The choroid plexus, located in the cerebral ventricles, is a highly vascularised tissue, in which blood microvessels are enclosed by a single layer of cubical epithelial cells . Choroid plexus epithelial cells are closely connected to each other by tight junctions and constitute the structural basis of the blood-CSF barrier . The barrier function can be subjected to modulation and thereby regulates the entry of physiologically important substances. However, choroid plexus epithelial cells are also an important target organ for polypeptides, with capacity to produce and secrete numerous biologically active neurotrophic factors into the CSF. In mammalian, brain CSF is produced by the choroid plexus, which not only regulates homeostasis in central nervous system (CNS), but also participates in neurohumoral brain modulation as well as in neuroimmune interaction. Several peptides are shown to be actively transported by the choroidal epithelial cells to the CSF and most of the transported hormones evidently have, at least, a hypothalamic destination. These peptides circulate throughout the brain and spinal cord, maintaining neuronal networks and associated cells .
Studies in the past few years have promoted insight into the molecular structure and function of the choroid plexus. Even modest changes in the choroid plexus can have far reaching effects and changes in the anatomy and physiology of the choroid plexus, and have been linked to several neurological disorders such as Alzheimer's disease or multiple sclerosis. Given a widely postulated role in neuronal cell survival, the p75NTR is also though to be associated with many neurodegenerative diseases.
The choroid plexus is involved in a variety of neurological disorders, including neurodegenerative, inflammatory, infectious, traumatic, neoplastic, and systemic diseases. It is well known that Aβ is accumulated in the choroid plexus of Alzheimer's disease patients ; there is also evidence that circulating Insulin-Like Growth Factor-I (IGF-I) participates in brain Aβ clearance by modulating choroid plexus function . It has been published that substances, such as transthyrretin (TTR) or apolipoprotein J (ApoJ), are produced by the choroid plexus and secreted into the CSF . In multiple sclerosis and in animal models of multiple sclerosis, the choroid plexus is the main route of leukocyte entry into the brain .
Many studies have demonstrated the presence of proteins and mRNA for a large number of cytokines, growth factors and hormones in the choroid plexus, for example: interleukin-1β , interleukin-6 , Tumor Necrosis Factor (TNF)-α , IGF-I , NGF , IGF-II , Transforming Growth Factor (TGF)-α , TGF-β , Vascular Endothelial Growth Factor (VEGF) , transferrin , TTR [6, 18], gelsolin  and vasopressin . Most of these substances have their own receptors in the choroid plexus . The production and secretion of all these substances and their receptors are strongly associated with the health of central nervous system (CNS).
The mammalian neurotrophins comprise a family of related secreted factors required for differentiation, survival, development, and death of specific populations of neurons and non-neuronal cells. The effects of the neurotrophins (NGF, Brain-Derived Neurotrophin Factor (BDNF), NT-3, NT-4) are mediated by binding to TrkA, TrkB and TrkC receptor tyrosine kinases and to the p75NTR [22, 23]. The Trk receptors play critical roles in mediating neuronal survival, growth and synaptic function . The p75NTR serves as a receptor for the four mentioned neurotrophins and it is an important component of distinct cell surface signaling platforms, which induce apoptosis and neuronal growth inhibition .
The presence of the neurotrophins and their receptors has been investigated in the choroid plexus of rats and humans. The choroid plexus contains high levels of NGF, NT-4, and TrkB, and low levels of NT-3 and BDNF, while TrkA and TrkC levels remain undetectable .
Regarding the neurotrophins receptors in the choroid plexus, the expression of p75NTR has not been fully investigated. The present research shows the distribution of p75NTR in the epithelial cells of the choroid plexus and its possible role in the normal transportation of molecules of the blood-CSF-barrier. The importance and novelty of this expression expands a new role of p75NTR.