New roles for dopamine D2 and D3 receptors in pancreatic beta cell insulin secretion.

Although long-studied in the central nervous system, there is increasing evidence that dopamine (DA) has important roles in the periphery including in metabolic regulation. Insulin-secreting pancreatic ß-cells express the machinery for DA synthesis and catabolism, as well as all five DA receptors. In these cells, DA functions as a negative regulator of glucose-stimulated insulin secretion (GSIS), which is mediated by DA D2-like receptors including D2 (D2R) and D3 (D3R) receptors. However, the fundamental mechanisms of DA synthesis, storage, release, and signaling in pancreatic ß-cells and their functional relevance in vivo remain poorly understood. Here, we assessed the roles of the DA precursor L-DOPA in ß-cell DA synthesis and release in conjunction with the signaling mechanisms underlying DA's inhibition of GSIS. Our results show that the uptake of L-DOPA is essential for establishing intracellular DA stores in ß-cells. Glucose stimulation significantly enhances L-DOPA uptake, leading to increased DA release and GSIS reduction in an autocrine/paracrine manner. Furthermore, D2R and D3R act in combination to mediate dopaminergic inhibition of GSIS. Transgenic knockout mice in which ß-cell D2R or D3R expression is eliminated exhibit diminished DA secretion during glucose stimulation, suggesting a new mechanism where D2-like receptors modify DA release to modulate GSIS. Lastly, ß-cell-selective D2R knockout mice exhibit marked postprandial hyperinsulinemia in vivo. These results reveal that peripheral D2R and D3R receptors play important roles in metabolism through their inhibitory effects on GSIS. This opens the possibility that blockade of peripheral D2-like receptors by drugs including antipsychotic medications may significantly contribute to the metabolic disturbances observed clinically.

A novel optical tracer for VMAT2 applied to live cell measurements of vesicle maturation in cultured human ß-cells.

The islet ß-cells integrate external signals to modulate insulin secretion to better regulate blood glucose levels during periods of changing metabolic demand. The vesicular monoamine transporter type 2 (VMAT2), an important regulator of CNS neurotransmission, has an analogous role in the endocrine pancreas as a key control point of insulin secretion, with additional roles in regulating ß-cell differentiation and proliferation. Here we report on the synthesis and biological characterisation of a fluorescent ligand for VMAT2 suitable for live cell imaging. Staining for VMAT2 and dopamine in live ß-cell cultures show colocalisation in specific vesicles and reveal a heterogeneous population with respect to cell size, shape, vesicle number, size, and contents. Staining for VMAT2 and zinc ion, as a surrogate for insulin, reveals a wide range of vesicle sizes. Immunohistochemistry shows larger ß-cell vesicles enriched for proinsulin, whereas smaller vesicles predominantly contain the processed mature insulin. In ß-cell cultures obtained from nondiabetic donors, incubation at non-stimulatory glucose concentrations promotes a shift in vesicle diameter towards the more mature insulin vesicles at the expense of the larger immature insulin secretory vesicle population. We anticipate that this probe will be a useful reagent to identify living ß-cells within complex mixtures for further manipulation and characterisation.

A role for foregut tyrosine metabolism in glucose tolerance.

OBJECTIVE: We hypothesized that DA and L-DOPA derived from nutritional tyrosine and the resultant observed postprandial plasma excursions of L-DOPA and DA might affect glucose tolerance via their ability to be taken-up by beta cells and inhibit glucose-stimulated ß-cell insulin secretion. METHODS: To investigate a possible circuit between meal-stimulated 3,4-dihydroxy-L-phenylalanine (L-DOPA) and dopamine (DA) production in the GI tract and pancreatic ß-cells, we: 1) mapped GI mucosal expression of tyrosine hydroxylase (TH) and aromatic amino acid decarboxylase (AADC); 2) measured L-DOPA and DA content of GI mucosal tissues following meal challenges with different L-tyrosine (TYR) content, 3) determined whether meal TYR content impacts plasma insulin and glucose excursions; and 4) characterized postprandial plasma excursions of L-DOPA and DA in response to meal tyrosine content in rodents and a population of bariatric surgery patients. Next, we characterized: 1) the metabolic transformation of TYR and L-DOPA into DA in vitro using purified islet tissue; 2) the metabolic transformation of orally administrated stable isotope labeled TYR into pancreatic DA, and 3) using a nuclear medicine technique, we studied endocrine beta cells in situ release and binding of DA in response to a glucose challenge. RESULTS: We demonstrate in rodents that intestinal content and circulatory concentrations L-DOPA and DA, plasma glucose and insulin are responsive to the tyrosine (TYR) content of a test meal. Intestinal expression of two enzymes, Tyrosine hydroxylase (TH) and Aromatic Amino acid Decarboxylase (AADC), essential to the transformation of TYR to DA was mapped and the metabolism of metabolism of TYR to DA was traced in human islets and a rodent beta cell line in vitro and from gut to the pancreas in vivo. Lastly, we show that ß cells secrete and bind DA in situ in response to glucose stimulation. CONCLUSIONS: We provide proof-of-principle evidence for the existence of a novel postprandial circuit of glucose homeostasis dependent on nutritional tyrosine. DA and L-DOPA derived from nutritional tyrosine may serve to defend against hypoglycemia via inhibition of glucose-stimulated ß-cell insulin secretion as proposed by the anti-incretin hypothesis.

Cross-sectional and Test-Retest Characterization of PET with [(18)F]FP-(+)-DTBZ for ß Cell Mass Estimates in Diabetes.

PURPOSE: The vesicular monoamine transporter, type 2 (VMAT2) is expressed by insulin producing ß cells and was evaluated as a biomarker of ß cell mass (BCM) by positron emission tomography (PET) with [(18)F]fluoropropyl-dihydrotetrabenazine ([(18)F]FP-(+)-DTBZ). PROCEDURES: We evaluated the feasibility of longitudinal pancreatic PET VMAT2 quantification in the pancreas in two studies of healthy controls and patients with type 1 or 2 diabetes. VMAT2 binding potential (BPND) was estimated voxelwise using a reference tissue method in a cross-sectional study, followed by assessment of reproducibility using a test-retest paradigm. Metabolic function was evaluated by stimulated c-peptide measurements. RESULTS: Pancreatic BPND was significantly decreased in patients with type 1 diabetes relative to controls and the test-retest variability was 9.4%. CONCLUSIONS: Pancreatic VMAT2 content is significantly reduced in long-term diabetes patients relative to controls and repeat scans are sufficiently reproducible to suggest the feasibility clinically VMAT2 measurements in longitudinal studies of new onset diabetes.

Gastrointestinal dopamine as an anti-incretin and its possible role in bypass surgery as therapy for type 2 diabetes with associated obesity.

The objective of this review was to summarize and integrate specific clinical observations from the field of gastric bypass surgery and recent findings in beta cell biology. When considered together, these data sets suggest a previously unrecognized physiological mechanism which may explain how Roux-en-Y gastric bypass (RYGB) surgery mediates the early rapid reversal of hyperglycemia, observed before weight loss, in certain type 2 diabetes mellitus (T2DM) patients. The novel mechanism is based on a recently recognized inhibitory circuit of glucose stimulated insulin secretion driven by DA stored in ß-cell vesicles and the gut. We propose that DA and glucagon-like peptide 1 (GLP-1) represent two opposing arms of a glucose stimulated insulin secretion (GSIS) regulatory system and hypothesize that dopamine represents the "anti-incretin" hypothesized to explain the beneficial effects of bariatric surgery on T2DM. These new hypotheses and the research driven by them may directly impact our understanding of: 1) the mechanisms underlying improved glucose homeostasis seen before weight loss following bariatric surgery; and 2) the regulation of glucose stimulated insulin secretion within islets. On a practical level, these studies may result in the development of novels drugs to modulate insulin secretion and/or methods to quantitatively asses in real time beta cell function and mass.

Anti-incretin, Anti-proliferative Action of Dopamine on ß-Cells.

Human islet ß-cells exploit an autocrine dopamine (DA)-mediated inhibitory circuit to regulate insulin secretion. ß-Cells also express the DA active transporter and the large neutral amino acid transporter heterodimer enabling them to import circulating DA or its biosynthetic precursor, L-3,4-dihydroxyphenylalanine (L-DOPA). The capacity to import DA or L-DOPA from the extracellular space possibly indicates that DA may be an endocrine signal as well. In humans, a mixed meal stimulus is accompanied by contemporary serum excursions of incretins, DA and L-DOPA, suggesting that DA may act as an anti-incretin as postulated by the foregut hypothesis proposed to explain the early effects of bariatric surgery on type 2 diabetes. In this report, we take a translational step backwards and characterize the kinetics of plasma DA and incretin production after a mixed meal challenge in a rat model and study the integration of incretin and DA signaling at the biochemical level in a rodent ß-cell line and islets. We found that there are similar excursions of incretins and DA in rats, as those reported in humans, after a mixed meal challenge and that DA counters incretin enhanced glucose-stimulated insulin secretion and intracellular signaling at multiple points from dampening calcium fluxes to inhibiting proliferation as well as apoptosis. Our data suggest that DA is an important regulator of insulin secretion and may represent 1 axis of a gut level circuit of glucose and ß-cell mass homeostasis.

Co-regulated expression of alpha and beta mRNAs encoding HLA-DR surface heterodimers is mediated by the MHCII RNA operon.

Major histocompatibility complex class II (MHCII) molecules are heterodimeric surface proteins involved in the presentation of exogenous antigens during the adaptive immune response. We demonstrate the existence of a fine level of regulation, coupling the transcription and processing of mRNAs encoding α and ß chains of MHCII molecules, mediated through binding of their Untraslated Regions (UTRs) to the same ribonucleoproteic complex (RNP). We propose a dynamic model, in the context of the 'MHCII RNA operon' in which the increasing levels of DRA and DRB mRNAs are docked by the RNP acting as a bridge between 5'- and 3'-UTR of the same messenger, building a loop structure and, at the same time, joining the two chains, thanks to shared common predicted secondary structure motifs. According to cell needs, as during immune surveillance, this RNP machinery guarantees a balanced synthesis of DRA and DRB mRNAs and a consequent balanced surface expression of the heterodimer.

Dopamine-mediated autocrine inhibitory circuit regulating human insulin secretion in vitro.

We describe a negative feedback autocrine regulatory circuit for glucose-stimulated insulin secretion in purified human islets in vitro. Using chronoamperometry and in vitro glucose-stimulated insulin secretion measurements, evidence is provided that dopamine (DA), which is loaded into insulin-containing secretory granules by vesicular monoamine transporter type 2 in human ß-cells, is released in response to glucose stimulation. DA then acts as a negative regulator of insulin secretion via its action on D2R, which are also expressed on ß-cells. We found that antagonism of receptors participating in islet DA signaling generally drive increased glucose-stimulated insulin secretion. These in vitro observations may represent correlates of the in vivo metabolic changes associated with the use of atypical antipsychotics, such as increased adiposity.

Contrasting effects of IFNα on MHC class II expression in professional vs. nonprofessional APCs: Role of CIITA type IV promoter.

We previously demonstrated that, in ex vivo cultures, IFNα downregulates the expression of MHC class II (MHCII) genes in human non-professional APCs associated with pancreatic islets. IFNα has an opposing effect on MHCII expression in professional APCs. In this study, we found that the mechanism responsible for the IFNα-mediated MHCII's downregulation in human MHCII-positive non-professional antigen presenting human non-hematopoietic cell lines is the result of the negative feedback system that regulates cytokine signal transduction, which eventually inhibits promoters III and IV of CIITA gene. Because the CIITA-PIV isoform is mostly responsible for the constitutive expression of MHCII genes in non-professional APCs, we pursued and achieved the specific knockdown of CIITA-PIV mRNA in our in vitro system, obtaining a partial silencing of MHCII molecules similar to that obtained by IFNα. We believe that our results offer a new understanding of the potential significance of CIITA-PIV as a therapeutic target for interventional strategies that can manage autoimmune disease and allograft rejection with little interference on the function of professional APCs of the immune system.

EBP1 and DRBP76/NF90 binding proteins are included in the major histocompatibility complex class II RNA operon.

Major histocompatibility complex class II mRNAs encode heterodimeric proteins involved in the presentation of exogenous antigens during an immune response. Their 3'UTRs bind a protein complex in which we identified two factors: EBP1, an ErbB3 receptor-binding protein and DRBP76, a double-stranded RNA binding nuclear protein, also known as nuclear factor 90 (NF90). Both are well-characterized regulatory factors of several mRNA molecules processing. Using either EBP1 or DRBP76/NF90-specific knockdown experiments, we established that the two proteins play a role in regulating the expression of HLA-DRA, HLA-DRB1 and HLA-DQA1 mRNAs levels. Our study represents the first indication of the existence of a functional unit that includes different transcripts involved in the adaptive immune response. We propose that the concept of 'RNA operon' may be suitable for our system in which MHCII mRNAs are modulated via interaction of their 3'UTR with same proteins.

Islet grafting and imaging in a bioengineered intramuscular space.

BACKGROUND: Because the hepatic portal system may not be the optimal site for islet transplantation, several extrahepatic sites have been studied. Here, we examine an intramuscular transplantation site, bioengineered to better support islet neovascularization, engraftment, and survival, and we demonstrate that at this novel site, grafted beta cell mass may be quantitated in a real-time noninvasive manner by positron emission tomography (PET) imaging. METHODS: Streptozotocin-induced rats were pretreated intramuscularly with a biocompatible angiogenic scaffold received syngeneic islet transplants 2 weeks later. The recipients were monitored serially by blood glucose and glucose tolerance measurements and by PET imaging of the transplant site with [11C] dihydrotetrabenazine. Parallel histopathologic evaluation of the grafts was performed using insulin staining and evaluation of microvasularity. RESULTS: Reversal of hyperglycemia by islet transplantation was most successful in recipients pretreated with bioscaffolds containing angiogenic factors when compared with those who received no bioscaffolds or bioscaffolds not treated with angiogenic factors. PET imaging with [11C] dihydrotetrabenazine, insulin staining, and microvascular density patterns were consistent with islet survival, increased levels of angiogenesis, and with reversal of hyperglycemia. CONCLUSIONS: Induction of increased neovascularization at an intramuscular site significantly improves islet transplant engraftment and survival compared with controls. The use of a nonhepatic transplant site may avoid intrahepatic complications and permit the use of PET imaging to measure and follow transplanted beta cell mass in real time. These findings have important implications for effective islet implantation outside of the liver and offer promising possibilities for improving islet survival, monitoring, and even prevention of islet loss.

11C-dihydrotetrabenazine PET of the pancreas in subjects with long-standing type 1 diabetes and in healthy controls.

UNLABELLED: Type 2 vesicular monoamine transporter (VMAT2), found in the brain, is also expressed by beta-cells of the pancreas in association with insulin. Preclinical experiments suggested that (11)C-dihydrotetrabenazine PET-measured VMAT2 binding might serve as a biomarker of beta-cell mass. We evaluated the feasibility of (11)C-dihydrotetrabenazine PET quantification of pancreatic VMAT2 binding in healthy subjects and patients with long-standing type 1 diabetes. METHODS: (11)C-Dihydrotetrabenazine PET was performed on 6 patients and 9 controls. VMAT2 binding potential (BP(ND)) was estimated voxelwise by using the renal cortex as reference tissue. As an index of total pancreatic VMAT2, the functional binding capacity (the sum of voxel BP(ND) x voxel volume) was calculated. Pancreatic BP(ND), functional binding capacity, and stimulated insulin secretion measurements were compared between groups. RESULTS: The pancreatic mean BP(ND) was decreased in patients (1.86 +/- 0.05) to 86% of control values (2.14 +/- 0.08) (P = 0.01). In controls, but not in patients, BP(ND) correlated with stimulated insulin secretion (r(2) = 0.50, P = 0.03). The average functional binding capacity was decreased by at least 40% in patients (P = 0.001). The changes in functional binding capacity and BP(ND) were less than the near-complete loss of stimulated insulin secretion observed in patients (P = 0.001). CONCLUSION: These results suggest that (11)C-dihydrotetrabenazine PET allows quantification of VMAT2 binding in the human pancreas. However, BP(ND) and functional binding capacity appear to overestimate beta-cell mass given the near-complete depletion of beta-cell mass in long-standing type 1 diabetes, which may be due to higher nonspecific binding in the pancreas than in the renal cortex.

Role of vesicular monoamine transporter type 2 in rodent insulin secretion and glucose metabolism revealed by its specific antagonist tetrabenazine.

Despite different embryological origins, islet beta-cells and neurons share the expression of many genes and display multiple functional similarities. One shared gene product, vesicular monoamine transporter type 2 (VMAT2, also known as SLC18A2), is highly expressed in human beta-cells relative to other cells in the endocrine and exocrine pancreas. Recent reports suggest that the monoamine dopamine is an important paracrine and/or autocrine regulator of insulin release by beta-cells. Given the important role of VMAT2 in the economy of monoamines such as dopamine, we investigated the possible role of VMAT2 in insulin secretion and glucose metabolism. Using a VMAT2-specific antagonist, tetrabenazine (TBZ), we studied glucose homeostasis, insulin secretion both in vivo and ex vivo in cultures of purified rodent islets. During intraperitoneal glucose tolerance tests, control rats showed increased serum insulin concentrations and smaller glucose excursions relative to controls after a single intravenous dose of TBZ. One hour following TBZ administration we observed a significant depletion of total pancreas dopamine. Correspondingly, exogenous L-3,4-dihydroxyphenylalanine reversed the effects of TBZ on glucose clearance in vivo. In in vitro studies of rat islets, a significantly enhanced glucose-dependent insulin secretion was observed in the presence of dihydrotetrabenazine, the active metabolite of TBZ. Together, these data suggest that VMAT2 regulates in vivo glucose homeostasis and insulin production, most likely via its role in vesicular transport and storage of monoamines in beta-cells.

VMAT2 gene expression and function as it applies to imaging beta-cell mass.

Diabetes mellitus is a metabolic disorder characterized by hyperglycemia. The two main forms of the disease are distinguished by different pathogenesis, natural histories, and population distributions and indicated as either type 1 (T1DM) or type 2 diabetes mellitus (T2DM). It is well established that T1DM is an autoimmune disease whereby beta-cells of pancreatic islets are destroyed leading to loss of endogenous insulin production. Albeit less dramatic, beta-cell mass (BCM) also drops in T2DM. Therefore, it is realistic to expect that noninvasive measures of BCM might provide useful information in the diabetes-care field. Preclinical studies have demonstrated that BCM measurements by positron emission tomography scanning, using the vesicular monoamine transporter type 2 (VMAT2) as a tissue-specific surrogate marker of insulin production and [11C] Dihydrotetrabenazine (DTBZ) as the radioligand specific for this molecule, is feasible in animal models. Unfortunately, the mechanisms underlying beta-cell-specific expression of VMAT2 are still largely unexplored, and a much better understanding of the regulation of VMAT2 gene expression and of its function in beta-cells will be required before the full utility of this technique in the prediction and treatment of individuals with diabetes can be understood. In this review, we summarize much of what is understood about the regulation of VMAT2 and identify questions whose answers may help in understanding what measurements of VMAT2 density mean in the context of diabetes.

The active translation of MHCII mRNA during dendritic cells maturation supplies new molecules to the cell surface pool.

The transition of human dendritic cells (DCs) from the immature to the mature phenotype is characterized by an increased density of MHC class II (MHCII) molecules on the plasma membrane, a key requirement of their competence as professional antigen presenting cells (APCs). MHCII molecules on the cell surface derive from newly synthesized as well as from preexisting proteins. So far, all the studies done on DCs during maturation, to establish the relative contribution of newly synthesized MHCII molecules to the cell surface pool did not produced a clear, unified scenario. We report that, in human DCs stimulated ex vivo with LPS, the changes in the RNA accumulation specific for at least two MHCII genes (HLA-DRA and HLA-DQA1) due to transcriptional upregulation, is associated with the active translation at high rate of these transcripts. Our finding reveals that, across the 24h of the maturation process in human DCs, newly synthesized MHCII proteins are supplied to the APCs cell surface pool.

Targeting vesicular monoamine transporter Type 2 for noninvasive PET-based ß-cell mass measurements.

The common pathology in both Types 1 and 2 diabetes is insufficient ß-cell mass to meet the metabolic needs of insulin production. The rising worldwide incidence of diabetes, combined with the lack of reliable endpoints of the body's true capacity to produce insulin, constitute a serious dilemma facing healthcare professionals and the pharmaceutical industry. Recent advances in imaging science and molecular imaging chemistry, as well as a broader understanding of basic islet biology, now allow the collection of quantitative information about ß cells deep within the pancreas. The ability to noninvasively measure the mass of insulin-producing cells will most likely be of value towards characterizing new drugs and refining the diagnosis and treatment of this burdensome disease.

Current progress in non-invasive imaging of beta cell mass of the endocrine pancreas.

The increasing incidence of diabetes requires a better understanding of the pathogenesis of the clinical disease. Studies in prevention and treatment have been hampered by the single end-point of diagnosis of diabetes and hyperglycemia. The common pathology in both type 1 and type 2 diabetes is insufficient beta-cell mass to meet the metabolic demand. Unfortunately, current diagnostic methods rely on metabolic responses that do not accurately reflect true beta-cell mass. Recent advances in beta-cell imaging have utilized multiple modalities in experimental and clinical settings. While no "gold-standard" exists to measure beta-cell mass, modalities such as single photon emission computed tomography, optical and fluorescent imaging, magnetic resonance imaging, and positron emission tomography have been used with mixed success. Many of the methods are limited by the inability to translate to the clinical setting, poor discrimination between the exocrine and endocrine pancreas, or a poor measurement of beta-cell mass. However, promising new "neurofunctional imaging" approaches have emerged as improved measures of beta-cell mass. We review the current understanding of the pathogenesis and evaluation of diabetes, as well as experimental approaches to assessing beta-cell mass.

Visualizing pancreatic beta-cell mass with [11C]DTBZ.

Beta-cell mass (BCM) influences the total amount of insulin secreted, varies by individual and by the degree of insulin resistance, and is affected by physiologic and pathologic conditions. The islets of Langerhans, however, appear to have a reserve capacity of insulin secretion and, overall, assessments of insulin and blood glucose levels remain poor measures of BCM, beta-cell function and progression of diabetes. Thus, novel noninvasive determinations of BCM are needed to provide a quantitative endpoint for novel therapies of diabetes, islet regeneration and transplantation. Built on previous gene expression studies, we tested the hypothesis that the targeting of vesicular monoamine transporter 2 (VMAT2), which is expressed by beta cells, with [11C]dihydrotetrabenazine ([11C]DTBZ), a radioligand specific for VMAT2, and the use of positron emission tomography (PET) can provide a measure of BCM. In this report, we demonstrate decreased radioligand uptake within the pancreas of Lewis rats with streptozotocin-induced diabetes relative to their euglycemic historical controls. These studies suggest that quantitation of VMAT2 expression in beta cells with the use of [11C]DTBZ and PET represents a method for noninvasive longitudinal estimates of changes in BCM that may be useful in the study and treatment of diabetes.

Effect of interferon alpha on MHC class II gene expression in ex vivo human islet tissue.

Type 1 diabetes (T1D) is caused by autoimmune destruction of the insulin-producing beta-cells of the islets of Langerhans. One still open question is where naive islet-reactive T cells encounter antigens and become stimulated. In this report we have re-examined the expression of MHC class II (MHCII) genes in human islets to further explore the possibility that non-professional antigen presenting cells (APCs) within islets contribute to autoimmunity. Since development of T1D has been linked to viral infections, we also studied ex-vivo MHCII expression in response to interferon-alpha (IFNalpha) in islet tissue and in different APCs. The findings are: first, MHCII genes expression in human islets is linked with the expression of the class II transactivator isoform transcribed from the promoter IV, similar to that described in non-professional APCs. Second, there is IFNalpha-mediated lineage-specific regulation of MHCII genes expression, seen as a decrease in the accumulation of MHCII transcripts in pancreatic islets opposite to an increase in dendritic cells and B-lymphoblastoid cell lines. Third, there is allele-specific regulation of the HLA-DQA1 gene by IFNalpha in islet tissue. These findings may begin to explain the molecular events that create favorable conditions for organ-specific autoimmunity and explain the incomplete penetrance of T1D susceptibility alleles.

Longitudinal noninvasive PET-based beta cell mass estimates in a spontaneous diabetes rat model.

Diabetes results from an absolute or relative reduction in pancreatic beta cell mass (BCM) leading to insufficient insulin secretion and hyperglycemia. Measurement of insulin secretory capacity is currently used as a surrogate measure of BCM. However, serum insulin concentrations provide an imprecise index of BCM, and no reliable noninvasive measure of BCM is currently available. Type 2 vesicular monoamine transporters (VMAT2) are expressed in human islet beta cells, as well as in tissues of the CNS. [11C]Dihydrotetrabenazine ([11C]DTBZ) binds specifically to VMAT2 and is a radioligand currently used in clinical imaging of the brain. Here we report the use of [11C]DTBZ to estimate BCM in a rodent model of spontaneous type 1 diabetes (the BB-DP rat). In longitudinal PET studies of the BB-DP rat, we found a significant decline in pancreatic uptake of [11C]DTBZ that anticipated the loss of glycemic control. Based on comparison of standardized uptake values (SUVs) of [11C]DTBZ and blood glucose concentrations, loss of more than 65% of the original SUV correlated significantly with the development of persistent hyperglycemia. These studies suggest that PET-based quantitation of VMAT2 receptors provides a noninvasive measurement of BCM that could be used to study the pathogenesis of diabetes and to monitor therapeutic interventions.