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See Inside Scientific American Mind Volume 25, Issue 4

Common Prescription Drugs Alter Blood’s Flow

Medicines can get the blood going, but some side effects remain unclear
vasculature function


Brain blood vasculature as a function of blood flow
Credit: Caitlin Sedwick via Wikimedia Commons

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Many drugs can alter vascular tone, or the degree of constriction in blood vessels. When the blood vessels tighten, blood pressure increases, and when it is too high it can lead to heart attack or stroke. Conversely, when blood pressure is too low people may experience lethargy, sexual dysfunction and other problems. Some drugs designed to affect vascular tone can also have cascading effects in the brain. Whether these unintended outcomes are helpful or harmful remains an open question.

Drug

Example

How They Affect Vasculature

Changes in the Brain

ACE inhibitors

Perindopril, Captopril

ACE inhibitors lower blood pressure by suppressing the action of an enzyme that encourages blood vessels to constrict.

Older generation ACE inhibitors can cross the blood-brain barrier and may stave off cognitive decline in people with Alzheimer’s. Some evidence suggests that the drugs might slow the buildup of harmful proteins in the brain. Newer types of ACE inhibitors, however, don’t cross the barrier, and they have been found to have the opposite effect: they might increase the risk of developing dementia.

Antihistamines

Benadryl, Claritin, Allegra

During an allergic reaction, histamines bind to receptors along the blood vessels, making them expand and become more permeable so white blood cells can attack the pathogen. Antihistamines block this mechanism, in effect narrowing the blood vessels.

In the brain, histamines are important in regulating the sleep-wake cycle. They have also been associated with learning, memory and attention. Older antihistamines can suppress brain activity, leading to drowsiness and learning problems. Newer antihistamines, such as Zyrtec, work more selectively and have fewer side effects.

Beta-blockers

Propranolol, Carvedilol, Atenolol

Beta-blockers prevent the stress hormone epinephrine from binding to receptors in the nervous system. Typically, epinephrine triggers the “fight or flight” response, which causes blood vessels to contract to pump more blood to the heart.

Doctors often prescribe beta-blockers off label for people with anxiety disorders, and actors and musicians sometimes take them to calm stage fright. In addition, a recent study suggests that beta-blockers may reduce the risk of dementia by releasing the overly tight blood vessels in the brain. Beta-blockers can also cause side effects of fatigue, insomnia and, potentially, depression.

Calcium channel blockers

Nifedipine

Calcium channel blockers prevent calcium ions from entering cell membranes in the heart and arteries, a process that normally signals the blood vessels to contract. By reducing the force of contraction, these drugs allow the heart to pump blood with less effort.

Some scientists propose that an excess of calcium channels in the brain can contribute to Parkinson’s disease, as too much calcium can be toxic to dopamine neurons. Epidemiological studies hint that taking calcium channel blockers that cross the blood brain barrier (not all do) reduces the likelihood of developing Parkinson’s.

Decongestants

Ephedrine, Sudafed

Decongestants vanquish runny noses by causing swollen blood vessels in the mucus membrane of the nose and sinuses to tighten. When taken in pill form, this effect extends throughout the body.

Pseudoephedrine, the active ingredient in most decongestants, also causes tightening in the blood vessels in the brain, which can trigger insomnia, restlessness and anxiety, as well as headache if the blood vessels narrow too quickly.

Nonsteroidal anti-inflammatory drugs (NSAIDs)
 

Ibuprofen (Motrin, Advil)

Naproxen (Aleve)

NSAIDs reduce pain and inflammation by preventing the production of prostaglandins and thromboxane, chemical compounds that cause blood vessels to constrict.

Prostaglandins and thromboxane are synthesized from a fatty substance in the brain and muscle tissue that is involved in cellular repair. Some studies suggest that NSAIDs protect against brain atrophy and memory loss in the early stages of Alzheimer’s disease by blocking the breakdown of this fatty substance.

Sildenafil

Viagra, Cialis, Levitra

Stimulating the penis produces cyclic guanosine monophosphate (cGNP), a chemical that dilates blood vessels. Viagra inhibits an enzyme that destroys cGNP, enabling more blood to flow to the organ.

Blocking the enzyme that destroys cGNP also increases the amount of oxytocin in the brain. Sometimes called the “love hormone,” oxytocin facilitates emotional bonding. In addition, animal studies indicate that Viagra can increase blood flow and promote nerve growth in the brain, potentially alleviating depression.

 

This article was originally published with the title "Out For Blood."

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