<P> The measurement of brain uptake of acetamide, antipyrine, benzyl alcohol, butanol, caffeine, cytosine, phenytoin, ethanol, ethylene glycol, heroin, mannitol, methanol, phenobarbital, propylene glycol, thiourea, and urea in ether - anesthetized newborn vs. adult rabbits shows that newborn rabbit and adult rabbit brain endothelia are functionally similar with respect to lipid - mediated permeability . These data confirmed that no differences in permeability could be detected between newborn and adult BBB capillaries . No difference in brain uptake of glucose, amino acids, organic acids, purines, nucleosides, or choline was observed between adult and newborn rabbits . These experiments indicate that the newborn BBB has restrictive properties similar to that of the adult . In contrast to suggestions of an immature barrier in young animals, these studies indicate that a sophisticated, selective BBB is operative at birth . </P> <P> The blood--brain barrier acts effectively to protect the brain from circulating pathogens . Accordingly, blood - borne infections of the brain are rare . Infections of the brain that do occur are often difficult to treat . Antibodies are too large to cross the blood--brain barrier, and only certain antibiotics are able to pass . In some cases, a drug has to be administered directly into the cerebrospinal fluid (CSF) where it can enter the brain by crossing the blood--cerebrospinal fluid barrier . </P> <P> The blood - brain barrier may become leaky in select neurological diseases, such as amyotrophic lateral sclerosis, epilepsy, brain trauma and edema, and in systemic diseases, such as liver failure . The blood--brain barrier becomes more permeable during inflammation, allowing antibiotics and phagocytes to move across the BBB . However, this also allows bacteria and viruses to infiltrate the blood - brain barrier . Examples of pathogens that can traverse the blood - brain barrier include toxoplasma gondii which causes toxoplasmosis, spirochetes like Borrelia (Lyme disease), Group B streptococci which causes meningitis in newborns, and Treponema pallidum which causes syphilis . Some of these harmful bacteria gain access by releasing cytotoxins like pneumolysin which have a direct toxic effect on brain microvascular endothelium and tight junctions . </P> <P> Circumventricular organs (CVOs) are individual structures located adjacent to the fourth ventricle or third ventricle in the brain, and are characterized by dense capillary beds with permeable endothelial cells unlike those of the blood - brain barrier . Included among CVOs having highly permeable capillaries are the area postrema, subfornical organ, vascular organ of the lamina terminalis, median eminence, pineal gland, and three lobes of the pituitary gland . </P>

How do viruses cross the blood brain barrier
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