What is dopamine? It's a neurotransmitter. It controls communication in the brain - it's a chemical that can tell a neuron to fire off a signal or not, and modulates the signals. Dopamine is ancient - found in lizard brains and every other animal along the evolutionary tree up to homo sapiens. But humans have a great deal of dopamine, and over many generations we have evolved to have more and more.
Control of dopamine and where it ends up in the brain isn't just determined by straight up mendalian genetics. As I discussed in this post, our mothers' neurochemical environment had a lot to do with how our dopamine machinery migrates and works in our brains.
Another special thing about humans is our bipedalism. Being upright while mom is pregnant exposes our fetal brains to different vestibular environments than other primates, so the theory is this elevated the dopamine levels in the left hemisphere of most people's brains. I know. Go with it for a minute. It's just a theory.
Humans also eat a lot of meat and fish compared to other primates - meat and fish give us more dopamine precursors. More dopamine is also associated with both greater competitiveness, aggression, and impulse control - one could see how that particular combination of traits would be selected for over human evolution.
Serotonin, another neurotransmitter, is our oldest neurotransmitter and the original antioxidant - dopamine is what made humans so successful.
Now the biochemistry. Dopamine is a type of neurotransmitter called a catecholamine. Catecholamines have, not surprisingly, a catechol chemical group attached to an amine.
How do we get dopamine? We eat it. The precursor amino acid from the protein we eat is called tyrosine. Tyrosine becomes dopa via the enzyme tyrosine hydroxylase, and dopa becomes dopamine via the actions of dopa decarboxylase. (One more chemical reaction can turn dopamine into its best buddy neurotransmitter, norepinephrine, but more on that later). As is the case with serotonin and its precursor tryptophan, tyrosine can cross the blood brain barrier, but dopamine itself cannot. That means that the dopamine our brain needs must be manufactured from dopamine machinery and precursors in the brain.
Still with me? What happens without dopamine, or with screwy dopamine machinery or inefficient dopamine? Well, in development this lack can cause mental retardation, which is the case in a rare genetic disease called PKU and cretinism (a type of mental retardation caused by iodine deficiency). Dopamine problems are implicated in ADHD, Alzheimer's, Parkinson's, depression, bipolar disorders, and schizophrenia. Having too much dopamine in the wrong place can make you psychotic. Illicit drugs that dump loads of dopamine (or strongly inhibit its reuptake, which is similar to dumping loads of dopamine) include cocaine and methamphetamines. Therefore high amounts of dopamine can cause euphoria, aggression and intense sexual feelings.
We need dopamine in the right place at the right time in the right amounts. When it all comes together, we are the awesomest ape around. When it doesn't, problems ensue (not surprisingly). Dopamine is linked to everything interesting about metabolism, evolution, and the brain. There's a feast of scientific info out there. Hopefully we can make some sense of it.
Information from this post is taken mostly from The Dopaminergic Mind in Human Evolution and History by Fred Previc.
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