Immunohistochemistry of opioid peptides in the guinea pig endocrine pancreas

Enhanced dynorphin expression and secretion in pancreatic beta-cells under hyperglycemic conditions

OBJECTIVE

Dynorphin, an endogenous opioid peptide predominantly expressed in the central nervous system and involved in stress response, pain, and addiction, has intrigued researchers due to its expression in pancreatic β-cells. In this study, we aimed to characterize dynorphin expression in mouse and human islets and explore the mechanisms regulating its expression.

METHODS

We used primary mouse and human islets with unbiased published datasets to examine how glucose and other nutrients regulate dynorphin expression and secretion in islets.

RESULTS

The prodynorphin gene is significantly upregulated in β-cells under hyperglycemic conditions. In vitro studies revealed that increased glucose concentrations correlate with increased dynorphin expression, indicating a critical interplay involving Ca2+, CamKII, and CREB pathways in β-cells. Perifusion studies allowed us to measure the dynamic secretion of dynorphin in response to glucose from mouse and human islets for the first time. Furthermore, we confirmed that increased dynorphin content within the β-cells directly correlates with enhanced dynorphin secretion. Finally, our findings demonstrate a synergistic effect of palmitate in conjunction with high glucose, further amplifying dynorphin levels and secretion in pancreatic islets.

CONCLUSIONS

This study demonstrates that the opioid peptide prodynorphin is expressed in mouse and human β-cells. Prodynorphin levels are regulated in parallel with insulin in response to glucose, palmitate, and amino acids. Our findings elucidate the signaling pathways involved, with CamKII playing a key role in regulating prodynorphin levels in β-cells. Finally, our findings are the first to demonstrate active dynorphin secretion from mouse and human islets in response to glucose.

Effect of bovine beta-casomorphins on rat pancreatic beta cells (RIN-5F) under glucotoxic stress

Beta-casomorphins (BCMs) are the bio-active peptides having opioid properties which are formed by the proteolytic digestion of β-caseins in milk. BCM-7 forms when A1 milk is digested in the small intestine due to a histidine at the 67th position in β-casein, unlike A2 milk, which has proline at this position and produces BCM-9. BCM-7 has further degraded into BCM-5 by the dipeptidyl peptidase-IV (DPP-IV) enzyme in the intestine. The opioid-like activity of BCM-7 is responsible for eliciting signaling pathways which enable a wide range of physiological effects. The aim of our study was to find out the differential role of BCMs (BCM-7, BCM-9 and BCM-5) on pancreatic β-cell proliferation, insulin secretion, and opioid peptide binding receptors from β-cells (RIN-5F cell line) in normal (5.5 mM) and high glucose (27.5 mM) concentrations. Our results showed that BCM-7/9/5 did not affect β-cell viability, proliferation, and insulin secretion at normal glucose level. However, at higher glucose concentration, BCMs significantly protected β-cells from glucotoxicity but did not affect the insulin secretion. Interestingly, in the presence of Mu-opioid peptide receptor antagonist CTOP, BCMs did not protect β-cells from glucotoxicity. The results suggest that BCMs protect β-cells from glucotoxicity via non-opioid mediated pathways because BCMs did not modulate the gene expression of the mu, kappa and delta opioid peptide receptors.

The role of mu-opioid receptors in pancreatic islet alpha cells

30% of people in the United States have diabetes or pre-diabetes. Many of these individuals will develop diabetic neuropathy as a comorbidity, which is often treated with exogenous opioids like morphine, oxycodone, or tramadol. Although these opioids are effective analgesics, growing evidence indicates that they may directly impact the endocrine pancreas function in human and preclinical models. One common feature of these exogenous opioid ligands is their preference for the mu opioid receptor (MOPR), so we aimed to determine if endogenous MOPRs directly regulate pancreatic islet metabolism and hormone secretion. We show that pharmacological antagonism of MOPRs enhances glucagon secretion, but not insulin secretion, from human islets under high glucose conditions. This increased secretion is accompanied by increased cAMP signaling. mRNA expression of MOPRs is enriched in human islet α-cells, but downregulated in T2D islet donors, suggesting a link between metabolism and MOPR expression. Conditional genetic knockout of MOPRs in murine α-cells increases glucagon secretion in high glucose conditions without increasing glucagon content. Consistent with downregulation of MOPRs during metabolic disease, conditional MOPR knockout mice treated with a high fat diet show impaired glucose tolerance, increased glucagon secretion, increased insulin content, and increased islet size. Finally, we show that MOPR-mediated changes in glucagon secretion are driven, in part, by KATP channel activity. Together, these results demonstrate a direct mechanism of action for endogenous opioid regulation of endocrine pancreas.

Fentanyl inhibits glucose-stimulated insulin release from β-cells in rat pancreatic islets

AIM: To explore the effects of fentanyl on insulin release from freshly isolated rat pancreatic islets in static culture.

METHODS: Islets were isolated from the pancreas of mature Sprague Dawley rats by common bile duct intraductal collagenase V digestion and were purified by discontinuous Ficoll density gradient centrifugation. The islets were divided into four groups according to the fentanyl concentration: control group (0 ng/mL), group I (0.3 ng/mL), group II (3.0 ng/mL), and group III (30 ng/mL). In each group, the islets were co-cultured for 48 h with drugs under static conditions with fentanyl alone, fentanyl + 0.1 μg/mL naloxone or fentanyl + 1.0 μg/mL naloxone. Cell viability was assessed by the MTT assay. Insulin release in response to low and high concentrations (2.8 mmol/L and 16.7 mmol/L, respectively) of glucose was investigated and electron microscopy morphological assessment was performed.

RESULTS: Low- and high-glucose-stimulated insulin release in the control group was significantly higher than in groups II and III (62.33 ± 9.67 μIU vs 47.75 ± 8.47 μIU, 39.67 ± 6.18 μIU and 125.5 ± 22.04 μIU vs 96.17 ± 14.17 μIU, 75.17 ± 13.57 μIU, respectively, P < 0.01) and was lowest in group III (P < 0.01). After adding 1 μg/mL naloxone, insulin release in groups II and III was not different from the control group. Electron microscopy studies showed that the islets were damaged by 30 ng/mL fentanyl.

CONCLUSION: Fentanyl inhibited glucose-stimulated insulin release from rat islets, which could be prevented by naloxone. Higher concentrations of fentanyl significantly damaged β-cells of rat islets.

The MOR-1 opioid receptor regulates glucose homeostasis by modulating insulin secretion

In addition to producing analgesia, opioids have also been proposed to regulate glucose homeostasis by altering insulin secretion. A considerable controversy exists, however, regarding the contribution of the mu-opioid receptor (MOR-1) to insulin secretion dynamics. We employed congenic C57BL/6J MOR-1 knockout (KO) mice to clarify the role of MOR in glucose homeostasis. We first found that both sexes of MOR-1 KO mice weigh more than wild-type mice throughout postnatal life and that this increase includes preferentially increased fat deposition. We also found that MOR-1 KO mice exhibit enhanced glucose tolerance that results from insulin hypersecretion that reflects increased beta-cell mass and increased secretory dynamics in the MOR-1 mutant mice compared with wild type. Analysis of the isolated islets indicated that islet insulin hypersecretion is mediated directly by MOR expressed on islet cells via a mechanism downstream of ATP-sensitive K(+) channel activation by glucose. These findings indicate that MOR-1 regulates body weight by a mechanism that involves insulin secretion and thus may represent a novel target for new diabetes therapies.

Association of the mu-opioid receptor gene with type 2 diabetes mellitus in an African American population

African Americans (AA) are at increased risk for developing type 2 diabetes mellitus (T2DM) relative to European Americans. We previously detected linkage of T2DM to 6q24-q27 (LOD 2.26) at 163.5 cM, closest to marker D6S1035, in a genome-wide scan of AA families. The mu-opioid receptor gene (OPRM1) is located within the LOD-1 support interval of this linkage peak. OPRM1 is an attractive positional candidate gene for T2DM susceptibility since agonists of OPRM1 affect glucose-induced insulin release and OPRM1 knockout mice have a more rapid induction of insulin resistance than wild-type. Twenty-two SNPs in this gene, at an average spacing of 3.9 kb, were genotyped in 380 AA T2DM cases and 276 AA controls. In single SNP association analyses, rs648007 demonstrated significant evidence of association with T2DM (P=0.013). Four blocks of high linkage disequilibrium were detected across the OPRM1 gene. Association analyses of haplotypes in each of these blocks revealed two haplotype blocks with significant overall P values (P=0.007 and 0.046). Significant, but rare, risk and protective haplotypes were identified as driving these associations with T2DM (P=0.034-0.047). These associations suggest that the OPRM1 gene plays a role in T2DM susceptibility in African Americans.

Role of mu-opioid receptors in insulin release in the presence of inhibitory and excitatory secretagogues

In mouse pancreatic islets incubated under static conditions, the inhibitory effects on glucose-evoked insulin release induced by adrenaline (1 microM), clonidine (2 microM) and UK 14,304 (brimonidine, 0.001-1 microM) were abolished by naloxone (30 nM). Only CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2), 0.1 microM), a very selective mu-opioid receptor antagonist, blocked the response to UK 14,304. Glucose-induced insulin secretion was attenuated by both beta-endorphin (0.01 microM) and endomorphin-1 (0.1 microM). Naloxone and CTOP prevented these inhibitory responses. The stimulatory effect of glibenclamide (1 microM) was also reduced by endomorphin-1. However, when islets were incubated in the presence of K(+) (30 mM), carbachol (100 microM) or forskolin (0.1 microM), neither the inhibitory effect induced by UK 14,304 was reversed by naloxone, nor endomorphin-1 altered the responses promoted by the excitatory agents. Thus, alpha(2)-adrenoceptor stimulation might inhibit glucose-induced insulin secretion by releasing endogenous opioids. Mu-Opioid receptor activation and opening of K(ATP) channels could be involved in the response.

Immunohistochemical localization of neuropeptides in bovine pancreas

The occurrence and density of distribution of nerves and endocrine cells that are immunoreactive for neuropeptides in the bovine pancreas were studied by immunohistochemistry. The six neuropeptides localized were galanin (GAL), substance P (SP), methionine-enkephalin (MENK), neuropeptide Y (NPY), calcitonin gene-related peptide (CGRP) and vasoactive intestinal polypeptide (VIP). The exocrine pancreas was shown to have an appreciable number of GAL- and SP-immunoreactive nerve fibres but few fibres showing immunoreactivity for VIP and CGRP. Numerous MENK-, GAL-, SP-, and NPY-immunoreactive nerve fibres were seen in the endocrine portion of the pancreas. Nerve cell bodies in the intrapancreatic ganglia showed immunoreactivity for all of the neuropeptides except CGRP. Endocrine cells showing immunoreactivity for GAL and SP were observed in the large islets and islets of Langerhans, respectively. The present results indicate a characteristic distribution of neuropeptides in the bovine pancreas, which may regulate both exocrine and endocrine secretions of pancreas.

Characterization of pancreatic endocrine cells of the European common frog Rana temporaria

To characterize the endocrine cell types of the pancreas of Rana temporaria, conventional staining, silver impregnation, and immunocytochemical methods for light and electron microscopy have been applied to paraffin, thin and semithin sections, many of them serial pairs. Quantitative data on the frequency and distribution (insular, extrainsular among the exocrine cells, or within the pancreatic ducts) of each endocrine cell type are also reported. Four distinct endocrine cell types have been identified: insulin (B) cells, which are also immunoreactive for [Met]enkephalin; glucagon/PP (A/PP) cells, also immunoreactive for GLP1; somatostatin (D) cells; and a fourth endocrine-like cell type (X cells) of unknown content and function. X cells display characteristic ultrastructure and tinctorial traits but are nonimmunoreactive for all of the 37 antisera tested. The presence of [Met]enkephalin in amphibian pancreatic endocrine cells is now reported for the first time. Almost half (44.9 +/- 7.9) of the total endocrine cell population lies outside the islets, mainly spread among the exocrine cells. Approximately 37.2 +/- 4.6% of the total endocrine cell population was immunoreactive for insulin, 48.8 +/- 6.9% was immunoreactive for glucagon/PP, and 14.0 +/- 4.9% was immunoreactive for somatostatin; 79.2 +/- 6.4% of glucagon/PP cells are found within the exocrine parenchyma, representing the majority (86.4 +/- 4.3%) of extrainsular endocrine component. On the contrary, most B cells (94.2 +/- 2.1%) are located within the islets; 30.8 +/- 12.9% of D cells are found outside the islets.

PP/PYY cells from endocrine pancreas of the scincid lizard Eumeces inexpectatus synthesize ACTH- and alpha-MSH-like molecules

The endocrine pancreas of the scincid lizard Eumeces inexpectatus secretes four major hormones, insulin, glucagon, somatostatin, and pancreatic polypeptide (PP); in addition, other peptides and neuropeptides, often colocalized in one of the principal cell types (A, B, D, and PP), were detected by light and ultrastructural immunocytochemistry. In particular, the pancreas is rich in peptide tyrosine tyrosine (PYY), ACTH, and alpha-MSH immunoreactivity. When single- and double-immunolabeled serial sections were compared for immunostaining for PP, PYY, ACTH, and alpha-MSH, there was broad coincidence with PP, termed PP/PYY, cells in view of the extensive colocalization of these two peptides. Furthermore, ultrastructural morphometric studies revealed similar secretory granules for PP immunoreactive (ir) and ACTH ir cells, while the endocrine cells express pro-opiomelanocortin (POMC) mRNA, indicating an active, extrapituitary synthesis of the POMC-derived peptides in these cells. In conclusion, the presence of POMC-derived peptides in the endocrine pancreatic cells suggests that they may regulate insulin secretion.

Glucose stimulation of pancreatic beta-cell lines induces expression and secretion of dynorphin

To investigate adaptive responses of pancreatic beta-cells to hyperglycemia, genes induced by glucose stimulation were identified by subtraction cloning. Among 53 clones representing differentially expressed genes, 20 encoded the endogenous opioid precursor, prodynorphin. The amino acid sequence of murine prodynorphin is identical to the rat protein in sequences comprising the opioid peptides and 86% identical in the remainder of the molecule. Stimulation of MIN6 cells increased prodynorphin RNA levels to more than 20-fold in proportion to physiological glucose concentrations. Similar induction levels were observed in murine betaTC3 and rat Rinm5F beta-cell lines. Prodynorphin RNA expression increased within 1 h of glucose stimulation, achieved maximal levels by 4 h, and remained elevated for at least 24 h. By using RIA, MIN6 cells were shown to contain and secrete increased amounts of dynorphin-A following glucose stimulation. Treatment of MIN6 cells with KCl, forskolin, or isobutyl-methyl-xanthine strongly induced prodynorphin RNA expression, suggesting that induction may be related to secretion-coupled signaling pathways. The induction of prodynorphin in several beta-cell lines is consistent with previous demonstrations of beta-cell synthesis of other endogenous opioids, including beta-endorphin, and suggests that opioids may have a potentially significant role in regulating beta-cell secretion.

Distribution of immunoreactive neuropeptides in the pancreas of the bullfrog, Rana catesbeiana, demonstrated by immunofluorescence

Indirect double immunofluorescence labelling for eight neuropeptides in the pancreas of the bullfrog, Rana catesbeiana, demonstrated the occurrence, distribution, and coexistence of certain neuropeptides in the exocrine and endocrine pancreas. Immunoreactivity of substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), FMRFamide (FMRF), and galanin (GAL) was localized in nerve fibers distributed between the acini and around the duct system and vasculature of the exocrine pancreas. In these regions, CGRP-immunoreactive fibers were more numerous than those containing the other five peptides. Almost all SP fibers showed coexistence of SP with CGRP, and about one third of fibers also showed coexistence of SP with VIP, NPY, FMRF, and GAL. In the endocrine pancreas, SP, CGRP, VIP, and GAL were recognized in the nerve fibers around and within the islets of Langerhans, and VIP and GAL fibers were more numerous than SP and CGRP fibers. All CGRP fibers, and about half of the VIP and GAL fibers were immunoreactive for SP. NPY- and FMRF-immunoreactive cells were found at the periphery of the islets. These findings suggest that the exocrine and endocrine pancreatic functions of the bullfrog are under the control of peptidergic innervation.

Opioids in neural and nonneural tissues

The endogenous opioid peptides (EOP) are grouped in three families, each deriving from the posttranslational processing of a distinct precursor molecule and exhibiting high affinity for a specific opioid receptor. The genes of EOPs are expressed in a wide variety of sites, including many nerve, neurosecretory, and endocrine cells. In reviewing the vast literature on this subject, a few patterns begin to emerge. First, the distribution of EOPs in tissues appears to be a distinct characteristic of each family of opioids. Second, the EOP producing cells can be grouped into two broad categories: those expressing only one and those expressing multiple EOP genes. Most EOP-producing nerve and neurosecretory cells fall into the first category, that is, they express one EOP gene, whereas most nonneural cells fall into the second category, that is, they express multiple EOP genes. Third, it appears that there is a relationship between opioids, proliferation rate, and state of differentiation of cells, since it has been shown that (a) mitogenic factors may change the EOP profile of a cell, and that (b) opioids may inhibit the proliferation rate of normal or neoplastic cells. The physiologic implication of these observations is briefly discussed.

Beta-endorphin-immunoreactive cells in the human fetal pancreas

This investigation has been carried out on 50 samples of fetal pancreata from the 10th to the 32nd week of gestation using the PAP technique. beta-Endorphin-reactive cells were morphometrically recorded by means of the point-counting method. beta-Endorphin reactivity occurred for the first time during the 15th week. During further development, beta-endorphin cells were found inside and outside the islets. From the 18th to 23rd week, these cells were primarily localized in the islet periphery. From the 24th week, they rearranged and occurred in irregular positions mixed with other islet cells. This rearrangement took place with a 4 week delay compared with the basic cell types of the islet organ. The extrainsular portion of these cells in the exocrine parenchyma varied between 0.3% in the 27th week and up to 10% in the 22nd week. Concerning the adult human pancreas, it has been suggested whether beta-endorphin cells may be a 6th basic cell type of the islet organ. Previous studies on the coexistence of somatostatin, glucagon and beta-endorphin in the same islet cell and the morphometric analysis would support this assumption. Biochemical examinations indicate that beta-endorphin is a modulator of insulin, glucagon, and somatostatin secretion in the islet organ. This is supported by the fact that beta-endorphin cells have extended cell bodies which is typical of cells with paracrine function.

Possible regulatory role of dynorphin A in the urinary bladder

Muscle strips from rat and human detrusor were studied using indirect immunofluorescence and electrical nerve stimulation in an organ bath. Immunoreactivity towards dynorphin was observed in varicose nerve fibres in the detrusor muscle and around immunonegative nerve cell bodies in the prevesical ganglia of the rat. In vitro, dynorphin A (1-13) (10(-13)-10(-6) M) strongly facilitated detrusor contraction induced by electrical field stimulation (EFS). This facilitation was counteracted by morphine (10(-10) and 10(-8) M) and naloxone (10(-10) and 10(-8) M) in a competitive manner. The facilitation could also be counteracted by the addition of the kappa-receptor antagonist M(r) 2266 (10(-7) M). Muscarinic blockade, achieved with atropine (10(-6) M), did not alter the effect of dynorphin A (1-13). Addition of phentolamine mesylate (10(-6) M), and propranolol (10(-6) M) per se facilitated the EFS-induced contractions. Both adrenergic blockade as well as the addition of the substance P blocker spantide, counteracted the facilitating effect of dynorphin A (1-13).

IN CONCLUSION

Dynorphin A immunoreactive material was found to be present in nerves in the rat detrusor and in prevesical ganglia. Dynorphin A (1-13) facilitated the detrusor contraction, possibly via actions on kappa-opioid receptors and interaction with non-cholinergic nerves.