Expert: John Locke - 6/28/2011

Question
Hello,

My question is how do biochemicals affect the body and the brain at the same time if there is a blood/brain barrier. For example, Morphine has an overlapping effect on both the brain and body (relieves physical pain in body and creates euphoric high in mind).

Similarly, upon completing a meal the peptide Cholecystokinin (CCK) is released into the bloodstream where it binds with receptors in the brain and in the gut, activating a different cellular response in each. When CCK binds with receptors in the small intestine it stimulates the release of enzymes necessary for digestion; while the binding of CCK with neural receptors produces feelings of satisfaction and satiety.

Another example of a peptide that intelligently synchronizes the behavior of the brain and body is oxytocin, a peptide that synchronizes the process of having children.

Before birth: Research has suggested that oxytocin may be the biochemical that inhibits fear and anxiety during sexual activity, without which orgasm (and conception) would be impossible.  During birth: Oxytocin binds with receptors in the uterus where it activates the cells and systems that produce the uterine contractions necessary for childbirth. After birth: Oxytocin stimulates the cells necessary for the release of milk during breastfeeding. Oxytocin also acts to initiate maternal behavior and may also enhance the bonding mechanism between mother and child.

Answer
Thanks for using AllExperts. You are quite correct in noting that the blood-brain barriers exists to protect the brain and central nervous system from exposure to many potentially noxious agents that could gain entry to the human body. By various mechanisms, however, substances may cross the barrier (often termed the BBB) to affect the CNS, which I will briefly review here. To read an explanation of the physiology of the BBB, I would suggest reading this site:
http://www.uic.edu/classes/neuros/neurosvascular1/chapter12.pdf

1. The BBB is not complete. Various parts of the brain are not covered by the BBB, allowing for the transport of materials across it. The ventricular system of the brain is the largest of these; various organs within the ventricular system are exposed directly to the systemic circulation with no shielding from the BBB. These include the pituitary system, the pineal gland, and the area postrema, which is responsible for producing vomiting in response to noxious substances. A few other such areas exist that are beyond the scope of my answer here, but they function similarly to these examples.

There is a good reason for this absence of the BBB in each of these cases. The pituitary system and pineal gland both secrete hormones that must gain access to the systemic circulation to have an effect, so they cannot be shielded behind the BBB. The area postrema must be able to sample the bloodsteam to detect if any noxious, toxic substances are present.

2. Some substances can cross the BBB by virtue of their intrinsic chemistry. Large molecules and highly polar molecules are both restricted from crossing--this includes many of the drugs that are commonly used in medical practice. Small, nonpolar molecules can pass with varying degrees of ease. This includes gases that obviously need to be exchanged, such as oxygen and carbon dioxide; it also includes small molecules such as hormones--CCK, oxytocin, etc.--and the opioid drugs (morphine, heroin, etc). Thus, anything that is able to cross the BBB will potentially have some effect on the central nervous system.

If you have any further questions, please let me or one of the other experts know.