The impairment of the hippocampal neuro-vascular unit precedes changes in spatial cognition in naturally aged rats

Ageing is the major risk factor for the most neurodegenerative diseases, such as Alzheimer's disease (AD). Damage to neurovascular components (microvessels, glia, and neurons) occurs with ageing and is suspected to exacerbate or cause mild cognitive impairment (MCI), vascular dementia, and AD. However, whether vascular cells, glia, and neurons change synchronously or asynchronously during ageing is unclear, and the relationship between complex dynamic pathophysiological changes in the brain and cognitive ability needs to be further studied. We used male Sprague-Dawley (SD) rats of three different ages (2 months, 12 months, and 24 months) and explored changes in the neurovascular unit (cerebral vessels, microglia, astrocytes, and neurons) and spatial memory upon normal ageing by the Morris water maze (MWM) test and immunofluorescence staining. We found that the impairments of microvessels, glia, neurons, and spatial memory were age-dependent in the rat hippocampus. In middle-aged (12-month-old) rats, some neurovascular unit components have become abnormal: the density and length of microvessels, pyramidal neuron, and SST (Somatostatin) neuron number was decreased, the number of astrocytes was increased in the hippocampus. The diameter of microvessels and PV (Parvalbumin) neuron numbers were decreased, the microglial number was increased and spatial learning was deficit at 24 months of age. In conclusion, we found that the impairment of the hippocampal neuro-vascular unit precedes changes in spatial cognition in naturally aged rats.