How MAOI Antidepressants work

Now let us look carefully at different types of antidepressants. But before doing this, I'd strongly recommens you to read the preface (Parts 1, 2, 3) before you continue to specific types of medicines.

Mono-Amine Oxidase Inhibitors

As we understood from the preface,

"There are many chemical messengers (or "neurotransmitters") called "monoamines" which occur naturally in the body. One of the effects that monoamines have is on mood. If the levels of monoamines in the body are high we may feel 'high', and if they are low we may feel 'low'. MAOIs stop the breakdown of these monoamines. They block the monoamine oxidase enzyme which breaks down the transmitter, so the next time an impulse comes along, there is more transmitter, a stronger message is passed, and activity in that part of the brain is increased. By stopping the breakdown of these, the MAOIs may bring back the amounts of monoamines to normal. This helps to improve mood in people who are feeling low or depressed."

"Reduced" nerve activity but with (mono-amine oxidase) enzyme blocked (or inhibited), and increased messages passed:

Side-effects:
Some antidepressants e.g. the tricyclics, block the reuptake of serotonin and noradrenaline. Others mainly block the reuptake of just serotonin e.g. the SSRIs. The MAOIs block the monoamine enzyme which breaks down noradrenaline, serotonin and some other transmitters. They are Monoamine Oxidase Inhibitors.
Unfortunately, the MAOIs also block an enzyme in the body which breaks down other compounds. One of these is an amino acid called tyramine. Tyramine is an essential compound which the body needs, and is found in many foods. However, if you have too much tyramine in the body, it can make your blood pressure rise. Foods such as cheese, yeast and meat extracts etc contain lots of tyramine. If you eat any of these foods while taking an MAOI, your body can not break down (or metabolise) the tyramine. You then get an excess of tyramine in the body, which increases your blood pressure very quickly. This can at cause headaches initially but can be very dangerous and has caused many deaths.
The MAOIs also affect other transmitters which are broken down by the MAO enzyme.
- Affecting your noradrenaline may also sometimes upset your blood pressure e.g. you may feel dizzy when you stand up etc.

Source.