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Ligands can also be attached to the surface of a nanoparticle to target certain receptors located within the brain. Once the nanoparticle is through the blood brain barrier it releases the drug into the brain. [5] A specific example of this solution is the delivery of anti-HIV drugs to the central nervous system by TAT-conjugated nanoparticles. [6]
The blood–brain barrier is formed by the brain capillary endothelium and excludes from the brain 100% of large-molecule neurotherapeutics and more than 98% of all small-molecule drugs. [23] Overcoming the difficulty of delivering therapeutic agents to specific regions of the brain presents a major challenge to treatment of most brain disorders.
The capillaries in the portal system are fenestrated (have many small channels with high vascular permeability) which allows a rapid exchange between the hypothalamus and the pituitary. The main hormones transported by the system include gonadotropin-releasing hormone , corticotropin-releasing hormone , growth hormone–releasing hormone , and ...
The choroid plexus regulates the production and composition of cerebrospinal fluid (CSF), that provides the protective buoyancy for the brain. [2] [10] CSF acts as a medium for the glymphatic filtration system that facilitates the removal of metabolic waste from the brain, and the exchange of biomolecules and xenobiotics into and out of the brain.
Furthermore, they allow the formation of the blood–brain barrier by inhibiting the effects of CNS immune cells (which can damage the formation of the barrier) and by reducing the expression of molecules that increase vascular permeability. [25] Aside from blood–brain barrier formation, pericytes also play an active role in its functionality.
Firstly, the brain can be flooded with molecules that are floating through the blood stream that are usually blocked by the barrier. Secondly, when the tight junctions loosen, the homeostasis of the brain can also be thrown off which can result in seizures and the compromised function of the brain. [8]
Circumventricular organs contain capillary networks that vary between one another and within individual organs both in density and permeability, with most CVO capillaries having a permeable endothelial cell layer, except for those in the subcommissural organ. [1] [16] Furthermore, all CVOs contain neural tissue, enabling a neuroendocrine role.
This is called the precapillary sphincter. The precapillary sphincter has now also been found in the brain, where it regulates blood flow to the capillary bed. [3] The sphincter can open and close the entrance to the capillary, by which contraction causes blood flow in a capillary to change as vasomotion occurs. [4] [unreliable source?