Effect of Astroglial Necrosis on Blood-Brain Barrier Maintenance and Development in vivo

171 In vitro, brain-derived endothelium loses several blood-brain barrier (BBB) characteristics such as-glutamyl transpeptidase, tight junctions and the polarized transport of amino acids. However, these features are regained during co-culture with astroglial cells or their conditioned medium. To determine if similar BBB deficits would occur in situ when micro-vessels were denuded of their astrocytic complement, chronic and acute perivascular astroglial destruction was effected by subcutaneous injection of the gliotoxin 6-aminonieotinamide (6-AN) into neonatal and adult Wistar rats. Animals bearing neocortical transplants were also exposed to the toxin to test whether glial and/or barrier defects would occur within the grafts. 6-AN is a nicotinamide antagonist which causes a blockade of the pentose-phosphate pathway. Both astrocytes and oligodendrocytes are particularly susceptible to this toxin at the dosages used in this study (10 mg/kg body weight), and they subsequently undergo cytotoxic edema followed by degener-ation. The mierovascular endothelium sustains no morphological damage after 6-AN administration. Neonatal animals received 4 intraperitoneal injections of 6-AN during the first postnatal week and were sacrificed at ages ranging from 6 12 days. Age-matched controls received injections of saline. 6-AN caused necrosis of glial cells throughout the entire CNS in neonates resulting in the appearance of spongy degeneration in both white and gray matter. Perivascular astro-glial processes were immensely swollen, with degenerating organelles and plasma membranes. The perivascular basement membrane appeared to be intact and stained normally with anti-laminin. Immunostaining for the glucose transporter protein and histochemical staining for-glutamyl transpeptidase revealed an expression of both BBB markers throughout the CNS of the experimental animals which was comparable to that of the age-matched controls. The BBB to endogenous protein remained intact as shown by immunostaining with rat serum albumin (RSA). Similarly, intravascularly injected horseradish peroxidase (HRP) did not leak from the microvasculature of experimental animals except in normally permeable areas and ultrastructural examination of these animals revealed a normal endocytotic uptake of HRP. Thus, despite widespread glial destruction throughout the neonatal CNS, the BBB characteristics of the developing vasculature were unaffected. Adult rats received a single intraperitoneal injection of 6-AN and were sacrificed after 1 5 days. Although mature rats exhibited glial necrosis only in specific regions of spinal cord, medulla, cerebellum and thalamus, the micro-vascular ultrastructure of the affected regions was similar to that of 6-AN-exposed neonates. Petechial hemorrhages associated with veins were observed within the lesioned areas. Perivascular laminin immunoreactivity appeared to be normal. Endothelial immunoreactivity for …