Substance P inhibits pancreatic exocrine secretion via a neural mechanism

Mapping and targeted viral activation of pancreatic nerves in mice reveal their roles in the regulation of glucose metabolism

A lack of comprehensive mapping of ganglionic inputs into the pancreas and of technology for the modulation of the activity of specific pancreatic nerves has hindered the study of how they regulate metabolic processes. Here we show that the pancreas-innervating neurons in sympathetic, parasympathetic and sensory ganglia can be mapped in detail by using tissue clearing and retrograde tracing (the tracing of neural connections from the synapse to the cell body), and that genetic payloads can be delivered via intrapancreatic injection to target sites in efferent pancreatic nerves in live mice through optimized adeno-associated viruses and neural-tissue-specific promoters. We also show that, in male mice, the targeted activation of parasympathetic cholinergic intrapancreatic ganglia and neurons doubled plasma-insulin levels and improved glucose tolerance, and that tolerance was impaired by stimulating pancreas-projecting sympathetic neurons. The ability to map the peripheral ganglia innervating the pancreas and to deliver transgenes to specific pancreas-projecting neurons will facilitate the examination of ganglionic inputs and the study of the roles of pancreatic efferent innervation in glucose metabolism.

Roles of the nervous system in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC), with its extremely poor prognosis, presents a substantial health problem worldwide. Outcomes have improved thanks to progress in surgical technique, chemotherapy, pre-/postoperative management, and centralization of patient care to high-volume centers. However, our goals are yet to be met. Recently, exome sequencing using PDAC surgical specimens has demonstrated that the most frequently altered genes were the axon guidance genes, indicating involvement of the nervous system in PDAC carcinogenesis. Moreover, perineural invasion has been widely identified as one poor prognostic factor. The combination of innovative technologies and extensive clinician experience with the nervous system come together here to create a new treatment option. However, evidence has emerged that suggests that the relationship between cancer and nerves in PDAC, the underlying mechanism, is not fully understood. In an attempt to tackle this lethal cancer, this review summarizes the anatomy and physiology of the pancreas and discusses the role of the nervous system in the pathophysiology of PDAC.

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.

Ketamine and xylazine effects in murine model of acute pancreatitis

Ketamine and xylazine (Ket/Xyl) are anesthetic agents that target neural pathways and are commonly used in combination in mouse studies. Since neural pathways can modulate acute pancreatitis severity, we asked if Ket/Xyl affect disease severity. C57BL/6 mice were treated with six hourly injections of cerulein to induce mild acute pancreatitis. Mice were also treated with and without ketamine, xylazine, and Ket/Xyl before pancreatitis induction in vivo and in vitro. Ket/Xyl pretreatment in vivo increased selected parameters of pancreatitis severity such as trypsin activity and edema; these effects were predominantly mediated by xylazine. Ket/Xyl also changed markers of autophagy. These in vivo effects of Ket/Xyl were not attenuated by atropine. The drugs had no little to no effect on pancreatitis responses in isolated pancreatic cells or lobules. These findings suggest that Ket/Xyl administration can have substantial effect on acute pancreatitis outcomes through nonmuscarinic neural pathways. Given widespread use of this anesthetic combination in experimental animal models, future studies of inflammation and injury using Ket/Xyl should be interpreted with caution.NEW & NOTEWORTHY Ketamine and xylazine anesthetic agent administration before acute pancreatitis induction in mice lead to changes in pancreatitis responses independent of acute pancreatitis induction. Future studies should consider the potential effects of anesthesia administration when studying disease processes associated with inflammation and injury.

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.

Expression of NK-1 and NK-3 tachykinin receptors in pancreatic acinar cells after acute experimental pancreatitis in rats

Activation of neurokinin (NK)-1 receptors but not of NK-3 stimulates amylase release from isolated pancreatic acini of the rat. Immunofluorescence studies show that NK-1 receptors are more strongly expressed than NK-3 receptors on pancreatic acinar cells under basal conditions. No studies have examined the expression of the two NK receptor populations in pancreatic acini during pancreatitis in rats. We therefore investigated the relationships between expression of these two tachykinin receptors and experimental acute pancreatitis induced by stimulating pancreatic amylase with caerulein (CK) in rats. Hyperstimulation of the pancreas by CK caused an increase in plasma amylase and pancreatic water content and resulted in morphological evidence of cytoplasmic vacuolization. Immunofluorescence analysis revealed a similar percentage of NK-1 receptor antibody immunoreactive acinar cells in rats with pancreatitis and in normal rat tissue but a larger percentage of NK-3 receptor immunoreactive cells in acute pancreatitis than in normal pancreas. Western blot analysis of NK-1 and NK-3 receptor protein levels after CK-induced pancreatitis showed no change in NK-1 receptors but a stronger increase in NK-3 receptor expression in pancreatic acini compared with normal rats thus confirming the immunofluorescence data. These new findings support previous evidence that substance P-mediated functions within the pancreas go beyond sensory signal transduction contributing to neurogenic inflammation, and they suggest that substance P plays a role in regulating pancreatic exocrine secretion via acinar NK-1 receptors. The significant increase in NK-3 receptors during pancreatic stimulation suggests that NK-3 receptors also intervene in the pathogenesis of mild acute pancreatitis in rats.

Relationship between overexpression of NK-1R, NK-2R and intestinal mucosal damage in acute necrotizing pancreatitis

AIM: To study the expression of neurokinin-1 receptor (NK-1R) and neurokinin-2 receptor (NK-2R) in distal ileum of acute necrotizing pancreatitis (ANP) and to evaluate the relationship between expression of these two receptors and intestinal mucosal damage.

METHODS: A total of 130 adult Sprague-Dawley rats were randomly divided into two groups: the rats in ANP group (n = 80) were induced by the retrograde intraductal infusion of 30 g·L^-1 sodium taurocholate. And the rats in normal control group (n = 50) received laparotomy only. Sacrifices were made 6 h, 12 h, 24 h and 48 h later in ANP and normal control group after induction respectively. Intestinal mucosal permeability was studied by intrajejunal injection of 1.5mCi radioactive isotope ^99mTc-diethlene triamine pentacetic acid (DTPA) and the radioactivity of ^99mTc-DTPA content in urine was measured 6 h, 12 h, 24 h and 48 h after induction. Then the pancreas and intestine were prepared for pathology. Reverse transcription polymerase chain reaction (RT-PCR) was used to determine the mRNA expression of NK-1R and NK-2R, and Western blot was used to investigate the protein level of NK-1R and NK-2R.

RESULTS: In ANP rats, serious histologic damages in intestinal mucosa were observed, and the radioactivity of ^99mTc-DTPA in urine increased significantly in the ANP group. RT-PCR revealed that NK-1R and NK-2R mRNA level was overexpressed in the distal ileum of ANP as compared with the normal control group. Western blot discovered stronger NK-1R (14-fold increase) and NK-2R (9-fold increase) immunoreactivity in the intestinal mucosa of ANP rats. Moreover, the overexpression of NK-1R was associated with mucosal pathological score (r = 0.77, P < 0.01) and intestinal permeability (r = 0.68, P < 0.01) in ANP rats.

CONCLUSION: NK-1R and NK-2R contribute to disrupted neuropeptides loop balance, deteriorate intestinal damage, and are involved in pathophysiological changes in ANP.

Neurohormonal control of pancreatic exocrine secretion

The neurohormonal control of pancreatic exocrine secretion is a complex interaction of multiple pathways involving a large number of gut hormones, neurotransmitters, and neuropeptides. Recent studies have elucidated a role for cholecystokinin in the regulation of bicarbonate and fluid secretion from pancreatic duct cells and suggested that cholecystokinin stimulation of human pancreatic acinar cells is likely regulated by an indirect mechanism of stimulation of afferent neurons. Evidence supports the regulation of potassium channels in rat pancreatic acinar cells by the cyclic AMP-mediated agonist secretin. Mechanisms for the regulation of cholecystokinin and secretin release by releasing factors have also been elucidated. The area postrema has been implicated in the mediation of inhibition of pancreatic secretion by the gut hormones peptide YY and pancreatic polypeptide. The neurotransmitter serotonin has been demonstrated to play a role in acid-induced secretin release and in pancreatic secretion stimulated by luminal factors. The regulation of pancreatic exocrine secretion by purines, nitric oxide, and gamma-aminobutyric acid as well as by the neuropeptides pituitary adenylate cyclase-activating peptide, gastrin-releasing peptide, and substance P is reviewed. The role of the central nervous system in modulating pancreatic secretion is also described. This review highlights the recent advances in knowledge of the neurohormonal regulation of pancreatic exocrine secretion.

NK-1 receptor desensitization and neutral endopeptidase terminate SP-induced pancreatic plasma extravasation

Substance P (SP) induces plasma extravasation and neutrophil infiltration by activating the neurokinin-1 receptor (NK1-R). We characterized the mechanisms regulating this response in the rat pancreas. Anesthetized rats were continuously infused with SP, and plasma extravasation was quantified using Evans blue (EB) dye. Continuous infusion of SP (8 nmol. kg(-1). h(-1)) resulted in a threshold increase in EB at 15 min, a peak effect at 30 min (150% increase), and a return to baseline by 60 min. The NK1-R antagonist CP-96,345 blocked SP-induced plasma extravasation. After 60 min, the NK1-R was desensitized to agonist challenge. Resensitization was first detected at 20 min and increased until full recovery was seen at 30 min. Inhibition of the cell-surface protease neutral endopeptidase (NEP) by phosphoramidon potentiated the effect of exogenous SP; therefore endogenous NEP attenuates SP-induced plasma extravasation. Thus the continuous infusion of SP stimulates plasma extravasation in the rat pancreas via activation of the NK1-R, and these effects are terminated by both desensitization of the NK1-R and the cell-surface protease NEP.

Substance P mediates inflammatory oedema in acute pancreatitis via activation of the neurokinin-1 receptor in rats and mice

Pancreatic oedema occurs early in the development of acute pancreatitis, and the overall extent of fluid loss correlates with disease severity. The tachykinin substance P (SP) is released from sensory nerves, binds to the neurokinin-1 receptor (NK1-R) on endothelial cells and induces plasma extravasation, oedema, and neutrophil infiltration, a process termed neurogenic inflammation. We sought to determine the importance of neurogenic mechanisms in acute pancreatitis. Pancreatic plasma extravasation was measured using the intravascular tracers Evans blue and Monastral blue after administration of specific NK1-R agonists/antagonists in rats and NK1-R(+/+)/(-/-) mice. The effects of NK1-R genetic deletion/antagonism on pancreatic plasma extravasation, amylase, myeloperoxidase (MPO), and histology in cerulein-induced pancreatitis were characterized. In rats, both SP and the NK1-R selective agonist [Sar(9) Met(O(2))(11)]SP stimulated pancreatic plasma extravasation, and this response was blocked by the NK1-R antagonist CP 96,345. Selective agonists of the NK-2 or NK-3 receptors had no effect. In rats, cerulein stimulated pancreatic plasma extravasation and serum amylase. These responses were blocked by the NK1-R antagonist CP 96,345. In wildtype mice, SP induced plasma extravasation while SP had no effect in NK1-R knockout mice. In NK1-R knockout mice, the effects of cerulein on pancreatic plasma extravasation and hyperamylasemia were reduced by 60%, and pancreatic MPO by 75%, as compared to wildtype animals. Neurogenic mechanisms of inflammation are important in the development of inflammatory oedema in acute interstitial pancreatitis.