Biogenic amines in the guinea pig endocrine pancreas

VMAT2 gene expression and function as it applies to imaging beta-cell mass

Diabetes mellitus is a metabolic disorder characterized by hyperglycemia. The two main forms of the disease are distinguished by different pathogenesis, natural histories, and population distributions and indicated as either type 1 (T1DM) or type 2 diabetes mellitus (T2DM). It is well established that T1DM is an autoimmune disease whereby beta-cells of pancreatic islets are destroyed leading to loss of endogenous insulin production. Albeit less dramatic, beta-cell mass (BCM) also drops in T2DM. Therefore, it is realistic to expect that noninvasive measures of BCM might provide useful information in the diabetes-care field. Preclinical studies have demonstrated that BCM measurements by positron emission tomography scanning, using the vesicular monoamine transporter type 2 (VMAT2) as a tissue-specific surrogate marker of insulin production and [11C] Dihydrotetrabenazine (DTBZ) as the radioligand specific for this molecule, is feasible in animal models. Unfortunately, the mechanisms underlying beta-cell-specific expression of VMAT2 are still largely unexplored, and a much better understanding of the regulation of VMAT2 gene expression and of its function in beta-cells will be required before the full utility of this technique in the prediction and treatment of individuals with diabetes can be understood. In this review, we summarize much of what is understood about the regulation of VMAT2 and identify questions whose answers may help in understanding what measurements of VMAT2 density mean in the context of diabetes.

Serotonin (5-hydroxytryptamine, 5-HT) immunoreactive endocrine and neural elements in the chromaffin enteropancreatic system of amphibians and reptiles

The diffuse chromaffin enteropancreatic system of nine species of amphibians (newts, frogs) and reptiles (turtles, lizards, snakes) was investigated immunohistochemically for the presence and topographic distribution of serotonin (5-hydroxytryptamine, 5-HT). The study revealed various numbers of serotonin-producing cells in the pancreas and intestinal epithelium and also immunolabelled nerve profiles in the villi of all species studied. In addition, two different morphological populations of serotonin cells ("open" and "closed") were localized in the functional segments of the intestines in the representative species of all the taxa investigated. Semi-quantitative evaluation of the immunolabelled pancreatic and enteric cells revealed significantly different mean numbers of labelled cells in different amphibian and reptilian taxa, and also between the various successive gut segments of each taxon. The ratio between "open" and "closed" varieties of serotonin cells recorded along the intestines followed a decreasing trend, progressive in lizards and snakes and more abrupt in newts, frogs and turtles. The above findings may help resolve several key stages of the phylogenetic evolution of poikilothermic vertebrates.

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.

Noradrenergic innervation of rabbit pancreatic ganglia

Sympathetic nerve stimulation indirectly regulates pancreatic endocrine and exocrine secretion, in part, through actions on the cholinergic parasympathetic innervation of the secretory tissues. Earlier work identified noradrenergic nerves in pancreatic ganglia and demonstrated the effects of exogenous norepinephrine (NE) on synaptic transmission but no quantitative studies of ganglionic NE content and release exist. Therefore, the distribution and density of catecholamine (CA)-containing nerves in rabbit pancreatic ganglia were studied using paraformaldehyde/glutaraldehyde (FAGLU) staining and HPLC analysis of CA concentrations. Neural release of [3H]NE was measured in ganglia isolated from the head/neck or body regions of the pancreas. CA-containing nerves densely innervated most ganglia (86%) from both regions, while neural and non-neural CA-containing cell bodies were rarely found. Ganglia from the head/neck region contained significantly higher concentrations of NE. Both 40 mM K+ and veratridine evoked Ca2+-dependent [3H]NE release and tetrodotoxin inhibited 80% of veratridine-stimulated release. omega-Conotoxin GVIA alone antagonized veratridine-stimulated release by 40% but the addition of nifedipine or omega-agatoxin IVA caused no further inhibition. There were no apparent regional differences in the Ca2+-dependence or toxin-sensitivity of NE release. In conclusion, ganglia throughout the rabbit pancreas receive a dense, functional noradrenergic innervation and NE release is dependent upon N- but not P/Q- or L-type voltage-dependent Ca2+ channels. These noradrenergic nerves may indirectly regulate pancreatic secretion through actions on ganglionic transmission.

Hyperglycemia induced by acute central fluoxetine administration: role of the central CRH system and 5-HT3 receptors

Brain serotonin and CRH systems participate in the control of blood glucose levels. We have previously demonstrated that the pharmacological stimulation of central 5-HT3 receptors, the target for several therapeutic agents used as antiemetics in the course of chemotherapy, induces hyperglycemia. The aim of the present study was to investigate the participation of the brain CRH component and 5-HT3 receptors in basal blood glucose levels as well as in the hyperglycemia induced by third ventricle injections of fluoxetine, a serotonin reuptake inhibitor with a broad range of clinical use. In this study, we used fasted adult Wistar male rats (220 +/- 20 g) whose third ventricles were cannulated 7 days prior to the experiments. Acute third ventricle injections of fluoxetine caused a significant increase in plasma glucose levels throughout the experiment. Pretreatment with alpha-helical CRH, a selective CRH antagonist, significantly blunted fluoxetine-induced hyperglycemia. Also, pretreatment with two distinct selective 5-HT3 receptor antagonists (LY-278,584 and ondansetron) significantly impaired the rise in plasma glucose levels observed in fluoxetine-treated animals pretreated with isotonic saline solution. None of these antagonists was able to modify blood glucose levels when injected alone into the third ventricle. Animals receiving third ventricle injections of fluoxetine, in spite of being hyperglycemic, presented plasma insulin levels similar to those displayed by normoglycemic, saline-treated controls. It is suggested that the acute increase in brain serotonergic activity caused by third ventricle injections of fluoxetine induces a hyperglycemic response that requires the functional integrity of the brain CRH system and 5-HT3 receptors. Also, it is proposed that the absence of a compensatory increase in plasma insulin levels may contribute to the generation of a hyperglycemic response after central fluoxetine administration.

Expression of the two isoforms of the vesicular monoamine transporter (VMAT1 and VMAT2) in the endocrine pancreas and pancreatic endocrine tumors

The uptake of monoamines into the secretory granules of monoamine-storing neuroendocrine cells is mediated by vesicular monoamine transporter protein 1 or 2 (VMAT1 or VMAT2). This study analyzed the expression of VMAT1 and VMAT2 in endocrine cells of normal human and monkey pancreas. The expression of VMAT1 and VMAT2 was also examined in infants with hyperinsulinemic hypoglycemia and in adults with pancreatic endocrine tumors (PETs). Using immunohistochemistry (IHC) and in situ hybridization (ISH), we demonstrated the mutually exclusive expression of VMAT1 in endocrine cells of the duct system and of VMAT2 in many cells of the islets of Langerhans. By confocal laser scanning microscopy, VMAT1-positive cells were identified as enterochromaffin (EC) cells and VMAT2-positive cells as beta-cells. In PETs, VMAT1 was found exclusively in all serotonin-containing tumors. In contrast, VMAT2 expression was lost in many insulinomas, independent of their biological behavior. VMAT2 was expressed by some non-insulin-producing tumors. The mutually exclusive expression of VMAT1 in EC cells and of VMAT2 in beta-cells suggests that both cell types store monoamines. Monoamine storage mediated by VMAT1 in EC cells is apparently maintained in EC cell tumors. In contrast, many insulinomas appear to lose their ability to accumulate monoamines via VMAT2.

5-hydroxytryptamine strongly inhibits fluid secretion in guinea pig pancreatic duct cells

We studied the distribution of 5-hydroxytryptamine- (5-HT-) containing cells in the guinea pig pancreas and examined the effects of 5-HT on fluid secretion by interlobular pancreatic ducts. The 5-HT-immunoreactive cells with morphological characteristics of enterochromaffin (EC) cells were scattered throughout the duct system and were enriched in islets of Langerhans. The fluid secretory rate in the isolated interlobular ducts was measured by videomicroscopy. Basolateral applications of 5-HT strongly but reversibly reduced HCO(3)-dependent, as well as secretin- and acetylcholine- (ACh-) stimulated, fluid secretion, whereas 5-HT applied into the lumen had no such effects. Secretin-stimulated fluid secretion could be inhibited by a 5-HT(3) receptor agonist, but not by agonists of the 5-HT(1), 5-HT(2), or 5-HT(4) receptors. Under the stimulation with secretin, 5-HT decreased the intracellular pH (pH(i)) and reduced the rate of pH(i) recovery after acid loading with NH(4)(+), suggesting that 5-HT inhibits the intracellular accumulation of HCO3(-). The elevation of intraductal pressure in vivo reduced secretin-stimulated fluid secretion, an effect that could be attenuated by a 5-HT(3) receptor antagonist. Thus, 5-HT, acting through basolateral 5-HT(3) receptors, strongly inhibits spontaneous, secretin-, and ACh-stimulated fluid secretion by guinea pig pancreatic ducts. 5-HT released from pancreatic ductal EC cells on elevation of the intraductal pressure may regulate fluid secretion of neighboring duct cells in a paracrine fashion.

Mouse islets cultured with vasoactive intestinal polypeptide: effects on insulin release and immunoreactivity for tyrosine hydroxylase

Mouse islets cultured for 1 or 4 days with or without 10 nM vasoactive intestinal polypeptide (VIP) were stained for tyrosine hydroxylase (TH) and examined for insulin secretion during culture and in a postculture perifusion system. Exposure to exogenous VIP for 4 days increased the frequency of islet cells expressing TH-like immunoreactivity. Regardless of the culturing conditions, the islets exhibited significant insulin secretory responses to 16.7 mM glucose, the effect being potentiated by 10 nM VIP in the perifusion medium. The insulin-releasing action of glucose and the potentiating effect of VIP were less pronounced in islets cultured for 1 day with VIP than in islets cultured without this neuropeptide. The following conclusions are suggested: (a) VIP stimulates the expression of TH in mouse islet cells; (b) the latency of the VIP-induced TH is a postreceptor phenomenon; (c) islet cultures exposed to VIP represent a new instance of the association between increased functional demands on beta cells and enhanced expression of TH and a new instance of VIP having trophic effects.

Localization of monoamine oxidase A and B in human pancreas, thyroid, and adrenal glands

We studied monoamine oxidase (MAO) A and B localization in human pancreas, thyroid gland, and adrenal gland by immunohistochemistry. The primary antibodies used were mouse monoclonal anti-human MAO-A (6G11/E1) and anti-human MAO-B (3F12/G10/2E3). Samples were obtained from six routine autopsy cases and fixed in 2% paraformaldehyde. Exocrine pancreas showed a widespread distribution of MAO-A, whereas MAO-B was present only in centroacinar cells and epithelial cells of pancreatic ducts. In endocrine pancreas, MAO-A was observed in around 50% of islet cells, whereas MAO-B was less abundant and was restricted to the periphery of islets. Thyroid gland showed strong MAO-A immunoreactivity in all cell types and was MAO-B-negative. In adrenal gland, the capsule displayed MAO-A but not MAO-B immunoreactivity, whereas the cortex showed widespread MAO-A staining but was MAO-B-negative in interstitial cells. Finally, in the medulla only a few scattered cells showed either MAO-A or MAO-B immunoreactivity. To our knowledge, these data represent the first study of the cellular distribution of MAO-A and MAO-B in the three human tissues included.

Granin proteins (chromogranin A and secretogranin II C23-3 and C26-3) in the endocrine pancreas of amphibians

The occurrence and cellular distribution of chromogranin A (CgA) and of two synthetic secretogranin II (SgII)-fragments (termed C23-3 and C26-3) has been investigated immunohistochemically in the endocrine pancreas of five amphibian species. Immunoreactivity for CgA was detected only in specimens of the genus Rana, whereas for SgII it was found in all the urodeles and anurans studied. Either CgA or the SgII-fragment displayed its own cellular distribution patterns in the endocrine pancreas of a given species. Moreover, immunoreactivity for both regions (C23-3 and C26-3) of the SgII-molecule exhibited by the same endocrine cell population have been encountered in newt and frog organs. Besides the interspecific heterogeneous distribution of CgA and of the two SgII-fragments in relation to the insular cell types, a striking heterogeneity of their immunostaining density among the endocrine cells of the same type was also revealed. The above findings entirely support the concept of a good conservation of granins during phylogeny; they do not support, however, the previously ascribed usefulness of these anionic glycoproteins as markers for all neuro-endocrine cells.

Granin proteins (chromogranin A and secretogranin II C23-3 and C26-3) in the endocrine pancreas of reptiles

The endocrine pancreas of four reptile species belonging to the turtles, lizards and snakes was investigated immunohistochemically for the occurrence and cellular distribution of chromogranin A (CgA) and of two synthetic secretonin II (SgII)-peptides (C23-3 and C26-3). CgA-immunoreactivity was found only in the turtle pancreas, whereas that for SGIIC23-3 appeared both in the turtle and snake. None of the species studied displayed immunoreactivity for SgIIC26-3. The two detected granins showed different distributions in relation to the endocrine cell types. Conspicuous variations of the immunostaining density for either granin in the same endocrine cell population and even complete lack of the immunoreaction were recorded. The findings suggest that, despite the restricted presence in the endocrine pancreas of the reptiles investigated, the granins are relatively well conserved during phylogeny; they do not confirm, however, the previously accepted usefulness of the granin protein family as common markers of neuroendocrine cells.

Granin proteins (chromogranin A and secretogranin II C23-3 and C26-3) in the intestine of reptiles

The occurrence, distribution and the possible cellular co-localizations of chromogranin A (CgA) and of two synthetic secretogranin II-peptides (SgIIC23-3 and SgIIC26-3) with several enteric neuropeptides and serotonin have been investigated immunohistochemically in turtles, lizards and snakes. The distribution of CgA-immunoreactivity was restricted only to the enteroendocrine cells in all the reptiles studied. SgII-immunoreactivity--absent in turtle--revealed nerve cells and fibers, besides enteroendocrine cells in lizard and snake guts. Moreover, the two antisera (C23-3 and C26-3) raised against the different regions of the SgII-molecule yielded distinct distribution patterns of immunoreactivity both in the lizard and snake organs. Small amounts of enteric serotonin cells co-stored CgA or SgIIC23-3 in lizards and snakes and only SgIIC26-3-peptide in snakes. CgA was found co-stored with somatostatin in a few enterocytes of the turtle duodenum. In the same gut segment of lizards and throughout the snake organ, neurotensin and the SgIIC23-3-peptide co-existed in a small number of endocrine cells. The pancreatic polypeptide-containing cells were devoid of immunoreactivity both for CgA and SgII. Bombesin immunopositive cells were absent throughout the intestines of the reptiles investigated. The above findings entirely support the heterogenous distribution of granins in neuroendocrine organs and tissues and also within the same neuroendocrine cell population. They further support the concept of a good conservation of granins during phylogeny.

Granin proteins (chromogranin A and secretogranin II C23-3 and C26-3) in the intestine of amphibians

The occurrence, distribution and possible cellular colocalizations of chromogranin A (CgA) and of two synthetic secretogranin II peptides (SgIIC23-3 and SgIIC26-3) with serotonin, somatostatin, neurotensin, pancreatic polypeptide and bombesin have been investigated immunohistochemically in the amphibian gut. CgA or SgIIC26-3-immunostained enterocytes were found throughout along the frog intestine, while no immunoreaction for any of the tested antisera against granins was seen in the same organ of newts. Variable amounts of serotonin-immunoreactive cells co-storing CgA or SgIIC26-3, but never both granins, were encountered in all intestinal segments of the frogs investigated. In addition, CgA was co-localized with somatostatin in a few endocrine cells of the frog (genus Rana) duodenum and small intestine. In the duodenum of another frog (genus Xenopus) several enterocytes co-stored SgIIC26-3 and neurotensin. Pancreatic polypeptide- and bombesin-immunoreactive cells, the latter detected only in the duodenum of Xenopus, did not contain and granin. The results suggest that, in spite of their relatively restricted occurrence in the intestine of frogs and even of their absence in that of newts, the granins are well conserved during phylogeny. On the other hand, the heterogeneous distributions of these anionic glycoproteins, related to the entero-endocrine cell types, make their previously assigned usefulness as markers of all neuro-endocrine cells unlikely.