Yohimbine Science
Plant Remedies in Traditional Medicine
Fossil records suggest that the use of plants as therapeutic agents in ancient times goes back to some 60,000 years ago (Solecki, 1975). To date, plants are employed in herbal curative medicinal systems in developing countries (Lanfranco, 1992).
According to the World Health Organization (WHO), approximately 80% of the world population in developing countries are still relying on traditional medicine to supplement their fundamental well-being (Pires, 2011). Plants are also used for the synthesis of new and improved bioactive compounds. Plant-derived bioactive compounds such as Yohimbine can be used as pharmacological agents.
What is Yohimbine?
Yohimbine is an alpha-2 adrenergic receptor antagonist, also called alpha-2 blockers. It is also an indole alkaloid which is found in numerous botanical sources. Alpha-2 adrenoceptors are activated by the catecholamines norepinephrine and epinephrine. Alpha-2 adrenoceptors are implicated in diverse physiological functions in the heart, and presynaptic alpha-2 receptors inhibit the release of norepinephrine and other neurotransmitters in both the central and peripheral nervous systems. Alpha-2 adrenergic receptor antagonists inhibit or block the function of the alpha 2 adrenergic receptor.
Where is Yohimbine Found?
Yohimbine is obtained from the bark of the West African tree Pausinystalia yohimbe (Family Rubiaceae). It is also found in the bark of Richeria Grandis tree species in the family Phyllanthaceae, which ranges the West Indies and South America. In Trinidad, this tree is known as Bois Bande. The bark contains about 6% of a mixture of alkaloids, the principal one of which is Yohimbine.
It occurs in flat or slightly quilled pieces up to 75 cm long and 2 cm thick. The grey-brown cork has furrows and cracks and patches of lichen. The inner surface is reddish-brown and striated.
Yohimbine affects the Sympathetic Nervous System
* Norepinephrine also called noradrenaline. Epinephrine is also called adrenaline.
When you think of adrenaline or noradrenaline, you should also think of the sympathetic nervous system which is our fight or flight system. In the area of the spinal cord related to the sympathetic nervous system, there are two neurons that come out. The first is the presynaptic neuron and the second is the postsynaptic neuron. The postsynaptic neuron speaks to the specific organ that we want to stimulate in the sympathetic nervous system.
Alpha 2 receptors are located on all presynaptic membranes in the thoracic-lumbar region of the spinal cord. During a fight or flight response, two neurons are released from the thoracic region of the sympathetic nervous system. The first is the presynaptic neuron, which is released from the central nervous system, and the second called the postsynaptic neuron.
The presynaptic neuron communicates with the postsynaptic neuron by releasing the neurotransmitter Acetyl Choline. Acetyl Choline binds to the receptors located on the post-synaptic neuron and continues to send the signal down the postsynaptic neuron to communicate to the organs we want to talk to.
In order to stimulate the organs, the postsynaptic neuron communicates with the organ by releasing a neurotransmitter called noradrenaline (also called norepinephrine). This neurotransmitter (in this case of noradrenaline/norepinephrine) then binds to adrenergic receptors in order to stimulate that organ.
When the postsynaptic neuron releases noradrenaline, the first organ it communicates with is the blood vessels. The blood vessels are stimulated so that they constrict causing blood to move away from our surface and flow to our skeletal muscles and our deeper tissues. These physiological changes that occur during the fight or flight response are activated in order to give the body increased strength and speed in anticipation for muscular action.
When adrenaline is released it will stimulate alpha 1 and beta 1 receptors. Alpha 1 and Beta 1 receptors are both stimulatory receptors meaning they cause the organ to constrict. Alpha 1 receptors are located on all smooth muscle, organs, and glands associated with the sympathetic nervous system, with the exception of cardiac muscle and juxtaglomerular cells of our kidneys. Beta 1 receptors are located on cardiac muscle.
When adrenaline is released, if it binds to alpha 2 and beta 2 receptors they cause the glands, muscles, or tissues to dilate or loosen. Alpha 2 receptors are located on presynaptic and postsynaptic nerve terminals. Beta 2 receptors are found on all smooth muscle, organs, and glands that need to be dilated such as the lungs. However, Beta 2 receptors are not located on presynaptic membranes of the sympathetic nervous system. When adrenaline is released it binds to beta 2 receptors in our airways, causing our airways relax and open up.
Yohimbine causes an increase in norepinephrine
Yohimbine is an alpha-2 adrenergic blocker which means it blocks or inhibits the actions of the alpha-2 adrenergic receptors. There are alpha-2 adrenoceptors located on the presynaptic nerve terminals which inhibit the release of norepinephrine. Norepinephrine acts at these presynaptic alpha-2 adrenergic receptors to inhibit its own release, as a sort of negative feedback. Yohimbine blocks this action thereby causing an increase in norepinephrine release at the presynaptic nerve terminal.
There are also alpha-2 adrenoceptors located on postsynaptic nerve terminals which cause the organs to dilate or loosen when norepinephrine binds to them. Yohimbine blocks the actions of these postjunctional alpha-2 adrenoceptors and prevents norepinephrine from binding. This then causes vasoconstriction and eventually raises blood pressure.
There are two opposite actions happening at the presynaptic and postsynaptic alpha-2 adrenoceptors. In the former, there is an increase in norepinephrine and in the latter there norepinephrine is prevented from binding. However, the increased norepinephrine at the presynaptic nerve terminals bind to the beta adrenergic receptors of the heart which eventually causes an increase in heart rate and contractility. This means that heart can now beat faster and harder to pump blood to our skeletal muscles and deep tissues in preparation for action.