Neuropeptides and the Regulation of Islet Function
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《糖尿病学杂志》
the Department of Clinical Sciences, Lund University, Lund, Sweden, and the Department of Experimental Medical Sciences, Lund University, Lund, Sweden
CART, cocaine- and amphetamine-regulated transcript; CGRP, calcitonin gene–related polypeptide; GRP, gastrin-releasing polypeptide; GSIS, glucose-stimulated insulin secretion; NPY, neuropeptide Y; PACAP, pituitary adenylate cyclase–activating polypeptide; PP, pancreatic polypeptide; TH, tyrosine hydroxylase; VIP, vasoactive intestinal polypeptide
ABSTRACT
The pancreatic islets are richly innervated by autonomic nerves. The islet parasympathetic nerves emanate from intrapancreatic ganglia, which are controlled by preganglionic vagal nerves. The islet sympathetic nerves are postganglionic with the nerve cell bodies located in ganglia outside the pancreas. The sensory nerves originate from dorsal root ganglia near the spinal cord. Inside the islets, nerve terminals run close to the endocrine cells. In addition to the classic neurotransmitters acetylcholine and norepinephrine, several neuropeptides exist in the islet nerve terminals. These neuropeptides are vasoactive intestinal polypeptide, pituitary adenylate cyclase–activating polypeptide, gastrin-releasing polypeptide, and cocaine- and amphetamine-regulated transcript in parasympathetic nerves; neuropeptide Y and galanin in the sympathetic nerves; and calcitonin gene–related polypeptide in sensory nerves. Activation of the parasympathetic nerves and administration of their neurotransmitters stimulate insulin and glucagon secretion, whereas activation of the sympathetic nerves and administration of their neurotransmitters inhibit insulin but stimulate glucagon secretion. The autonomic nerves contribute to the cephalic phase of insulin secretion, to glucagon secretion during hypoglycemia, to pancreatic polypeptide secretion, and to the inhibition of insulin secretion, which is seen during stress. In rodent models of diabetes, the number of islet autonomic nerves is upregulated. This review focuses on neural regulation of islet function, with emphasis on the neuropeptides.
Since the discovery of nerves in the pancreatic islet by Paul Langerhans in his thesis from 1869, the neural-islet axis has been explored by a number of neuroanatomists, physiologists, and endocrinologists (rev. in 1–3). It is currently known that branches of the parasympathetic and sympathetic as well as the sensory nervous system innervate the islets with nerve terminals ending closely to the islet endocrine cells. It is also known that these nerves affect islet hormone secretion. The classic neurotransmitters in the islet autonomic nerves are acetylcholine and norepinephrine. During the last decades, the contribution to neural regulation of islet function also by neuropeptides has been established (rev. in 4). Several neuropeptides are localized to islet nerve terminals, are released from the pancreatic nerves upon nerve stimulation, and influence islet hormone secretion. It is also known that the islet innervation is altered in animal models of type 2 diabetes (5). The autonomic nervous system has also been suggested to be involved in the regulation of islet mass (6). The present review highlights the role of neuropeptides in islet function.
PARASYMPATHETIC NERVOUS SYSTEM
Anatomy and effects.
The parasympathetic nerves innervating the pancreatic islets emanate from the pancreatic ganglia, which are innervated by preganglionic parasympathetic nerves originating in the dorsal motor nucleus of the vagus. Activation of the parasympathetic nerves enhances insulin and glucagon secretion (1,3). These stimulatory effects are of physiological relevance under at least three conditions: 1) for the cephalic phase of insulin secretion during meal ingestion, 2) for the glucagon response to hypoglycemia, and 3) for pancreatic polypeptide (PP) secretion.
Parasympathetic nerves and cephalic phase of insulin secretion.
The cephalic phase of insulin secretion leads to the rapid and early increase in insulin levels during the first minutes after food ingestion (7). This is due to activation of olfactory-gustatory sensory receptors in association with psychological stimuli, which activates central parasympathetic nerves that stimulate insulin secretion. The importance of the cephalic phase of insulin secretion was recently examined by an approach to block the autonomic ganglia with the ganglionic blocker, trimetophane, in healthy subjects (7). In this approach, trimetophane was infused intravenously to inhibit the autonomic ganglia. A meal was served during the trimetophane infusion, and the trimetophane infusion was stopped 15 min later. It was first demonstrated that circulating insulin increased within the first 10 min after meal ingestion, which is before any rise in circulating glucose was observed. Of more importance in this context, however, was the second finding that this 10-min insulin response to meal ingestion was reduced by 75% by trimetophane (Fig. 1). This demonstrates that a neurally mediated cephalic phase of meal-related insulin secretion exists in humans. This reduction was accompanied by impairment of glucose elimination, even though the inhibited insulin response was limited to the first 15 min. Hence, the rapid and early insulin secretion contributes substantially to the glucose tolerance after meal ingestion. This may in turn be ascribed to the inhibition of hepatic glucose production by insulin.
Parasympathetic nerves and stimulation of glucagon secretion during hypoglycemia.
The parasympathetic nerves might also be of physiological importance for the stimulation of glucagon secretion during hypoglycemia (8). Glucagon secretion during counterregulation might be mediated by low glucose, low islet insulin, and high epinephrine, which all are consequences of hypoglycemia and all stimulate glucagon secretion. A possible contribution by the islet nerves was examined using the trimetophane protocol to inhibit autonomic ganglia in the presence of insulin-induced hypoglycemia in healthy subjects (9). It was found that the glucagon response to hypoglycemia (2.5 mmol/l glucose) was markedly reduced by trimetophane (Fig. 2). Hence, a significant proportion of the glucagon response to hypoglycemia depends on autonomic nerves.
Parasympathetic nerves and PP secretion.
The islet parasympathetic nerves seem of importance for PP secretion. This is evident from findings that vagus nerve stimulation increases PP secretion (10,11) and that the PP response to hypoglycemia is reduced by autonomic ganglionic blockade, as demonstrated in humans (9). In fact, the coupling of PP secretion to parasympathetic activity has suggested that plasma PP levels may be used as a marker or index of parasympathetic activity (12). However, the physiological relevance of the PP response to parasympathetic activation is not known.
Islet parasympathetic neurotransmitters.
It is well known that acetylcholine stimulates insulin secretion through a direct action on the islet -cells (3). In addition, several neuropeptides are localized to islet parasympathetic nerve terminals and therefore potentially contribute to the islet effects of parasympathetic activation. These neuropeptides are vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase–activating polypeptide (PACAP), gastrin-releasing polypeptide (GRP), neuropeptide Y (NPY), and cocaine- and amphetamine-regulated transcript (CART) peptide (2–4,13–15). Figure 3 illustrates immunofluorescence of rodent pancreatic tissue showing the close proximity of these nerves with the pancreatic islets by showing VIP as an example: nerves co-harboring VIP and the cholinergic marker vesicular acetylcholine transporter (VAchT) enter the islets to terminate in close proximity to the islet endocrine cells (Figs. 3A–C). Figure 3 also shows nerves harboring GRP (Fig. 3G) and CART (Fig. 3H) in islets. Figure 3I–J shows that a great proportion of the CART-containing fibers are also VIP immunoreactive. Similar findings have previously been reported for PACAP (16) and, in the pig, for GRP (17) and, recently, for CART peptide (14,15). Therefore, VIP, PACAP, GRP, and CART are parasympathetic co-transmitter neuropeptides in autonomic nerve endings in the islets.
Effects of parasympathetic co-transmitters on insulin and glucagon secretion in mice.
VIP, PACAP, and GRP stimulate insulin and glucagon secretion when administered both in vivo in several species, including humans, and in vitro in isolated islets or the perfused pancreas (3,4,16–19). Here we report that VIP, PACAP, and GRP all potentiate glucose-stimulated insulin secretion (GSIS) and arginine-stimulated glucagon secretion, both when examined in isolated mouse islets (Figs. 4A and 5A) and when intravenously administered to mice together with glucose (Fig. 4C) or arginine (Fig. 5C). Increased insulin secretion has also been documented in transgenic mice overexpressing the VIP gene (20) or the PACAP gene (21) in the islet -cells. The molecular basis of their effects is still far from fully understood, although it is established that VIP and PACAP signal through cAMP.
The importance of VIP and PACAP for islet physiology has been explored in model experiments using specific receptor antagonists and peptide ligand or receptor knockout mice. A study using PACAP–/– mice showed impaired GSIS in these mice (22). Also VIP–/– mice have been generated (23), but insulin secretion in these animals remains to be examined. Another strategy to explore the physiology of VIP and PACAP is to inhibit the activity or expression of their receptors. The two neuropeptides activate both VPAC1 and VPAC2 receptors and PACAP in addition to PAC1 receptors. Of these receptors, VPAC2 and PAC1 are expressed in islet cells (16). By disrupting the PAC1 receptor gene in mice, expression of truncated PAC1 receptors, which do not bind PACAP, evolves (24). PAC1R–/– mice display a marked reduction in the insulin response to both oral and intravenous glucose, showing that PACAP is of importance for a normal GSIS. Furthermore, the insulin response to intravenous administration of 2-deoxy-glucose (2-DG), is reduced in PAC1R–/– mice (Fig. 6). 2-deoxy-glucose competes with glucose for phosphorylation, which results in neuroglycopenia. This in turn activates the autonomic nerves, which results in a stimulation of insulin secretion after 2 min (25). Other studies have shown that PAC1 receptor antagonists pharmacologically inhibit the insulin response to oral glucose in mice (26) and to vagal nerve activation in the pig pancreas (27). A recent study compared the reduction in insulin secretion after gastric glucose versus intravenous glucose in PAC1R–/– mice by matching the glucose levels under these two conditions. The reduction was more marked after gastric glucose, again suggesting that PACAP contributes to the insulin response to oral glucose (28). The impairment of the insulin response in these mice suggests that PACAP may contribute to the cephalic phase of insulin secretion. In contrast, mice with VPAC2 gene deletion have a normal glucose tolerance during an oral glucose tolerance test in association with a reduced insulin response (29). This suggests increased insulin sensitivity in VPAC2–/– mice with an appropriate downregulation of the insulin response to maintain normal glucose tolerance, which indirectly would support normal -cell function. A recent study has also demonstrated that PAC1R–/– mice display impaired glucagon secretion during hypoglycemia (30). Thus, PACAP may contribute to the parasympathetic involvement in the glucagon response to counterregulation, in addition to its potential physiological importance in regulating insulin secretion after 1) glucose stimulation, 2) vagal nerve activation, and 3) meal intake.
Because VIP and PACAP both strongly stimulate insulin secretion, they may be of potential interest in the treatment of type 2 diabetes. PACAP has been shown to reduce the hyperglycemia in rodent diabetes (high fat–fed mice and GK rats) (31). However, one problem is that PACAP stimulates glucagon secretion and the peptide has potent vasoactive effects, which would limit its usefulness in treatment. Instead, specific activation of VPAC2 receptors would be advantageous. Recently, a specific VPAC2 receptor agonist was described (32): it augments GSIS in both rodent and human islets and potentiates insulin secretion and glucose disposal in rats, thereby offering a novel target for treatment of type 2 diabetes based on islet neuropeptides.
The islet parasympathetic nerve endings also harbor GRP. GRP is released from the pancreas during parasympathetic nerve stimulation and stimulates insulin and glucagon secretion (3,4,17,19). It has been shown that the GRP receptor is expressed in islets (33), suggesting a direct action of the neuropeptide on islet cells. A study in GRP receptor gene–deficient mice has shown that the insulin response to oral glucose is impaired (34), which would support a role in the meal-related insulin response. It was also shown that insulin secretion in response to endogenous nerve activation in mice (by 2-deoxyglucose) is impaired in these mice, suggesting that GRP contributes to neurally mediated islet hormone secretion. These findings suggest that islet neuronal GRP, like VIP and PACAP, is of physiological importance.
It was recently also demonstrated that CART, an anorexigenic peptide that is highly expressed in the brain (rev. in 35), is also a neuropeptide of the rat and mouse pancreas (14,15, rev. in 36). Figure 3H–J shows the location of CART-containing nerves within an islet. Colocalization with VIP demonstrates the parasympathetic identity of the majority of the CART fibers. Recent studies have shown that CART affects islet hormone secretion (37). CART inhibits GSIS from isolated rat islets. On the other hand, CART potentiates GSIS augmented by glucagon-like peptide 1. Although there is hitherto no CART receptor identified, recent data suggest that CART exerts the potentiating effect on glucagon-like peptide 1–mediated GSIS via increased cAMP and the protein kinase A–dependent pathway (37). Furthermore, the potential impact of CART of islet function was demonstrated by using CART–/– mice (14). These mice displayed blunted GSIS both in vivo and in vitro, together with impaired glucose elimination.
SYMPATHETIC NERVOUS SYSTEM
Anatomy and function.
The islet sympathetic nerves are postganglionic, with their nerve cell bodies mainly located in the celiac ganglion or in the paravertebral sympathetic ganglia. Electrical stimulation of the sympathetic nerves inhibits insulin secretion and stimulates glucagon secretion (1–4). This is of physiological importance during stress and physical exercise to elicit a hyperglycemic response by increasing hepatic glucose delivery. An experimental tool for the study of function of the sympathetic nerves is administration of 6-hydroxydopamine to rodents. 6-hydroxydopamine is taken up by way of vesicular monoamine transporter localized to nerve endings of sympathetic neurons and selectively destroys sympathetic nerve terminals. As a consequence, islet nerves staining for the sympathetic marker tyrosine hydroxylase (TH) are absent in mice at 48 h after administration of 6-hydroxydopamine (38). This is associated with augmented GSIS (39,40), reduced insulin gene expression, and increased -cell mass (38). These findings therefore suggest that the sympathetic nerves are of importance both for insulin secretion, insulin gene expression, and islet -cell mass.
Islet sympathetic neurotransmitters.
The classic sympathetic neurotransmitter is norepinephrine, which inhibits GSIS and stimulates glucagon secretion (3,39). However, combined - and -adrenoceptor blockade does not prevent sympathetic nerve activation from inhibiting insulin secretion (41). This suggests that neurotransmitters other than norepinephrine contribute to some sympathetic islet effects. Neuropeptides localized to islet sympathetic nerve terminals are NPY and galanin (1–4). They are both released from the pancreas during sympathetic nerve activation (13,39) and, like sympathetic nerve stimulation, inhibit insulin secretion and stimulate glucagon secretion (3,4). Figure 3D–E illustrates that NPY nerves are localized to islets and the colocalization with TH illustrates the sympathetic nature of the NPY-containing nerve endings. It should be emphasized, however, that NPY is localized also to nerve endings that do not harbor TH. Also, galanin is localized to TH-containing nerve endings in the islets in a number of species (2,42). Both NPY and galanin are released from the pancreas during sympathetic nerve activation (13,43) and, like sympathetic nerve stimulation, inhibit insulin secretion and stimulate glucagon secretion (3,4). Figures 4B and D and 5B and D show that norepinephrine, NPY, and galanin all inhibit GSIS and augment arginine-stimulated glucagon secretion in isolated mouse islets and in vivo in mice. Because NPY and galanin thus mimic the effects of sympathetic nerve stimulation, they might contribute to sympathetic islet effects.
In the dog, galanin has been suggested to be of major importance in contributing to the inhibitory influence of sympathetic nerve activation on insulin secretion (43). Also in mice, galanin seems to be of physiological impact because immunoneutralization of galanin prevents the inhibition of insulin secretion, which is seen during a stress model (swimming) in mice (44). To study the physiological impact of galanin on islet function in more detail, mice with a loss-of-function mutation in the galanin gene have been examined (45). The inhibition of insulin secretion that is seen after administration of 2-deoxyglucose (and that reflects activation of sympathetic nerves) was impaired in galanin–/– mice. This further supports a contribution by galanin of the response to sympathetic activation.
SENSORY NERVES
The islets are innervated by sensory nerves that harbor calcitonin gene–related polypeptide (CGRP) (1–4). The fibers leave the pancreas along the sympathetic fibers with the splanchnic nerves and reach the spinal cord. Figure 3H illustrates an islet fiber harboring CGRP. The relevance of these nerves for islet function is far from understood. These sensory nerves may be targeted by the use of the toxin capsaicin, which causes degeneration of small unmyelinated C-fibers. Capsaicin is an agonist for the transient receptor potential vanilloid receptor (TRPV1); acutely, it activates the receptors, which leads to a release of the neurotransmitters. After the acute effect, however, capsaicin leads to loss of unmyelinated sensory fibers in conjunction with a substantial number of CGRP nerves (46). If capsaicin is given to neonatal rodents, the sensory deactivation is permanent, whereas if given to adults, the deactivation is transient. One effect of capsaicin-induced sensory deactivation is increased insulin secretion (47). This would suggest that sensory nerve activation in islets inhibits insulin secretion.
Islet sensory neurotransmitters.
CGRP nerves are scattered through the pancreas but with particular density around small blood vessels and islets (48). Furthermore, exogenous administration of CGRP inhibits GSIS (48). This suggests that sensory nerves inhibit insulin secretion; however, the physiological importance of this remains elusive. It should be mentioned in this context that CART is also present in CGRP-containing fibers in rat and mouse pancreas (14,15). The biological significance of CART in these fibers needs further investigation.
Sensory deafferentation and treatment of diabetes.
Since sensory nerves apparently inhibit insulin secretion, possibly through CGRP, and since sensory deafferentation by capsaicin increases insulin secretion, it has been proposed that sensory deactivation might be a novel target for treatment of type 2 diabetes. This hypothesis was substantiated in a recent study in obese Zucker rats (49). This novel potential neurally based therapeutic approach has been further explored by using the toxin resiniferatoxin. This is a vanilloid that has the same mechanism in causing deactivation/degeneration of C-fibers (and A-fibers) as capsaicin but is less toxic. By administering resiniferatoxin to obese Zucker rats (50) and to Zucker diabetic fatty (ZDF) rats (51), improved glycemia has been observed, together with improved insulin secretion, as demonstrated in ZDF rats.
ISLET NERVES AND DIABETES
Several studies have indicated altered islet neurohormonal influences in models of insulin resistance and type 2 diabetes, and therefore it has been speculated whether islet nerve dysfunction may contribute to the development of type 2 diabetes. A study in high fat–fed rats, which is a model of glucose intolerance and type 2 diabetes, disclosed an increased islet innervation (52). Furthermore, insulin resistance in high fat–fed mice is accompanied by augmented insulin secretion after cholinergic activation as a sign of cholinergic hypersensitivity (53) and as a sign that the hyperinsulinemia in ob/ob mice is highly sensitive to atropine (54). Indeed, cholinergic activation by carbachol normalizes insulin secretion in high fat–fed mice (55). It was therefore of interest that reduced islet innervation was evident in a model of type 2 diabetes, the Chinese hamster (5).
We present here immunocytochemical data of islets from two rodent models of diabetes. The models are the Goto-Kakizaki (GK) rats and the db/db mice. The GK rat model is one of the best-described models of type 2 diabetes, as recently was reviewed (56). Its basis is a -cell defect, which occurs through changes in several independent genes, leading to impaired insulin secretion in combination with metabolic impairment reducing -cell neogenesis and a potential acquired loss of -cell differentiation due to glucotoxicity. The db/db mouse is characterized by a leptin receptor deficiency, resulting in insulin resistance and impaired islet function (57). Figures 7 and 8 show that in both these two rodent models of diabetes, the islet innervation is upregulated. This emphasizes that islet innervation is of importance for the islet dysfunction in diabetes. We also found that certain neuropeptides, e.g., VIP and CART (Table 1) (37), are expressed in islet endocrine cells in the diabetic rodents. This is similar to our previous observation of marked expression of NPY in islet -cells in dexamethasone-induced insulin resistance in rats (58). We propose that augmented endocrine cell expression of neurotransmitters in the islets is a sign of islet adaptation for normalization of glucose homeostasis. We suggest the term "neuro-islet plasticity" for this phenomenon; a thorough examination of which needs to be undertaken now.
A novel potential effect of parasympathetic nerves, which may be of pathophysiological relevance for type 2 diabetes, is stimulation of -cell mass. Regulation of -cell mass has recently come into focus because of studies suggesting that -cell mass is reduced in type 2 diabetes (6,59). In adult life, -cell growth and renewal is mainly regulated by replication and differentiation from islet precursor cells within islets and within the duct epithelium (60). An important mechanism for increase in -cell mass may therefore be the increased demand created by insulin resistance; therefore, islets are enlarged in, for instance, the pre-diabetic state and obesity. On the other hand, in overt type 2 diabetes, -cell mass seems to be reduced, which may be ascribed to exaggerated apoptosis triggered by, for instance, glucotoxicity or lipotoxicity. However, neural effects may also contribute to these perturbations. Thus, it has been demonstrated that vagotomy attenuates the islet hyperplasia in ob/ob mice, the mechanism of which needs to be further explored (61). From a therapeutic point of view, the potential of the incretin hormone glucagon-like peptide 1 to augment islet -cell mass, presumably by both increasing neogenesis and inhibiting apoptosis, has been extensively discussed (60). However, islet neuropeptides may also be of interest in this context; for example, exogenous administration of PACAP has been shown to maintain -cell mass in an experimental diabetes model in mice, which is due to increased -cell apoptosis (-cell overexpression of calmodulin [62]).
CONCLUSIONS AND OUTSTANDING ISSUES
The pancreatic islets are innervated by the autonomic nervous system, which affect islet function through both classic neurotransmitters and neuropeptides. Table 1 summarizes the neuropeptides, which are of greatest importance in this context. The islet nerves seem of physiological relevance for 1) the insulin secretion during the cephalic phase of meal ingestion, 2) the glucagon response to hypoglycemia, 3) the PP release, and 4) the islet stress response. The islet nerves may also be of significance for the regulation of islet mass. Novel potential targets for treatment of diabetes may evolve from studies of islet effects of autonomic nerves and neuropeptides. Of most interest at the moment is stimulation of insulin secretion by 1) cholinergic agonism, 2) VPAC2 receptor agonists, 3) PACAP, 4) CART in combination with glucagon-like peptide 1, and 5) sensory nerve deactivation. However, much remains to be established regarding the potential role for the autonomic nerves in relation to islet function. Complex issues relate to mechanisms of activation of the autonomic nerves and regulation of release of the neurotransmitters in relation to each other and the possibility of synergistic action of the neurotransmitters. Other issues relate to the involvement of neuropeptides in islet physiology in humans as well as to potential importance of the autonomic nerves for the islet compensation to insulin resistance, to diabetes development, and as targets for treatment of diabetes. Therefore, although almost 140 years have passed since the first description of islet nerves by Paul Langerhans, much effort has yet to be paid to understand the role for these nerves in islet physiology and pathophysiology.
ACKNOWLEDGMENTS
These studies have been supported by the Swedish Research Council (grants 6834 and 4499), Swedish Diabetes Association, Albert Phlson Foundation, The Royal Physiographic Society, Novo Nordisk Foundation, Tore Nilsson, ke Wiberg and Gyllenstiernska-Krapperup Foundations, Region Skne, and the Faculty of Medicine, Lund University.
FOOTNOTES
This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educational grant from Servier.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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CART, cocaine- and amphetamine-regulated transcript; CGRP, calcitonin gene–related polypeptide; GRP, gastrin-releasing polypeptide; GSIS, glucose-stimulated insulin secretion; NPY, neuropeptide Y; PACAP, pituitary adenylate cyclase–activating polypeptide; PP, pancreatic polypeptide; TH, tyrosine hydroxylase; VIP, vasoactive intestinal polypeptide
ABSTRACT
The pancreatic islets are richly innervated by autonomic nerves. The islet parasympathetic nerves emanate from intrapancreatic ganglia, which are controlled by preganglionic vagal nerves. The islet sympathetic nerves are postganglionic with the nerve cell bodies located in ganglia outside the pancreas. The sensory nerves originate from dorsal root ganglia near the spinal cord. Inside the islets, nerve terminals run close to the endocrine cells. In addition to the classic neurotransmitters acetylcholine and norepinephrine, several neuropeptides exist in the islet nerve terminals. These neuropeptides are vasoactive intestinal polypeptide, pituitary adenylate cyclase–activating polypeptide, gastrin-releasing polypeptide, and cocaine- and amphetamine-regulated transcript in parasympathetic nerves; neuropeptide Y and galanin in the sympathetic nerves; and calcitonin gene–related polypeptide in sensory nerves. Activation of the parasympathetic nerves and administration of their neurotransmitters stimulate insulin and glucagon secretion, whereas activation of the sympathetic nerves and administration of their neurotransmitters inhibit insulin but stimulate glucagon secretion. The autonomic nerves contribute to the cephalic phase of insulin secretion, to glucagon secretion during hypoglycemia, to pancreatic polypeptide secretion, and to the inhibition of insulin secretion, which is seen during stress. In rodent models of diabetes, the number of islet autonomic nerves is upregulated. This review focuses on neural regulation of islet function, with emphasis on the neuropeptides.
Since the discovery of nerves in the pancreatic islet by Paul Langerhans in his thesis from 1869, the neural-islet axis has been explored by a number of neuroanatomists, physiologists, and endocrinologists (rev. in 1–3). It is currently known that branches of the parasympathetic and sympathetic as well as the sensory nervous system innervate the islets with nerve terminals ending closely to the islet endocrine cells. It is also known that these nerves affect islet hormone secretion. The classic neurotransmitters in the islet autonomic nerves are acetylcholine and norepinephrine. During the last decades, the contribution to neural regulation of islet function also by neuropeptides has been established (rev. in 4). Several neuropeptides are localized to islet nerve terminals, are released from the pancreatic nerves upon nerve stimulation, and influence islet hormone secretion. It is also known that the islet innervation is altered in animal models of type 2 diabetes (5). The autonomic nervous system has also been suggested to be involved in the regulation of islet mass (6). The present review highlights the role of neuropeptides in islet function.
PARASYMPATHETIC NERVOUS SYSTEM
Anatomy and effects.
The parasympathetic nerves innervating the pancreatic islets emanate from the pancreatic ganglia, which are innervated by preganglionic parasympathetic nerves originating in the dorsal motor nucleus of the vagus. Activation of the parasympathetic nerves enhances insulin and glucagon secretion (1,3). These stimulatory effects are of physiological relevance under at least three conditions: 1) for the cephalic phase of insulin secretion during meal ingestion, 2) for the glucagon response to hypoglycemia, and 3) for pancreatic polypeptide (PP) secretion.
Parasympathetic nerves and cephalic phase of insulin secretion.
The cephalic phase of insulin secretion leads to the rapid and early increase in insulin levels during the first minutes after food ingestion (7). This is due to activation of olfactory-gustatory sensory receptors in association with psychological stimuli, which activates central parasympathetic nerves that stimulate insulin secretion. The importance of the cephalic phase of insulin secretion was recently examined by an approach to block the autonomic ganglia with the ganglionic blocker, trimetophane, in healthy subjects (7). In this approach, trimetophane was infused intravenously to inhibit the autonomic ganglia. A meal was served during the trimetophane infusion, and the trimetophane infusion was stopped 15 min later. It was first demonstrated that circulating insulin increased within the first 10 min after meal ingestion, which is before any rise in circulating glucose was observed. Of more importance in this context, however, was the second finding that this 10-min insulin response to meal ingestion was reduced by 75% by trimetophane (Fig. 1). This demonstrates that a neurally mediated cephalic phase of meal-related insulin secretion exists in humans. This reduction was accompanied by impairment of glucose elimination, even though the inhibited insulin response was limited to the first 15 min. Hence, the rapid and early insulin secretion contributes substantially to the glucose tolerance after meal ingestion. This may in turn be ascribed to the inhibition of hepatic glucose production by insulin.
Parasympathetic nerves and stimulation of glucagon secretion during hypoglycemia.
The parasympathetic nerves might also be of physiological importance for the stimulation of glucagon secretion during hypoglycemia (8). Glucagon secretion during counterregulation might be mediated by low glucose, low islet insulin, and high epinephrine, which all are consequences of hypoglycemia and all stimulate glucagon secretion. A possible contribution by the islet nerves was examined using the trimetophane protocol to inhibit autonomic ganglia in the presence of insulin-induced hypoglycemia in healthy subjects (9). It was found that the glucagon response to hypoglycemia (2.5 mmol/l glucose) was markedly reduced by trimetophane (Fig. 2). Hence, a significant proportion of the glucagon response to hypoglycemia depends on autonomic nerves.
Parasympathetic nerves and PP secretion.
The islet parasympathetic nerves seem of importance for PP secretion. This is evident from findings that vagus nerve stimulation increases PP secretion (10,11) and that the PP response to hypoglycemia is reduced by autonomic ganglionic blockade, as demonstrated in humans (9). In fact, the coupling of PP secretion to parasympathetic activity has suggested that plasma PP levels may be used as a marker or index of parasympathetic activity (12). However, the physiological relevance of the PP response to parasympathetic activation is not known.
Islet parasympathetic neurotransmitters.
It is well known that acetylcholine stimulates insulin secretion through a direct action on the islet -cells (3). In addition, several neuropeptides are localized to islet parasympathetic nerve terminals and therefore potentially contribute to the islet effects of parasympathetic activation. These neuropeptides are vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase–activating polypeptide (PACAP), gastrin-releasing polypeptide (GRP), neuropeptide Y (NPY), and cocaine- and amphetamine-regulated transcript (CART) peptide (2–4,13–15). Figure 3 illustrates immunofluorescence of rodent pancreatic tissue showing the close proximity of these nerves with the pancreatic islets by showing VIP as an example: nerves co-harboring VIP and the cholinergic marker vesicular acetylcholine transporter (VAchT) enter the islets to terminate in close proximity to the islet endocrine cells (Figs. 3A–C). Figure 3 also shows nerves harboring GRP (Fig. 3G) and CART (Fig. 3H) in islets. Figure 3I–J shows that a great proportion of the CART-containing fibers are also VIP immunoreactive. Similar findings have previously been reported for PACAP (16) and, in the pig, for GRP (17) and, recently, for CART peptide (14,15). Therefore, VIP, PACAP, GRP, and CART are parasympathetic co-transmitter neuropeptides in autonomic nerve endings in the islets.
Effects of parasympathetic co-transmitters on insulin and glucagon secretion in mice.
VIP, PACAP, and GRP stimulate insulin and glucagon secretion when administered both in vivo in several species, including humans, and in vitro in isolated islets or the perfused pancreas (3,4,16–19). Here we report that VIP, PACAP, and GRP all potentiate glucose-stimulated insulin secretion (GSIS) and arginine-stimulated glucagon secretion, both when examined in isolated mouse islets (Figs. 4A and 5A) and when intravenously administered to mice together with glucose (Fig. 4C) or arginine (Fig. 5C). Increased insulin secretion has also been documented in transgenic mice overexpressing the VIP gene (20) or the PACAP gene (21) in the islet -cells. The molecular basis of their effects is still far from fully understood, although it is established that VIP and PACAP signal through cAMP.
The importance of VIP and PACAP for islet physiology has been explored in model experiments using specific receptor antagonists and peptide ligand or receptor knockout mice. A study using PACAP–/– mice showed impaired GSIS in these mice (22). Also VIP–/– mice have been generated (23), but insulin secretion in these animals remains to be examined. Another strategy to explore the physiology of VIP and PACAP is to inhibit the activity or expression of their receptors. The two neuropeptides activate both VPAC1 and VPAC2 receptors and PACAP in addition to PAC1 receptors. Of these receptors, VPAC2 and PAC1 are expressed in islet cells (16). By disrupting the PAC1 receptor gene in mice, expression of truncated PAC1 receptors, which do not bind PACAP, evolves (24). PAC1R–/– mice display a marked reduction in the insulin response to both oral and intravenous glucose, showing that PACAP is of importance for a normal GSIS. Furthermore, the insulin response to intravenous administration of 2-deoxy-glucose (2-DG), is reduced in PAC1R–/– mice (Fig. 6). 2-deoxy-glucose competes with glucose for phosphorylation, which results in neuroglycopenia. This in turn activates the autonomic nerves, which results in a stimulation of insulin secretion after 2 min (25). Other studies have shown that PAC1 receptor antagonists pharmacologically inhibit the insulin response to oral glucose in mice (26) and to vagal nerve activation in the pig pancreas (27). A recent study compared the reduction in insulin secretion after gastric glucose versus intravenous glucose in PAC1R–/– mice by matching the glucose levels under these two conditions. The reduction was more marked after gastric glucose, again suggesting that PACAP contributes to the insulin response to oral glucose (28). The impairment of the insulin response in these mice suggests that PACAP may contribute to the cephalic phase of insulin secretion. In contrast, mice with VPAC2 gene deletion have a normal glucose tolerance during an oral glucose tolerance test in association with a reduced insulin response (29). This suggests increased insulin sensitivity in VPAC2–/– mice with an appropriate downregulation of the insulin response to maintain normal glucose tolerance, which indirectly would support normal -cell function. A recent study has also demonstrated that PAC1R–/– mice display impaired glucagon secretion during hypoglycemia (30). Thus, PACAP may contribute to the parasympathetic involvement in the glucagon response to counterregulation, in addition to its potential physiological importance in regulating insulin secretion after 1) glucose stimulation, 2) vagal nerve activation, and 3) meal intake.
Because VIP and PACAP both strongly stimulate insulin secretion, they may be of potential interest in the treatment of type 2 diabetes. PACAP has been shown to reduce the hyperglycemia in rodent diabetes (high fat–fed mice and GK rats) (31). However, one problem is that PACAP stimulates glucagon secretion and the peptide has potent vasoactive effects, which would limit its usefulness in treatment. Instead, specific activation of VPAC2 receptors would be advantageous. Recently, a specific VPAC2 receptor agonist was described (32): it augments GSIS in both rodent and human islets and potentiates insulin secretion and glucose disposal in rats, thereby offering a novel target for treatment of type 2 diabetes based on islet neuropeptides.
The islet parasympathetic nerve endings also harbor GRP. GRP is released from the pancreas during parasympathetic nerve stimulation and stimulates insulin and glucagon secretion (3,4,17,19). It has been shown that the GRP receptor is expressed in islets (33), suggesting a direct action of the neuropeptide on islet cells. A study in GRP receptor gene–deficient mice has shown that the insulin response to oral glucose is impaired (34), which would support a role in the meal-related insulin response. It was also shown that insulin secretion in response to endogenous nerve activation in mice (by 2-deoxyglucose) is impaired in these mice, suggesting that GRP contributes to neurally mediated islet hormone secretion. These findings suggest that islet neuronal GRP, like VIP and PACAP, is of physiological importance.
It was recently also demonstrated that CART, an anorexigenic peptide that is highly expressed in the brain (rev. in 35), is also a neuropeptide of the rat and mouse pancreas (14,15, rev. in 36). Figure 3H–J shows the location of CART-containing nerves within an islet. Colocalization with VIP demonstrates the parasympathetic identity of the majority of the CART fibers. Recent studies have shown that CART affects islet hormone secretion (37). CART inhibits GSIS from isolated rat islets. On the other hand, CART potentiates GSIS augmented by glucagon-like peptide 1. Although there is hitherto no CART receptor identified, recent data suggest that CART exerts the potentiating effect on glucagon-like peptide 1–mediated GSIS via increased cAMP and the protein kinase A–dependent pathway (37). Furthermore, the potential impact of CART of islet function was demonstrated by using CART–/– mice (14). These mice displayed blunted GSIS both in vivo and in vitro, together with impaired glucose elimination.
SYMPATHETIC NERVOUS SYSTEM
Anatomy and function.
The islet sympathetic nerves are postganglionic, with their nerve cell bodies mainly located in the celiac ganglion or in the paravertebral sympathetic ganglia. Electrical stimulation of the sympathetic nerves inhibits insulin secretion and stimulates glucagon secretion (1–4). This is of physiological importance during stress and physical exercise to elicit a hyperglycemic response by increasing hepatic glucose delivery. An experimental tool for the study of function of the sympathetic nerves is administration of 6-hydroxydopamine to rodents. 6-hydroxydopamine is taken up by way of vesicular monoamine transporter localized to nerve endings of sympathetic neurons and selectively destroys sympathetic nerve terminals. As a consequence, islet nerves staining for the sympathetic marker tyrosine hydroxylase (TH) are absent in mice at 48 h after administration of 6-hydroxydopamine (38). This is associated with augmented GSIS (39,40), reduced insulin gene expression, and increased -cell mass (38). These findings therefore suggest that the sympathetic nerves are of importance both for insulin secretion, insulin gene expression, and islet -cell mass.
Islet sympathetic neurotransmitters.
The classic sympathetic neurotransmitter is norepinephrine, which inhibits GSIS and stimulates glucagon secretion (3,39). However, combined - and -adrenoceptor blockade does not prevent sympathetic nerve activation from inhibiting insulin secretion (41). This suggests that neurotransmitters other than norepinephrine contribute to some sympathetic islet effects. Neuropeptides localized to islet sympathetic nerve terminals are NPY and galanin (1–4). They are both released from the pancreas during sympathetic nerve activation (13,39) and, like sympathetic nerve stimulation, inhibit insulin secretion and stimulate glucagon secretion (3,4). Figure 3D–E illustrates that NPY nerves are localized to islets and the colocalization with TH illustrates the sympathetic nature of the NPY-containing nerve endings. It should be emphasized, however, that NPY is localized also to nerve endings that do not harbor TH. Also, galanin is localized to TH-containing nerve endings in the islets in a number of species (2,42). Both NPY and galanin are released from the pancreas during sympathetic nerve activation (13,43) and, like sympathetic nerve stimulation, inhibit insulin secretion and stimulate glucagon secretion (3,4). Figures 4B and D and 5B and D show that norepinephrine, NPY, and galanin all inhibit GSIS and augment arginine-stimulated glucagon secretion in isolated mouse islets and in vivo in mice. Because NPY and galanin thus mimic the effects of sympathetic nerve stimulation, they might contribute to sympathetic islet effects.
In the dog, galanin has been suggested to be of major importance in contributing to the inhibitory influence of sympathetic nerve activation on insulin secretion (43). Also in mice, galanin seems to be of physiological impact because immunoneutralization of galanin prevents the inhibition of insulin secretion, which is seen during a stress model (swimming) in mice (44). To study the physiological impact of galanin on islet function in more detail, mice with a loss-of-function mutation in the galanin gene have been examined (45). The inhibition of insulin secretion that is seen after administration of 2-deoxyglucose (and that reflects activation of sympathetic nerves) was impaired in galanin–/– mice. This further supports a contribution by galanin of the response to sympathetic activation.
SENSORY NERVES
The islets are innervated by sensory nerves that harbor calcitonin gene–related polypeptide (CGRP) (1–4). The fibers leave the pancreas along the sympathetic fibers with the splanchnic nerves and reach the spinal cord. Figure 3H illustrates an islet fiber harboring CGRP. The relevance of these nerves for islet function is far from understood. These sensory nerves may be targeted by the use of the toxin capsaicin, which causes degeneration of small unmyelinated C-fibers. Capsaicin is an agonist for the transient receptor potential vanilloid receptor (TRPV1); acutely, it activates the receptors, which leads to a release of the neurotransmitters. After the acute effect, however, capsaicin leads to loss of unmyelinated sensory fibers in conjunction with a substantial number of CGRP nerves (46). If capsaicin is given to neonatal rodents, the sensory deactivation is permanent, whereas if given to adults, the deactivation is transient. One effect of capsaicin-induced sensory deactivation is increased insulin secretion (47). This would suggest that sensory nerve activation in islets inhibits insulin secretion.
Islet sensory neurotransmitters.
CGRP nerves are scattered through the pancreas but with particular density around small blood vessels and islets (48). Furthermore, exogenous administration of CGRP inhibits GSIS (48). This suggests that sensory nerves inhibit insulin secretion; however, the physiological importance of this remains elusive. It should be mentioned in this context that CART is also present in CGRP-containing fibers in rat and mouse pancreas (14,15). The biological significance of CART in these fibers needs further investigation.
Sensory deafferentation and treatment of diabetes.
Since sensory nerves apparently inhibit insulin secretion, possibly through CGRP, and since sensory deafferentation by capsaicin increases insulin secretion, it has been proposed that sensory deactivation might be a novel target for treatment of type 2 diabetes. This hypothesis was substantiated in a recent study in obese Zucker rats (49). This novel potential neurally based therapeutic approach has been further explored by using the toxin resiniferatoxin. This is a vanilloid that has the same mechanism in causing deactivation/degeneration of C-fibers (and A-fibers) as capsaicin but is less toxic. By administering resiniferatoxin to obese Zucker rats (50) and to Zucker diabetic fatty (ZDF) rats (51), improved glycemia has been observed, together with improved insulin secretion, as demonstrated in ZDF rats.
ISLET NERVES AND DIABETES
Several studies have indicated altered islet neurohormonal influences in models of insulin resistance and type 2 diabetes, and therefore it has been speculated whether islet nerve dysfunction may contribute to the development of type 2 diabetes. A study in high fat–fed rats, which is a model of glucose intolerance and type 2 diabetes, disclosed an increased islet innervation (52). Furthermore, insulin resistance in high fat–fed mice is accompanied by augmented insulin secretion after cholinergic activation as a sign of cholinergic hypersensitivity (53) and as a sign that the hyperinsulinemia in ob/ob mice is highly sensitive to atropine (54). Indeed, cholinergic activation by carbachol normalizes insulin secretion in high fat–fed mice (55). It was therefore of interest that reduced islet innervation was evident in a model of type 2 diabetes, the Chinese hamster (5).
We present here immunocytochemical data of islets from two rodent models of diabetes. The models are the Goto-Kakizaki (GK) rats and the db/db mice. The GK rat model is one of the best-described models of type 2 diabetes, as recently was reviewed (56). Its basis is a -cell defect, which occurs through changes in several independent genes, leading to impaired insulin secretion in combination with metabolic impairment reducing -cell neogenesis and a potential acquired loss of -cell differentiation due to glucotoxicity. The db/db mouse is characterized by a leptin receptor deficiency, resulting in insulin resistance and impaired islet function (57). Figures 7 and 8 show that in both these two rodent models of diabetes, the islet innervation is upregulated. This emphasizes that islet innervation is of importance for the islet dysfunction in diabetes. We also found that certain neuropeptides, e.g., VIP and CART (Table 1) (37), are expressed in islet endocrine cells in the diabetic rodents. This is similar to our previous observation of marked expression of NPY in islet -cells in dexamethasone-induced insulin resistance in rats (58). We propose that augmented endocrine cell expression of neurotransmitters in the islets is a sign of islet adaptation for normalization of glucose homeostasis. We suggest the term "neuro-islet plasticity" for this phenomenon; a thorough examination of which needs to be undertaken now.
A novel potential effect of parasympathetic nerves, which may be of pathophysiological relevance for type 2 diabetes, is stimulation of -cell mass. Regulation of -cell mass has recently come into focus because of studies suggesting that -cell mass is reduced in type 2 diabetes (6,59). In adult life, -cell growth and renewal is mainly regulated by replication and differentiation from islet precursor cells within islets and within the duct epithelium (60). An important mechanism for increase in -cell mass may therefore be the increased demand created by insulin resistance; therefore, islets are enlarged in, for instance, the pre-diabetic state and obesity. On the other hand, in overt type 2 diabetes, -cell mass seems to be reduced, which may be ascribed to exaggerated apoptosis triggered by, for instance, glucotoxicity or lipotoxicity. However, neural effects may also contribute to these perturbations. Thus, it has been demonstrated that vagotomy attenuates the islet hyperplasia in ob/ob mice, the mechanism of which needs to be further explored (61). From a therapeutic point of view, the potential of the incretin hormone glucagon-like peptide 1 to augment islet -cell mass, presumably by both increasing neogenesis and inhibiting apoptosis, has been extensively discussed (60). However, islet neuropeptides may also be of interest in this context; for example, exogenous administration of PACAP has been shown to maintain -cell mass in an experimental diabetes model in mice, which is due to increased -cell apoptosis (-cell overexpression of calmodulin [62]).
CONCLUSIONS AND OUTSTANDING ISSUES
The pancreatic islets are innervated by the autonomic nervous system, which affect islet function through both classic neurotransmitters and neuropeptides. Table 1 summarizes the neuropeptides, which are of greatest importance in this context. The islet nerves seem of physiological relevance for 1) the insulin secretion during the cephalic phase of meal ingestion, 2) the glucagon response to hypoglycemia, 3) the PP release, and 4) the islet stress response. The islet nerves may also be of significance for the regulation of islet mass. Novel potential targets for treatment of diabetes may evolve from studies of islet effects of autonomic nerves and neuropeptides. Of most interest at the moment is stimulation of insulin secretion by 1) cholinergic agonism, 2) VPAC2 receptor agonists, 3) PACAP, 4) CART in combination with glucagon-like peptide 1, and 5) sensory nerve deactivation. However, much remains to be established regarding the potential role for the autonomic nerves in relation to islet function. Complex issues relate to mechanisms of activation of the autonomic nerves and regulation of release of the neurotransmitters in relation to each other and the possibility of synergistic action of the neurotransmitters. Other issues relate to the involvement of neuropeptides in islet physiology in humans as well as to potential importance of the autonomic nerves for the islet compensation to insulin resistance, to diabetes development, and as targets for treatment of diabetes. Therefore, although almost 140 years have passed since the first description of islet nerves by Paul Langerhans, much effort has yet to be paid to understand the role for these nerves in islet physiology and pathophysiology.
ACKNOWLEDGMENTS
These studies have been supported by the Swedish Research Council (grants 6834 and 4499), Swedish Diabetes Association, Albert Phlson Foundation, The Royal Physiographic Society, Novo Nordisk Foundation, Tore Nilsson, ke Wiberg and Gyllenstiernska-Krapperup Foundations, Region Skne, and the Faculty of Medicine, Lund University.
FOOTNOTES
This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educational grant from Servier.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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Filipsson K, Holst JJ, Ahren B: PACAP contributes to insulin secretion after gastric glucose gavage in mice. Am J Physiol 279:R424–R432, 2000
Torne K, Hannibal J, Fahrenkrug J, Holst JJ: PACAP-(1-39) as neurotransmitter in pig pancreas: receptor activation revealed by the antagonist PACAP-(6-38). Am J Physiol 273:G436–G446, 1997
Ahren B: The insulin response to gastric glucose is reduced in PAC1 and GRP receptor gene deleted mice. Nutr Metab Cardiovasc Dis 16:S17–S21, 2006
Asnicar MA, Kster A, Heiman ML, Tinsley F, Smith DP, Galbreath E, Fox N, Linda Y, Blum WF, Hsiung HM: Vasoactive intestinal polypeptide/pituitary adenylate cyclase-activating peptide receptor 2 deficiency in mice results in growth retardation and increased basal metabolic rate. Endocrinology 143:3994–3006, 2002
Persson K, Ahren B: The neuropeptide PACAP contributes to the glucagon response to insulin-induced hypoglycemia in mice. Acta Physiol Scand 175:24–28, 2002
Yada T, Sakurada M, Filipsson K, Kikuchi M, Ahren B: Intraperitoneal PACAP administration decreases blood glucose in GK rats, and in normal and high-fat diet mice. Ann N Y Acad Sci 921:259–263, 2000
Tsutsumi M, Claus TH, Liang Y, Li Y, Yang L, Zhu J, Dela Cruz F, Peng X, Cheng H, Yung SL, Hamren DS, Livingston JN, Pan CQ: A potent and highly selective VPAC2 agonist enhances glucose-induced insulin release and glucose disposal. Diabetes 51:1453–1460, 2002
Persson K, Pacini G, Sundler F, Ahren B: Islet function phenotype in gastrin-releasing peptide gene deficient mice. Endocrinology 143:3717–3726, 2002
Persson K, Gingerich RL, Nayak S, Wada K, Wada E, Ahren B: Reduced GLP-1 and insulin responses and glucose intolerance after gastric glucose in GRP receptor deleted mice. Am J Physiol 279:E956–E962, 2000
Hunter RG, Kuhar MJ: CART peptides as targets for CNS drug development. Curr Drug Target CNS Neurol Disord 3:201–205, 2003
Wierup N, Sundler F: CART is a novel islet regulatory peptide. Peptides 27:2031–2036, 2006
Wierup N, Bjrkquist M, Kuhar MJ, Mulder H, Sundler F: CART regulates islet hormone secretion and is expressed in the -cells of type 2 diabetic rats. Diabetes 55:305–311, 2006
Kvist Reimer M, Sundler F, Ahren B: Effects of chemical sympathectomy by means of 6-hydroxydopamine on insulin secretion and islet morphology in alloxan-diabetic mice. Cell Tissue Res 307:203–209, 2002
Karlsson S, Myrsen U, Nieuwenhuizen A, Sundler F, Ahren B: Presynaptic sympathetic mechanism in the insulinostatic effect of epinephrine in mouse pancreatic islets. Am J Physiol 272:R1371–R1378, 1997
Ahren B, Lundquist I: Adrenalectomy and chemical sympathectomy by 6-hydroxydopamine: effects on basal and stimulated insulin secretion. Pflügers Arch 390:17–21, 1981
Dunning BE, Ahren B, Veith RC, Taborsky GJ Jr: Non-adrenergic sympathetic neural influences on basal pancreatic hormone secretion. Am J Physiol 255:E785–E790, 1988
Ahren B, Lindskog S: Galanin and the regulation of islet hormone secretion. Int J Pancreatol 11:147–160, 1992
Dunning BE, Taborsky GJ Jr: Galanin release during pancreatic nerve stimulation is sufficient to influence islet function. Am J Physiol 256:E191–E198, 1989
Dunning BE, Karlsson S, Ahren B: Contribution of galanin to stress-induced impairment of insulin secretion in swimming mice. Acta Physiol Scand 143:145–152, 1991
Ahren B, Pacini G, Wynick D, Wierup N, Sundler F: Loss-of-function mutation of the galanin gene is associated with perturbed islet function in mice. Endocrinology 145:3190–3196, 2004
Karlsson S, Sundler F, Ahren B: Neonatal capsaicin-treatment in mice: effects on pancreatic peptidergic nerves and 2-deoxy-D-glucose-induced insulin and glucagon secretion. J Autonom Nerv Syst 39:51–60, 1992
Karlsson S, Scheurink AJW, Steffens AB, Ahren B: Involvement of capsaicin-sensitive nerves in regulation of insulin secretion and glucose tolerance in conscious mice. Am J Physiol 267:R1071–R1077, 1993
Ahren B, Pettersson M: Calcitonin gene-related peptide (CGRP) and amylin and the endocrine pancreas. Int J Pancreatol 6:1–15, 1990
Gram DX, Hansen AJ, Wilken M, Elm T, Svendsen O, Carr RD, Ahren B, Brand CL: Plasma calcitonin gene-related peptide is increased prior to obesity, and sensory nerve desensitization by capsaicin improves oral glucose tolerance in obese Zucker rats. Eur J Endocrinol 153:963–969, 2005
Moesgaard SG, Brand CL, Sturis J, Ahren B, Wilken M, Fleckner J, Carr RD, Svendsen O, Hansen AJ, Gram DX: Sensory nerve inactivation by resiniferatoxin improves insulin sensitivity in male obese Zucker rats. Am J Physiol 288:E1137–E1145, 2005
Gram DX, Hansen AJ, Deacon CF, Brand CL, Ribel U, Wilken M, Carr RD, Svendsen O, Ahren B: Sensory nerve desensitization by resiniferatoxin improves glucose tolerance and increases insulin secretion in Zucker diabetic fatty rats and is associated with reduced plasma activity of dipeptidyl peptidase IV. Eur J Pharmacol 509:211–217, 2005
Ahren B, Gudbjartsson T, Naser Al-Amin A, Mrtensson H, Myrsen-Axcrona U, Karlsson S, Mulder H, Sundler F: Islet perturbations in insulin resistant high-fat fed rats. Pancreas 18:75–83, 1999
Ahren B, Simonsson E, Scheurink AJW, Mulder H, Myrsen U, Sundler F: Dissociated insulinotropic sensitivity to glucose and carbachol in high-fat diet-induced insulin resistance in C57BL/6J mice. Metabolism 46:97–106, 1997
Ahren B, Lundquist I: Modulation of basal insulin secretion in the obese, hyperglycemic mouse. Metabolism 31:172–179, 1982
Ahren B, Sauerberg P, Thomsen C: Increased insulin secretion and normalization of glucose tolerance by cholinergic agonism in high-fat fed C57BL/6J mice. Am J Physiol 277:E93–E102, 1999
Portha B: Programmed disorders of -cell development and function as one cause of type 2 diabetes The GK rat paradigm. Diabetes Metab Res Rev 21:495–504, 2005
Ktorza A, Bernard C, Parent V, Penicaud L, Froguel P, Lathrop M, Gaugier D: Are animal models of diabetes relevant to study of the genetics of non-insulin-dependent diabetes in humans Diabetes Metab 23 (Suppl. 2):38–46, 1997
Myrsen U, Ahren B, Sundler F: Dexamethasone-induced NPY expression in rat islet endocrine cells: rapid reversibility and partial prevention by insulin. Diabetes 45:1306–1316, 1996
Butler AE, Janson J, Bonner-Weir S, Ritzel R, Rizza RA, Butler PC: -Cell deficit and increased -cell apoptosis in humans with type 2 diabetes. Diabetes 52:102–110, 2003
Perfetti R, Hui H: The role of GLP-1 in the life and death of pancreatic beta cells. Horm Metab Res 36:804–810, 2004
Edvell A, Lindstrm P: Vagotomy in young obese hyperglycemic mice: effects on syndrome development and islet proliferation. Am J Physiol 274:E1034–E1039, 1998
Tsunekawa S, Miura Y, Yamamoto N, Itoh N, Ariyoshi Y, Senda T, Oiso Y, Niki I: Systemic administration of pituitary adenylate cyclase-activating polypeptide maintains beta-cell mass and retards onset of hyperglycaemia in beta-cell-specific calmodulin-overexpressing transgenic mice. Eur J Endocrinol 152:805–811, 2005(Bo Ahren, Nils Wierup, and Frank Sundler)