Astrocyte dysfunction alters GABAergic communication and ammonia metabolism in the streptozotocin-induced sporadic Alzheimer's disease model

Background: In the sporadic model of Alzheimer's disease (AD), induced by intracerebroventricular streptozotocin (STZ) administration, cognitive impairment is accompanied by specific astrocytic changes in the hippocampus prior to amyloid deposition. Objective: Hypothesizing that the synthesis of GABA, via MAO-B, contributes to ammonia elevation, thereby compromising antioxidant defense and ATP synthesis, and possibly contributing to cognitive damage, we determined the hippocampal levels of glutamine synthetase (GS), monoamine oxidase B (MAO-B) and other enzymes related to GABA metabolism. Methods: Immunoblotting and RT-PCR assays were carried out in hippocampal samples of Wistar rats, at 4 and 16 weeks post-STZ, in the sporadic STZ-induced AD model, corresponding to the pre-amyloid and amyloid phases, respectively. Results: We observed a reduction in GS activity and increased MAO-B content, both in 4 weeks and in 16 weeks, reinforcing the idea that astroglial dysfunction precedes the amyloid phase. These alterations were accompanied by an increase in the content of ornithine decarboxylase 1 (ODC1), which catalyzes the synthesis of putrescine (substrate for GABA synthesis, via MAO-B), and a reduction in the gene expression of arginine-glycine amidinotransferase (AGAT), an enzyme involved in the synthesis of creatine, and in the generation of GABA agonists. These changes were only seen in the amyloid phase of the AD model. Conclusions: Our findings contribute to explain the greater damage that occurs in energy metabolism at this stage, in addition to the greater GABAergic loss. The changes reinforce the importance of the STZ model and further our understanding of the changes in both AD phases.