Sympathomimetic

Sympathomimetic

Sympathomimetic drugs mimic the effects of transmitter substances of the sympathetic nervous system such as catecholamines, epinephrine (adrenaline), norepinephrine (noradrenaline), dopamine, etc. Such drugs are used to treat cardiac arrest and low blood pressure, or even delay premature labor, among other things.

These drugs act at the postganglionic sympathetic terminal,[1] either directly activating postsynaptic receptors, blocking breakdown and reuptake, or stimulating production and release of catecholamines.

Mechanisms of action

The mechanisms of sympathomimetic drugs can be direct-acting, such as α-adrenergic agonists, β-adrenergic agonists, and dopaminergic agonists; or indirect-acting, such as MAOIs, COMT inhibitors, release stimulants, and reuptake inhibitors that increase the levels of endogenous catecholamines.

Structure-activity relationship

For maximum sympathomimetic activity a drug must have the following; 1. Amine group two carbons away from an aromatic group. 2. A hydroxyl group at the chiral beta position in the R-configuration 3. Hydroxyl groups in the meta and para position of the aromatic ring to form a Catechol which is essential for receptor binding

The structure can be modified to alter binding. If the amine is primary or secondary it will have direct action, but if the amine is tertiary it will have poor direct action. Also if the amine has bulky substituents then it will have greater Beta adrenergic receptor activity, but it the substituent is non-bulky then it will favor the alpha adrenergic receptors.

Direct-acting

Adrenergic receptor agonists

Main article: Adrenergic agonist

Direct stimulation of the α- and β-adrenergic receptors can produce sympathomimetic effects. Salbutamol is a very commonly used direct-acting β2-agonist. Other examples include phenylephrine, isoproterenol, and dobutamine.

Dopaminergic agonists

Stimulation of the D1 receptor by dopaminergic agonists such as fenoldopam is used intravenously to treat hypertensive crisis.

Indirect-acting

Norepinephrine and Dopamine transporter blockade

Classical sympathomimetic drugs are amphetamines (including MDMA), ephedrine, and cocaine, which act by blocking and reversing norepinephrine transporter (NET) activity. NET is a transport protein expressed on the surface of some cells that clears noradrenaline and adrenaline from the extracellular space and into cells, terminating the signaling effects.

Structure-activity relationship

A primary or secondary aliphatic amine separated by 2 carbons from a substituted benzene ring is minimally required for high agonist activity. The pKa of the amine is approximately 8.5-10.[2] 1- Substitution on aromatic ring-

The presence of hydroxy group in the benzene ring at 3rd and 4th position shows maximum alpha and beta activity

Cross-reactivity

Substances like cocaine also affect dopamine, and some substances like MDMA affect serotonin.

Norepinephrine is synthesized by the body from the amino acid tyrosine,[3] and is used in the synthesis of epinephrine, which is a stimulating neurotransmitter of the central nervous system.[4] Thus, all sympathomimetic amines fall into the larger group of stimulants (see psychoactive drug chart). In addition to intended therapeutic use, many of these stimulants have abuse potential, can induce tolerance, and possibly physical dependence, although not by the same mechanism(s) as opioids or sedatives. The symptoms of physical withdrawal from stimulants are commonly described as feeling tired, sluggish, or generally unmotivated. Physical withdrawal from most sedatives can be potentially lethal. Opioid withdrawal is very uncomfortable, often described as a bad case of the flu, but not lethal unless the user has an underlying condition.

Comparison

"Parasympatholytic" and "sympathomimetic" are similar, but not identical. For example, both cause mydriasis, but parasympatholytics reduce accommodation (cycloplegia) while sympathomimetics do not.

Examples

See also

References

External links

  • Medical Subject Headings (MeSH)
  • eMedicine Dictionary
  • eMedicine Dictionary

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