Chemical stressors refer to toxic agents such as gasses like ozone and sulfur dioxide, heavy metals, and pesticides/herbicides. These substances induce change – usually damage – in living organisms like the human body. Many chemical stressors occur naturally or have natural counterparts.
However, what all chemical stressors have in common: they work by crossing the blood-brain barrier. Inhaling ozone, whose concentration in the air increases significantly on hot sunny days, can lead to chest pain, sore throat, airway inflammation, and more frequent asthma attacks. People with the highest risk of experiencing the negative health impact of ozone exposure are those with asthma, children, the elderly, and outdoor workers. Similarly, mercury exposure will cause decreased coordination, increased tremors, decreased sensation, and damage to the nervous system, kidneys, liver, and the immune system in the long run.
Pesticides/herbicides, another chemical stressor, can also cross the blood-brain barrier and result in numerous terrible diseases and health problems with regular exposure. One of the most dangerous herbicides is paraquat, sold in the United States under plenty of brand names, such as Gramoxone, Blanco, Parazone, and Firestorm. It is a potent herbicide developed to destroy weeds and grasses that have become resistant to glyphosate, another toxic substance commonly used across the country.
The disease most often associated with extensive paraquat exposure is Parkinson’s disease. Nevertheless, short-term exposure can lead to excess fluid in the lungs and gastrointestinal problems such as nausea, vomiting, diarrhea, heart failure, and kidney failure. People exposed to paraquat and maneb, a fungicide, have a whopping 250% greater risk of developing Parkinson’s disease than the rest of the population. Let’s find out if and how herbicide exposure can cause Parkinson’s disease.
What Is the Blood-Brain Barrier, and Can Chemical Stressors such as Herbicides Cross It?
The blood-brain barrier surrounds most blood vessels in the brain and is highly selective, which means it only allows certain substances to cross it and reach the brain. Its function is to protect the brain from toxins, pathogens, and circulating neurotransmitters such as glutamate that can damage neurons if their levels increase too much.
The blood-brain barrier acts as a boundary between the circulating blood and the brain’s extracellular space. Only water, some gasses like oxygen, and lipid-soluble substances can easily cross it. Dangerous chemical stressors can sometimes cross the blood-brain barrier, leading to serious health consequences. To enter the central nervous system, chemical stressors such as herbicides directly interact with the luminal side of the cerebral endothelium, which makes up the blood-brain barrier.
As a fundamental component of the neurovascular unit, the blood-brain barrier acts as the interface of the brain, facilitating communication between the central nervous system and the peripheral nervous system. Its purpose is to protect the brain against circulating toxins and pathogens that can cause brain infections, including chemical stressors, while allowing essential nutrients to reach the brain.
When it comes to herbicides like paraquat, several old medical studies found that the toxic substance cannot cross the blood-brain barrier. Therefore it wouldn’t be able to cause Parkinson’s disease because it cannot reach the part of the brain responsible for releasing dopamine known as the substantia nigra. Paraquat manufacturers have been heavily relying on these outdated studies, bringing them up every time Parkinson’s disease as a result of paraquat exposure was brought up.
In previous rat experiments, it was seen that paraquat is poorly absorbed from the gastrointestinal tract. Consequently, roughly 50% of a single dose ingested by rats settles in their gut within 32 hours after administration, and only 5% to 10% of it is absorbed. Excretion occurs in urine and feces, with approximately 45% of a single dose eliminated after 48 hours. Because of these findings and some early evidence that paraquat is mostly excluded from the brain, it has been assumed that the herbicide would not cross the blood-brain barrier to such a great extent that it would cause Parkinson’s disease. Nevertheless, recent studies have shown that paraquat can actually cross the blood-brain barrier and lead to Parkinson’s.
How Can Paraquat Cross the Blood-Brain Barrier and Cause Parkinson’s Disease?
Recent medical studies were carried out to determine whether paraquat can cross the blood-brain barrier and cause Parkinson’s disease. Paraquat is structurally similar to N-methyl-4-phenyl pyridinium (MPP+), a neurotoxin that interferes with oxidative cellular processes and eventually leads to cell death. This neurotoxin ultimately causes parkinsonism by destroying certain neurons that produce dopamine in the substantia nigra. Using a brain microdialysis technique, it was shown that the herbicide could cross the blood-brain barrier. This technique entails sampling and collecting small-molecular-weight substances from interstitial spaces, like the space between the brain and the blood-brain barrier. Nowadays, it is a widely used method in neuroscience, being one of the few techniques available that allow the quantification of neurotransmitters such as dopamine.
Paraquat induces oxidative stress, followed by neuronal loss in the substantia nigra. Oxidative stress and the imbalance between free radicals and antioxidants in the body may cause cellular damage, impair the DNA repair system, and mitochondrial dysfunction, leading to neurodegenerative disorders such as Parkinson’s disease. The symptoms of Parkinson’s disease stem primarily from low levels of dopamine. This neurotransmitter plays a crucial role in sending messages to the part of the brain that controls movement and coordination. Impaired movement and coordination are hallmarks of Parkinson’s disease, along with symptoms such as rigid muscles, tremors in the hands, and speech changes.
Paraquat can reach the central nervous system through neutral amino acid transporters associated with the blood-brain barrier. The neurotoxicity of paraquat may be related to the herbicide altering the blood-brain barrier’s permeability. The processes governing the entrance and release of chemical stressors through the blood-brain barrier are biological. They operate according to principles applicable in toxicology and pharmacology. The blood-brain barrier cannot differentiate between helpful substances and toxicants crossing it.
New studies focusing on the link between lack of dopamine and Parkinson’s disease have also revealed that other brain chemicals may play a role in developing this neurological disorder. Nevertheless, a low level of dopamine remains the main culprit behind Parkinson’s disease.
About the Author
As the case manager at Atraxia Law, Miguel Leyva specializes in gathering and organizing relevant information about the injuries people suffered due to herbicide exposure. He also provides support to those struggling with Parkinson’s disease due to paraquat exposure.