CHANGES IN THE POPULATION OF PARVALBUMIN-EXPRESSING INTERNEURONS IN THE HUMAN HIPPOCAMPAL FORMATION IN ALZHEIMER'S DISEASE: A ROSTROCAUDAL STUDY

Ageing and neurodegenerative pathologies such as Alzheimer's disease (AD) involve progressive decrease in brain volume and loss of cell density. One of the most affected areas is the hippocampus, a complex structure organized in different cellular and non-cellular layers and a key region for memory processing and learning. Neuronal types include interneurons, with a crucial role in regulating the activity of principal neurons. Some populations of interneurons can be detected by the expression of calcium-binding proteins such as parvalbumin (PV). The hippocampal population of PV-immunoreactive (PV-IR) interneurons may be affected by ageing and neurodegenerative diseases, thus we aim to analyze in detail by immunohistochemistry the changes in this interneuron population along the rostrocaudal length of the hippocampus in cases diagnosed with mild to moderate AD (Braak III-IV stages) compared to age-matched brain donors without neurological disease (Braak I stage). The results show a non-uniform rostrocaudal density of PV-IR cells in both groups, with the highest values found in tail. Braak III-IV cases show selective decreases of the PV-IR population compared to Braak I donors, with the head being the most affected segment and tail the most preserved. CA1 was the field showing the highest percentage of PV-IR cell density decrease in mild to moderate AD cases. This population changes depending on the layer/field/segment. Since the three rostro-caudal segments of the hippocampus (head, body and tail) are associated with different functions, the results may serve as a morphological basis to explain some of the alterations observed in patients with neurodegenerative pathologies.