurons and nonneuronal cells in the central nervous system (CNS).

Scope of review

This review aims to show role of de novo ceramide synthesis in the CNS in controling key physiological processes, including food intake, energy expenditure, and thermogenesis. The corollary is a condition that leads to a dysfunction in ceramide metabolism in these central regions that can have major consequences on the physiological regulation of energy balance.

Major conclusions

Excessive hypothalamic de novo ceramide synthesis has been shown to result in the establishment of central insulin resistance, endoplasmic reticulum stress, and inflammation. Additionally, excessive hypothalamic de novo ceramide synthesis has also been associated with changes in the activity of the autonomic nervous system. Such dysregulation of hypothalamic de novo ceramide synthesis forms the key starting point for the initiation of pathophysiological conditions such as obesity – which may or may not be associated with type 2 diabetes.

Glutamatergic plasticity within neurocircuits of the dorsal vagal complex and the regulation of gastric functions

The meticulous regulation of the gastrointestinal (GI) tract is required for the coordination of gastric motility and emptying, intestinal secretion, absorption, and transit as well as for the overarching management of food intake and energy homeostasis. Disruption of GI functions is associated with the development of severe GI disorders and the alteration of food intake and caloric balance. Functional GI disorders as well as the dysregulation of energy balance and food intake are frequently associated with, or result from, alterations in the central regulation of GI control. The faithful and rapid transmission of information from the stomach and upper GI tract to second-order neurons of the nucleus of the tractus solitarius (NTS) relies on the delicate modulation of excitatory glutamatergic transmission, as does the relay of integrated signals from the NTS to parasympathetic efferent neurons of the dorsal motor nucleus of the vagus (DMV). Many studies have focused on understanding the physiological and pathophysiological modulation of these glutamatergic synapses, although their role in the control and regulation of GI functions has lagged behind that of cardiovascular and respiratory functions. The purpose of this review is to examine the current literature exploring the role of glutamatergic transmission in the DVC in the regulation of GI functions.

Excitatory Impact of Dental Occlusion on Dorsal Motor Nucleus of Vagus

Neurons in the trigeminal mesencephalic nucleus (Vme) have axons that branch peripherally to innervate the orofacial region and project centrally to several motor nuclei in brainstem. The dorsal motor nucleus of vagus nerve (DMV) resides in the brainstem and takes a role in visceral motor function such as pancreatic exocrine secretion. The present study aimed to demonstrate the presence of Vme-DMV circuit, activation of which would elicit a trigeminal neuroendocrine response. A masticatory dysfunctional animal model termed unilateral anterior crossbite (UAC) model created by disturbing the dental occlusion was used. Cholera toxin B subunit (CTb) was injected into the inferior alveolar nerve of rats to help identify the central axon terminals of Vme neurons around the choline acetyltransferase (ChAT) positive motor neurons in the DMV. The level of vesicular glutamate transporter 1 (VGLUT1) expressed in DMV, the level of acetylcholinesterase (AChE) expressed in pancreas, the level of glucagon and insulin expression in islets and serum, and the blood glucose level were detected and compared between UAC and the age matched sham-operation control mice. Data indicated that compared with the controls, there were more CTb/VGLUT1 double labeled axon endings around the ChAT positive neurons in the DMV of UAC groups. Mice in UAC group expressed a higher VGLUT1 protein level in DMV, AChE protein level in pancreas, glucagon and insulin level in islet and serum, and higher postprandial blood glucose level, but lower fasting blood glucose level. All these were reversed at 15-weeks when UAC cessation was performed from 11-weeks (all, P < 0.05). Our findings demonstrated Vme-DMV circuit via which the aberrant occlusion elicited a trigeminal neuroendocrine response such as alteration in the postprandial blood glucose level. Dental occlusion is proposed as a potential therapeutic target for reversing the increased postprandial glucose level.

Pancreatic β-Cells Communicate With Vagal Sensory Neurons

BACKGROUND AND AIMS

Destroying visceral sensory nerves impacts pancreatic islet function, glucose metabolism, and diabetes onset, but how islet endocrine cells interact with sensory neurons has not been studied.

METHODS

We characterized the anatomical pattern of pancreatic sensory innervation by combining viral tracing, immunohistochemistry, and reporter mouse models. To assess the functional interactions of β-cells with vagal sensory neurons, we recorded Ca²⁺ responses in individual nodose neurons in vivo while selectively stimulating β-cells with chemogenetic and pharmacologic approaches.

RESULTS

We found that pancreatic islets are innervated by vagal sensory axons expressing Phox2b, substance P, calcitonin-gene related peptide, and the serotonin receptor 5-HT₃R. Centrally, vagal neurons projecting to the pancreas terminate in the commissural nucleus of the solitary tract. Nodose neurons responded in vivo to chemogenetic stimulation of β-cells and to pancreas infusion with serotonin, but were not sensitive to insulin. Responses to chemogenetic and pharmacologic stimulation of β-cells were blocked by a 5-HT₃R antagonist and were enhanced by increasing serotonin levels in β-cells. We further confirmed directly in living pancreas slices that sensory terminals in the islet were sensitive to serotonin.

CONCLUSIONS

Our study establishes that pancreatic β-cells communicate with vagal sensory neurons, likely using serotonin signaling as a transduction mechanism. Serotonin is coreleased with insulin and may therefore convey information about the secretory state of β-cells via vagal afferent nerves.

Optical Imaging of Pancreatic Innervation

At the time of Ivan Pavlov, pancreatic innervation was studied by looking at pancreas secretions in response to electrical stimulation of nerves. Nowadays we have ways to visualize neuronal activity in real time thanks to advances in fluorescent reporters and imaging techniques. We also have very precise optogenetic and pharmacogenetic approaches that allow neuronal manipulations in a very specific manner. These technological advances have been extensively employed for studying the central nervous system and are just beginning to be incorporated for studying visceral innervation. Pancreatic innervation is complex, and the role it plays in physiology and pathophysiology of the organ is still not fully understood. In this review we highlight anatomical aspects of pancreatic innervation, techniques for pancreatic neuronal labeling, and approaches for imaging pancreatic innervation in vitro and in vivo.

Vagal mechanisms as neuromodulatory targets for the treatment of metabolic disease

With few effective treatments available, the global rise of metabolic diseases, including obesity, type 2 diabetes mellitus, and cardiovascular disease, seems unstoppable. Likely caused by an obesogenic environment interacting with genetic susceptibility, the pathophysiology of obesity and metabolic diseases is highly complex and involves crosstalk between many organs and systems, including the brain. The vagus nerve is in a key position to bidirectionally link several peripheral metabolic organs with the brain and is increasingly targeted for neuromodulation therapy to treat metabolic disease. Here, we review the basics of vagal functional anatomy and its implications for vagal neuromodulation therapies. We find that most existing vagal neuromodulation techniques either ignore or misinterpret the rich functional specificity of both vagal efferents and afferents as demonstrated by a large body of literature. This lack of specificity of manipulating vagal fibers is likely the reason for the relatively poor beneficial long‐term effects of such therapies. For these therapies to become more effective, rigorous validation of all physiological endpoints and optimization of stimulation parameters as well as electrode placements will be necessary. However, given the large number of function‐specific fibers in any vagal branch, genetically guided neuromodulation techniques are more likely to succeed.

Prader- Willi syndrome: An uptodate on endocrine and metabolic complications

Prader-Willi syndrome (PWS) is a genetic disorder characterized by short stature, low lean body mass, muscular hypotonia, mental retardation, behavioral abnormalities, dysmorphic features, and excessive appetite with progressive obesity. It is caused by lack of expression of genes on the paternally inherited chromosome 15q11.2-q13. This genetic disorder has an estimated prevalence that ranges between 1/10,000-1/30,000. Hypothalamic dysfunction is a common finding in PWS and it has been implicated in several manifestations of this syndrome such as hyperphagia, temperature instability, high pain threshold, sleep disordered breathing, and multiple endocrine abnormalities. These include growth hormone deficiency, central adrenal insufficiency, hypogonadism, hypothyroidism, and obesity often complicated by type 2 diabetes. The aim of this manuscript is to overview the current literature on metabolic and endocrine complications of PWS, focusing on human studies and providing insights on the physio pathological mechanisms. A careful management of metabolic and endocrine complications can contribute to improve quality of life, prevent complications, and prolong life expectancy of PW patients.

Implication of neurohormonal-coupled mechanisms of gastric emptying and pancreatic secretory function in diabetic gastroparesis

Recently, diabetic gastroparesis (DGP) has received much attention as its prevalence is increasing in a dramatic fashion and management of patients with DGP represents a challenge in the clinical practice due to the limited therapeutic options. DGP highlights an interrelationship between the gastric emptying and pancreatic secretory function that regulate a wide range of digestive and metabolic functions, respectively. It well documented that both gastric emptying and pancreatic secretion are under delicate control by multiple neurohormonal mechanisms including extrinsic parasympathetic pathways and gastrointestinal (GI) hormones. Interestingly, the latter released in response to various determinants that related to the rate and quality of gastric emptying. Others and we have provided strong evidence that the central autonomic nuclei send a dual output (excitatory and inhibitory) to the stomach and the pancreas in response to a variety of hormonal signals from the abdominal viscera. Most of these hormones released upon gastric emptying to provide feedback, and control this process and simultaneously regulate pancreatic secretion and postprandial glycemia. These findings emphasize an important link between gastric emptying and pancreatic secretion and its role in maintaining homeostatic processes within the GI tract. The present review deals with the neurohormonal-coupled mechanisms of gastric emptying and pancreatic secretory function that implicated in DGP and this provides new insights in our understanding of the pathophysiology of DGP. This also enhances the process of identifying potential therapeutic targets to treat DGP and limit the complications of current management practices.

Pancreatic neuro-insular network in young mice revealed by 3D panoramic histology

AIMS/HYPOTHESIS

It has been proposed that the neuro-insular network enables rapid, synchronised insulin secretion. However, to date, acquiring the pancreatic tissue map to study the neural network remains a challenging task as there is a lack of feasible approaches for large-scale tissue analysis at the organ level. Here, we have developed 3-dimensional (3D) panoramic histology to characterise the pancreatic neuro-insular network in young mice.

METHODS

Pancreases harvested from young wild-type B6 mice (3 and 8 weeks old) and db/db mice (3 weeks old; db/db vs db/+) were used to develop 3D panoramic histology. Transparent pancreases were prepared by optical clearing to enable deep-tissue, tile-scanning microscopy for qualitative and quantitative analyses of islets and the pancreatic tissue network in space.

RESULTS

3D panoramic histology reveals the pancreatic neurovascular network and the coupling of ganglionic and islet populations via the network. This integration is identified in both 3- and 8-week-old mice, featuring the peri-arteriolar neuro-insular network and islet-ganglionic aggregation. In weaning hyperphagic db/db mice, the 3D image data identifies the associated increases in weight, adipose tissue attached to the pancreas, density of large islets (major axis > 150 μm) and pancreatic sympathetic innervation compared with db/+ mice.

CONCLUSIONS/INTERPRETATION

Our work provides insight into the neuro-insular integration at the organ level and demonstrates a new approach for investigating previously unknown details of the pancreatic tissue network in health and disease.

High fat diet attenuates glucose-dependent facilitation of 5-HT3 -mediated responses in rat gastric vagal afferents

KEY POINTS

Glucose regulates the density and function of 5-HT3 receptors on gastric vagal afferent neurones. Diet-induced obesity compromises the excitability and responsiveness of vagal afferents. In this study, we assessed whether exposure to a high fat diet (HFD) compromises the glucose-dependent modulation of 5-HT responses in gastric vagal afferents prior to the development of obesity. We show that HFD does not alter the response of gastric vagal afferent nerves and neurones to 5-HT but attenuates the ability of glucose to amplify 5-HT3-induced responses. These results suggest that glucose-dependent vagal afferent signalling is compromised by relatively short periods of exposure to HFD well in advance of the development of obesity or glycaemic dysregulation. Glucose regulates the density and function of 5-HT3 receptors on gastric vagal afferent neurones. Since diet-induced obesity attenuates the responsiveness of gastric vagal afferents to several neurohormones, the aim of the present study was to determine whether high fat diet (HFD) compromises the glucose-dependent modulation of 5-HT responses in gastric vagal afferents prior to the development of obesity. Rats were fed control or HFD (14% or 60% kilocalories from fat, respectively) for up to 8 weeks. Neurophysiological recordings assessed the ability of 5-HT to increase anterior gastric vagal afferent nerve (VAN) activity in vivo before and after acute hyperglycaemia, while electrophysiological recordings from gastric-projecting nodose neurones assessed the ability of glucose to modulate the 5-HT response in vitro. Immunocytochemical studies determined alterations in the neuronal distribution of 5-HT3 receptors. 5-HT and cholecystokinin (CCK) induced dose-dependent increases in VAN activity in all rats; HFD attenuated the response to CCK, but not 5-HT. The 5-HT-induced response was amplified by acute hyperglycaemia in control, but not HFD, rats. Similarly, although 5-HT induced an inward current in both control and HFD gastric nodose neurones in vitro, the 5-HT response and receptor distribution was amplified by acute hyperglycaemia only in control rats. These data suggest that, while HFD does not affect the response of gastric-projecting vagal afferents to 5-HT, it attenuates the ability of glucose to amplify 5-HT effects. This suggests that glucose-dependent vagal afferent signalling is compromised by short periods of exposure to HFD well in advance of obesity or glycaemic dysregulation.

Neural control of the endocrine pancreas

The autonomic nervous system affects glucose metabolism partly through its connection to the pancreatic islet. Since its discovery by Paul Langerhans, the precise innervation patterns of the islet has remained elusive, mainly because of technical limitations. Using 3-dimensional reconstructions of axonal terminal fields, recent studies have determined the innervation patterns of mouse and human islets. In contrast to the mouse islet, endocrine cells within the human islet are sparsely contacted by autonomic axons. Instead, the invading sympathetic axons preferentially innervate smooth muscle cells of blood vessels. This innervation pattern suggests that, rather than acting directly on endocrine cells, sympathetic nerves may control hormone secretion by modulating blood flow in human islets. In addition to autonomic efferent axons, islets also receive sensory innervation. These axons transmit sensory information to the brain but also have the ability to locally release neuroactive substances that have been suggested to promote diabetes pathogenesis. We discuss recent findings on islet innervation, the connections of the islet with the brain, and the role islet innervation plays during the progression of diabetes.

Role of metabotropic glutamate receptors in the regulation of pancreatic functions

The pancreas consists of two major divisions, the exocrine and the endocrine pancreas. Recent data from our laboratory have shown that the functions of the two divisions are under modulatory regulation by separate neurocircuits that originate in the dorsal motor nucleus of the vagus (DMV). Metabotropic glutamate receptors (mGluRs) are expressed throughout the central nervous system and have been implicated in the modulation of synaptic transmission. mGluRs consist of three groups of receptors, which can be distinguished based on their pharmacological properties and second messenger systems. Group I mGluRs predominantly increase, whereas group II and III mGluRs decrease synaptic transmission. Group II and group III mGluRs are present on excitatory and inhibitory synaptic terminals impinging on pancreas-projecting DMV neurons. We have shown that group II mGluRs regulate both exocrine pancreatic secretions and insulin release, whereas group III mGluRs only regulate insulin release. Several mGluR agonists and antagonists have been shown to have clinical uses for disorders accompanied by abnormal synaptic transmission, including anxiety and Parkinson's disease. Moreover, a negative allosteric modulator of Group I mGluRs is effective in alleviating symptoms of gastro-esophageal reflux disease (GERD). Since the role of the three mGluR groups in mediating different gastrointestinal (GI) functions appears to be highly specific, the use of agonists or antagonists directed at a single receptor group could potentially provide highly selective targets for the treatment of GI disorders including GERD, functional dyspepsia and acute pancreatitis.

Intrapancreatic ganglia neurons receive projection fibers from melanocortin-4 receptor-expressing neurons in the dorsal motor nucleus of the vagus nerve of the mouse

Melanocortin-4 receptor (MC4R)-expressing neurons are widely distributed in the central nervous system and play a crucial role in a variety of physiological functions including energy and glucose/insulin homeostasis. However, their neural pathways remain to be elucidated. In the present study, we examined a possible pathway from MC4R-expressing neurons in the dorsal motor nucleus of the vagus nerve (DMV) to the intrapancreatic ganglia using transgenic mice that express green fluorescent protein (GFP) under the control of the MC4R-promoter. Using immunofluorescence labeling, we demonstrated that GFP-immunoreactive (ir) nerve fibers were distributed in the intrapancreatic ganglia closely associated with the islets as well as among the acini. These GFP-ir fibers with bouton-like varicosities were frequently observed to surround ganglion cells immunoreactive for vasoactive intestinal polypeptide, a marker for postganglionic parasympathetic neurons. Using the pre-embedding immunoperoxidase method, we clearly showed that GFP-ir terminals formed synapses predominantly with dendrites and additionally with somata of the ganglion cells. Moreover, bilateral subdiaphragmatic vagotomy caused a marked loss of GFP immunoreactivity in the pancreas. Using a combination of retrograde tracing and immunohistochemistry, we finally demonstrated that nearly half of the pancreas-projecting DMV neurons were immunoreactive for GFP. These results suggest that MC4R-expressing DMV neurons may participate in the regulation of glucose/insulin homeostasis through their projections to the intrapancreatic ganglia.

The dorsal motor nucleus of the vagus and regulation of pancreatic secretory function

Recent investigation of the factors and pathways that are involved in regulation of pancreatic secretory function (PSF) has led to development of a pancreatic vagovagal reflex model. This model consists of three elements, including pancreatic vagal afferents, the dorsal motor nucleus of the vagus (DMV) and pancreatic vagal efferents. The DMV has been recognized as a major component of this model and so this review focuses on the role of this nucleus in regulation of PSF. Classically, the control of the PSF has been viewed as being dependent on gastrointestinal hormones and vagovagal reflex pathways. However, recent studies have suggested that these two mechanisms act synergistically to mediate pancreatic secretion. The DMV is the major source of vagal motor output to the pancreas, and this output is modulated by various neurotransmitters and synaptic inputs from other central autonomic regulatory circuits, including the nucleus of the solitary tract. Endogenously occurring excitatory (glutamate) and inhibitory amino acids (GABA) have a marked influence on DMV vagal output to the pancreas. In addition, a variety of neurotransmitters and receptors for gastrointestinal peptides and hormones have been localized in the DMV, emphasizing the direct and indirect involvement of this nucleus in control of PSF.

Obesity surgery and gut-brain communication

The prevalence of obesity, and the cluster of serious metabolic diseases it is associated with, continues to rise globally, and hopes for effective treatment with drugs have been considerably set back. Thus, success with bariatric surgeries to induce sustained body weight loss and effectively cure most of the associated co-morbidities appears almost "miraculous" and systematic investigation of the mechanisms at work has gained momentum. Here, we will discuss the basic organization of gut-brain communication and review clinical and pre-clinical investigations on the potential mechanisms by which gastric bypass surgery leads to its beneficial effects on energy balance and glucose homeostasis. Although a lot has been learned regarding changes in energy intake and expenditure, secretion of gut hormones, and improvement in glucose homeostasis, there has not yet been the "breakthrough observation" of identifying a key signaling component common to the beneficial effects of the surgery. However, given the complexity and redundancy of gut-brain signaling and gut signaling to other relevant organs, it is perhaps more realistic to expect a number of key signaling changes that act in concert to bring about the "miracle".

Effects of nitric oxide synthase blockade on dorsal vagal stimulation-induced pancreatic insulin secretion

We and others have previously shown that the dorsal motor nucleus of the vagus (DMV) is involved in regulation of pancreatic exocrine secretion. Many pancreatic preganglionic neurons within the DMV are inhibited by pancreatic secretagogues suggesting that an inhibitory pathway may participate in the control of pancreatic exocrine secretion. Accordingly, the present study examined whether chemical stimulation of the DMV activates the endocrine pancreas and whether an inhibitory pathway is involved in this response. All experiments were conducted in overnight fasted isoflurane/urethane-anesthetized Sprague Dawley rats. Activation of the DMV by bilateral microinjection of bicuculline methiodide (BIM, GABA(A) receptor antagonist, 100 pmol/25 nl; 4 mM) resulted in a significant and rapid increase in glucose-induced insulin secretion (9.2±0.1 ng/ml peak response) compared to control microinjection (4.0±0.6 ng/ml). Activation of glucose-induced insulin secretion by chemical stimulation of the DMV was inhibited (2.1±1.1 ng/ml and 1.6±0.1 ng/ml 5 min later) in the presence of the muscarinic receptor antagonist atropine methonitrate (100 μg/kg/min, i.v.). On the other hand, the nitric oxide (NO) synthesis inhibitor l-nitroarginine methyl ester (30 mg/kg, i.v.) significantly increased the excitatory effect of DMV stimulation on glucose-induced insulin secretion to 15.3±3.0 ng/ml and 16.1±3.1 ng/ml 5 min later. These findings suggest that NO may play an inhibitory role in the central regulation of insulin secretion.

The vagus nerve, food intake and obesity

Food interacts with sensors all along the alimentary canal to provide the brain with information regarding its composition, energy content, and beneficial effect. Vagal afferents innervating the gastrointestinal tract, pancreas, and liver provide a rapid and discrete account of digestible food in the alimentary canal, as well as circulating and stored fuels, while vagal efferents, together with the sympathetic nervous system and hormonal mechanisms, codetermine the rate of nutrient absorption, partitioning, storage, and mobilization. Although vagal sensory mechanisms play a crucial role in the neural mechanism of satiation, there is little evidence suggesting a significant role in long-term energy homeostasis. However, increasing recognition of vagal involvement in the putative mechanisms making bariatric surgeries the most effective treatment for obesity should greatly stimulate future research to uncover the many details regarding the specific transduction mechanisms in the periphery and the inter- and intra-neuronal signaling cascades disseminating vagal information across the neuraxis.

Glucagon-like peptide-1 modulates synaptic transmission to identified pancreas-projecting vagal motoneurons

Using a brainstem slice preparation, we aimed to study the pre- and postsynaptic effects of glucagon-like peptide-1 (GLP-1) on synaptic transmission to identified pancreas-projecting vagal motoneurons. Following blockade of GABAergic mediated currents with bicuculline, perfusion with 100 nM GLP-1 increased both amplitude and frequency of excitatory postsynaptic currents (EPSCs) in 21 of 52 neurons. Perfusion with the GLP-1 selective agonist exendin-4 (100 nM), also increased the frequency of spontaneous EPSCs, while pretreatment with the GLP-1 selective antagonist, exendin 9-39, prevented the effects of GLP-1. In the presence of kynurenic acid to block ionotropic glutamatergic currents, perfusion with GLP-1 increased the frequency of inhibitory postsynaptic currents (IPSCs) in 28 of 74 neurons; in 14 of these responsive neurons, GLP-1 also increased IPSC amplitude, indicating actions at both pre- and postsynaptic sites. Perfusion with exendin-4 increased the frequency of spontaneous IPSCs, while pretreatment with exendin 9-39 prevented the effects of GLP-1. These results suggest that GLP-1 modulates both excitatory and inhibitory synaptic inputs to pancreas-projecting vagal motoneurons.

Glucagon-like peptide-1 excites pancreas-projecting preganglionic vagal motoneurons

Glucagon-like peptide-1 (GLP-1) increases pancreatic insulin secretion via a direct action on pancreatic beta-cells. A high density of GLP-1-containing neurons and receptors is also present in brain stem vagal circuits; therefore, the aims of the present study were to investigate 1) whether identified pancreas-projecting neurons of the dorsal motor nucleus of the vagus (DMV) respond to exogenously applied GLP-1, 2) the mechanism(s) of action of GLP-1, and 3) whether the GLP-1-responsive neurons (putative modulators of endocrine secretion) could be distinguished from DMV neurons responsive to peptides that modulate pancreatic exocrine secretion, specifically pancreatic polypeptide (PP). Whole cell recordings were made from identified pancreas-projecting DMV neurons. Perfusion with GLP-1 induced a concentration-dependent depolarization in approximately 50% of pancreas-projecting DMV neurons. The GLP-1 effects were mimicked by exendin-4 and antagonized by exendin-(9-39). In approximately 60% of the responsive neurons, the GLP-1-induced depolarization was reduced by tetrodotoxin (1 microM), suggesting both pre- and postsynaptic sites of action. Indeed, the GLP-1 effects were mediated by actions on potassium currents, GABA-induced currents, or both. Importantly, neurons excited by GLP-1 were unresponsive to PP and vice versa. These data indicate that 1) GLP-1 may act on DMV neurons to control pancreatic endocrine secretion, 2) the effects of GLP-1 on pancreas-projecting DMV neurons are mediated both via a direct excitation of their membrane as well as via an effect on local circuits, and 3) the GLP-1-responsive neurons (i.e., putative endocrine secretion-controlling neurons) could be distinguished from neurons responsive to PP (i.e., putative exocrine secretion-controlling neurons).

Microinjection of exogenous somatostatin in the dorsal vagal complex inhibits pancreatic secretion via somatostatin receptor-2 in rats

Previous studies have suggested that somatostatin inhibits pancreatic secretion at a central vagal site, and the dorsal vagal complex (DVC) is involved in central feedback inhibition of the exocrine pancreas. The aim of this study was to investigate the effect of exogenous somatostatin in the DVC on pancreatic secretion and the somatostatin receptor subtype(s) responsible for the effect. The effects of somatostatin microinjected into the DVC on pancreatic secretion stimulated by cholecystokinin octapeptide (CCK-8) or 2-deoxy-d-glucose (2-DG) were examined in anesthetized rats. To investigate the somatostatin inhibitory action site, a somatostatin receptor antagonist [SRA; cyclo(7-aminoheptanoyl-Phe-d-Trp-Lys-Thr)] was microinjected into the DVC before intravenous infusion of somatostatin and CCK-8/2-DG. The effects of injection of a somatostatin receptor-2 agonist (seglitide) and combined injection of somatostatin and a somatostatin receptor-2 antagonist (CYN 154806) in the DVC on the pancreatic secretion were also investigated. Somatostatin injected into the DVC significantly inhibited pancreatic secretion evoked by CCK-8 or 2-DG in a dose-dependent manner. SRA injected into the DVC completely reversed the inhibitory effect of intravenous administration of somatostatin. Seglitide injected into the DVC also inhibited CCK-8/2-DG-induced pancreatic protein secretion. However, combined injection of somatostatin and CYN 154806 did not affect the CCK-8/2-DG-induced pancreatic secretion. Somatostatin in the DVC inhibits pancreatic secretion via somatostatin receptor-2, and the DVC is the action site of somatostatin for its inhibitory effect.

Autonomic pathways regulating pancreatic exocrine secretion

The parasympathetic (PNS) and sympathetic (SNS) and nervous systems densely innervate the exocrine pancreas. Efferent PNS pathways, consisting of central dorsal motor nucleus of the vagus (DMV) and peripheral pancreatic neurons, stimulate exocrine secretion. The DMV integrates cortical (olfactory, gustatory) and gastric, and intestinal vagal afferent input to determine central PNS outflow during cephalic, gastric and intestinal phases of exocrine secretion. Pancreatic neurons integrate DMV input with peripheral enteric, sympathetic, and, possibly, afferent axon reflexes to determine final PNS input to all exocrine effectors. Gut and islet hormones appear to modulate both central and peripheral PNS pathways. Preganglionic sympathetic neurons in the intermediolateral (IML) column of the spinal cord receive inputs from brain centers, some shared with the PNS, and innervate postganglionic neurons, mainly in prevertebral ganglia. Sympathetic innervation of the exocrine pancreas is primarily indirect, and inhibits secretion by decreasing blood flow and inhibiting transmission in pancreatic ganglia. Interactions between SNS and PNS pathways appear to occur in brain, spinal cord, pancreatic and prevertebral ganglia, and at neuroeffector synapses. Thus, the PNS and SNS pathways regulating the exocrine pancreas are directly or indirectly antagonistic at multiple sites: the state of exocrine secretion reflects the balance of these influences. Despite over a century of study, much remains to be understood about the connections of specific neurons forming pancreatic pathways, their processes of neurotransmission, and how disruption of these pathways contributes to pancreatic disease.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Modulation of hormone secretion by functional electrical stimulation of the intact and incompletely dysfunctional dog pancreas

The purpose of the present study was to modulate the secretion of insulin and glucagon in Beagle dogs by stimulation of nerves innervating the intact and partly dysfunctional pancreas. Three 33-electrode spiral cuffs were implanted on the vagus, splanchnic and pancreatic nerves in each of two animals. Partial dysfunction of the pancreas was induced with alloxan. The nerves were stimulated using rectangular, charge-balanced, biphasic, and constant current pulses (200 micros, 1 mA, 20 Hz, with a 100-micros delay between biphasic phases). Blood samples from the femoral artery were drawn before the experiment, at the beginning of stimulation, after 5 min of stimulation, and 5 min after the end of stimulation. Radioimmunoassay data showed that in the intact pancreas stimulation of the vagal nerve increased insulin (+99.2 microU/ml) and glucagon (+18.7 pg/ml) secretion and decreased C-peptide secretion (-0.15 ng/ml). Splanchnic nerve stimulation increased insulin (+1.7 microU/ml), C-peptide (+0.01 ng/ml), and glucagon (+50 pg/ml) secretion, whereas pancreatic nerve stimulation did not cause a marked change in any of the three hormones. In the partly dysfunctional pancreas, vagus nerve stimulation increased insulin (+15.5 microU/ml), glucagon (+11 pg/ml), and C-peptide (+0.03 ng/ml) secretion. Splanchnic nerve stimulation reduced insulin secretion (-2.5 microU/ml) and increased glucagon (+58.7 pg/ml) and C-peptide (+0.39 ng/ml) secretion, and pancreatic nerve stimulation increased insulin (+0.2 microU/ml), glucagon (+5.2 pg/ml), and C-peptide (+0.08 ng/ml) secretion. It was concluded that vagal nerve stimulation can significantly increase insulin secretion for a prolonged period of time in intact and in partly dysfunctional pancreas.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Electrophysiological effects of GABA on cat pancreatic neurons

In mammalian peripheral sympathetic ganglia GABA acts presynaptically to facilitate cholinergic transmission and postsynaptically to depolarize membrane potential. The GABA effect on parasympathetic pancreatic ganglia is unknown. We aimed to determine the effect of locally applied GABA on cat pancreatic ganglion neurons. Ganglia with attached nerve trunks were isolated from cat pancreata. Conventional intracellular recording techniques were used to record electrical responses from ganglion neurons. GABA pressure microejection depolarized membrane potential with an amplitude of 17.4 +/- 0.7 mV. Electrically evoked fast excitatory postsynaptic potentials were significantly inhibited (5.4 +/- 0.3 to 2.9 +/- 0.2 mV) after GABA application. GABA-evoked depolarizations were mimicked by the GABA(A) receptor agonist muscimol and abolished by the GABA(A) receptor antagonist bicuculline and the Cl(-) channel blocker picrotoxin. GABA was taken up and stored in ganglia during preincubation with 1 mM GABA; beta-aminobutyric acid application after GABA loading significantly (P < 0.05) increased depolarizing response to GABA (15.6 +/- 1.0 vs. 7.8 +/- 0.8 mV without GABA preincubation). Immunolabeling with antibodies to GABA, glial cell fibrillary acidic protein, protein gene product 9.5, and glutamic acid decarboxylase (GAD) immunoreactivity showed that GABA was present in glial cells, but not in neurons, and that glial cells did not contain GAD, whereas islet cells did. The data suggest that endogenous GABA released from ganglionic glial cells acts on pancreatic ganglion neurons through GABA(A) receptors.

Vagal-enteric interface: vagal activation-induced expression of c-Fos and p-CREB in neurons of the upper gastrointestinal tract and pancreas

Many gastrointestinal and pancreatic functions are under strong modulatory control by the brain via the vagus nerve. To start identifying location and neurochemical phenotype of the enteric neurons receiving functional vagal efferent input, we activated vagal preganglionic neurons either by electrical or chemical stimulation and examined the expression of phosphorylated CREB (c-AMP response element binding protein) and the immediate early gene c-Fos. There was no spontaneous expression of both markers in the pancreas and considerable spontaneous expression of p-CREB but not Fos in the upper GI-tract. Unilateral electrical vagal stimulation-induced p-CREB was found in 40% of neurons in the head of the pancreas. Fos expression was found in 70-90% of neurons in the esophagus and stomach, in 20-30% of myenteric plexus neurons and 5-15% in submucosal neurons of the proximal duodenum. Double-labeling experiments showed that a majority of pancreatic neurons and about 25-35% of neurons in the stomach and duodenum contain NADPH-diaphorase and that many of these receive functional vagal input. Other neurons that can be vagally activated contain gastrin-releasing peptide or calretinin. Chemical stimulation of the dorsal surface of the caudal brainstem with the stable TRH analog RX77368 resulted in selective activation of vagal efferents with expression of Fos in a small number of gastric myenteric plexus neurons. The results demonstrate the suitability of this method to investigate magnitude and local distribution of vagal input to the enteric nervous system as well as specificity of its neurochemically coded pathways. They represent the first step in the identification of function-specific units of parasympathetic vagal outflow.

Functional and chemical anatomy of the afferent vagal system

The results of neural tracing studies suggest that vagal afferent fibers in cervical and thoracic branches innervate the esophagus, lower airways, heart, aorta, and possibly the thymus, and via abdominal branches the entire gastrointestinal tract, liver, portal vein, billiary system, pancreas, but not the spleen. In addition, vagal afferents innervate numerous thoracic and abdominal paraganglia associated with the vagus nerves. Specific terminal structures such as flower basket terminals, intraganglionic laminar endings and intramuscular arrays have been identified in the various organs and organ compartments, suggesting functional specializations. Electrophysiological recording studies have identified mechano- and chemo-receptors, as well as temperature- and osmo-sensors. In the rat and several other species, mostly polymodal units, while in the cat more specialized units have been reported. Few details of the peripheral transduction cascades and the transmitters for signal propagation in the CNS are known. Glutamate and its various receptors are likely to play an important role at the level of primary afferent signaling to the solitary nucleus. The vagal afferent system is thus in an excellent position to detect immune-related events in the periphery and generate appropriate autonomic, endocrine, and behavioral responses via central reflex pathways. There is also good evidence for a role of vagal afferents in nociception, as manifested by affective-emotional responses such as increased blood pressure and tachycardia, typically associated with the perception of pain, and mediated via central reflex pathways involving the amygdala and other parts of the limbic system. The massive central projections are likely to be responsible for the antiepileptic properties of afferent vagal stimulation in humans. Furthermore, these functions are in line with a general defensive character ascribed to the vagal afferent, paraventricular system in lower vertebrates.

Functional vagal input to chemically identified neurons in pancreatic ganglia as revealed by Fos expression

The importance of neural elements in the control of both endocrine and exocrine pancreatic secretory functions and their coordination with gastrointestinal, hepatic, and general homeostatic functions is increasingly recognized. To better characterize the vagal efferent input to the pancreas, the capacity of electrical vagal stimulation to induce expression of c-Fos in neurochemically identified neurons of intrapancreatic ganglia was investigated. At optimal stimulation parameters, unilateral stimulation of either the left or right cervical vagus induced Fos expression in approximately 30% of neurons in the head and 10-20% of neurons in the body and tail of the pancreas. There was no Fos expression if no stimulation or stimulation with a distally cut vagus was applied. Large proportions of neurons contained nitric oxide synthase as assessed with NADPH diaphorase histochemistry (88%) and choline acetyltransferase. The proportion of nitrergic and nonnitrergic neurons receiving vagal input was not different. It is concluded that a significant proportion of pancreatic neurons receives excitatory synaptic input from vagal preganglionic axons and that many of these vagal postganglionic neurons can produce nitric oxide and acetylcholine.

Characterization of alterations in glucose and insulin metabolism in Prader-Willi subjects

Obesity is a common component of non-insulin-dependent diabetes mellitus (NIDDM) and plays an important role in the development of insulin resistance and hyperinsulinemia. Prader-Willi syndrome (PWS) has been associated with morbid obesity and an increased propensity for early development of NIDDM. It has been assumed that the etiology for this increased rate of NIDDM is related to the morbid obesity and concomitant insulin resistance, but this remains controversial. To shed light on the glucoregulatory mechanisms in PWS, we studied both pediatric and adult PWS patients with normoglycemia. The objectives of our study were (1) to examine glucose, insulin, and C-peptide responses to oral (OGTT) and intravenous (IVGTT) glucose tolerance tests; (2) to characterize acute first- and second-phase insulin secretion during an IVGTT; (3) to assess hepatic insulin extraction (HIE) and insulin clearance (IC) in PWS subjects; and (4) to determine whether beta-cell function in PWS is age-dependent. These results in PWS were compared with values obtained in age-, sex-, and body mass index (BMI)-matched non-PWS obese controls. Three groups were studied. Group I consisted of nine PWS subjects under the age of 13 years and 22 age-, sex-, weight-, and puberty stage-matched obese subjects who underwent OGTT. Group II consisted of 14 adult PWS subjects and 10 age-, weight-, and BMI-matched obese adults who underwent OGTT. Group III consisted of nine adult PWS subjects and eight age-, sex-, and weight-matched obese adults who underwent frequently sampled IVGTT (FSIVGTT). During the OGTT in the pediatric group, fasting (86 +/- 3 v 89 +/- 2 mg/dL), peak (144 +/- 11 v 147 +/- 4 mg/dL), and total area under the curve (AUC) (6,984 +/- 1,320 v 6,963 +/- 615 mg/dL x min) glucose levels were not significantly different in PWS versus obese children, respectively. In contrast, fasting (20 +/- 6 v 37 +/- 4 microU/mL), peak (114 +/- 24 v 214 +/- 23 microU [correction of mU]/mL), and total AUC (12,673 +/- 2,176 v 26,734 +/- 2,608 microU/mL microU/mL min) insulin levels were significantly lower in pediatric PWS. During the OGTT in the adult groups, neither fasting insulin (16.7 +/- 2.8 v 13.5 +/- 2.5 microU/mL) nor total AUC for insulin (10,664 +/- 1,955 v 11,623 +/- 1,584 microU/mL x min) were significantly different in adult PWS and obese groups. During the IVGTT in adults, both first-phase (138 +/- 42 v 454 +/- 102 microU/mL x min) and second-phase (295 +/- 66 v 1,015 +/- 231 microU/mL x min) insulin release were significantly reduced in PWS subjects despite similar glucose levels. Similarly, first-phase (8.6 +/- 2.3 v 21 +/- 4.6 ng/dL x min) and second-phase (47 +/- 4.6 v 75 +/- 14 ng/dL x min) C-peptide responses were also significantly reduced in PWS subjects. In contrast, mean HIE and IC was 33% higher in PWS subjects versus obese controls (15.4 +/- 1.5 v 10.3 +/- 1.6). Similarly, poststimulation HIE and IC was significantly greater (5.2 +/- 0.8 v 2.4 +/- 0.4) in the PWS group compared with the obese group (P < .01). In summary, this study demonstrates that nondiabetic PWS subjects manifest (1) a reduced beta-cell response to glucose stimulation, (2) a significantly increased HIE compared with obese controls, and (3) a dissociation of obesity and insulin resistance, in contrast to normal obese subjects. We conclude that glucoregulatory mechanisms are different in obese PWS versus non-PWS subjects.

Interaction between parasympathetic and sympathetic nerves in prevertebral ganglia: morphological evidence for vagal efferent innervation of ganglion cells in the rat

Vagal efferent preganglionic neurons were anterogradely labeled by injecting either DiI or DiA, fluorescent lipophilic carbocyanine dyes, into the dorsal motor nucleus of the vagus of the rat. All neurons of the peripheral nervous system (outside the blood-brain barrier) were then fluorescently counterstained in vivo by injecting Fluorogold (Fluorochrome, Inc., Englewood, CO) intraperitoneally. The upper abdominal prevertebral ganglia, including the numerous microganglia associated with the periarterial plexuses of the celiac and superior mesenteric arteries, were identified and dissected in formalin-fixed tissue under ultraviolet light and stereomicroscopic guidance. In 14 of 15 animals analyzed (93%), labeled vagal efferent fibers were found to penetrate into both the left and right celiac ganglia and the superior mesenteric ganglion, as well as into some of the associated microganglia. These projections formed varicose terminal-like structures, highly suggestive of synaptic contacts surrounding individual ganglion cells. In about half the animals, such vagal innervation was also seen in the left and right suprarenal ganglia. The specificity of vagal efferent labeling was confirmed by control experiments, which included injections in vagotomized animals and direct selective labeling of vagal afferents from the nodose ganglia. It is concluded that vagal efferent preganglionics innervate principal ganglion cells of prevertebral ganglia. These vagal contacts may either directly modulate the postganglionic outflow or else gate some or all of the potential modulatory inputs to these postganglionic neurons, thus allowing the vagal system to exert a more selective influence on sympathetic outflow. Finally, the use of laser scanning confocal microscopy and the in toto Fluorogold staining method for investigations of the peripheral nervous system are discussed.

Pancreatic polypeptide, microinjected into the dorsal vagal complex, potentiates glucose-stimulated insulin secretion in the rat

Specific binding sites for circulating pancreatic polypeptide (PP) have been found within the dorsal vagal complex (DVC) in the caudal medulla oblongata. Therefore, the effects of rat PP on pancreatic hormone secretion upon its microinjection into the DVC in halothane-anesthetized rats at doses of 0.4-40 pmol were investigated. At this range of doses, the changes in plasma concentrations of insulin, glucagon and glucose over basal levels did not differ from those after vehicle microinjection. In a separate series of experiments, vehicle and PP at doses of 0.4 and 4 pmol were microinjected into the right DVC 40 min after the continuous infusion of D-glucose had been started. In animals receiving continuous infusion of D-glucose, PP microinjected into the DVC (4 pmol), resulted in markedly higher insulin levels at corresponding time points compared to those with vehicle microinjected into the DVC. These data indicate, for the first time, that microinjection of PP into the DVC may potentiate glucose-stimulated insulin secretion in halothane-anesthetized rats.

The nucleus raphe obscurus controls pancreatic hormone secretion in the rat

Until recently, the dorsal vagal complex (DVC) was considered as the only brain stem regulatory center for the vagal control of the endocrine pancreas. Because the nucleus raphe obscurus (NRO) maintains anatomic connections via the DVC to the pancreas, a functional significance of these findings was investigated in the present study. Kainic acid and vehicle were microinjected into the right DVC and the NRO of alpha-chloralose-anesthetized rats, and plasma concentrations of rat insulin, glucagon, and glucose were determined before and 5, 15, 30, and 60 min after injections. Chemical stimulation of neurons in the DVC by kainic acid at a dose of 200 pmol evoked increases in concentrations of insulin, with a peak at 15 min, and glucagon, with a peak at 30 min. Microinjection of kainic acid into the NRO at a dose of 200 pmol, but not at a dose of 20 pmol, produced increases in plasma concentrations of insulin, with a peak at 30 min, and glucagon, with a peak at 60 min. Plasma glucose levels on microinjection of kainic acid into the NRO at a dose of 20 pmol were decreased, whereas no changes on microinjection of kainic acid at a dose of 200 pmol were observed. The effects of kainic acid on insulin and glucagon secretion in the NRO were abolished by bilateral vagotomy. The study demonstrates for the first time that the NRO can contribute to vagal control of pancreatic endocrine function, although the exact circuitry and neurotransmitters involved in this response remain unknown.

Three-dimensional reconstructions of autonomic projections to the gastrointestinal tract

Three-dimensional reconstruction protocols in confocal microscopy are typically considered in terms of rendering separate stacks of optical sections. Single stacks, however, include volumes that are often too small to permit descriptions of entire neurons, complete axonal arbors, or complex neural networks. Furthermore, traditional tissue preparation protocols generally yield specimens too limited to permit reconstructions of complex neural systems. For 3-D analyses of extensive networks such as the autonomic nervous system projections within the viscera, it is critical to incorporate appropriate tissue techniques, including suitable tracer protocols, into the reconstruction strategy. This report summarizes complementary technologies, including whole mount procedures, tracer techniques for identifying single fibers in situ, and methods of examining stacks of optical images, which make it practical to describe the complete terminal field of an individual axon in the gastrointestinal tract. Such methods establish that vagal motor axons travel long distances within their target organs, collateralize frequently, and ramify extensively. Vagal afferents have extensive, complex, and, in some cases, polytopic arbors within target tissues.

Neuronal and endothelial nitric oxide synthase immunoreactivity and NADPH-diaphorase staining in rat and human pancreas: influence of fixation

In this study, we wished to clarify the distribution and co-localization of nitric oxide synthase and NA-DPH-diaphorase (NADPH-d) in nerve cells, nerve fibres and parenchymal cells in exocrine and endocrine pancreas, and to assess the influence of fixation on the staining pattern obtained. For this purpose, we applied nitric oxide synthase immunocytochemistry and NADPH-d histochemistry to rat and human pancreas under different fixation conditions. Antibodies to neuronal and endothelial nitric oxide synthase were similarly applied. We found complete co-localization of neuronal nitric oxide synthase and NADPH-d in ganglion cells, and in nerve fibres around acini, excretory ducts, blood vessels and in islets of Langerhans of rat and human pancreas. Immunoreactivity for endothelial nitric oxide synthase was co-localized with NADPH-d in endothelial cells. However, in NADPH-d reactive islet and ductal epithelial cells we could detect neither brain nor endothelial nitric oxide synthase immunoreactivity with any fixation protocol applied. There were marked differences in NADPH-d staining of both neurons and parenchymal cells under different fixation conditions. These results indicate the existence of different types of NADPH-d, which are associated or not associated with nitric oxide synthase(s), and which are differently influenced by various fixation procedures in rat and human pancreas.

Vagal innervation of the rat pylorus: an anterograde tracing study using carbocyanine dyes and laser scanning confocal microscopy

In an attempt to identify the distribution and structure of vagal fibers and terminals in the gastroduodenal junction, vagal efferents were labeled in vivo by multiple injections of the fluorescent carbocyanine dye DiA into the dorsal motor nucleus (dmnX), and vagal afferents were anterogradely labeled by injections of DiI into the nodose ganglia of the same or separate rats. Thick frontal cryostat sections were analysed either with conventional or laser scanning confocal microscopy, using appropriate filter combinations and/or different wavelength laser excitation to distinguish the fluorescent tracers. Vagal efferent terminal-like structures were present in small ganglia within the circular sphincter muscle, which, in the absence of a well-developed, true myenteric plexus at this level, represent the myenteric ganglia. Furthermore, vagal efferent terminals were also present in submucosal ganglia, but were absent from mucosa, Brunner's glands and circular muscle fibers. Vagal afferent fibers and terminal-like structures were more abundant than efferents. The most prominent afferent terminals were profusely branching, large net-like aggregates of varicose fibers running within the connective tissue matrix predominantly parallel to the circular sphincter muscle bundles. Profusely arborizing, highly varicose endings were also present in large myenteric ganglia of the antrum and duodenum, in the modified intramuscular ganglia, and in submucosal ganglia. Additionally, afferent fibers and terminals were present throughout the mucosal lining of the gastroduodenal junction. The branching patterns of some vagal afferents suggested that individual axons produced multiple collaterals in different compartments. NADPH-diaphorase positive, possibly nitroxergic neurons were present in myenteric ganglia of the immediately adjacent antrum and duodenum, and fine varicose fibers entered the sphincter muscle from both sides, delineating the potential vagal inhibitory postganglionic innervation. These morphological results support the view of a rich and differentiated extrinsic neural control of this important gut region as suggested by functional studies.

Characterization of vagal innervation to the rat celiac, suprarenal and mesenteric ganglia

In order to shed light on the controversial issue of vagal innervation of the solar plexus ganglia, vagal efferent preganglionic fibers were anterogradely labeled by injecting the fluorescent carbocyanine dye Dil into the dorsal motor nucleus (dmnX). Additionally, Fluorogold was used to label the ganglia in toto, providing a counterstain and the possibility of UV light-guided dissection of the various ganglia. Using optical sectioning of whole mounted intact ganglia by means of laser scanning confocal microscopy, a considerable number of Dil-labeled vagal terminal-like structures were found in the major ganglia (celiac, superior mesenteric and suprarenal). Additionally, vagal efferent terminals were regularly found in microganglia associated with the periarterial plexuses of the celiac and superior mesenteric arteries, and in a few cases in small ganglia of the intermesenteric and renal plexuses. By using animals with prior selective vagal branch vagotomies, leaving only one (or a pair) of the three major abdominal divisions intact, it was concluded that the two celiac branches contribute the bulk of this vagal innervation, with the two gastric and the unpaired hepatic branch providing a small contribution mostly limited to the celiac ganglia. From control experiments, which involved Dil injections (1) into the dmnX in animals whose visceral afferents had been previously destroyed by capsaicin; (2) into the nodose ganglia, in order to anterogradely label vagal afferents; and (3) into the cervical vagus nerve as a control for uptake by fibers of passage, it was concluded that the identified terminal-like structures were vagal efferents and not inadvertently labeled afferents. We suggest that these vagal terminals have to be regarded either as ectopic parasympathetic junctions, or as part of a vagal mechanism for gating of sympathetic ganglionic transmission. Functionally, the parasympathetic innervation of the solar plexus may provide not only the classic vagal influence on gastrointestinal targets, but also vagal control of the adrenal glands and possibly other abdominal organs that have not been traditionally regarded as vagal targets.

An anterograde tracing study of the vagal innervation of rat liver, portal vein and biliary system

In order to investigate the distribution and structure of the vagal liver innervation, abdominal vagal afferents and efferents were selectively labeled by injecting WGA-HRP or Dil into the nodose ganglia, and DiA into the dorsal motor nucleus, respectively. Vagal afferent fibers produced characteristic terminal-like structures at three locations in the liver hilus: 1. Fine varicose endings preferentially surrounding, but not entering, the numerous peribiliary glands in the larger intra and extrahepatic bile ducts 2. Large, cup-shaped terminals in almost all paraganglia 3. Fine varicose endings in the portal vein adventitia. No fibers and terminals were found in the hepatic parenchyma. While about two thirds of the vagal afferent fibers that originate in the left nodose ganglion, and are contained in the hepatic branch, bypass the liver hilus area on their way to the gastroduodenal artery, a significant number (approx. 10% of the total) of vagal afferents that do innervate the area, originates from the right nodose ganglion, and projects to the periarterial plexus of the common hepatic artery and liver pedicle most likely through the dorsal celiac branch. Varicose vagal efferent fibers were present within the fascicles of the vagal hepatic branch and fine terminal-like structures in a small fraction of the paraganglia. No efferents were found to terminate in the hepatic parenchyma or on the few neurons embedded in nerves or paraganglia. In contrast to the paucity of vagal terminals in the hepatic parenchyma, an abundance of vagal efferent and afferent fibers and terminals with distinctive distribution patterns and structural characteristics was present in esophagus and gastrointestinal tract.(ABSTRACT TRUNCATED AT 250 WORDS)

Vagal afferent innervation of the rat fundic stomach: morphological characterization of the gastric tension receptor

Although the gastric tension receptor has been characterized behaviorally and electrophysiologically quite well, its location and structure remains elusive. Therefore, the vagal afferents to the rat fundus (forestomach or nonglandular stomach) were anterogradely labeled in vivo with injections of the carbocyanine dye Dil into the nodose ganglia, and the nerves and ganglia of the enteric nervous system were labeled in toto with intraperitoneal Fluorogold injection. Dissected layers and cryostat cross sections of the fundic wall were mounted in glycerin and analyzed by means of conventional and laser scanning confocal microscopy. Particularly in the longitudinal, and to a lesser extent in the circular, smooth muscle layers, Dil-labeled fibers and terminals were abundant. These processes, which originated from fibers coursing through the myenteric ganglia and connectives, entered either muscle coat and then ran parallel to the respective muscle fibers, often for several millimeters. They ran in close association with the Fluorogold-labeled network of interstitial cells of Cajal, upon which they appeared to form multiple spiny appositions or varicosities. In the myenteric plexus, two different types of afferent vagal structures were observed. Up to 300 highly arborizing endings forming dense accumulations of small puncta similar to the esophageal intraganglionic laminar endings (Rodrigo et al., '75 Acta Anat. 92:79-100) were found in the fundic wall ipsilateral to the injected nodose ganglion. They often covered small clusters of myenteric neurons or even single isolated ganglion cells (mean = 5.8 neurons) and tended to extend throughout the neuropil of the ganglia. In a second pattern, fine varicose fibers with less profuse arborizations innervated mainly the central regions of myenteric ganglia. Camera lucida analyses established that single vagal afferent fibers had separate collaterals in both a smooth muscle layer and the myenteric ganglia. Finally, Dil-labeled afferent vagal fibers were also found in the submucosa and mucosa. Control experiments in rats with supranodose vagotomy as well as rats with Dil injections directly in the distal cervical vagus ruled out the possibility of colabeling of afferent fibers of passage. In triple labeling experiments, in conjunction with Dil labeling of afferents and Fluorogold labeling of enteric neurons, the carbocyanine dye DiA was injected into the dorsal motor nucleus of the vagus to anterogradely label the efferent vagal fibers and terminals. The different distributions and morphological characteristics of the vagal afferents and efferents could be simultaneously compared. In some instances the same myenteric ganglion was apparently innervated by an afferent laminar ending and an efferent terminal.(ABSTRACT TRUNCATED AT 400 WORDS)