Failure of the Brain Glucagon-Like Peptide-1-Mediated Control of Intestinal Redox Homeostasis in a Rat Model of Sporadic Alzheimer’s Disease

Abstract

The gastrointestinal system may be involved in the etiopathogenesis of the insulin-resistant brain state (IRBS) and Alzheimer’s disease (AD). Gastrointestinal hormone glucagon-like peptide-1 (GLP-1) is being explored as a potential therapy as activation of brain GLP-1 receptors (GLP-1R) exerts neuroprotection and controls peripheral metabolism. Intracerebroventricular administration of streptozotocin (STZ-icv) is used to model IRBS and GLP-1 dyshomeostasis seems to be involved in the development of neuropathological changes. The aim was to explore (i) gastrointestinal homeostasis in the STZ-icv model (ii) assess whether the brain GLP-1 is involved in the regulation of gastrointestinal redox homeostasis and (iii) analyze whether brain-gut GLP-1 axis is functional in the STZ-icv animals. Acute intracerebroventricular treatment with exendin-3(9-39)amide was used for pharmacological inhibition of brain GLP-1R in the control and STZ-icv rats, and oxidative stress was assessed in plasma, duodenum and ileum. Acute inhibition of brain GLP-1R increased plasma oxidative stress. TBARS were increased, and low molecular weight thiols (LMWT), protein sulfhydryls (SH), and superoxide dismutase (SOD) were decreased in the duodenum, but not in the ileum of the controls. In the STZ-icv, TBARS and CAT were increased, LMWT and SH were decreased at baseline, and no further increment of oxidative stress was observed upon central GLP-1R inhibition. The presented results indicate that (i) oxidative stress is increased in the duodenum of the STZ-icv rat model of AD, (ii) brain GLP-1R signaling is involved in systemic redox regulation, (iii) brain-gut GLP-1 axis regulates duodenal, but not ileal redox homeostasis, and iv) brain-gut GLP-1 axis is dysfunctional in the STZ-icv model.

The effect of acute oral galactose administration on the redox system of the rat small intestine

“…Lipid peroxidation was assessed based on quantification of lipid peroxidation end products using the thiobarbituric acid reactive substances (TBARS) assay as described previously [5,23]. Supernatants of tissue homogenates (12 μL) were mixed with 120 μL of the TBA-TCA reagent (0.038% thiobarbituric acid (Kemika, Croatia) in 15% trichloroacetic acid (Merck, USA).…”

“…Nitrocellulose redox permanganometry (NRP) was utilized for determination of tissue reductive capacity [5,23,32,33]. 1 μL of each homogenate was loaded onto the nitrocellulose membrane (Amersham Protran 0.45; GE Healthcare Life Sciences, Chicago, IL, USA) and left to dry out at room temperature.…”

“…Superoxide dismutase (SOD) activity was determined based on the inhibition of the 1,2,3-trihydroxybenzene (THB) autooxidation rate [5,23,36]. 3 μL of each sample was incubated with 100 μL of the reaction buffer containing 80 μL of the THB solution (60 mM THB dissolved in 1 mM HCl) added to 4000 μL of the SOD reaction buffer (0.05 M Tris-HCl, and 1 mM Na2EDTA (pH 8.2)) or MnSOD reaction buffer (0.05 M Tris-HCl, 1 mM Na2EDTA, 2mM KCN (pH 8.2)) right before incubation.…”

“…Catalase (CAT) activity was determined by measuring the dissociation rate of H2O2 using the adaptation of the method proposed by Hadwan [5,23,40]. 18 μL of each homogenate was incubated with 40 μL of 10 mM H2O2 in 1 x PBS and the reaction was stopped at t0 = 0 s (to assess baseline concentration of H2O2) and t1 = 60 s (to assess the final concentration of H2O2) with 100 μL of the Co(NO3)2 working solution (5 mL of Co(NO3)2 x 6 H2O (0.2 g dissolved in 10 mL ddH2O) mixed with 5 mL of (NaPO3)6 (0.1 g dissolved in 10 mL ddH2O) added to 90 mL of NaHCO3 (9 g dissolved in 100 mL ddH2O)).…”

“…Importantly, in contrast to the majority of studies utilizing galactose for the induction of agingrelated pathophysiological processes in which galactose was standardly administered via the parenteral route, health-promoting effects of galactose in the rat model of sAD have been repeatedly observed following long-term administration of galactose dissolved in drinking water (200 mg/kg/day) and available ad libitum [20,21] indicating that i) the gastrointestinal tract might be a critical factor mediating the observed beneficial effects, and ii) galactose might be beneficial only in small "tolerable" doses when tissue galactose metabolic capacity is not saturated. The possibility that the gastrointestinal tract mediates the protective effects of galactose is especially interesting in the context of recent findings of intestinal redox dyshomeostasis [23], dysfunctional cell turnover and suppressed apoptosis of the intestinal epithelium [24], and altered function of the brain-gut GLP-1 [23] and glucose-dependent insulinotropic polypeptide (GIP) [24] axes in the STZ-icv rat model of sAD. Considering the increasingly recognized importance of the gastrointestinal tract in the etiopathogenesis of neurodegeneration [25][26][27][28], the relevance of gut redox homeostasis for the maintenance of intestinal barrier and function [29][30][31], and a unique biochemical role of galactose in the context of redox regulation [5], the present aim was to explore the effects of acute oral administration of D-galactose on the individual mediators of redox homeostasis and their relationships in the rat small intestine.…”