Changes of activity and ultrastructural localization of alkaline phosphatase in cerebral cortical microvessels of rat after single intraperitoneal administration of mercuric chloride.

Inorganic mercury salts administered systemically at low mg/ml doses produce neurotoxic effects without penetrating the cerebral microvascular endothelial cells which form the blood-brain barrier (BBB). This phenomenon promoted investigations testing a hypothesis relating inorganic mercury-induced brain dysfunction to its interference with the BBB transport. In the present study, we tested the effect of a single i.p. administration of mercuric chloride (MC) (6 mg/kg body weight) on the activity and ultrastructural localization of cerebral alkaline phosphatase (AP), a cerebromicrovascular marker enzyme primarily located on luminal plasmalemma of endothelial cells. At 1h after MC administration, light microscopy revealed a virtual absence of AP in cerebral cortical layers II and III, and its dramatic reduction in the remaining layers. Electron microscopy confirmed the disappearance of the AP reaction product from luminal endothelial cell membranes, and luminal phasmalemma revealed pinocytic vesicles and invaginations likely to manifest changes in BBB transport. At 18h post-treatment, a moderate enzyme activity appeared on abluminal endothelial plasmalemma and on basement membrane, but remained absent from luminal plasmalemma. A similar picture persisted through day 5 post-treatment. The inhibition and subsequent translocation of AP activity from luminal to abluminal site and the accompanying ultrastructural changes are typical of the formation of "leaky" microvessels, previously reported for a variety of neuropathological conditions associated with BBB damage.