• Gaining or losing weight
• Eating more or less than usual almost every day
• Sleeping too much or not enough almost every day
• Feeling restless and unable to sit still
• Feeling that moving takes great effort
• Feeling tired or as if you have no energy almost every day
• Feeling unworthy or guilty nearly every day
• Having low self-esteem or feeling down on yourself
• Finding it hard to focus, remember things, or make decisions nearly every day
• Feeling anxious, worried, or nervous
• Drinking more alcohol or caffeine
• Taking more of a prescription or over-the-counter medication than is directed
• Smoking or using tobacco more often

There are three basic brain chemicals, neurotransmitter molecules, known chemically as monoamines, which are believed to be involved in mood regulation. Serotonin, Norepinephrine and Dopamine.
It's probably more complicated than this. I have several family members who's careers involve the brain and/or mood. I'll have them give me an update some time.

Classes of Antidepressant Medications

• Selective Serotonin Reuptake Inhibitors (SSRI) - e.g. Prozac (fluoxetine), Paxil (paroxetine), Zoloft (sertraline)
  Work by inhibiting the reuptake of serotonin, allowing more serotonin to remain available.
• Benzodiazepines - work by altering the effect of a chemical messenger, or neurotransmitter, called gamma-aminobutyric acid (GABA).e.g. Valium (Diazepam), Xanax (Alprazolam)
• Serotonin and Norepinephrine Reuptake Inhibitors (SNRIs) - e.g.Effexor (venlafaxine)
• Monoamine Oxidase Inhibitors (MAOIs)- e.g. Nardil (phenelzine)
• Tricyclic Antidepressants (TCIs)- e.g. Elavil (amitryptyline)