Effects of gamma-aminobutyric acid on secretagogue-induced exocrine secretion of isolated, perfused rat pancreas

Computational prediction of MicroRNAs targeting GABA receptors and experimental verification of miR-181, miR-216 and miR-203 targets in GABA-A receptor

BACKGROUND

GABA receptors are well known as the inhibitory receptors in the central nervous system and are also found in peripheral tissues. We have previously shown that GABA receptors are involved in lung development and fluid homeostasis. However, the microRNAs that regulate GABA receptors have not yet been identified.

RESULTS

In this study, we used the online software, TargetScan and miRanda, to query the microRNAs that directly target GABA receptors and then selected some of them to verify experimentally using 3'-UTR reporter assays. Computational approaches predict many microRNA binding sites on the 3'-UTR of GABAA receptors, but not on GABAC receptors. 3'-UTR reporter assays only verified miR-181, miR-216, and miR-203 as the microRNAs that target GABA receptor α1-subunit among 10 microRNAs tested.

CONCLUSIONS

Our studies reinforce that microRNA target prediction needs to be verified experimentally. The identification of microRNAs that target GABA receptors provides a basis for further studies of post-transcriptional regulation of GABA receptors.

Glucagon-like peptide-1 stimulates GABA formation by pancreatic beta-cells at the level of glutamate decarboxylase

Pancreatic beta-cells are the major extraneural site of glutamate decarboxylase expression (GAD). During culture of isolated beta-cells, the GAD product gamma-aminobutyrate (GABA) is rapidly released in the medium, independently of insulin. It is considered as a possible mediator of beta-cell influences on alpha-cells, acinar cells, and/or infiltrating lymphocytes. In this perspective, we investigated the regulation of GABA release by rat beta-cells during a 24-h culture period. Glucose was previously reported to inhibit GABA release by diverting cellular GABA to mitochondrial breakdown through activation of GABA transferase (GABA-T). In the present study, glucagon-like peptide-1 (GLP-1) was shown to stimulate GABA formation at the level of GAD, its effect being suppressed by the GAD inhibitor allylglycine and remaining unaltered by the GABA-T inhibitor gamma-vinyl-GABA. The stimulatory action of GLP-1 is cAMP dependent, being reproduced by the adenylate cyclase activator forskolin and the cAMP analog N(6)-benzoyladenosine-3',5'-cAMP and inhibited by a PKA inhibitor. It is dependent on protein synthesis and associated with an increased expression of GAD67 but not GAD65. The GLP-1-induced stimulation of GAD activity in beta-cells can elevate medium GABA levels in conditions of glucose-driven intracellular GABA breakdown and thus maintain GABA-mediated beta-cell influences on neighboring cells.

γ-aminobutyric acid secreted from islet β-cells modulates exocrine secretion in rat pancreas

AIM: To investigate the role of endogenous γ-amino-butyric acid (GABA) in pancreatic exocrine secretion.

METHODS: The isolated, vascularly perfused rat pancreas was employed in this study to eliminate the possible influences of extrinsic nerves and hormones. Cholecystokinin (CCK; 10 pmol/L) was intra-arterially given to stimulate exocrine secretion of the pancreas.

RESULTS: Glutamine, a major precursor of GABA, which was given intra-arterially at concentrations of 1, 4 and 10 mmol/L, dose-dependently elevated the CCK-stimulated secretions of fluid and amylase in the normal pancreas. Bicuculline (10 μmol/L), a GABA_A receptor antagonist, blocked the enhancing effect of glutamine (4 mmol/L) on the CCK-stimulated exocrine secretions. Glutamine, at concentrations of 1, 4 and 10 mmol/L, dose-dependently increased the GABA concentration in portal effluent of the normal pancreas. The effects of glutamine on the CCK-stimulated exocrine secretion as well as the GABA secretion were markedly reduced in the streptozotocin-treated pancreas.

CONCLUSION: GABA could be secreted from β-cells into the islet-acinar portal system after administration of glutainine, and could enhance the CCK-stimulated exocrine secretion through GABA_A receptors. Thus, GABA in islet β-cells is a hormone modulating pancreatic exocrine secretion.

Differential expression of GABAA receptor π subunit in cultured rat alveolar epithelial cells

Although type A gamma-aminobutyric acid (GABA) receptors (ligand-gated Cl(-) channels) have been extensively studied in the central nervous system, no information is available on this receptor in lung cells. We have examined the expression of GABA(A) receptor pi-subunit (GABRP) during the trans-differentiation between rat alveolar epithelial type II cells and type I cells. Rat alveolar type II cells, when cultured on plastic plates, gradually trans-differentiated into type-I-like cells and lost their GABRP mRNA expression. However, the GABRP mRNA was partially retained in the type II cells cultured on Matrigel. Keratinocyte growth factor (a mitogen of type II cells) increased GABRP expression. A detached collagen gel maintained the GABRP mRNA to a level close to that of the freshly isolated type II cells. An air-liquid interface culture system, mimicking in vivo conditions in the lung, significantly up-regulated the expression of GABRP mRNA and protein. mRNAs of the GABA(A) receptor alpha1-, alpha3-, beta2-, gamma2-, and gamma3-subunits were also detected in rat type II cells. These results suggest that GABRP expression is differentially regulated by culture substrata, growth factor, detached gel, and an air-apical surface.

Effects of gamma-aminobutyric acid on action of gastrin-releasing peptidergic neurons in exocrine secretion of isolated, perfused rat pancreas

INTRODUCTION

gamma-Aminobutyric acid (GABA) has been reported to enhance exocrine secretion evoked by intrinsic neuronal excitation in the pancreas.

AIM

To see the effect of GABA on the action of gastrin-releasing peptide (GRP)ergic neurons in exocrine secretion of the pancreas.

METHODOLOGY

Pancreatic neurons were excited by electrical field stimulation (EFS) in the isolated, perfused rat pancreas. GRP in the pancreatic circulation was neutralized by an anti-GRP antiserum to block GRPergic neuronal action on pancreatic exocrine secretion.

RESULTS

GABA (3, 10, 30 microM), given intra-arterially, elevated the EFS-evoked pancreatic secretions of fluid and amylase dose-dependently. An anti-GRP antiserum (10 microL/mL: titer of 1:66,000) reduced the GABA (10 microM)-enhanced EFS-evoked pancreatic secretions. Synthetic porcine GRP-27 (30, 100, 300 p ) increased the pancreatic secretions dose-dependently, and these were further elevated by GABA (10 microM). The anti-GRP antiserum also reduced the GABA-enhanced GRP (100 p )-induced pancreatic secretions. Bicuculline (10 microM) reduced the enhancing effect of GABA on pancreatic secretions evoked by EFS as well as GRP.

CONCLUSION

GABA enhances pancreatic secretions evoked by EFS as well as GRP, which is reduced by the anti-GRP antiserum. The enhancing effects of GABA on the EFS- and GRP-induced pancreatic secretions are diminished by bicuculline. The results indicate that GABA enhances intrinsic GRPergic neuronal action on exocrine secretion via the GABA(A) receptors in the rat pancreas.

Neurohormonal control of pancreatic exocrine secretion

The neurohormonal control of pancreatic exocrine secretion is a complex interaction of multiple pathways involving a large number of gut hormones, neurotransmitters, and neuropeptides. Recent studies have elucidated a role for cholecystokinin in the regulation of bicarbonate and fluid secretion from pancreatic duct cells and suggested that cholecystokinin stimulation of human pancreatic acinar cells is likely regulated by an indirect mechanism of stimulation of afferent neurons. Evidence supports the regulation of potassium channels in rat pancreatic acinar cells by the cyclic AMP-mediated agonist secretin. Mechanisms for the regulation of cholecystokinin and secretin release by releasing factors have also been elucidated. The area postrema has been implicated in the mediation of inhibition of pancreatic secretion by the gut hormones peptide YY and pancreatic polypeptide. The neurotransmitter serotonin has been demonstrated to play a role in acid-induced secretin release and in pancreatic secretion stimulated by luminal factors. The regulation of pancreatic exocrine secretion by purines, nitric oxide, and gamma-aminobutyric acid as well as by the neuropeptides pituitary adenylate cyclase-activating peptide, gastrin-releasing peptide, and substance P is reviewed. The role of the central nervous system in modulating pancreatic secretion is also described. This review highlights the recent advances in knowledge of the neurohormonal regulation of pancreatic exocrine secretion.

Neurohormonal control of exocrine pancreas

The exocrine pancreas is regulated by various hormonal factors derived from the gut through hormone-hormonal and neurohormonal interactions. Physiologic stimuli entering the upper small intestine elicit the release of intestinal hormones and activate sensory reflex mechanisms from the intestinal mucosa to stimulate or inhibit exocrine pancreatic secretion. In addition, the endocrine pancreas, intrapancreatic nerves, and some extrapancreatic neural pathways, with or without mediation by the vagus nerve, are known to participate in regulation of exocrine pancreatic secretion. It has been established that two key intestinal hormones, secretin and cholecystokinin (CCK), in physiologic doses, act through the vagal afferent pathway and interact with each other as well as with other gut hormones. The releases of these two hormones are mediated through the corresponding releasing peptides. In the past few years, the roles of secretin- and CCK-releasing peptides have become more clearly defined. The participation of several neurotransmitters and regulatory peptides in the regulation of exocrine pancreatic secretion has also been established. In addition, neurotransmitters and neuropeptides released from the central nervous system may participate in the regulation of pancreatic secretion. It is conceivable that a few neurotransmitters and neuropeptides are involved in each neural regulatory pathway. However, their roles and sites of action in each pathway remain to be determined.