Mechanism of the neuroprotective effect of GLP-1 in a rat model of Parkinson's with pre-existing diabetes

Several studies have suggested the association between neurodegenerative diseases and diabetes mellitus (DM), DM causes cognitive impairment with age, but its effect is not well known in Parkinson's disease (PD). As a member of the incretin family, Glucagon-like peptide-1 (GLP-1) has glycemic regulation functions. It also exerts many additional effects on different tissues through its receptor's widespread expression. our aim is to investigate the effect of pre-existing diabetes on the severity of PD in male albino rats, and to find out whether GLP-1 could improve PD symptoms in diabetic animals in addition to its hypoglycemic effect, and how it could do that. 75 adult male albino rats were equally divided into: Control, Parkinson's, Diabetic Parkinson's, Diabetic Parkinson's + low dose exenatide (GLP-1 receptor agonist), Diabetic Parkinson's + high dose exenatide group. Blood glucose and insulin, striatal dopamine, some striatal oxidative stress and inflammatory markers, and the catalepsy score were measured. Pre-existing of diabetes before initiation of PD raises the severity of PD shown by the more significant increase in catalepsy score, and the more significant decrease in striatal dopamine level. GLP-1 effects extend beyond their hypoglycemic effects only since it has a direct anti-oxidant, and anti-inflammatory neuronal effect with increasing the striatal dopamine and improving the catalepsy score in a dose dependent manner. Diabetes increases the severity of impairment in PD, and GLP-1 improve it through its direct neuronal effect in addition to its indirect effect through producing hypoglycemia.