Category Archives: Autonomic Pharmacology

Alpha receptors

Alpha receptors can be classified as a subtype of adrenergic receptor, their activation triggers a complex range of autonomic responses that will be detailed in the following article.  We recommend our readers to check our article on beta receptors for further information on autonomic pharmacology.

Article contents:

Alpha 1 receptors

The image below shows the synaptic cleft, presynaptic and postsynaptic membranes. Alpha 1 receptors are located on the postsynaptic membrane of effector organs.

Activation of alpha 1 receptors increases vascular smooth muscle contraction, producing increases in blood presure. These receptors also influence activity of genitourinary smooth muscle.

alpha _1_receptors
Effects of alpha 1 receptor activation on different tissues and systems:

  • Vascular smooth muscle. Activation of vascular smooth muscle receptors causes vasoconstriction, leading to an increase in peripheral resistance and in blood pressure. Alpha 1 antagonists are used in the treatment of hypertension because of their properties as peripheral vasodilators.
  • Mydriasis. Mydriasis is mediated by both alpha 1 and 2 activation.
  • Genitourinary tract smooth muscle. Alpha 1A receptors are located at the external sphincter of the bladder. Blockade of alpha 1A receptors decreases tone in the smooth muscle of the bladder neck and prostate, thus improving urinary flow. Tamsulosine is used for the treatment of benign prostatic hyperplasia because of its ability to selectively block alpha 1A receptors.

Alpha 2 receptors

Although alpha 2 receptors are found on both presynaptic neurons and postynaptic cells, they work mainly as autoreceptors to mediate feedback inhibition of sympathetic transmission.

In addition to neurons, alpha 2 receptors are located in other regions, like pancreatic beta cells and platelets.
The diagram below shows how when activated, these receptors act as inhibitory autoreceptors (they inhibit norepinephine release from adrenergic neurons) and as inhibitory heteroreceptors (they inhibit acetylcholine release from cholinergic neurons).
In addition, activation of alpha 2 receptors on pancreatic beta cells membranes inhibits insulin release.

Drugs affecting alpha receptors

Alpha 1 agonists

Because of their properties as vasoconstrictive agents they are used to reduce edema and inflammation. Common decongestant drugs include naphazoline, phenylephrine and propylhexedrine.

Alpha 1 antagonists

These drugs act by causing vasodilation and decreased peripheral resistance, therefore they are used in the treatment of hypertension (prazosin). Treatment of benign prostatic hyperplasia is another clinical use of alpha 1 antagonists: tamsulosin is a subtype-selective alpha 1A receptor that has more specificity toward smooth muscle in genitourinary tract; thus tamsulosin has lower incidence of orthostatic hypotension.

Alpha 2 agonists

By activating central alpha 2 receptors, alpha 2 adrenergic agonists inhibit sympathetic outflow from CNS. Clonidine is used for the rteatment of hypertension and symptoms associated with opioid withdrawal. Alpha-methyldopa is a precursor to the alpha 2 agonist alpha-methylnorepinephrine, it is the drug of choice for the treatment of hypertension during pregnancy.

Alpha 2 antagonists

Yohimbine blockade of alpha 2 receptors leads to increased release of norepinephrine with susequent stimulation of cardiac beta 1 receptors and peripheral vasculature alpha 1 receptors. Yohimbine was used in the past to treat erectile dysfunction.

Collection of PowerPoint lectures on ANS pharmacology

The following set of lectures has been created by Dr. Edwin Jackson and were published in the Division for Cardiovascular Pharmacology, a branch of the American Society for Pharmacology and Experimental Therapeutics (ASPET). Why study autonomic pharmacology? Outline of slides: Parasympathetic Ganglionic Synapse, Parasympathetic Organ Synapse, Sympathetic Ganglionic Synapse, Sympathetic Ganglionic Synapse, How do drugs…Continue Reading

Potential new indication of beta blockers: improve wound healing

A recent article in PLoS Medicine shows that epinephrine impairs wound healing and proposes beta blockers as a potential medication. Stress-Mediated Increases in Systemic and Local Epinephrine Impair Skin Wound Healing: Potential New Indication for Beta Blockers Raja K. Sivamani1, Christine E. Pullar1¤, Catherine G. Manabat-Hidalgo1, David M. Rocke2, Richard C. Carlsen3, David G. Greenhalgh4,5,…

USMLE-like pharmacology quiz. Part 5: Autonomic pharmacology. Sympathetic nervous system

The following are multiple choice questions in a style similar to those included in the USMLE step 1. The main topic here is: Autonomic Pharmacology: Sympathetic Nervous System. This quiz has been uploaded to Scribd by medical student James Lamberg. The correct answers can be found in link at the end of this post. Note:…Continue Reading

Alfuzosin does not improve symptoms of chronic prostatitis- chronic pelvic pain syndrome

From JournalWatch: Alpha-Blocker Does Not Ease Symptoms of Chronic Prostatitis Alfuzosin, an alpha-adrenergic–receptor blocker, does not improve symptoms in men with chronic prostatitis–chronic pelvic pain syndrome, according to a New England Journal of Medicine study.Continue Reading

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