Expression of functional GABAA receptors in neuroendocrine gastropancreatic cells

Neuroendocrine neoplasia of the gastrointestinal tract revisited: towards precision medicine

Over the past 5 years, a number of notable research advances have been made in the field of neuroendocrine cancer, specifically with regard to neuroendocrine cancer of the gastrointestinal tract. The aim of this Review is to provide an update on current knowledge that has proven effective for the clinical management of patients with these tumours. For example, for the first time in the tubular gastrointestinal tract, well-differentiated high-grade (grade 3) tumours and mixed neuroendocrine-non-neuroendocrine neoplasms (MiNENs) are defined in the WHO classification. This novel classification enables efficient identification of the most aggressive well-differentiated neuroendocrine tumours and helps in defining the degree of aggressiveness of MiNENs. The Review also discusses updates to epidemiology, cell biology (including vesicle-specific components) and the as-yet-unresolved complex genetic background that varies according to site and differentiation status. The Review summarizes novel diagnostic instruments, including molecules associated with the secretory machinery, novel radiological approaches (including pattern recognition techniques), novel PET tracers and liquid biopsy combined with DNA or RNA assays. Surgery remains the treatment mainstay; however, peptide receptor radionuclide therapy with novel radioligands and new emerging medical therapies (including vaccination and immunotherapy) are evolving and being tested in clinical trials, which are summarized and critically reviewed here.

Lactobacillus fermentum HP3-Mediated Fermented Hericium erinaceus Juice as a Health Promoting Food Supplement to Manage Diabetes Mellitus

The current study investigated the antidiabetic property of Lactobacillus fermentum HP3–mediated fermented Hericium erinaceus juice (FHJ) using male Wistar rats with streptozotocin-induced diabetes mellitus (DM). FHJ was prepared using boiled mushroom juice and L. fermentum HP3. Amino acid and γ-aminobutyric acid (GABA) content of FHJ was analyzed. Streptozotocin-induced DM rats were supplemented with FHJ in a pre- and posttreatment method. The changes in plasma insulin, plasma glucose level, glycated hemoglobin (HbA1c), representative cytokines, and the antioxidant system were assessed in experimental rats using spectrophotometric methods and enzyme-linked immunosorbent assay. The supplementation of FHJ improved the body mass, insulin level, and recovery progress of hyperglycemia. HbA1c level was altered by the FHJ intervention. The inflammatory cytokines level was suppressed in FHJ supplemented group compared with control. Intervention of FHJ and insulin improved the production of interleukin-10 and transforming growth factor-–β1 in DM rat. The study suggested that fermented H erinaceus juice may be used as one of the food-based health-promoting supplement to manage DM along with medication.

Rat submandibular gland perfusion method for clarifying inhibitory regulation of GABAA receptor

GABA is an inhibitory transmitter found in rat salivary gland. However, the inhibitory potential of GABA on salivary secretion is unclear. Using an in vivo cannulation method, intraperitoneal administration of GABA was ineffective in the absence of gabaculine, a GABA transaminase inhibitor, on pilocarpine-induced salivary secretion, suggesting that GABA was rendered metabolically inactive before reaching the salivary gland. We hypothesized that the action of a drug on the salivary glands could be measured directly using a submandibular gland perfusion system. The submandibular gland artery, veins, and duct were cannulated in situ so that physiological functions such as innervation would not be compromised. Hank's balanced salt solution (pH 7.4) was perfused at a rate of 0.5 ml/min together with 1 μM carbachol (CCh) over a 5-min period every 30 min. Amount of secreted saliva showed no change to the recurrent addition of CCh to the perfusate. GABA or muscimol dose-dependently inhibited CCh-induced salivary secretion. This effect was blocked by bicuculline, a GABA(A)-receptor (GABA(A)-R) antagonist, and enhanced by clonazepam, a central-type benzodiazepine-receptor agonist. These results suggest that salivary secretion is suppressed by GABA(A)-R in rat salivary gland and that the perfusion method used was effective in clarifying inhibitory regulation of GABA(A)-R.

Role of γ-aminobutyric acid and its receptors in carcinogenesis

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain and it is also expressed in the central nervous system, peripheral nerves and peripheral non-neural tissues. Recent studies have shown that GABA is involved in the proliferation and migration of tumor cells and other processes of tumor development. According to different sensitivity to agonists and antagonists, GABA receptors have been classified into three types: A, B and C. GABA receptors and their receptor subunits are involved in complicated regulation of tumor cells. Many studies have demonstrated that GABA binding to its receptors can activate or inhibit the cAMP signaling pathway and the MAPK/ERK pathway, and regulate cancer cell proliferation and migration. The potential value of GABA in cancer diagnosis, prognostic prediction and biotherapy has been gradually revealed. In the present article, we reviewed the recent progress in understanding the role of GABA and its receptors in carcinogenesis.

The role of cell lines in the study of neuroendocrine tumors

Cell lines originating from neuroendocrine tumors (NETs) represent useful experimental models to assess the control of synthesis and release of different hormones and hormone-like peptides, to evaluate the mechanisms of action of these agents in target tissues at the cellular and subcellular levels, and to study cell proliferation and tumor development, as well as the effect of different drugs on these complex processes. To date, the understanding of NET biology (with regard to their mechanisms of hormone secretion, cell proliferation and metastatic spread) has been hampered by the lack of appropriate animal models or cell lines for their study. In the present review, we aim to summarize the recent in vitro/in vivo data regarding cell lines derived from NETs which are most frequently employed in experimental neuroendocrinology.

GABA coordinates with insulin in regulating secretory function in pancreatic INS-1 β-cells

Pancreatic islet β-cells produce large amounts of γ-aminobutyric acid (GABA), which is co-released with insulin. GABA inhibits glucagon secretion by hyperpolarizing α-cells via type-A GABA receptors (GABA(A)Rs). We and others recently reported that islet β-cells also express GABA(A)Rs and that activation of GABA(A)Rs increases insulin release. Here we investigate the effects of insulin on the GABA-GABA(A)R system in the pancreatic INS-1 cells using perforated-patch recording. The results showed that GABA produces a rapid inward current and depolarizes INS-1 cells. However, pre-treatment of the cell with regular insulin (1 µM) suppressed the GABA-induced current (I(GABA)) by 43%. Zinc-free insulin also suppressed I(GABA) to the same extent of inhibition by regular insulin. The inhibition of I(GABA) occurs within 30 seconds after application of insulin. The insulin-induced inhibition of I(GABA) persisted in the presence of PI3-kinase inhibitor, but was abolished upon inhibition of ERK, indicating that insulin suppresses GABA(A)Rs through a mechanism that involves ERK activation. Radioimmunoassay revealed that the secretion of C-peptide was enhanced by GABA, which was blocked by pre-incubating the cells with picrotoxin (50 µM, p<0.01) and insulin (1 µM, p<0.01), respectively. Together, these data suggest that autocrine GABA, via activation of GABA(A)Rs, depolarizes the pancreatic β-cells and enhances insulin secretion. On the other hand, insulin down-regulates GABA-GABA(A)R signaling presenting a feedback mechanism for fine-tuning β-cell secretion.

Expression of neurexin, neuroligin, and their cytoplasmic binding partners in the pancreatic beta-cells and the involvement of neuroligin in insulin secretion

The composition of the beta-cell exocytic machinery is very similar to that of neuronal synapses, and the developmental pathway of beta-cells and neurons substantially overlap. beta-Cells secrete gamma-aminobutyric acid and express proteins that, in the brain, are specific markers of inhibitory synapses. Recently, neuronal coculture experiments have identified three families of synaptic cell-surface molecules (neurexins, neuroligins, and SynCAM) that drive synapse formation in vitro and that control the differentiation of nascent synapses into either excitatory or inhibitory fully mature nerve terminals. The inhibitory synapse-like character of the beta-cells led us to hypothesize that members of these families of synapse-inducing adhesion molecules would be expressed in beta-cells and that the pattern of expression would resemble that associated with neuronal inhibitory synaptogenesis. Here, we describe beta-cell expression of the neuroligins, neurexins, and SynCAM, and show that neuroligin expression affects insulin secretion in INS-1 beta-cells and rat islet cells. Our findings demonstrate that neuroligins and neurexins are expressed outside the central nervous system and help confer an inhibitory synaptic-like phenotype onto the beta-cell surface. Analogous to their role in synaptic neurotransmission, neurexin-neuroligin interactions may play a role in the formation of the submembrane insulin secretory apparatus.

Glucose-dependent regulation of gamma-aminobutyric acid (GABA A) receptor expression in mouse pancreatic islet alpha-cells

The mechanism(s) by which glucose regulates glucagon secretion both acutely and in the longer term remain unclear. Added to isolated mouse islets in the presence of 0.5 mmol/l glucose, gamma-aminobutyric acid (GABA) inhibited glucagon release to a similar extent (46%) as 10 mmol/l glucose (55%), and the selective GABA(A) receptor (GABA(A)R) antagonist SR95531 substantially reversed the inhibition of glucagon release by high glucose. GABA(A)R alpha4, beta3, and gamma2 subunit mRNAs were detected in mouse islets and clonal alphaTC1-9 cells, and immunocytochemistry confirmed the presence of GABA(A)Rs at the plasma membrane of primary alpha-cells. Glucose dose-dependently increased GABA(A)R expression in both islets and alphaTC1-9 cells such that mRNA levels at 16 mmol/l glucose were approximately 3.0-fold (alpha4), 2.0-fold (beta3), or 1.5-fold (gamma2) higher than at basal glucose concentrations (2.5 or 1.0 mmol/l, respectively). These effects were mimicked by depolarizing concentrations of K(+) and reversed by the L-type Ca(2+) channel blocker nimodipine. We conclude that 1) release of GABA from neighboring beta-cells contributes substantially to the acute inhibition of glucagon secretion from mouse islets by glucose and 2) that changes in GABA(A)R expression, mediated by changes in intracellular free Ca(2+) concentration, may modulate this response in the long term.

Serum gastrin in Zollinger-Ellison syndrome: II. Prospective study of gastrin provocative testing in 293 patients from the National Institutes of Health and comparison with 537 cases from the literature. evaluation of diagnostic criteria, proposal of new criteria, and correlations with clinical and tumoral features

In two-thirds of patients with Zollinger-Ellison syndrome (ZES), fasting serum gastrin (FSG) levels overlap with values seen in other conditions. In these patients, gastrin provocative tests are needed to establish the diagnosis of ZES. Whereas numerous gastrin provocative tests have been proposed, only the secretin, calcium, and meal tests are widely used today. Many studies have analyzed gastrin provocative test results in ZES, but they are limited by small patient numbers and methodologic differences. To address this issue, we report the results of a prospective National Institutes of Health (NIH) study of gastrin provocative tests in 293 patients with ZES and compare these data with those from 537 ZES and 462 non-ZES patients from the literature. In 97%-99% of gastrinoma patients, an increase in serum gastrin post secretin (Delta secretin) or post calcium (Delta calcium) occurred. In NIH ZES patients with <10-fold increase in FSG, the sensitivity/specificity of the widely used criteria were as follows: Delta secretin > or =200 pg/mL (83%/100%), Delta secretin >50% (86%/93%), Delta calcium > or =395 pg/mL (54%/100%), and Delta calcium >50% (78%/83%). A systematic analysis of the sensitivity and specificity of other possible criteria for a positive secretin or calcium test allowed us to identify a new criterion for secretin testing (Delta > or =120 pg/mL) with the highest sensitivity/specificity (94%/100%) and to confirm the commonly used criterion for calcium tests (Delta > or =395 pg/mL) (62%/100%). This analysis further showed that the secretin test was more sensitive than the calcium test (94% vs. 62%). Our results suggest that secretin stimulation should be used as the first-line provocative test because of its greater sensitivity and simplicity and lack of side effects. In ZES patients with a negative secretin test, 38%-50% have a positive calcium test. Therefore the calcium test should be considered in patients with a strong clinical suspicion of ZES but a negative secretin test. Furthermore, we found that some clinical (diarrhea, duration of medical treatment), laboratory (basal acid output), and tumoral (size, extent) characteristics correlate with the serum gastrin increase post secretin and post calcium. However, using the proposed criteria, the result of these provocative tests (that is, positive or negative) is minimally influenced by these factors, so secretin and calcium provocative tests are reliable in patients with different clinical, laboratory, and tumor characteristics. A systematic analysis of meal testing showed that 54%-77% of ZES patients have a <50% postprandial serum gastrin increase. However, 9%-20% of ZES patients had a >100% increase post meal, causing significant overlap with antral syndromes. Furthermore, we could not confirm the usefulness of meal tests for localization of duodenal gastrinomas. We conclude that the secretin test is a crucial element in the diagnosis of most ZES patients, the calcium test may be useful in selected patients, but the meal test is not helpful in the management of ZES. For secretin testing, the criterion with the highest sensitivity and specificity is an increase of > or =120 pg/mL, which should replace other criteria commonly used today.

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.

Gamma-aminobutyric acid up- and downregulates insulin secretion from beta cells in concert with changes in glucose concentration

AIMS/HYPOTHESIS

The role of gamma-aminobutyric acid (GABA) and A-type GABA receptors (GABA(A)Rs) in modulating islet endocrine function has been actively investigated since the identification of GABA and GABA(A)Rs in the pancreatic islets. However, the reported effects of GABA(A)R activation on insulin secretion from islet beta cells have been controversial.

METHODS

This study examined the hypothesis that the effect of GABA on beta cell insulin secretion is dependent on glucose concentration.

RESULTS

Perforated patch-clamp recordings in INS-1 cells demonstrated that GABA, at concentrations ranging from 1 to 1,000 micromol/l, induced a transmembrane current (I(GABA)) which was sensitive to the GABA(A)R antagonist bicuculline. The current-voltage relationship revealed that I(GABA) reversed at -42+/-2.2 mV, independently of glucose concentration. Nevertheless, the glucose concentration critically controlled the membrane potential (V (M)), i.e., at low glucose (0 or 2.8 mmol/l) the endogenous V (M) of INS-1 cells was below the I(GABA) reversal potential and at high glucose (16.7 or 28 mmol/l), the endogenous V (M) of INS-1 cells was above the I(GABA) reversal potential. Therefore, GABA dose-dependently induced membrane depolarisation at a low glucose concentration, but hyperpolarisation at a high glucose concentration. Consistent with electrophysiological findings, insulin secretion assays demonstrated that at 2.8 mmol/l glucose, GABA increased insulin secretion in a dose-dependent fashion (p<0.05, n=7). This enhancement was blocked by bicuculline (p<0.05, n=4). In contrast, in the presence of 28 mmol/l glucose, GABA suppressed the secretion of insulin (p<0.05, n=5).

CONCLUSIONS/INTERPRETATION

These findings indicate that activation of GABA(A)Rs in beta cells regulates insulin secretion in concert with changes in glucose levels.

Molecular physiology of vesicular glutamate transporters in the digestive system

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system (CNS). Packaging and storage of glutamate into glutamatergic neuronal vesicles require ATP-dependent vesicular glutamate uptake systems, which utilize the electrochemical proton gradient as a driving force. Three vesicular glutamate transporters (VGLUT1-3) have been recently identified from neuronal tissue where they play a key role to maintain the vesicular glutamate level. Recently, it has been demonstrated that glutamate signaling is also functional in peripheral neuronal and non-neuronal tissues, and occurs in sites of pituitary, adrenal, pineal glands, bone, GI tract, pancreas, skin, and testis. The glutamate receptors and VGLUTs in digestive system have been found in both neuronal and endocrinal cells. The glutamate signaling in the digestive system may have significant relevance to diabetes and GI tract motility disorders. This review will focus on the most recent update of molecular physiology of digestive VGLUTs.

Regulated exocytosis of GABA-containing synaptic-like microvesicles in pancreatic beta-cells

We have explored whether γ-aminobutyric acid (GABA) is released by regulated exocytosis of GABA-containing synaptic-like microvesicles (SLMVs) in insulin-releasing rat pancreatic β-cells. To this end, β-cells were engineered to express GABA_A-receptor Cl⁻-channels at high density using adenoviral infection. Electron microscopy indicated that the average diameter of the SLMVs is 90 nm, that every β-cell contains ∼3,500 such vesicles, and that insulin-containing large dense core vesicles exclude GABA. Quantal release of GABA, seen as rapidly activating and deactivating Cl⁻-currents, was observed during membrane depolarizations from −70 mV to voltages beyond −40 mV or when Ca²⁺ was dialysed into the cell interior. Depolarization-evoked GABA release was suppressed when Ca²⁺ entry was inhibited using Cd²⁺. Analysis of the kinetics of GABA release revealed that GABA-containing vesicles can be divided into a readily releasable pool and a reserve pool. Simultaneous measurements of GABA release and cell capacitance indicated that exocytosis of SLMVs contributes ∼1% of the capacitance signal. Mathematical analysis of the release events suggests that every SLMV contains 0.36 amol of GABA. We conclude that there are two parallel pathways of exocytosis in pancreatic β-cells and that release of GABA may accordingly be temporally and spatially separated from insulin secretion. This provides a basis for paracrine GABAergic signaling within the islet.

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.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Subunit composition and function of GABAA receptors of rat spermatozoa

GABA triggers mammalian sperm acrosome reaction (AR). Here, evidence is presented, showing that rat spermatozoa contain GABAA receptors, composed of alpha5, beta1 and beta3 subunits. The effects of GABAA receptor agonist and antagonist on the induction of AR in rat spermatozoa were assessed using the chlortetracycline assay. Muscimol, a GABAA receptor agonist, triggered AR; whereas bicuculline, a GABAA receptor antagonist and picrotoxin, a GABAA receptor/Cl- channel blocker, inhibited the ability of GABA or progesterone to induce AR. In conclusion, GABAA receptors appear to mediate the action of progesterone in inducing AR in rat spermatozoa.

GABA in the endocrine pancreas: cellular localization and function in normal and diabetic rats

Gamma amino butyric acid (GABA) and its related enzymes have been demonstrated in pancreatic beta cells of normal rat. Antibodies against GABA-synthesizing enzymes have been implicated in the pathogenesis of Type I diabetes. In spite of the importance of GABA in the aetiology of diabetes mellitus, detailed morphological data on the pattern of distribution of GABA in the pancreas of normal and diabetic rats are lacking. Diabetes mellitus (DM) was induced by a single dose of streptozotocin (STZ) given intraperitoneally (60 mg kg body weight(-1)). Four weeks after the induction of DM, normal (n = 6) and diabetic (n = 6) rats were anesthetized with chloral hydrate and their pancreata were removed and processed for the localization and effect of GABA on insulin secretion using immunohistochemistry and radioimmunoassay techniques. The number of GABA-like immunoreactive (GABA-LIR) cells in the pancreatic islets of STZ-diabetic rats decreased significantly (P<0.0001) when compared to non-diabetic control rats. The pattern and percentage distribution of GABA in the islet of Langerhans of normal and diabetic rat was similar to that of insulin. GABA induced a significant (P<0.0007) increase in insulin secretion from the pancreas of normal rats. In diabetic pancreas, GABA evoked a higher but not significant (P<0.1) increase in insulin secretion. These findings showed that the number of GABA-LIR cells is reduced significantly in diabetes. Moreover, GABA is a strong secretagogue of insulin from the pancreas of normal rat.

An osmotic-sensitive taurine pool is localized in rat pancreatic islet cells containing glucagon and somatostatin

Previous reports have dealt with the hypoglycemic properties of taurine and its effects on insulin secretion by adult and fetal isolated islets. We have studied the presence and cellular distribution of taurine in rat islets, the conditions to evoke its release, and its possible modulatory action on insulin secretion. We localized taurine by techniques of double immunolabeling in most glucagon-positive cells and in some somatostatin-positive cells, whereas insulin-positive cells were not labeled with the taurine antibody. Although high-glucose stimulation did not evoke any taurine release, a hyposmotic solution (17% osmolarity reduction) induced a specific phasic release of taurine and GABA (34 and 52% increase on their basal release rate). On the other hand, taurine (10 mmol/l) application slightly reduced the second phase of insulin secretion induced by glucose stimulation. In conclusion, taurine is highly concentrated in glucagon-containing cells of the islet periphery. It is not liberated by glucose stimulation but is strongly released under hyposmotic conditions. All of these data suggest that taurine plays an osmoregulatory role in alpha-cells.

GABA(A)-receptor expression in glioma cells is triggered by contact with neuronal cells

The expression of functional GABA(A)-receptors in glioma cells correlates with low malignancy of tumours and cell lines from glioma lack these receptors. Here we show that contact with neurons induces the expression of functional GABA(A)-receptors. C6 and F98 glioma cell lines were labelled by recombinant expression of enhanced green fluorescent protein injected into rat brain and studied in acute slices after two to three weeks of tumour growth. The cells responded to GABA or the specific agonist, muscimol with a current typical for GABA(A)-receptors, as studied with the patch-clamp technique. To get insight into the mechanism of GABA(A) receptor induction, the C6 or F98 cells were co-cultured with neurons, astrocytes, oligodendrocytes and microglia. Glioma cells expressed functional GABA(A) receptors within 24 h only in cultures where physical contact to neurons occurred. Activation of GABA(A)-receptors in the co-cultures attenuated glioma cell metabolism while blockade of the receptors increased metabolism. We conclude that with this form of interaction, neurons can influence tumour behaviour in the brain.

Biochemical characteristics of the gamma-aminobutyric acid system in the insulinoma cell lines HIT-T15, RIN-m5F, betaTC3, and comparison with rat brain

BACKGROUND

gamma-aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in the mammalian brain. Both GABA and its synthesizing enzyme, L-glutamate decarboxylase (GAD), are also present in the insulin-secreting pancreatic beta cells, in which its physiologic role is unclear. We have studied several aspects of the GABA system in the insulinoma cell lines HIT-T15, RIN-m5F, and betaTC3 in comparison with rat brain tissue.

METHODS

Insulinoma cell lines and embryonic rat brain cortex neurons were cultured. GAD activity was determined by a radioenzymatic method and the presence of GAD(67) protein was assessed by immunocytochemistry. Amino acid content and the effect of different conditions on the release of endogenous GABA were measured by HPLC and fluorometric detection after o-phthaldialdehyde derivatization. [3H]GABA was used for measuring the uptake of the amino acid in the insulinoma cultures and in rat forebrain synaptosomes.

RESULTS

The three insulinoma lines possess GABA and GAD activity at levels of approximately 20% compared with adult rat brain cortex. Dissimilar from the latter, in insulinoma cultures enzyme activity was not enhanced by addition of an excess of the coenzyme pyridoxal-5'-phosphate. Immunocytochemical visualization of GAD showed that the cells in both neuronal cultures and insulinoma lines were GAD(67)-positive, similar to Purkinje cell somata of adult rat cerebellar cortex. [3H]GABA uptake in the cell lines was approximately 10% of that in rat forebrain synaptosomes and showed less ionic and temperature dependence. In both cultured cerebral neurons and RINm5F cells, the addition of arginine induced the release of GABA, whereas neither high K(+) concentration nor glucose had any effect.

CONCLUSIONS

The insulinoma cell lines studied possess the same GAD(67) form of the enzyme present in brain. RIN line cells are capable of transporting glutamate. In these cells as well as in cultured cortical neurons, arginine stimulates the release of GABA and glutamate probably as the result of its electrogenic transport. Insulinoma cell lines may therefore be useful to study GABA metabolism and function in pancreatic beta cells.

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

Because GABA and its related enzymes have been determined in beta-cells of pancreas islets, effects of GABA on pancreatic exocrine secretion were investigated in the isolated, perfused rat pancreas. GABA, given intra-arterially at concentrations of 3, 10, 30, and 100 microM, did not exert any influence on spontaneous or secretin (12 pM)-induced pancreatic exocrine secretion. However, GABA further elevated CCK (10 pM)-, gastrin-releasing peptide (100 pM)-, or electrical field stimulation-induced pancreatic secretions of fluid and amylase dose dependently. The GABA (30 microM)-enhanced CCK-induced pancreatic secretions were completely blocked by bicuculline (10 microM), a GABA(A) receptor antagonist, but were not affected by saclofen (10 microM), a GABA(B) receptor antagonist. The enhancing effects of GABA (30 microM) on CCK-induced pancreatic secretions were not changed by tetrodotoxin (1 microM) but were partially reduced by cyclo-(7-aminoheptanonyl-Phe-D-Trp-Lys-Thr[BZL]) (10 nM), a somatostatin antagonist. In conclusion, GABA enhances pancreatic exocrine secretion induced by secretagogues, which predominantly induce enzyme secretion, via GABA(A) receptors in the rat pancreas. The enhancing effect of GABA is partially mediated by inhibition of islet somatostatin release.

Subunit composition and pharmacological characterization of gamma-aminobutyric acid type A receptors in frog pituitary melanotrophs

The frog pars intermedia is composed of a single population of endocrine cells directly innervated by gamma-aminobutyric acid (GABA)ergic nerve terminals. We have previously shown that GABA, acting through GABA(A) receptors, modulates both the electrical and secretory activities of frog pituitary melanotrophs. The aim of the present study was to take advantage of the frog melanotroph model to determine the relationship between the subunit composition and the pharmacological properties of native GABA(A) receptors. Immunohistochemical labeling revealed that in situ and in cell culture, frog melanotrophs were intensely stained with alpha2-, alpha3-, gamma2-, and gamma3-subunit antisera and weakly stained with a gamma1-subunit antiserum. Melanotrophs were also immunolabeled with a monoclonal antibody to the beta2/beta3-subunit. In contrast, frog melanotrophs were not immunoreactive for the alpha1-, alpha5-, and alpha6-isoforms. The effects of allosteric modulators of the GABA(A) receptor on GABA-activated chloride current were tested using the patch-clamp technique. Among the ligands acting at the benzodiazepine-binding site, clonazepam (EC50, 5 x 10(-9) M), diazepam (EC50, 10(-8) M), zolpidem (EC50, 3 x 10(-8) M), and beta-carboline-3-carboxylic acid methyl ester (EC50, 10(-6) M) were found to potentiate the whole cell GABA-evoked current in a dose-dependent manner. Methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (IC50, 3 x 10(-5) M) inhibited the current, whereas Ro15-4513 had no effect. Among the ligands acting at other modulatory sites, etomidate (EC50, 2 x 10(-6) M) enhanced the GABA-evoked current, whereas 4'-chlorodiazepam (IC50, 4 x 10(-7) M), ZnCl2 (IC50, >5 x 10(-5) M), and furosemide (IC50, >3 x 10(-4) M) depressed the response to GABA. PK 11195 did not affect the GABA-evoked current or its inhibition by 4'-chlorodiazepam. The results indicate that the native GABA(A) receptors in frog melanotrophs are formed by combinations of alpha2-, alpha3-, beta2/3-, gamma1-, gamma2-, and gamma3-subunits. The data also demonstrate that clonazepam is the most potent, and zolpidem is the most efficient positive modulator of the native receptors. Among the inhibitors, 4'-chlorodiazepam is the most potent, whereas ZnCl2 is the most efficient negative modulator of the GABA(A) receptors. The present study provides the first correlation between subunit composition and the functional properties of native GABA(A) receptors in nontumoral endocrine cells.

Expression of functional GABAA receptors in isolated human insulinoma cells

Pancreatic islets contain and release high concentrations of GABA. GABA is thought to play a paracrine role in beta-cells. Searching for a paracrine function of GABA in neoplastic beta-cells we performed patch-clamp studies in isolated human insulinoma cells. We show that human insulinoma cells can express functional GABAA receptors. Activation of GABAA receptors caused a reversible membrane depolarization in a subgroup of insulinoma cells. Membrane depolarization resulted in transmembraneous calcium influx through voltage-gated calcium channels and stimulation of insulin secretion. Insulin secretion was increased by the GABAA receptor agonist muscimol (50 microM) by about 280%. Thus, GABAA receptors can be expressed in human insulinoma cells and can regulate their insulin release.

Expression of functional GABAA receptors in cholecystokinin-secreting gut neuroendocrine murine STC-1 cells

1. Gastrointestinal neuroendocrine (NE) cells synthesize, store and secrete gamma-aminobutyric acid (GABA). Recently, an autocrine-paracrine function of GABA has been proposed for secretion from NE cells. 2. To search for functional GABAA receptors in NE gut cells, we performed whole-cell and perforated-patch-clamp studies in the intestinal cholecystokinin (CCK)-secreting NE cell line STC-1. 3. Application of GABA evoked currents in STC-1 cells. These effects were mimicked by muscimol, an agonist of GABAA receptors, and blocked by picrotoxin or bicuculline, antagonists of GABAA receptors. The GABA- or muscimol-activated currents reversed near 0 mV, which under the recording conditions used was consistent with the activation of the GABAA receptor-Cl- channel complex. 4. In contrast to the effect on most neurons, GABA as well as muscimol led to a (reversible) depolarization of the membrane potential of STC-1 cells. Membrane depolarization in turn activated voltage-gated Ca2+ channels and increased intracellular Ca2+ concentrations in STC-1 cells. 5. In accordance with the observed membrane depolarization and activation of voltage-gated Ca2+ channels, both GABA and muscimol stimulated Ca2+-dependent CCK release. In contrast, bicuculline inhibited the GABA-induced secretion of CCK. 6. Using the reverse transcription-polymerase chain reaction (RT-PCR), mRNA of the GABAA receptor subunits alpha2, alpha3, alpha5, beta1, beta3 and delta could be detected in STC-1 cells. 7. In summary, we have shown that the CCK-secreting gut NE cell line STC-1 expresses functional GABAA receptors and that GABA stimulates CCK release. Thus, GABA is involved in the fine tuning of CCK secretion from the gut NE cell line STC-1.

Immortalized gonadotropin-releasing hormone neurons secrete gamma-aminobutyric acid-evidence for an autocrine regulation

The immortalized hypothalamic neuronal cell lines GT1-1 and GT1-7 represent unique model systems to investigate the physiological control of gonadotropin-releasing hormone (GnRH) secretion. Using immunofluorescence microscopy, key proteins of regulated exocytosis, e.g. synaptotagmin, synaptobrevin and SNAP-25 (synaptosomal associated protein of 25 kDa) were found in GT1 neurons. In addition, GT1 neurons contained synaptophysin, a marker protein for small synaptic vesicles (SSVs) which are responsible for the storage of neurotransmitters such as gamma-aminobutyric acid (GABA). Upon subcellular fractionation, a lighter vesicle population characterized by synaptophysin separated from a denser vesicle population containing GnRH. Both vesicle populations contained synaptobrevin and synaptotagmin. Besides GnRH, GT1 neurons expressed glutamic acid decarboxylase at the mRNA-level and synthesized GABA. More importantly, GT1 neurons took up and stored 3H-GABA. The stored GABA was released after stimulation with increasing K+ concentrations and by alpha-latrotoxin. Reducing the extracellular Ca2+-concentration abolished stimulated secretion, indicating that GABA was released by regulated exocytosis. Hormone secretion from GT1 neurons is controlled by GABA via GABA(A) and GABA(B) receptors reflecting the situation in vivo. Our data provide the first evidence that GT1 neurons possess a second regulated secretory pathway sustained by SSVs storing and releasing GABA. The released GABA influences GnRH secretion by an auto- or paracrine loop.

Functional GABA(A) receptors on human glioma cells

Glioma cells in acute slices and in primary culture, and glioma-derived human cell lines were screened for the presence of functional GABA(A) receptors. Currents were measured in whole-cell voltage clamp in response to gamma-aminobutyric acid (GABA). While cells from the most malignant glioma, the glioblastoma multiforme, did not respond to GABA, an inward current (under our experimental conditions with high Cl- concentration in the pipette) was induced in gliomas of lower grades, namely in 71% of oligodendroglioma cells and in 62% of the astrocytoma cells. Glioma cell lines did not express functional GABA(A) receptors, irrespective of the malignancy of the tumour they originate from. The currents elicited by application of GABA were due to activation of GABA(A) receptors; the specific agonist muscimol mimicked the response, the antagonists bicuculline and picrotoxin blocked the GABA-activated current and the benzodiazepine receptor agonist flunitrazepam augmented the GABA-induced current and the benzodiazepine inverse agonist DMCM decreased the GABA current. Cells were heterogeneous with respect to the direction of the current flow as tested in gramicidin perforated patches: in some cells GABA hyperpolarized the membrane, while in the majority it triggered a depolarization. Moreover, GABA triggered an increase in [Ca2+]i in the majority of the tumour cells due to the activation of Ca2+ channels. Our results suggest a link between the expression of GABA receptors and the growth of glioma cells as the disappearance of functional GABA(A) receptors parallels unlimited growth typical for malignant tumours and immortal cell lines.

Differential expression of glutamate receptor subtypes in rat pancreatic islets

Immunocytochemistry was carried out on sections of rat pancreas to localize the expression of glutamate receptor subunits and the major pancreatic peptide hormones. Glutamate receptor expression was concentrated in pancreatic islets, and each islet cell type expressed different neuronal glutamate receptors of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate classes. AMPA receptor subunits were expressed in alpha, beta, and pancreatic polypeptide cells, whereas kainate receptors were found predominantly in alpha and delta cells. Patch clamp electrophysiology was used to measure the functional properties of islet cell glutamate receptors. L-glutamate and other glutamate receptor agonists evoked currents in islet cells that were blocked by the selective AMPA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione and potentiated by cyclothiazide in a manner indistinguishable from that of neuronal AMPA receptors. Activation of islet cell AMPA receptors produced steady-state cation currents that depolarized the cells an average of 20.7 +/- 5.4 mV (n = 6). Currents mediated by functional kainate receptors were also observed in a line of transformed pancreatic alpha cells. Thus, L-glutamate probably regulates islet physiology via actions at both AMPA and kainate receptor classes. The pattern of receptor expression suggests that glutamate receptor activation may have multiple, complex consequences for islet physiology.