Islet sympathetic innervation and islet neuropathology in patients with type 1 diabetes.

Dysregulation of glucagon secretion in type 1 diabetes (T1D) involves hypersecretion during postprandial states, but insufficient secretion during hypoglycemia. The sympathetic nervous system regulates glucagon secretion. To investigate islet sympathetic innervation in T1D, sympathetic tyrosine hydroxylase (TH) axons were analyzed in control non-diabetic organ donors, non-diabetic islet autoantibody-positive individuals (AAb), and age-matched persons with T1D. Islet TH axon numbers and density were significantly decreased in AAb compared to T1D with no significant differences observed in exocrine TH axon volume or lengths between groups. TH axons were in close approximation to islet α-cells in T1D individuals with long-standing diabetes. Islet RNA-sequencing and qRT-PCR analyses identified significant alterations in noradrenalin degradation, α-adrenergic signaling, cardiac ß-adrenergic signaling, catecholamine biosynthesis, and additional neuropathology pathways. The close approximation of TH axons at islet α-cells supports a model for sympathetic efferent neurons directly regulating glucagon secretion. Sympathetic islet innervation and intrinsic adrenergic signaling pathways could be novel targets for improving glucagon secretion in T1D.

Human pancreatic afferent and efferent nerves: mapping and 3-D illustration of exocrine, endocrine, and adipose innervation.

The pancreas consists of both the exocrine (acini and ducts) and endocrine (islets) compartments to participate in and regulate the body's digestive and metabolic activities. These activities are subjected to neural modulation, but characterization of the human pancreatic afferent and efferent nerves remains difficult because of the lack of three-dimensional (3-D) image data. Here we prepare transparent human donor pancreases for 3-D histology to reveal the pancreatic microstructure, vasculature, and innervation in a global and integrated fashion. The pancreatic neural network consists of the substance P (SP)-positive sensory (afferent) nerves, the vesicular acetylcholine transporter (VAChT)-positive parasympathetic (efferent) nerves, and the tyrosine hydroxylase (TH)-positive sympathetic (efferent) nerves. The SP+ afferent nerves were found residing along the basal domain of the interlobular ducts. The VAChT+ and TH+ efferent nerves were identified at the peri-acinar and perivascular spaces, which follow the blood vessels to the islets. In the intrapancreatic ganglia, the SP+ (scattered minority, ~7%) and VAChT+ neurons co-localize, suggesting a local afferent-efferent interaction. Compared with the mouse pancreas, the human pancreas differs in 1) the lack of SP+ afferent nerves in the islet, 2) the lower ganglionic density, and 3) the obvious presence of VAChT+ and TH+ nerves around the intralobular adipocytes. The latter implicates the neural influence on the pancreatic steatosis. Overall, our 3-D image data reveal the human pancreatic afferent and efferent innervation patterns and provide the anatomical foundation for future high-definition analyses of neural remodeling in human pancreatic diseases.NEW & NOTEWORTHY Modern three-dimensional (3-D) histology with multiplex optical signals identifies the afferent and efferent innervation patterns of human pancreas, which otherwise cannot be defined with standard histology. Our 3-D image data reveal the unexpected association of sensory and parasympathetic nerves/neurons in the intrapancreatic ganglia and identify the sympathetic and parasympathetic nerve contacts with the infiltrated adipocytes. The multiplex approach offers a new way to characterize the human pancreas in remodeling (e.g., fatty infiltration and duct lesion progression).

Acute pancreatitis decreases the sensitivity of pancreas-projecting dorsal motor nucleus of the vagus neurones to group II metabotropic glutamate receptor agonists in rats.

Recent studies have shown that pancreatic exocrine secretions (PES) are modulated by dorsal motor nucleus of the vagus (DMV) neurones, whose activity is finely tuned by GABAergic and glutamatergic synaptic inputs. Group II metabotropic glutamate receptors (mGluR) decrease synaptic transmission to pancreas-projecting DMV neurones and increase PES. In the present study, we used a combination of in vivo and in vitro approaches aimed at characterising the effects of caerulein-induced acute pancreatitis (AP) on the vagal neurocircuitry modulating pancreatic functions. In control rats, microinjection of bicuculline into the DMV increased PES, whereas microinjections of kynurenic acid had no effect. Conversely, in AP rats, microinjection of bicuculline had no effect, whereas kynurenic acid decreased PES. DMV microinjections of the group II mGluR agonist APDC and whole cell recordings of excitatory currents in identified pancreas-projecting DMV neurones showed a reduced functional response in AP rats compared to controls. Moreover, these changes persisted up to 3 weeks following the induction of AP. These data demonstrate that AP increases the excitatory input to pancreas-projecting DMV neurones by decreasing the response of excitatory synaptic terminals to group II mGluR agonist.

Modulation of pancreatic exocrine and endocrine secretion.

PURPOSE OF REVIEW: Recent advances in the regulation of pancreatic secretion by secretagogues, modulatory proteins and neural pathways are discussed. RECENT FINDINGS: Downstream events involved in secretagogue stimulation of pancreatic secretion have been elucidated through characterization of the Src kinase pathway. An additional mechanism regulating vagus nerve effects on the pancreas involves Group II and III metabotropic glutamate receptors that are located presynaptically on certain vagal pancreas-projecting neurons. Hypothalamic neurons perceive glucose and regulate insulin release by direct communication with islets, and activation of proopiomelanocortin neurons by leptin enhances insulin secretion and modulates glucose but not energy homeostasis. Ghrelin and somatostatin mediate glucose-stimulated insulin secretion by differential receptor signaling that is dependent on the amount of ghrelin and state of receptor heterodimerization. Endoplasmic reticulum (ER) stress and loss-of-function mutations of a key ER stress protein are associated with disruption of membrane translocation and reduction in insulin secretion. The importance of hormones, neuropeptides, amino acids, cytokines and regulatory proteins in pancreatic secretion and the pathophysiology of type 2 diabetes are also discussed. SUMMARY: The biomolecular pathways regulating pancreatic secretions are still not fully understood. New secretagogues and mechanisms continue to be identified and this information will aid in drug discovery and development of new and improved therapy for pancreatic disorders.

The islet-acinar axis of the pancreas: more than just insulin.

Although the role of the islets in the regulation of acinar cell function seemed a mystery to investigators who observed their dispersion among pancreatic acini, over time an appreciation for this intricate and unique structural arrangement has developed. The last three decades have witnessed a steadily growing understanding of the interrelationship of the endocrine and the exocrine pancreas. The islet innervation and vascular anatomy have been more fully characterized and provide an appropriate background for our current understanding. The interrelationship between the endocrine and exocrine pancreas is mediated by islet-derived hormones such as insulin and somatostatin, other humoral factors including pancreastatin and ghrelin, and also neurotransmitters (nitric oxide, peptide YY, substance P, and galanin) released by the nerves innervating the pancreas. Although considerable progress has been achieved, further work is required to fully delineate the complex interplay of the numerous mechanisms involved. This review aims to provide a comprehensive update of the current literature available, bringing together data gleaned from studies addressing the actions of individual hormones, humoral factors, and neurotransmitters on the regulation of amylase secretion from the acinar cell. This comprehensive view of the islet-acinar axis of the pancreas while acknowledging the dominant role played by insulin and somatostatin on exocrine secretion sheds light on the influence of the various neuropeptides on amylase secretion.

Natriuretic peptides as regulatory mediators of secretory activity in the digestive system.

Atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are members of the natriuretic peptide family best known for their role in blood pressure regulation. However, in recent years all the natriuretic peptides and their receptors have been described in the gastrointestinal tract, digestive glands and central nervous system, as well as implicated in the regulation of digestive gland functions. The current review highlights the regulatory role of ANP and CNP in pancreatic and other digestive secretions. ANP and CNP stimulate basal as well as induced pancreatic secretion and modify bicarbonate and chloride secretions. Whereas ANP and CNP exert effects directly on pancreatic cells, CNP also acts through a vago-vagal reflex. At high doses both peptides attenuate pancreatic secretion induced by high doses of secretin through the PLC/PKC pathway. With regards to other digestive secretions, ANP and CNP decrease bile secretion in the rat. ANP does not induce salivation by itself but enhances stimulated salivary secretion and modifies salivary composition in rat parotid as well as submandibular glands. In rat pancreatic, hepatic, parotid and submandibular tissues, the NPR-C receptor mediates mostly peripheral responses whereas NPR-A and NPR-B receptors, which are coupled to guanylate cyclase, likely mediate the central response. In addition, ANP modulates gastric acid secretion via a vagal-dependent mechanism. In the intestine, ANP and CNP decrease water and sodium chloride absorption through an increase in cGMP levels. Overall, these findings indicate that ANP and CNP are members of the large group of regulatory peptides affecting digestive secretions.

Secretion from acinar cells of the exocrine pancreas: role of enteropancreatic reflexes and cholecystokinin.

Although the molecular machinery and mechanism of cell secretion in acinar cells of the exocrine pancreas is well documented and clear, only recently has the pharmacophysiology of pancreatic exocrine secretion come to light. Therefore, we focus in this article on the current understanding of the pharmacophysiology of pancreatic exocrine secretion. The pancreatic secretory response to ingestion of a meal is mediated via a complex interplay of neural, humoral and paracrine mediators. A major role in the control of the intestinal phase of pancreatic secretion is attributed to vago-vagal enteropancreatic reflexes. In the scheme of this control mechanism, afferents originating in the duodenal mucosa, and efferents mediating central input on the pancreatic ganglia, activate intrapancreatic postganglionic neurons. Experiments utilizing specific receptor antagonists demonstrate the involvement of both muscarinic M1 and M3 receptors expressed in pancreatic acinar cells. Cholecystokinin (CCK), originally implicated in the humoral secretion of pancreatic enzymes, through a direct action on acinar CCK receptors, is also essential to the enteropancreatic reflex mechanism. CCK stimulation of the exocrine pancreatic secretion through excitation of sensory afferents of the enteropancreatic reflexes, is a paracrine mode of CCK action, and is probably the only one in humans and the predominant one in rats. In dogs, however, CCK acts on the pancreas via both the humoral and a paracrine route. More recent experiments suggest further possible sites of CCK action. Additionally, at the brain stem, vago-vagal enteropancreatic reflexes may be modulated by input from higher brain centres, particularly the hypothalamic-cholinergic system in the tonic stimulation of preganglionic neurons of the dorsal motor nucleus of the vagus projecting into the pancreas.

Activation of the dorsal vagal nucleus increases pancreatic exocrine secretion in the rat.

Pancreatic secretion is regulated by the dorsal vagal nucleus (DVN) which is modulated by several neurotransmitters and diverse synaptic inputs. The inhibitory neurotransmitter GABA is a major modulator of the vagal output to the gastrointestinal tract. The present study investigated the effects of GABA(A) receptor blockade in the DVN, using bicuculline methiodide (BIM, GABA(A) receptor antagonist, 100 pmol/25 nl), on pancreatic exocrine secretion (PES). Male Sprague-Dawley rats anaesthetised with isoflurane were used in all experiments. PES was collected from the common bile-pancreatic duct and was used to determine the pancreatic protein output (PPO). PES and PPO were measured prior to, and after, microinjection of BIM into the DVN. Bilateral microinjection of BIM into the DVN significantly increased PES and PPO from 23.4+/-3.2 microl/h to 66.1+/-17.5 microl/h and 19.3+/-1.7 microg/h to 35.7+/-3.0 microg/h (P<0.05), respectively. Atropine methonitrate (100 microg/(kg min), i.v.) blocked the excitatory effect of BIM microinjection on PES and PPO. These results suggest that activation of DVN neurons stimulates pancreatic secretion via a cholinergic muscarinic mechanism.

C-type natriuretic peptide stimulates pancreatic exocrine secretion in the rat: role of vagal afferent and efferent pathways.

We previously reported that C-type natriuretic peptide (CNP) increases amylase release in isolated pancreatic acini through natriuretic peptide receptor C activation and enhances pancreatic exocrine secretion via vagal pathways when applied to the brain. In the present study we sought to establish whether CNP was involved in the peripheral regulation of pancreatic secretion. Anesthetized rats were prepared with pancreatic duct cannulation, pyloric ligation and bile diversion into the duodenum. CNP dose-dependently enhanced pancreatic flow, chloride and protein excretion but did not modify bicarbonate output. A selective natriuretic peptide receptor C agonist enhanced pancreatic flow and mimicked CNP-evoked protein output but failed to modify chloride secretion. Truncal vagotomy, perivagal application of capsaicin and hexamethonium reduced CNP-evoked pancreatic flow and abolished chloride excretion but did not affect protein output. Furthermore, pre-treatment with atropine reduced both CNP-stimulated pancreatic flow and chloride excretion but failed to modify protein excretion. Partial muscarinic blockade of CNP-evoked chloride output suggested that mediators other than acetylcholine were involved. However, CNP response was unaltered by cholecystokinin and vasoactive intestinal peptide receptor blockade or by nitric oxide synthase inhibition. In conclusion, CNP-stimulated pancreatic flow through the activation of the natriuretic peptide receptor C and the vago-vagal reflex but it increased protein output only by natriuretic peptide receptor C activation and chloride excretion by vago-vagal reflexes. Present results suggest that CNP may play a role as a local regulator of the exocrine pancreas.

Vagally mediated, nonparacrine effects of cholecystokinin-8s on rat pancreatic exocrine secretion.

Cholecystokinin (CCK) has been proposed to act in a vagally dependent manner to increase pancreatic exocrine secretion via actions exclusively at peripheral vagal afferent fibers. Recent evidence, however, suggests the CCK-8s may also affect brain stem structures directly. We used an in vivo preparation with the aims of 1) investigating whether the actions of intraduodenal casein perfusion to increase pancreatic protein secretion also involved direct actions of CCK at the level of the brain stem and, if so, 2) determining whether, in the absence of vagal afferent inputs, CCK-8s applied to the dorsal vagal complex (DVC) can also modulate pancreatic exocrine secretion (PES). Sprague-Dawley rats (250-400 g) were anesthetized and the common bile-pancreatic duct was cannulated to collect PES. Both vagal deafferentation and pretreatment with the CCK-A antagonist lorglumide on the floor of the fourth ventricle decreased the casein-induced increase in PES output. CCK-8s microinjection (450 pmol) in the DVC significantly increased PES; the increase was larger when CCK-8s was injected in the left side of the DVC. Protein secretion returned to baseline levels within 30 min. Microinjection of CCK-8s increased PES (although to a lower extent) also in rats that underwent complete vagal deafferentation. These data indicate that, as well as activating peripheral vagal afferents, CCK-8s increases pancreatic exocrine secretion via an action in the DVC. Our data suggest that the CCK-8s-induced increases in PES are due mainly to a paracrine effect of CCK; however, a relevant portion of the effects of CCK is due also to an effect of the peptide on brain stem vagal circuits.

'Look for the right mentor'. An interview with Prof. Manfred v. Singer.

Prof. Manfred V. Singer, a pioneer in the field of neuro-hormonal regulation of pancreatic exocrine secretion, is interviewed. He provided the first experimental proof of the existence of neural reflexes that control pancreatic functions. Here, Prof. Singer shares his life experience in pancreatic research and points out key aspects of a successful mentor-mentee relationship.

Neurohormonal control of exocrine pancreatic secretion.

PURPOSE OF REVIEW: Investigations into the neural and hormonal control of pancreatic exocrine function have led to many exciting discoveries over the past year. This review seeks to identify those articles that further our understanding into the complex relation of the varying factors regulating pancreatic secretion. RECENT FINDINGS: Major findings include the new insights into the regulation of the pancreas through receptor-mediated mechanisms, investigations of pancreatic exocytosis, impairment of pancreatic exocrine function by insulin deficiency, the effects of surgical interventions for the treatment of chronic pancreatitis on pancreatic exocrine function, how exocrine function is altered by the cause of acute pancreatitis, and clinical observations relating to management of pancreatic disease and investigations of pancreatic function testing. SUMMARY: Over the past year, substantial new information has been published on the neurohormonal control of pancreatic exocrine function. These data provide insights into the physiology and pathophysiology of pancreatic secretion and diseases of exocrine insufficiency.

Catecholamines and 5-hydroxytryptamine in tissues of the rabbit exocrine pancreas.

OBJECTIVES: Norepinephrine (NE), dopamine (DA), epinephrine (Epi), and 5-hydroxytryptamine (5-HT) all modulate pancreatic exocrine secretion, yet their concentrations in specific tissues of the exocrine pancreas are unknown. METHODS: Concentrations of catecholamines and 5-HT in rabbit pancreatic ganglia, acini, ducts and ampullae, and arteries and veins were measured using HPLC. RESULTS: Concentrations of NE in ganglia from the head/neck region were significantly higher than those from the body (1620 +/- 220 vs. 778 +/- 179 pmol/mg protein). Acini contained little NE, DA, or 5-HT (9 +/- 2, 0.9 +/- 0.2, 13 +/- 5 pmol/mg protein). Ducts and ampullae contained NE (314 +/- 74 and 156 +/- 24 pmol/mg protein), DA (43 +/- 14 and 13 +/- 4 pmol/mg protein), Epi (63 +/- 29 and 39 +/- 6 pmol/mg protein), and 5-HT (696 +/- 151 and 3563 +/- 288 pmol/mg protein). Arteries and veins contained the highest concentrations of NE (1962 +/- 463 and 736 +/- 80 pmol/mg protein, respectively). CONCLUSIONS: Pancreatic ganglia and blood vessels, rather than acini, are the main sites of noradrenergic sympathetic innervation of the rabbit exocrine pancreas. These nerves preferentially target ganglionic transmission in the head/neck versus the body. Serotonergic nerves provide little or no innervation of rabbit pancreatic ganglia or acini.