Blood-brain barrier

The barrier is a feature of the body's blood vessels, not an outcome. It comes mainly from the endothelial cells that form the vessel walls, through a mix of physical, transport and metabolic properties [1]. Other vascular, immune and nerve cells help regulate those properties [1]. This tight control keeps conditions stable for neurons and shields neural tissue from toxins and pathogens [1].

A 2021 review describes the barrier as a dynamic interface between blood and brain [3]. It is also selective. A 2005 review estimated that it keeps out about 100% of large-molecule brain drugs and more than 98% of small-molecule drugs [2]. The 2021 review adds that many therapeutic agents cannot reach the brain at effective doses while the barrier is intact [3].

Whether a compound crosses the barrier is an early question for any claim about the brain. Some answers are clear: serotonin made in the body cannot readily cross it [4]. Others are open. For GABA, a 2015 review found the studies on barrier passage often contradictory and their methods widely different [5].

Crossing the barrier, on its own, does not show an effect on any outcome measured in people. It is one step in a mechanism. On our compound pages, statements about barrier passage are labeled as mechanism, with the species or model named, and grades rest on outcomes measured in people.

• Neurotransmitter
• Serotonin
• GABA