Drug Receptors N. S.

Drug Action

Drugs of C.N.S
 Elmhurst College
Nervous System Cholinergic Drugs II Adrenergic Drugs II  Chemistry Department
Cholinergic Drugs I Adrenergic Drugs I Adrenergic Drugs III  Virtual ChemBook

Adrenergic Stimulants

 Structure

 Use, Receptor Site  

alpha

Vassopressor, bronchodilator, nasal decongestant

alpha or beta

 Neo-Synephrine vasopressor nasal decongestant

alpha

Propadrine,
nasal decongestant

alpha or beta

 vasopressor, bronchodilator, nasal decongestant

alpha or beta

Sudafed vasopressor, bronchodilator, nasal decongestant

alpha or beta
 

 Afrin, Duration, nasal decongestant

Adrenergic Drugs II

Adrenergic Stimulants:

Adrenergic drugs stimulate the adrenergic nerves directly by mimicking the action of norepinephrine. Therapeutically, these drugs are used in nasal decongestants as the main example given here.

The structures of the stimulants closely mimic the structure of the neurotransmitters and are thus able to interact with the receptor site. Adrenergic stimulants may have three modes of action: direct interaction with specific receptors (examples are epinephrine and phenylephrine); indirect action by stimulating release of neurotransmitters; or a mixed action involving both of the above (examples are phenylpropanolamine and ephedrine).

1. Bronchodilators:

Epinephrine and ephedrine are mainly used in the treatment of bronchial asthma. This respiratory disease results from a spasmodic contraction of the smooth muscles of the bronchi. In addition there is an increase in bronchial secretions of mucus. In an acute asthmatic attack, manifested by great difficulties in breathing, epinephrine or isoproternol is administered by injection or inhalation. These drugs act directly on beta receptors to relax the smooth muscles (bronchodilation). In addition epinephrine by an alpha stimulating action, constricts the bronchial mucosa and reduces secretions.

Ephedrine, although 100 times less potent than epinephrine, has prolonged bronchodilatory effects after oral administration. This action as well as giving relief from nasal congestion make it a valuable drug for the prevention of asthmatic attacks. Pseudoephedrine is the (-) optical isomer of (+) ephedrine. The activities of the optical isomers of ephedrine show the rather precise nature of the receptor-drug interactions. Pseudoephedrine is much less potent than ephedrine. See the graphic below for details of the interaction with the receptor site.

2. Nasal Decongestants and Vasopressors:

The swelling of blood vessels of the mucous membranes of the nasal passage causes nasal discharge and obstruction. Vasoconstrictor agents or nasal decongestants act on local sympathetic nerves and shrink swollen nasal membranes. Decongestants are either topically administered as drops and inhalants or are orally ingested. Although topically administered nasal decongestants do shrink membranes, the relief is only temporary and is followed by "rebound congestion". Phenylephrine and ephedrine are used topically and provide relief for 3-4 hours. The response to rebound congestion may lead to repeated use, damage to nasal mucosa, and chronic congestion. Xylometazoline and oxymetazoline have a longer lasting effect (10-12 hours).

Oral administration of nasal decongestants such as phenylpropanolamine, pseudoephedrine, and phenylephrine provide a more effective means of reaching the nerves via the blood stream. This route does not disrupt the mucosa environment, is longer acting, and reduces the incidence of "rebound congestion".

3. Cardiac Activation:

Ephinephrine can be injected directly into the heart to stimulate it after it as stopped beating due to drowning, suffocation, shock, electrocution, and anesthesia. The epinephrine dramatically restores the heart beat. In cases of shock, norepinephrine has been used to restore and maintain sufficient blood pressure and ensure adequate blood flow to vital organs.

When local anesthetics are used to reduce or eliminate pain in a specific area, epinephrine is frequently used in conjunction with these agents to constrict the blood vessels at the area and prevent drug diffusion from that area.