Cholinergic innervation of the pancreas in the sheep

Neural control of pancreatic peptide hormone secretion

Pancreatic peptide hormone secretion is inextricably linked to maintenance of normal levels of blood glucose. In animals and man, pancreatic peptide hormone secretion is controlled, at least in part, by input from parasympathetic (vagal) premotor neurons that are found principally in the dorsal motor nucleus of the vagus (DMV). Iatrogenic (insulin-induced) hypoglycaemia evokes a homeostatic response commonly referred to as the glucose counter-regulatory response. This homeostatic response is of particular importance in Type 1 diabetes in which episodes of hypoglycaemia are common, debilitating and lead to suboptimal control of blood glucose. Glucagon is the principal counterregulatory hormone but for reasons unknown, its secretion during insulin-induced hypoglycaemia is impaired. Pancreatic parasympathetic neurons are distinguishable electrophysiologically from those that control other (e.g. gastric) functions and are controlled by supramedullary inputs from hypothalamic structures such as the perifornical region. During hypoglycaemia, glucose-sensitive, GABAergic neurons in the ventromedial hypothalamus are inhibited leading to disinhibition of perifornical orexin neurons with projections to the DMV which, in turn, leads to increased secretion of glucagon.

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.

Localisation of S100 Protein and Acetylated Tubulin in Sheep Pancreas

The pancreas plays a critical role in the control of nutritional homeostasis. It consists of two major parts, the exocrine pancreas, and the endocrine pancreas. In the present study S100 protein and acetylated α-tubulin were used to identify positive structures in both the exocrine and endocrine part of the ovine pancreas. In the exocrine part of the pancreas, a positive reaction to S100 protein was confined to centroacinar cells, intercalated, and intralobular ducts cells. In addition, the S100 protein was localized in the Schwann cells of nerve fibres. On the pancreatic islets, the S100 protein has been observed in Schwann cells of nerve axons, where they form a fine envelope that invests the islet surface. Inside the pancreatic islets, the Schwann cells positive for S100 protein envelope the endocrine cells of the islets. The difference in positivity of the S100 protein was found in relation to the endocrine cells. The relationship between endocrine cell positivity and positive exocrine duct cells was discussed. Acetylated α-tubulin (AT) was restricted to axons of the nerve fibres and was located within the connective tissue accompanying intralobular and inter-lobular ducts, and between secretory acini in close contact with secretory cells.

Dual Mode of Action of Acetylcholine on Cytosolic Calcium Oscillations in Pancreatic Beta and Acinar Cells In Situ

Cholinergic innervation in the pancreas controls both the release of digestive enzymes to support the intestinal digestion and absorption, as well as insulin release to promote nutrient use in the cells of the body. The effects of muscarinic receptor stimulation are described in detail for endocrine beta cells and exocrine acinar cells separately. Here we describe morphological and functional criteria to separate these two cell types in situ in tissue slices and simultaneously measure their response to ACh stimulation on cytosolic Ca2+ oscillations [Ca2+]c in stimulatory glucose conditions. Our results show that both cell types respond to glucose directly in the concentration range compatible with the glucose transporters they express. The physiological ACh concentration increases the frequency of glucose stimulated [Ca2+]c oscillations in both cell types and synchronizes [Ca2+]c oscillations in acinar cells. The supraphysiological ACh concentration further increases the oscillation frequency on the level of individual beta cells, inhibits the synchronization between these cells, and abolishes oscillatory activity in acinar cells. We discuss possible mechanisms leading to the observed phenomena.

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.

Pituitary adenylate cyclase-activating peptide-27 (PACAP-27) is co-stored with galanin, substance P and corticotropin releasing factor (CRF) in intrapancreatic ganglia of the sheep

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuropeptide existing in two variant forms (of either 27 or 38 residues), widely present in numerous organs and evoking multiple effects both in the central and peripheral nervous systems. The present study was undertaken to evaluate the distribution pattern of PACAP-27 expression in the ovine pancreas. Using double immunohistochemical stainings co-localizations of PACAP-27 with galanin, SP or CRF were studied in intrapancreatic neurons. In intrapancreatic ganglia, immunoreactivty to PACAP-27 was found in 87.6 ± 5.4% of PGP 9.5-positive intrapancreatic neurons but not in intraganglionic nerve fibres. Numerous PACAP-27-immunoreactive nerve terminals were also observed between pancreatic acini and around small arterioles. No immunoreactivity to PACAP-27 was found in the endocrine pancreas. In 42.9 ± 6.2% of PACAP-27-immunoreactive intrapancreatic neurons the expression of galanin was also found. Statistically lower subpopulation (12.4 ± 4.0%) of intrapancreatic neurons exhibited simultaneously the immunoreactivity to PACAP-27 and SP. The expression of CRF was detected in the relatively smallest group (3.2 ± 1.4%) of PACAP-27-positive intrapancreatic neurons. The present results suggest that in the ovine pancreas PACAP-27 may play an important role as mediator of pancreatic functions. In PACAP-related pancreatic activities, a modulatory role of galanin, SP and to a lower extend of CRF is also likely.

Expression of Neuronal Nitric Oxide Synthase in the Pancreas of the Sheep

In numerous mammals, nitric oxide (NO) influences the activity of the exocrine and endocrine pancreas. In this study, immunocytochemistry was utilized to investigate the expression of neuronal nitric oxide synthase (nNOS) in the pancreas of sheep. In double immunocytochemical staining, the co-localization of nNOS with vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY) or substance P (SP) was studied. The presence of nNOS was confined to the intrapancreatic neurones (9.6 +/- 1.3%) as well as to nerve fibres of the endocrine pancreas and intrapancreatic ganglia. nNOS-immunoreactive (IR) neurones were round and oval in shape and predominantly (83.3 +/- 2.6%) belonged to the middle-size group (25-50 mum). Numerous, fine islets supplying nNOS-IR nerve terminals were devoid of VIP, SP or NPY. Moderately numerous, non-varicose nNOS-IR nerve fibres of intrapancreatic ganglia frequently expressed VIP or NPY, but not SP; 2.2 +/- 0.6% of nNOS-IR intrapancreatic neurones displayed lack of VIP, whereas 7.5 +/- 0.8% were VIP-IR. All nNOS-IR neurones were devoid of SP. The frequencies of nNOS-IR/NPY-IR and nNOS-IR/NPY-negative intrapancreatic neurones were 2.2 +/- 0.4% and 6.1 +/- 1.1%, respectively. Comparison with other mammals indicated that nitrergic innervation of the ovine pancreas is species-determined and may be a reflection of the ruminants' digestion specificity. The possible origin of nNOS-IR nerve fibres and functional significance of NO in the pancreas of sheep were discussed.