Chronic Microglial Activation and Ageing Reduces the Numbers of Medial Septal Cholinergic Cells and Alters Their Morphology

Pathological ageing is associated with brain inflammation that increases the risk of neurodegenerative disease like Alzheimer's disease (AD). Chronic activation of microglia, the resident immune cells of the central nervous system (CNS), lead to "neuroinflammation" and result in gradual neurodegeneration. Pro-inflammatory cytokine interleukin-6 (IL-6) is a common early inflammatory marker in AD. Medial septum (MS) of the basal forebrain (BF) provides most of the cholinergic innervations into the hippocampus and the cortex. While BF cholinergic cell loss and reduced cortical choline acetyltransferase (ChAT) activity is associated with cognitive decline, the underlying mechanism leading to cholinergic cell loss is still unknown.Aged, 12 (n=5) and 24 (n=5) months GFAP-IL6 (presenting continuous IL-6 expression under the control of GFAP promoter, contributing to chronic microglia activation), and aged, 12 (n=5) and 24 (n=5) months ChATBAC -eGFP (GFP tagged cholinergic neurones to represent the effect of healthy ageing) mice were used. Immunohistochemistry combined with unbiased stereology and 3D morphology analysis were used to estimate the number and morphology of Iba1+ microglia and ChAT+ cholinergic cells, in the MS.Stereological estimation of MS microglia number displayed a significant 25% increase GFAP-IL6 (12M) mice compared to control ChATBAC -eGFP (12M) mice. A significant 30% increase was obtained for GFAP-IL6 (24M) mice compared to control (24M) mice. Furthermore, a significant 23% increase in microglia was obtained for GFAP-IL6 (24M) mice compared to the GFAP-IL6 (12M) mice. Meanwhile, Cholinergic cell number displayed a significant 31% decrease in GFAP-IL6 (12M) mice compared to control (12M) mice. While a significant 30% decrease was displayed in GFAP-IL6 (24M) mice compared to control (24M) mice. There was a tendency for the cholinergic cell number to decrease with ageing among control and GFAP-IL6 cohorts. A significant change in microglial morphology was also observed with ageing and neuroinflammation, that resembled reactive microglial morphology. Cholinergic cell morphology displayed visible effects of ageing and neuroinflammation with GFAP-IL-6 cohorts displaying patterns like neuronal degeneration.These findings demonstrate a potential direct effect of chronic microglia activation on cholinergic cell number in the MS which can exacerbate throughout ageing leading to neurodegenerative impairments.