Noradrenergic innervation of rabbit pancreatic ganglia

Prenatal development of sympathetic innervation of the human pancreas

BACKGROUND

The sympathetic nervous system plays an important role in the regulation of pancreatic exocrine and endocrine secretion. The results of experimental studies also demonstrate the involvement of the sympathetic nervous system in the regulation of endocrine cell differentiation and islet formation during the development of the pancreas. However, the prenatal development of sympathetic innervation of the human pancreas has not yet been studied.

MATERIAL AND METHODS

Pancreatic autopsy samples from 24 human fetuses were examined using immunohistochemistry with antibodies to tyrosine hydroxylase (TH). The density, concentration, and size (width, length, perimeter and area) of the TH-positive sympathetic nerves were compared in four developmental periods: pre-fetal (8-11 weeks post conception (w.p.c.), n = 6), early fetal (13-20 gestational weeks (g.w.), n = 7), middle fetal (21-28 g.w., n = 6) and late fetal (29-40 g.w., n = 5) using morphometric methods and statistical analysis (Multiple Comparisons p values). Double immunofluorescence with antibodies to TH and either insulin or glucagon and confocal microscopy were applied to analyze the interaction between the sympathetic nerves and endocrine cells, and the co-localization of TH with hormones.

RESULTS

TH-positive sympathetic nerves were detected in the fetal pancreas starting from the early stages (8 w.p.c.). The developmental dynamics of sympathetic nerves was follows: from the pre-fetal period, the amount of TH-positive nerves gradually increased and their branching occurred reaching the highest density and concentration in the middle fetal period, followed by a decrease in these parameters in the late fetal period. From the 14th g.w. onwards, thin TH-positive nerve fibers were mainly distributed in the vicinity of blood vessels and around the neurons of intrapancreatic ganglia, which is similar in adults. There were only rare TH-positive nerve fibers adjacent to acini or located at the periphery of some islets. The close interactions between the TH-positive nerve fibers and endocrine cells were observed in the neuro-insular complexes. Additionally, non-neuronal TH-containing cells were found in the pancreas of fetuses from the pre-fetal and early fetal periods. Some of these cells simultaneously contained glucagon.

CONCLUSIONS

The results demonstrate that sympathetic innervation of the human pancreas, including the formation of perivascular and intraganglionic nerve plexuses, extensively develops during prenatal period, while some processes, such as the formation of sympathetic innervation of islet capillaries, may occur postnatally. Non-neuronal TH-containing cells, as well as the interactions between the sympathetic terminals and endocrine cells observed in the fetal pancreas may be necessary for endocrine pancreas development in humans.

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.

Intrapancreatic Ganglia and Neural Regulation of Pancreatic Endocrine Secretion

Extrapancreatic nerves project to pancreatic islets directly or converge onto intrapancreatic ganglia. Intrapancreatic ganglia constitute a complex information-processing center that contains various neurotransmitters and forms an endogenous neural network. Both intrapancreatic ganglia and extrapancreatic nerves have an important influence on pancreatic endocrine function. This review introduces the histomorphology, innervation, neurochemistry, and electrophysiological properties of intrapancreatic ganglia/neurons, and summarizes the modulatory effects of intrapancreatic ganglia and extrapancreatic nerves on endocrine function.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

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.

Purinergic signalling in the pancreas in health and disease

Pancreatic cells contain specialised stores for ATP. Purinergic receptors (P2 and P1) and ecto-nucleotidases are expressed in both endocrine and exocrine calls, as well as in stromal cells. The pancreas, especially the endocrine cells, were an early target for the actions of ATP. After the historical perspective of purinergic signalling in the pancreas, the focus of this review will be the physiological functions of purinergic signalling in the regulation of both endocrine and exocrine pancreas. Next, we will consider possible interaction between purinergic signalling and other regulatory systems and their relation to nutrient homeostasis and cell survival. The pancreas is an organ exhibiting several serious diseases - cystic fibrosis, pancreatitis, pancreatic cancer and diabetes - and some are associated with changes in life-style and are increasing in incidence. There is upcoming evidence for the role of purinergic signalling in the pathophysiology of the pancreas, and the new challenge is to understand how it is integrated with other pathological processes.

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.

Alpha-adrenergic modulation of synaptic transmission in rabbit pancreatic ganglia

Pancreatic ganglia contain noradrenergic nerve terminals whose role in ganglionic transmission is unknown. Intracellular recordings from rabbit pancreatic neurons were used to study the effects of alpha-adrenergic agonists and antagonists on ganglionic transmission and to determine if endogenously released norepinephrine contributed to synaptic depression. Significant regional differences in alpha adrenergic effects were observed. In neurons from ganglia of the head/neck region norepinephrine or selective alpha(2) agonists presynaptically inhibited ganglionic transmission and this effect was antagonized by the alpha(2) antagonist yohimbine. In the majority of cells membrane hyperpolarization accompanied presynaptic inhibition during superfusion of alpha(2) agonists. Repetitive nerve stimulation evoked a presynaptic post-train depression (PTD) of ganglionic transmission in all neurons tested. A combination of nisoxetine (selective inhibitor of the norepinephrine transporter) and tyramine (releaser of endogenous catecholamines) increased PTD. Pretreatment with clonidine inhibited synaptic transmission and abolished PTD while yohimbine did not affect it. Pretreatment with guanethidine (>or=3.5 h) also failed reduce PTD while neurons unresponsive to alpha(2) adrenoceptor agonists routinely exhibited PTD, implying the presence of other inhibitory neurotransmitters sharing a common presynaptic mechanism with alpha(2) agonists. In the majority of neurons from ganglia of the body region superfusion of norepinephrine or the selective alpha(1) agonist phenylephrine evoked membrane depolarization and facilitated ganglionic transmission. These effects were antagonized by the alpha(1) antagonist prazosin. The remaining neurons exhibited either alpha(2)-mediated synaptic inhibition or no-response. In conclusion, inhibitory alpha(2) and excitatory alpha(1) adrenoceptors exist in pancreatic ganglia and predominate in the head/neck and body, respectively. Norepinephrine, released during repetitive nerve stimulation, may contribute to synaptic depression in the head/neck region and appeared to share a common mechanism with other, unidentified neurotransmitters mediating synaptic depression in both regions. These differences indicate a functional heterogeneity of pancreatic sympathetic innervation that may reflect the reported regional differences in exocrine and endocrine cells.

Short-term synaptic plasticity in rabbit pancreatic ganglia

The extrinsic innervation of the pancreas converges on a plexus of intrinsic pancreatic ganglia whose cholinergic neurons innervate acini, ducts, islets and blood vessels. Therefore, understanding ganglionic transmission is essential for understanding neural control of pancreatic secretion. Intracellular recordings of nicotinic fast excitatory postsynaptic potentials (fEPSPs) and action potentials (APs) were used to characterize and compare transmission in ganglia from the head/neck and body regions of the rabbit pancreas. Paired-pulse facilitation (PPF) or depression (PPD) of fEPSPs was observed in ganglia from both regions with PPF peaking and disappearing at shorter inter-stimulus intervals than PPD. PPF was most frequent in the head/neck (60%) and PPD (50%) in the body. Repetitive stimulation (10 Hz/5 s) evoked multiple forms of mid- and post-train plasticity. Facilitation during the first 1-2 s of train stimulation was reduced or reversed with continued stimulation due to development of synaptic depression and mid-train depression was of greater magnitude in the head/neck region. A brief (approximately 10 s) post-train augmentation was followed by a 1-2 min post-train depression that appeared to result from inhibition of ACh release. Regional differences in the frequency, magnitude, or duration of all forms of synaptic plasticity suggested regional differences in the extrinsic innervation patterns and possibly the function of pancreatic ganglia. In conclusion, rabbit pancreatic ganglia exhibit multiple forms of short-term synaptic plasticity that markedly alter the probability of postsynaptic firing, consistent with these ganglia being critical sites of synaptic integration and autonomic regulation of pancreatic secretion.