Involvement of P2Y signaling in the restoration of glucose-induced insulin exocytosis in pancreatic ß cells exposed to glucotoxicity.

Purinergic P2Y receptors, by binding adenosine triphosphate (ATP), are known for enhancing glucose-stimulated insulin secretion (GSIS) in pancreatic ß cells. However, the impact of these receptors in the actin dynamics and insulin granule exocytosis in these cells is not established, neither in normal nor in glucotoxic environment. In this study, we investigate the involvement of P2Y receptors on the behavior of insulin granules and the subcortical actin network dynamics in INS-1 832/13 ß cells exposed to normal or glucotoxic environment and their role in GSIS. Our results show that the activation of P2Y purinergic receptors by ATP or its agonist increase the insulin granules exocytosis and the reorganization of the subcortical actin network and participate in the potentiation of GSIS. In addition, their activation in INS-1832/13 ß-cells, with impaired insulin secretion following exposure to elevated glucose levels, restores GSIS competence through the distal steps of insulin exocytosis. These results are confirmed ex vivo by perifusion experiments on islets from type 2 diabetic (T2D) Goto-Kakizaki (GK) rats. Indeed, the P2Y receptor agonist restores the altered GSIS, which is normally lost in this T2D animal model. Moreover, we observed an improvement of the glucose tolerance, following the acute intraperitoneal injection of the P2Y agonist concomitantly with glucose, in diabetic GK rats. All these data provide new insights into the unprecedented therapeutic role of P2Y purinergic receptors in the pathophysiology of T2D.

Type 2 Diabetes Mellitus and Alzheimer's Disease: Shared Molecular Mechanisms and Potential Common Therapeutic Targets.

The global prevalence of diabetes mellitus and Alzheimer's disease is increasing alarmingly with the aging of the population. Numerous epidemiological data suggest that there is a strong association between type 2 diabetes and an increased risk of dementia. These diseases are both degenerative and progressive and share common risk factors. The amyloid cascade plays a key role in the pathophysiology of Alzheimer's disease. The accumulation of amyloid beta peptides gradually leads to the hyperphosphorylation of tau proteins, which then form neurofibrillary tangles, resulting in neurodegeneration and cerebral atrophy. In Alzheimer's disease, apart from these processes, the alteration of glucose metabolism and insulin signaling in the brain seems to induce early neuronal loss and the impairment of synaptic plasticity, years before the clinical manifestation of the disease. The large amount of evidence on the existence of insulin resistance in the brain during Alzheimer's disease has led to the description of this disease as "type 3 diabetes". Available animal models have been valuable in the understanding of the relationships between type 2 diabetes and Alzheimer's disease, but to date, the mechanistical links are poorly understood. In this non-exhaustive review, we describe the main molecular mechanisms that may link these two diseases, with an emphasis on impaired insulin and IGF-1 signaling. We also focus on GSK3ß and DYRK1A, markers of Alzheimer's disease, which are also closely associated with pancreatic ß-cell dysfunction and type 2 diabetes, and thus may represent common therapeutic targets for both diseases.

Serotonin 2B Receptors in Mesoaccumbens Dopamine Pathway Regulate Cocaine Responses.

Addiction is a maladaptive pattern of behavior following repeated use of reinforcing drugs in predisposed individuals, leading to lifelong changes. Common among these changes are alterations of neurons releasing dopamine in the ventral and dorsal territories of the striatum. The serotonin 5-HT2B receptor has been involved in various behaviors, including impulsivity, response to antidepressants, and response to psychostimulants, pointing toward putative interactions with the dopamine system. Despite these findings, it remains unknown whether 5-HT2B receptors directly modulate dopaminergic activity and the possible mechanisms involved. To answer these questions, we investigated the contribution of 5-HT2B receptors to cocaine-dependent behavioral responses. Male mice permanently lacking 5-HT2B receptors, even restricted to dopamine neurons, developed heightened cocaine-induced locomotor responses. Retrograde tracing combined with single-cell mRNA amplification indicated that 5-HT2B receptors are expressed by mesolimbic dopamine neurons. In vivo and ex vivo electrophysiological recordings showed that 5-HT2B-receptor inactivation in dopamine neurons affects their neuronal activity and increases AMPA-mediated over NMDA-mediated excitatory synaptic currents. These changes are associated with lower ventral striatum dopamine activity and blunted cocaine self-administration. These data identify the 5-HT2B receptor as a pharmacological intermediate and provide mechanistic insight into attenuated dopamine tone following exposure to drugs of abuse.SIGNIFICANCE STATEMENT Here we report that mice lacking 5-HT2B receptors totally or exclusively in dopamine neurons exhibit heightened cocaine-induced locomotor responses. Despite the sensitized state of these mice, we found that associated changes include lower ventral striatum dopamine activity and lower cocaine operant self-administration. We described the selective expression of 5-HT2B receptors in a subpopulation of dopamine neurons sending axons to the ventral striatum. Increased bursting in vivo properties of these dopamine neurons and a concomitant increase in AMPA synaptic transmission to ex vivo dopamine neurons were found in mice lacking 5-HT2B receptors. These data support the idea that the chronic 5-HT2B-receptor inhibition makes mice behave like animals already exposed to cocaine with higher cocaine-induced locomotion associated with changes in dopamine neuron reactivity.

Preconceptional exposure of adult male rats to bisphenol S impairs insulin sensitivity and glucose tolerance in their male offspring.

Since the use of bisphenol A (BPA) has been restricted because of its endocrine disruptor properties, bisphenol S (BPS) has been widely used as a substitute of BPA. However, BPS exerts similar effects on metabolic health as BPA. The effects of maternal exposure to BPA and BPS on the metabolic health of offspring have been largely documented during the past decade. However, the impact of preconceptional paternal exposure to BPS on progenies remains unexplored. In this study we investigated the impact of paternal exposure to BPS before conception, on the metabolic phenotype of offspring. Male Wistar rats were administered BPS through drinking water at the dose of 4 µg/kg/day (BPS-4 sires) or 40 µg/kg/day (BPS-40 sires) for 2 months before mating with females. The progenies (F1) were studied at fetal stage and in adulthood. We showed that preconceptional paternal exposure to BPS for 2 months did not alter the metabolic status of sires. The female offspring of sires exposed to lower or higher doses of BPS showed no alteration of their metabolic phenotype compared to females from control sires. In contrast, male offspring of BPS-4 sires exhibited increased body weight and body fat/lean ratio, decreased insulin sensitivity and increased glucose-induced insulin secretion at adult age, compared to the male offspring of control sires. Moreover, male offspring of BPS-4 sires developed glucose intolerance later in life. None of these effects were apparent in male offspring of BPS-40 sires. In conclusion, our study provides the first evidence of the non-monotonic and sex-specific effects of preconceptional paternal exposure to BPS on the metabolic health of offspring, suggesting that BPS is not a safe BPA substitute regarding the inter-generational transmission of metabolic disorders through the paternal lineage.

Lithium treatment mitigates the diabetogenic effects of chronic cortico-therapy.

BACKGROUND AND PURPOSE: Glucocorticoids (GCs) are the main treatment for autoimmune and inflammatory disorders and are also used as immunosuppressive therapy for patients with organ transplantation. However, these treatments have several side effects, including metabolic disorders. Indeed, cortico-therapy may induce insulin resistance, glucose intolerance, disrupted insulin and glucagon secretion, excessive gluconeogenesis, leading to diabetes in susceptible individuals. Recently, lithium has been shown to alleviate deleterious effects of GCs in various diseased conditions. EXPERIMENTAL APPROACH: In this study, using two rat models of GC-induced metabolic disorders, we investigated the effects of Lithium Chloride (LiCl) in the mitigation of deleterious effects of GCs. Rats were treated either with corticosterone or dexamethasone, and with or without LiCl. Animals were then assessed for glucose tolerance, insulin sensitivity, in vivo and ex vivo glucose-induced insulin secretion and hepatic gluconeogenesis. KEY RESULTS: We showed that in rats chronically treated with corticosterone, lithium treatment markedly reduced insulin resistance. In addition, in rats treated with dexamethasone, lithium administration improved glucose tolerance, associated with enhanced insulin secretion in vivo. Moreover, liver gluconeogenesis was reduced upon LiCl treatment. The improvement of insulin secretion in vivo appeared to be due to an indirect regulation of ß cell function, since the ex vivo assessment of insulin secretion and ß cell mass in islets from animals treated with LiCl revealed no difference compared to untreated animals. CONCLUSION AND IMPLICATIONS: Collectively, our data provide evidences for the beneficial effects of lithium to mitigate the adverse metabolic effects of chronic cortico-therapy.

Prolonged nicotine exposure reduces aversion to the drug in mice by altering nicotinic transmission in the interpeduncular nucleus.

Nicotine intake is likely to result from a balance between the rewarding and aversive properties of the drug, yet the individual differences in neural activity that control aversion to nicotine and their adaptation during the addiction process remain largely unknown. Using a two-bottle choice experiment, we observed considerable heterogeneity in nicotine-drinking profiles in isogenic adult male mice, with about half of the mice persisting in nicotine consumption even at high concentrations, whereas the other half stopped consuming. We found that nicotine intake was negatively correlated with nicotine-evoked currents in the interpeduncular nucleus (IPN), and that prolonged exposure to nicotine, by weakening this response, decreased aversion to the drug, and hence boosted consumption. Lastly, using knock-out mice and local gene re-expression, we identified ß4-containing nicotinic acetylcholine receptors of IPN neurons as molecular and cellular correlates of nicotine aversion. Collectively, our results identify the IPN as a substrate for individual variabilities and adaptations in nicotine consumption.

A versatile system for the neuronal subtype specific expression of lentiviral vectors.

Lentiviral expression vectors are powerful tools for gene therapy and long-term gene expression/repression in the mammalian brain. However, no specificity of transduction has been reported so far in the central nervous system. Here we have developed a novel system to achieve a neuronal subtype specific expression in either dopaminergic (DA) or GABAergic neurons. We employed a delivery strategy by which the transgene is not expressed until its activation by Cre recombinase. We successfully tested the system in vitro and then used this novel lentivector, containing loxP sites, in 2 different transgenic mouse lines expressing Cre either in DA or in GABAergic neurons. In both lines the reporter gene was detected exclusively in Cre-positive cells, demonstrating that with this experimental approach we were able to achieve completely specific expression of transgenes delivered by lentiviral vectors. This universal system can be applied to all neural subtypes making use of the growing number of specific Cre driver lines.- Tolu, S., Avale, M. E., Nakatani, H., Pons, S., Parnaudeau, S., Tronche, F., Vogt, A., Monyer, H., Vogel, R., de Chaumont, F., Olivo-Marin, J.-C., Changeux, J.-P., Maskos, U. A versatile system for the neuronal subtype specific expression of lentiviral vectors.

Paternal High-Protein Diet Programs Offspring Insulin Sensitivity in a Sex-Specific Manner.

The impact of maternal nutrition on offspring is well documented. However, the implication of pre-conceptional paternal nutrition on the metabolic health of the progeny remains underexplored. Here, we investigated the impact of paternal high-protein diet (HPD, 43.2% protein) consumption on the endocrine pancreas and the metabolic phenotype of offspring. Male Wistar rats were given HPD or standard diet (SD, 18.9% protein) for two months. The progenies (F1) were studied at fetal stage and in adulthood. Body weight, glycemia, glucose tolerance (GT), glucose-induced insulin secretion in vivo (GIIS) and whole-body insulin sensitivity were assessed in male and female F1 offspring. Insulin sensitivity, GT and GIIS were similar between F1 females from HPD (HPD/F1) and SD fathers (SD/F1). Conversely, male HPD/F1 exhibited increased insulin sensitivity (p < 0.05) and decreased GIIS (p < 0.05) compared to male SD/F1. The improvement of insulin sensitivity in HPD/F1 was sustained even after 2 months of high-fat feeding. In male HPD/F1, the ß cell mass was preserved and the ß cell plasticity, following metabolic challenge, was enhanced compared to SD/F1. In conclusion, we provide the first evidence of a sex-specific impact of paternal HPD on the insulin sensitivity and GIIS of their descendants, demonstrating that changes in paternal nutrition alter the metabolic status of their progeny in adulthood.

Underlying mechanisms of glucocorticoid-induced ß-cell death and dysfunction: a new role for glycogen synthase kinase 3.

Glucocorticoids (GCs) are widely prescribed for their anti-inflammatory and immunosuppressive properties as a treatment for a variety of diseases. The use of GCs is associated with important side effects, including diabetogenic effects. However, the underlying mechanisms of GC-mediated diabetogenic effects in ß-cells are not well understood. In this study we investigated the role of glycogen synthase kinase 3 (GSK3) in the mediation of ß-cell death and dysfunction induced by GCs. Using genetic and pharmacological approaches we showed that GSK3 is involved in GC-induced ß-cell death and impaired insulin secretion. Further, we unraveled the underlying mechanisms of GC-GSK3 crosstalk. We showed that GSK3 is marginally implicated in the nuclear localization of GC receptor (GR) upon ligand binding. Furthermore, we showed that GSK3 regulates the expression of GR at mRNA and protein levels. Finally, we dissected the proper contribution of each GSK3 isoform and showed that GSK3ß isoform is sufficient to mediate the pro-apoptotic effects of GCs in ß-cells. Collectively, in this work we identified GSK3 as a viable target to mitigate GC deleterious effects in pancreatic ß-cells.

Metabolic Diseases and Down Syndrome: How Are They Linked Together?

Down syndrome is a genetic disorder caused by the presence of a third copy of chromosome 21, associated with intellectual disabilities. Down syndrome is associated with anomalies of both the nervous and endocrine systems. Over the past decades, dramatic advances in Down syndrome research and treatment have helped to extend the life expectancy of these patients. Improved life expectancy is obviously a positive outcome, but it is accompanied with the need to address previously overlooked complications and comorbidities of Down syndrome, including obesity and diabetes, in order to improve the quality of life of Down syndrome patients. In this focused review, we describe the associations between Down syndrome and comorbidities, obesity and diabetes, and we discuss the understanding of proposed mechanisms for the association of Down syndrome with metabolic disorders. Drawing molecular mechanisms through which Type 1 diabetes and Type 2 diabetes could be linked to Down syndrome could allow identification of novel drug targets and provide therapeutic solutions to limit the development of metabolic and cognitive disorders.

Nicotine inhibits the VTA-to-amygdala dopamine pathway to promote anxiety.

Nicotine stimulates dopamine (DA) neurons of the ventral tegmental area (VTA) to establish and maintain reinforcement. Nicotine also induces anxiety through an as yet unknown circuitry. We found that nicotine injection drives opposite functional responses of two distinct populations of VTA DA neurons with anatomically segregated projections: it activates neurons that project to the nucleus accumbens (NAc), whereas it inhibits neurons that project to the amygdala nuclei (Amg). We further show that nicotine mediates anxiety-like behavior by acting on ß2-subunit-containing nicotinic acetylcholine receptors of the VTA. Finally, using optogenetics, we bidirectionally manipulate the VTA-NAc and VTA-Amg pathways to dissociate their contributions to anxiety-like behavior. We show that inhibition of VTA-Amg DA neurons mediates anxiety-like behavior, while their activation prevents the anxiogenic effects of nicotine. These distinct subpopulations of VTA DA neurons with opposite responses to nicotine may differentially drive the anxiogenic and the reinforcing effects of nicotine.

Circulating Triglycerides Gate Dopamine-Associated Behaviors through DRD2-Expressing Neurons.

Energy-dense food alters dopaminergic (DA) transmission in the mesocorticolimbic (MCL) system and can promote reward dysfunctions, compulsive feeding, and weight gain. Yet the mechanisms by which nutrients influence the MCL circuitry remain elusive. Here, we show that nutritional triglycerides (TGs), a conserved post-prandial metabolic signature among mammals, can be metabolized within the MCL system and modulate DA-associated behaviors by gating the activity of dopamine receptor subtype 2 (DRD2)-expressing neurons through a mechanism that involves the action of the lipoprotein lipase (LPL). Further, we show that in humans, post-prandial TG excursions modulate brain responses to food cues in individuals carrying a genetic risk for reduced DRD2 signaling. Collectively, these findings unveil a novel mechanism by which dietary TGs directly alter signaling in the reward circuit to regulate behavior, thereby providing a new mechanistic basis by which energy-rich diets may lead to (mal)adaptations in DA signaling that underlie reward deficit and compulsive behavior.

Nicotine enhances alcohol intake and dopaminergic responses through ß2* and ß4* nicotinic acetylcholine receptors.

Alcohol and nicotine are the most widely co-abused drugs. Both modify the activity of dopaminergic (DA) neurons of the Ventral Tegmental Area (VTA) and lead to an increase in DA release in the Nucleus Accumbens, thereby affecting the reward system. Evidences support the hypothesis that distinct nicotinic acetylcholine receptors (nAChRs), the molecular target of acetylcholine (ACh) and exogenous nicotine, are also in addition implicated in the response to alcohol. The precise molecular and neuronal substrates of this interaction are however not well understood. Here we used in vivo electrophysiology in the VTA to characterise acute and chronic interactions between nicotine and alcohol. Simultaneous injections of the two drugs enhanced their responses on VTA DA neuron firing and chronic exposure to nicotine increased alcohol-induced DA responses and alcohol intake. Then, we assessed the role of ß4 * nAChRs, but not ß2 * nAChRs, in mediating acute responses to alcohol using nAChR subtypes knockout mice (ß2-/- and ß4-/- mice). Finally, we showed that nicotine-induced modifications of alcohol responses were absent in ß2-/- and ß4-/- mice, suggesting that nicotine triggers ß2* and ß4 * nAChR-dependent neuroadaptations that subsequently modify the responses to alcohol and thus indicating these receptors as key mediators in the complex interactions between these two drugs.

Role of ß4* Nicotinic Acetylcholine Receptors in the Habenulo-Interpeduncular Pathway in Nicotine Reinforcement in Mice.

Nicotine exerts its psychopharmacological effects by activating the nicotinic acetylcholine receptor (nAChR), composed of alpha and/or beta subunits, giving rise to a diverse population of receptors with a distinct pharmacology. ß4-containing (ß4*) nAChRs are located almost exclusively in the habenulo-interpeduncular pathway. We examined the role of ß4* nAChRs in the medial habenula (MHb) and the interpeduncular nucleus (IPN) in nicotine reinforcement using behavioral, electrophysiological, and molecular techniques in transgenic mice. Nicotine intravenous self-administration (IVSA) was lower in constitutive ß4 knockout (KO) mice at all doses tested (7.5, 15, 30, and 60 µg/kg/infusion) compared with wild-type (WT) mice. In vivo microdialysis showed that ß4KO mice have higher extracellular dopamine (DA) levels in the nucleus accumbens than in WT mice, and exhibit a differential sensitivity to nicotine-induced DA outflow. Furthermore, electrophysiological recordings in the ventral tegmental area (VTA) demonstrated that DA neurons of ß4KO mice are more sensitive to lower doses of nicotine than that of WT mice. Re-expression of ß4* nAChRs in IPN neurons fully restored nicotine IVSA, and attenuated the increased sensitivity of VTA DA neurons to nicotine. These findings suggest that ß4* nAChRs in the IPN have a role in maintaining nicotine IVSA.

Nicotinic receptors in the ventral tegmental area promote uncertainty-seeking.

Cholinergic neurotransmission affects decision-making, notably through the modulation of perceptual processing in the cortex. In addition, acetylcholine acts on value-based decisions through as yet unknown mechanisms. We found that nicotinic acetylcholine receptors (nAChRs) expressed in the ventral tegmental area (VTA) are involved in the translation of expected uncertainty into motivational value. We developed a multi-armed bandit task for mice with three locations, each associated with a different reward probability. We found that mice lacking the nAChR ß2 subunit showed less uncertainty-seeking than their wild-type counterparts. Using model-based analysis, we found that reward uncertainty motivated wild-type mice, but not mice lacking the nAChR ß2 subunit. Selective re-expression of the ß2 subunit in the VTA was sufficient to restore spontaneous bursting activity in dopamine neurons and uncertainty-seeking. Our results reveal an unanticipated role for subcortical nAChRs in motivation induced by expected uncertainty and provide a parsimonious account for a wealth of behaviors related to nAChRs in the VTA expressing the ß2 subunit.

A concurrent excitation and inhibition of dopaminergic subpopulations in response to nicotine.

Midbrain dopamine (DA) neurons are key players in motivation and reward processing. Increased DA release is thought to be central in the initiation of drug addiction. Whereas dopamine neurons are generally considered to be activated by drugs such as nicotine, we report here that nicotine not only induces excitation of ventral tegmental area (VTA) DA cells but also induces inhibition of a subset of VTA DA neurons that are anatomically segregated in the medial part of the VTA. These opposite responses do not correlate with the inhibition and excitation induced by noxious stimuli. We show that this inhibition requires D2 receptor (D2-R) activation, suggesting that a dopaminergic release is involved in the mechanism. Our findings suggest a principle of concurrent excitation and inhibition of VTA DA cells in response to nicotine. It promotes unexplored roles for DA release in addiction contrasting with the classical views of reinforcement and motivation, and give rise to a new interpretation of the mode of operation of the reward system.

Peroxisome proliferator-activated receptors-alpha modulate dopamine cell activity through nicotinic receptors.

BACKGROUND: Modulation of midbrain dopamine neurons by nicotinic acetylcholine receptors (nAChRs) plays an important role in behavior, cognition, motivation, and reward. Specifically, nAChRs containing beta2 subunits (beta2-nAChRs) switch dopamine cells from a resting to an excited state. However, how beta2-nAChRs can be modulated and thereby how dopamine firing activity is affected remains elusive. Because changes in dopamine cell activity are reflected in the dynamics of microcircuits generating altered responses to stimuli and inputs, factors regulating their state are fundamental. Among these, endogenous ligands to the nuclear receptor-transcription factor peroxisome proliferator-activated receptors type-alpha (PPARalpha) have been recently found to suppress nicotine-induced responses of dopamine neurons. METHODS: We used both in vitro and in vivo electrophysiological techniques together with behavioral analysis to investigate on the effects of modulation of PPARalpha in Sprague-Dawley rat and C57BLJ/6 mouse dopamine neurons and their interactions with beta2-nAChRs. To this aim, we took advantage of a selective reexpression of beta2-nAChR exclusively in dopamine cells by stereotaxically injecting a lentiviral vector in the mouse ventral tegmental area. RESULTS: We found that activation of PPARalpha decreases in vitro both dopamine cell activity and ventral tegmental area net output through negative modulation of beta2-nAChRs. Additionally, PPARalpha activation in vivo reduces both the number of spontaneously active dopamine neurons and nicotine-induced increased locomotion. CONCLUSIONS: Our combined findings suggest PPARalpha ligands as important negative modulators of beta2-nAChRs on dopamine neurons. Thus, PPARalpha ligands might prove beneficial in treating disorders in which dopamine dysfunction plays a prominent role, such as schizophrenia and nicotine addiction.