Probiotics treatment improves diabetes-induced impairment of synaptic activity and cognitive function: Behavioral and electrophysiological proofs for microbiome–gut–brain axis

Abstract

Diabetes mellitus-induced metabolic disturbances underlie the action of many systems including some higher functions of the brain such as learning and memory. Plenty of evidence supports the effects of probiotics on the function of many systems including the nervous system. Here we report the effect of probiotics treatment on the behavioral and electrophysiological aspects of learning and memory disorders. Diabetic rats were made through intraperitoneal injection of streptozocin. The control and diabetic rats were fed with either normal regimen (control rats recieving normal regimen (CO) and diabetics rats receiving normal regimen (DC), respectively) or normal regimen plus probiotic supplementation for 2 months (control rats receiving probiotic supplementation (CP) and diabetics rats recieving probiotic supplementation (DP), respectively). The animals were first introduced to spatial learning task in the Morris water maze. Then, in electrophysiological experiments, stimulating the Schaffer collaterals the basic and potentiated excitatory postsynaptic potential (EPSPs) were recorded in the CA1 area of the hippocampus. Finally, the serum levels of glucose, insulin, superoxide dismutase and 8-hydroxy-2′-deoxyguanosine (8-OHdG) were measured. We found that probiotics administration considerably improved the impaired spatial memory in the diabetic animals. The probiotics supplementation in the diabetic rats recovered the declined basic synaptic transmission and further restored the hippocampal long-term potentiation (LTP). While the probiotics administration enhanced the activation of superoxide dismutase and increased the insulin level of serum it decreased both the glucose level of serum and the 8-OHdG factor. From the present results we concluded that probiotics efficiently reverse deteriorated brain functions in the levels of cognitive performances and their proposed synaptic mechanisms in diabetes mellitus. These considerations imply on the necessity of an optimal function of the microbiome–gut–brain axis in the behavioral as well as electrophysiological aspects of brain action.

Introduction

Diabetes mellitus is a metabolic disease caused by absolute or relative insulin deficiency (Vicentini et al., 2011). It is well documented that oxidative stress is a basic mechanism behind the development of the diabetic state (Aly and Mantawy, 2012). Hyperglycemia in diabetes mellitus (DM) may be one of the most important factors responsible for the development of oxidative stress, which promotes the main complications in DM patients (Vicentini et al., 2011). An imbalance between oxidants and antioxidants in favor of the oxidants, potentially leading to damage, is termed ‘oxidative stress’ (Sies, 1997). Oxidative stress and the damage that it causes have been implicated in a wide variety of natural and pathological processes, including diabetes, aging, cancer, atherosclerosis, neurological degeneration, schizophrenia, and autoimmune disorders such as arthritis (Shackelford et al., 2000). Oxidative stress can be derived from a variety of sources that include events such as the production of reactive oxygen species by mitochondrial oxidative phosphorylation, ionizing radiation exposure, and metabolism of exogenous compounds (Shackelford et al., 2000). In addition to these sources of oxidative stress, a decrease in the activity of antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase, and catalase may contribute to some disease states (Sagara et al., 1998).

Several biomarkers to estimate oxidative stress have been suggested, however, most of them have failed to reach clinical significance. One successful discovery of the late 1980s was the level of 8-hydroxydeoxyguanosine (8-hydroxy-2′-deoxyguanosine; 8-OHdG) which has been proven to be increased in the serum or urine of patients who have oxidative stress-associated disease (Lunec et al., 2002). Reactive oxygen species attacks guanine bases in DNA easily and form 8-OHdG which can bind to thymidine rather than cytosine. The level of 8-OHdG is generally regarded as a biomarker of mutagenesis consequent to oxidative stress (Ock et al., 2012).

Moderate disturbances of learning and memory and complex information processes have been reported in both type 1 and 2 diabetic patients (Biessels and Gispen, 2005). Evidence indicates that oxidative stresses produced in diabetes cause both memory and cognitive disorders (Selvarajah et al., 2011).

Long-term potentiation (LTP) and long-term depression (LTD) are activity-dependent changes in synaptic efficacy which have been considered for cellular mechanisms of learning and memory (Luscher and Malenka, 2012). The disturbed hippocampal LTP and LTD in streptozotocin (STZ)-induced rats has been reported to be correlated well with the learning disturbance (Kamal et al., 2000).

Probiotics are live microorganisms which, when administered in adequate amounts, confer a health benefit on the host (Guarner et al., 2005). Widely accepted probiotics contain different lactic acid producing bacteria of human origin including bifidobacteria, lactobacilli or enterococci (Songisepp et al., 2005). In order to improve the efficacy of probiotics, combinations of different bacterial strains can be used (Karimi and Pena, 2003). A common choice is a mixture of Lactobacillus and Bifidobacteria (Karimi and Pena, 2003). Probiotics are used in the treatment of a range of diseases such as infections, allergies, and inflammatory disorders (Isolauri and Salminen, 2008).

Through decreased inflammatory damages and increased level of antioxidant enzymes such as SOD and glutathione peroxidase probiotics suppress the production of the oxidative stresses (D’Souza et al., 2010).

STZ-induced DM offers a very cost-effective and expeditious technique that can be used in most strains of rodents, opening the field of DM research to an array of genotypic and phenotypic options that would otherwise be inaccessible (Deeds et al., 2011).

In the present study we asked several questions: Does a food regimen containing probiotics affect the serum level of glucose and insulin? How do the behavioral tasks respond to the probiotics treatment? What is the effect of the probiotics administration on the induction of LTP?