Neprilysin deficiency reduces hepatic gluconeogenesis in high fat-fed mice

Neprilysin is a peptidase that cleaves glucoregulatory peptides, including glucagon-like peptide-1 (GLP-1) and cholecystokinin (CCK). Some studies suggest that its inhibition in diabetes and/or obesity improves glycemia, and that this is associated with enhanced insulin secretion, glucose tolerance and insulin sensitivity. Whether reduced neprilysin activity also improves hepatic glucose metabolism has not been explored. We sought to determine whether genetic deletion of neprilysin suppresses hepatic glucose production (HGP) in high fat-fed mice. Nep+/+ and Nep-/- mice were fed high fat diet for 16 weeks, and then underwent a pyruvate tolerance test (PTT) to assess hepatic gluconeogenesis. Since glycogen breakdown in liver can also yield glucose, we assessed liver glycogen content in fasted and fed mice. In Nep-/- mice, glucose excursion during the PTT was reduced when compared to Nep+/+ mice. Further, liver glycogen levels were significantly greater in fasted but not fed Nep-/- versus Nep+/+ mice. Since gut-derived factors modulate HGP, we tested whether gut-selective inhibition of neprilysin could recapitulate the suppression of hepatic gluconeogenesis observed with whole-body inhibition, and this was indeed the case. Finally, the gut-derived neprilysin substrates, GLP-1 and CCK, are well-known to suppress HGP. Having previously demonstrated elevated plasma GLP-1 levels in Nep-/- mice, we now measured plasma CCK bioactivity and reveal an increase in Nep-/- versus Nep+/+ mice, suggesting GLP-1 and/or CCK may play a role in reducing HGP under conditions of neprilysin deficiency. In sum, neprilysin modulates hepatic gluconeogenesis and strategies to inhibit its activity may reduce HGP in type 2 diabetes and obesity.

Acute Inhibition of Intestinal Neprilysin Enhances Insulin Secretion via GLP-1 Receptor Signaling in Male Mice

The peptidase neprilysin modulates glucose homeostasis by cleaving and inactivating insulinotropic peptides, including some produced in the intestine such as glucagon-like peptide-1 (GLP-1). Under diabetic conditions, systemic or islet-selective inhibition of neprilysin enhances beta-cell function through GLP-1 receptor (GLP-1R) signaling. While neprilysin is expressed in intestine, its local contribution to modulation of beta-cell function remains unknown. We sought to determine whether acute selective pharmacological inhibition of intestinal neprilysin enhanced glucose-stimulated insulin secretion under physiological conditions, and whether this effect was mediated through GLP-1R. Lean chow-fed Glp1r+/+ and Glp1r-/- mice received a single oral low dose of the neprilysin inhibitor thiorphan or vehicle. To confirm selective intestinal neprilysin inhibition, neprilysin activity in plasma and intestine (ileum and colon) was assessed 40 minutes after thiorphan or vehicle administration. In a separate cohort of mice, an oral glucose tolerance test was performed 30 minutes after thiorphan or vehicle administration to assess glucose-stimulated insulin secretion. Systemic active GLP-1 levels were measured in plasma collected 10 minutes after glucose administration. In both Glp1r+/+ and Glp1r-/- mice, thiorphan inhibited neprilysin activity in ileum and colon without altering plasma neprilysin activity or active GLP-1 levels. Further, thiorphan significantly increased insulin secretion in Glp1r+/+ mice, whereas it did not change insulin secretion in Glp1r-/- mice. In conclusion, under physiological conditions, acute pharmacological inhibition of intestinal neprilysin increases glucose-stimulated insulin secretion in a GLP-1R-dependent manner. Since intestinal neprilysin modulates beta-cell function, strategies to inhibit its activity specifically in the intestine may improve beta-cell dysfunction in type 2 diabetes.

Neprilysin inhibition improves intravenous but not oral glucose-mediated insulin secretion via GLP-1R signaling in mice with -cell dysfunction

Type 2 diabetes is associated with the upregulation of neprilysin, a peptidase capable of cleaving glucoregulatory peptides such as glucagon-like peptide-1 (GLP-1). In humans, use of the neprilysin inhibitor sacubitril in combination with an angiotensin II receptor blocker was associated with increased plasma GLP-1 levels and improved glycemic control. Whether neprilysin inhibition per se is mediating these effects remains unknown. We sought to determine whether pharmacological neprilysin inhibition on its own confers beneficial effects on glycemic status and β-cell function in a mouse model of reduced insulin secretion, and whether any such effects are dependent on GLP-1 receptor (GLP-1R) signaling. High-fat-fed male wild-type (Glp1r^+/+) and GLP-1R knockout (Glp1r^-/-) mice were treated with low-dose streptozotocin (STZ) to recapitulate type 2 diabetes-associated β-cell dysfunction, or vehicle as control. Mice were continued on high-fat diet alone or supplemented with the neprilysin inhibitor sacubitril for 8 wk. At the end of the study period, β-cell function was assessed by oral or intravenous glucose-tolerance test. Fasting and fed glucose were significantly lower in wild-type mice treated with sacubitril, although active GLP-1 levels and insulin secretion during oral glucose challenge were unchanged. In contrast, insulin secretion in response to intravenous glucose was significantly enhanced in sacubitril-treated wild-type mice, and this effect was blunted in Glp1r^-/- mice. Similarly, sacubitril enhanced insulin secretion in vitro in islets from STZ-treated Glp1r^+/+ but not Glp1r^-/- mice. Together, our data suggest the insulinotropic effects of pharmacological neprilysin inhibition in a mouse model of β-cell dysfunction are mediated via intra-islet GLP-1R signaling.NEW & NOTEWORTHY The neprilysin inhibitor, sacubitril, improves glycemic status in a mouse model of reduced insulin secretion. Sacubitril enhances intravenous but not oral glucose-mediated insulin secretion. The increased glucose-mediated insulin secretion is GLP-1 receptor-dependent. Neprilysin inhibition does not raise postprandial circulating active GLP-1 levels.

SGLT2-i improves markers of islet endothelial cell function in db/db diabetic mice

Islet endothelial cells produce paracrine factors important for islet beta-cell function and survival. Under conditions of type 2 diabetes, islet endothelial cells exhibit a dysfunctional phenotype including increased expression of genes involved in cellular adhesion and inflammation. We sought to determine whether treatment of hyperglycemia with the sodium glucose co-transporter 2 inhibitor empagliflozin, either alone or in combination with metformin, would improve markers of endothelial cell function in islets, assessed ex vivo, and if such an improvement is associated with improved insulin secretion in a mouse model of diabetes in vivo. For these studies, db/db diabetic mice and non-diabetic littermate controls were treated for 6 weeks with empagliflozin or metformin, either alone or in combination. For each treatment group, expression of genes indicative of islet endothelial dysfunction was quantified. Islet endothelial and beta-cell area was assessed by morphometry of immunochemically stained pancreas sections. Measurements of plasma glucose and insulin secretion during an intravenous glucose tolerance test were performed on vehicle and drug treated diabetic animals. We found that expression of endothelial dysfunction marker genes is markedly increased in diabetic mice. Treatment with either empagliflozin or metformin lowered expression of the dysfunction marker genes ex vivo, which correlated with improved glycemic control, and increased insulin release in vivo. Empagliflozin treatment was more effective than metformin alone, with a combination of the two drugs demonstrating the greatest effects. Improving islet endothelial function through strategies such as empagliflozin/metformin treatment may provide an effective approach for improving insulin release in human type 2 diabetes.

Co-impairment of autonomic and glucagon responses to insulin-induced hypoglycemia in dogs with naturally occurring insulin-dependent diabetes mellitus

This study aimed to investigate the contributions of two factors potentially impairing glucagon response to insulin-induced hypoglycemia (IIH) in insulin-deficient diabetes: 1) loss of paracrine disinhibition by intra-islet insulin and 2) defects in the activation of the autonomic inputs to the islet. Plasma glucagon responses during hyperinsulinemic-hypoglycemic clamps ([Formula: see text]40 mg/dL) were assessed in dogs with spontaneous diabetes (n = 13) and in healthy nondiabetic dogs (n = 6). Plasma C-peptide responses to intravenous glucagon were measured to assess endogenous insulin secretion. Plasma pancreatic polypeptide, epinephrine, and norepinephrine were measured as indices of parasympathetic and sympathoadrenal autonomic responses to IIH. In 8 of the 13 diabetic dogs, glucagon did not increase during IIH (diabetic nonresponder [DMN]; ∆ = -6 ± 12 pg/mL). In five other diabetic dogs (diabetic responder [DMR]), glucagon responses (∆ = +26 ± 12) were within the range of nondiabetic control dogs (∆ = +27 ± 16 pg/mL). C-peptide responses to intravenous glucagon were absent in diabetic dogs. Activation of all three autonomic responses were impaired in DMN dogs but remained intact in DMR dogs. Each of the three autonomic responses to IIH was positively correlated with glucagon responses across the three groups. The study conclusions are as follows: 1) Impairment of glucagon responses in DMN dogs is not due to generalized impairment of α-cell function. 2) Loss of tonic inhibition of glucagon secretion by insulin is not sufficient to produce loss of the glucagon response; impairment of autonomic activation is also required. 3) In dogs with major β-cell function loss, activation of the autonomic inputs is sufficient to mediate an intact glucagon response to IIH.NEW & NOTEWORTHY In dogs with naturally occurring, insulin-dependent (C-peptide negative) diabetes mellitus, impairment of glucagon responses is not due to generalized impairment of α-cell function. Loss of tonic inhibition of glucagon secretion by insulin is not sufficient, by itself, to produce loss of the glucagon response. Rather, impaired activation of the parasympathetic and sympathoadrenal autonomic inputs to the pancreas is also required. Activation of the autonomic inputs to the pancreas is sufficient to mediate an intact glucagon response to insulin-induced hypoglycemia in dogs with naturally occurring diabetes mellitus. These results have important implications that include leading to a greater understanding and insight into the pathophysiology, prevention, and treatment of hypoglycemia during insulin treatment of diabetes in companion dogs and in human patients.

Early sympathetic islet neuropathy in autoimmune diabetes: lessons learned and opportunities for investigation

This review outlines the current state of knowledge regarding a unique neural defect of the pancreatic islet in autoimmune diabetes, one that we have termed early sympathetic islet neuropathy (eSIN). We begin with the findings that a majority of islet sympathetic nerves are lost near the onset of type 1, but not type 2, diabetes and that this nerve loss is restricted to the islet. We discuss later work demonstrating that while the loss of islet sympathetic nerves and the loss of islet beta cells in type 1 diabetes both require infiltration of the islet by lymphocytes, their respective mechanisms of tissue destruction differ. Uniquely, eSIN requires the activation of a specific neurotrophin receptor and we propose two possible pathways for activation of this receptor during the immune attack on the islet. We also outline what is known about the functional consequences of eSIN, focusing on impairment of sympathetically mediated glucagon secretion and its application to the clinical problem of insulin-induced hypoglycaemia. Finally, we offer our view on the important remaining questions regarding this unique neural defect.

Human Type 1 Diabetes Is Characterized by an Early, Marked, Sustained, and Islet-Selective Loss of Sympathetic Nerves

In humans, the glucagon response to moderate-to-marked insulin-induced hypoglycemia (IIH) is largely mediated by the autonomic nervous system. Because this glucagon response is impaired early in type 1 diabetes, we sought to determine if these patients, like animal models of autoimmune diabetes, have an early and severe loss of islet sympathetic nerves. We also tested whether this nerve loss is a permanent feature of type 1 diabetes, is islet-selective, and is not seen in type 2 diabetes. To do so, we quantified pancreatic islet and exocrine sympathetic nerve fiber area from autopsy samples of patients with type 1 or 2 diabetes and control subjects without diabetes. Our central finding is that patients with either very recent onset (<2 weeks) or long duration (>10 years) of type 1 diabetes have a severe loss of islet sympathetic nerves (Δ = -88% and Δ = -79%, respectively). In contrast, patients with type 2 diabetes lose no islet sympathetic nerves. There is no loss of exocrine sympathetic nerves in either type 1 or type 2 diabetes. We conclude that patients with type 1, but not type 2, diabetes have an early, marked, sustained, and islet-selective loss of sympathetic nerves, one that may impair their glucagon response to IIH.

Short-term diabetic hyperglycemia suppresses celiac ganglia neurotransmission, thereby impairing sympathetically mediated glucagon responses

Short-term hyperglycemia suppresses superior cervical ganglia neurotransmission. If this ganglionic dysfunction also occurs in the islet sympathetic pathway, sympathetically mediated glucagon responses could be impaired. Our objectives were 1) to test for a suppressive effect of 7 days of streptozotocin (STZ) diabetes on celiac ganglia (CG) activation and on neurotransmitter and glucagon responses to preganglionic nerve stimulation, 2) to isolate the defect in the islet sympathetic pathway to the CG itself, and 3) to test for a protective effect of the WLD(S) mutation. We injected saline or nicotine in nondiabetic and STZ-diabetic rats and measured fos mRNA levels in whole CG. We electrically stimulated the preganglionic or postganglionic nerve trunk of the CG in nondiabetic and STZ-diabetic rats and measured portal venous norepinephrine and glucagon responses. We repeated the nicotine and preganglionic nerve stimulation studies in nondiabetic and STZ-diabetic WLD(S) rats. In STZ-diabetic rats, the CG fos response to nicotine was suppressed, and the norepinephrine and glucagon responses to preganglionic nerve stimulation were impaired. In contrast, the norepinephrine and glucagon responses to postganglionic nerve stimulation were normal. The CG fos response to nicotine, and the norepinephrine and glucagon responses to preganglionic nerve stimulation, were normal in STZ-diabetic WLD(S) rats. In conclusion, short-term hyperglycemia's suppressive effect on nicotinic acetylcholine receptors of the CG impairs sympathetically mediated glucagon responses. WLD(S) rats are protected from this dysfunction. The implication is that this CG dysfunction may contribute to the impaired glucagon response to insulin-induced hypoglycemia seen early in type 1 diabetes.

Glucose intolerance induced by blockade of central FGF receptors is linked to an acute stress response

Objective

Central administration of ligands for fibroblast growth factor receptors (FGFRs) such as fibroblast growth factor-19 (FGF19) and FGF21 exert glucose-lowering effects in rodent models of obesity and type 2 diabetes (T2D). Conversely, intracerebroventricular (icv) administration of the non-selective FGFR inhibitor (FGFRi) PD173074 causes glucose intolerance, implying a physiological role for neuronal FGFR signaling in glucose homeostasis. The current studies were undertaken to identify neuroendocrine mechanisms underlying the glucose intolerance induced by pharmacological blockade of central FGFRs.

Methods

Overnight fasted, lean, male, Long-Evans rats received icv injections of either PD173074 or vehicle (Veh) followed 30 min later by performance of a frequently sampled intravenous glucose tolerance test (FSIGT). Minimal model analysis of glucose and insulin data from the FSIGT was performed to estimate insulin-dependent and insulin-independent components of glucose disposal. Plasma levels of lactate, glucagon, corticosterone, non-esterified free fatty acids (NEFA) and catecholamines were measured before and after intravenous (iv) glucose injection.

Results

Within 20 min of icv PD173074 injection (prior to the FSIGT), plasma levels of lactate, norepinephrine and epinephrine increased markedly, and each returned to baseline rapidly (within 8 min) following the iv glucose bolus. In contrast, plasma glucagon levels were not altered by icv FGFRi at either time point. Consistent with a previous report, glucose tolerance was impaired following icv PD173074 compared to Veh injection and, based on minimal model analysis of FSIGT data, this effect was attributable to reductions of both insulin secretion and the basal insulin effect (BIE), consistent with the inhibitory effect of catecholamines on pancreatic β-cell secretion. By comparison, there were no changes in glucose effectiveness at zero insulin (GEZI) or the insulin sensitivity index (S_I). To determine if iv glucose (given during the FSIGT) contributed to the rapid resolution of the sympathoadrenal response induced by icv FGFRi, we performed an additional study comparing groups that received iv saline or iv glucose 30 min after icv FGFRi. Our finding that elevated plasma catecholamine levels returned rapidly to baseline irrespective of whether rats subsequently received an iv bolus of saline or glucose indicates that the rapid reversal of sympathoadrenal activation following icv FGFRi was unrelated to the subsequent glucose bolus.

Conclusions

The effect of acute inhibition of central FGFR signaling to impair glucose tolerance likely involves a stress response associated with pronounced, but transient, sympathoadrenal activation and an associated reduction of insulin secretion. Whether this effect is a true consequence of FGFR blockade or involves an off-target effect of the FGFR inhibitor requires additional study.

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icv FGFR antagonist causes glucose intolerance in rats.

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This effect is associated with robust sympathoadrenal activation.

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The sympathoadrenal response is rapid in onset, but clears rapidly.

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Reduced insulin secretion contributes to FGFR inhibitor-induced glucose intolerance.

The search for the mechanism of early sympathetic islet neuropathy in autoimmune diabetes

This review outlines our search for the mechanism causing the early loss of islet sympathetic nerves in autoimmune diabetes. Since our previous work has documented the importance of autonomic stimulation of glucagon secretion during hypoglycaemia, the loss of these nerves may contribute to the known impairment of this specific glucagon response early in human type 1 diabetes. We therefore briefly review the contribution that autonomic activation, and sympathetic neural activation in particular, makes to the subsequent glucagon response to hypoglycaemia. We also detail evidence that animal models of autoimmune diabetes mimic both the early loss of islet sympathetic nerves and the impaired glucagon response seen in human type 1 diabetes. Using data from these animal models, we examine mechanisms by which this loss of islet nerves could occur. We provide evidence that it is not due to diabetic hyperglycaemia, but is related to the lymphocytic infiltration of the islet. Ablating the p75 neurotrophin receptor, which is present on sympathetic axons, prevents early sympathetic islet neuropathy (eSIN), but, interestingly, not diabetes. Thus, we appear to have separated the immune-related loss of islet sympathetic nerves from the immune-mediated destruction of islet β-cells. Finally, we speculate on a way to restore the sympathetic innervation of the islet.

The p75 neurotrophin receptor is required for the major loss of sympathetic nerves from islets under autoimmune attack

Our goal was to determine the role of the p75 neurotrophin receptor (p75NTR) in the loss of islet sympathetic nerves that occurs during the autoimmune attack of the islet. The islets of transgenic (Tg) mice in which β-cells express a viral glycoprotein (GP) under the control of the insulin promotor (Ins2) were stained for neuropeptide Y before, during, and after virally induced autoimmune attack of the islet. Ins2-GP(Tg) mice injected with lymphocytic choriomeningitis virus (LCMV) lost islet sympathetic nerves before diabetes development but coincident with the lymphocytic infiltration of the islet. The nerve loss was marked and islet-selective. Similar nerve loss, chemically induced, was sufficient to impair sympathetically mediated glucagon secretion. In contrast, LCMV-injected Ins2-GP(Tg) mice lacking the p75NTR retained most of their islet sympathetic nerves, despite both lymphocytic infiltration and development of diabetes indistinguishable from that of p75NTR wild-type mice. We conclude that an inducible autoimmune attack of the islet causes a marked and islet-selective loss of sympathetic nerves that precedes islet collapse and hyperglycemia. The p75NTR mediates this nerve loss but plays no role in mediating the loss of islet β-cells or the subsequent diabetes. p75NTR-mediated nerve loss may contribute to the impaired glucose counterregulation seen in type 1 diabetes.

Minireview: The role of the autonomic nervous system in mediating the glucagon response to hypoglycemia

In type 1 diabetes, the impairment of the glucagon response to hypoglycemia increases both its severity and duration. In nondiabetic individuals, hypoglycemia activates the autonomic nervous system, which in turn mediates the majority of the glucagon response to moderate and marked hypoglycemia. The first goal of this minireview is therefore to illustrate and document these autonomic mechanisms. Specifically we describe the hypoglycemic thresholds for activating the three autonomic inputs to the islet (parasympathetic nerves, sympathetic nerves, and adrenal medullary epinephrine) and their magnitudes of activation as glucose falls from euglycemia to near fatal levels. The implication is that their relative contributions to this glucagon response depend on the severity of hypoglycemia. The second goal of this minireview is to discuss known and suspected down-regulation or damage to these mechanisms in diabetes. We address defects in the central nervous system, the peripheral nervous system, and in the islet itself. They are categorized as either functional defects caused by glucose dysregulation or structural defects caused by the autoimmune attack of the islet. In the last section of the minireview, we outline approaches for reversing these defects. Such reversal has both scientific and clinical benefit. Scientifically, one could determine the contribution of these defects to the impairment of glucagon response seen early in type 1 diabetes. Clinically, restoring this glucagon response would allow more aggressive treatment of the chronic hyperglycemia that is linked to the debilitating long-term complications of this disease.

Loss of islet sympathetic nerves and impairment of glucagon secretion in the NOD mouse: relationship to invasive insulitis

AIMS/HYPOTHESIS

We hypothesised that non-obese diabetic mice (NOD) mice have an autoimmune-mediated loss of islet sympathetic nerves and an impairment of sympathetically mediated glucagon responses. We aimed: (1) to determine whether diabetic NOD mice have an early impairment of the glucagon response to insulin-induced hypoglycaemia (IIH) and a coincident loss of islet sympathetic nerves; (2) to determine whether invasive insulitis is required for this nerve loss; and (3) to determine whether sympathetically mediated glucagon responses are also impaired.

METHODS

We measured glucagon responses to both IIH and tyramine in anaesthetised mice. We used immunohistochemistry to quantify islet sympathetic nerves and invasive insulitis.

RESULTS

The glucagon response to IIH was markedly impaired in NOD mice after only 3 weeks of diabetes (change, -70%). Sympathetic nerve area within the islet was also markedly reduced at this time (change, -66%). This islet nerve loss was proportional to the degree of invasive insulitis. More importantly, blocking the infiltration prevented the nerve loss. Mice with autoimmune diabetes had an impaired glucagon response to sympathetic nerve activation, whereas those with non-autoimmune diabetes did not.

CONCLUSIONS/INTERPRETATION

The invasive insulitis seen in diabetic NOD mice causes early sympathetic islet neuropathy. Further studies are needed to confirm that early sympathetic islet neuropathy is responsible for the impaired glucagon response to tyramine.

Impaired activation of celiac ganglion neurons in vivo after damage to their sympathetic nerve terminals

Because damage to sympathetic nerve terminals occurs in a variety of diseases, we tested the hypothesis that nerve terminal damage per se is sufficient to impair ganglionic neurotransmission in vivo. First, we measured the effect of nerve terminal damage produced by the sympathetic nerve terminal toxin 6-hydroxydopamine (6-OHDA) on ganglionic levels of several neurotrophins thought to promote neurotransmission. 6-OHDA-induced nerve terminal damage did not decrease the expression of neurotrophin-4 or brain-derived neurotrophic factor mRNA in the celiac ganglia but did decrease the ganglionic content of both nerve growth factor protein (nadir = -63%) and the mRNA of the alpha-3 subunit of the nicotinic cholinergic receptor (nadir = -49%), a subunit required for neurotransmission. Next, we tested whether this degree of receptor deficiency was sufficient to impair activation of celiac ganglia neurons. Impaired fos mRNA responses to nicotine administration in the celiac ganglia of 6-OHDA-pretreated rats correlated temporally with suppressed expression of functional nicotinic receptors. We verified by Fos protein immunohistochemistry that this ganglionic impairment was specific to principal ganglionic neurons. Last, we tested whether centrally initiated ganglionic neurotransmission is also impaired following nerve terminal damage. The principal neurons in rat celiac ganglia were reflexively activated by 2-deoxy-glucose-induced glucopenia, and the Fos response in the celiac ganglia was markedly inhibited by pretreatment with 6-OHDA. We conclude that sympathetic nerve terminal damage per se is sufficient to impair ganglionic neurotransmission in vivo and that decreased nicotinic receptor production is a likely mediator.

Direct stimulation of ghrelin secretion by sympathetic nerves

The hormone ghrelin is secreted mainly from the gut, rises in peripheral plasma before meals, and is implicated in stimulating hunger, initiating meals, and developing obesity. We hypothesize that activation of the sympathetic nervous system contributes to preprandial ghrelin surges. The present studies in isoflurane-anesthetized Wistar rats were designed to determine whether sympathetic nerves and neurohormones are capable of stimulating ghrelin secretion. We activated gut sympathetic nerves by two methods: electrical sympathetic nerve stimulation (SNS) and chemical sympathetic nerve activation with iv tyramine (TYR) administration. Portal venous blood was sampled before and during a 10-min sympathetic stimulation. Successful activation of gut sympathetic nerves was verified by increments in portal venous norepinephrine. SNS increased portal ghrelin by 206 +/- 50%. In contrast, simply isolating gut sympathetic nerves without applying current had a minimal effect on ghrelin levels. TYR also increased portal ghrelin [change (Delta), +52 +/- 11%], whereas saline infusion had little effect. We next determined whether the neural stimulation of ghrelin secretion was mediated indirectly via the suppression of insulin secretion during SNS and TYR. Streptozotocin-induced diabetes prevented a fall in insulin during TYR, yet the portal ghrelin response (Delta = +47 +/- 18%) was similar to that in nondiabetic rats. Lastly, to test for humoral stimulation of ghrelin, we infused the sympathetic neurohormone, epinephrine, to achieve levels found during severe stress. Epinephrine failed to stimulate ghrelin secretion (Delta = +4 +/- 35%). We conclude that the neural, but not the neurohumoral, branch of the sympathetic nervous system can directly stimulate ghrelin secretion.

Increased galanin expression in the celiac ganglion of BB diabetic rats

BB rats lose >50% of their islet sympathetic nerve terminals soon after diabetes onset, markedly impairing the glucagon response to activation of these nerves. In this study, we sought evidence that this degree of disease-induced nerve terminal damage affected their neuronal cell bodies. Increased galanin expression was used as a marker of the change of phenotype that occurs in neuronal cell bodies when their axons are severely damaged. The celiac ganglion (CG) was analyzed because it is a major source of the sympathetic nerves that project to the pancreatic islets. But we first needed to determine if damaging nerve terminals could increase galanin expression in this ganglion and, if so, when that expression was maximal. Severe, global nerve terminal damage produced a dramatic increase of CG galanin expression which was maximal 5 days later. We next determined if a global, but partial, nerve terminal loss would also increase galanin expression and found a significant increase of galanin mRNA and its peptide in the CG. Finally, we determined if the disease-induced, partial and islet-selective loss of nerve terminals seen in BB diabetic rats was sufficient to increase galanin: we, again, found a significant increase of galanin mRNA and its peptide in their CG. These increases did not occur in their superior cervical ganglia. We conclude that the selective damage to islet sympathetic nerve terminals seen in BB diabetic rats, rather than the systemic factors of diabetic hyperglycemia or insulin deficiency, causes the increased galanin expression observed in the CG of this animal model of type 1 diabetes.

Impaired glucagon response to sympathetic nerve stimulation in the BB diabetic rat: effect of early sympathetic islet neuropathy

We investigated the functional impact of a recently described islet-specific loss of sympathetic nerves that occurs soon after the autoimmune destruction of beta-cells in the BB diabetic rat (35). We found that the portal venous (PV) glucagon response to sympathetic nerve stimulation (SNS) was markedly impaired in newly diabetic BB rats (BB D). We next found a normal glucagon response to intravenous epinephrine in BB D, eliminating the possibility of a generalized secretory defect of the BB D alpha-cell as the mediator of the impaired glucagon response to SNS. We then sought to determine whether the glucagon impairment to SNS in BB D was due solely to their loss of islet sympathetic nerve terminals or whether other effects of autoimmune diabetes contributed. We therefore reproduced, in nondiabetic Wistar rats, an islet nerve terminal loss similar to that in BB D with systemic administration of the sympathetic neurotoxin 6-hydroxydopamine. The impairment of the glucagon response to SNS in these chemically denervated, nondiabetic rats was similar to that in the spontaneously denervated BB D. We conclude that the early sympathetic islet neuropathy of BB D causes a functional defect of the sympathetic pathway to the alpha-cell that can, by itself, account for the impaired glucagon response to postganglionic SNS.

Autonomic mechanism and defects in the glucagon response to insulin-induced hypoglycaemia

In summary, this article briefly reviews the evidence that three separate autonomic inputs to the islet are capable of stimulating glucagon secretion and that each is activated during IIH. We have reviewed our evidence that these autonomic inputs mediate the glucagon response to IIH, both in non-diabetic animals and humans. Finally, we outline our new preliminary data suggesting an eSIN in an autoimmune animal model of T1DM. We conclude that the glucagon response to IIH is autonomically mediated in non-diabetic animals and humans. We further suggest that at least one of these autonomic inputs, the sympathetic innervation of the islet, is diminished in autoimmune T1DM. These data raise the novel possibility that an autonomic defect contributes to the loss of the glucagon response to IIH in T1DM.

Early, selective, and marked loss of sympathetic nerves from the islets of BioBreeder diabetic rats

To discover whether islet sympathetic nerves are damaged during the autoimmune destruction of islet B-cells, we immunostained sections of pancreas from BioBreeder (BB) diabetic rats, using antibodies against vesicular monoamine transporter 2 (VMAT2), a marker of sympathetic nerve terminals. We found a marked decrease in the VMAT2-positive fiber area in the islets of BB rats that had been diabetic for only 1-2 weeks compared with their nondiabetic controls. In contrast, there was no significant decrease in the VMAT2-positive fiber area in the exocrine pancreas in these early diabetic BB rats. Furthermore, streptozotocin-diabetic rats showed no decrease in VMAT2-positive fiber area in their islets compared with controls. The classical diabetic autonomic neuropathy (DAN) that eventually occurs in the heart was not present in BB diabetic rats at this early stage as evidenced by normal cardiac VMAT2 immunostaining and normal cardiac norepinephrine content. Also, in contrast to DAN, this islet neuropathy did not worsen with duration of diabetes. These data provide evidence of a heretofore unrecognized early sympathetic islet neuropathy (eSIN). Because eSIN occurs selectively in the islet, is rapid in onset, and is associated with autoimmune but not chemically induced diabetes, it is distinct from DAN in location, time course, and mechanism.

Fos expression in rat celiac ganglion: an index of the activation of postganglionic sympathetic nerves

To develop an index of the activation of abdominal sympathetic nerves, we used Fos immunostaining of the celiac ganglion (CG) taken from rats receiving nicotine, preganglionic nerve stimulation, or glucopenic agents. Subcutaneous nicotine injection moderately increased Fos expression in the principal ganglionic cells of the CG (17 +/- 4 Fos+ per mm(2), approximately 12% of all principal CG cells), whereas subcutaneous saline had no effect (0 +/- 0 Fos+ per mm(2); n = 7; P < 0.01). Greater Fos expression was obtained by applying nicotine topically to the CG (71 +/- 8 Fos+ per mm(2); 52% of all principal CG cells, n = 5; P < 0.01 vs. topical saline, n = 4) and by preganglionic nerve stimulation (126 +/- 9 Fos+ per mm(2); 94% of all principal CG cells, n = 11; P < 0.01 vs. nerve isolation, n = 7). Moderate Fos expression was also observed in the CG after intraperitoneal 2-deoxy-D-glucose (2DG) injection (21 +/- 2 Fos+ per mm(2); 16% of all principal CG cells, n = 5; P < 0.01 vs. saline ip) or insulin injection (16 +/- 2 Fos+ per mm(2); 12% of all principal CG cells, n = 6; P < 0.01 vs. saline ip). Furthermore, Fos expression induced by 2DG was dose and time dependent. These data demonstrate significant Fos expression in the CG in response to chemical, electrical, and reflexive stimulation. Thus Fos expression in the CG may be a useful index to describe various levels of activation of its postganglionic sympathetic neurons.

Parasympathetic inhibition of sympathetic neural activity to the pancreas

The present study tested the hypothesis that activation of the parasympathetic nervous system could attenuate sympathetic activation to the pancreas. To test this hypothesis, we measured pancreatic norepinephrine (NE) spillover (PNESO) in anesthetized dogs during bilateral thoracic sympathetic nerve stimulation (SNS; 8 Hz, 1 ms, 10 mA, 10 min) with and without (randomized design) simultaneous bilateral cervical vagal nerve stimulation (VNS; 8 Hz, 1 ms, 10 mA, 10 min). During SNS alone, PNESO increased from the baseline of 431 +/- 88 pg/min to an average of 5,137 +/- 1,075 pg/min (P < 0.05) over the stimulation period. Simultaneous SNS and VNS resulted in a significantly (P < 0.01) decreased PNESO response [from 411 +/- 61 to an average of 2,760 +/- 1,005 pg/min (P < 0.05) over the stimulation period], compared with SNS alone. Arterial NE levels increased during SNS alone from 130 +/- 11 to approximately 600 pg/ml (P < 0.05); simultaneous SNS and VNS produced a significantly (P < 0.05) smaller response (142 +/- 17 to 330 pg/ml). Muscarinic blockade could not prevent the effect of VNS from reducing the increase in PNESO or arterial NE in response to SNS. It is concluded that parasympathetic neural activity opposes sympathetic neural activity not only at the level of the islet but also at the level of the nerves. This neural inhibition is not mediated via muscarinic mechanisms.

Meal-induced insulin secretion in dogs is mediated by both branches of the autonomic nervous system

We investigated the relationship between autonomic activity to the pancreas and insulin secretion in chronically catheterized dogs when food was shown, during eating, and during the early absorptive period. Pancreatic polypeptide (PP) output, pancreatic norepinephrine spillover (PNESO), and arterial epinephrine (Epi) were measured as indexes for parasympathetic and sympathetic nervous activity to the pancreas and for adrenal medullary activity, respectively. The relation between autonomic activity and insulin secretion was confirmed by autonomic blockade. Showing food to dogs initiated a transient increase in insulin secretion without changing PP output or PNESO. Epi did increase, suggesting beta(2)-adrenergic mediation, which was confirmed by beta-adrenoceptor blockade. Eating initiated a second transient insulin response, which was only totally abolished by combined muscarinic and beta-adrenoceptor blockade. During absorption, insulin increased to a plateau. PP output showed the same pattern, suggesting parasympathetic mediation. PNESO decreased by 50%, suggesting withdrawal of inhibitory sympathetic neural tone. We conclude that 1) the insulin response to showing food is mediated by the beta(2)-adrenergic effect of Epi, 2) the insulin response to eating is mediated both by parasympathetic muscarinic stimulation and by the beta(2)-adrenergic effect of Epi, and 3) the insulin response during early absorption is mediated by parasympathetic activation, with possible contribution of withdrawal of sympathetic neural tone.

Differential action of hepatic sympathetic neuropeptides: metabolic action of galanin, vascular action of NPY

Activation of hepatic nerves increases both hepatic glucose production (HGP) and hepatic arterial vasoconstriction, the latter best described by a decrease of hepatic arterial conductance (HAC). Because activation of canine hepatic nerves releases the neuropeptides galanin and neuropeptide Y (NPY) as well as the classical neurotransmitter norepinephrine (NE), we sought to determine the relative role of these neuropeptides vs. norepinephrine in mediating metabolic and vascular responses of the liver. We studied the effects of local exogenous infusions of galanin and NPY on HGP and HAC to predict the metabolic and vascular function of endogenously released neuropeptide. Galanin (n = 8) or NPY (n = 4) was infused with and without NE directly into the common hepatic artery of halothane-anesthetized dogs, and we measured changes in HGP and HAC. A low dose of exogenous galanin infused directly into the hepatic artery potentiated the HGP response to NE yet had little effect on HGP when infused alone. The same dose of galanin infused into a peripheral vein (n = 8) did not potentiate the HGP response to NE, suggesting that the locally infused galanin acted directly on the liver to modulate NE's metabolic action. In contrast, a large dose of exogenous NPY failed to influence HGP when infused either alone or in combination with NE. Finally, NPY, but not galanin, tended to decrease HAC when infused alone; neither neuropeptide potentiated the HAC response to NE. Therefore, both hepatic neuropeptides may contribute to the action of sympathetic nerves on liver metabolism and blood flow. It is likely that endogenous hepatic galanin acts directly on the liver to selectively modulate norepinephrine's metabolic action, whereas endogenous hepatic NPY acts independently of NE to cause vasoconstriction.

The canine sympathetic neuropeptide galanin: a neurotransmitter in pancreas, a neuromodulator in liver

Our laboratory has investigated the role of the neuropeptide galanin in the sympathetic neural control of both the canine endocrine pancreas and liver. Galanin mRNA and peptide were found in the neuronal cell bodies of the celiac ganglion, which projects fibers to both organs. Galanin fibers formed dense networks around the islets. Galanin was released from these nerves and the amount released appeared sufficient to markedly inhibit basal insulin secretion. We therefore propose that galanin is a sympathetic neurotransmitter in canine endocrine pancreas. Galanin was also found in hepatic nerves usually co-localized with tyrosine hydroxylase, a sympathetic marker. Further, intraportal administration of the sympathetic neurotoxin, 6-hydroxydopamine, abolished galanin staining in the hepatic parenchyma. We evaluated the role of galanin in mediating the actions of sympathetic nerves to increase hepatic glucose production and decrease hepatic arterial conductance. Local infusion of synthetic galanin had little effect by itself, but it did potentiate the action of norepinephrine to stimulate hepatic glucose production, demonstrating a neuromodulatory action. In contrast, galanin had no effect on hepatic arterial blood flow. We therefore propose that in the liver galanin functions as a neuromodulator of norepinephrine's metabolic action.

Galanin is localized in sympathetic neurons of the dog liver

Stimulation of canine hepatic nerves releases the neuropeptide galanin from the liver; therefore, galanin may be a sympathetic neurotransmitter in the dog liver. To test this hypothesis, we used immunocytochemistry to determine if galanin is localized in hepatic sympathetic nerves and we used hepatic sympathetic denervation to verify such localization. Liver sections from dogs were immunostained for both galanin and the sympathetic enzyme marker tyrosine hydroxylase (TH). Galanin-like immunoreactivity (GALIR) was colocalized with TH in many axons of nerve trunks as well as individual nerve fibers located both in the stroma of hepatic blood vessels and in the liver parenchyma. Neither galanin- nor TH-positive cell bodies were observed. Intraportal 6-hydroxydopamine (6-OHDA) infusion, a treatment that selectively destroys hepatic adrenergic nerve terminals, abolished the GALIR staining in parenchymal neurons but only moderately diminished the GALIR staining in the nerve fibers around blood vessels. To confirm that 6-OHDA pretreatment proportionally depleted galanin and norepinephrine in the liver, we measured both the liver content and the hepatic nerve-stimulated spillover of galanin and norepinephrine from the liver. Pretreatment with 6-OHDA reduced the content and spillover of both galanin and norepinephrine by > 90%. Together, these results indicate that galanin in dog liver is primarily colocalized with norepinephrine in sympathetic nerves and may therefore function as a hepatic sympathetic neurotransmitter.

Activation of hepatic sympathetic nerves during hypoxic, hypotensive and glucopenic stress

To investigate the potential for neural regulation of liver function, we sought to determine whether hepatic sympathetic nerves are activated during stress. Hepatic norepinephrine spillover (HNESO) was measured in halothane-anesthetized dogs before, during and after glucopenia, hypoxia and hemorrhage. HNESO increased during 2-deoxyglucose (2-DG, 600 mg/kg plus 13.5 mg/kg/min, IV)-induced glucopenia from a baseline of 9 +/- 3 ng/min to 83 +/- 24 ng/min (delta = + 74 +/- 23 ng/min, p < 0.01). During hypoxia (partial pressure of oxygen in arterial blood = 23 +/- 2 mmHg), HNESO increased by 142 +/- 47 ng/min (p < 0.025), and HNESO increased by 84 +/- 22 ng/min (p < 0.01) during hemorrhage (mean arterial blood pressure = 40 +/- 1 mmHg), suggesting activation of hepatic sympathetic nerves during all three stresses. To validate the use of HNESO as an index of hepatic sympathetic nerve activity, we repeated the stresses of hypoxia and hemorrhage in dogs following chemical sympathetic denervation of the liver induced by prior intraportal 6-hydroxy-dopamine infusion. Hepatic denervation reduced the HNESO responses to hypoxia and hemorrhage by more than 90%. In addition to hepatic neural responses to stress, the sympathetic responses of the adrenal medulla and of systemic sympathetic nerves were monitored using changes in the arterial concentration of epinephrine and norepinephrine, respectively. Arterial epinephrine and norepinephrine increased by varying degrees during all three stresses, suggesting general sympatho-adrenal activation. As expected, 6-hydroxydopamine pretreatment did not alter the epinephrine response to hypoxia or hemorrhage. The arterial norepinephrine responses to hypoxia and hemorrhage were modestly reduced in hepatically sympathectomized animals, suggesting a small hepatic contribution to the elevated arterial level of norepinephrine during these stresses. We conclude that: (1) the stresses of glucopenia, hypoxia and hemorrhage activate the sympathetic nerves of the liver and (2) HNESO is a valid index of hepatic sympathetic nerve activity. Finally, we speculate that such activation may influence liver function.

Pancreatic sympathetic nerves contribute to increased glucagon secretion during severe hypoglycemia in dogs

To determine if pancreatic sympathetic nerves can contribute to increased glucagon secretion during hypoglycemia, plasma glucagon and pancreatic glucagon secretion in situ were measured before and during insulin-induced hypoglycemia in three groups of halothane-anesthetized dogs. All dogs were bilaterally vagotomized to eliminate the input from pancreatic parasympathetic nerves. One group of dogs received only vagotomy (VAGX). A second group was vagotomized and adrenalectomized (VAGX + ADX). A third group received vagotomy, adrenalectomy, plus surgical denervation of the pancreas (VAGX + ADX + NERVX) to prevent activation of pancreatic sympathetic nerves. In dogs with VAGX only, hypoglycemia increased plasma epinephrine (Epi), pancreatic norepinephrine (NE) output (+320 +/- 140 pg/min, P < 0.05), arterial plasma glucagon (+28 +/- 12 pg/ml, P < 0.01), and pancreatic glucagon output (+1,470 +/- 370 pg/min, P < 0.01). The addition of ADX eliminated the increase of Epi but did not increase pancreatic NE output (+370 +/- 190 pg/min, P < 0.025), arterial plasma glucagon (+20 +/- 5 pg/ml, P < 0.01), or pancreatic glucagon output (+810 +/- 200 pg/min, P < 0.01). In contrast, the addition of pancreatic denervation eliminated the increase of pancreatic NE output (-20 +/- 40 pg/min, P < 0.05 vs. VAGX), the arterial glucagon (+1 +/- 2 pg/ml, P < 0.01 vs. VAGX), and pancreatic glucagon output responses (+210 +/- 280 pg/min, P < 0.025 vs. VAGX) to hypoglycemia. Thus activation of pancreatic sympathetic nerves can contribute to the increased glucagon secretion during severe insulin-induced hypoglycemia in dogs.

Corelease of galanin and NE from pancreatic sympathetic nerves during severe hypoglycemia in dogs

To determine whether norepinephrine (NE) and galanin are coreleased during reflex activation of the sympathetic nervous system by hypoglycemia, we administered insulin to halothane-anesthetized (0.8%) dogs and measured the spillover of NE and galanin-like immunoreactivity (GLIR) into pancreatic venous plasma. Insulin injection produced hypoglycemia [plasma glucose (PG) = 34 +/- 3 mg/dl] but did not activate pancreatic noradrenergic (delta pancreatic NE output = +20 +/- 130 pg/min) or galaninergic nerves (delta GLIR output = +40 +/- 50 fmol/min). To determine whether more severe hypoglycemia would activate these nerves, insulin was administered to dogs infused with somatostatin (SS; 2.5 micrograms/min) to block the counterregulatory increase of glucagon secretion. SS reduced the glucagon response to hypoglycemia by greater than 90%, which allowed PG to decrease to 14 +/- 1 mg/dl. Pancreatic NE output increased by 470 +/- 140 pg/min (P less than 0.005); however, pancreatic GLIR output did not increase significantly (delta = +70 +/- 50 fmol/min). When SS was discontinued, pancreatic NE output increased by 490 +/- 200 pg/min (P less than 0.025), and GLIR output increased by an additional +160 +/- 70 fmol/min (P less than 0.025; total delta from baseline = +230 +/- 90 fmol/min, P less than 0.025), suggesting that SS may restrain pancreatic NE and galanin release. Pancreatic NE and GLIR spillover were also increased during severe hypoglycemia when ganglionic neurotransmission was partially impaired with hexamethonium but not when the neural pathway was interrupted by spinal cord transection. We conclude that NE and galanin are coreleased from pancreatic sympathetic nerves when these nerves are centrally activated during severe hypoglycemia in halothane-anesthetized dogs.

Alpha 2-adrenergic regulation of galanin and norepinephrine release from canine pancreas

We found previously that electrical stimulation of the mixed autonomic pancreatic nerves (MPNS) is anesthesized dogs elicits marked and rapid increases of pancreatic output of both norepinephrine (NE) and galanin, and on that basis hypothesized a role for galanin as a sympathetic cotransmitter in the endocrine pancreas. In the present study, direct evidence was sought for the co-release of galanin and NE from canine pancreas by determining whether pancreatic galanin output is subject to modulation by presynaptic alpha 2-adrenergic mechanisms as has been established for NE. During MPNS (8 Hz, 1 ms, 10 mA, 10 min) in anesthesized dogs, both pancreatic NE and galanin outflow were increased with similar temporal patterns during consecutive stimulations. Blockade of presynaptic alpha 2-adrenoceptors with yohimbine increased and stimulation of presynaptic alpha 2-adrenoceptors with clonidine reduced NE and galanin outflow. Over all experiments, pancreatic spillover of galanin was highly correlated with that of NE. It is concluded that presynaptic alpha 2-adrenergic mechanisms modulate not only NE but also pancreatic galanin release, suggesting that galanin is co-released with NE from noradrenergic nerves in the endocrine pancreas.