Response of neurons and microglia/macrophages in the area postrema of adult rats following exposure to hypobaric hypoxia.

The response of neurons and microglia/macrophages in the area postrema (AP) was examined in adult rats following exposure to hypobaric hypoxia. In this connection, immunoexpression of complement type 3 (CR3) receptors, major histocompatibility complex (MHC) class I and II antigens and ED1 antigens on the macrophages/microglia was downregulated immediately after the hypoxic insult. However, it showed an upregulation at 7-14 days and was comparable to the controls thereafter. At the ultrastructural level, swollen axons showing disruption of their myelin sheaths were observed between 7 and 14 days. At this time interval microglia/macrophages in the AP were observed to phagocytose such axons. Neurons did not show any structural alteration at any time interval following hypoxic exposure and appeared comparable to the neurons in the control AP. It is suggested that alterations in CR3 receptors, ED1, and MHC I and II antigens on the macrophages/microglia in hypobaric hypoxia were in response to axonal changes. Increased permeability of blood vessels following hypoxia may also play a role in activation of these cells as they would be involved in the clearance of extravasated serum derived substances.