It was going to be a special day in Tom’s science class today, with no lesson, but with a guest lecturer. Ross Evans, a professor from the school of pharmacy at the local university, was coming to talk about medications, and Tom’s teacher was offering extra credit to students who showed initiative in asking questions and participating in the discussion. Tom had watched a TV program the night before about the brain. On the show, it was mentioned that many drugs affected the brain, sometimes in a way that helped and other times in a harmful way. Tom had some questions to ask.
At the end of the talk, Tom posed this one: “How do drugs get into the brain, and once they are in there, what do they do?”
“That’s a very good question,” replied Dr. Evans. “Drugs are given for the most part by mouth or by an injection or shot. The drug enters the bloodstream and is taken by the blood to many parts of the body, so drugs can easily gain access to parts of the body such as the liver, heart, or kidney. The brain, however, is not as easy to penetrate. The brain is a very important part of the body, responsible for controlling almost all of the body’s everyday functions. Can anyone tell me what one of these functions is?”
Susie replied, “The brain controls breathing. You don’t even need to think about breathing. The brain just does it for you.”
The Blood-Brain Barrier
“Correct,” replied Dr. Evans. “Breathing is one of the most important functions of the brain. Because the brain is so important, it has a special protective barrier. This barrier is known as the blood-brain barrier. The blood-brain barrier is the major roadblock for drugs getting from the bloodstream into the brain. Only certain types of drugs can get past the barrier.”
“I know that alcohol gets into the brain,” said Tom. “We’re told that all the time.”
“True,” replied Dr. Evans. “Alcohol and many other drugs get into the brain very easily. But other drugs, such as tetracycline, an antibiotic drug, do not penetrate the blood-brain barrier very well at all.”
Dr. Evans went on to explain that there are two factors that control whether a drug will cross the blood-brain barrier. The first is the size of a single molecule of the drug. Drugs made up of very small single molecules, such as alcohol, cross the blood-brain barrier very easily. Drugs with very large molecules, such as tetracycline, do not cross as easily.
The other major factor involved in a drug getting into the brain is fat solubility. Dr. Evans explained that fat solubility refers to the ability of the drug to dissolve in fatty-type substances, like oil. Some drugs are very fat-soluble, which makes them able to cross the blood-brain barrier with ease. A drug like this would be diazepam, commonly known by its brand name, Valium. Other drugs tend to be more water soluble, for example pencillin. Getting these drugs to cross the blood-brain barrier is like trying to mix water in oil – it just doesn’t work very well.
Binding to a Receptor
Dr. Evans went to answer the second part of Tom’s question about what drugs do once they get into the brain. “Once a drug gets into the brain,” he said, “the drug binds to a receptor.”
Dr. Evans explained that a receptor is like an electrical plug and socket. When you plug a light into an electrical socket and turn it on, you cause electricity to flow, and the light goes on. Similarly, when something binds to a receptor, an action takes places. An example of this is a receptor in the heart. When the drug adrenaline binds to receptors in the heart, the heart beats faster.
In the brain, many different substances, called neurotransmitters, can bind to receptors. Adrenaline binding to certain receptors in the brain may cause a person to feel nervous or anxious. Receptors have different actions depending on where they are located.
Neurotransmitters bind to receptors the way a key fits a lock. A particular kind of receptor will only bind to a certain neurotransmitter. Scientists who make drugs often try to figure out what the “key” looks like and design the drug to look like the key so that it will fit into the lock. In this way, drug are able to bind to receptors in the body.
Stimulate or Block?
Drugs can have many different actions on receptors. Drugs can stimulate the receptor and cause more of the action the receptor is responsible for. For example, amphetamines (stimulant drugs) can stimulate receptors and make the person feel nervous, anxious, and full of energy.
In contrast, drugs can also block the action of the receptor. For instance, antihistamines can block certain receptors and make a person feel drowsy. These actions can be beneficial or unwanted. For example, an antihistamine can bind to receptors in the nose to stop a person from having allergy symptoms, and this is their beneficial effect. At the same time, the antihistamine can bind to receptors in the brain that make the person feel drowsy, and this is an unwanted effect of the drug.
“As you can see,” Dr. Evans concluded, “drugs are very complex chemicals. The brain is very complex, and an important part of the body. The way that drugs get into the brain and effect the receptors of the brain is important for you to understand. This way you can know how drugs cause many of the effects that are important in treating many medical problems.”