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Site of action » Michael Bell

Posted by ed_uk on March 12, 2006, at 9:46:28

In reply to No, the main site of action is not the brain » ed_uk, posted by Michael Bell on March 12, 2006, at 3:26:41

Hi Michael

>Most Mao-A, which has more of an antidepressant than Mao-B, is found in the intestine, liver and gut area. Mao-B, on the other hand, like what deprenyl causes inhibition of at low doses, is found mainly in the brain (despite what the smart-drugs website claims). Nardil's main site of action is in the small intestine, NOT the brain.

Phenelzine acts an an antidepressant and anxiolytic by acting on the brain, not the small intestine. Although MAO-A is undoubtedly present in the intestine, inhibiting this enzyme does not result in antidepressant efficacy.

MAO is present in the intestine because it is needed to metabolise dietary tyramine. Drugs which inhibit intestinal MAO allow large amounts of tyramine to enter the blood stream, potentially resulting in a hypertensive crisis.

MAO-A and MAO-B is present in the brain to metabolise various neurotransmitters eg. serotonin etc. Inhibition of MAO-A and MAO-B in the brain potentiates the activity of these neurotransmitter systems and results in an antidepressant/anxiolytic effect.

If it were possible to administer phenelzine transdermally (as a skin patch), it would remain and effective AD because it would still inhibit MAO-A and MAO-B in the brain. The risk of hypertensive crisis would be reduced because there would be less inhibition of MAO in the intestine.

In summary, phenelzine inhibitits MAO in the intestine, but this is an unwanted effect, it does not result in any antidepressant efficacy. Phenelzine acts an an AD by inhibiting MAO-A and MAO-B in the brain.

>Most Mao-A, which has more of an antidepressant than Mao-B, is found in the intestine, liver and gut area. Mao-B, on the other hand, like what deprenyl causes inhibition of at low doses, is found mainly in the brain (despite what the smart-drugs website claims).

MAO-A and MAO-B are both present in the brain. That large quantities of MAO-A are found in the intestine is not relevent to phenelzine's efficacy. Phenelzine is a more consistently effective AD then selegiline (deprenyl) because it is non-selective ie. it readily inhibits both forms of MAO in the brain. You are correct that phenelzine inhibits MAO in the gut, but this is not responsible for its antidepressant efficacy. It is, in fact, an unwanted effect because it increases the likelihood of suffering a hypertensive crisis.

An ideal MAOI antidepressant would potently inhibit both MAO-A and MAO-B in the brain without affecting MAO enzymes in the intestine.

All antidepressant owe their efficacy to their actions on the brain. It is the brain which is responsible for our depression and anxiety. Like phenelzine, many ADs act on other parts of the body - this is responsible for many of their side effects. The TCAs are particularly notorious for this because they block the effect of acetylcholine (ie. they are anticholinergic) in many organ systems.

SSRIs also act on the intestine, this may result in diarrhea, abdo pain or constipation. It is often said that there is more serotonin in the intestine than in the brain. This does not mean that SSRIs are effective ADs due to their actions on the intestine! SSRIs also act on the platelets in the blood. Neither of these properties results in AD efficacy. Like phenelzine, the SSRIs act as antidepressants by acting on the brain. Their other actions are unwanted. It is very difficult to synthesise brain-selective drugs! Most psych drugs interfere with intestinal activity, but this is not responsible for their efficacy in mental health problems! All psych drugs owe their efficacy to their actions on the brain.

Phenelzine inhibits MAO-A in the brain. This is one of its most important effects in the treatment of depression and anxiety.

Regards

Ed


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