Effect of exogenous Apelin-13 via Neuropeptide Y on neuroprotective, learning and memory in glutamate excitotoxicity

Neurodegenerative diseases (ND) are associated with progressive neuronal network loss in the central or peripheral nervous system and are characterized by loss of learning-memory, behavior, sensory and/or motor functions. Excessive glutamate-mediated excitotoxicity plays an important role in the etiology of ND by causing calcium dyshomeostasis, mitochondrial dysfunction, apoptosis and neuronal death. The aim of our study is to investigate whether apelin-13 and NPY activate common signaling pathways and play a neuroprotective role against excitotoxic damage and learning-memory impairments. In our study, a D-Glutamic acid-mediated excitotoxicity model was created in 42 male Sprague Dawley rats, 6-8 weeks old and weighing 200-250 g. Animals were divided into 6 experimental groups, 7 in each group, as Control ((K)-%0.9 NaCl, i.p), D-Glutamic Acid ((D)-4 mg/kg, i.p), Apelin-13 ((A)-300 µg/kg, i.p), D-Glutamic Acid+Apelin-13 (GA), GA+NPY2 receptor antagonist ((GAN2)-1.5 mg/kg, i.p) and GA+NPY5 (1.5 mg/kg, i.p). Following the injection, learning-memory performance and locomotor activity were evaluated with Novel object recognition, Open field, Morris water maze tests, and animals were sacrificed on the 22nd day according to ethical rules. Serum, hippocampus and prefrontal cortex tissues were taken and biochemical and histochemical analyses were performed, and learning-memory tests were evaluated. In group D, an increase in Caspase-3 level, a very significant decrease in Extracellular Signal Regulatory Kinase (ERK1/2) and Protein Kinase B-1 (AKT-1) levels (p<0.001), an increase in oxidant parameters related to oxidative stress and a decrease in antioxidant parameters indicated neurodegeneration due to mitochondrial dysfunction caused by excitotoxicity. An increase in the number and latency of target quadrant entries in Morris water maze (p<0.001), a decrease in the discrimination index of the novel object recognition test (p<0.001) were found, and learning-memory performance was impaired, supported by histochemical staining and scoring. Apelin-13 played a neuroprotective role by alleviating damage in the GA group and could not activate its role in the presence of NPY2R antagonist, and since NPY2R activation was not prevented when NPY5R antagonist was administered, a significant level of protection was detected in the GAN5 group compared to GAN2. Learning-memory performances, histochemical staining-scorings also showed that Apelin-13's protection was mediated by NPY2R. It is possible to say that Apelin-13's neuroprotective effects in D-Glutamic acid-mediated excitotoxicity pathophysiology are achieved by reducing oxidative stress, inhibiting apoptosis and preserving allocentric memory. In addition, it can be stated that the synergistic therapeutic effects of Apelin-13 are more pronounced mediated by NPY2R and more limited mediated by NPY5R. Keywords: Excitotoxicity, D-Glutamic Acid, Apelin-13, Neuropeptide Y, Neurodegeneration