Newsgroups: alt.drugs
From: William E. White 
Subject: Re: Yohimbe
Date: Mon, 3 Oct 1994 00:18:34 GMT

Yohimbine, the active constituent in yohimbe bark (there may be others,
since yohimbine alone does not seem to produce the same effects as the
bark extract), is an alpha-2 adrenergic antagonist.  It is not, to my
knowledge, an MAOI (if someone knows differently, please inform me).
[Erowid Note: According to Quinton RM. B J Pharmacology 1963 21:51-66, 
yohimbine is in fact a very weak MAOI, but this may not be clinically relevant. 
Yohimbine has been shown to be potentially dangerous in combination with
other drugs such as SSRIs and amphetamines, which is similar to contraindications
for MAOIs, but the problems may not be related to yohimbine's MAO-Inhibition.  
Also, Yohimbe products should NOT be taken with other MAOIs.]

The alpha-2 receptor is an autoreceptor (presynaptic, I think) which
acts as a kind of "thermostat" to adrenergic activity.  That is, some
of the noradrenaline released by a neuron goes back to the alpha-2
autoreceptor, which then REDUCES the amount of noradrenaline secreted.
This is similar to a thermostat, which registers the temperature and
reduces the amount of heat produced when the air warms up.

By blocking the alpha-2 autoreceptor, yohimbine increases the amount of
adrenergic activity.  Additionally, it does so in a different way than
a simple agonist (which would universally activate receptors, rather
than amplifying existing noradrenergic activity) would do.

Yohimbine is used medically to treat impotence, as increased adrenergic
activity seems to help.  I think "Yocon" is the brand name.

Yohimbe / yohimbine are contraindicated in people with high blood
pressure, heart problems, anxiety, and panic attacks, all of which can
be made worse with adrenergic activity.  It should not be taken with
any stimulant.  Avoid MAOIs with yohimbine.

>understanding, MAO inhibitors keep certain receptor points in your brain
>open, making other substances *much* more potent.

MAOIs prevent monoamine oxidase, an enzyme (actually two enzymes) from
degrading certain substances.  For example, orally ingested DMT is normally 
broken down by MAO before it ever reaches the brain (if you smoke it,
some will make it there).  By inhibiting MAO, the DMT stays around until
it is degraded by other (much slower) pathways.

The troubles with MAOIs are twofold:  First, some chemicals are degraded
by MAOI, but when undegraded can be dangerous; an example of this is
tyramine, an amino acid present in cheese, wine, etc., which is an
indirectly acting sympathomimetic (like amphetamine).  Normally no
tyramine makes it to your brain when you eat cheese, but if you are
taking a MAOI, you can end up in a hypertensive crisis.
Second, and more insidiously, sometimes there is an enzyme pathway from
a harmless substance to a more harmful one, which is "uncovered" when
the (more active) MAO pathway is inhibited.