Effect of oestradiol on neurotrophin receptor expression in cholinergic neurons in vivo

Abstract

The basal forebrain is home to the largest population of the cholinergic neurons in the brain. These neurons are involved in a number of cognitive functions including attention, learning and memory. Basal forebrain cholinergic neurons (BFCNs) are particularly vulnerable in a number of neurological diseases with the most notable being Alzheimer’s disease, with evidence illustrating the link between decreasing cholinergic markers and the degree of cognitive impairment. The neurotrophin growth factor system is present on these BFCNs and has been shown to promote survival and differentiation on these neurons. Clinical and neurodegenerative studies have demonstrated the neuroprotective effects of 17β-oestradiol (E2) on neurodegeneration in BFCNs. It is believed that E2 interacts with neurotrophin signalling on cholinergic neurons to mediate these beneficial effects. In this study we aimed to observe if E2 influences the neurotrophin system by examining the effect of short-term ovariectomy (OVX) on neurotrophin receptor expression. Then, to determine if this effect is mediated by oestrogen receptor α (ERα), co-localisation between neurotrophin receptors and oestrogen receptor α was examined. Finally, neurotrophin receptor expression in neuron-specific ERα knockout mice was studied. To investigate the effect of E2 on neurotrophin receptor expression, coronal sections of the basal forebrain from OVX, OVX with E2 subcutaneous (s.c.) implant or sham operated cholinergic acetyltransferase-eGFP (ChAT-eGFP) mice were labelled using immunohistochemistry for neurotrophin receptors. Results confirmed that the loss of E2 in OVX mice resulted in a significant reduction in neurotrophin receptor expression in BFCN populations, with effects depending on neurotrophin receptor and region examined. The involvement of ERα in this relationship was then studied using double labelling immunohistochemistry staining for neurotrophin receptors and ERα in ChAT-eGFP mice. Results from this indicated that ERα co-localised with all the neurotrophin receptors investigated, however co-localisation between ERα and neurotrophin receptors was below 20% in the majority of basal forebrain regions. In the final experiment a similar protocol to the one used in experiment 1 was conducted on neuron-specific ERα knockout mice. Results indicated that OVX failed to affect neurotrophin receptor expression on BFCNs in knockout mice and in some regions basal neurotrophin expression was significantly lower than expression seen in wild-type C57BL/6 mice. Evidence presented in this study confirms that altering the levels of circulating E2 levels via OVX and E2 replacement significantly affects the expression of the neurotrophin receptors on BFCN, with effects differing depending on neurotrophin receptor and brain region examined. ERα was found to co-localise to a small extent with neurotrophin receptors on these cholinergic neurons, suggesting that there is potential for direct regulation of neurotrophin receptors through ERα, however indirect mechanisms are probably involved as well. The final experiment provided some evidence of an ERα mediated effect, however further studies are required to confirm its role and importance beside the effects mediated by other oestrogen receptors.