A study of intrathecal, cerebrospinal fluid-to-brain exchange

Lipid Nanoparticles Enhance mRNA Delivery to the Central Nervous System Upon Intrathecal Injection

Lipid nanoparticle-messenger RNA formulations have garnered significant attention for their therapeutic potential in infectious diseases, cancer and genetic disorders. However, effective mRNA delivery to the central nervous system (CNS) remains a formidable challenge. To overcome this limitation, a class of brain-targeting lipids (BLs) is developed by incorporating brain-targeting small molecules with amino lipids and formulated them with helper lipids to generate brain-targeting lipid nanoparticles (BLNPs) for mRNA delivery. Screening studies led to a lead formulation, TD5 BLNPs, outperforming FDA-approved DLin-MC3-DMA LNPs in delivering mRNA to the brain upon intrathecal injection. Specifically, a single intrathecal injection of TD5 BLNP-GFP mRNA led to GFP expression in 29.6% of neurons and 38.1% of astrocytes across the brain. In an Ai14 mouse model, TD5 BLNP-Cre recombinase mRNA treatment induced tdTomato expression in ≈30% of neurons and 40% of astrocytes across major brain regions. Notably, delivery of Cas9 mRNA/sgRNA complex using TD5 BLNPs achieved effective genome editing in the brain. Additionally, TD5 BLNPs showed comparable safety profiles to MC3 LNPs, indicating promising biocompatibility. Overall, this TD5 BLNP formulation effectively delivers mRNA to brain tissues via intrathecal injection and facilitates efficient expression in both neurons and astrocytes, presenting a potential strategy for treating CNS diseases.

Characterization of Ultrapotent Chemogenetic Ligands for Research Applications in Nonhuman Primates

Chemogenetics is a technique for obtaining selective pharmacological control over a cell population by expressing an engineered receptor that is selectively activated by an exogenously administered ligand. A promising approach for neuronal modulation involves the use of "Pharmacologically Selective Actuator Modules" (PSAMs); these chemogenetic receptors are selectively activated by ultrapotent "Pharmacologically Selective Effector Molecules" (uPSEMs). To extend the use of PSAM/PSEMs to studies in nonhuman primates, it is necessary to thoroughly characterize the efficacy and safety of these tools. We describe the time course and brain penetrance in rhesus monkeys of two compounds with promising binding specificity and efficacy profiles in in vitro studies, uPSEM792 and uPSEM817, after systemic administration. Rhesus monkeys received subcutaneous (s.c.) or intravenous (i.v.) administration of uPSEM817 (0.064 mg/kg) or uPSEM792 (0.87 mg/kg), and plasma and cerebrospinal fluid samples were collected over 48 h. Both compounds exhibited good brain penetrance, relatively slow washout, and negligible conversion to potential metabolites─varenicline or hydroxyvarenicline. In addition, we found that neither of these uPSEMs significantly altered the heart rate or sleep. Our results indicate that both compounds are suitable candidates for neuroscience studies using PSAMs in nonhuman primates.

Exploring the role of nanomedicines for the therapeutic approach of central nervous system dysfunction: At a glance

In recent decades, research scientists, molecular biologists, and pharmacologists have placed a strong emphasis on cutting-edge nanostructured materials technologies to increase medicine delivery to the central nervous system (CNS). The application of nanoscience for the treatment of neurodegenerative diseases (NDs) such as Alzheimer’s disease (AD), Parkinson’s disease (PD), multiple sclerosis (MS), Huntington’s disease (HD), brain cancer, and hemorrhage has the potential to transform care. Multiple studies have indicated that nanomaterials can be used to successfully treat CNS disorders in the case of neurodegeneration. Nanomedicine development for the cure of degenerative and inflammatory diseases of the nervous system is critical. Nanoparticles may act as a drug transporter that can precisely target sick brain sub-regions, boosting therapy success. It is important to develop strategies that can penetrate the blood–brain barrier (BBB) and improve the effectiveness of medications. One of the probable tactics is the use of different nanoscale materials. These nano-based pharmaceuticals offer low toxicity, tailored delivery, high stability, and drug loading capacity. They may also increase therapeutic effectiveness. A few examples of the many different kinds and forms of nanomaterials that have been widely employed to treat neurological diseases include quantum dots, dendrimers, metallic nanoparticles, polymeric nanoparticles, carbon nanotubes, liposomes, and micelles. These unique qualities, including sensitivity, selectivity, and ability to traverse the BBB when employed in nano-sized particles, make these nanoparticles useful for imaging studies and treatment of NDs. Multifunctional nanoparticles carrying pharmacological medications serve two purposes: they improve medication distribution while also enabling cell dynamics imaging and pharmacokinetic study. However, because of the potential for wide-ranging clinical implications, safety concerns persist, limiting any potential for translation. The evidence for using nanotechnology to create drug delivery systems that could pass across the BBB and deliver therapeutic chemicals to CNS was examined in this study.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Blood-brain barrier delivery for lysosomal storage disorders with IgG-lysosomal enzyme fusion proteins

The majority of lysosomal storage diseases affect the brain. Treatment of the brain with intravenous enzyme replacement therapy is not successful, because the recombinant lysosomal enzymes do not cross the blood-brain barrier (BBB). Biologic drugs, including lysosomal enzymes, can be re-engineered for BBB delivery as IgG-enzyme fusion proteins. The IgG domain of the fusion protein is a monoclonal antibody directed against an endogenous receptor-mediated transporter at the BBB, such as the insulin receptor or the transferrin receptor. This receptor transports the IgG across the BBB, in parallel with the endogenous receptor ligand, and the IgG acts as a molecular Trojan horse to ferry into brain the lysosomal enzyme genetically fused to the IgG. The IgG-enzyme fusion protein is bi-functional and retains both high affinity binding for the BBB receptor, and high lysosomal enzyme activity. IgG-lysosomal enzymes are presently in clinical trials for treatment of the brain in Mucopolysaccharidosis.

Evolving new-age strategies to transport therapeutics across the blood-brain-barrier

A basic understanding of the blood-brain barrier (BBB) is essential for the novel advancements in targeting drugs specific to the brain. Neoplasm compromising the internal structure of BBB that results in impaired vasculature is called as blood tumor barrier (BTB). Besides, the BBB serves as a chief hindrance to the passage of a drug into the brain parenchyma. The small and hydrophilic drugs majorly display an absence of desired molecular characteristics required to cross the BBB. Furthermore, all classes of biologics have failed in the clinical trials of brain diseases over the past years since these biologics are large molecules that do not cross the BBB. Also, new strategies have been discovered that use the Trojan horse technology with the re-engineered biologics for BBB transport. Thus, this review delivers information about the different grades of tumors (I-IV) i.e. examples of BBB/BTB heterogenicity along with the different mechanisms for transporting the therapeutics into the brain tumors by crossing BBB. This review also provides insights into the emerging approaches of peptide delivery and the non-invasive and brain-specific molecular Trojan horse targeting technologies. Also, the several challenges in the clinical development of BBB penetrating IgG fusion protein have been discussed.

Dynamic disequilibrium of macromolecular transport as possible mechanism for hydrocephalus associated with long-term spaceflight

Hydrocephalus associated with long term spaceflight (HALS) for missions lasting over five months is well described but poorly understood. While structural changes of the brain due to microgravitational forces affecting the circulation of cerebrospinal fluid (CSF) have been described as one potential cause, we propose an alternative hypothesis based on dynamic disequilibrium of macromolecular transport across the blood brain barrier. We propose that factors altering physiology under conditions of spaceflight such as microgravity, hypercapnia, venous hypertension, medications, and dietary substances contribute to increased protein load in the ventricles and/or contribute to impairment of transport out of the ventricles that results in HALS. Individual variation in the genetic expression of efflux transporters (p-glycoprotein) has been shown to correlate with the presence and degree of hydrocephalus in animal studies. We describe the evidence behind this concept and propose how these factors can be studied in order to determine the underlying pathogenesis which is imperative in order to cure or prevent HALS.

Crossing the Chloride Channel: The Current and Potential Therapeutic Value of the Neuronal K+-Cl- Cotransporter KCC2

Chloride (Cl-) homeostasis is an essential process involved in neuronal signalling and cell survival. Inadequate regulation of intracellular Cl- interferes with synaptic signalling and is implicated in several neurological diseases. The main inhibitory neurotransmitter of the central nervous system is γ-aminobutyric acid (GABA). GABA hyperpolarises the membrane potential by activating Cl- permeable GABAA receptor channels (GABAAR). This process is reliant on Cl- extruder K+-Cl- cotransporter 2 (KCC2), which generates the neuron's inward, hyperpolarising Cl- gradient. KCC2 is encoded by the fifth member of the solute carrier 12 family (SLC12A5) and has remained a poorly understood component in the development and severity of many neurological diseases for many years. Recent advancements in next-generation sequencing and specific gene targeting, however, have indicated that loss of KCC2 activity is involved in a number of diseases including epilepsy and schizophrenia. It has also been implicated in neuropathic pain following spinal cord injury. Any variant of SLC12A5 that negatively regulates the transporter's expression may, therefore, be implicated in neurological disease. A recent whole exome study has discovered several causative mutations in patients with epilepsy. Here, we discuss the implications of KCC2 in neurological disease and consider the evolving evidence for KCC2's potential as a therapeutic target.

Pharmacological Modulation of Endogenous Opioid Activity to Attenuate Neuropathic Pain in Rats

We showed previously that spinal metabotropic glutamate receptor 1 (mGluR₁) signaling suppresses or facilitates (depending on the stage of estrous cycle) analgesic responsiveness to intrathecal endomorphin 2, a highly mu-opioid receptor-selective endogenous opioid. Spinal endomorphin 2 antinociception is suppressed during diestrus by mGluR₁ when it is activated by membrane estrogen receptor alpha (mERα) and is facilitated during proestrus when mGluR₁ is activated by glutamate. In the current study, we tested the hypothesis that in female rats subjected to spinal nerve ligation (SNL), the inhibition of spinal estrogen synthesis or blockade of spinal mERα/mGluR₁ would be antiallodynic during diestrus, whereas during proestrus, mGluR₁ blockade would worsen the mechanical allodynia. As postulated, following SNL, aromatase inhibition or mERα/mGluR₁ blockade during diestrus markedly lessened the mechanical allodynia. This was observed only on the paw ipsilateral to SNL and was eliminated by naloxone, implicating endogenous opioid mediation. In contrast, during proestrus, mGluR₁ blockade worsened the SNL-induced mechanical allodynia of the ipsilateral paw. Findings suggest menstrual cycle stage-specific drug targets for and the putative clinical utility of harnessing endogenous opioids for chronic pain management in women, as well as the value of, if not the necessity for, considering menstrual cycle stage in clinical trials thereof. PERSPECTIVE: Intrathecal treatments that enhance spinal endomorphin 2 analgesic responsiveness under basal conditions lessen mechanical allodynia in a chronic pain model. Findings provide a foundation for developing drugs that harness endogenous opioid antinociception for chronic pain relief, lessening the need for exogenous opioids and thus prescription opioid abuse.

Estrogens synthesized and acting within a spinal oligomer suppress spinal endomorphin 2 antinociception: ebb and flow over the rat reproductive cycle

The magnitude of antinociception elicited by intrathecal endomorphin 2 (EM2), an endogenous mu-opioid receptor (MOR) ligand, varies across the rat estrous cycle. We now report that phasic changes in analgesic responsiveness to spinal EM2 result from plastic interactions within a novel membrane-bound oligomer containing estrogen receptors (mERs), aromatase (aka estrogen synthase), metabotropic glutamate receptor 1 (mGluR1), and MOR. During diestrus, spinal mERs, activated by locally synthesized estrogens, act with mGluR1 to suppress spinal EM2/MOR antinociception. The emergence of robust spinal EM2 antinociception during proestrus results from the loss of mER-mGluR1 suppression, a consequence of altered interactions within the oligomer. The chemical pairing of aromatase with mERs within the oligomer containing MOR and mGluR1 allows estrogens to function as intracellular messengers whose synthesis and actions are confined to the same signaling oligomer. This form of estrogenic signaling, which we term "oligocrine," enables discrete, highly compartmentalized estrogen/mER-mGluR1 signaling to regulate MOR-mediated antinociception induced by EM2. Finally, spinal neurons were observed not only to coexpress MOR, mERα, aromatase, and mGluR1 but also be apposed by EM2 varicosities. This suggests that modulation of spinal analgesic responsiveness to exogenous EM2 likely reflects changes in its endogenous analgesic activity. Analogous suppression of spinal EM2 antinociception in women (eg, around menses, comparable with diestrus in rats) as well as the (pathological) inability to transition out of that suppressed state at other menstrual cycle stages could underlie, at least in part, the much greater prevalence and severity of chronic pain in women than men.

Receptor Targeted Polymeric Nanostructures Capable of Navigating across the Blood-Brain Barrier for Effective Delivery of Neural Therapeutics

The window of neurological maladies encompasses 600 known neurological disorders. In the past few years, an inordinate upsurge in the incidences of neuronal ailments with increased mortality rate has been witnessed globally. Despite noteworthy research in the discovery and development of neural therapeutics, brain drug delivery still encounters limited success due to meager perviousness of most of the drug molecules through the blood-brain barrier (BBB), a tight layer of endothelial cells that selectively impedes routing of the molecules across itself. In this Review, we have tried to present a comprehensive idea on the recent developments in nanoparticle based BBB delivery systems, with a focus on the advancements in receptor targeted polymeric nanoparticles pertaining to BBB delivery. We have also attempted to bridge the gap between conventional brain delivery strategies and nanoparticle based BBB delivery for in-depth understanding. Various strategies are being explored for simplifying delivery of molecules across the BBB; however, they have their own limitations such as invasiveness and need for hospitalization and surgery. Introduction of nanotechnology can impressively benefit brain drug delivery. Though many nanoparticles are being explored, there are still several issues that need to be analyzed scrupulously before a real and efficient BBB traversing nanoformulation is realized.

Lysosomal enzyme replacement therapies: Historical development, clinical outcomes, and future perspectives

Lysosomes and lysosomal enzymes play a central role in numerous cellular processes, including cellular nutrition, recycling, signaling, defense, and cell death. Genetic deficiencies of lysosomal components, most commonly enzymes, are known as "lysosomal storage disorders" or "lysosomal diseases" (LDs) and lead to lysosomal dysfunction. LDs broadly affect peripheral organs and the central nervous system (CNS), debilitating patients and frequently causing fatality. Among other approaches, enzyme replacement therapy (ERT) has advanced to the clinic and represents a beneficial strategy for 8 out of the 50-60 known LDs. However, despite its value, current ERT suffers from several shortcomings, including various side effects, development of "resistance", and suboptimal delivery throughout the body, particularly to the CNS, lowering the therapeutic outcome and precluding the use of this strategy for a majority of LDs. This review offers an overview of the biomedical causes of LDs, their socio-medical relevance, treatment modalities and caveats, experimental alternatives, and future treatment perspectives.

Nanomedicine in Central Nervous System (CNS) Disorders: A Present and Future Prospective

Purpose: For the past few decades central nervous system disorders were considered as a major strike on human health and social system of developing countries. The natural therapeutic methods for CNS disorders limited for many patients. Moreover, nanotechnology-based drug delivery to the brain may an exciting and promising platform to overcome the problem of BBB crossing. In this review, first we focused on the role of the blood-brain barrier in drug delivery; and second, we summarized synthesis methods of nanomedicine and their role in different CNS disorder. Method: We reviewed the PubMed databases and extracted several kinds of literature on neuro nanomedicines using keywords, CNS disorders, nanomedicine, and nanotechnology. The inclusion criteria included chemical and green synthesis methods for synthesis of nanoparticles encapsulated drugs and, their in-vivo and in-vitro studies. We excluded nanomedicine gene therapy and nanomaterial in brain imaging. Results: In this review, we tried to identify a highly efficient method for nanomedicine synthesis and their efficacy in neuronal disorders. SLN and PNP encapsulated drugs reported highly efficient by easily crossing BBB. Although, these neuro-nanomedicine play significant role in therapeutics but some metallic nanoparticles reported the adverse effect on developing the brain. Conclusion: Although impressive advancement has made via innovative potential drug development, but their efficacy is still moderate due to limited brain permeability. To overcome this constraint,powerful tool in CNS therapeutic intervention provided by nanotechnology-based drug delivery methods. Due to its small and biofunctionalization characteristics, nanomedicine can easily penetrate and facilitate the drug through the barrier. But still, understanding of their toxicity level, optimization and standardization are a long way to go.

A Single Injection of Recombinant Adeno-Associated Virus into the Lumbar Cistern Delivers Transgene Expression Throughout the Whole Spinal Cord

The lack of methods to deliver transgene expression in spinal cord has hampered investigation of gene function and therapeutic targets for spinal cord diseases. Here, we report that a single intrathecal injection of recombinant adeno-associated virus rhesus-10 (rAAVrh10) into the lumbar cistern led to transgene expression in 60 to 90 % of the cells in the spinal cord. The transgene was expressed in all cell types, including neurons, glia, ependymal cells, and endothelial cells. Additionally, the transgene was expressed in some brain areas up to the frontal cortex and the olfactory bulb. The rAAV was distributed predominantly in the spinal cord, where its genome copy was over ten times that of the peripheral organs. Compared with intravenous injection, another method for rAAV delivery to the broad central nervous system (CNS), the intrathecal injection reduced the dosage of rAAV required to achieve similar or higher levels of transgene expression in the CNS by ~100-fold. Finally, the transduced areas were co-localized with the perivascular spaces of Virchow-Robin, from which the rAAV spreads further into the CNS parenchyma, thus suggesting that rAAV penetrated the CNS parenchyma through this pathway. Taken together, we have defined a fast and efficient method to deliver widespread transgene expression in mature spinal cord in mice. This method can be applied to stably overexpress or silence gene expression in the spinal cord to investigate gene functions in mammalian CNS. Additionally, this method can be applied to validate therapeutic targets for spinal cord diseases.

A proposed role for efflux transporters in the pathogenesis of hydrocephalus

Hydrocephalus is a common brain disorder that is treated only with surgery. The basis for surgical treatment rests on the circulation theory. However, clinical and experimental data to substantiate circulation theory have remained inconclusive. In brain tissue and in the ventricles, we see that osmotic gradients drive water diffusion in water-permeable tissue. As the osmolarity of ventricular CSF increases within the cerebral ventricles, water movement into the ventricles increases and causes hydrocephalus. Macromolecular clearance from the ventricles is a mechanism to establish the normal CSF osmolarity, and therefore ventricular volume. Efflux transporters, (p-glycoprotein), are located along the blood brain barrier and play an important role in the clearance of macromolecules (endobiotics and xenobiotics) from the brain to the blood. There is clinical and experimental data to show that macromolecules are cleared out of the brain in normal and hydrocephalic brains. This article summarizes the existing evidence to support the role of efflux transporters in the pathogenesis of hydrocephalus. The location of p-gp along the pathways of macromolecular clearance and the broad substrate specificity of this abundant transporter to a variety of different macromolecules are reviewed. Involvement of p-gp in the transport of amyloid beta in Alzheimer disease and its relation to normal pressure hydrocephalus is reviewed. Finally, individual variability of p-gp expression might explain the variability in the development of hydrocephalus following intraventricular hemorrhage.

Strategies for delivery of therapeutics into the central nervous system for treatment of lysosomal storage disorders

Lysosomal storage disorders (LSDs) are a group of about fifty life-threatening conditions caused by genetic defects affecting lysosomal components. The underscoring molecular deficiency leads to widespread cellular dysfunction through most tissues in the body, including peripheral organs and the central nervous system (CNS). Efforts during the last few decades have rendered a remarkable advance regarding our knowledge, medical awareness, and early detection of these genetic defects, as well as development of several treatment modalities. Clinical and experimental strategies encompassing enzyme replacement, gene and cell therapies, substrate reduction, and chemical chaperones are showing considerable potential in attenuating the peripheral pathology. However, a major drawback has been encountered regarding the suboptimal impact of these approaches on the CNS pathology. Particular anatomical and biochemical constraints of this tissue pose a major obstacle to the delivery of therapeutics into the CNS. Approaches to overcome these obstacles include modalities of local administration, strategies to enhance the blood-CNS permeability, intranasal delivery, use of exosomes, and those exploiting targeting of transporters and transcytosis pathways in the endothelial lining. The later two approaches are being pursued at the time by coupling therapeutic agents to affinity moieties and drug delivery systems capable of targeting these natural transport routes. This approach is particularly promising, as using paths naturally active at this interface may render safe and effective delivery of LSD therapies into the CNS.

Comparison of the effects of pre-anesthetic administration of normal saline, ringer and voluven on the spread of sensory block with hyperbaric bupivacaine spinal anesthesia

BACKGROUND

Spinal anesthesia is an important and commonly used method for surgical anesthetic in operating rooms. However, even with identical drug dosage and administration mode, the extent of drug distribution in vivo is highly variable and difficult to control. Preanesthetic administration of fluids immediately before spinal anesthesia (preload) is normal practice. The choice of fluid type may affect drug distribution as well as the duration and level of the block.

OBJECTIVES

We examined whether preloads of normal saline, Ringer, or hydroxyethyl starch has different effects on the time it takes to reach maximum block, and the distribution and duration of spinal block level.

PATIENTS AND METHODS

This was a randomized trial and the 150 patients selected were evenly divided into three groups and given; normal saline, Ringer, or hydroxyethyl starch 130/0.4f luids. Preload was given at 10 mL/kg for the normal saline and Ringer groups, and 5 mL/kg for the hydroxyethyl starch group, 10 min before the spinal anesthesia. Sensory block levels were recorded every 5 min until 30 min after spinal anesthesia and then at 60 and 90 min. Time taken to reach maximum and median sensory block, maximum and median level of block, duration of block, and hemodynamic status were recorded.

RESULTS

There were no statistically significant differences in the demographic characteristics between the three groups. Maximum block was higher in normal saline compared to Ringer (P = 0.029). Time taken to reach maximum block was greater in Ringer compared to both normal saline (P = 0.001) and hydroxyethyl starch (P = 0.003). Normal saline had a longer duration of sensory block T10 compared to Ringer and hydroxyethyl starch (P = 0.03).

CONCLUSIONS

Preload fluids have an impact on the level, distribution and duration of sensory block in spinal block. Of the three fluids, normal saline produced the greatest maximum and longest duration of block, whereas time taken to reach maximum block was longer in the Ringer group.

Drug transport across the blood-brain barrier

The blood-brain barrier (BBB) prevents the brain uptake of most pharmaceuticals. This property arises from the epithelial-like tight junctions within the brain capillary endothelium. The BBB is anatomically and functionally distinct from the blood-cerebrospinal fluid barrier at the choroid plexus. Certain small molecule drugs may cross the BBB via lipid-mediated free diffusion, providing the drug has a molecular weight <400 Da and forms <8 hydrogen bonds. These chemical properties are lacking in the majority of small molecule drugs, and all large molecule drugs. Nevertheless, drugs can be reengineered for BBB transport, based on the knowledge of the endogenous transport systems within the BBB. Small molecule drugs can be synthesized that access carrier-mediated transport (CMT) systems within the BBB. Large molecule drugs can be reengineered with molecular Trojan horse delivery systems to access receptor-mediated transport (RMT) systems within the BBB. Peptide and antisense radiopharmaceuticals are made brain-penetrating with the combined use of RMT-based delivery systems and avidin-biotin technology. Knowledge on the endogenous CMT and RMT systems expressed at the BBB enable new solutions to the problem of BBB drug transport.

Paracrine regulation of neural stem cells in the subependymal zone

Stem cells maintain their self-renewal and multipotency capacities through a self-organizing network of transcription factors and intracellular pathways activated by extracellular signaling from the microenvironment or "niche" in which they reside in vivo. In the adult mammalian brain new neurons continue to be generated throughout life of the organisms and this lifelong process of neurogenesis is supported by a reservoir of neural stem cells in the germinal regions. The discovery of adult neurogenesis in the mammalian brain has sparked great interest in defining the conditions that guide neural stem cell (NSC) maintenance and differentiation into the great variety of neuronal and glial subtypes. Here we review current knowledge regarding the paracrine regulation provided by the components of the niche and its function, focusing on the main germinal region of the adult central nervous system (CNS), the subependymal zone (SEZ).

Medicinal chemistry based approaches and nanotechnology-based systems to improve CNS drug targeting and delivery

The central nervous system (CNS) is protected by various barriers, which regulate nervous tissue homeostasis and control the selective and specific uptake, efflux, and metabolism of endogenous and exogenous molecules. Among these barriers is the blood-brain barrier (BBB), a physical and physiological barrier that filters very efficiently and selectively the entry of compounds from the blood to the brain and protects nervous tissue from harmful substances and infectious agents present in the bloodstream. The BBB also prevents the entry of potential drugs. As a result, various drug targeting and delivery strategies are currently being developed to enhance the transport of drugs from the blood to the brain. Following a general introduction, we briefly overview in this review article the fundamental physiological properties of the BBB. Then, we describe current strategies to bypass the BBB (i.e., invasive methods, alternative approaches, and temporary opening) and to cross it (i.e., noninvasive approaches). This section is followed by a chapter addressing the chemical and technological solutions developed to cross the BBB. A special emphasis is given to prodrug-targeting approaches and targeted nanotechnology-based systems, two promising strategies for BBB targeting and delivery of drugs to the brain.

Clinical translation of stem cell therapy in traumatic brain injury: the potential of encapsulated mesenchymal cell biodelivery of glucagon-like peptide-1

Traumatic brain injury remains a major cause of death and disability; it is estimated that annually 10 million people are affected. Preclinical studies have shown the potential therapeutic value of stem cell therapies. Neuroprotective as well as regenerative properties of stem cells have been suggested to be the mechanism of action in preclinical studies. However, up to now stem cell therapy has not been studied extensively in clinical trials. This article summarizes the current experimental evidence and points out hurdles for clinical application. Focusing on a cell therapy in the acute stage of head injury, the potential of encapsulated cell biodelivery as a novel cell-therapeutic approach will also be discussed.

Therapeutic concentrations of glucagon-like peptide-1 in cerebrospinal fluid following cell-based delivery into the cerebral ventricles of cats

Background

Neuropeptides may have considerable potential in the treatment of acute and chronic neurological diseases. Encapsulated genetically engineered cells have been suggested as a means for sustained local delivery of such peptides to the brain. In our experiments, we studied human mesenchymal stem cells which were transfected to produce glucagon-like peptide-1 (GLP-1).

Methods

Cells were packed in a water-permeable mesh bag containing 400 polymeric microcapsules, each containing 3000 cells. The mesh bags were either transplanted into the subdural space, into the brain parenchyma or into the cerebral ventricles of the cat brain. Mesh bags were explanted after two weeks, and cell viability, as well as GLP-1 concentration in the cerebrospinal fluid (CSF), was measured.

Results

Viability of cells did not significantly differ between the three implantation sites. However, CSF concentration of GLP-1 was significantly elevated only after ventricular transplantation with a maximum concentration of 73 pM (binding constant = 70 pM).

Conclusions

This study showed that ventricular cell-based delivery of soluble factors has the capability to achieve concentrations in the CSF which may become pharmacologically active. Despite the controversy about the pharmacokinetic limitations of ventricular drug delivery, there might be a niche in this for encapsulated cell biodelivery of soluble, highly biologically-effective neuropeptides of low molecular weight like GLP-1.

Drug transport in brain via the cerebrospinal fluid

The human brain has no lymphatic system, but produces over a half-liter each day of cerebrospinal fluid. The cerebrospinal fluid is secreted at the choroid plexus and occupies the cavities of the four ventricles, as well as the cranial and spinal sub-arachnoid space. The cerebrospinal fluid moves over the surfaces of the brain and spinal cord and is rapidly absorbed into the general circulation. The choroid plexus forms the blood-cerebrospinal fluid barrier, and this barrier is functionally distinct from the brain microvascular endothelium, which forms the blood-brain barrier. Virtually all non-cellular substances in blood distribute into cerebrospinal fluid, and drug entry into cerebrospinal fluid is not an index of drug transport across the blood-brain barrier. Drug injected into the cerebrospinal fluid rapidly moves into the blood via bulk flow, but penetrates into brain tissue poorly owing to the limitations of diffusion. Drug transport into cerebrospinal fluid vs. brain interstitial fluid requires knowledge of the relative expression of transporters at the choroid plexus versus the brain microvascular endothelium.

Nanotechnology for delivery of drugs to the brain for epilepsy

Epilepsy results from aberrant electrical activity that can affect either a focal area or the entire brain. In treating epilepsy with drugs, the aim is to decrease seizure frequency and severity while minimizing toxicity to the brain and other tissues. Antiepileptic drugs (AEDs) are usually administered by oral and intravenous routes, but these drug treatments are not always effective. Drug access to the brain is severely limited by a number of biological factors, particularly the blood-brain barrier, which impedes the ability of AEDs to enter and remain in the brain. To improve the efficacy of AEDs, new drug delivery strategies are being developed; these methods fall into the three main categories: drug modification, blood-brain barrier modification, and direct drug delivery. Recently, all three methods have been improved through the use of drug-loaded nanoparticles.

Neural cell adhesion molecule (NCAM) and prealbumin in cerebrospinal fluid from depressed patients

The size of the soluble form of the human cerebrospinal fluid (CSF) neural cell adhesion molecule, NCAM-sol, was by gel permeation chromatography estimated to 160-250 kDa. Within the CSF the concentration of NCAM-sol was found about 15-25% increased in lumbar fluid and 25% increased in ventricular fluid, both compared to cisternal fluid. Whereas prealbumin was found evenly distributed in CSF, albumin was relatively enriched in lumbar fluid. The concentrations of NCAM-sol and prealbumin were measured in lumbar CSF from psychiatric patients. Prealbumin was increased 7.2% and NCAM-sol was decreased 15.1% in depressed patients. The changes were partially normalized during recovery from the depression. The findings can be explained by hypothesizing that endogenous depression is associated with an increased choroid plexus activity and CSF production.

Drug targeting to the brain

The goal of brain drug targeting technology is the delivery of therapeutics across the blood-brain barrier (BBB), including the human BBB. This is accomplished by re-engineering pharmaceuticals to cross the BBB via specific endogenous transporters localized within the brain capillary endothelium. Certain endogenous peptides, such as insulin or transferrin, undergo receptor-mediated transport (RMT) across the BBB in vivo. In addition, peptidomimetic monoclonal antibodies (MAb) may also cross the BBB via RMT on the endogenous transporters. The MAb may be used as a molecular Trojan horse to ferry across the BBB large molecule pharmaceuticals, including recombinant proteins, antibodies, RNA interference drugs, or non-viral gene medicines. Fusion proteins of the molecular Trojan horse and either neurotrophins or single chain Fv antibodies have been genetically engineered. The fusion proteins retain bi-functional properties, and both bind the BBB receptor, to trigger transport into brain, and bind the cognate receptor inside brain to induce the pharmacologic effect. Trojan horse liposome technology enables the brain targeting of non-viral plasmid DNA. Molecular Trojan horses may be formulated with fusion protein technology, avidin-biotin technology, or Trojan horse liposomes to target to brain virtually any large molecule pharmaceutical.

Drug targeting to the brain

The central nervous system (CNS) is a sanctuary site and is protected by various barriers. These regulate brain homeostasis and the transport of endogenous and exogenous compounds by controlling their selective and specific uptake, efflux, and metabolism in the brain. Unfortunately, potential drugs for the treatment of most brain diseases are therefore often not able to cross these barriers. As a result, various drug delivery and targeting strategies are currently being developed to enhance the transport and distribution of drugs into the brain. Here we discuss briefly the biology and physiology of the blood-brain barrier (BBB) and the blood-cerebro-spinal-fluid barrier (BCSFB), and, in more detail, the possibilities for delivering large-molecular-weight drugs by local and global delivery and by viral and receptor-mediated nonviral drug delivery to the (human) brain.

Blood-brain barrier delivery

Neuropharmaceutics is the largest potential growth sector of the pharmaceutical industry. However, this growth is blocked by the problem of the blood-brain barrier (BBB). Essentially 100% of large-molecule drugs and >98% of small-molecule drugs do not cross the BBB. The BBB can be traversed because there are multiple endogenous transporters within this barrier. Therefore, brain drug development programs of the future need to be re-configured so that drugs are formulated to enable transport into the brain via endogenous BBB transporters.

Blood-brain barrier and chemotherapeutic treatment of brain tumors

The blood-brain barrier (BBB) is of pivotal importance to maintain homeostasis of the CNS, as it closely regulates the composition of the interstitial fluid in the brain. Unfortunately, malignancies that grow within the CNS may evade chemotherapeutic drugs using the same barrier, making this disease refractory to most chemotherapy regimens. This review will outline the impact of the BBB in brain cancer and discuss the efforts that have been made to enhance the drug exposure of brain tumors. Although this review will focus on the role of the BBB in primary brain cancer (malignant glioma), its impact on brain metastases will also be briefly discussed.

Limited brain diffusion of the glucocorticoid receptor agonist RU28362 following i.c.v. administration: implications for i.c.v. drug delivery and glucocorticoid negative feedback in the hypothalamic-pituitary-adrenal axis

The experiments described herein present a method for tracking diffusion of the glucocorticoid receptor agonist RU28362 in brain following i.c.v. drug administration. A useful property of glucocorticoid receptor is that it is primarily cytoplasmic when unbound and rapidly translocates to the nucleus when bound by ligand. Thus, removal of endogenous glucocorticoids by adrenalectomy allows us to identify brain regions with activated glucocorticoid receptor after i.c.v. glucocorticoid receptor agonist treatment by examining the presence or absence of nuclear glucocorticoid receptor immunostaining. We have previously demonstrated that an i.p. injection of 150 microg/kg RU28362 1 h prior to restraint stress is sufficient to suppress stress-induced hypothalamic-pituitary-adrenal axis hormone secretion [Ginsberg AB, Campeau S, Day HE, Spencer RL (2003) Acute glucocorticoid pretreatment suppresses stress-induced hypothalamic-pituitary-adrenal axis hormone secretion and expression of corticotropin-releasing hormone hnRNA but does not affect c-fos mRNA or fos protein expression in the paraventricular nucleus of the hypothalamus. J Neuroendocrinol 15:1075-1083]. We report here, however, that in rats i.c.v. treatment with a high-dose of RU28362 (1 microg) 1 h prior to stressor onset does not suppress stress-induced hypothalamic-pituitary-adrenal axis activity. We then performed a series of experiments to examine the possible differences in glucocorticoid receptor activation patterns in brain and pituitary after i.c.v. or i.p. treatment with RU28362. In a dose-response study we found that 1 h after i.c.v. injection of RU28362 (0.001, 0.1 and 1.0 microg) glucocorticoid receptor nuclear immunoreactivity was only evident in brain tissue immediately adjacent to the lateral or third ventricle, including the medial but not more lateral portion of the medial parvocellular paraventricular nucleus of the hypothalamus. In contrast, i.p. injection of RU28362 produced a uniform predominantly nuclear glucocorticoid receptor immunostaining pattern throughout all brain tissue. I.c.v. injection of the endogenous glucocorticoid receptor agonist, corticosterone (1 microg) also had limited diffusion into brain tissue. Time-course studies indicated that there was not a greater extent of nuclear glucocorticoid receptor immunostaining present in brain after shorter (10 or 30 min) or longer (2 or 3 h) intervals of time after i.c.v. RU28362 injection. Importantly, time-course studies found that i.c.v. RU28362 produced significant increases in nuclear glucocorticoid receptor immunostaining in the anterior pituitary that were evident within 10 min after injection and maximal after 1 h. These studies support an extensive literature indicating that drugs have very limited ability to diffuse out of the ventricles into brain tissue after i.c.v. injection, while at the same time reaching peripheral tissue sites. In addition, these studies indicate that significant occupancy of some glucocorticoid receptor within the paraventricular nucleus of the hypothalamus and pituitary is not necessarily sufficient to suppress stress-induced hypothalamic-pituitary-adrenal axis activity.

Microscopic morphology and histology of the human meninges

The meninges comprise the dura mater and the leptomeninges (arachnoid and pia mater). Dura forms an outer endosteal layer related to the bones of the skull and spine and an inner layer closely applied to the arachnoid mater. Leptomeninges have multiple functions and anatomical relationships. The outer parietal layer of arachnoid is impermeable to CSF due to tight intercellular junctions; elsewhere leptomeningeal cells form demosomes and gap junctions. Trabeculae of leptomeninges compartmentalize the subarachnoid space and join the pia to arachnoid mater. In bacterial meningitis leptomeningeal cells secrete cytokines. Pia mater is reflected from the surface of the brain and spinal cord onto arteries and veins, thus separating the subarachnoid space from the brain and cord. A sheath of leptomeninges accompanies arteries into the brain and is related to the pathways for the drainage of interstitial fluid that play a role in inflammatory responses in the brain and appear to be blocked by amyloid-beta in Alzheimer's disease. Specialised leptomeningeal cells in the stroma of the choroid plexus form collagen whorls that become calcified with age. Leptomeningeal cells also form channels in the core and apical cap of arachnoid granulations for the drainage of CSF into venous sinuses. In the spine, leptomeninges form highly perforated intermediate sheets of arachnoid and delicate ligaments that compartmentalize the subarachnoid space; dentate ligaments anchor subpial collagen to the dura mater and stabilize the spinal cord. Despite the multiple anatomical arrangements and physiological functions, leptomeningeal cells retain many histological features that are similar from site to site.

New neurons follow the flow of cerebrospinal fluid in the adult brain

In the adult brain, neuroblasts born in the subventricular zone migrate from the walls of the lateral ventricles to the olfactory bulb. How do these cells orient over such a long distance and through complex territories? Here we show that neuroblast migration parallels cerebrospinal fluid (CSF) flow. Beating of ependymal cilia is required for normal CSF flow, concentration gradient formation of CSF guidance molecules, and directional migration of neuroblasts. Results suggest that polarized epithelial cells contribute important vectorial information for guidance of young, migrating neurons.

Targeted delivery across the blood–brain barrier

The safest and most effective way of targeting drugs to the entire brain is via delivery systems directed at endogenous receptor-mediated uptake mechanisms present at the cerebral capillaries. Such systems have been shown to be effective in animal models including primates, but no clinical trials have been performed so far. This review focuses on the well-characterised transferrin and insulin receptor-targeted systems, as well as on the more recently described systems that use the low-density lipoprotein-related protein 1 receptor, the low-density lipoprotein-related protein 2 receptor (also known as megalin and glycoprotein 330) or the diphtheria toxin receptor (which is the membrane-bound precursor of heparin-binding epidermal growth factor-like growth factor). The possibilities and limitations of these systems are compared and their future for human application is discussed.

Considerations in the use of cerebrospinal fluid pharmacokinetics to predict brain target concentrations in the clinical setting: implications of the barriers between blood and brain

In the clinical setting, drug concentrations in cerebrospinal fluid (CSF) are sometimes used as a surrogate for drug concentrations at the target site within the brain. However, the brain consists of multiple compartments and many factors are involved in the transport of drugs from plasma into the brain and the distribution within the brain. In particular, active transport processes at the level of the blood-brain barrier and blood-CSF barrier, such as those mediated by P-glycoprotein, may lead to complex relationships between concentrations in plasma, ventricular and lumbar CSF, and other brain compartments. Therefore, CSF concentrations may be difficult to interpret and may have limited value. Pharmacokinetic data obtained by intracerebral microdialysis monitoring may be used instead, providing more valuable information. As non-invasive alternative techniques, positron emission tomography or magnetic resonance spectroscopy may be of added value.

Vector-mediated drug delivery to the brain

As a consequence of the growing ageing population, many neurodegenerative diseases, cancer and infections of the brain will become more prevalent. Despite major advances in neuroscience, many potential therapeutic agents are denied access to the central nervous system (CNS) because of the existence of the blood-brain barrier (BBB). This barrier is formed by the endothelial cells of the brain capillaries and its primary characteristic is the impermeability of the capillary wall due to the presence of complex tight junctions and a low endocytic activity. The BBB behaves as a continuous lipid bilayer and prevents the passage of polar and lipid-insoluble substances. The BBB is, therefore, the major obstacle to drugs that are potentially useful for combating diseases affecting the CNS. Extensive efforts have been made to develop CNS drug delivery strategies in order to enhance delivery of therapeutic molecules across the BBB. The current challenge is to develop drug-delivery strategies that will allow the passage of therapeutic drugs through the BBB in a safe and effective manner. This review focuses specifically on the strategies developed to enhance drug delivery across the BBB with an emphasis on the vector-mediated strategy.

Brain drug delivery technologies: novel approaches for transporting therapeutics

The blood-brain barrier (BBB) denies many therapeutic agents access to brain tumours and other diseases of the central nervous system (CNS). Despite remarkable advances in our understanding of the mechanisms involved in the development of the brain diseases and the actions of neuroactive agents, drug delivery to the brain remains a challenge. For more than 20 years, extensive efforts have been made to enhance delivery of therapeutic molecules across vascular barriers of the CNS. The current challenge is to develop drug-delivery strategies that will allow the passage of drug molecules through the BBB in a safe and effective manner, and this review will provide an insight into some of the strategies developed to enhance drug delivery across the BBB.

Do centrally administered neuropeptides access cognate receptors?: an analysis in the central corticotropin-releasing factor system

To determine the extent to which centrally administered corticotropin-releasing factor (CRF) activates neurons that express CRF receptors (CRF-Rs), we followed the kinetics and distribution (relative to those of CRF-Rs) of Fos induction seen in response to intracerebroventricular (icv) injection of the peptide (1-10 microg). CRF provoked widespread Fos expression: its strength was dose-related, it peaked at 2 hr after injection, and it was antagonized in a dose-dependent manner by coinjection of CRF-R antagonists. The activation pattern closely mimicked the distribution of CRF-R1 mRNA, in including widespread Fos induction throughout the cortical mantle, in cell groups involved in sensory information processing, and in the cerebellum and several of its major afferents and targets. Dual labeling revealed extensive correspondence of CRF-stimulated Fos-immunoreactivity (Fos-ir) and CRF-R1 mRNA at these and other loci. Unique sites of CRF-R2 expression were relatively unresponsive to CRF but were more so after icv administration of urocortin (UCN), a new mammalian CRF-related peptide. Both CRF and UCN elicited activational responses in cell groups that are involved in central autonomic control but that express neither CRF-R, including the central amygdaloid and paraventricular hypothalamic nuclei, and brainstem catecholaminergic cell groups. The results support an ability of CRF-related peptides in the ventricular system to access receptor-expressing cells directly but leave open questions as to the basis for the recruitment of central autonomic structures, many of which have been identified as stress-related sites of CRF action.

Microdialysis for pharmacokinetic analysis of drug transport to the brain

The intracerebral microdialysis technique represents an important tool for monitoring free drug concentrations in brain extracellular fluid (brain(EcF)) as a function of time. With knowledge of associated free plasma concentrations, it provides information on blood-brain barrier (BBB) drug transport. However, as the implantation of the microdialysis probe evokes tissue reactions, it should be established if the BBB characteristics are maintained under particular microdialysis experimental conditions. Several studies have been performed to evaluate the use of intracerebral microdialysis as a technique to measure drug transport across the BBB and to measure regional pharmacokinetics of drugs in the brain. Under carefully controlled conditions, the intracerebral microdialysis data did reflect passive BBB transport under normal conditions, as well as changes induced by hyperosmolar opening or by the presence of a tumor in the brain. Studies on active BBB transport by the mdr1a-encoded P-glycoprotein (Pgp) were performed, comparing mdr1a(-/-) with wild-type mice. Microdialysis surgery and experimental procedures did not affect Pgp functionality, but the latter did influence in vivo concentration recovery, which was in line with theoretical predictions. It is concluded that intracerebral microdialysis provides meaningful data on drug transport to the brain, only if appropriate methods are applied to determine in vivo concentration recovery.

Methodological considerations of intracerebral microdialysis in pharmacokinetic studies on drug transport across the blood-brain barrier

For the study of the pharmacokinetics of drugs in the brain a number of in vivo techniques is available, including autoradiography, imaging techniques, cerebrospinal fluid sampling and in vivo voltammetry, which all have their specific advantages and limitations. Intracerebral microdialysis is a relatively new in vivo technique. It permits monitoring of local concentrations of drugs and metabolites at specific sites in the brain which makes it an attractive tool for pharmacokinetic research. In the use of this technique a number of factors should be considered. These include: type of probe, surgical trauma, post-surgery interval, perfusion flow rate, as well as composition and temperature of the perfusion medium. In particular in studies on drug transport across the blood-brain barrier (BBB), effects of insertion of the probe on BBB functionality is important. It appears that BBB functionality is not significantly affected if surgical and experimental conditions are well-controlled. The relationship between dialysate concentrations and those in the extracellular fluid of the periprobe tissue, the recovery of the drug, depends on periprobe processes governing the actual concentration of the drug at that site. These include extracellular-microvascular exchange, metabolism, and diffusion of the drug. Several methods have been proposed to determine recovery values. In particular the no net flux method and the extended no net flux method are useful in practice. Several microdialysis studies on BBB transport of drugs are presented showing that intracerebral microdialysis is capable to assess local BBB transport profiles. Compared with other in vivo techniques, intracerebral microdialysis is the only (affordable) technique that offers the possibility to monitor local BBB transport of drugs in unanaesthetized animals, under physiological and pathological conditions.

The ependyma: a protective barrier between brain and cerebrospinal fluid

This review summarizes the current scientific literature concerning the ependymal lining of the cerebral ventricles of the brain with an emphasis on selective barrier function and protective roles for the common ependymal cell. Topics covered include the development, morphology, protein and enzyme expression including reactive changes, and pathology. Some cells lining the neural tube are committed at an early stage to becoming ependymal cells. They serve a secretory function and perhaps act as a cellular/axonal guidance system, particularly during fetal development. In the mature mammalian brain ependymal cells possess the structural and enzymatic characteristics necessary for scavenging and detoxifying a wide variety of substances in the CSF, thus forming a metabolic barrier at the brain-CSF interface.

New approaches to drug delivery through the blood-brain barrier

The development of recombinant proteins, monoclonal antibodies, or antisense oligonucleotides as pharmaceuticals for the brain will require the parallel development of practical strategies for delivery of these pharmaceuticals in vivo through the endothelial wall of capillaries in the brain, the blood-brain barrier. The brain and spinal cord constitute the only organ to be perfused by capillaries having such a barrier, which excludes the uptake into the brain of circulating molecules that do not have access to several specialized transport systems within the barrier. The current challenge for biotechnology is to develop effective drug-delivery strategies to the brain in parallel with the ongoing drug-discovery programs for this organ.

Drug transport across the blood--brain barrier. I. Anatomical and physiological aspects

This review describes various aspects of the transport of drugs across the blood-brain barrier and comprises three parts. In this first part, the anatomical and physiological aspects of blood-brain transport are discussed. It appears that the blood-brain barrier has an anatomical basis at the endothelium of the capillary wall. This endothelium is characterized by the presence of very tight junctions. As a result, the transport by passive diffusion of drugs with a low lipophilicity, is restricted. For certain classes of closely related relatively hydrophilic compounds, however, the presence of specialized carrier systems has been demonstrated which may facilitate transport. Also evidence is presently available, that the permeability of the blood-brain barrier may be under active regulatory control. It is expected that improved knowledge of the anatomical and physiological aspects of the blood-brain barrier and its regulation will provide a scientific basis for the development of strategies to improve the transport of drugs into the central nervous system.

Pathways of fluid drainage from the brain--morphological aspects and immunological significance in rat and man

There is firm physiological evidence for the lymphatic drainage of interstitial fluid and cerebrospinal fluid from the brains of rats, rabbits and cats. The object of this review, is to describe firstly the morphological aspects of lymphatic drainage pathways from the rat brain and secondly, to explore through scanning and transmission electron microscope techniques, the possibility of similar lymphatic drainage pathways in man. Interstitial and oedema fluid spreads diffusely through the white matter in the rat and appears to drain into the ventricular cerebrospinal fluid. In grey matter, however, tracers pass along perivascular spaces to the surface of the brain and into the cerebrospinal fluid. Paravascular compartments in the subarachnoid space follow the course of major arterial branches to the circle of Willis and thence along the ethmoidal arteries to the cribriform plate of the ethmoid bone. Particulate tracers, such as Indian ink, enter channels in the arachnoid beneath the olfactory bulbs and connect directly with nasal lymphatics through channels which pass through holes in the cribriform plate. Proteins and other solutes may also drain along other cranial nerves. Thus, there is a bulk flow pathway for interstitial and cerebrospinal fluid from the rat brain into cervical lymphatics. In man, it is probable that diffuse interstitial drainage of fluid from the white matter occurs in a similar way to that in the rat. Furthermore, the anatomical pathways exist by which bulk drainage of fluid could occur along perivascular spaces from the grey matter into perivascular spaces of the leptomeningeal arteries and thence into the cerebrospinal fluid (CSF).(ABSTRACT TRUNCATED AT 250 WORDS)