Sex-specific effects of naturally occurring variants in the dopamine receptor D2 locus on insulin secretion and type 2 diabetes susceptibility.

AIMS: Modulation of dopamine receptor D2 (DRD2) activity affects insulin secretion in both rodents and isolated pancreatic ß-cells. We hypothesized that single nucleotide polymorphisms in the DRD2/ANKK1 locus may affect susceptibility to type 2 diabetes in humans. METHODS: Four potentially functional variants in the coding region of the DRD2/ANKK1 locus (rs1079597, rs6275, rs6277, rs1800497) were genotyped and analysed for type 2 diabetes susceptibility in up to 25 000 people (8148 with type 2 diabetes and 17687 control subjects) from two large independent Dutch cohorts and one Danish cohort. In addition, 340 Dutch subjects underwent a 2-h hyperglycaemic clamp to investigate insulin secretion. Since sexual dimorphic associations related to DRD2 polymorphisms have been previously reported, we also performed a gender-stratified analysis. RESULTS: rs1800497 at the DRD2/ANKK1 locus was associated with a significantly increased risk for type 2 diabetes in women (odds ratio 1.14 (1.06-1.23); P = 4.1*104) but not in men (odds ratio 1.00 (95% CI 0.93-1.07); P = 0.92) or the combined group. Although rs1800497 was not associated with insulin secretion, we did find another single nucleotide polymorphism in this locus, rs6275, to be associated with increased first-phase glucose-stimulated insulin secretion in women (P = 5.5*104) but again not in men (P = 0.34). CONCLUSION: The present data identify DRD2/ANKK1 as a potential sex-specific type 2 diabetes susceptibility gene.

Pharmacological modulation of dopamine receptor D2-mediated transmission alters the metabolic phenotype of diet induced obese and diet resistant C57Bl6 mice.

High fat feeding induces a variety of obese and lean phenotypes in inbred rodents. Compared to Diet Resistant (DR) rodents, Diet Induced Obese (DIO) rodents are insulin resistant and have a reduced dopamine receptor D2 (DRD2) mediated tone. We hypothesized that this differing dopaminergic tone contributes to the distinct metabolic profiles of these animals. C57Bl6 mice were classified as DIO or DR based on their weight gain during 10 weeks of high fat feeding. Subsequently DIO mice were treated with the DRD2 agonist bromocriptine and DR mice with the DRD2 antagonist haloperidol for 2 weeks. Compared to DR mice, the bodyweight of DIO mice was higher and their insulin sensitivity decreased. Haloperidol treatment reduced the voluntary activity and energy expenditure of DR mice and induced insulin resistance in these mice. Conversely, bromocriptine treatment tended to reduce bodyweight and voluntary activity, and reinforce insulin action in DIO mice. These results show that DRD2 activation partly redirects high fat diet induced metabolic anomalies in obesity-prone mice. Conversely, blocking DRD2 induces an adverse metabolic profile in mice that are inherently resistant to the deleterious effects of high fat food. This suggests that dopaminergic neurotransmission is involved in the control of metabolic phenotype.

Blocking dopamine D2 receptors by haloperidol curtails the beneficial impact of calorie restriction on the metabolic phenotype of high-fat diet induced obese mice.

Calorie restriction is the most effective way of expanding life-span and decreasing morbidity. It improves insulin sensitivity and delays the age-related loss of dopamine receptor D(2) (DRD2) expression in the brain. Conversely, high-fat feeding is associated with obesity, insulin resistance and a reduced number of DRD2 binding sites. We hypothesised that the metabolic benefit of calorie restriction involves the preservation of appropriate DRD2 transmission. The food intake of wild-type C57Bl6 male mice was restricted to 60% of ad lib. intake while they were treated with the DRD2 antagonist haloperidol or vehicle using s.c. implanted pellets. Mice with ad lib. access to food receiving vehicle treatment served as controls. All mice received high-fat food throughout the experiment. After 10 weeks, an i.p. glucose tolerance test was performed and, after 12 weeks, a hyperinsulinaemic euglycaemic clamp. Hypothalamic DRD2 binding was also determined after 12 weeks of treatment. Calorie-restricted (CR) vehicle mice were glucose tolerant and insulin sensitive compared to ad lib. (AL) fed vehicle mice. CR mice treated with haloperidol were slightly heavier than vehicle treated CR mice. Haloperidol completely abolished the beneficial impact of calorie restriction on glucose tolerance and partly reduced the insulin sensitivity observed in CR vehicle mice. The metabolic differences between AL and CR vehicle mice were not accompanied by alterations in hypothalamic DRD2 binding. In conclusion, blocking DRD2 curtails the metabolic effects of calorie restriction. Although this suggests that the dopaminergic system could be involved in the metabolic benefits of calorie restriction, restricting access to high-fat food does not increase (hypothalamic) DRD2 binding capacity, which argues against this inference.

The dopamine receptor D2 agonist bromocriptine inhibits glucose-stimulated insulin secretion by direct activation of the alpha2-adrenergic receptors in beta cells.

Treatment with the dopamine receptor D2 (DRD2) agonist bromocriptine improves metabolic features in obese patients with type 2 diabetes by a still unknown mechanism. In the present study, we investigated the acute effect of bromocriptine and its underlying mechanism(s) on insulin secretion both in vivo and in vitro. For this purpose, C57Bl6/J mice were subjected to an intraperitoneal glucose tolerance test (ipGTT) and a hyperglycemic (HG) clamp 60min after a single injection of bromocriptine or placebo. The effects of bromocriptine on glucose-stimulated insulin secretion (GSIS), cell membrane potential and intracellular cAMP levels were also determined in INS-1E beta cells. We report here that bromocriptine increased glucose levels during ipGTT in vivo, an effect associated with a dose-dependent decrease in GSIS. During the HG clamp, bromocriptine reduced both first-phase and second-phase insulin response. This inhibitory effect was also observed in INS-1E beta cells, in which therapeutic concentrations of bromocriptine (0.5-50nM) decreased GSIS. Mechanistically, neither cellular energy state nor cell membrane depolarization was affected by bromocriptine whereas intracellular cAMP levels were significantly reduced, suggesting involvement of G-protein-coupled receptors. Surprisingly, the DRD2 antagonist domperidone did not counteract the effect of bromocriptine on GSIS, whereas yohimbine, an antagonist of the alpha2-adrenergic receptors, completely abolished bromocriptine-induced inhibition of GSIS. In conclusion, acute administration of bromocriptine inhibits GSIS by a DRD2-independent mechanism involving direct activation of the pancreatic alpha2-adrenergic receptors. We suggest that treatment with bromocriptine promotes beta cells rest, thereby preventing long-lasting hypersecretion of insulin and subsequent beta cell failure.

Four weeks high fat feeding induces insulin resistance without affecting dopamine release or gene expression patterns in the hypothalamus of C57Bl6 mice.

Obesity is associated with diminished dopaminergic neurotransmission. It remains unclear whether this is a cause or a consequence of the obese state. We hypothesized that high fat feeding, a well known trigger of obesity in diet sensitive mice, would blunt dopaminergic neurotransmission prior to the development of insulin resistance. We monitored in vivo dopamine release in the dorsomedial region of the hypothalamus, and determined hypothalamic gene expression patterns of dopamine receptors 1 and 2 (DRD1 and 2), tyrosine hydroxylase (TH) and the dopamine transporter (DAT) in C57Bl6 mice maintained on a high fat diet for 4 weeks. Also, a hyperinsulinemic euglycemic clamp was performed to evaluate the metabolic status of the mice. Mice maintained on a low fat diet served as controls. The high fat diet did not alter dopamine release in the dorsomedial hypothalamus of fed or fasted mice or the dopaminergic response to refeeding. Furthermore, gene expression levels of DRD1, DRD2, TH and DAT were not affected by high fat feeding. However, the high fat diet did hamper insulin action as evidenced by diminished glucose disposal during hyperinsulinemia (p<0.05). We show here that short term high fat feeding does not affect dopaminergic neurotransmission in the hypothalamus, whereas it does impair insulin action. This suggests that reduced dopaminergic neurotransmission in the hypothalamus of obese animal models is due to mechanism(s) that are not directly triggered by diet composition.