Effect of parasympathetic denervation of liver and pancreas on glucose kinetics in man

Vagal pathways for systemic regulation of glucose metabolism

Maintaining blood glucose at an appropriate physiological level requires precise coordination of multiple organs and tissues. The vagus nerve bidirectionally connects the central nervous system with peripheral organs crucial to glucose mobilization, nutrient storage, and food absorption, thereby presenting a key pathway for the central control of blood glucose levels. However, the precise mechanisms by which vagal populations that target discrete tissues participate in glucoregulation are much less clear. Here we review recent advances unraveling the cellular identity, neuroanatomical organization, and functional contributions of both vagal efferents and vagal afferents in the control of systemic glucose metabolism. We focus on their involvement in relaying glucoregulatory cues from the brain to peripheral tissues, particularly the pancreatic islet, and by sensing and transmitting incoming signals from ingested food to the brain. These recent findings - largely driven by advances in viral approaches, RNA sequencing, and cell-type selective manipulations and tracings - have begun to clarify the precise vagal neuron populations involved in the central coordination of glucose levels, and raise interesting new possibilities for the treatment of glucose metabolism disorders such as diabetes.

Mapping and targeted viral activation of pancreatic nerves in mice reveal their roles in the regulation of glucose metabolism

A lack of comprehensive mapping of ganglionic inputs into the pancreas and of technology for the modulation of the activity of specific pancreatic nerves has hindered the study of how they regulate metabolic processes. Here we show that the pancreas-innervating neurons in sympathetic, parasympathetic and sensory ganglia can be mapped in detail by using tissue clearing and retrograde tracing (the tracing of neural connections from the synapse to the cell body), and that genetic payloads can be delivered via intrapancreatic injection to target sites in efferent pancreatic nerves in live mice through optimized adeno-associated viruses and neural-tissue-specific promoters. We also show that, in male mice, the targeted activation of parasympathetic cholinergic intrapancreatic ganglia and neurons doubled plasma-insulin levels and improved glucose tolerance, and that tolerance was impaired by stimulating pancreas-projecting sympathetic neurons. The ability to map the peripheral ganglia innervating the pancreas and to deliver transgenes to specific pancreas-projecting neurons will facilitate the examination of ganglionic inputs and the study of the roles of pancreatic efferent innervation in glucose metabolism.

Unravelling innervation of pancreatic islets

The central and peripheral nervous systems play critical roles in regulating pancreatic islet function and glucose metabolism. Over the last century, in vitro and in vivo studies along with examination of human pancreas samples have revealed the structure of islet innervation, investigated the contribution of sympathetic, parasympathetic and sensory neural pathways to glucose control, and begun to determine how the structure and function of pancreatic nerves are disrupted in metabolic disease. Now, state-of-the art techniques such as 3D imaging of pancreatic innervation and targeted in vivo neuromodulation provide further insights into the anatomy and physiological roles of islet innervation. Here, we provide a summary of the published work on the anatomy of pancreatic islet innervation, its roles, and evidence for disordered islet innervation in metabolic disease. Finally, we discuss the possibilities offered by new technologies to increase our knowledge of islet innervation and its contributions to metabolic regulation.

Antipsychotic-induced metabolic and cardiovascular side effects in schizophrenia: a novel mechanistic hypothesis

The use of antipsychotics is hindered by the frequent occurrence of metabolic and cardiovascular side effects, resulting in worsened quality of life and greater mortality as a result of cardiovascular and cerebrovascular disorders in schizophrenia patients than the comparable general population. The various antipsychotics induce extrapyramidal symptoms, impaired glucose and lipid metabolism, weight gain, hypertension and arrhythmias, with variable frequency. Second-generation antipsychotics appear to have several advantages over first-generation antipsychotics, including a claimed better action on cognitive function and the negative symptoms of schizophrenia, and lower frequency of extrapyramidal side effects; however, their use is associated with a greater frequency of metabolic and cardiovascular disturbances. The mechanisms of these important side effects are not well understood, and generic approaches (psychoeducational programmes and symptomatic therapies) have been proposed to limit their severity. Extensive data from the literature indicate that autonomic nervous system dysfunction--intrinsic to schizophrenia and strongly exacerbated by antipsychotic treatment--is the cause of the pervasive metabolic and vascular dysfunctions associated with schizophrenia. In this article, we marshal further literature data to argue that the metabolic and cardiovascular side effects of antipsychotics are primarily mediated by their ability to block peripheral dopamine receptors, which physiologically modulate sympathetic activity. We also propose that these effects might be overcome by providing peripheral dopaminergic stimulation.

Oesophageal dysmotility, delayed gastric emptying and autonomic neuropathy correlate to disturbed glucose homeostasis

AIMS/HYPOTHESIS

Among diabetic patients, glucose homeostasis may be affected by abnormal gastrointestinal motility and autonomic neuropathy. This study analysed whether oesophageal dysmotility, delayed gastric emptying or autonomic neuropathy affect glucose homeostasis.

MATERIALS AND METHODS

Oesophageal manometry and gastric emptying scintigraphy were performed in 20 diabetic patients. Heart-rate variation during deep breathing (expiration/inspiration [E/I] ratio) and continuous subcutaneous glucose concentrations for a period of 72 h were also monitored in the same patients.

RESULTS

Oesophageal dysmotility was found in eight of 14 patients. Eleven of 20 patients had delayed gastric emptying (abnormal gastric emptying half-time [T (50)]) and nine of 18 had an abnormal E/I ratio. Complaints of abdominal fullness were predictive of delayed gastric emptying. A low peristaltic speed of the oesophagus was associated with impaired T (50) (r ( s )=-0.67; p=0.02). One hour after breakfast, subcutaneous glucose levels decreased in patients with delayed gastric emptying but continued to rise in those with normal emptying. Consequently, the median glucose level 2.5 h after breakfast was lower in the former (9.1 [4.2-12.5] vs 14.3 [11.2-17.7] mmol/l; p<0.05). Glucose fluctuations during the 72 h were significantly higher in patients with an abnormal E/I ratio than in those with a normal E/I ratio (coefficient of variation: 41 [46-49] vs 28 [27-34]%; p=0.008).

CONCLUSIONS/INTERPRETATION

Abdominal fullness predicted delayed gastric emptying that was associated with diminished glucose uptake after breakfast. Low oesophageal peristaltic speed was associated with slow gastric emptying whereas parasympathetic neuropathy was associated with increased glucose variations.

Activation of the parasympathetic nervous system is necessary for normal meal-induced insulin secretion in rhesus macaques

Meal-induced insulin secretion is thought to be regulated primarily by absorbed nutrients and incretin hormones released from the gastrointestinal tract. In addition, the parasympathetic nervous system (PNS) is known to mediate preabsorptive, or cephalic phase, insulin secretion. Despite evidence that the PNS remains activated during the absorptive phase of the meal, its role in mediating postprandial insulin secretion has not been established. To study the role of the PNS in absorptive phase insulin release, we measured plasma concentrations of glucose as well as islet hormones and incretins in six healthy rhesus monkeys before and for 60 min after meals while they were infused with saline (control), atropine (muscarinic blockade), or trimethaphan (nicotinic blockade). During the infusion of saline, plasma levels of glucose, pancreatic polypeptide (PP), insulin, glucose-dependent insulinotropic polypeptide, and glucagon-like peptide-1 increased promptly after meal ingestion and remained elevated throughout the 60 min of the study. The PP response was nearly abolished in animals treated with trimethaphan, indicating functional blockade of PNS input to the islet, and in contrast to the control study, there were minimal changes in plasma concentrations of glucose, incretin hormones, and insulin. Because trimethaphan inhibited glycemic and incretin stimuli in addition to blocking PNS input to the islet, it was not possible to discern the relative roles of these factors in the stimulation of insulin secretion. Atropine also significantly decreased PNS transmission to the islet, as reflected by PP levels similar to those observed with trimethaphan. Unlike the trimethaphan study, plasma glucose levels rose normally during atropine treatment and were similar to those in the control study over the course of the experiments (114 +/- 22 and 132 +/- 23 mmol/L.60 min, respectively). In addition, the rise in plasma glucagon-like peptide-1 following the meal was not suppressed by atropine, and the glucose-dependent insulinotropic polypeptide responses were only modestly decreased. Despite the significant increases in circulating glucose and incretins, plasma insulin levels were greatly attenuated by atropine, so that the 60 min responses were more comparable to those during trimethaphan treatment than to those in the control study (atropine, 3,576 +/- 1,284; trimethaphan, 4,128 +/- 2,616; control, 15,834 +/- 5,586 pmol/L.60 min; P: < 0.05). Thus, muscarinic blockade markedly suppressed the meal-induced insulin response despite normal postprandial glycemia and significant elevations of incretins. These results indicate that activation of the PNS during the absorptive phase of meals contributes significantly to the postprandial insulin secretory response.

Thermogenic effect of slight hyperglycemia during a lipid infusion

Resistance to the glucoregulatory action of insulin is a common finding in obesity and may affect thermogenesis. In 13 healthy subjects, we studied the influence of acute insulin resistance induced by a lipid infusion on thermogenesis without any glucose load (n = 4) or during a euglycemic-hyperinsulinemic clamp (n = 5) and an oral glucose tolerance test (OGTT, n = 8). When substrates were not administered at the same time, the energy cost of storage was significantly (P < .05) lower for lipids (3.9%+/-0.9%) than for glucose (11.9%+/-0.5% during the clamp and 14.9%+/-4.0% during the OGTT, NS). The lipid infusion decreased glucose storage during the clamp (control, 3.99+/-0.40 mg x kg(-1) x min(-1); lipid infusion, 0.92+/-0.39; P < .05) but increased it during the OGTT (control, 1.76+/-0.22 mg x kg(-1) x min(-1); lipid infusion, 2.94+/-0.27; P < .05). Infused lipids were stored more (clamp, 3.31+/-0.16; OGTT, 2.65+/-0.11 mg x kg(-1) x min(-1); P < .01) and oxidized less (clamp, 0.64+/-0.21; OGTT, 1.02+/-0.09 mg x kg(-1) x min(-1); P < .05) during the clamp than during the OGTT. When lipids were infused, the energy cost of substrate storage was lower during the clamp versus the OGTT (clamp, 3.2%+/-0.8%; OGTT, 7.3%+/-1.0%; P < .05). This effect was attributed to a lipid-induced impairment of glucose tolerance, which overcomes the inhibitory effect of lipid infusion on glucose storage observed in euglycemia. A slight elevation of plasma glucose in response to a lipid infusion impairs thermogenesis by redirecting the storage of substrates from lipids to glucose, which has a higher energy cost.

Autonomic neuropathy in Type 2 diabetic patients is associated with hyperinsulinaemia and hypertriglyceridaemia

AIMS

To clarify whether parasympathetic neuropathy in Type 2 diabetic patients is associated with features of the insulin resistance syndrome.

METHODS

Blood pressures, glycaemic control (HbA1c), plasma lipids, residual beta-cell function (fasting plasma C-peptide), autonomic nerve function, urinary albumin excretion and glomerular filtration rate (Cr-EDTA clearance) were evaluated in 82 Type 2 diabetic patients (age 63+/-years) 5 years after diagnosis of diabetes.

RESULTS

Parasympathetic neuropathy (an abnormal age corrected E/I ratio) was found in 24/82 (29%) patients. After adjustment for body mass index (BMI), patients with parasympathetic neuropathy had elevated fasting plasma C-peptide (P < 0.001) and triglyceride (Tg) (P < 0.05) levels compared with patients without parasympathetic neuropathy. In addition, the age corrected E/I ratio correlated inversely with Tg (r=-0.31; P<0.01) and fasting plasma C-peptide (r=-0.32; P < 0.01) in the Type 2 diabetic patients.

CONCLUSION

Autonomic neuropathy 5 years after diagnosis of Type 2 diabetes is associated with an unfavourable metabolic risk profile.