Dairy consumption is associated with lower plasma dihydroceramides in women from the D.E.S.I.R. cohort.

AIM: In the D.E.S.I.R. cohort, higher consumption of dairy products was associated with lower incidence of hyperglycaemia, and dihydroceramide concentrations were higher in those who progressed to diabetes. Our aim here was to study the relationships between dairy consumption and concentrations of dihydroceramides and ceramides. METHODS: In the D.E.S.I.R. cohort, men and women aged 30-65 years, volunteers from West-Central France, were included in a 9-year follow-up with examinations every 3 years, including food-frequency questionnaires. Two items concerned dairy products (cheese, other dairy products except cheese). At each examination, dihydroceramides and ceramides were determined by mass spectrometry in a cohort subset; in the present study, the 105 people who did not progress to type 2 diabetes were analyzed, as the disorder per se might be a confounding factor. RESULTS: Higher consumption of dairy products (except cheese) was associated with total plasma dihydroceramides during the follow-up, but only in women (P=0.01 for gender interaction). In fact, dihydroceramide levels were lower in women with high vs low consumption (P=0.03), and were significantly increased during follow-up (P=0.01) in low consumers only. There was also a trend for lower ceramides in women with high dairy (except cheese) intakes (P=0.08). Cheese was associated with dihydroceramide and ceramide changes during follow-up (P=0.04 for both), but no clear trend was evident in either low or high consumers. CONCLUSION: These results show that, in women, there is an inverse association between fresh dairy product consumption and predictive markers (dihydroceramides) of type 2 diabetes.

A novel, double intra-carotid cannulation technique to study the effect of central nutrient sensing on glucose metabolism in the rat.

BACKGROUND: The hypothalamus plays a key role in central nutrient sensing and glucose homeostasis. Due to its position next to the third ventricle, intracerebroventricular (ICV) injections or osmotic minipumps are widely applied techniques in studying effects of hormones and other molecules on the hypothalamus and glucose metabolism. NEW METHODS: The intracarotid catheter technique in which a catheter is placed in the carotid artery, pointing towards the brain, provides a physiological route to centrally infuse blood-borne molecules in an undisturbed animal. To measure effects of central interventions on peripheral glucose metabolism, endogenous glucose production (EGP) and insulin sensitivity can be measured using a stable isotope technique. To combine both techniques, it is necessary to combine different catheters. We here describe a novel cannulation technique for the carotid artery, enabling stress-free infusions towards the brain and blood sampling from the carotid artery concomitantly, and infuse a stable isotope via the jugular vein. RESULTS: We showed accurate EGP measurements when intracarotically infusing saline towards the brain. The stress-hormone corticosterone, as well as energy expenditure, did not alter upon central infusion. COMPARISON EXISTING METHOD(S): ICV infusions bypass the blood-brain-barrier (BBB) and are thus a less physiological approach when studying central effects of blood-borne factors. Furthermore, ICV injections can elicit a stress response which can interfere with outcomes of glucose metabolism. We described a stress-free, physiological method to study effects of central infusions on peripheral parameters. CONCLUSIONS: This technique provides new opportunities for studying central effects of, for instance, hormones and nutrients, on glucose metabolism.

Physiological and pathophysiological implications of lipid sensing in the brain.

Fatty acid (FA)-sensitive neurons are present in the brain, especially the hypothalamus, and play a key role in the neural control of energy homeostasis. Through neuronal output, FA may modulate feeding behaviour as well as insulin secretion and action. Subpopulations of neurons in the ventromedial and arcuate hypothalamic nuclei are selectively either inhibited or activated by FA. Molecular effectors of these FA effects probably include chloride or potassium ion channels. While intracellular metabolism and activation of the ATP-sensitive K⁺ channel appear to be necessary for some of the signalling effects of FA, at least half of the FA responses in ventromedial hypothalamic neurons are mediated by interaction with FAT/CD36, an FA transporter/receptor that does not require intracellular metabolism to activate downstream signalling. Thus, FA or their metabolites can modulate neuronal activity as a means of directly monitoring ongoing fuel availability by brain nutrient-sensing neurons involved in the regulation of energy and glucose homeostasis. Recently, the role of lipoprotein lipase in FA sensing has also been shown in animal models not only in hypothalamus, but also in hippocampus and striatum. Finally, FA overload might impair neural control of energy homeostasis through enhanced ceramide synthesis and may contribute to obesity and/or type 2 diabetes pathogenesis in predisposed subjects.

Comparative effects of Citrullus colocynthis, sunflower and olive oil-enriched diet in streptozotocin-induced diabetes in rats.

Citrullus colocynthis (colocynth) seeds are traditionally used as antidiabetic medication in Mediterranean countries. The present study evaluated the differential effects of diets enriched with C. colocynthis, sunflower or olive oils on the pancreatic beta-cell mass in streptozotocin (STZ)-induced diabetes in rats. STZ injection induced rapid hyperglycaemia in all animals. However, 2 months later, hyperglycaemia was significantly less pronounced in the rats fed a C. colocynthis oil-enriched diet compared with other rat groups (7.9mM versus 12mM and 16mM with colocynth versus olive and sunflower oils, respectively). Assessment of insulin sensitivity using the homoeostasis model assessment (HOMA) method also indicated less insulin resistance in the rats fed a C. colocynthis oil-enriched diet versus the other rats. Finally, 2 months after STZ injection, the pancreatic beta-cell mass was similar in both the STZ-treated rats fed the colocynth oil-enriched diet and their controls fed the same diet. In contrast, the pancreatic beta-cell mass remained lower in the STZ-induced diabetic rats fed with olive oil- and sunflower oil-enriched diets compared with the C. colocynthis group. We conclude that C. colocynthis oil supplementation may have a beneficial effect by partly preserving or restoring pancreatic beta-cell mass in the STZ-induced diabetes rat model.

Fatty acid sensing and nervous control of energy homeostasis.

Nutrient sensitive neurons (glucose and fatty acids, FA) are present in both the hypothalamus and the brainstem and play a key role in nervous control of energy homeostasis. Through neuronal output, especially the autonomic nervous system, it is now evidenced that FA may modulate food behaviour and both insulin secretion and action. For example, central administration of oleate inhibits both food intake and hepatic glucose production in rats. This suggests that a slight increase in plasma FA concentrations in the postprandial state might be detected by the central nervous system as a satiety signal. At cellular levels, subpopulations of FA-sensitive neurons (either excited or inhibited by FA) are now identified within the hypothalamus. However molecular effectors of FA effects remain unclear. They probably include ionic channels such as chloride or potassium. FA metabolism seems also required to induce neuronal response. Thus, FA per se or their metabolites modulate neuronal activity, as a mean of directly monitoring ongoing fuel availability by CNS nutrient-sensing neurons involved in the regulation of insulin secretion. Beside these physiological effects, FA overload or dysfunction of their metabolism could impair nervous control of energy homeostasis and contribute to development of obesity and/or type 2 diabetes in predisposed subjects.

Effects of oleic acid on distinct populations of neurons in the hypothalamic arcuate nucleus are dependent on extracellular glucose levels.

Pharmacological manipulation of fatty acid metabolism in the hypothalamic arcuate nucleus (ARC) alters energy balance and glucose homeostasis. Thus, we tested the hypotheses that distinctive populations of ARC neurons are oleic acid (OA) sensors that exhibit a glucose dependency, independent of whether some of these OA sensors are also glucose-sensing neurons. We used patch-clamp recordings to investigate the effects of OA on ARC neurons in brain slices from 14- to 21-day-old Sprague-Dawley (SD) rats. Additionally, we recorded spontaneous discharge rate in ARC neurons in 8-wk-old fed and fasted SD rats in vivo. Patch-clamp studies showed that in 2.5 mM glucose 12 of 94 (13%) ARC neurons were excited by 2 microM OA (OA-excited or OAE neurons), whereas six of 94 (6%) were inhibited (OA-inhibited2.5 or OAI2.5 neurons). In contrast, in 0.1 mM glucose, OA inhibited six of 20 (30%) ARC neurons (OAI0.1 neurons); none was excited. None of the OAI0.1 neurons responded to OA in 2.5 mM glucose. Thus OAI2.5 and OAI0.1 neurons are distinct. Similarly, in seven of 20 fed rats (35%) the overall response was OAE-like, whereas in three of 20 (15%) it was OAI-like. In contrast, in fasted rats only OAI-like response were observed (three of 15; 20%). There was minimal overlap between OA-sensing neurons and glucose-sensing neurons. In conclusion, OA regulated three distinct subpopulations of ARC neurons in a glucose-dependent fashion. These data suggest that an interaction between glucose and fatty acids regulates OA sensing in ARC neurons.

Early changes in insulin secretion and action induced by high-fat diet are related to a decreased sympathetic tone.

To evaluate the relationship between the development of obesity, nervous system activity, and insulin secretion and action, we tested the effect of a 2-mo high-fat diet in rats (HF rats) on glucose tolerance, glucose-induced insulin secretion (GIIS), and glucose turnover rate compared with chow-fed rats (C rats). Moreover, we measured pancreatic and hepatic norepinephrine (NE) turnover, as assessment of sympathetic tone, and performed hypothalamic microdialysis to quantify extracellular NE turnover. Baseline plasma triglyceride, free fatty acid, insulin, and glucose concentrations were similar in both groups. After 2 days of diet, GIIS was elevated more in HF than in C rats, whereas plasma glucose time course was similar. There was a significant increase in basal pancreatic NE level of HF rats, and a twofold decrease in the fractional turnover constant was observed, indicating a change in sympathetic tone. In ventromedian hypothalamus of HF rats, the decrease in NE extracellular concentration after a glucose challenge was lower compared with C rats, suggesting changes in overall activity. After 7 days, insulin hypersecretion persisted, and glucose intolerance appeared. Later (2 mo), there was no longer insulin hypersecretion, whereas glucose intolerance worsened. At all times, HF rats also displayed hepatic insulin resistance. On day 2 of HF diet, GIIS returned to normal after treatment with oxymetazoline, an alpha(2A)-adrenoreceptor agonist, thus suggesting the involvement of a low sympathetic tone in insulin hypersecretion in response to glucose in HF rats. In conclusion, the HF diet rapidly results in an increased GIIS, at least in part related to a decreased sympathetic tone, which can be the first step of a cascade of events leading to impaired glucose homeostasis.

Beta oxidation in the brain is required for the effects of non-esterified fatty acids on glucose-induced insulin secretion in rats.

AIMS/HYPOTHESIS: NEFA play a key role in the setting of insulin resistance and hyperinsulinaemia, which are both features of the prediabetic state. In addition to the direct effects on pancreas and peripheral tissues, NEFA have been reported to act via changes in autonomic nervous system activity. The present study was aimed at studying the effects of a local increase in NEFA in the brain on glucose-induced insulin secretion (GIIS) and on insulin action. We hypothesised that cerebral NEFA beta oxidation is a prerequisite for these central effects. METHODS: Male Wistar rats were infused with Intralipid/heparin for 24 h through the carotid artery towards the brain (IL rats), after which we performed the GIIS test, a euglycaemic-hyperinsulinaemic clamp and c-fos immunochemistry. In another series of experiments, Intralipid/heparin infusion was coupled with lateral ventricular infusion of etomoxir, a CPT1 inhibitor, which was initiated 5 days previously. RESULTS: During the infusion period, there were no changes in plasma NEFA, insulin or glucose concentrations. IL rats displayed an increased GIIS compared with control rats (C rats) infused with saline/heparin, and their liver insulin sensitivity was decreased. Furthermore, lipid infusion induced a significant decrease in c-fos-like immunoreactive neurons in medial hypothalamic nuclei, and an increase in lateral hypothalamus. Neuronal activation profile was almost normalised in IL rats infused with etomoxir, and GIIS was strongly decreased, possibly because of the concomitant normalisation of hepatic glucose output. CONCLUSIONS/INTERPRETATION: These results strongly suggest that beta oxidation is required for the central effects of NEFA on GIIS.