Cholinergic system and synaptic vesicles alterations in brain areas of an animal model of metabolic syndrome

The metabolic syndrome (MetS) refers to the concomitant presence of obesity, dyslipidaemia, insulin resistance and hypertension, that induces Type-2 diabetes mellitus (T2DM), cardiovascular and cerebrovascular diseases. Moreover, obesity and MetS are recognised as risk factors in the development of cognitive impairment such as Alzheimer’s disease (AD) and vascular dementia (VaD). These neurodegenerative diseases affect the cholinergic system with a decrease of acetylcholine (ACh) levels and activity of biosynthetic enzyme choline acetyltransferase (ChAT). This study has investigated cholinergic system of obese Zucker rats (OZRs) compared with non -obese cohort lean Zucker rats (LZRs) to demonstrate a relationship between obesity and brain disorders. The OZRs, with a mutation of leptin receptor, represents a model of obesity related to T2DM. Male OZRs and the littermate LZRs of 12, 16 and 20 weeks of age were used. The rats were monitored for body weight, food intake, blood pressure and blood levels of triglycerides, cholesterol and glucose. The OZRs were hyperphagic, hyperlipidemic, hyperglycaemic and exhibiting a moderate degree of arterial hypertension respect to controls LZRs. Behavioural tests are performed and revealed that in OZRs there are no changes in anxiety and emotional learning tasks. In the brain, immunochemical and immunohistochemical analysis were performed for vesicular acetylcholine transporter (VAChT),ChAT, acetylcholinesterase (AChE), nicotinic (nAChRα7) receptor, muscarinic (mAChR) receptor subtypes (mAChR1, mAChR3, and mAChR5) and isoforms A, B and C of synaptic vesicle 2 proteins (SV2). SV2 are integral proteins localized on the surface of synaptic vesicles in different neurons, and are involved in exocytosis and neurotransmitter release. SV2B has been reported to control synaptic vesicle release dynamics. A decrease of this protein was found in hippocampus and cortex of AD patients. Colocalization of SV2C and ChAT was found in different brain areas. Our results confirm the presence of VAChT in the cerebral areas involved in cognitive functions, such as the frontal cortex and hippocampus, with its reduction in 20-weeks-old OZRs. Furthermore, a lower expression of AChE in OZR at the same age suggests a possible physiological compensatory mechanism. A reduction in the expression of nAChRα7 was found, and this could be involved in the modulation of the inflammatory response at the central level. Different mAChRs expressions were detected in two investigated areas with a decrease of m1AChR subtype in hippocampus in 20 weeks OZRs. It could be related to the induction of brain damage linked to obesity. Moreover a decrease expression of SV2B in frontal cortex and hippocampus of 20 weeks-old OZRs was found and this could represent an alteration of synaptic vesicle release. The results demonstrate that modulation of the cholinergic system and synaptic vesicle release in obese rats, are extremely complex phenomena. They could be related both to inflammation and processes of cognitive impairment induced by obesity