Dopaminergic neurotoxin administration to the area of the suprachiasmatic nuclei induces insulin resistance

Tyrosine Hydroxylase Knockdown at the Hypothalamic Supramammillary Nucleus Area Induces Obesity and Glucose Intolerance

INTRODUCTION

The supramammillary nucleus (SuMN) exerts influences on a wide range of brain functions including feeding and feeding-independent fuel metabolism. However, which specific neuronal type(s) within the SuMN manifest this influence has not been delineated. This study investigated the effect of SuMN tyrosine hydroxylase (TH) (rate-limiting enzyme in dopamine synthesis) knockdown (THx) on peripheral fuel metabolism.

METHODS

SuMN-THx was accomplished using a virus-mediated shRNA to locally knockdown TH gene expression at the SuMN. The impact of SuMN-THx was examined over 35-72 days in rats least prone to developing metabolic syndrome (MS) - female Sprague-Dawley rats resistant to the obesogenic effect of high fat diet (HFDr) and fed regular chow (RC) - upon body weight/fat, feeding, glucose tolerance, and insulin sensitivity. The influence of HFD, gender, and long-term response of SuMN-THx was subsequently investigated in female HFDr rats fed HFD, male HFDr rats fed RC, and female HFD-sensitive rats fed RC over 1 year, respectively.

RESULTS

SuMN-THx induced obesity and glucose intolerance, elevated plasma leptin and triglycerides, increased hepatic mRNA levels of gluconeogenic, lipogenic, and pro-inflammatory genes, reduced white adipose fatty acid oxidation rate, and altered plasma corticosterone level and hepatic circadian gene expression. Moreover, SuMN-THx increased feeding during the natural resting/fasting period and altered ghrelin feeding response suggesting ghrelin resistance. This MS-inducing effect was enhanced by HFD feeding, similarly observed in male rats and persisted over 1 year.

DISCUSSION/CONCLUSION

SuMN-THx induced long-term, gender-nonspecific, multiple pathophysiological changes leading to MS suggesting SuMN dopaminergic circuits communicating with other brain metabolism and behavior control centers modulate peripheral fuel metabolism.

Brain Dopamine-Clock Interactions Regulate Cardiometabolic Physiology: Mechanisms of the Observed Cardioprotective Effects of Circadian-Timed Bromocriptine-QR Therapy in Type 2 Diabetes Subjects

Despite enormous global efforts within clinical research and medical practice to reduce cardiovascular disease(s) (CVD), it still remains the leading cause of death worldwide. While genetic factors clearly contribute to CVD etiology, the preponderance of epidemiological data indicate that a major common denominator among diverse ethnic populations from around the world contributing to CVD is the composite of Western lifestyle cofactors, particularly Western diets (high saturated fat/simple sugar [particularly high fructose and sucrose and to a lesser extent glucose] diets), psychosocial stress, depression, and altered sleep/wake architecture. Such Western lifestyle cofactors are potent drivers for the increased risk of metabolic syndrome and its attendant downstream CVD. The central nervous system (CNS) evolved to respond to and anticipate changes in the external (and internal) environment to adapt survival mechanisms to perceived stresses (challenges to normal biological function), including the aforementioned Western lifestyle cofactors. Within the CNS of vertebrates in the wild, the biological clock circuitry surveils the environment and has evolved mechanisms for the induction of the obese, insulin-resistant state as a survival mechanism against an anticipated ensuing season of low/no food availability. The peripheral tissues utilize fat as an energy source under muscle insulin resistance, while increased hepatic insulin resistance more readily supplies glucose to the brain. This neural clock function also orchestrates the reversal of the obese, insulin-resistant condition when the low food availability season ends. The circadian neural network that produces these seasonal shifts in metabolism is also responsive to Western lifestyle stressors that drive the CNS clock into survival mode. A major component of this natural or Western lifestyle stressor-induced CNS clock neurophysiological shift potentiating the obese, insulin-resistant state is a diminution of the circadian peak of dopaminergic input activity to the pacemaker clock center, suprachiasmatic nucleus. Pharmacologically preventing this loss of circadian peak dopaminergic activity both prevents and reverses existing metabolic syndrome in a wide variety of animal models of the disorder, including high fat-fed animals. Clinically, across a variety of different study designs, circadian-timed bromocriptine-QR (quick release) (a unique formulation of micronized bromocriptine-a dopamine D2 receptor agonist) therapy of type 2 diabetes subjects improved hyperglycemia, hyperlipidemia, hypertension, immune sterile inflammation, and/or adverse cardiovascular event rate. The present review details the seminal circadian science investigations delineating important roles for CNS circadian peak dopaminergic activity in the regulation of peripheral fuel metabolism and cardiovascular biology and also summarizes the clinical study findings of bromocriptine-QR therapy on cardiometabolic outcomes in type 2 diabetes subjects.

Dopamine receptor agonists mechanism of actions on glucose lowering and their connections with prolactin actions

Robust experiment evidence suggests that prolactin can enhance beta-cell proliferation and increase insulin secretion and sensitivity. Apart from acting as an endocrine hormone, it also function as an adipokine and act on adipocytes to modulate adipogenesis, lipid metabolism and inflammation. Several cross-sectional epidemiologic studies consistently showed that circulating prolactin levels positive correlated with increased insulin sensitivity, lower glucose and lipid levels, and lower prevalence of T2D and metabolic syndrome. Bromocriptine, a dopamine receptor agonist used to treat prolactinoma, is approved by Food and Drug Administration for treatment in type 2 diabetes mellitus since 2009. Prolactin lowering suppress insulin secretion and decrease insulin sensitivity, therefore dopamine receptor agonists which act at the pituitary to lower serum prolactin levels are expected to impair glucose tolerance. Making it more complicating, studies exploring the glucose-lowering mechanism of bromocriptine and cabergoline have resulted in contradictory results; while some demonstrated actions independently on prolactin status, others showed glucose lowering partly explained by prolactin level. Previous studies showed that a moderate increase in central intraventricular prolactin levels stimulates hypothalamic dopamine with a decreased serum prolactin level and improved glucose metabolism. Additionally, sharp wave-ripples from the hippocampus modulates peripheral glucose level within 10 minutes, providing evidence for a mechanistic link between hypothalamus and blood glucose control. Central insulin in the mesolimbic system have been shown to suppress dopamine levels thus comprising a feedback control loop. Central dopamine and prolactin levels plays a key role in the glucose homeostasis control, and their dysregulation could lead to the pathognomonic central insulin resistance depicted in the “ominous octet”. This review aims to provide an in-depth discussion on the glucose-lowering mechanism of dopamine receptor agonists and on the diverse prolactin and dopamine actions on metabolism targets.

Bromocriptine-QR Therapy Reduces Sympathetic Tone and Ameliorates a Pro-Oxidative/Pro-Inflammatory Phenotype in Peripheral Blood Mononuclear Cells and Plasma of Type 2 Diabetes Subjects

Bromocriptine-QR is a sympatholytic dopamine D2 agonist for the treatment of type 2 diabetes that has demonstrated rapid (within 1 year) substantial reductions in adverse cardiovascular events in this population by as yet incompletely delineated mechanisms. However, a chronic state of elevated sympathetic nervous system activity and central hypodopaminergic function has been demonstrated to potentiate an immune system pro-oxidative/pro-inflammatory condition and this immune phenotype is known to contribute significantly to the advancement of cardiovascular disease (CVD). Therefore, the possibility exists that bromocriptine-QR therapy may reduce adverse cardiovascular events in type 2 diabetes subjects via attenuation of this underlying chronic pro-oxidative/pro-inflammatory state. The present study was undertaken to assess the impact of bromocriptine-QR on a wide range of immune pro-oxidative/pro-inflammatory biochemical pathways and genes known to be operative in the genesis and progression of CVD. Inflammatory peripheral blood mononuclear cell biology is both a significant contributor to cardiovascular disease and also a marker of the body’s systemic pro-inflammatory status. Therefore, this study investigated the effects of 4-month circadian-timed (within 2 h of waking in the morning) bromocriptine-QR therapy (3.2 mg/day) in type 2 diabetes subjects whose glycemia was not optimally controlled on the glucagon-like peptide 1 receptor agonist on (i) gene expression status (via qPCR) of a wide array of mononuclear cell pro-oxidative/pro-inflammatory genes known to participate in the genesis and progression of CVD (OXR1, NRF2, NQO1, SOD1, SOD2, CAT, GSR, GPX1, GPX4, GCH1, HMOX1, BiP, EIF2α, ATF4, PERK, XBP1, ATF6, CHOP, GSK3β, NFkB, TXNIP, PIN1, BECN1, TLR2, TLR4, TLR10, MAPK8, NLRP3, CCR2, GCR, L-selectin, VCAM1, ICAM1) and (ii) humoral measures of sympathetic tone (norepinephrine and normetanephrine), whole-body oxidative stress (nitrotyrosine, TBARS), and pro-inflammatory factors (IL-1β, IL-6, IL-18, MCP-1, prolactin, C-reactive protein [CRP]). Relative to pre-treatment status, 4 months of bromocriptine-QR therapy resulted in significant reductions of mRNA levels in PBMC endoplasmic reticulum stress-unfolded protein response effectors [GRP78/BiP (34%), EIF2α (32%), ATF4 (29%), XBP1 (25%), PIN1 (14%), BECN1 (23%)], oxidative stress response proteins [OXR1 (31%), NRF2 (32%), NQO1 (39%), SOD1 (52%), CAT (26%), GPX1 (33%), GPX4 (31%), GCH1 (30%), HMOX1 (40%)], mRNA levels of TLR pro-inflammatory pathway proteins [TLR2 (46%), TLR4 (20%), GSK3β (19%), NFkB (33%), TXNIP (18%), NLRP3 (32%), CCR2 (24%), GCR (28%)], mRNA levels of pro-inflammatory cellular receptor proteins CCR2 and GCR by 24% and 28%, and adhesion molecule proteins L-selectin (35%) and VCAM1 (24%). Relative to baseline, bromocriptine-QR therapy also significantly reduced plasma levels of norepinephrine and normetanephrine by 33% and 22%, respectively, plasma pro-oxidative markers nitrotyrosine and TBARS by 13% and 10%, respectively, and pro-inflammatory factors IL-18, MCP1, IL-1β, prolactin, and CRP by 21%,13%, 12%, 42%, and 45%, respectively. These findings suggest a unique role for circadian-timed bromocriptine-QR sympatholytic dopamine agonist therapy in reducing systemic low-grade sterile inflammation to thereby reduce cardiovascular disease risk.

Therapeutic potential of dopamine agonists in the treatment of type 2 diabetes mellitus

Diabetes is a global health concern that has affected almost 415 million people globally. Bromocriptine is a dopamine D2 agonist, which is a Food and Drug Administration (FDA)-approved drug to treat type 2 diabetes mellitus (T2DM) patients. However, it is considered that a novel treatment therapy is required which can be used in the treatment of diabetes with or without other antidiabetic agents. Dopamine agonists are usually used in neurological disorders like Parkinson's disease (PD), restless leg syndrome, and hyperprolactinemia. However, dopamine agonists including bromocriptine and cabergoline are also effective in reducing the glycemic level in T2DM patients. Bromocriptine was formerly used for the treatment of PD, hyperprolactinemia, and restless leg syndrome, but now it is used for improving glycemic levels as well as reducing free fatty acids and triglycerides. In addition, cabergoline has been found to be effective in glycemic control, but this drug is yet to be approved by the FDA due to its limitations and lack of study. Findings of the clinical trials of bromocriptine have suggested that it reduces almost 0.4-0.8% glycated hemoglobin and cardiovascular risk by 40% in insulin-resistant patients. Moreover, the safe use of bromocriptine in obese T2DM patients makes it a more attractive option as it causes weight loss. Indeed, bromocriptine is a novel therapy for T2DM patients, as its mechanism of action is unique in T2DM patients with minimal adverse effects. This review summarizes the potential of dopamine agonists in the treatment of T2DM.

Diabetes mellitus and Parkinson's disease: dangerous liaisons between insulin and dopamine

The relationship between diabetes mellitus and Parkinson's disease has been described in several epidemiological studies over the 1960s to date. Molecular studies have shown the possible functional link between insulin and dopamine, as there is strong evidence demonstrating the action of dopamine in pancreatic islets, as well as the insulin effects on feeding and cognition through central nervous system mechanism, largely independent of glucose utilization. Therapies used for the treatment of type 2 diabetes mellitus appear to be promising candidates for symptomatic and/or disease-modifying action in neurodegenerative diseases including Parkinson's disease, while an old dopamine agonist, bromocriptine, has been repositioned for the type 2 diabetes mellitus treatment. This review will aim at reappraising the different studies that have highlighted the dangerous liaisons between diabetes mellitus and Parkinson's disease.

Identification of novel key molecular signatures in the pathogenesis of experimental diabetic retinopathy

Deep mining of the molecular mechanisms underlying diabetic retinopathy (DR) is critical for the development of novel therapeutic targets. This study aimed to identify key molecular signatures involved in experimental DR on the basis of integrated bioinformatics analysis. Four datasets consisting of 37 retinal samples were downloaded from the National Center of Biotechnology Information Gene Expression Omnibus. After batch-effect adjustment, bioinformatics tools such as Networkanalyst, Enrichr, STRING, and Metascape were used to evaluate the differentially expressed genes (DEGs), perform enrichment analysis, and construct protein-protein interaction networks. The hub genes were identified using Cytoscape software. The DEGs of interest from the meta-analysis were confirmed by quantitative reverse transcription-polymerase chain reaction in diabetic rats and a high-glucose-treated retinal cell model, respectively. A total of 743 DEGs related to lens differentiation, insulin resistance, and high-density lipoprotein (HDL) cholesterol metabolism were obtained using the meta-analysis. Alterations of dynamic gene expression in the chloride ion channel, retinol metabolism, and fatty acid metabolism were involved in the course of DR in rats. Importantly, H3K27m3 modifications regulated the expression of most DEGs at the early stage of DR. Using an integrated bioinformatics approach, novel molecular signatures were obtained for different stages of DR progression, and the findings may represent distinct therapeutic strategies for DR patients.

Time-of-Day-Dependent Effects of Bromocriptine to Ameliorate Vascular Pathology and Metabolic Syndrome in SHR Rats Held on High Fat Diet

The treatment of type 2 diabetes patients with bromocriptine-QR, a unique, quick release micronized formulation of bromocriptine, improves glycemic control and reduces adverse cardiovascular events. While the improvement of glycemic control is largely the result of improved postprandial hepatic glucose metabolism and insulin action, the mechanisms underlying the drug's cardioprotective effects are less well defined. Bromocriptine is a sympatholytic dopamine agonist and reduces the elevated sympathetic tone, characteristic of metabolic syndrome and type 2 diabetes, which potentiates elevations of vascular oxidative/nitrosative stress, known to precipitate cardiovascular disease. Therefore, this study investigated the impact of bromocriptine treatment upon biomarkers of vascular oxidative/nitrosative stress (including the pro-oxidative/nitrosative stress enzymes of NADPH oxidase 4, inducible nitric oxide (iNOS), uncoupled endothelial nitric oxide synthase (eNOS), the pro-inflammatory/pro-oxidative marker GTP cyclohydrolase 1 (GTPCH 1), and the pro-vascular health enzyme, soluble guanylate cyclase (sGC) as well as the plasma level of thiobarbituric acid reactive substances (TBARS), a circulating marker of systemic oxidative stress), in hypertensive SHR rats held on a high fat diet to induce metabolic syndrome. Inasmuch as the central nervous system (CNS) dopaminergic activities both regulate and are regulated by CNS circadian pacemaker circuitry, this study also investigated the time-of-day-dependent effects of bromocriptine treatment (10 mg/kg/day at either 13 or 19 h after the onset of light (at the natural waking time or late during the activity period, respectively) among animals held on 14 h daily photoperiods for 16 days upon such vascular biomarkers of vascular redox state, several metabolic syndrome parameters, and mediobasal hypothalamic (MBH) mRNA expression levels of neuropeptides neuropeptide Y (NPY) and agouti-related protein (AgRP) which regulate the peripheral fuel metabolism and of mRNA expression of other MBH glial and neuronal cell genes that support such metabolism regulating neurons in this model system. Such bromocriptine treatment at ZT 13 improved (reduced) biomarkers of vascular oxidative/nitrosative stress including plasma TBARS level, aortic NADPH oxidase 4, iNOS and GTPCH 1 levels, and improved other markers of coupled eNOS function, including increased sGC protein level, relative to controls. However, bromocriptine treatment at ZT 19 produced no improvement in either coupled eNOS function or sGC protein level. Moreover, such ZT 13 bromocriptine treatment reduced several metabolic syndrome parameters including fasting insulin and leptin levels, as well as elevated systolic and diastolic blood pressure, insulin resistance, body fat store levels and liver fat content, however, such effects of ZT 19 bromocriptine treatment were largely absent versus control. Finally, ZT 13 bromocriptine treatment reduced MBH NPY and AgRP mRNA levels and mRNA levels of several MBH glial cell/neuronal genes that code for neuronal support/plasticity proteins (suggesting a shift in neuronal structure/function to a new metabolic control state) while ZT 19 treatment reduced only AgRP, not NPY, and was with very little effect on such MBH glial cell genes expression. These findings indicate that circadian-timed bromocriptine administration at the natural circadian peak of CNS dopaminergic activity (that is diminished in insulin resistant states), but not outside this daily time window when such CNS dopaminergic activity is naturally low, produces widespread improvements in biomarkers of vascular oxidative stress that are associated with the amelioration of metabolic syndrome and reductions in MBH neuropeptides and gene expressions known to facilitate metabolic syndrome. These results of such circadian-timed bromocriptine treatment upon vascular pathology provide potential mechanisms for the observed marked reductions in adverse cardiovascular events with circadian-timed bromocriptine-QR therapy (similarly timed to the onset of daily waking as in this study) of type 2 diabetes subjects and warrant further investigations into related mechanisms and the potential application of such intervention to prediabetes and metabolic syndrome patients as well.

Rapid and dramatic glucose-lowering effect of bromocriptine in an inadequately controlled type 2 diabetes patient with prolactinoma

Dopamine receptor agonists are typically used to treat Parkinson's disease and certain pituitary tumors, such as prolactinoma or a growth hormone-producing tumor. A 53-year-old woman with a history of prolactinoma was referred to Kumamoto University Hospital (Kumamoto, Japan) with poorly controlled type 2 diabetes. Her glycated hemoglobin and serum prolactin levels were increased (8.8% and 160.3 ng/mL, respectively). Bromocriptine, a dopamine D₂ receptor agonist, was administered to reduce her serum prolactin level. Because bromocriptine-QR (quick release) has been approved for the treatment of type 2 diabetes mellitus in the USA, a continuous glucose monitoring system, FreeStyle Libre Pro, was utilized to examine the effect of bromocriptine on glycemic control. After the initial administration of bromocriptine, glucose levels were rapidly and dramatically ameliorated, and the time in range (70-180 mg/dL) improved from <50% to >90% between 1 week before and after the initial administration of bromocriptine.

Bromocriptine mesylate improves glucose tolerance and disposal in a high-fat-fed canine model

Bromocriptine mesylate treatment was examined in dogs fed a high fat diet (HFD) for 8 wk. After 4 wk on HFD, daily bromocriptine (Bromo; n = 6) or vehicle (CTR; n = 5) injections were administered. Oral glucose tolerance tests were performed before beginning HFD (OGTT1), 4 wk after HFD began (Bromo only), and after 7.5 wk on HFD (OGTT3). After 8 wk on HFD, clamp studies were performed, with infusion of somatostatin and intraportal replacement of insulin (4× basal) and glucagon (basal). From 0 to 90 min (P1), glucose was infused via peripheral vein to double the hepatic glucose load; and from 90 to 180 min (P2), glucose was infused via the hepatic portal vein at 4 mg·kg^-1·min^-1, with the HGL maintained at 2× basal. Bromo decreased the OGTT glucose ΔAUC_0-30 and ΔAUC_0-120 by 62 and 27%, respectively, P < 0.05 for both) without significantly altering the insulin response. Bromo dogs exhibited enhanced net hepatic glucose uptake (NHGU) compared with CTR (~33 and 21% greater, P1 and P2, respectively, P < 0.05). Nonhepatic glucose uptake (non-HGU) was increased ~38% in Bromo in P2 (P < 0.05). Bromo vs. CTR had higher (P < 0.05) rates of glucose infusion (36 and 30%) and non-HGU (~40 and 27%) than CTR during P1 and P2, respectively. In Bromo vs. CTR, hepatic 18:0/16:0 and 16:1/16:0 ratios tended to be elevated in triglycerides and were higher (P < 0.05) in phospholipids, consistent with a beneficial effect of bromocriptine on liver fat accumulation. Thus, bromocriptine treatment improved glucose disposal in a glucose-intolerant model, enhancing both NHGU and non-HGU.

Circadian-timed dopamine agonist treatment reverses high-fat diet-induced diabetogenic shift in ventromedial hypothalamic glucose sensing

INTRODUCTION

Within the ventromedial hypothalamus (VMH), glucose inhibitory (GI) neurons sense hypoglycaemia while glucose excitatory (GE) neurons sense hyperglycaemia to initiate counter control mechanisms under normal conditions. However, potential electrophysiological alterations of these two neuronal types in vivo in insulin-resistant states have never been simultaneously fully documented. Further, the anti-diabetic effect of dopamine agonism on this VMH system under insulin resistance has not been studied.

METHODS

This study examined the impact of a high-fat diet (HFD) on in vivo electrophysiological recordings from VMH GE and GI neurons and the ability of circadian-timed dopamine agonist therapy to reverse any adverse effect of the HFD on such VMH activities and peripheral glucose metabolism.

RESULTS

HFD significantly inhibited VMH GE neuronal electrophysiological response to local hyperglycaemia (36.3%) and augmented GI neuronal excitation response to local hypoglycaemia (47.0%). Bromocriptine (dopamine agonist) administration at onset of daily activity (but not during the daily sleep phase) completely reversed both VMH GE and GI neuronal aberrations induced by HFD. Such timed treatment also normalized glucose intolerance and insulin resistance. These VMH and peripheral glucose metabolism effects of circadian-timed bromocriptine may involve its known effect to reduce elevated VMH noradrenergic activity in insulin-resistant states as local VMH administration of norepinephrine was observed to significantly inhibit VMH GE neuronal sensing of local hyperglycaemia in insulin-sensitive animals on regular chow diet (52.4%).

CONCLUSIONS

HFD alters VMH glucose sensing in a manner that potentiates hyperglycaemia and this effect on the VMH can be reversed by appropriately circadian-timed dopamine agonist administration.

Antipsychotics and glucose metabolism: how brain and body collide

Since the serendipitous discovery of the first antipsychotic (AP) drug in the 1950s, APs remain the cornerstone of treatment for schizophrenia. A shift over the past two decades away from first-generation, conventional APs to so-called "atypical" (or 2nd/3rd generation) APs parallels acknowledgment of serious metabolic side-effects associated in particular with these newer agents. As will be reviewed, AP drugs and type 2 diabetes are now inextricably linked, contributing to the three- to fivefold increased risk of type 2 diabetes observed in schizophrenia. However, this association is not straightforward. Biological and lifestyle-related illness factors contribute to the association between type 2 diabetes and metabolic disease independently of AP treatment. In addition, APs have a well-established weight gain propensity which could also account for elevated risk of insulin resistance and type 2 diabetes. However, compelling preclinical and clinical evidence now suggests that these drugs can rapidly and directly influence pathways of glucose metabolism independently of weight gain and even in absence of psychiatric illness. Mechanisms of these direct effects remain poorly elucidated but may involve central and peripheral antagonism of neurotransmitters implicated not only in the therapeutic effects of APs but also in glucose homeostasis, possibly via effects on the autonomic nervous system. The clinical relevance of studying "direct" effects of these drugs on glucose metabolism is underscored by the widespread use of these medications, both on and off label, for a growing number of mental illnesses, extending safety concerns well beyond schizophrenia.

Circadian peak dopaminergic activity response at the biological clock pacemaker (suprachiasmatic nucleus) area mediates the metabolic responsiveness to a high-fat diet

Among vertebrate species of the major vertebrate classes in the wild, a seasonal rhythm of whole body fuel metabolism, oscillating from a lean to obese condition, is a common biological phenomenon. This annual cycle is driven in part by annual changes in the circadian dopaminergic signalling at the suprachiasmatic nuclei (SCN), with diminution of circadian peak dopaminergic activity at the SCN facilitating development of the seasonal obese insulin-resistant condition. The present study investigated whether such an ancient circadian dopamine-SCN activity system for expression of the seasonal obese, insulin-resistant phenotype may be operative in animals made obese amd insulin resistant by high-fat feeding and, if so, whether reinstatement of the circadian dopaminergic peak at the SCN would be sufficient to reverse the adverse metabolic impact of the high-fat diet without any alteration of caloric intake. First, we identified the supramammillary nucleus as a novel site providing the majority of dopaminergic neuronal input to the SCN. We further identified dopamine D2 receptors within the peri-SCN region as being functional in mediating SCN responsiveness to local dopamine. In lean, insulin-sensitive rats, the peak in the circadian rhythm of dopamine release at the peri-SCN coincided with the daily peak in SCN electrophysiological responsiveness to local dopamine administration. However, in rats made obese and insulin resistant by high-fat diet (HFD) feeding, these coincident circadian peak activities were both markedly attenuated or abolished. Reinstatement of the circadian peak in dopamine level at the peri-SCN by its appropriate circadian-timed daily microinjection to this area (but not outside this circadian time-interval) abrogated the obese, insulin-resistant condition without altering the consumption of the HFD. These findings suggest that the circadian peak of dopaminergic activity at the peri-SCN/SCN is a key modulator of metabolism and the responsiveness to adverse metabolic consequences of HFD consumption.

Interaction between the obesity-risk gene FTO and the dopamine D2 receptor gene ANKK1/TaqIA on insulin sensitivity

AIMS/HYPOTHESIS

Variations in FTO are the strongest common genetic determinants of adiposity, and may partly act by influencing dopaminergic signalling in the brain leading to altered reward processing that promotes increased food intake. Therefore, we investigated the impact of such an interaction on body composition, and peripheral and brain insulin sensitivity.

METHODS

Participants from the Tübingen Family study (n = 2245) and the Malmö Diet and Cancer study (n = 2921) were genotyped for FTO SNP rs8050136 and ANKK1 SNP rs1800497. Insulin sensitivity in the caudate nucleus, an important reward area in the brain, was assessed by fMRI in 45 participants combined with intranasal insulin administration.

RESULTS

We found evidence of an interaction between variations in FTO and an ANKK1 polymorphism that associates with dopamine (D2) receptor density. In cases of reduced D2 receptor availability, as indicated by the ANKK1 polymorphism, FTO variation was associated with increased body fat and waist circumference and reduced peripheral insulin sensitivity. Similarly, altered central insulin sensitivity was observed in the caudate nucleus in individuals with the FTO obesity-risk allele and diminished D2 receptors.

CONCLUSIONS/INTERPRETATION

The effects of variations in FTO are dependent on dopamine D2 receptor density (determined by the ANKK1 polymorphism). Carriers of both risk alleles might, therefore, be at increased risk of obesity and diabetes.

Impact of bromocriptine-QR therapy on cardiovascular outcomes in type 2 diabetes mellitus subjects on metformin

OBJECTIVES

Type 2 diabetes mellitus (T2DM) is associated with a substantially increased risk of cardiovascular disease (CVD). Bromocriptine-QR (B-QR), a quick release sympatholytic dopamine D₂ receptor agonist, is a FDA-approved therapy for T2DM which may provide CVD risk reduction. Metformin is considered to be an agent with a potential cardioprotective benefit. This large placebo controlled clinical study assessed the impact of B-QR addition to existing metformin therapy on CVD outcomes in T2DM subjects.

METHODS

1791 subjects (1208 B-QR; 583 placebo) on metformin ± another anti-diabetes therapy at baseline derived from the Cycloset Safety Trial, a 12-month, randomized, multicenter, placebo-controlled, double-blind study in T2DM, were included in this study. The primary CVD endpoint evaluated was treatment impact on CVD event rate, prespecified as a composite of time to first myocardial infarction, stroke, coronary revascularization, or hospitalization for unstable angina/congestive heart failure. Impact on glycemic control was evaluated as a secondary analysis.

RESULTS

The composite CVD end point occurred in 16/1208 B-QR treated (1.3%) and 18/583 placebo treated (3.1%) subjects resulting in a 55% CVD hazard risk reduction (intention-to-treat, Cox regression analysis; HR: 0.45 [0.23-0.88], p = 0.028). Kaplan-Meier curves demonstrated a significantly lower cumulative incidence rate of the CVD endpoint in the B-QR treatment group (Log-Rank p = 0.017). In subjects with poor glycemic control (HbA1c ≥ 7.5) at baseline, B-QR therapy relative to placebo resulted in a significant mean %HbA1c reduction of -0.59 at week 12 and -0.51 at week 52 respectively (p < 0.001 for both) and a 10 fold higher percent of subjects achieving HbA1c goal of ≤7% by week 52 (B-QR 30%, placebo 3%; p = 0.003).

CONCLUSION

These findings suggest that in T2DM subjects on metformin, BQR therapy may represent an effective strategy for reducing CVD risk. Cycloset Safety Trial registration: ClinicalTrials.gov Identifier: NCT00377676.

Evidence-based practice use of quick-release bromocriptine across the natural history of type 2 diabetes mellitus

OBJECTIVES

To provide an evidence-based practice overview on the clinical use of bromocriptine-quick release (QR) across the natural history of type 2 diabetes mellitus (T2DM).

METHODS

Articles for inclusion were selected after a comprehensive literature search of English-language PubMed articles and identification of other relevant references through other sources. Inclusion criteria were animal studies examining the mechanism of action and efficacy of bromocriptine, and clinical studies examining the safety and efficacy of bromocriptine-QR in patients with T2DM, without a time limitation.

RESULTS

The brain plays a key role in total body metabolism, in particular ensuring that sufficient levels of glucose are available for proper neural functioning. The hypothalamic suprachiasmatic nucleus (SCN), the body's biological clock, plays a key role in the regulation of seasonal and diurnal variations of insulin sensitivity. A daily surge of dopaminergic activity in the SCN upon waking enables insulin sensitivity throughout the day. When this is disrupted (e.g. by a high fat/sugar diet, stress, altered [diminished] exercise, altered sleep/wake cycle, diabetes), insulin resistance persists throughout the day and overnight. Improving the morning surge in dopaminergic activity with the short-acting dopamine D2 receptor agonist bromocriptine-QR can safely and effectively improve glycemic control, while improving cardiovascular disease risk factors and related adverse events, and reducing sympathetic tone, as demonstrated by 5 reports of the Cycloset Safety Trial and 3 additional clinical studies of bromocriptine-QR.

CONCLUSIONS

In patients with T2DM, the dopamine D2 receptor agonist bromocriptine-QR has been shown to be well tolerated, efficacious, and a logical treatment option.

Premature ejaculation in type II diabetes mellitus patients: association with glycemic control

Background

Premature ejaculation (PE) is a highly prevalent sexual dysfunction among patients with diabetes mellitus (DM). Despite this, the underlying mechanism of this association is poorly understood. In this study, we aimed to investigate the prevalence of PE in a group of patients with DM and explore possible associations linking both conditions together.

Methods

This was a prospective study of subjects recruited with advertisement pamphlets and whose sexual function was assessed using the international index of erectile function-5 (IIEF-5) and the Arabic index of premature ejaculation (AIPE) questionnaires together with stopwatch measured intravaginal ejaculatory latency time (ELT). Participants were divided into two groups; group A subjects had DM and group B were healthy adult males.

Results

A total of 488 subjects were recruited. Group A included 199 (40.8%) subjects, while group B included 289 (59.2%). The prevalence of PE and ED was significantly higher in group A subjects (P<0.001). Mean ELT ± standard deviation (SD) was 3.6±2.7 in group A versus 4.3±2.8 in group B (P<0.014). Diabetic patients with erectile dysfunction (ED) showed a significantly higher incidence of PE with significantly shorter ELT.

Conclusions

PE is more prevalent in diabetic patients. DM is a multi-systemic disorder with complications that could help explain the pathophysiology of PE.

Bromocriptine-QR therapy for the management of type 2 diabetes mellitus: developmental basis and therapeutic profile summary

An extended series of studies indicate that endogenous phase shifts in circadian neuronal input signaling to the biological clock system centered within the hypothalamic suprachiasmatic nucleus (SCN) facilitates shifts in metabolic status. In particular, a diminution of the circadian peak in dopaminergic input to the peri-SCN facilitates the onset of fattening, insulin resistance and glucose intolerance while reversal of low circadian peak dopaminergic activity to the peri-SCN via direct timed dopamine administration to this area normalizes the obese, insulin resistant, glucose intolerant state in high fat fed animals. Systemic circadian-timed daily administration of a potent dopamine D2 receptor agonist, bromocriptine, to increase diminished circadian peak dopaminergic hypothalamic activity across a wide variety of animal models of metabolic syndrome and type 2 diabetes mellitus (T2DM) results in improvements in the obese, insulin resistant, glucose intolerant condition by improving hypothalamic fuel sensing and reducing insulin resistance, elevated sympathetic tone, and leptin resistance. A circadian-timed (within 2 hours of waking in the morning) once daily administration of a quick release formulation of bromocriptine (bromocriptine-QR) has been approved for the treatment of T2DM by the U.S. Food and Drug Administration. Clinical studies with such bromocriptine-QR therapy (1.6 to 4.8 mg/day) indicate that it improves glycemic control by reducing postprandial glucose levels without raising plasma insulin. Across studies of various T2DM populations, bromocriptine-QR has been demonstrated to reduce HbA1c by -0.5 to -1.7. The drug has a good safety profile with transient mild to moderate nausea, headache and dizziness as the most frequent adverse events noted with the medication. In a large randomized clinical study of T2DM subjects, bromocriptine-QR exposure was associated with a 42% hazard ratio reduction of a pre-specified adverse cardiovascular endpoint including myocardial infarction, stroke, hospitalization for congestive heart failure, revascularization surgery, or unstable angina. Bromocriptine-QR represents a novel method of treating T2DM that may have benefits for cardiovascular disease as well.

Rethinking energy in parkinsonian motor symptoms: a potential role for neural metabolic deficits

Parkinson’s disease (PD) is characterized as a chronic and progressive neurodegenerative disorder that results in a variety of debilitating symptoms, including bradykinesia, resting tremor, rigidity, and postural instability. Research spanning several decades has emphasized basal ganglia dysfunction, predominantly resulting from dopaminergic (DA) cell loss, as the primarily cause of the aforementioned parkinsonian features. But, why those particular features manifest themselves remains an enigma. The goal of this paper is to develop a theoretical framework that parkinsonian motor features are behavioral consequence of a long-term adaptation to their inability (inflexibility or lack of capacity) to meet energetic demands, due to neural metabolic deficits arising from mitochondrial dysfunction associated with PD. Here, we discuss neurophysiological changes that are generally associated with PD, such as selective degeneration of DA neurons in the substantia nigra pars compacta (SNc), in conjunction with metabolic and mitochondrial dysfunction. We then characterize the cardinal motor symptoms of PD, bradykinesia, resting tremor, rigidity and gait disturbance, reviewing literature to demonstrate how these motor patterns are actually energy efficient from a metabolic perspective. We will also develop three testable hypotheses: (1) neural metabolic deficits precede the increased rate of neurodegeneration and onset of behavioral symptoms in PD; (2) motor behavior of persons with PD are more sensitive to changes in metabolic/bioenergetic state; and (3) improvement of metabolic function could lead to better motor performance in persons with PD. These hypotheses are designed to introduce a novel viewpoint that can elucidate the connections between metabolic, neural and motor function in PD.

Impact of Bromocriptine-QR Therapy on Glycemic Control and Daily Insulin Requirement in Type 2 Diabetes Mellitus Subjects Whose Dysglycemia Is Poorly Controlled on High-Dose Insulin: A Pilot Study

BACKGROUND

The concurrent use of a postprandial insulin sensitizing agent, such as bromocriptine-QR, a quick release formulation of bromocriptine, a dopamine D2 receptor agonist, may offer a strategy to improve glycemic control and limit/reduce insulin requirement in type 2 diabetes (T2DM) patients on high-dose insulin. This open label pilot study evaluated this potential utility of bromocriptine-QR.

METHODS

Ten T2DM subjects on metformin (1-2 gm/day) and high-dose (TDID ≥ 65 U/day) basal-bolus insulin were enrolled to receive once daily (morning) bromocriptine-QR (1.6-4.8 mg/day) for 24 weeks. Subjects with at least one postbaseline HbA1c measurement (N = 8) were analyzed for change from baseline HbA(1c), TDID, and postprandial glucose area under the curve of a four-hour mixed meal tolerance test (MMTT).

RESULTS

Compared to the baseline, average HbA1c decreased 1.76% (9.74 ± 0.56 to 7.98 ± 0.36, P = 0.01), average TDID decreased 27% (199 ± 33 to 147 ± 31, P = 0.009), and MMTT AUC(60-240) decreased 32% (P = 0.04) over the treatment period. The decline in HbA(1c) and TDID was observed at 8 weeks and sustained over the remaining 16-week study duration.

CONCLUSION

In this study, bromocriptine-QR therapy improved glycemic control and meal tolerance while reducing insulin requirement in T2DM subjects poorly controlled on high-dose insulin therapy.

Timed Bromocriptine-QR Therapy Reduces Progression of Cardiovascular Disease and Dysglycemia in Subjects with Well-Controlled Type 2 Diabetes Mellitus

BACKGROUND

Type 2 diabetes (T2DM) patients, including those in good glycemic control, have an increased risk of cardiovascular disease (CVD). Maintaining good glycemic control may reduce long-term CVD risk. However, other risk factors such as elevated vascular sympathetic tone and/or endothelial dysfunction may be stronger potentiators of CVD. This study evaluated the impact of bromocriptine-QR, a sympatholytic dopamine D2 receptor agonist, on progression of metabolic disease and CVD in T2DM subjects in good glycemic control (HbA1c ≤ 7.0%).

METHODS

1834 subjects (1219 bromocriptine-QR; 615 placebo) with baseline HbA1c ≤ 7.0% derived from the Cycloset Safety Trial (this trial is registered with ClinicalTrials.gov Identifier: NCT00377676), a 12-month, randomized, multicenter, placebo-controlled, double-blind study in T2DM, were evaluated. Treatment impact upon a prespecified composite CVD endpoint (first myocardial infarction, stroke, coronary revascularization, or hospitalization for angina/congestive heart failure) and the odds of losing glycemic control (HbA1c >7.0% after 52 weeks of therapy) were determined.

RESULTS

Bromocriptine-QR reduced the CVD endpoint by 48% (intention-to-treat; HR: 0.52 [0.28-0.98]) and 52% (on-treatment analysis; HR: 0.48 [0.24-0.95]). Bromocriptine-QR also reduced the odds of both losing glycemic control (OR: 0.63 (0.47-0.85), p = 0.002) and requiring treatment intensification to maintain HbA1c ≤ 7.0% (OR: 0.46 (0.31-0.69), p = 0.0002).

CONCLUSIONS

Bromocriptine-QR therapy slowed the progression of CVD and metabolic disease in T2DM subjects in good glycemic control.

Age-dependent association of serum prolactin with glycaemia and insulin sensitivity in humans

The dopamine agonist bromocriptine has been approved for the treatment of type 2 diabetes in the United States. Bromocriptine inhibits prolactin secretion, and patients with hyperprolactinaemia display impaired insulin sensitivity. We therefore hypothesized that low prolactin levels are associated with lower glycaemia and higher insulin sensitivity in healthy subjects. Prolactin levels were determined from fasting serum in participants without diabetes from the cross-sectional Tübingen family study for type 2 diabetes (m/f = 562/1,121, age = 40 ± 13 years, BMI = 30 ± 9 kg/m(2)). A 75 g oral glucose tolerance test was performed, and the area under the glucose curve (AUC(0-120)Glucose) and insulin sensitivity index were calculated. A subgroup (n = 494) underwent hyperinsulinaemic-euglycaemic clamp tests. Prolactin associated positively with insulin sensitivity (p = 0.001, adjusted for gender, age, and BMI). Age strongly interacted (p < 0.0001) with the effect of prolactin on insulin sensitivity, inverting the positive relationship to a negative one in younger participants. Glycated haemoglobin (HbA1c) and AUC(0-120)Glucose correlated negatively with prolactin, and an interaction with age was found as well. Higher prolactin levels are associated with improved insulin sensitivity and lower glucose in individuals without diabetes. This relationship turns to its opposite in younger persons. As prolactin is a proxy for the dopaminergic tone in the central nervous system, these associations may indicate an age-dependent influence of the brain on peripheral insulin sensitivity.

Neuroendocrine and metabolic components of dopamine agonist amelioration of metabolic syndrome in SHR rats

BACKGROUND

The hypertensive, pro-inflammatory, obese state is strongly coupled to peripheral and hepatic insulin resistance (in composite termed metabolic syndrome [MS]). Hepatic pro-inflammatory pathways have been demonstrated to initiate or exacerbate hepatic insulin resistance and contribute to fatty liver, a correlate of MS. Previous studies in seasonally obese animals have implicated an important role for circadian phase-dependent increases in hypothalamic dopaminergic tone in the maintenance of the lean, insulin sensitive condition. However, mechanisms driving this dopaminergic effect have not been fully delineated and the impact of such dopaminergic function upon the above mentioned parameters of MS, particularly upon key intra-hepatic regulators of liver inflammation and lipid and glucose metabolism have never been investigated.

OBJECTIVE

This study therefore investigated the effects of timed daily administration of bromocriptine, a potent dopamine D2 receptor agonist, on a) ventromedial hypothalamic catecholamine activity, b) MS and c) hepatic protein levels of key regulators of liver inflammation and glucose and lipid metabolism in a non-seasonal model of MS - the hypertensive, obese SHR rat.

METHODS

Sixteen week old SHR rats maintained on 14 hour daily photoperiods were treated daily for 16 days with bromocriptine (10 mg/kg, i.p.) or vehicle at 1 hour before light offset and, subsequent to blood pressure recordings on day 14, were then utilized for in vivo microdialysis of ventromedial hypothalamic catecholamine activity or sacrificed for the analyses of MS factors and regulators of hepatic metabolism. Normal Wistar rats served as wild-type controls for hypothalamic activity, body fat levels, and insulin sensitivity.

RESULTS

Bromocriptine treatment significantly reduced ventromedial hypothalamic norepinephrine and serotonin levels to the normal range and systolic and diastolic blood pressures, retroperitoneal body fat level, plasma insulin and glucose levels and HOMA-IR relative to vehicle treated SHR controls. Such treatment also reduced plasma levels of C-reactive protein, leptin, and norepinephrine and increased that of plasma adiponectin significantly relative to SHR controls. Finally, bromocriptine treatment significantly reduced hepatic levels of several pro-inflammatory pathway proteins and of the master transcriptional activators of lipogenesis, gluconeogenesis, and free fatty acid oxidation versus control SHR rats.

CONCLUSION

These findings indicate that in SHR rats, timed daily dopamine agonist treatment improves hypothalamic and neuroendocrine pathologies associated with MS and such neuroendocrine events are coupled to a transformation of liver metabolism potentiating a reduction of elevated lipogenic and gluconeogenic capacity. This liver effect may be driven in part by concurrent reductions in hyperinsulinemia and sympathetic tone as well as by reductions in intra-hepatic inflammation.

Central dopaminergic activity influences metabolic parameters in healthy men

BACKGROUND/AIMS

Central dopaminergic activity is probably linked to regulation of glucose and lipid metabolism and weight maintenance. The aim of our study was to evaluate the relationship between central dopaminergic activity measured using the apomorphine challenge test and metabolic parameters in healthy men.

METHODS

Forty-two healthy men (average age 43.5 ± 7.4 years, body mass index, BMI, 27.4 ± 5.7) were examined anthropometrically and biochemically (glycemia, lipids, glycated hemoglobin). Central dopaminergic activity was assessed as the area under the curve (AUC) of prolactin (PRL) and growth hormone (GH) responses to the apomorphine challenge test after sublingual administration of apomorphine in a dose of 0.033 mg/kg. Insulin resistance was quantified by calculation of glucose disposal and metabolic clearance rate during a euglycemic hyperinsulinemic clamp on two insulin levels (1 and 10 mIU/kg/min). Linear regression was used for statistical analysis.

RESULTS

Hormonal responses correlated negatively with age (for AUC/GH r = -0.33; p = 0.031) and BMI (AUC/GH r = -0.41; p = 0.007). After adjustment for age and BMI, a statistically significant negative correlations between AUC/PRL and total cholesterol (r = -0.41; p = 0.007), AUC/GH and HbA1c (r = -0.37; p = 0.016) and AUC/GH and HOMA (homeostasis model assessment; r = -0.345; p = 0.025) were observed.

CONCLUSION

Central dopaminergic activity declines with age and BMI. Higher total cholesterol, glycated hemoglobin and insulin resistance parameters are connected with lower central dopamine tone.

Bromocriptine in type 2 diabetes mellitus

Bromocriptine mesylate quick-release was approved by the Food and Drug Administration (FDA) in May 2009, for the treatment of type 2 diabetes. Bromocriptine is thought to act on the circadian neuronal activities in the hypothalamus, to reset an abnormally elevated hypothalamic drive for increased plasma glucose, free fatty acids, and triglycerides in insulin-resistant patients. Randomized controlled trials have shown that bromocriptine-QR lowers glycated hemoglobin by 0.4 - 0.8% either as monotherapy or in combination with other anti-diabetes medications. The doses used to treat diabetes (up to 4.8 mg daily) are much lower than those used to treat Parkinson's disease, and apart from nausea, the drug is well-tolerated. The novel mechanism of action, good side effect profile, and its effects to reduce cardiovascular event rates make it an attractive option for the treatment of type 2 diabetes.

Randomized clinical trial of quick-release bromocriptine among patients with type 2 diabetes on overall safety and cardiovascular outcomes

OBJECTIVE

Quick-release bromocriptine (bromocriptine-QR), a D2 dopamine receptor agonist, is indicated as a treatment for type 2 diabetes. The Cycloset Safety Trial, a 52-week, randomized, double-blind, multicenter trial, evaluated the overall safety and cardiovascular safety of this novel therapy for type 2 diabetes.

RESEARCH DESIGN AND METHODS

A total of 3,095 patients with type 2 diabetes were randomized 2:1 to bromocriptine-QR or placebo in conjunction with the patient's usual diabetes therapy (diet controlled only or up to two antidiabetes medications, including insulin). The all-cause-safety end point was the occurrence of any serious adverse event (SAE), with a hazard ratio (HR) noninferiority margin of 1.5. In a prespecified analysis, the frequency of cardiovascular disease (CVD) events defined as a composite of myocardial infarction, stroke, coronary revascularization, and hospitalization for angina or congestive heart failure was evaluated using modified intent-to-treat analysis (clinicaltrials.gov, NCT00377676).

RESULTS

In the bromocriptine-QR group, 176 (8.6%) people reported SAEs compared with 98 (9.6%) in the placebo group (HR 1.02 [96% one-sided CI 1.27]). Fewer people reported a CVD end point in the bromocriptine-QR group versus the placebo group (37 [1.8%] vs. 32 [3.2%], respecively) (HR 0.60 [95% two-sided CI 0.35-0.96]). Nausea was the most commonly reported adverse event in the bromocriptine-QR group.

CONCLUSIONS

The frequency of SAEs was comparable between the treatment arms. Compared with patients in the placebo arm, fewer patients taking bromocriptine-QR experienced a cardiovascular end point.

Bromocriptine--unique formulation of a dopamine agonist for the treatment of type 2 diabetes

IMPORTANCE TO THE FIELD

There is a large unmet need for new therapies to treat type 2 diabetes (T2DM) which reduce fasting and postprandial glucose without increasing insulin levels and which are not associated with weight gain or hypoglycemia. The quick-release formulation of bromocriptine (bromocriptine-QR; Cycloset) represents such a therapy.

AREAS COVERED IN THE REVIEW

Bromocriptine-QR's proposed mechanism of action, unique formulation and clinical efficacy and safety will be discussed. A Medline search was conducted using the terms: bromocriptine quick-release, circadian rhythms, treatment type 2 diabetes, insulin resistance, beta-cell dysfunction (years 1985 - 2009).

WHAT THE READER WILL GAIN

The reader will gain an understanding of the importance of the brain as a target for the treatment of type 2 diabetes. In addition the safety, efficacy and indication for use of a first-in-class dopamine agonist as a treatment option for type 2 diabetes are discussed.

TAKE HOME MESSAGE

Bromocriptine-QR is indicated to be used alone or in conjunction with all available treatments for type 2 diabetes. Although the mechanism of action is not fully understood, bromocriptine-QR's action points to a central target in the brain (hypothalamus) which may explain the observed peripheral improvements in metabolic parameters.

Disruption of the dopamine d2 receptor impairs insulin secretion and causes glucose intolerance

The relationship between antidopaminergic drugs and glucose has not been extensively studied, even though chronic neuroleptic treatment causes hyperinsulinemia in normal subjects or is associated with diabetes in psychiatric patients. We sought to evaluate dopamine D2 receptor (D2R) participation in pancreatic function. Glucose homeostasis was studied in D2R knockout mice (Drd2(-/-)) mice and in isolated islets from wild-type and Drd2(-/-) mice, using different pharmacological tools. Pancreas immunohistochemistry was performed. Drd2(-/-) male mice exhibited an impairment of insulin response to glucose and high fasting glucose levels and were glucose intolerant. Glucose intolerance resulted from a blunted insulin secretory response, rather than insulin resistance, as shown by glucose-stimulated insulin secretion tests (GSIS) in vivo and in vitro and by a conserved insulin tolerance test in vivo. On the other hand, short-term treatment with cabergoline, a dopamine agonist, resulted in glucose intolerance and decreased insulin response to glucose in wild-type but not in Drd2(-/-) mice; this effect was partially prevented by haloperidol, a D2R antagonist. In vitro results indicated that GSIS was impaired in islets from Drd2(-/-) mice and that only in wild-type islets did dopamine inhibit GSIS, an effect that was blocked by a D2R but not a D1R antagonist. Finally, immunohistochemistry showed a diminished pancreatic beta-cell mass in Drd2(-/-) mice and decreased beta-cell replication in 2-month-old Drd2(-/-) mice. Pancreatic D2Rs inhibit glucose-stimulated insulin release. Lack of dopaminergic inhibition throughout development may exert a gradual deteriorating effect on insulin homeostasis, so that eventually glucose intolerance develops.

Investigational therapies in the treatment of obesity

Obesity is a major public health concern and environmental factors are involved in its development. The hypothalamus is a primary site for the integration of signals for the regulation of energy homeostasis. Dysregulation of these pathways can lead to weight loss or gain. Some drugs in development can have favourable effects on body weight, acting on some of these pathways and leading to responses resulting in weight loss. Strategies for the management of weight reduction include exercise, diet, behavioural therapy, drug therapy and surgery. Investigational antiobesity medications can modulate energy homeostasis by stimulating catabolic or inhibiting anabolic pathways. Investigational drugs stimulating catabolic pathways consist of leptin, agonists of melanocortin receptor-4, 5-HT and dopamine; bupropion, growth hormone fragments, cholecystokinin subtype 1 receptor agonist, peptide YY3-36, oxyntomodulin, ciliary neurotrophic factor analogue, beta3-adrenergic receptor agonists, adiponectin derivatives and glucagon-like peptide-1. On the other hand, investigational drugs inhibiting anabolic pathways consist of the ghrelin receptor, neuropeptide Y receptor and melanin-concentrating hormone-1 antagonists; somatostatin analogues, peroxisome proliferator-activated receptor-gamma and -beta/delta antagonists, gastric emptying retardation agents, pancreatic lipase inhibitors, topiramate and cannabinoid-1 receptor antagonists. These differing approaches are reviewed and commented on in this article.

Reduced dopaminergic tone in hypothalamic neural circuits: expression of a "thrifty" genotype underlying the metabolic syndrome?

The thrifty genotype hypothesis postulates that the genetically determined ability to grow obese and insulin resistant in times of food abundance confers a survival advantage in times of famine. Obviously, this ability poses a major health threat in modern times, where food is always available in large quantities. In the last 10-15 years, many genes encoding pathways that orchestrate energy balance and fuel flux have been discovered. This paper summarizes the evidence that diminished dopaminergic tone in hypothalamic nuclei contributes to the "thrifty" genotype/phenotype. Reduced dopaminergic neurotransmission in the suprachiasmatic nucleus of seasonally obese animals appears to drive noradrenalin and NPY mediated transmissions in other nuclei to induce the obesity syndrome at the appropriate time of year. Treatment with dopamine D(2) receptor agonists can fully reverse the metabolic syndrome in these animals. Similar mechanisms are operative in non-seasonal obese animal models. In man, treatment with dopamine D(2) receptor antagonists induces obesity and type 2 diabetes mellitus, whereas dopamine D(2) receptor activation ameliorates the metabolic profile in obese nondiabetic and diabetic humans. Various loss of function mutations of the dopamine D(2) receptor gene are associated with overweight in humans. In concert, the data support the notion that diminution of dopaminergic (dopamine D(2) receptor mediated) transmission in relevant hypothalamic nuclei sets the stage for efficient partitioning of ingested nutrients to contribute to a phenotype that is not so thrifty anymore.

Modulation of Monoaminergic Neural Circuits

A plethora of data from experimental animals provide strong support for the concept that reduced dopaminergic neuronal activity and enhanced noradrenergic tone in specific hypothalamic nuclei are involved in the pathogenesis of the metabolic syndrome. The available information on these neurotransmitter systems in insulin-resistant humans with obesity is in keeping with the postulate that analogous mechanisms may underlie their adverse metabolic profile. Treatment with bromocriptine, which has dopaminergic (D2 receptor agonist) and sympatholytic (alpha2-adrenoceptor agonistic and an alpha1-adrenoceptor antagonistic) actions, can reverse the metabolic anomalies in a variety of obese mammalian species. Combined D1/D2 receptor activation appears to exert even more powerful effects on fuel metabolism in various animal models of the metabolic syndrome. The currently available data on the metabolic effects of bromocriptine in humans with obesity and type 2 diabetes mellitus point in the same direction. Bromocriptine favorably affects glucose metabolism and various other components of the metabolic syndrome simultaneously to ameliorate the risk of damage to eyes, neural tissue, kidneys and the cardiovascular system in patients with type 2 diabetes mellitus. Moreover, a substantial number of studies indicate that bromocriptine lowers blood pressure in animals and humans with hypertension via its sympatholytic capacities. However, the effects of bromocriptine alone are relatively modest, the metabolic mechanism of action in humans remains uncertain, and the long-term efficacy and safety profiles of this compound are unknown. It seems important to seek for ways to boost the action of bromocriptine, by combining dopaminergic D2 and D1 receptor activation, for example. Notably, there is no antidiabetic drug that acts through central (dopaminergic) mechanisms. This novel approach may, therefore, result in synergistic actions with other available agents to favorably impact the risk of tissue damage in patients with type 2 diabetes mellitus.

Association of the antidiabetic effects of bromocriptine with a shift in the daily rhythm of monoamine metabolism within the suprachiasmatic nuclei of the Syrian hamster

Bromocriptine, a dopamine D2 agonist, inhibits seasonal fattening and improves seasonal insulin resistance in Syrian hamsters. Alterations in daily rhythms of neuroendocrine activities are involved in the regulation of seasonal metabolic changes. Changes in circadian neuroendocrine activities that regulate metabolism are believed to be modulated by central circadian oscillators within the hypothalamic suprachiasmatic nuclei (SCN) of seasonal animals. We examined the association of metabolic responses to bromocriptine with its effects on the daily rhythms of metabolic hormones and daily monoamine profiles within the SCN, a primary circadian pacemaker known to regulate metabolism, in Syrian hamsters. Obese glucose-intolerant male Syrian hamsters (body weight [BW] 185 +/- 10 g) held on 14h daily photoperiods were treated at light onset with bromocriptine (800 microg/animal/day, ip) or vehicle for 2 weeks. Animals were then subjected to a glucose tolerance test (GTT) (3 g/kg BW, ip). Different subsets of animals (n = 6) from each treatment group were sacrificed at 0h/24h, 5h, 10h, 15h, or 20h after light onset for analyses of SCN monoamines, plasma insulin, prolactin, cortisol, thyroxin (T4), triiodothyronine (T3), glucose, and free fatty acids (FFAs). Compared with control values, bromocriptine treatment significantly reduced weight gain (14.9 vs. -2.9 g, p < .01) and the areas under the GTT glucose and insulin curves by 29% and 48%, respectively (p < .05). Basal plasma insulin concentration was markedly reduced throughout the day in bromocriptine-treated animals without influencing plasma glucose levels. Bromocriptine reduced the daily peak in FFA by 26% during the late light span (p < .05). Bromocriptine significantly shifted the daily plasma cortisol peak from the early dark to the light period of the day, reduced the plasma prolactin (mean 1.8 vs. 39.4 ng/dL) and T4 throughout the day (mean 1.6 vs. 3.8 microg/dL), and selectively reduced T3 during the dark period of the day (p < .01). Concurrently, bromocriptine treatment significantly reduced SCN dopamine turnover during the light period and shifted daily peaks of SCN serotonin and 5-hydroxy-indoleacetic acid (5-HIAA) content by 12h from the light to the dark period of the day (p < .05). This was confirmed by a further in vivo microdialysis study in which bromocriptine increased SCN extracellular 5-HIAA of glucose-intolerant hamsters during the dark phase (47% increase, p < .05) toward levels observed in normal glucose-tolerant hamsters. Thus, bromocriptine-induced resetting of daily patterns of SCN neurotransmitter metabolism is associated with the effects of bromocriptine on attenuation of the obese insulin-resistant and glucose-intolerant condition. A large body of corroborating evidence suggests that such bromocriptine-induced changes in SCN monoamine metabolism may be functional in its effects on metabolism.

Chronic infusion of norepinephrine into the VMH of normal rats induces the obese glucose-intolerant state

Increases in ventromedial hypothalamic (VMH) norepinephrine (NE) levels and/or activities have been observed in a variety of animal models of the obese insulin-resistant condition. This study examined the metabolic effects of chronic NE infusion (25 nmol/h) into the unilateral VMH of normal rats. Within 4 days, VMH NE infusion significantly increased plasma insulin (140%), glucagon (45%), leptin (300%), triglyceride (100%), abdominal fat pad weight (50%), and white adipocyte lipogenic (100%) and lipolytic (100%) activities relative to vehicle-infused rats. Furthermore, isolated islet insulin secretory response to glucose (15 mM) within 4 days of such treatment was increased over twofold (P < 0.05). Among treated animals, fat stores continued to increase over time and plateaued at approximately 2 wk (3-fold increase), remaining elevated to the end of the study (5 wk). By week 4 of treatment, NE infusion induced glucose intolerance as evidenced by a 32% increase in plasma glucose total area under the glucose tolerance test curve (P < 0.01). Whole body fat oxidation rate measured after 5 wk of infusion was significantly increased among treated animals as evidenced by a reduced respiratory quotient (0.87 +/- 0.01) relative to controls (0. 90 +/- 0.01). VMH NE infusion induced hyperphagia (30%) only during the first week and did not affect body weight over the 5-wk period. Increases in VMH NE activity that are common among obese insulin-resistant animal models can cause the development of this obese glucose-intolerant (metabolic) syndrome.