Adrenergic nerves release norepinephrine as the neurotransmitter
for the sympathetic nervous system. The sympathetic system activates
and prepares the body for vigorous muscular activity, stress,
and emergencies. Adrenergic drugs stimulate the adrenergic nerves
directly by mimicking the action of norepinephrine or indirectly
by stimulating the release of norepinephrine.
Therapeutically, these drugs are used to combat life-threatening
disorders, which include acute attacks of bronchial asthma, shock,
cardiac arrest, and allergic reactions. In addition these drugs
are used in nasal decongestants and appetite suppressants.
Adrenergic Nerve Transmissions:
Adrenergic nerves release the neurotransmitters: Norepinephrine
(noradrenaline, NE), epinephrine, EP, and dopamine DA. The synthesis
of the neurotransmitters DA and NE and EP and the hormones NE
and EP takes place by a pathway that involves 5 enzymes. Tyrosine
is generally considered the starting point to synthesize DOPA
(1a), DA (1b), and NE (1c). Norepinephrine is stored at (2).
Norepinephrine is released from the nerve ending in response
to a nerve impulse or drug (3). NE interacts with alpha and beta
receptor sites at (4). Its receptor action is terminated by recapture
and storage in the original nerve ending or inactivated by an
enzyme. For example chloropheniramine, an antihistamine, can
inhibit the mechanism for uptake and recapture of norepinephrine.
There are at least two adrenergic receptor sites (alpha or
beta). Norepinephrine activates primarily alpha receptors and
epinephrine activates primarily beta receptors, although it may
also activate alpha receptors. Stimulation of alpha receptors
is associated with constriction of small blood vessels in the
bronchial mucosa and relaxation of smooth muscles of the intestinal
tract. Beta receptor activation relaxes bronchial smooth muscles
which cause the bronchi of the lungs to dilate.
In addition beta receptor stimulatory effects cause an increase
in the rate and force of heart contractions. As a result increased
amounts of blood leave the heart and is diverted from nonactive
organs to areas that actively participate in the body's reaction
to stress such as skeletal muscles, brain, and liver.