BBB functions and physiology: The central nervous system (CNS) necessitates a highly controlled environment for its proper functioning. Blood brain barrier is the system which maintains the homeostasis of the CNS. It works by separating and shielding the brain from the constant changes in the nutrient and chemical composition of systemic blood. It protects the brain tissue from harmful chemicals that might be present in the blood. The key functions of the BBB are transport of nutrients from the blood to brain and metabolites in opposite direction to systemic circulation. Only a small number of molecules are allowed access through the BBB system into brain. BBB also contains phase I and II metabolic enzymes that carry out modification of molecules that pass through. Some drugs that are able to cross the BBB are removed by means of transporter mechanism located on the abluminal side (brain side of BBB). BBB is composed of three elements, namely, the endothelial cells of capillaries surrounding the brain, the perivascular end feet of the astrocyte cells and the basement membrane1. The endothelial cells surrounding the brain differ from other endothelial cells in having tight junctions (TJs) between them, absence of pores, high concentration of mitochondria and lack of pinocytic vacuoles2. The BBB is highly impervious due to the presence of TJs. The adjacent endothelial cells of BBB are joined together by fragments known as zona occludens2. TJs are formed by transmembrane proteins such as claudin, junction adhesion molecule etc. The pericytes that surround blood capillary endothelial cells have claw-like projections that entangle the capillary in BBB2.
BBB as transport mechanism and barrier to toxic agents: Transport of nutrients, drugs and toxic compounds across the BBB depend on their lipophilicity and size. Water passes through the BBB by simple diffusion. Other small molecules such as O2 and CO2 can also diffuse easily through it. Highly lipophilic (lipid soluble) molecules such as nicotine and diazepam can diffuse into the brain easily. Molecules that are polar or charged require transporter mechanism and do not diffuse/pass through BBB. Water soluble nutrients such as glucose and amino acids are transported by facilitated diffusion or transport by carrier proteins without expenditure of energy3. Figure 1 demonstrates a simplified representation of transport of substances through the lipid bilayer of cell membrane4. The addictive nature of many drugs stems from the fact that they are lipophilic, can easily diffuse through the BBB bind to the receptors and generate feeling of euphoria and psychoactive effects. It has been recently demonstrated that 6-monoacetyl morphine (6MAM), the habit forming primary metabolite of heroin formed in the blood can pass through BBB due to its lipophilic nature and low molecular weight. It has a greater half life than heroin (25 min. vs. 7.8 min) and binds to opioid receptor. Morphine, due to two hydroxyl groups cannot penetrate BBB easily. Following a single 12 mg intravenous mg dose of heroin, a peak heroin concentration of 0.141 mg/L was obtained at 2 minutes, while the 6-acetylmorphine and concentration was 0.151mg/mL5. Diphenoxylate, which is commonly old as Lomotil is used in treatment of diarrhea. It can cross the blood–brain barrier (MW 452) and binds to opioid receptor in the brain making it a habit forming substance. Lomotil is classified as Schedule V substance under controlled substance act6.
Fig 1. Transport across lipid bilayer.
In contrast to some carrier proteins that transport molecules into the BBB, many proteins are also present in the cerebral capillaries that prevent the entry of compounds to brain or cause efflux from the brain. These transporter proteins remove a number of drugs and toxic substances against concentration gradient by active transport following expenditure of energy (ATP). ATP-binding cassette (ABC) family of transport proteins such as the P-glycoprotein7, multidrug resistance proteins (MDR) and breast cancer resistance protein (BCRP) carry out efflux of many drug molecules, toxic substances and xenobiotics. Solute carrier family (SLC) of transporters is another family of transporter proteins utilized for removal of organic anions such as steroid conjugates, cardiac glycosides etc. Antioxidant enzymes are also found in the BBB cells that form enzymatic barrier for oxidizing agents and xenobiotics.
Placental physiology and effectiveness against drugs and chemicals
Physiology of placenta and function: Placenta is the organ that provides structural and nutritional support to the developing fetus. The key functions of the placenta are transport of nutrients and gases to and removal of waste products from the fetus. It is formed by fetal (chorion frondosum) and maternal (decidua basalis) tissues8. Implantation of blastocyst triggers the beginning of placenta development. Trophoblast that forms the outer layer of placenta is formed off the outer layer of blastocyst. Trophoblast is divided into two layers: cytotrophoblast and syncytiotrophoblast. Syncytiotrophoblast is a continuous layer of cells covering the placental surface. Cytotrophoblast cells undergo fusion and differentiation to form syncytiotrophoblast. The trophoblast, syncytiotrophoblast, cytotrophoblast and capillary endothelium cells form the blood placental barrier8,9. Placental barrier separates the blood supply between the mother and her fetus. The factors that control the permeability of compounds through the placental barrier are size, lipid solubility and degree of ionization. Small lipophilic molecules diffuse freely across the placental membrane, while large hydrophilic molecules require unique transporter mechanism to pass through. Water molecules diffuse through the placental membrane. Gas molecules such as O2 and CO2 are able to pass through simple diffusion. Maternal blood has high oxygen partial pressure than fetal blood and in response to difference in partial pressure O2 diffuses to fetal side while CO2 diffuses to maternal side. Nutrient such as glucose is transported by non insulin dependent hexose transporter via facilitated diffusion. Certain xenobiotics that are structurally similar to endogenous molecules are transferred into fetal blood by facilitated diffusion. Transport of amino acids from maternal side to fetal side occurs by active transport against concentration gradient. This stems from the fact that fetal blood has higher amino acid concentration than maternal blood. Excretion of xenobiotics from fecal into maternal side occurs across concentration gradient by active transport and requires expenditure of energy. Many of the teratogenic agents such as thalidomide, ethanol can simply diffuse from maternal to fetal side due to lipophilic nature. Infectious agents such as polio, rubella and measles causing virus can easily pass across the placenta towards the fetus. Another example of transport across the placental barrier of harmful substance is Agent Orange. Agent Orange, an herbicide, used in Vietnam War was contaminated with a very toxic digoxin analog. When a pregnant mother gets exposed to dioxin, the teratogen enters the fetus after passing through placental barrier. Dioxin is taken up by the cells by binding to a protein transcription factor called the aryl hydrocarbon receptor. Following binding of digoxin to AhR, the protein moves to the nucleus, where it influences gene expression and can result in birth defects10. A few compounds pass through the placental barrier towards the fetus through phagocytosis or pinocytosis9. Pregnant women on exposure to heavy metals such as Hg, Pb, and Cd can transmit these metals to fetus. Concentration of Pb was found to be around 100 ng/g ww in the placenta in the US in 1970s, however, it went down <50 ng/g ww since 1980s11.
Protective role of placenta: A key fetoprotective mechanism involves the presence of drug transporter proteins throughout the blood placental barrier. The protein transporters present in the blood placental barrier include the ATP-binding cassette transporter (ABC) family, ATP- independent transporters in solute carrier protein family, monocarboxylate and nucleoside transporters12. Members of ABC family namely the P-glycoprotein, BCRP and multidrug resistance associated protein ABCC1-3. These transporter proteins are critical for efflux of xenobiotics, chemotherapeutic agents, organic anions and teratogens from the fetuses to the maternal blood. Umbenhauer et.al. have demonstrated that lack of P-gp in placenta of mouse fetuses increase their susceptibility to teratogenic effect of pesticide ivermectin13.
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