Definition: The chemical substances to which people are referring when they mention "brain chemistry," neurotransmitters pass messages from one neuron (nerve cell in the brain or spinal cord) to the next.
Common neurotransmitters and their functions include:
Depression is often blamed at least in part on reduced levels of serotonin in the brain. This is why people with true clinical depressions don't just feel moody or sad...they also have sleep and appetite problems as well. Medications like Prozac, Zoloft, Paxil, Celexa, and Lexapro inhibit the reuptake of serotonin in the brain. In other words, they force the brain to use the serotonin it's making over and over instead of just recycling it right away, which makes the brain behave as if it were making enough in the first place.
Norepinephrine is the other brain chemical most often implicated in depression. Low norepinephrine levels explain problems with motivation and concentration in people with depression. Drugs like Wellbutrin, Cymbalta, Buspar, and Effexor inhibit the reuptake of norepinephrine and serotonin in the brain.
Drugs like cocaine make the brain re-use dopamine over and over, creating feelings of pleasure. However, too much dopamine in the brain has also been linked to psychosis and schizophrenia. Too little dopamine in the brain causes diseases like Parkinson's.
People who have anxiety disorders may not be using GABA (Gamma-AminoButyric Acid) correctly; anti-anxiety drugs like Valium and Xanax increase GABA levels in the brain.
Botox inhibits acetylcholine, which is why the muscles into which it's injected are paralyzed!
Neurotransmitters are manufactured by neurons, which "ship out" their product in packages called synaptic vesicles. Synaptic vesicles travel to the "shipping docks," axon terminals, and then burst open to release the neurotransmitters into the synaptic space, the space between neurons.
The neurotransmitters cross the space and plug into the "receiving docks," receptor sites, on the next neuron. This sends whatever message is associated with the neurotransmitter in question.
So serotonin neurotransmitters send messages about sleep and appetite and mood, while norepinephrine neurotransmitters send messages about concentration, motivation, and alertness.
Once they've done their job, the neurotransmitters return to the original neuron. They go back across the synaptic space and are sucked up by re-uptake pumps, where the "factory" in the neuron recycles them.
Selective Serotonin Reuptake Inhibitors like Prozac keep the pumps from sucking up the neurotransmitters as quickly. So the neurotransmitters go back and do their jobs again, returning to the receptors of the receiving neuron and sending the message again. This makes the brain behave the same way it would as if it were making enough serotonin in the first place, and using each neurotransmitter only once.