system
Given the following post, respond as if you're commenting on Reddit:
human
What do dopamine, serotonin, etc. do in the brain?

I got no useful answers in e li5.
gpt
Dopamine and serotonin are both a type of chemical known as a neurotransmitter. There are many different types of neurotransmitter, and many different types of each type. The brain is made up of long cells known as neurons, which pass electrical currents from one to the next. In between each neuron is an extremely small gap called a synapse. In order to send the signal from one neuron to the next, the electrical signal needs to jump this gap - but it can't. In order to do this, the neuron sending the signal releases neurotransmitters into the synapse, which float across and bind to the receiving neuron. This then effects the activity of the receiving neuron, either by increasing or decreasing its electrical activity. 

If you are asking what is the function of serotonin or dopamine, it depends on where in the brain they are being released. Both chemicals are released in a wide variety of brain regions, and the effect of their release depends on the types of neurons and their wiring in those different regions. 

Dopamine is quite often tagged as the 'reward' chemical, because it is released in the 'reward pathway'. It is not a coincidence that recreational drugs also bind to the same receptor sites as dopamine! But it has also been shown to be involved in arousal, motor control and even lactation.

The release of serotonin is thought to be often involved in apetite and mood. The widely used anti-depressent, Prozac, is a serotnin re-uptake inhibitor. When you take this drug, it slows down the rate that serotonin is re-absorbed after being released into the synapse. This means there is more serotonin floating around in the synapse, and so it is more likely to bind to, and effect the neurons involved in making us feel depressed.