Answer: Cocaine enhances the permeability of the blood-brain barrier.
The blood-brain barrier is an anatomical adaptation that separates the bloodstream from nervous tissue, protecting the brain and spinal cord from contaminants or toxins found in the blood. The barrier itself is made up of endothelial cells, and is very selectively permeable, allowing gases and some chemicals to pass.
In addition to centrally mediated rewarding effects through the action on monoamine reuptake inhibition, cocaine is also known to disrupt the integrity of the blood-brain barrier, compromising its ability to prevent other chemicals from entering the brain. When cocaine is in the bloodstream, the increased permeability allows other possible contaminants or parasites to enter the brain.
One of the major risks with cocaine usage is the increased likelihood to engage in risky sexual behaviors. This factor, in combination with the weakened blood-brain barrier, can lead to increased access of HIV into the brain.
The main culprit for how cocaine disrupts the blood-brain barrier is a signaling molecule called platelet-derived growth factor. Cocaine causes PDGF to bind to the endothelial cells that make up the blood brain barrier near the small blood vessels in the brain. Following this, mitogen-activated protein kinases and Egr-1 signaling pathways are activated, which result in greater expression of the PDGF. These results were published in the journal Blood (Cocaine-mediated induction of platelet-derived growth factor: implication for increased vascular permeability)