Intraperitoneal PACAP administration decreases blood glucose in GK rats, and in normal and high fat diet mice

The functional heterogeneity of PACAP: Stress, learning, and pathology

Pituitary adenylate cyclase-activating peptide (PACAP) is a highly conserved and widely expressed neuropeptide that has emerged as a key regulator of multiple neural and behavioral processes. PACAP systems, including the various PACAP receptor subtypes, have been implicated in neural circuits of learning and memory, stress, emotion, feeding, and pain. Dysregulation within these PACAP systems may play key roles in the etiology of pathological states associated with these circuits, and PACAP function has been implicated in stress-related psychopathology, feeding and metabolic disorders, and migraine. Accordingly, central PACAP systems may represent important therapeutic targets; however, substantial heterogeneity in PACAP systems related to the distribution of multiple PACAP isoforms across multiple brain regions, as well as multiple receptor subtypes with several isoforms, signaling pathways, and brain distributions, provides both challenges and opportunities for the development of new clinically-relevant strategies to target the PACAP system in health and disease. Here we review the heterogeneity of central PACAP systems, as well as the data implicating PACAP systems in clinically-relevant behavioral processes, with a particular focus on the considerable evidence implicating a role of PACAP in stress responding and learning and memory. We also review data suggesting that there are sex differences in PACAP function and its interactions with sex hormones. Finally, we discuss both the challenges and promise of harnessing the PACAP system in the development of new therapeutic avenues and highlight PACAP systems for their critical role in health and disease.

PACAP attenuates hepatic lipid accumulation through the FAIM/AMPK/IRβ axis during overnutrition

Objective

Pituitary adenylate cyclase-activating polypeptide (PACAP) was reported to attenuate hepatic lipid accumulation in overnutrition-related metabolic disorder, mediated by up-regulation of fas apoptosis inhibitory molecule (FAIM). However, how PACAP regulates FAIM in metabolic tissues remains to be addressed. Here we investigated the underlying mechanism on the role of PACAP in ameliorating metabolic disorder and examined the potential therapeutic effects of PACAP in preventing the progression of metabolic associated fatty liver disease (MAFLD).

Methods

Mouse models with MAFLD induced by high-fat diet were employed. Different doses of PACAP were intraperitoneally administrated. Western blot, luciferase assay, lentiviral-mediated gene manipulations and animal metabolic phenotyping analysis were performed to explore the signaling pathway involved in PACAP function.

Results

PACAP ameliorated the excessive hepatic lipid accumulation and inhibited lipogenesis in HFD-fed C57BL/6J mice. Mechanistically, PACAP activated the FAIM-AMPK-IRβ axis to inhibit the expression of lipid synthesis genes, and FAIM mediated the effects of PACAP. FAIM suppression via lentiviral-mediated shRNA inhibited the activation of AMPK, whereas FAIM overexpression promoted AMPK activation. PACAP increased the promoter activity of FAIM gene through activating PKA-CREB signaling pathway.

Conclusion

Our work demonstrated that the administration of PACAP represented a feasible approach for treating hepatic lipid accumulation in MAFLD. The findings reveal the molecular mechanism that PACAP increase FAIM expression and activates the FAIM/AMPK/IRβ signaling axis, thus inhibits lipogenesis to mediate its beneficial effects.

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PACAP ameliorates hepatic lipid accumulation through the AMPK pathway.

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AMPK is a downstream mediator of FAIM.

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FAIM is transcriptionally activated by CREB and regulated by PACAP.

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PACAP regulates the FAIM-AMPK-IRβ axis to treat fatty liver phenotype.

The PACAP/PAC1 Receptor System and Feeding

Pituitary adenylyl cyclase activating polypeptide (PACAP) belongs to the vasoactive intestinal polypeptide (VIP)/secretin/glucagon superfamily. PACAP is present in two forms (PACAP-38 and PACAP-27) and binds to three guanine-regulatory (G) protein-coupled receptors (PAC1, VPAC1, and VPAC2). PACAP is expressed in the central and peripheral nervous systems, with high PACAP levels found in the hypothalamus, a brain region involved in feeding and energy homeostasis. PAC1 receptors are high-affinity and PACAP-selective receptors, while VPAC1 and VPAC2 receptors show a comparable affinity to PACAP and VIP. PACAP and its receptors are expressed in the central and peripheral nervous systems with moderate to high expression in the hypothalamus, amygdala, and other limbic structures. Consistent with their expression, PACAP is involved in several physiological responses and pathological states. A growing body of literature suggests that PACAP regulates food intake in laboratory animals. However, there is no comprehensive review of the literature on this topic. Thus, the purpose of this article is to review the literature regarding the role of PACAP and its receptors in food intake regulation and to synthesize how PACAP exerts its anorexic effects in different brain regions. To achieve this goal, we searched PubMed and reviewed 68 articles regarding the regulatory action of PACAP on food intake. Here, we present the literature regarding the effect of exogenous PACAP on feeding and the role of endogenous PACAP in this process. We also provide evidence regarding the effect of PACAP on the homeostatic and hedonic aspects of food intake, the neuroanatomical sites where PACAP exerts its regulatory action, which PACAP receptors may be involved, and the role of various signaling pathways and neurotransmitters in hypophagic effects of PACAP.

The EndoC-βH1 cell line is a valid model of human beta cells and applicable for screenings to identify novel drug target candidates

OBJECTIVE

To characterize the EndoC-βH1 cell line as a model for human beta cells and evaluate its beta cell functionality, focusing on insulin secretion, proliferation, apoptosis and ER stress, with the objective to assess its potential as a screening platform for identification of novel anti-diabetic drug candidates.

METHODS

EndoC-βH1 was transplanted into mice for validation of in vivo functionality. Insulin secretion was evaluated in cells cultured as monolayer and as pseudoislets, as well as in diabetic mice. Cytokine induced apoptosis, glucolipotoxicity, and ER stress responses were assessed. Beta cell relevant mRNA and protein expression were investigated by qPCR and antibody staining. Hundreds of proteins or peptides were tested for their effect on insulin secretion and proliferation.

RESULTS

Transplantation of EndoC-βH1 cells restored normoglycemia in streptozotocin induced diabetic mice. Both in vitro and in vivo, we observed a clear insulin response to glucose, and, in vitro, we found a significant increase in insulin secretion from EndoC-βH1 pseudoislets compared to monolayer cultures for both glucose and incretins. Apoptosis and ER stress were inducible in the cells and caspase 3/7 activity was elevated in response to cytokines, but not affected by the saturated fatty acid palmitate. By screening of various proteins and peptides, we found Bombesin (BB) receptor agonists and Pituitary Adenylate Cyclase-Activating Polypeptides (PACAP) to significantly induce insulin secretion and the proteins SerpinA6, STC1, and APOH to significantly stimulate proliferation. ER stress was readily induced by Tunicamycin and resulted in a reduction of insulin mRNA. Somatostatin (SST) was found to be expressed by 1% of the cells and manipulation of the SST receptors was found to significantly affect insulin secretion.

CONCLUSIONS

Overall, the EndoC-βH1 cells strongly resemble human islet beta cells in terms of glucose and incretin stimulated insulin secretion capabilities. The cell line has an active cytokine induced caspase 3/7 apoptotic pathway and is responsive to ER stress initiation factors. The cells' ability to proliferate can be further increased by already known compounds as well as by novel peptides and proteins. Based on its robust performance during the functionality assessment assays, the EndoC-βH1 cell line was successfully used as a screening platform for identification of novel anti-diabetic drug candidates.

Tackling dipeptidyl peptidase IV in neurological disorders

Dipeptidyl peptidase IV (DPP-IV) is a serine protease best known for its role in inactivating glucagon-like peptide-1 (GLP-1), pituitary adenylate cyclase-activating polypeptide (PACAP) and glucose-dependent insulinotropic peptide (GIP), three stimulators of pancreatic insulin secretion with beneficial effects on glucose disposal. Owing to the relationship between DPP-IV and these peptides, inhibition of DPP-IV enzyme activity is considered as an attractive treatment option for diabetic patients. Nonetheless, increasing studies support the idea that DPP-IV might also be involved in the development of neurological disorders with a neuroinflammatory component, potentially through its non-incretin activities on immune cells. In this review article, we aim at highlighting recent literature describing the therapeutic value of DPP-IV inhibitors for the treatment of such neurological conditions. Finally, we will illustrate some of the promising results obtained using berberine, a plant extract with potent inhibitory activity on DPP-IV.

A novel selective VPAC2 agonist peptide-conjugated chitosan modified selenium nanoparticles with enhanced anti-type 2 diabetes synergy effects

A novel neuroendocrine peptide, pituitary adenylate cyclase activating peptide (PACAP), was found to have an important role in carbohydrate or lipid metabolism and was susceptible to dipeptidyl peptidase IV degradation. It can not only mediate glucose-dependent insulin secretion and lower blood glucose by activating VPAC2 receptor, but also raise blood glucose by promoting glucagon production by VPAC1 receptor activation. Therefore, its therapeutic application is restricted by the exceedingly short-acting half-life and the stimulatory function for glycogenolysis. Herein, we generated novel peptide-conjugated selenium nanoparticles (SeNPs; named as SCD), comprising a 32-amino acid PACAP-derived peptide DBAYL that selectively binds to VPAC2, and chitosan-modified SeNPs (SeNPs-CTS, SC) as slow-release carrier. The circulating half-life of SCD is 14.12 h in mice, which is 168.4-and 7.1-fold longer than wild PACAP (~5 min) and DBAYL (~1.98 h), respectively. SCD (10 nmol/L) significantly promotes INS-1 cell proliferation, glucose uptake, insulin secretion, insulin receptor expression and also obviously reduces intracellular reactive oxygen species levels in H₂O₂-injured INS-1 cells. Furthermore, the biological effects of SCD are stronger than Exendin-4 (a clinically approved drug through its insulinotropic effect), DBAYL, SeNPs or SC. A single injection of SCD (20 nmol/kg) into db/db mice with type 2 diabetes leads to enhanced insulin secretion and sustained hypoglycemic effect, and the effectiveness and duration of SCD in enhancing insulin secretion and reducing blood glucose levels are much stronger than Exendin-4, SeNPs or SC. In db/db mice, chronic administration of SCD by daily injection for 12 weeks markedly improved glucose and lipid profiles, insulin sensitivity and the structures of pancreatic and adipose tissue. The results indicate that SC can play a role as a carrier for the slow release of bioactive peptides and SCD could be a hopeful therapeutic against type 2 diabetes through the synergy effects of DBAYL and SeNPs.

A recombinant slow-release PACAP-derived peptide alleviates diabetes by promoting both insulin secretion and actions

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuroendocrine factor that activates both the receptor VPAC1 and VPAC2. Although PACAP possesses insulinotropic activity, its therapeutic application is limited by the extremely short acting half-life and the stimulatory effects on glucagon production via a VPAC1-dependent mechanism. Here we have generated a recombinant PACAP-derived peptide (named as MHDBAY) comprising a 7-mer albumin-binding peptide identified by phage display screening (WQRPSSW), a cleavage peptide for Factor Xa (FXa) and dipeptidyl peptidase IV (DPP IV), and a 31-amino acid PACAP-derived peptide (DBAY) that can specifically bind to the VPAC2 receptor. MHDBAY binds to albumin both in vitro and in animals, thereby leading to an orderly slow release of the active peptide DBAY via the protease cleavage. In db/db mice and New Zealand rabbits, the circulating half-life of MHDBAY is approximately 12.2 h, which is 146-fold longer than DBAY (∼5 min). A single injection of MHDBAY into db/db diabetic mice markedly increases insulin secretion, thereby leading to sustained alleviation of hyperglycemia. The potency and duration of MHDBAY in increasing insulin secretion and decreasing blood glucose levels is much greater than Exendin-4, an anti-diabetic drug via its insulinotropic actions. Furthermore, chronic administration of MHDBAY by daily injection for 8 weeks significantly improves both glucose and lipid profiles and also greatly increases insulin sensitivity in db/db mice. These findings suggest that serum albumin may act as a reservoir for slow-release of small bioactive peptides, and MHDBAY may represent a promising therapeutic peptide for diabetes.

Pleiotropic mechanisms for the glucose-lowering action of DPP-4 inhibitors

Dipeptidyl peptidase (DPP)-4 inhibition is a glucose-lowering treatment for type 2 diabetes. The classical mechanism for DPP-4 inhibitors is that they inhibit DPP-4 activity in peripheral plasma, which prevents the inactivation of the incretin hormone glucagon-like peptide (GLP)-1 in the peripheral circulation. This in turn increases circulating intact GLP-1, which results in stimulated insulin secretion and inhibited glucagon secretion, in turn increasing glucose utilization and diminishing hepatic glucose production, which, through reduction in postprandial and fasting glucose, reduces HbA1c. However, recent experimental studies in mainly rodents but also to a limited degree in humans have found additional mechanisms for DPP-4 inhibitors that may contribute to their glucose-lowering action. These nonclassical mechanisms include 1) inhibition of gut DPP-4 activity, which prevents inactivation of newly released GLP-1, which in turn augments GLP-1-induced activations of autonomic nerves and results in high portal GLP-1 levels, resulting in inhibited glucose production through portal GLP-1 receptors; 2) inhibition of islet DPP-4 activity, which prevents inactivation of locally produced intact GLP-1 in the islets, thereby augmenting insulin secretion and inhibiting glucagon secretion and possibly preventing islet inflammation; and 3) prevention of the inactivation of other bioactive peptides apart from GLP-1, such as glucose-dependent insulinotropic polypeptide, stromal-derived factor-1α, and pituitary adenylate cyclase-activating polypeptide, which may improve islet function. These pleiotropic effects may contribute to the effects of DPP-4 inhibition. This Perspectives in Diabetes outlines and discusses these nonclassical mechanisms of DPP-4 inhibition.

Molecular mechanisms underlying the Nephroprotective effects of PACAP in diabetes

Diabetic nephropathy is the leading cause of end-stage renal failure and accounts for 30-40 % of patients entering renal transplant programmes. The nephroprotective effects of the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP38) against diabetes have been shown previously, but the molecular mechanisms responsible for these effects remain unknown. In the present study, we showed that PACAP treatment counteracted the diabetes-induced increase in the level of the proapoptotic pp38MAPK and cleaved caspase-3 and also decreased the p60 subunit of NFκB. The examined antiapoptotic factors, including pAkt and pERK1/2, showed a slight increase in the diabetic kidneys, while PACAP treatment resulted in a notable elevation of these proteins. PCR and Western blot revealed the downregulation of fibrotic markers, like collagen IV and TGF-β1 in the kidney. PACAP treatment resulted in increased expression of the antioxidant glutathione. We conclude that the nephroprotective effect of PACAP in diabetes is, at least partly, due to its antiapoptotic, antifibrotic and antioxidative effect in addition to the previously described antiinflammatory effect.

Therapeutic potential of VIP vs PACAP in diabetes

Type 2 diabetes (T2D) is characterized by chronic insulin resistance and a progressive decline in beta-cell function. Although rigorous glucose control can reduce morbidity and mortality associated with diabetes, achieving optimal long-term glycemic control remains to be accomplished in many diabetic patients. As beta-cell mass and function inevitably decline in T2D, exogenous insulin administration is almost unavoidable as a final outcome despite the use of oral antihyperglycemic agents in many diabetic patients. Pancreatic islet cell death, but not the defect in new islet formation or beta-cell replication, has been blamed for the decrease in beta-cell mass observed in T2D patients. Thus, therapeutic approaches designed to protect islet cells from apoptosis could significantly improve the management of T2D, because of its potential to reverse diabetes not just ameliorate glycemia. Therefore, an ideal beta-cell-preserving agent is expected to protect beta cells from apoptosis and stimulate postprandial insulin secretion along with increasing beta-cell replication and/or islet neogenesis. One such potential agent, the islet endocrine neuropeptide vasoactive intestinal peptide (VIP) strongly stimulates postprandial insulin secretion. Because of its broad spectrum of biological functions such as acting as a potent anti-inflammatory factor through suppression of Th1 immune response, and induction of immune tolerance via regulatory T cells, VIP has emerged as a promising therapeutic agent for the treatment of many autoimmune diseases including diabetes.

Early changes in pituitary adenylate cyclase-activating peptide, vasoactive intestinal peptide and related receptors expression in retina of streptozotocin-induced diabetic rats

The retinal expression and distribution of pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP) and their receptors was investigated in early streptozotocin (STZ)-induced diabetic rats. Diabetes was induced in rats by STZ injection (60 mg/kg i.p.). PACAP, VIP and their receptors in nondiabetic control and diabetic retinas were assayed by quantitative real-time PCR and Western blot 1 and 3 weeks after STZ injection. Effects of intravitreal treatment with PACAP38 on the expression of the two apoptotic-related genes Bcl-2 and p53 were also evaluated. PACAP and VIP, as well as VPAC1 and VPAC2 receptors, but not PAC1 mRNA levels, were transiently induced in retinas 1 week following STZ. These findings were confirmed by immunoblot analyses. Three weeks after the induction of diabetes, significant decreases in the expression of peptides and their receptors were observed, Bcl-2 expression decreased and p53 expression increased. Intravitreal injection of PACAP38 restored STZ-induced changes in retinal Bcl-2 and p53 expression to nondiabetic levels. The initial upregulation of PACAP, VIP and related receptors and the subsequent downregulation in retina of diabetic rats along with the protective effects of PACAP38 treatment, suggest a role for both peptides in the pathogenesis of diabetic retinopathy.

Compensatory recovery of blood glucose levels in KKA(y) mice fed a high-fat diet: insulin-sparing effects of PACAP overexpression in β cells

Inadequate compensatory insulin secretion is observed during the development of type 2 diabetes and deteriorates over time in a manner that is difficult to reverse. Here, we found that plasma glucose levels in genetically diabetic KKA(y) mice fed a high-fat diet were markedly increased in young mice. However, the levels started to decrease at 22 weeks of age and returned to normal levels at around 40 weeks of age. These changes were accompanied by a marked increase in insulin levels from week 25 onwards. Decreased energy intake and suppressed fat pad accumulation were observed at 44-45 weeks of age compared with those at 19-22 weeks of age. β cell-specific overexpression of pituitary adenylate cyclase-activating polypeptide (PACAP), an insulinotropic neuropeptide, decreased the insulin levels required to compensate for hyperglycemia. Glucose disposal was significantly enhanced despite impaired insulin sensitivity in 41-44-week-old A(y) mice without or with PACAP overexpression. In conclusion, the present results provide further evidence that PACAP is involved in the regulation of hyperinsulinemia and islet hyperplasia in type 2 diabetes. Our results also indicate that A(y) mice fed a high-fat diet constitute an animal model suitable to study compensatory islet hyperplasia.

Trophic effects of PACAP on pancreatic islets: a mini-review

Progressive beta-cell insufficiency in the pancreas is a hallmark of both types I and II diabetes, and agents that protect against beta-cell dysfunction are potential drug targets for diabetes mellitus. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a strong secretagogue of insulin from pancreatic islets and is suggested to be involved in physiological blood glucose homeostasis and the pathology of diabetes. Recent studies in genetically engineered animal models have shown that PACAP stimulates pancreatic functions, especially in cooperation with other regulatory factors including glucose. Furthermore, chronic activation of PACAP signaling regulates pancreatic islet mass in a context-dependent manner. Accumulating in vivo and in vitro evidence suggest that PACAP has trophic effects and regulates both proliferation and cell viability of beta-cells and thereby contributes to protection against diabetes. This review focuses on such trophic actions of PACAP on pancreatic beta-cells and discusses the pathophysiological significance of pancreatic PACAP, with the aim to provide information for future development of treatment for diabetes.

Effect of pituitary adenylate cyclase-activating polypeptide in islet transplantation

INTRODUCTION

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an islet substance serving as an intra-islet amplifier of glucose-induced insulin secretion similar to exendin-4. It has been reported that systemic administration of PACAP maintained beta-cell mass, delayed the onset of hyperglycemia, and protected beta cells from glucose toxicity. Moreover, PACAP increases glucose-stimulated insulin release in vitro and in vivo. In this study, we investigated the possibility of PACAP use in human islet transplantation.

METHODS

Human islets were cultured in the presence or absence of PACAP (10(-12) mol/L) for 48 hours. We assessed beta-cell viability using FACS, cellular composition analysis by iCys/LSC, and glucose-stimulated insulin secretion. In vivo, islets were transplanted beneath the kidney capsule of Streptozotocin-induced diabetic immunodeficient mice. An intravenous glucose tolerance test (IVGTT) was also performed in the presence or absence of PACAP (Peptide International, Louisville, Ky, United States; 1.3 nmol/kg).

RESULTS

There were significant improvements in terms of beta-cell viability and cellular composition between islets cultured with or without PACAP, respectively (P < .05). Moreover, glucose-stimulated insulin secretion significantly improved in islets cultured with PACAP compared with controls, respectively (P < .05). Treatment of recipient mice with PACAP resulted in beneficial effects on insulin secretion (PACAP vs control, 13.2 vs 1.9 mU/L), in IVGTT. However, no significant difference was observed in glucose levels between the 2 groups.

CONCLUSIONS

Our study indicated that PACAP significantly improved beta-cell viability and survival during culture, and increased insulin secretion in vitro and in vivo. However, blood glucose levels in vivo after an IVGTT did not significantly improve, probably due to increased glucagon secretion from alpha cells. PACAP supplementation to culture medium could be of assistance to improve clinical islet transplantation outcomes.

Receptors for NPY and PACAP differ in expression and activity during adipogenesis in the murine 3T3-L1 fibroblast cell line

BACKGROUND AND PURPOSE

Neuropeptides are involved in the regulation of food intake in the central nervous system, but they might also act on peripheral fat tissue via neuropeptide receptors.

EXPERIMENTAL APPROACH

We investigated the receptor expression and activity of pituitary adenylate cyclase-activating polypeptide (PACAP) and of neuropeptide Y at the mRNA and protein levels in the 3T3-L1 fibroblast line during differentiation into adipocytes. Intracellular calcium concentration was measured by calcium imaging.

KEY RESULTS

The PACAP receptors PAC(1) and VPAC(2) as well as the neuropeptide Y(1) receptor were expressed at the mRNA level in fibroblasts, pre-adipocytes and adipocytes. The mRNA profile of the PAC(1) receptor isoforms showed the HOP sequence, whereas the HIP-isoform was present in subconfluent 3T3-L1 fibroblasts only. At the protein level, the mature 3T3-L1 adipocytes produced the PAC(1) and Y(1) receptors; only the PAC(1) receptor showed carbohydrate residues. Both neuropeptides induced an increase of intracellular calcium in mature adipocytes, which was absent in the precursor cells. These changes in calcium were mediated by Y(1) and PAC(1) receptors as demonstrated by the effects of specific receptor agonists and antagonists.

CONCLUSIONS AND IMPLICATIONS

As the PAC(1)-HOP receptor variant seems to be responsible for PACAP-mediated calcium influx in many cell types, the HOP sequence might also mediate the increase in intracellular calcium in adipocytes. Because a high intracellular calcium level is associated with lipogenesis, peptidergic innervation of adipose tissue might be involved in stress-induced obesity.

Islet G protein-coupled receptors as potential targets for treatment of type 2 diabetes

Islet dysfunction - characterized by a combination of defective insulin secretion, inappropriately high glucagon secretion and reduced beta-cell mass - has a central role in the pathophysiology of type 2 diabetes. Several G protein-coupled receptors (GPCRs) expressed in islet beta-cells are known to be involved in the regulation of islet function, and therefore are potential therapeutic targets. This is evident from the recent success of glucagon-like peptide 1 (GLP1) mimetics and dipeptidyl peptidase 4 (DPP4) inhibitors, which promote activation of the GLP1 receptor to stimulate insulin secretion and inhibit glucagon secretion, and also have the potential to increase beta-cell mass. Other islet beta-cell GPCRs that are involved in the regulation of islet function include the glucose-dependent insulinotropic peptide (GIP) receptor, lipid GPCRs, pleiotropic peptide GPCRs and islet biogenic amine GPCRs. This Review summarizes islet GPCR expression, signalling and function, and highlights their potential as targets for the treatment of type 2 diabetes.

Role of pituitary adenylate cyclase-activating polypeptide in the pancreatic endocrine system

In the pancreatic islets, pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed in beta cells and autonomic nerve terminals; the majority of these nerve terminals are parasympathetic. PACAP binds to three types of G protein-coupled receptors (GPCRs): VPAC1 receptors, VPAC2 receptors, and PAC1 receptors. All these receptor types are expressed in pancreatic islets. PACAP stimulates insulin and glucagon secretion. These actions are achieved in part through increased formation of cAMP after activation of adenylate cyclase and in part through increase in cytosolic calcium, achieved through increase in calcium uptake and release from intracellular calcium stores. Deletion of PAC1 receptors or VPAC2 receptors results in impaired insulin secretion and glucose intolerance. Studies in PAC1 receptor gene deleted mice have suggested that PACAP may be of physiological importance in mediating prandial insulin secretion and in contributing to the glucagon response to hypoglycemia. Animal studies have also suggested that activation of the receptors, in particular VPAC2 receptors, may be used as a therapeutic approach for the treatment of type 2 diabetes. Hence, PACAP is an islet neuropeptide with a potential role in islet physiology and as a basis for development of islet-promoting therapy in type 2 diabetes.

Pituitary adenylate cyclase-activating polypeptide attenuates streptozotocin-induced apoptotic death of RIN-m5F cells through regulation of Bcl-2 family protein mRNA expression

Oxidative stress, followed by the apoptotic death of pancreatic beta cells, is considered to be one of causative agents in the evolution of the type 2 diabetic state; therefore, the protection of beta cells can comprise an efficacious strategy for preventing type 2 diabetes. In the present study, RIN-m5F cells (i.e. the rat insulinoma beta cell line) were stimulated with streptozotocin, resulting in a time- and concentration-dependent release of lactate dehydrogenase. There appeared to be significant apoptotic cell death after 2 h of treatment with streptozotocin at 10 mM, as demonstrated by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling staining and 2.6-fold activation of cellular caspase-3, an apoptotic enzyme. By contrast, some neuropeptides of the glucagon-secretin family and coenzyme Q(10), an endogenous mitochondrial antioxidant, could attenuate streptozotocin cytotoxicity, and especially pituitary adenylate cyclase-activating polypeptide (PACAP), at a concentration of 10(-7) M, exhibited 34% attenuation of lactate dehydrogenase release from streptozotocin-treated RIN-m5F cells. Quantitative RT-PCR experiments indicated the inhibitory effect of PACAP on streptozotocin-evoked up-regulation of pro-apoptotic factor (Noxa and Bax) and a 2.3-fold enhancement of Bcl-2 mRNA expression, a pro-survival protein, was also observed after addition of PACAP. The data obtained suggest the anti-apoptotic role of PACAP in streptozotocin-treated RIN-m5F cells through the regulation of pro-apoptotic and pro-survival factors.

Long-term administration of maxadilan improves glucose tolerance and insulin sensitivity in mice

Maxadilan and its truncated variant, M65, are agonist and antagonist specific, respectively, for the PAC1 receptor. PAC1 is the specific receptor for the pituitary adenylate cyclase-activating peptide (PACAP), which is not shared by vasoactive intestinal peptide (VIP). PACAP is a ubiquitous peptide of the glucagon superfamily that is involved in glucose homeostasis and regulation of insulin secretion. This study employed the recombinant maxadilan and M65 to evaluate the PAC1 receptor-mediated effects on energy metabolism using NIH mice. First, the acute effect of maxadilan-induced hyperglycemia was blocked by M65. In long-term studies, NIH mice were given daily intraperitoneal injections with maxadilan, M65, or vehicle for 21 days. Maxadilan suppressed feeding and enhanced water intake significantly for the first several days. After that period, maxadilan treatment continued to promote food and water intake. Long-term administration of maxadilan led to an increase in body weight (P<0.01), decrease in body fat (P<0.01), down-regulation of basal plasma glucose (P<0.01), upregulation of basal plasma insulin (P<0.01) and improved glucose tolerance (P<0.01) and insulin sensitivity (P<0.01). An elevation in plasma LDL (P<0.01) was also observed in the maxadilan group. However, M65 displayed no significant adverse effects on the aforementioned parameters except basal plasma glucose (P<0.05). The significant changes induced by maxadilan indicate that the PAC1 receptor plays multiple key roles in carbohydrate metabolism, lipid metabolism and energy homeostasis in mice.

Feeding and metabolism in mice lacking pituitary adenylate cyclase-activating polypeptide

Disruption of the pituitary adenylate cyclase-activating polypeptide (PACAP) gene in mice has demonstrated a role for this highly conserved neuropeptide in the regulation of metabolism and temperature control. Localization of PACAP neurons within hypothalamic nuclei that regulate appetite suggest PACAP may affect feeding and thus energy balance. We used PACAP-null mice to address this question, examining both food intake and energy expenditure. PACAP-null mice were leaner than wild-type littermates due to decreased adiposity and displayed increased insulin sensitivity. The lean phenotype in the PACAP-null mice was completely eliminated if animals were fed a high-fat diet or housed near thermoneutrality (28 C). Further metabolic analyses of PACAP-null mice housed at 21 C indicated that the reduced body weight could not be explained by decreased food intake, increased metabolic rate, or increased locomotor activity. The thyroid hormone axis of PACAP-null mice was affected, because mRNA levels of hypothalamic TRH and brown adipose tissue type 2 deiodinase were reduced in PACAP-null mice housed at room temperature, and brain deiodinase activity was lower in PACAP-null mice after an acute cold challenge compared with wild-type controls. These results demonstrate that PACAP is not required for the regulation of food intake yet is necessary to maintain normal energy homeostasis, likely playing a role in central cold-sensing mechanisms.

Role of VIP and PACAP in islet function

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are two closely related neuropeptides that are expressed in islets and in islet parasympathetic nerves. Both peptides bind to their common G-protein-coupled receptors, VPAC1 and VPAC2, and PACAP, in addition to the specific receptor PAC1, all three of which are expressed in islets. VIP and PACAP stimulate insulin secretion in a glucose-dependent manner and they both also stimulate glucagon secretion. This action is achieved through increased formation of cAMP after activation of adenylate cyclase and stimulation of extracellular calcium uptake. Deletion of PAC1 receptors or VPAC2 receptors results in glucose intolerance. These peptides may be of importance in mediating prandial insulin secretion and the glucagon response to hypoglycemia. Animal studies have also suggested that activation of the receptors, in particular VPAC2 receptors, may be used as a therapeutic approach for the treatment of type 2 diabetes. This review summarizes the current knowledge of the potential role of VIP and PACAP in islet function.

Long-term administration of PACAP receptor antagonist, PACAP(6-27), impairs glucose tolerance and insulin sensitivity in obese diabetic ob/ob mice

Pituitary adenylate cyclase-activating peptide (PACAP) is a ubiquitous peptide of the glucagon superfamily that is involved in glucose homeostasis and regulation of insulin secretion. This study employed the PACAP receptor antagonist, PACAP(6-27) to evaluate the role of endogenous PACAP in genetic obesity-related diabetes and related metabolic abnormalities using ob/ob mice. Acute in vivo antagonistic potency of PACAP(6-27) was confirmed in ob/ob mice by blockade of the insulin-releasing action but not hyperglycaemia. In longer-term studies, ob/ob mice were given once daily injections of PACAP(6-27) or vehicle for 14 days. Feeding activity, body weight, basal plasma glucose and plasma insulin concentrations were not significantly affected by chronic PACAP(6-27) treatment. However, PACAP(6-27) treatment impaired glucose tolerance, insulin sensitivity and the glycaemic response to feeding. Plasma glucagon and lipids were unchanged. These observations indicate a role of endogenous PACAP for normal glucose homeostasis, but indicate a minor involvement in the regulation of insulin secretion in ob/ob mice.

Pituitary adenylate cyclase-activating peptide (PACAP): assessment of dipeptidyl peptidase IV degradation, insulin-releasing activity and antidiabetic potential

Pituitary adenylate cyclase-activating peptide (PACAP) is a member of the glucagon family of peptides. Like other members, most notably glucagon-like peptide-1 (GLP-1), PACAP is rapidly degraded by dipeptidylpeptidase IV (DPP IV). This study investigated how degradation by DPP IV affected the insulinotropic activity of PACAP, and whether PACAP exerted acute antihyperglycemic properties in normal or ob/ob mice. DPP IV degradation of PACAP(1-27) over 18 h led to the formation of PACAP(3-27), PACAP(5-27) and ultimately PACAP(6-27). In contrast to 1.4-1.8-fold concentration-dependent stimulation of insulin secretion by PACAP(1-27), these peptide fragments lacked insulinotropic activity. While PACAP(1-27) and PACAP(1-38) generated significant insulin responses when given alone or together with glucose in ob/ob and normal mice, they also elevated plasma glucose. These actions were eliminated following degradation of the peptide by incubation with DPP IV. The hyperglycemic effects may be explained at least partly by a potent glucagon-releasing action in ob/ob and normal mice. In conclusion, PACAP is inactivated by DPP IV and despite insulin-releasing effects, its actions on glucagon secretion and glucose homeostasis do not make it a good therapeutic tool for the treatment of type 2 diabetes.

Therapeutic approaches to preserve islet mass in type 2 diabetes

Type 2 diabetes is characterized by hyperglycemia resulting from insulin resistance in the setting of inadequate beta-cell compensation. Currently available therapeutic agents lower blood glucose through multiple mechanisms but do not directly reverse the decline in beta-cell mass. Glucagon-like peptide-1 (GLP-1) receptor agonists, exemplified by Exenatide (exendin-4), not only acutely lower blood glucose but also engage signaling pathways in the islet beta-cell that lead to stimulation of beta-cell replication and inhibition of beta-cell apoptosis. Similarly, glucose-dependent insulinotropic polypeptide (GIP) receptor activation stimulates insulin secretion, enhances beta-cell proliferation, and reduces apoptosis. Moreover, potentiation of the endogenous postprandial levels of GLP-1 and GIP via inhibition of dipeptidyl peptidase-IV (DPP-IV) also expands beta-cell mass via related mechanisms. The thiazolidinediones (TZDs) enhance insulin sensitivity, reduce blood glucose levels, and also preserve beta-cell mass, although it remains unclear whether TZDs affect beta-cell mass via direct mechanisms. Complementary approaches to regeneration of beta-cell mass involve combinations of factors, exemplified by epidermal growth factor and gastrin, which promote islet neogenesis and ameliorate diabetes in rodent studies. Considerable preclinical data support the concept that one or more of these therapeutic approaches, alone or in combination, may potentially reverse the decline in beta-cell mass that is characteristic of the natural history of type 2 diabetes.

Potential protective action of pituitary adenylate cyclase-activating polypeptide (PACAP38) on in vitro and in vivo models of myeloma kidney injury

The most common type of renal injury in multiple myeloma is chronic tubulointerstitial nephropathy associated with casts in tubule lumens, an entity referred to as “myeloma kidney” that often progresses to end-stage kidney diseases. Myeloma kidney is associated with a significant increase in all-cause mortality, yet no effective intervention, except a limited use of steroid, is available. Here, we report that pituitary adenylate cyclase-activating polypeptide with 38 residues (PACAP38) dramatically prevents injury of cultured renal proximal tubule cells caused by myeloma light chains through suppression of proinflammatory cytokines production, by inhibiting p38 MAPK and translocation of NFκB via both PAC1 and VPAC1 receptors. The suppressive effects of PACAP was as effective as dexamethasone in all of their cytokine assays and demonstrated both in vitro and in vivo. Furthermore, PACAP38 inhibits myeloma cell growth directly and may also indirectly by suppressing production of the growth factor, IL-6, from bone marrow stromal cells, that is stimulated by adhesion of myeloma cells. These findings render PACAP38 worth evaluation as a promising candidate for an effective and safe renoprotectant in myeloma kidney, and possibly other nephropathy, and also as a new antitumor agent in multiple myeloma.

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

OBJECTIVE

Pituitary adenylate cyclase-activating polypeptide (PACAP) has been shown to play an important role in the regulation of islet function. We investigated its effects in beta-cell-specific calmodulin-overexpressing diabetic (CaMTg) mice, in which we consider that apoptosis of beta cells is the primary defect leading to basal hyperglycaemia.

METHODS

CaMTg mice were treated with continuous s.c. infusions of PACAP from 2 to 4 weeks after birth, and were evaluated against littermate non-transgenic (nTg) and saline-treated CaMTg mice as to plasma glucose levels, insulin content, islet function and morphological features.

RESULTS

Remarkable and progressive hyperglycaemia was observed in CaMTg mice, and PACAP treatment blunted this elevation. Insulin secretion from isolated islets demonstrated an impaired response to glucose in CaMTg mice, and PACAP treatment did not cause any improvement. The total pancreatic insulin content in CaMTg mice decreased significantly to 19.1% of that in nTg mice. PACAP treatment of CaMTg mice increased the content to 158% of the value in saline-treated CaMTg mice. The insulin content in isolated islets from CaMTg mice also decreased to 15.9% of that in nTg mice, while PACAP treatment caused a doubling of the value. Immunohistochemical investigation revealed that the insulin-positive islet area was markedly smaller in CaMTg mice and that PACAP treatment significantly expanded the insulin-positive islet area.

CONCLUSIONS

These findings indicate that PACAP treatment retards the onset of hyperglycaemia in CaMTg mice by maintaining beta-cell mass and PACAP treatment may potentially be a therapeutic measure for preventing beta-cell exhaustion during hyperglycaemia.

Cytosolic Ca2+ responses to sub-picomolar and nanomolar PACAP in pancreatic beta-cells are mediated by VPAC2 and PAC1 receptors

Pituitary adenylate cyclase-activating polypeptide (PACAP) potentiates glucose-induced insulin release and increases cytosolic Ca2+ concentration ([Ca2+]i) in islet beta-cells in a concentration-dependent manner with two peaks at 10(-13) and 10(-9) M. PAC1 receptor (PAC1-R) and VPAC2 receptor (VPAC2-R) are expressed in pancreatic beta-cells and thought to be involved in insulin release. We aimed to determine the receptor types involved in the [Ca2+]i responses to 10(-13) and 10(-9) M PACAP. We measured [Ca2+]i in beta-cells and examined comparative effects of PAC1-R-selective agonist maxadilan, its antagonist M65, VPAC2-R-selective agonist Ro25-1553, and native ligands of PACAP and VIP. In the presence of 8.3 mM glucose, maxadilan, Ro25-1553, PACAP, and VIP at 10(-13) and 10(-9) M all increased [Ca2+]i. PACAP and maxadilan elicited greater effects at 10(-9) M than at 10(-13) M both in the incidence and amplitude of [Ca2+]i responses. For VIP and Ro25-1553, in contrast, the effects at 10(-9) and 10(-13) M were comparable. Furthermore, the amplitude of [Ca2+]i responses to 10(-9) M PACAP, but not 10(-13) M PACAP, was suppressed by M65. The results suggest that VPAC2-R and PAC1-R contribute equally to [Ca2+]i responses to sub-picomolar concentrations of PACAP, while PAC1-R has greater contribution to [Ca2+]i responses to nanomolar concentrations of this peptide.

Possible pathomechanisms of sudden infant death syndrome: key role of chronic hypoxia, infection/inflammation states, cytokine irregularities, and metabolic trauma in genetically predisposed infants

Chronic hypoxia, viral infections/bacterial toxins, inflammation states, biochemical disorders, and genetic abnormalities are the most likely trigger of sudden infant death syndrome (SIDS). Autopsy studies have shown increased pulmonary density of macrophages and markedly more eosinophils in the lungs accompanied by increased T and B lymphocytes. The elevated levels of immunoglobulins, about 20% more muscle in the pulmonary arteries, increased airway smooth muscle cells, and increased fetal hemoglobin and erythropoietin are evidence of chronic hypoxia before death. Other abnormal findings included mucosal immune stimulation of the tracheal wall, duodenal mucosa, and palatine tonsils, and circulating interferon. Low normal or higher blood levels of cortisol often with petechiae on intrathoracic organs, depleted maternal IgG antibodies to endotoxin core (EndoCAb) and early IgM EndoCAb triggered, partial deletions of the C4 gene, and frequent IL-10-592*A polymorphism in SIDS victims as well as possible hypoxia-induced decreased production of antiinflammatory, antiimmune, and antifibrotic cytokine IL-10, may be responsible for the excessive reactions to otherwise harmless infections. In SIDS infants, during chronic hypoxia and times of infection/inflammation, several proinflammatory cytokines are released in large quantities, sometimes also representing a potential source of tissue damage if their production is not sufficiently well controlled, eg, by pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP). These proinflammatory cytokines down-regulate gene expression of major cytochrome P-450 and/or other enzymes with the specific effects on mRNA levels, protein expression, and enzyme activity, thus affecting metabolism of several endogenous lipophilic substances, such as steroids, lipid-soluble vitamins, prostaglandins, leukotrienes, thromboxanes, and exogenous substances. In SIDS victims, chronic hypoxia, TNF-alpha and other inflammatory cytokines, and arachidonic acid (AA) as well as n-3 polyunsaturated fatty acids (FA), stimulated and/or augmented superoxide generation by polymorphonuclear leukocytes, which contributed to tissue damage. Chronic hypoxia, increased amounts of nonheme iron in the liver and adrenals of these infants, enhanced activity of CYP2C9 regarded as the functional source of reactive oxygen species (ROS) in some endothelial cells, and nicotine accumulation in tissues also intensified production of ROS. These increased quantities of proinflammatory cytokines, ROS, AA, and nitric oxide (NO) also resulted in suppression of many CYP450 and other enzymes, eg, phosphoenolpyruvate carboxykinase (PEPCK), an enzyme important in the metabolism of FA during gluconeogenesis and glyceroneogenesis. PEPCK deficit found in SIDS infants (caused also by vitamin A deficiency) and eventually enhanced by PACAP lipolysis of adipocyte triglycerides resulted in an increased FA level in blood because of their impaired reesterification to triacylglycerol in adipocytes. In turn, the overproduction and release of FA into the blood of SIDS victims could lead to the metabolic syndrome and an early phase of type 2 diabetes. This is probably the reason for the secondary overexpression of the hepatic CYP2C8/9 content and activity reported in SIDS infants, which intensified AA metabolism. Pulmonary edema and petechial hemorrhages often present in SIDS victims may be the result of the vascular leak syndrome caused by IL-2 and IFN-alpha. Chronic hypoxia with the release of proinflammatory mediators IL-1alpha, IL-1beta and IL-6, and overloading of the cardiovascular and respiratory systems due to the narrowing airways and small pulmonary arteries of these children could also contribute to the development of these abnormalities. Moreover, chronic hypoxia of SIDS infants induced also production of hypoxia-inducible factor 1alpha (HIF-1alpha), which stimulated synthesis and release of different growth factors by vascular endothelial cells and intensified subclinical inflammatory reactions in the central nervous system, perhaps potentiated also by PACAP and VIP gene mutations. These processes could lead to the development of brainstem gliosis and disorders in the release of neuromediators important for physiologic sleep regulation. All these changes as well as eventual PACAP abnormalities could result in disturbed homeostatic control of the cardiovascular and respiratory responses of SIDS victims, which, combined with the nicotine effects and metabolic trauma, finally lead to death in these often genetically predisposed children.

Inhibitors of dipeptidyl peptidase IV: a novel approach for the prevention and treatment of Type 2 diabetes?

Inhibitors of the enzyme dipeptidyl peptidase IV (DPP IV) are of increasing interest to both diabetologists and the pharmaceutical industry alike, as they may become established as the next member of the oral antidiabetic class of therapeutic agents, designed to lower blood glucose and, possibly, prevent the progressive impairment of glucose metabolism in patients with impaired glucose tolerance and Type 2 diabetes. DPP IV has become a focus of attention for drug design, as it has a pivotal role in the rapid degradation of at least two of the hormones released during food ingestion, a property that has warranted the design of inhibitor-based drugs. At the molecular level, DPP IV cleaves two amino acids from the N-terminus of the intact, biologically active forms of both so-called incretin hormones, glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide (formerly known as gastric inhibitory polypeptide), resulting in truncated metabolites, which are largely inactive. Inhibition of the enzyme, therefore, is thought to increase levels of the active forms of both incretin hormones, culminating in an increase in insulin release after a meal, in a fully glucose-dependent manner. DPP IV inhibitors combine several features of interest to the drug design process. They can be readily optimised for their target and be designed as low molecular weight, orally active entities compatible with once-daily administration.

[Physiological and therapeutic roles of PACAP in glucose metabolism and diabetes]

Pituitary adenylate cyclase activating polypeptide (PACAP) is a ubiquitous neuropeptide in the central and peripheral nervous systems. Previously we reported that PACAP38 is localized in pancreatic islets and serves as an endogenous amplifier of glucose-induced insulin secretion. PACAP activates Gs-cAMP system, stimulates voltage-dependent Ca(2+) channels, and increases cytosolic Ca(2+) concentration in beta-cells. On the other hand, PAC1 receptor is expressed in adipocytes. PACAP enhances insulin-stimulated glucose uptake in an adipocyte cell-line, 3T3-L1 cells. PACAP does not alter the tyrosine phosphorylation of insulin receptor and IRS-1, but increases the activity of PI-3 kinase, a distal site of insulin signaling. PACAP also promotes differentiation of 3T3-L1 cells from fibroblasts to adipocytes. In GK rats, an animal model of type 2 diabetes, daily i.p. injection of PACAP38 (6 pmol/kg) from the age of 3 weeks prevents development of hyperglycemia between 3 to 8 weeks. These results demonstrate that PACAP enhances glucose-stimulated insulin secretion in islets, enhances insulin action inadipocytes, and prevents hyperglycemia in diabetic animals. This finding presents a possible therapeutic use of PACAP in the treatment of diabetes.

Functional roles of the neuropeptide PACAP in brain and pancreas

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide implicated in a broad variety of physiological processes. To assess PACAP's function in vivo, we recently generated PACAP knockout mice (PACAP(-/-)) and transgenic mice overexpressing PACAP specifically in the pancreas (PACAP-Tg). In PACAP(-/-) mice, we have demonstrated a marked phenotypic changes including a high early mortality rate, increased novelty-seeking behavior and abnormal explosive jumping in a novel environment, as well as reduced female fertility. In this paper, we reevaluated these phenotypes in terms of the genetic background of the mice. Genetic background appears to modulate critically the magnitude but not the general nature of the PACAP-null phenotype. In PACAP-Tg mice, we have recently demonstrated that enhanced glucose-induced insulin secretion with normal glucose tolerance, amelioration of streptozotocin-induced diabetes with increased beta-cell proliferation, and a trend towards an increase in total islet mass with age. Here we show that PACAP(-/-) mice exhibit significantly impaired glucose-induced insulin secretion but still have normal glucose tolerance. These observations suggest that PACAP may play important roles in and beyond the regulation of insulin release. Taken together, the mutant phenotypes revealed both expected and unexpected roles of PACAP in the brain and pancreatic functions.

Overexpression of PACAP in transgenic mouse pancreatic beta-cells enhances insulin secretion and ameliorates streptozotocin-induced diabetes

Pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the vasoactive intestinal peptide/secretin/glucagon family, stimulates insulin secretion from islets in a glucose-dependent manner at femtomolar concentrations. To assess PACAP's pancreatic function in vivo, we generated transgenic mice overexpressing PACAP in the pancreas under the control of human insulin promoter. Northern blot and immunohistochemical analyses showed that PACAP is overexpressed in pancreatic islets, specifically in transgenic mice. Plasma glucose and glucagon levels during a glucose tolerance test were not different between PACAP transgenic mice and nontransgenic littermates. However, plasma insulin levels in transgenic mice were higher after glucose loading. Also, increases of streptozotocin-induced plasma glucose were attenuated in transgenic compared with nontransgenic mice. Notably, an increase in 5-bromo-2-deoxyuridine-positive beta-cells in the streptozotocin-treated transgenic mice was observed but without differences in the staining patterns by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling. Morphometric analysis revealed that total islet mass tends to increase in 12-month-old transgenic mice but showed no difference between 12-week-old transgenic and nontransgenic littermates. This is the first time that PACAP has been observed to play an important role in the proliferation of beta-cells.

The neuropeptide pituitary adenylate cyclase-activating polypeptide and islet function

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is ubiquitously distributed in both the central and peripheral nervous systems and exerts a variety of effects. PACAP is a neuropeptide in pancreatic islets, where it has been suggested as a parasympathetic and sensory neurotransmitter. PACAP stimulates insulin secretion in a glucose-dependent manner, by an effect executed mainly through augmenting the formation of cAMP and stimulating the uptake of calcium. Accumulating evidence in animal studies points to a physiological importance of PACAP in the regulation of the insulin response to feeding. This review summarizes the current knowledge of islet actions and mechanisms and the function of PACAP.