Distinct patterns of cerebral extravasation by Evans blue and sodium fluorescein in rats.

The Evans blue dye (EBD; 961 Da) and the sodium fluorescein dye (NaF; 376 Da) are commonly used inert tracers in blood-brain barrier (BBB) research. They are both highly charged low molecular weight (LMW) tracers with similar lipophobic profiles. Nevertheless, the EBD binds to serum albumin (69,000 Da) to become a high molecular weight (HMW) protein tracer when injected into the circulation, whereas the NaF remains an unbound small molecule in the circulation. In this study, rats were injected with equal doses of either EBD or NaF to monitor their blood and tissue distribution. The EBD was largely confined to the circulation with little accumulation in the peripheral organ and even less accumulation in the central tissue, whereas the NaF distributed more evenly between the blood and the peripheral organ but was also largely excluded from the central tissue. Importantly, the EBD crossed the BBB most effectively at the prefrontal cortex and the cerebellum, and most poorly at the striatum. In marked contrast, the NaF was evenly distributed throughout the brain. Finally, the EBD exhibited this same peculiar tissue distribution profile when administered by either bolus injection or slow infusion. Our study suggests that different regions of the brain are equally permeable to LMW inert dyes like the NaF, but are markedly different in permeability to HMW proteins such as EBD-labelled serum albumin.

Intravenous mannitol does not increase blood-brain barrier permeability to inert dyes in the adult rat forebrain.

Intravenous mannitol (IV-M) is widely administered in the clinic to lower intracranial pressure in patients with brain trauma and stroke. However, intracarotid arterial mannitol (ICA-M) is known to potently open the blood-brain barrier (BBB) to serum protein tracers such as the Evans blue dye (EBD). In this study, we aimed to determine the potential effect of IV-M on BBB permeability to EBD and a small molecular tracer sodium fluorescein dye (NaF). Rats received intravenous EBD/NaF injections, and after a 30-min equilibration time, they received mannitol (20%, 0.5 g/kg) through either route of administration. At 90 min after the mannitol injection, the rats were perfused to rid their circulations of the tracers, and the tracers extravasated into the brain parenchyma were measured by photospectrometry. As expected, ICA-M considerably increased EBD extravasation into the rat forebrain regions, including the motor cortex (P=0.0069), the striatum (P=0.0097), and the hippocampus (P=0.0281; student's t-test). In marked contrast, IV-M exerted no effect on EBD extravasation into these forebrain regions. To increase the power of the IV-M study, we repeated the experiments in two independent trials of experiments (n=6-9/group/trial) and found the same result. Finally, consistent with no effect on EBD extravasation, IV-M had no effect on NaF extravasation into the rat forebrain. In conclusion, we report direct evidence that IV-M, at a dose used clinically, in contrast to the same dose of ICA-M, exerted no effect on BBB permeability to protein and small molecular tracers.