Blood-borne substances can invade into the extracellular spaces of the brain via endothelial cells in sites without the blood-brain barrier (BBB), and can travel through the interstitial fluid (ISF) of the brain parenchyma adjacent to non-BBB sites. It has been shown that cerebrospinal fluid (CSF) drains directly into the blood via the arachnoid villi and also into lymph nodes via the subarachnoid spaces of the brain, while ISF drains into the cervical lymph nodes through perivascular drainage pathways. In addition, the glymphatic pathway of fluids, characterized by para-arterial pathways, aquaporin4-dependent passage through astroglial cytoplasm, interstitial spaces, and paravenous routes, has been established. Meningeal lymphatic vessels along the superior sagittal sinus were very recently discovered. It is known that, in mice, blood-borne substances can be transferred to areas with intact BBB function, such as the medial regions of the hippocampus, presumably through leaky vessels in non-BBB sites. In the present paper, we review the clearance mechanisms of interstitial substances, such as amyloid-β peptides, as well as summarize models of BBB deterioration in response to different types of insults, including acute ischemia followed by reperfusion, hypertension, and chronic hypoperfusion. Lastly, we discuss the relationship between perivascular clearance and brain disorders.

New findings on flow or drainage pathways of brain interstitial fluid and cerebrospinal fluid have been made. The interstitial fluid flow has an effect on the passage of blood-borne substances in the brain parenchyma, especially in areas near blood-brain barrier (BBB)-free regions. Actually, blood-borne substances can be transferred in areas with intact BBB function, such as the hippocampus, the corpus callosum, periventricular areas, and medial portions of the amygdala, presumably through leaky vessels in the subfornical organs or the choroid plexus. Increasing evidence indicates that dysfunction of the BBB function may play a significant role in the pathogenesis of vascular dementia. Accordingly, we have examined which insults seen in patients suffering from vascular dementia have an effect on the BBB using experimental animal models exhibiting some phenotypes of vascular dementia. The BBB in the hippocampus was clearly deteriorated in Mongolian gerbils exposed to acute ischemia followed by reperfusion and also in stroke-prone spontaneously hypertensive rats (SHRSP) showing hypertension. The BBB in the corpus callosum was clearly deteriorated in Wistar rats with permanent ligation of the bilateral common carotid arteries showing chronic hypoperfusion. The BBB in the hippocampus and the olfactory bulb was mildly deteriorated in aged senescence accelerated prone mice (SAMP8) showing cognitive dysfunction. The BBB in the hippocampus was mildly deteriorated in aged animals with hydrocephalus. Mild endothelial damage was seen in hyperglycemic db/db mice. In addition, mRNA expression of osteopontin, matrix metalloproteinase-13 (MMP-13), and CD36 was increased in vessels showing BBB damage in hypertensive SHRSP. As osteopontin, MMP-13 and CD36 are known to be related to brain injury and amyloid β accumulation or clearance, BBB damage followed by increased gene expression of these molecules not only contributes to the pathogenesis of vascular dementia, but also bridges the gap between vascular dementia and Alzheimer's disease.

Diabetes mellitus (DM) is now recognized as one of the risk factors for Alzheimer's disease (AD), and the diseasemodifying effects of anti-diabetic drugs on AD have recently been attracting great attention. Sodium/glucose cotransporter 2 (SGLT2) inhibitors are a new class of antidiabetic drugs targeting the SGLT2/solute carrier family 5 member 2 (SLC5A2) protein, which is known to localize exclusively in the brush border membrane of early proximal tubules in the kidney. However, recent data suggest that it is also expressed in other tissues. In the present study, we investigated the expression of SGLT2/SLC5A2 in human and mouse brains. Immunohistochemical staining of paraffin sections from autopsied human brains and C3H/He mouse brains revealed granular cytoplasmic immunoreactivity in choroid plexus epithelial cells and ependymal cells. Immunoblot analysis of the membrane fraction of mouse choroid plexus showed distinct immunoreactive bands at 70 and 26 kDa. Band patterns around 70 kDa in the membrane fraction of the choroid plexus were different from those in the kidney. Reverse transcription-polymerase chain reaction analysis confirmed the expression of Slc5a2 mRNA in the mouse choroid plexus. Our results provide in vivo evidence that SGLT2/ SLC5A2 is expressed in cells facing the cerebrospinal fluid, in addition to early proximal tubular epithelial cells. These findings suggest that SGLT2 inhibitors may have another site of action in the brain. The effects of SGLT2 inhibitors on brain function and AD progression merit further investigation to develop better treatment options for DM patients.

The entry of blood-borne macromolecular substances into the brain parenchyma from cerebral vessels is blocked by the blood–brain barrier (BBB) function. Accordingly, increased permeability of the vessels induced by insult noted in patients suffering from vascular dementia likely contributes to the cognitive impairment. On the other hand, blood-borne substances can enter extracellular spaces of the brain via endothelial cells at specific sites without the BBB, and can move to brain parenchyma, such as the hippocampus and periventricular areas, adjacent to specific sites, indicating the contribution of increased permeability of vessels in the specific sites to brain function. It is necessary to consider influx and efflux of interstitial fluid (ISF) and cerebrospinal fluid (CSF) in considering effects of brain transfer of intravascular substances on brain function. Two pathways of ISF and CSF are recently being established. One is the intramural peri-arterial drainage (IPAD) pathway of ISF. The other is the glymphatic system of CSF. Dysfunction of the two pathways could also contribute to brain dysfunction. We review the effects of several kinds of insult on vascular permeability and the failure of fluid clearance on the brain function.

The choroid plexus plays a central role in the regulation of the microenvironment of the central nervous system by secreting the majority of the cerebrospinal fluid and controlling its composition, despite that it only represents approximately 1% of the total brain weight. In addition to a variety of transporter and channel proteins for solutes and water, the choroid plexus epithelial cells are equipped with glucose, fructose, and urate transporters that are used as energy sources or antioxidative neuroprotective substrates. This review focuses on the recent advances in the understanding of the transporters of the SLC2A and SLC5A families (GLUT1, SGLT2, GLUT5, GLUT8, and GLUT9), as well as on the urate-transporting URAT1 and BCRP/ABCG2, which are expressed in choroid plexus epithelial cells. The glucose, fructose, and urate transporters repertoire in the choroid plexus epithelium share similar features with the renal proximal tubular epithelium, although some of these transporters exhibit inversely polarized submembrane localization. Since choroid plexus epithelial cells have high energy demands for proper functioning, a decline in the expression and function of these transporters can contribute to the process of age-associated brain impairment and pathophysiology of neurodegenerative diseases.