1) Yohimbe and sex activity
A study was done comparing several doses of yohimbine administered
directly into the stomach cavity or spinal fluid of male rats to see
the general, stimulatory and sexual effects of this compound. A clear-cut
'inverted-U' effect was seen with doses up to a certain point becoming
stimulative, but then with greater doses becoming depressive. At the
maximum stimulative dose there was a shortened time spent before sexual
mounting, between different sessions of mounting and before ejaculation
during mounting. These studies show that the nervous system effects
as well the more recognized blood circulatory effects are important
in the sexual activity promotin actions of yohimbine. Sala, et al.:
Central effect of yohimbine on sexual behavior in the rat. Physiol.
Behav. 47(1), 165-73, 1990
A study using yohimbine alkaloids (all found in the tree bark)
on the male dog ejaculatory response showed an inverted U-curve.
The ejaculatory volume peaked first with the erectile potency and
duration increasing until a very high dose, which the decreased
it. While the yohimbine isomer rauwolscine also was stimulatory,
another called corynanthine did not have this sexual effect. This
shows the importance of dosage in the sexual actions of yohimbine.
Yonezawa, A. et al.: Biphasic effects of yohimbine on the ejaculatory
response in the dog. Life Sci. 48 (20); 103-09, 1991
2) Yohimbe and nervous system effects
Yohimbe was tested for the type of nervous system effects it had,
serotonergic, noradrenergic or dopaminergic. It turned out to have
all three. This was done by administering yohimbine and measuring
the three types of metabolites before and after. It was found that
yohimbine increased noradrenergic (MHPG and VMA) and dopaminergic
(HVA) cerebrospinal metabolites while also having serotoninergic
effects. Dillen, L. et al.; Selected ion monitoring analysis of
monoamine metabolites in cerebrospinal fluid. Application to the
study of in vivo effects of alpha-2-agonists. Biomed. Environ. Mass
Spectrom., 14 (11) ; 675-82, 1987
3) Yohimbe alkaloid increases amphetamine effects
and toxicity
In a study several different yohimbine isomers were compared in
their effects when given before and after amphetamine. Here beta-yohimbine
was twice as toxic as yohimbine HCL which was five times as toxic
as corynanthine. The amphetamine potentiation by yohimbine HCL was
slight while the corynanthine actually reduced the toxicity of amphetamines.
Overall, the yohimbine alkaloids seem to increase amphetamine effect
and toxicity by serotoninergic or dopaminergic mechanisms. Lambert,
G. A. and Lang, W. J.: Interaction between yohimbine alkaloids and
amphetamine in mice. Psychopharmacology, 51(2); 209-12, 1977
4) Yohimbe alkaloid counteracts nitrous oxide and
ketamine analgesia
A study was done to see if nitrous oxide analgesia produced acute
tolerance and worked by alpha-2-adrenergic receptors. Nitrous oxide
was given to rats and the tail-flick test used to measure analgesia.
Some acute tolerance occured with 80% nitrous oxide after the effects
peaked at 30 min. .The administration of yohimbine (a known alpha-2-adrenergic
agent) completely blocked the nitrous oxide analgesia. This suggested
to researchers that alpha-2-adrenergic receptors are involved in nitrous
oxide analgesia. Ohara, A. et al.: Nitrous oxide analgesia: existence
of acute tolerance and complete antagonism by yohimbine. Masui to
Sosei 31 (1); 37-9, 1995
A combination of the drugs xylazine and ketamine are commonly used
for anesthesia during veterinary surgery. However several problems
arise due to their duration of action and the side effects associated
with them. Studies on the zebra finch found that yohimbine worked
very well to counteract the anesthesia after surgery and reduced the
occurance of the nerve gaps (cortical neuron vacuoles) commonly caused
by these drugs. The study concludes that yohimbine adminsitration
should be considered following all procedures using the ketamine/xylazine
combination. Kilander, Kerry and Heater-Williams: Yohimbine reduces
neuropathology induced by ketamine/xylazine anesthesia. PHYSIOL BEHAV
51 (3); 657-59, 1992
Yohimbine was also found to reverse the effects of the ketamine/xylazine
combination in raccoons (Procyon lotor). Six adult racoons were sedated
using the ketamine/xylazine combination and 20 minutes later were
given yohimbine. The yohimbine served to counteract the sedation.
Deresienski, Diane and Rupprecht: Yohimbine reversal of ketamine-xylazine
immobilization of racoons (Procyon lotor). J WILDL DIS 25 (2); 169-74,
1989
5) Yohimbe alkaloid is a MAOI
An alkaloid found in both yohimbe and East Indian Snakeroot (Rauwolfia
serpentina) named rauwolscine or alpha-yohimbine (an isomer of yohimbine)
was found to inhibit the enzyme monoamine oxidase. The mild MAOI type
effects of yohimbe bark have been anecdotally reported for years but
now a specific alkaloid has shown measurable activity in the rat liver.
It was found that the MAOI activity of alpha-yohimbine works by its
association with a specific type of receptor called an imidazoline
I-2 site. This appears to be distinct from the more typically recognized
type of MAO inhibition such as that caused by harmaline or pargyline.
Carpene, et al.: Inhibition of Amine Oxidase Activity by Derivatives
that Recognize Imidazoline I-2 sites. The Journal of Experimental
Therapueutics 272 (2), 1995
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