Loretta Lam

Insulin resistance and secretion in vivo: Effects of different antipsychotics in an animal model

Atypical antipsychotics now represent the mainstay of treatment for patients with schizophrenia. Unfortunately, as a class they have also been associated with an increased risk of weight gain and metabolic abnormalities, including type 2 diabetes. We have investigated the diabetogenic effects of a spectrum of antipsychotics, both atypical and typical. Healthy animals were treated acutely with clozapine (10 mg/kg), olanzapine (3.0 mg/kg), risperidone (1 mg/kg), ziprasidone (3 mg/kg) or haloperidol (0.25 mg/kg) and tested using the hyperinsulinemic-euglycemic and hyperglycemic clamp procedures. Clozapine and olanzapine had a rapid and potent effect on insulin sensitivity by lowering the glucose infusion rate and increasing hepatic glucose production. Both clozapine and olanzapine, as well as risperidone, decreased peripheral glucose utilization. Neither ziprasidone nor haloperidol had a significant impact on insulin sensitivity. In the hyperglycemic clamp, clozapine and olanzapine impaired beta cell function as reflected by a decrease in insulin secretion. Results confirm that 1) antipsychotic medications have an immediate impact on metabolic parameters and 2) the various atypical antipsychotics differ in their propensity to acutely induce metabolic side effects. Our data also support the preclinical use of these clamp procedures in screening putative antipsychotics.

Insulin Resistance and Decreased Glucose-stimulated Insulin Secretion After Acute Olanzapine Administration

The newer atypical antipsychotics, as a class, have been associated with an increased risk of weight gain and metabolic abnormalities. The mechanisms underlying this phenomenon are currently unclear, but there are data to suggest the possibility of an immediate (as opposed to chronic) effect of these drugs. The aim of the present study was to assess the acute effects of olanzapine on specific measures of insulin sensitivity and secretion. Healthy animals were tested in either the hyperinsulinemic-euglycemic or the hyperglycemic clamp. After reaching steady state in the hyperinsulinemic-euglycemic clamp, rats were injected with olanzapine (3 mg/kg sc) and monitored for an additional 130 minutes. In the hyperglycemic clamp, olanzapine was injected approximately 90 minutes before receiving a glucose bolus, and hyperglycemia was maintained via exogenous glucose infusion for an additional 90 minutes. Insulin and C-peptide levels were monitored throughout this clamp. Acute administration of olanzapine significantly lowered the glucose infusion rate due to an increase in hepatic glucose production and a decrease in glucose utilization. Olanzapine pretreatment induced hyperglycemia and markedly decreased plasma insulin and C-peptide in response to the glucose challenge. These findings indicate that olanzapine can directly induce metabolic changes that occur rapidly and well in advance of the changes that might be anticipated as a result of its weight-gain liability. We present novel findings highlighting an olanzapine-induced deficit in beta-cell functioning.

Atypical antipsychotics and effects of muscarinic, serotonergic, dopaminergic and histaminergic receptor binding on insulin secretion in vivo: An animal model

The atypical antipsychotics (AAPs) have been associated with increased risk of type-2 diabetes. Evidence suggests direct, drug-related effects independent of weight gain and although mechanisms underlying this phenomenon are unclear, it has been suggested that the heterogeneous receptor binding profile of the AAPs may influence receptors implicated in glucose metabolism. This study aimed to clarify weight gain-independent mechanisms of AAP-induced changes in insulin secretion by deconstructing their binding profile with representative antagonists. Healthy rats were pretreated with a single subcutaneous dose of darifenacin 6 mg/kg (n=10), a selective M(3) muscarinic antagonist; ketanserin 2mg/kg (n=10), a 5HT(2A) antagonist; raclopride 0.3mg/kg (n=11) a selective D(2)/D(3) antagonist; terfenadine 20mg/kg (n=9) a selective H(1) antagonist; or, vehicle (n=11). Hyperglycemic clamps were employed following injection, providing an index of secretory capacity of pancreatic β-cells. Acute treatment with darifenacin and ketanserin significantly decreased insulin response to glucose challenge as compared to controls, which was confirmed in the darifenacin group by reduced C-peptide levels. Treatment with raclopride resulted in an increased insulin response and a strong tendency to increased C-peptide levels. H(1) blockade did not result in effects on insulin or C-peptide. Results suggest that the effects of antipsychotics on glucose dysregulation may be related to direct inhibitory effects of muscarinic (M(3)) and serotonergic (5HT(2)) antagonism on insulin secretion. Based on the expression of D(2)-like receptors in β-cells, which mediate inhibition of insulin secretion, we propose that prolonged D(2) blockade with antipsychotics may predispose to depletion of insulin stores and an eventual defect in pancreatic compensation.

Atypical antipsychotics and effects of adrenergic and serotonergic receptor binding on insulin secretion in-vivo: An animal model

Atypical antipsychotics (AAPs) are associated with several metabolic sequelae including increased risk of type 2 diabetes. Growing evidence points to a direct drug effect of these compounds on glucose homeostasis, independent of weight gain. While the responsible mechanisms have yet to be elucidated, the heterogeneous binding profiles of AAPs likely include receptors involved in glucose metabolism. This study aimed to clarify weight-gain independent mechanisms of AAP-induced alterations in insulin secretion. Deconstruction of the receptor binding profiles of these agents was done using representative antagonists. Healthy rats were pre-treated with a single subcutaneous dose of prazosin 0.25mg/kg (n = 16), a selective α1 antagonist; idazoxan 0.5mg/kg (n = 10), a selective α2 antagonist; SB242084 0.5mg/kg (n = 10), a selective 5HT2C antagonist; WAY100635 0.1mg/kg (n = 10), a selective 5HT1A antagonist; MDL100907 0.5mg/kg (n = 8), a selective 5HT2A antagonist; or vehicle: 0.9% NaCl saline (n = 8), DMSO (n = 8), or cyclodextrin (n = 5). Hyperglycemic clamps were employed following injection, providing an index of secretory capacity of pancreatic β-cells. Treatment with prazosin and MDL100907 resulted in significant decreases in both insulin and C-peptide secretion compared to their respective controls, DMSO and saline. These findings were corroborated with decreased glucose infusion rate and disposition index in the prazosin group. Results suggest that α1 and 5HT2A receptor antagonism may be involved in glucose dysregulation with AAP treatment, however, the exact mechanisms involved remain unknown.

Effects of intracerebroventricular (ICV) olanzapine on insulin sensitivity and secretion in vivo: An animal model

The atypical antipsychotics (AAPs) have been associated with an increased risk of type 2 diabetes. While weight gain associated with AAPs is a risk factor for diabetes, preclinical work suggests that among these medications, olanzapine, when given peripherally in a single dose, causes pronounced effects on insulin sensitivity and secretion. Given a critical role of the hypothalamus in control of glucose metabolism, we examined the effect of central administration of olanzapine. Sprague-Dawley rats were treated with a single 75 μg intracerebroventricular (ICV) dose of olanzapine and tested using separate hyperinsulinemic-euglycemic and hyperglycemic clamps. Dosing of olanzapine was established based on inhibition of amphetamine-induced locomotion. In contrast to the single dosing peripheral paradigm, there was no effect of central olanzapine on insulin sensitivity, either with respect to hepatic glucose production or peripheral glucose uptake. Analogous to the peripheral model, a single ICV dose of olanzapine followed by the hyperglycemic clamp decreased insulin (p=0.0041) and C-peptide response (p=0.0039) to glucose challenge as compared to vehicle, mirrored also by a decrease in the steady state glucose infusion rate required to maintain hyperglycemia (p=0.002). In conclusion, we demonstrate novel findings that at least part of the effect of olanzapine on beta-cell function in vivo is central.

Insulin inhibits and oral sucrose increases neointimal growth after arterial injury in rats.

Background/Aims: In our previous studies, rats on insulin treatment (5 U/day) and oral glucose to avoid hypoglycemia had reduced neointimal growth after arterial injury. However, plasma glucose in the insulin-treated rats was lower than normal and the effect of oral glucose remained undetermined. In this study, the effects of normoglycemic hyperinsulinemia and oral glucose or sucrose were investigated in the same model. Methods: Rats were divided into 6 groups: (1) control implants and tap water; (2) insulin implants (5 U/day) and oral glucose + i.p. glucose to avoid any glucose lowering; (3) insulin implants (4 U/day) and oral glucose; (4) insulin implants (4 U/day) and oral sucrose; (5) control implants and oral glucose, and (6) control implants and oral sucrose. Results: Insulin treatment at both doses reduced neointimal area (p < 0.001) 14 days after injury in rats receiving oral glucose but not in those receiving oral sucrose. Oral glucose, without insulin, had no effect on neointimal formation, whereas oral sucrose increased neointimal growth (p < 0.05). Oral sucrose (p < 0.05) but not oral glucose decreased insulin sensitivity measured with hyperinsulinemic clamps. Conclusions: (1) Insulin decreases neointimal growth after arterial injury independent of glucose-lowering or oral glucose administration and (2) oral sucrose per se affects neointimal growth.

In vivo Effect of Insulin to Decrease Matrix Metalloproteinase-2 and -9 Activity after Arterial Injury

In vitro, insulin has both growth-promoting and vasculoprotective effects. In vivo, the effect of insulin is mainly protective. Insulin treatment (3 U/day) decreases smooth muscle cell (SMC) migration and neointimal growth after carotid angioplasty in normal rats maintained at normoglycemia by oral glucose. SMC migration requires limited proteolysis of the extracellular matrix, which is mediated by matrix metalloproteinases (MMPs). In this study, we investigated the effects of normoglycemic hyperinsulinemia on MMP activity after balloon angioplasty. Rats were divided into three groups: (1) control implants and tap water; (2) control implants and oral glucose, and (3) insulin implants (3 U/day) and oral glucose. Results: Gelatin zymography revealed that insulin reduced the gelatinolytic activity of pro-MMP-2 by 46% (p < 0.05), MMP-2 by 44% (p < 0.05) and MMP-9 by 51% (p < 0.05) compared to controls after arterial injury. Insulin also reduced mRNA levels of MMP-2 (p < 0.05) and MMP-9 (p < 0.05) and protein levels of MMP-2 (p < 0.05). In contrast, there were no significant changes in membrane-type 1 MMP protein and tissue inhibitors of MMP activity after insulin treatment. Thus, these results suggest a mechanism by which insulin inhibits SMC migration and supports a vasculoprotective role for insulin in vivo.

In male rats, the ability of central insulin to suppress glucose production is impaired by olanzapine, whereas glucose uptake is left intact

Background Insulin receptors are widely expressed in the brain and may represent a crossroad between metabolic and cognitive disorders. Although antipsychotics, such as olanzapine, are the cornerstone treatment for schizophrenia, they are associated with high rates of type 2 diabetes and lack efficacy for illness-related cognitive deficits. Historically, this risk of diabetes was attributed to the weight gain propensity of antipsychotics, but recent work suggests antipsychotics can have weight-independent diabetogenic effects involving unknown brain-mediated mechanisms. Here, we examined whether antipsychotics disrupt central insulin action, hypothesizing that olanzapine would impair the well-established ability of central insulin to supress hepatic glucose production. Methods Pancreatic euglycemic clamps were used to measure glucose kinetics alongside a central infusion of insulin or vehicle into the third ventricle. Male rats were pretreated with olanzapine or vehicle per our established model of acute olanzapine-induced peripheral insulin resistance. Groups included (central–peripheral) vehicle–vehicle (n = 11), insulin–vehicle (n = 10), insulin–olanzapine (n = 10) and vehicle–olanzapine (n = 8). Results There were no differences in peripheral glucose or insulin levels. Unexpectedly, we showed that central insulin increased glucose uptake, and this effect was not perturbed by olanzapine. We replicated suppression of glucose production by insulin (clamp relative to basal: 77.9% ± 13.1%, all p < 0.05), an effect abolished by olanzapine (insulin–olanzapine: 7.7% ± 14%). Limitations This study used only male rats and an acute dose of olanzapine. Conclusion To our knowledge, this is the first study suggesting olanzapine may impair central insulin sensing, elucidating a potential mechanism of antipsychotic-induced diabetes and opening avenues of investigation related to domains of schizophrenia psychopathology.

High-dose metformin (420 mg/kg daily p.o.) increases insulin sensitivity but does not affect neointimal thickness in the rat carotid balloon injury model of restenosis

OBJECTIVE Our laboratory has shown that insulins effect to decrease neointimal thickness after arterial injury is greatly diminished in insulin resistant conditions. Thus, in these conditions, a better alternative to insulin could be to use an insulin sensitizing agent. Metformin, the most commonly prescribed insulin sensitizer, has a cardiovascular protective role. Therefore, the objective of this study was to investigate the potential benefit of metformin on neointimal area after arterial injury in a rat model of restenosis. METHODS Rats fed with either normal or high fat diet and treated with or without oral metformin (420mg/kg daily) underwent carotid balloon injury. Effects of metformin on clamp-determined insulin sensitivity, vessel AMPK (AMP-activated protein kinase) phosphorylation (activation marker) and neointimal area were evaluated. RESULTS Metformin increased insulin sensitivity, but did not affect neointimal thickness in either the normal fat or high fat diet-fed rats. Furthermore, metformin activated AMPK in uninjured but not in injured vessels. Similarly, 10mmol/L metformin inhibited proliferation and activated AMPK in smooth muscle cells of uninjured but not injured vessels, whereas 2mmol/L metformin did not have any effect. CONCLUSION In rats, metformin does not decrease neointimal growth after arterial injury, despite increasing whole body insulin sensitivity.