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

Glucose-sensing glucagon-like peptide-1 receptor neurons in the dorsomedial hypothalamus regulate glucose metabolism

Glucagon-like peptide-1 (GLP-1) regulates energy homeostasis via activation of the GLP-1 receptors (GLP-1Rs) in the central nervous system. However, the mechanism by which the central GLP-1 signal controls blood glucose levels, especially in different nutrient states, remains unclear. Here, we defined a population of glucose-sensing GLP-1R neurons in the dorsomedial hypothalamic nucleus (DMH), by which endogenous GLP-1 decreases glucose levels via the cross-talk between the hypothalamus and pancreas. Specifically, we illustrated the sufficiency and necessity of DMHGLP-1R in glucose regulation. The activation of the DMHGLP-1R neurons is mediated by a cAMP-PKA-dependent inhibition of a delayed rectifier potassium current. We also dissected a descending control of DMHGLP-1R -dorsal motor nucleus of the vagus nerve (DMV)-pancreas activity that can regulate glucose levels by increasing insulin release. Thus, our results illustrate how central GLP-1 action in the DMH can induce a nutrient state-dependent reduction in blood glucose level.

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.

Vagal neurocircuitry and its influence on gastric motility

A large body of research has been dedicated to the effects of gastrointestinal peptides on vagal afferent fibres, yet multiple lines of evidence indicate that gastrointestinal peptides also modulate brainstem vagal neurocircuitry, and that this modulation has a fundamental role in the physiology and pathophysiology of the upper gastrointestinal tract. In fact, brainstem vagovagal neurocircuits comprise highly plastic neurons and synapses connecting afferent vagal fibres, second order neurons of the nucleus tractus solitarius (NTS), and efferent fibres originating in the dorsal motor nucleus of the vagus (DMV). Neuronal communication between the NTS and DMV is regulated by the presence of a variety of inputs, both from within the brainstem itself as well as from higher centres, which utilize an array of neurotransmitters and neuromodulators. Because of the circumventricular nature of these brainstem areas, circulating hormones can also modulate the vagal output to the upper gastrointestinal tract. This Review summarizes the organization and function of vagovagal reflex control of the upper gastrointestinal tract, presents data on the plasticity within these neurocircuits after stress, and discusses the gastrointestinal dysfunctions observed in Parkinson disease as examples of physiological adjustment and maladaptation of these reflexes.

Exposure to a high fat diet during the perinatal period alters vagal motoneurone excitability, even in the absence of obesity

KEY POINTS

Obesity is recognized as being multifactorial in origin, involving both genetic and environmental factors. The perinatal period is known to be critically important in the development of neural circuits responsible for energy homeostasis and the integration of autonomic reflexes. Diet-induced obesity alters the biophysical, pharmacological and morphological properties of vagal neurocircuits regulating upper gastrointestinal tract functions, including satiety. Less information is available, however, regarding the effects of a high fat diet (HFD) itself on the properties of vagal neurocircuits. The present study was designed to test the hypothesis that exposure to a HFD during the perinatal period alters the electrophysiological, pharmacological and morphological properties of vagal efferent motoneurones innervating the stomach. Our data indicate that perinatal HFD decreases the excitability of gastric-projecting dorsal motor nucleus neurones and dysregulates neurotransmitter release from synaptic inputs and that these alterations occur prior to the development of obesity. These findings represent the first direct evidence that exposure to a HFD modulates the processing of central vagal neurocircuits even in the absence of obesity. The perinatal period is critically important to the development of autonomic neural circuits responsible for energy homeostasis. Vagal neurocircuits are vital to the regulation of upper gastrointestinal functions, including satiety. Diet-induced obesity modulates the excitability and responsiveness of both peripheral vagal afferents and central vagal efferents but less information is available regarding the effects of diet per se on vagal neurocircuit functions. The aims of this study were to investigate whether perinatal exposure to a high fat diet (HFD) dysregulated dorsal motor nucleus of the vagus (DMV) neurones, prior to the development of obesity. Whole cell patch clamp recordings were made from gastric-projecting DMV neurones in thin brainstem slices from rats that were exposed to either a control diet or HFD from pregnancy day 13. Our data demonstrate that following perinatal HFD: (i) DMV neurones had decreased excitability and input resistance with a reduced ability to fire action potentials; (ii) the proportion of DMV neurones excited by cholecystokinin (CCK) was unaltered but the proportion of neurones in which CCK increased excitatory glutamatergic synaptic inputs was reduced; (iii) the tonic activation of presynaptic group II metabotropic glutamate receptors on inhibitory nerve terminals was attenuated, allowing modulation of GABAergic synaptic transmission; and (iv) the size and dendritic arborization of gastric-projecting DMV neurones was increased. These results suggest that perinatal HFD exposure compromises the excitability and responsiveness of gastric-projecting DMV neurones, even in the absence of obesity, suggesting that attenuation of vago-vagal reflex signalling may precede the development of obesity.

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.

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.

Role of the vagus in the reduced pancreatic exocrine function in copper-deficient rats

Copper plays an essential role in the function and development of the central nervous system and exocrine pancreas. Dietary copper limitation is known to result in noninflammatory atrophy of pancreatic acinar tissue. Our recent studies have suggested that vagal motoneurons regulate pancreatic exocrine secretion (PES) by activating selective subpopulations of neurons within vagovagal reflexive neurocircuits. We used a combination of in vivo, in vitro, and immunohistochemistry techniques in a rat model of copper deficiency to investigate the effects of a copper-deficient diet on the neural pathways controlling PES. Duodenal infusions of Ensure or casein, as well as microinjections of sulfated CCK-8, into the dorsal vagal complex resulted in an attenuated stimulation of PES in copper-deficient animals compared with controls. Immunohistochemistry of brain stem slices revealed that copper deficiency reduced the number of tyrosine hydroxylase-immunoreactive, but not neuronal nitric oxide synthase- or choline acetyltransferase-immunoreactive, neurons in the dorsal motor nucleus of the vagus (DMV). Moreover, a copper-deficient diet reduced the number of large (>11 neurons), but not small, intrapancreatic ganglia. Electrophysiological recordings showed that DMV neurons from copper-deficient rats are less responsive to CCK-8 or pancreatic polypeptide than are DMV neurons from control rats. Our results demonstrate that copper deficiency decreases efferent vagal outflow to the exocrine pancreas. These data indicate that the combined selective loss of acinar pancreatic tissue and the decreased excitability of efferent vagal neurons induce a deficit in the vagal modulation of PES.

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.

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.

Vanilloid, purinergic, and CCK receptors activate glutamate release on single neurons of the nucleus tractus solitarius centralis

Baroreceptor inputs to nucleus of the tractus solitarius medialis (mNTS) neurons can be differentiated, among other features, by their response to vanilloid or purinergic agonists, active only on C- or A-fibers, respectively. A major aim of this study was to examine whether neurons of NTS centralis (cNTS), a subnucleus dominated by esophageal inputs, exhibit a similar dichotomy. Since it has been suggested that cholecystokinin (CCK), exerts its gastrointestinal (GI)-related effects via paracrine activation of vagal afferent C-fibers, we tested whether CCK-sensitive fibers impinging upon cNTS neurons are responsive to vanilloid but not purinergic agonists. Using whole cell patch-clamp recordings from cNTS, we recorded miniature excitatory postsynaptic currents (mEPSCs) to test the effects of the vanilloid agonist capsaicin, the purinergic agonist α,β-methylene-ATP (α,β-Met-ATP), and/or CCK-octapeptide (CCK-8s). α,β-Met-ATP, capsaicin; and CCK-8s increased EPSC frequency in 37, 71, and 46% of cNTS neurons, respectively. Approximately 30% of cNTS neurons were responsive to both CCK-8s and α,β-Met-ATP, to CCK-8s and capsaicin, or to α,β-Met-ATP and capsaicin, while 32% of neurons were responsive to all three agonists. All neurons responding to either α,β-Met-ATP or CCK-8s were also responsive to capsaicin. Perivagal capsaicin, which is supposed to induce a selective degeneration of C-fibers, decreased the number of cNTS neurons responding to capsaicin or CCK-8s but not those responding to α,β-Met-ATP. In summary, GI inputs to cNTS neurons cannot be distinguished on the basis of their selective responses to α,β-Met-ATP or capsaicin. Our data also indicate that CCK-8s increases glutamate release from purinergic and vanilloid responsive fibers impinging on cNTS neurons.

Neuropeptide Y receptor subtypes in the dorsal vagal complex under acute feeding adaptation in the adult rat

Neuropeptide Y (NPY), Peptide YY (PYY) and pancreatic polypeptides (PPs) belong to the same peptide family called the Y or NPY family. Central and peripheral injections of these peptides are implicated in the regulation of food intake at the level of the hypothalamus (central effects; increased food intake) and dorsal vagal complex (DVC) (peripheral effects; decreased food intake). The DVC of the brainstem is a satiety reflex key region, which includes the nucleus tractus solitarius (NTS), area postrema (AP) and dorso motor nucleus of the vagus (DMX). NPY binding sites were quantified on serial DVC sections using in vitro receptor autoradiography in two feeding adaptation models: fasting and inflammatory anorexia. Receptor autoradiography revealed that Y(1), Y(2), Y(4) and Y(5) receptor subtypes are present in all nuclei of the DVC. Additionally, we also observed significant amount of specific labelling remaining even after having blocked all known NPY receptor subtypes targeted by radioligands such as [(125)I][Leu(31), Pro(34)]PYY, [(125)I]PYY3-36 and [(125)I]hPP. This binding is referred as an atypical NPY site. Lipopolysaccharide (LPS) injection and food deprivation (24-48h) did not induce any change in the expression of NPY Y(1), Y(2,) Y(4) and Y(5) receptors at the level of the NTS and DMX. However, a significant decrease in [(125)I]PYY3-36/Y(2) and [(125)I]hPP/Y(4)- and Y(5)-insensitive binding sites (residual or atypical site) was observed in the AP. Together, these data could suggest that residual or atypical NPY binding site in the AP is modulated by food deprivation and may be physiologically relevant and implicated in feeding behaviors.

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.

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

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

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).

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.