Drugs & Blood Brain Barrier

It is recommended that you have an understanding of the Signal Transmission page before you move on to this page.

When a neurotransmitter is released, its conformation (shape) is what enables it to bind to the postsynaptic neuron's receptor. As long as the molecule has a conformation that "fits" the conformation of the receptor (a more precise understanding of this is given by biochemistry), it will be able to bind. However, molecules other than just the intended neurotransmitter may be able to bind to the receptor. In other words, any molecule with a conformation that fits the receptor's conformation will bind to the receptor and cause the postsynaptic neuron to have an action potential.

A simplistic model with keyboard characters follows.

Let's say that this is neurotransmitter A:     >

And let's say that this is receptor B:            >=== 

And call this molecule C:                            *->

You can see that neurotransmitter A can fit into receptor B like this: >>===.

But molecule C can also fit into receptor B: *->>===.

Imagine that the A-B connection occurs in a network responsible for dopamine release (which, among other things, causes happiness). A is usually only released under certain conditions (for example, receiving a good test score). If molecule C is introduced, it can bind to receptor B whether or not those conditions are met. This means that dopamine release (and as a result, happiness) can be triggered whenever molecule C is present. It is now possible to bypass the ordinary life events that create happiness, and instead have happiness whenever you inject or inhale molecule C.

Perhaps there are many molecules that would fit into receptors and could have psychoactive results. Not all of them are able to reach the receptors, though. Millions of years of evolution have prevented many molecules from being able to enter your brain because of their adverse effects. Only drugs that pass the blood brain barrier are able to reach neurons. The cells of the BBB are very close together and selectively permeable, meaning that very few molecules are able to cross it. There are many requirements to passing the blood brain barrier which can be found on this Wikipedia page. A few important sentences from that page about the types of molecules that can be let in are here:

"The blood–brain barrier allows the passage of water, some gases, and lipid soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids that are crucial to neural function. On the other hand, the blood–brain barrier may prevent the entry of lipophilic, potential neurotoxins by way of an active transport mechanism mediated by P-glycoprotein. "