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, 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.
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.
Adrenergic receptor agonists
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.
Stimulation of the D1 receptor by dopaminergic agonists such as fenoldopam is used intravenously to treat hypertensive crisis.
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.
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. 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
Substances like cocaine also affect dopamine, and some substances like MDMA affect serotonin.
Norepinephrine is synthesized by the body into epinephrine, causing central nervous system stimulation. Thus, all sympathomimetic amines fall into the larger group of stimulants (see psychoactive drug chart). Many of these stimulants have therapeutic use and abuse potential, can induce tolerance, and possibly physical dependence. Stimulates do not produce physical dependence in a way like opioids or sedatives will. The symptoms of physical withdrawal from stimulates are commonly described as being tired, sluggish, or general non-motivation. Physical withdrawal from most Sedatives can be potentially lethal. Opioid withdrawal is very uncomfortable, often describes as a bad case of the flu, but not lethal unless the user has an underlying condition.
"Parasympatholytic" and "sympathomimetic" are similar, but not identical. For example, both cause mydriasis, but parasympatholytics reduce accommodation (cycloplegia) while sympathomimetics do not.