Stress and the BBB
©Arlene R. Taylor PhD www.arlenetaylor.org
“Okay,” Harvey said, setting his laptop on the dining room table, “I’m off on an internet search about the BBB, as our visiting professor referred to it this morning. I need to write 500 words for class tomorrow. Good thing I can do a search right here at home.” Harvey powered up his Latitude.
“Are you talking about the Better Business Bureau?” his wife asked.
Harvey looked up from his laptop, a puzzled expression on his face. “The Better Business Bureau? What’s that got to do with our neurophysiology lab?”
“I thought that’s what BBB stood for,” said his wife.
“Oh! No!” said Harvey, dissolving into laughter. “Sorry. Blood Brain Barrier. That was a good guess, though. The prof explained the BBB as a method of ‘separating circulating blood in the capillaries from the extracellular fluid in the brain and central nervous system,’ as he put it.” And Harvey was heads down again surfing the Internet for relevant information. “Boy,” he exclaimed a few moments later. “This is really interesting stuff. Talk about a Star-Wars-type shield!”
Of the estimated 60,000 blood vessels (arteries, veins, and capillaries) that run throughout the human organism, about 400 miles of vessels are found within the human brain. What Harvey found in his internet search was that the Blood Brain Barrier or BBB, is a marvelous piece of workmanship. During the first trimester of gestation, all vertebrate brains develop a protective covering that occurs along all capillaries and is designed to create tight junctions that, in effect, join the membranes together so closely that they seal the capillary walls, forming in effect a virtually impermeable barrier to fluid.
Most capillaries contain slit pores so molecules can diffuse easily from the tiny blood vessels out into the surrounding tissues. They function much like a soaker hose would in your garden. BBB capillaries have no slit pores. Instead, they are lined with tight epithelial cells to help prevent molecules from diffusing out. In addition, glial cells, neuron helpers that may actually outnumber neurons in the brain, provide additional protection. The most abundant type of glial cells in the brain and central nervous system or CNS, these glial cells are known as astrocytes because of their characteristic star-shape. A matrix of fibers lining the interior surface of the blood vessels, also plays a part.
The specialized capillaries, and astrocytes (glial cells), and the fiber matrix are primary components of the BBB. It is designed to shield the brain from viruses, toxins, and other potentially harmful large molecules, as well as unpredictable fluctuations in a variety of body substances. The shield prevents stuff that is able to get into the blood stream from being passed into the brain and central nervous system.
The brain is the first body organ to recognize a stressor and it does so almost instantly. When this happens, the brain triggers the release of a cascade of stress hormones such as adrenalin and cortisol to assist in managing the stressful event. The brain and body are an integrated system, working together, hand in glove. Stress hormones were intended to help the brain and body respond effectively to stressful events; they were never intended to be triggered ongoing.
Within the brain itself, the hippocampus, the brain’s search engine, may be the most susceptible to these stress hormones. (Hippocampi, really; two little identical brain organs in the mammalian or second layer of the brain.) High levels of stress hormones can eventually kill brain cells in the hippocampus, which can interfere with its function. This may show up in a specific brain as a decreased ability to learn new information, to transfer information from short- to long-term memory, and to recall stored information.
During the Persian Gulf War, Israeli soldiers received pyridostigmine, a drug that attaches to receptors on nerves located outside the central nervous system (CNS). When chemical weapons invade the body, they are unable to find any open docking stations (bind to the receptors) because they are already occupied with the drug pyridostigmine. This limits the ability of chemical weapons to cause damage. Normally, only very small amounts of pyridostigmine make it across the BBB. During the war, about twenty-five percent of the soldiers who had received this drug complained of neurological symptoms including headaches and drowsiness. The symptoms would likely occur only if the substance had crossed the BBB. By way of comparison, during peacetime when researchers gave this drug to another group of soldiers, only eight percent reported any symptoms.
Physicians performed similar studies with mice. The research animals were forced to swim for two four-minute intervals. The control group were not required to swim. It required over 100 times more pyridostigmine to penetrate the brains of unstressed mice compared to the mice that were required to swim. The researchers repeated the study using a large molecule of blue dye and found similar results. Substances that should not have crossed the blood-brain barrier appeared to do so under conditions of stress.
Stress appears to increase the permeability of the BBB. Stated another way, stressors have been found to increase the ability of molecules to cross the BBB, sometimes rather dramatically. As the BBB becomes more porous, unwanted molecules are able to get into the brain and wreak all manner of havoc. This also leaves the brain vulnerable to unpredictable fluctuations in a variety of body substances including excess stress hormones, which can even increase one’s risk of stroke.
“So, Harvey,” his wife asked a couple hours later, “what did you learn?”
“How much time do you have?” asked Harvey, stretching happily. “For one, hormones are very powerful. For another, you must have stress hormones to live but too many of them appear able to weaken the BBB. The result of increased BBB permeability could even impact your longevity.”
Harvey was correct on all accounts. You cannot live without stress hormones but they can have a cumulative effect, similar to that of repeated exposure to radiation. And yes, hormones are exceedingly powerful. When adrenalin and cortisol are released in a sudden rush, they can create an internal chemical tsunami. And it can be equally problematic when they are released chronically in amounts that put the body’s levels out of balance.
“Here’s my closing sentence,” said Harvey, as he hit the print button on his laptop: “Learning to recognize stressors quickly and managing them appropriately, along with doing whatever you can to rebalance out-of-balance hormones, are critically important strategies for life and health. You can help protect your BBB!”