A novel preparation to study rat pancreatic spinal and vagal mechanosensitive afferents in vitro

Unravelling innervation of pancreatic islets

The central and peripheral nervous systems play critical roles in regulating pancreatic islet function and glucose metabolism. Over the last century, in vitro and in vivo studies along with examination of human pancreas samples have revealed the structure of islet innervation, investigated the contribution of sympathetic, parasympathetic and sensory neural pathways to glucose control, and begun to determine how the structure and function of pancreatic nerves are disrupted in metabolic disease. Now, state-of-the art techniques such as 3D imaging of pancreatic innervation and targeted in vivo neuromodulation provide further insights into the anatomy and physiological roles of islet innervation. Here, we provide a summary of the published work on the anatomy of pancreatic islet innervation, its roles, and evidence for disordered islet innervation in metabolic disease. Finally, we discuss the possibilities offered by new technologies to increase our knowledge of islet innervation and its contributions to metabolic regulation.

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.

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.

Neural plasticity in pancreatitis and pancreatic cancer

Pancreatic nerves undergo prominent alterations during the evolution and progression of human chronic pancreatitis and pancreatic cancer. Intrapancreatic nerves increase in size (neural hypertrophy) and number (increased neural density). The proportion of autonomic and sensory fibres (neural remodelling) is switched, and are infiltrated by perineural inflammatory cells (pancreatic neuritis) or invaded by pancreatic cancer cells (neural invasion). These neuropathic alterations also correlate with neuropathic pain. Instead of being mere histopathological manifestations of disease progression, pancreatic neural plasticity synergizes with the enhanced excitability of sensory neurons, with Schwann cell recruitment toward cancer and with central nervous system alterations. These alterations maintain a bidirectional interaction between nerves and non-neural pancreatic cells, as demonstrated by tissue and neural damage inducing neuropathic pain, and activated neurons releasing mediators that modulate inflammation and cancer growth. Owing to the prognostic effects of pain and neural invasion in pancreatic cancer, dissecting the mechanism of pancreatic neuroplasticity holds major translational relevance. However, current in vivo models of pancreatic cancer and chronic pancreatitis contain many discrepancies from human disease that overshadow their translational value. The present Review discusses novel possibilities for mechanistically uncovering the role of the nervous system in pancreatic disease progression.

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.

Pancreatic insulin and exocrine secretion are under the modulatory control of distinct subpopulations of vagal motoneurones in the rat

Brainstem vago-vagal neurocircuits modulate upper gastrointestinal functions. Derangement of these sensory-motor circuits is implicated in several pathophysiological states, such as gastroesophageal reflux disease (GERD), functional dyspepsia and, possibly, pancreatitis. While vagal circuits controlling the stomach have received more attention, the organization of brainstem pancreatic neurocircuits is still largely unknown. We aimed to investigate the in vitro and in vivo modulation of brainstem vagal circuits controlling pancreatic secretion. Using patch clamp techniques on identified vagal pancreas-projecting neurones, we studied the effects of metabotropic glutamate receptor (mGluR) agents in relation to the effects of exendin-4, a glucagon-like peptide 1 analogue, cholecystokinin (CCK) and pancreatic polypeptide (PP). An in vivo anaesthetized rat preparation was used to measure pancreatic exocrine secretion (PES) and plasma insulin following microinjection of metabotropic glutamate receptor (mGluR) agonists and exendin-4 in the brainstem. Group II and III mGluR agonists (2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4), respectively) decreased the frequency of miniature inhibitory and excitatory postsynaptic currents (mIPSCs and mEPSCs, respectively) in the majority of the neurones tested. All neurones responsive to L-AP4 were also responsive to APDC, but not vice versa. Further, in neurones where L-AP4 decreased mIPSC frequency, exendin-4 increased, while PP had no effect upon, mIPSC frequency. Brainstem microinjection of APDC or L-AP4 decreased plasma insulin secretion, whereas only APDC microinjections increased PES. Exendin-4 microinjections increased plasma insulin. Our results indicate a discrete organization of vagal circuits, which opens up promising avenues of research aimed at investigating the physiology of homeostatic autonomic neurocircuits.

Pancreatic nociception--revisiting the physiology and pathophysiology

BACKGROUND

Pain management of many pancreatic diseases remains a major clinical concern. This problem reflects our poor understanding of pain signaling from the pancreas.

OBJECTIVES

This review provides an overview of our current knowledge, with emphasis on current pain management strategies and recent experimental findings.

METHODS

A systematic search of the scientific literature was carried out using EMBASE, PubMed/MEDLINE, and the Cochrane Central Register of Controlled Trials for the years 1965-2011 to obtain access to all publications, especially randomized controlled trials, systematic reviews, and meta-analyses exploring pain and its management in disease states such as acute pancreatitis (AP), chronic pancreatitis (CP) and pancreatic cancer (PC).

RESULTS

Over the last decade, numerous molecular mediators such as nerve growth factor and the transient receptor potential (TRP) cation channel family have been implicated in afferent nerve signaling. More recent animal studies have indicated the location of the receptive fields for the afferent nerves in the pancreas and shown that these are activated by agents including cholecystokinin octapeptide, 5-hydroxytryptamine and bradykinin. Studies with PC specimens have shown that neuro-immune interactions occur and numerous agents including TRP cation channel V1, artemin and fractalkine have been implicated. Experimental studies in the clinical setting have demonstrated impairment of inhibitory pain modulation from supraspinal structures and implicated neuropathic pain mechanisms.

CONCLUSIONS

Our knowledge in this area remains incomplete. Characterization of the mediators and receptors/ion channels on the sensory nerve terminals are required in order to facilitate the development of new pharmaceutical treatments for AP and CP.

Nerve growth factor modulates TRPV1 expression and function and mediates pain in chronic pancreatitis

BACKGROUND & AIMS

The pathogenesis of pain in chronic pancreatitis (CP) is poorly understood and treatment remains difficult. We hypothesized that nerve growth factor (NGF) plays a key role in this process via its effects on the transient receptor potential vanilloid 1, TRPV1.

METHODS

CP was induced by intraductal injection of trinitrobenzene sulfonic acid in rats. After 3 weeks, anti-NGF antibody or control serum was administered daily for 1 week. Pancreatic hyperalgesia was assessed by nocifensive behavioral response to electrical stimulation of the pancreas as well as by referred somatic pain assessed by von Frey filament testing. TRPV1 currents in pancreatic sensory neurons were examined by patch-clamp. The expression and function of TRPV1 in pancreas-specific nociceptors was examined by immunostaining and quantification of messenger RNA levels.

RESULTS

Blockade of NGF significantly attenuated pancreatic hyperalgesia and referred somatic pain compared with controls. It also decreased TRPV1 current density and open probability and reduced the proportion of pancreatic sensory neurons that expressed TRPV1 as well as levels of TRPV1 in these neurons.

CONCLUSIONS

These findings emphasize a key role for NGF in pancreatic pain and highlight the role it plays in the modulation of TRPV1 expression and activity in CP.

Experimental models of visceral pain

Visceral pain models are used to study afferent nerve traffic during noxious stimulation at the level of the visceral organ. This chapter provides details on several in vitro and in vivo models of organs in the gastrointestinal and genitourinary tract that use electrophysiological recordings of afferent nerve fibres in order to directly characterize stimulus-response relationships. These models can also be used to investigate stimulus-response patterns during physiological (nonpainful) stimulation of the visceral organs or during exposure to pathological stimuli, such as inflammatory mediators during inflammation of the visceral organ.

Astrocytic activation in thoracic spinal cord contributes to persistent pain in rat model of chronic pancreatitis

One of the most important symptoms in chronic pancreatitis (CP) is constant and recurrent abdominal pain. However, there is still no ideal explanation and treatment on it. Previous studies indicated that pain in CP shared many characteristics of neuropathic pain. As an important mechanism underlying neuropathic pain, astrocytic activation is probably involved in pain of CP. Based on the trinitrobenzene sulfonic acid (TNBS)-induce rat CP model, we performed pancreatic histology to assess the severity of CP with semiquantitative scores and tested the nociceptive behaviors following induction of CP. Glial fibrillary acidic protein (GFAP) expressions in the thoracic spinal cord were observed by immunohistochemistry and real-time reverse transcription polymerase chain reaction (RT-PCR). Meanwhile, we injected intrathecally astrocytic specific inhibitor l-alpha-aminoadipate (LAA) and observed its effect on nociception induced by CP. Compared to the naive and sham group, TNBS produced long lasting pancreatitis, and persistent mechanical hypersensitivity in the abdomen that was evident 1 week after TNBS infusion and persisted up to 5 weeks. Compared with naive or sham operated rats, GFAP staining was significantly increased 5 weeks after CP induction. Real-time RT-PCR indicated that GFAP expression was significantly increased in TNBS treated rats compared to the sham group. TNBS-induced astrocytic activation was significantly attenuated by LAA, compared with the saline control. Treatment with LAA significantly, even though not completely, attenuated the allodynia. Our results provide for the first time that astrocytes may play a critical role in pain of CP. Some actions could be taken to prevent astrocytic activation to treat pain in CP patients.

Galanin inhibits caerulein-stimulated pancreatic amylase secretion via cholinergic nerves and insulin

Pancreatic exocrine secretion is affected by galanin, but the mechanisms involved are unclear. We aimed to determine the effect and elucidate the mechanism of action of exogenous galanin on basal and stimulated pancreatic amylase secretion in vitro. The effect of galanin on basal-, carbachol-, and caerulein-stimulated amylase secretion from isolated murine pancreatic lobules was measured. Carbachol and caerulein concentration-response relationships were established. Lobules were coincubated with galanin (10(-12) M to 10(-7) M), carbachol (10(-6) M), or caerulein (10(-10) M). Lobules were preincubated with atropine (10(-5) M), tetrodotoxin (10(-5) M), hexamethonium (10(-5) M), or diazoxide (10(-7) M and 10(-4) M) for 30 min followed by incubation with caerulein (10(-10) M) alone or combined with galanin (10(-12) M). Amylase secretion was expressed as percent of total lobular amylase. Immunohistochemical studies used the antigen retrieval technique and antisera for galanin receptor (GALR) 1, 2, and 3. Carbachol and caerulein stimulated amylase secretion in a concentration-dependent manner with maximal responses of two- and 1.7-fold over control evoked at 10(-6) M and 10(-10) M, respectively. Galanin (10(-12) M) completely inhibited caerulein-stimulated amylase secretion but had no effect on carbachol-stimulated or basal secretion. Atropine and tetrodotoxin pretreatment abolished the caerulein-stimulated amylase secretion, whereas hexamethonium had no significant effect. Diazoxide significantly reduced caerulein-stimulated amylase secretion by approximately 80%. Galanin did not affect caerulein-stimulated amylase secretion in the presence of hexamethonium or diazoxide. Glucose-stimulated amylase secretion was also inhibited by galanin. Immunohistochemistry revealed islet cells labeled for GALR2. These data suggest that galanin may modulate caerulein-stimulated amylase secretion by acting on cholinergic nerves and/or islet cells possibly via GALR2 to regulate insulin release.