A histochemical study of neuro-insular complexes in the pancreas of the rat

Neural pathways that control the glucose counterregulatory response

Glucose is an essential metabolic substrate for all bodily tissues. The brain depends particularly on a constant supply of glucose to satisfy its energy demands. Fortunately, a complex physiological system has evolved to keep blood glucose at a constant level. The consequences of poor glucose homeostasis are well-known: hyperglycemia associated with uncontrolled diabetes can lead to cardiovascular disease, neuropathy and nephropathy, while hypoglycemia can lead to convulsions, loss of consciousness, coma, and even death. The glucose counterregulatory response involves detection of declining plasma glucose levels and secretion of several hormones including glucagon, adrenaline, cortisol, and growth hormone (GH) to orchestrate the recovery from hypoglycemia. Low blood glucose leads to a low brain glucose level that is detected by glucose-sensing neurons located in several brain regions such as the ventromedial hypothalamus, the perifornical region of the lateral hypothalamus, the arcuate nucleus (ARC), and in several hindbrain regions. This review will describe the importance of the glucose counterregulatory system and what is known of the neurocircuitry that underpins it.

The ductal origin of structural and functional heterogeneity between pancreatic islets

Islets form in the pancreas after the first endocrine cells have arisen as either single cells or small cell clusters in the epithelial cords. These cords constitute the developing pancreas in one of its earliest recognizable stages. Islet formation begins at the time the cords transform into a branching ductal system, continues while the ductal system expands, and finally stops before the exocrine tissue of ducts and acini reaches its final expansion. Thus, islets continuously arise from founder cells located in the branching and ramifying ducts. Islets arising from proximal duct cells locate between the exocrine lobules, develop strong autonomic and sensory innervations, and pass their blood to efferent veins (insulo-venous efferent system). Islets arising from cells of more distal ducts locate within the exocrine lobules, respond to nerve impulses ending at neighbouring blood vessels, and pass their blood to the surrounding acini (insulo-acinar portal system). Consequently, the section of the ductal system from which an islet arises determines to a large extent its future neighbouring tissue, architecture, properties, and functions. We note that islets interlobular in position are frequently found in rodents (rats and mice), whereas intralobularly-located, peripheral duct islets prevail in humans and cattle. Also, we expound on bovine foetal Laguesse islets as a prominent foetal type of type 1 interlobular neuro-insular complexes, similar to neuro-insular associations frequently found in rodents. Finally, we consider the probable physiological and pathophysiological implications of the different islet positions within and between species.

Islet organogenesis, angiogenesis and innervation

The pancreas is characterized by a major component, an exocrine and ductal system involved in digestion, and a minor component, the endocrine islets represented by islet micro-organs that tightly regulate glucose homoeostasis. Pancreatic organogenesis is strictly co-ordinated by transcription factors that are expressed sequentially to yield functional islets capable of maintaining glucose homoeostasis. Angiogenesis and innervation complete islet development, equipping islets to respond to metabolic demands. Proper regulation of this triad of processes during development is critical for establishing functional islets.

Synchronization of pancreatic islet oscillations by intrapancreatic ganglia: a modeling study

Plasma insulin measurements from mice, rats, dogs, and humans indicate that insulin levels are oscillatory, reflecting pulsatile insulin secretion from individual islets. An unanswered question, however, is how the activity of a population of islets is coordinated to yield coherent oscillations in plasma insulin. Here, using mathematical modeling, we investigate the feasibility of a potential islet synchronization mechanism, cholinergic signaling. This hypothesis is based on well-established experimental evidence demonstrating intrapancreatic parasympathetic (cholinergic) ganglia and recent in vitro evidence that a brief application of a muscarinic agonist can transiently synchronize islets. We demonstrate using mathematical modeling that periodic pulses of acetylcholine released from cholinergic neurons is indeed able to coordinate the activity of a population of simulated islets, even if only a fraction of these are innervated. The role of islet-to-islet heterogeneity is also considered. The results suggest that the existence of cholinergic input to the pancreas may serve as a regulator of endogenous insulin pulsatility in vivo.

An immunocytochemical study of intrapancreatic ganglia, nerve fibres and neuroglandular junctions in Brockmann bodies of the tompot blenny (Blennius gattoruggine), a marine teleost

The innervation of the Brockmann bodies in the teleost fish, Blennius gattoruggine, was studied using immunocytochemical techniques at both the light and electron microscopy levels. Islet innervation consisted of intrapancreatic ganglia, generally localized inside the rim of the exocrine tissue of the Brockmann bodies, in proximity to the islet, nerve fibres and nerve terminals with synaptic complexes. The intrapancreatic ganglia were of variable size, with different numbers of ganglionic cells, that appeared unipolar in section. The cell bodies showed immunoreactivity to galanin, oxytocin, peptide tyrosine tyrosine and glucagon. The extrinsic and intrinsic nerve fibres passed through the exocrine parenchyma and crossed the connectival septa and islet connectival sheath, penetrating into the islets, where they became increasingly thinner. They terminated on the endocrine cells with dilated nerve terminals. At least three types of terminals were detected, depending on the different vesicle content: peptidergic, cholinergic or adrenergic. They presented specialized synaptic structures, the neuroglandular junctions, some of which contained neurosecretory granules immunogold labelled by galanin antiserum. This new finding confirms the role of galanin as a neurotransmitter. This rich supply of innervation may be important in the regulation and integration of islet secretion.

Ultrastructural characterization of a regular schwann-axon-islet complex after the autograft of pancreatic fragments into the spleen of the adult dog

After autotransplantation of pancreatic fragments into the dog's spleen, the morphogenesis of the reinnervating process has evolved as an highly differentiated model, reproducing the most peculiar and systematic relationships between schwann cells, axons, and islet cells reported in the dog's islet, despite it's modulation by the restrictive conditions derived from the intrasplenic location of the dispersed pancreatic tissue. The reinnervating process is described, emphasizing the peculiar ultrastructural features and topography of the schwann cells and of the axonal network that impose the concept of a true anatomical reinnervation, which make previsible the possibility of a very selective and direct neurochemical and/or electrotonic control of the engrafted islet cells. The schwann-axon-islet complexes are a very peculiar and regular arrangement between islet cells and nervous elements and are reproduced after the autotransplant without the engrafted ganglia, whose potential but aleatory contribution could not be unequivocally characterized. Axonal profiles or schwann cells on the abundant regenerated ductal-acinar structures were not identified.

Islet innervation of nondiabetic and diabetic Chinese hamsters. I. Acetylcholinesterase histochemistry and norepinephrine fluorescence

Cholinergic and adrenergic innervation of pancreatic islets from age- and sex-matched nondiabetic (M subline) and diabetic (AC subline) Chinese hamsters was analyzed by acetylcholinesterase (AChE) histochemistry and norepinephrine fluorescence. The AChE activity was significantly diminished in islets of diabetic animals compared with that of nondiabetic controls. The reduction in cholinergic innervation displayed an inverse relationship with respect to nonfasting plasma glucose and ketone levels. On the basis of qualitative analysis, adrenergic activity also appeared to be depressed in islets of diabetic animals. These date suggest that autonomic control of islet function is altered in diabetic Chinese hamsters when plasma glucose and ketone levels arae elevated and may exacerbate metabolic complications in this animal model.

Net insulin secretion and IRI response to 1 mg glucose during OGTT

Dogs in postabsorbtive state were anesthetized with IV nembutal. Their femoral arteries were catheterized, and the abdomens were entered via a midline incision. A small branch of the pancreatic artery and a corresponding small vein were catheterized. 133Xe was injected through the artery, and the the pancreatic blood flow rate (PBFR) was determined. Net insulin secretion was calculated using the insulin concentration of pancreatic vein-artery difference and pancreatic plasma flow rate (PPFR). After control studies an oral glucose tolerance test (2 g/kg body wt.) was performed. Under control studies the mean value of net insulin secretion was 3,753.2 +/- 699.1 microunits/100 g P/min and the maximum values were 10,610.2 +/- 3,658.7 and 11,108.0 +/- 2,852.6 microunits/100 g P/min at 20 and 60 min after glucose loading, respectively (P less than 0.05). Insulin response per mg of glucose was 83.2 +/- 12.2 microunits under control conditions. Twenty minutes after glucose loading this figure rose to 173.4 +/- 41.7 microunits/mg glucose and at 60 min 207.9 +/- 49.2 microunits/mg glucose.