Dopamine is a neurotransmitter that plays a critical role in neurological and psychiatric disorders, such as schizophrenia, Parkinson disease, and drug addiction (1). Increasing evidence also shows implication of dopamine in various physiological functions such as cell proliferation (2), gastrointestinal protection (3), and inhibition of prolactin secretion (4). Effects of dopamine on insulin secretion in general and on pancreatic beta cell function in particular have been poorly studied. Insulin exocytosis from the beta cell is primarily controlled by metabolismsecretion coupling. First, glucose equilibrates across the plasma membrane and is phosphorylated by glucokinase, initiating glycolysis (5). Subsequently, mitochondrial metabolism generates ATP, which promotes the closure of ATP-sensitive potassium channels and, as a consequence, depolarization of the plasma membrane (6). This leads to calcium influx through voltage-gated calcium channels and a rise in cytosolic calcium, triggering insulin exocytosis (6, 7). Additional signals participating in the amplifying pathway (8) are necessary to reproduce the sustained secretion elicited by glucose. Insulin secretion evoked by glucose metabolism can be further modulated by parasympathetic and sympathetic neurotransmitters (9).Treatment with dopamine precursor L-dopa in humans suffering from Parkinson disease reduces insulin secretion upon oral glucose tolerance test (10). In rodents, a single injection with L-dopa results in the accumulation of dopamine in beta cells and inhibition of the insulin secretory responses (11,12). In isolated islets, analogues of dopamine inhibit glucose-stimulated insulin release (13), whereas one study reports potentiation of insulin secretion upon acute dopamine accumulation (14). Taken as a whole, these previous studies suggest that beta cells might be directly responsive to dopamine. Here, we investigated the molecular mechanisms implicated in beta cell responses to dopamine action. In particular, the present data demonstrate the presence of dopamine receptors in beta cells. Moreover, the inhibitory effects of dopamine are predominantly ascribed to activation of the D2-like receptor family members. MATERIALS AND METHODS INS-1E Cells and Pancreatic Islets-INS-1Ecells, used as a well differentiated beta cell clone (15), were cultured in a humidified atmosphere containing 5% CO 2 in a medium composed of RPMI 1640 supplemented with 10 mM Hepes, 5% (v/v) heat-inactivated fetal calf serum, 2 mM glutamine, 100 units/ml penicillin, 100 g/ml streptomycin, 1 mM sodium pyruvate, and 50 M 2-mercaptoethanol. For rodent islets, Wistar rats or BALB/c mice weighing 200 -250 g and 25-30 g, respectively, were obtained from in-house breeding (CMU-Zootechnie, Geneva, Switzerland). We followed the principles of laboratory animal care, and the study was approved by the responsible ethics committee. Pancreatic islets were isolated by collagenase digestion and handpicking from male Wistar rats or BALB/c mice as described previously (16). Isolated islets w...

Insulin exocytosis is regulated in pancreatic ß-cells by a cascade of intracellular signals translating glucose levels into corresponding secretory responses. The mitochondrial enzyme glutamate dehydrogenase (GDH) is regarded as a major player in this process, although its abrogation has not been tested yet in animal models. Here, we generated transgenic mice, named ßGlud1 ؊/؊ , with ß-cell-specific GDH deletion. Our results show that GDH plays an essential role in the full development of the insulin secretory response. In situ pancreatic perfusion revealed that glucose-stimulated insulin secretion was reduced by 37% in ßGlud1 ؊/؊ . Furthermore, isolated islets with either constitutive or acute adenovirus-mediated knock-out of GDH showed a 49 and 38% reduction in glucose-induced insulin release, respectively. Adenovirus-mediated re-expression of GDH in ßGlud1 ؊/؊ islets fully restored glucose-induced insulin release. Thus, GDH appears to account for about 40% of glucose-stimulated insulin secretion and to lack redundant mechanisms. In ßGlud1 ؊/؊ mice, the reduced secretory capacity resulted in lower plasma insulin levels in response to both feeding and glucose load, while body weight gain was preserved. The results demonstrate that GDH is essential for the full development of the secretory response in ß-cells. However, maximal secretory capacity is not required for maintenance of glucose homeostasis in normo-caloric conditions.Pancreatic ß-cells produce the hormone insulin that is essential for glucose homeostasis. Upon nutrient stimulation, elevation of cytosolic calcium in the ß-cell is the primary and necessary signal for insulin exocytosis (1). Then, increasing the magnitude of the secretory response requires amplification of the calcium signal supported by metabolism-derived additive factors (2). The enzyme glutamate dehydrogenase (GDH, 2 EC 1.4.1.3) has been proposed to participate to the development of the secretory response. GDH is a homohexamer located in the mitochondrial matrix that catalyzes the reversible reaction: ␣-ketoglutarate ϩ NH 3 ϩ NAD(P)H % glutamate ϩ NAD(P) ϩ ; inhibited by GTP and activated by ADP (3, 4). Regarding ß-cell, allosteric activation of GDH by L-leucine or its non-metabolized analogue BCH has triggered most of the attention over the last three decades (5).To date, the role of GDH in ß-cell function remains unclear and debated. Specifically, GDH might play a role in glucoseinduced amplifying pathway through generation of glutamate (6, 7). GDH is also an amino acid sensor triggering insulin release upon glutamine stimulation in conditions of GDH allosteric activation (8 -10). Recently, the importance of GDH has been further highlighted by studies showing that SIRT4, a mitochondrial ADP-ribosyltransferase, down-regulates GDH activity and thereby modulates insulin secretion (11,12).GDH is encoded by a well-conserved 45-kb gene named GLUD1, which is organized into 13 exons (13). A decade ago, clinical data and associated genetic studies revealed GDH as a key enzyme for the control ...

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