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Thus, clonidine's agonism on α 2A adrenergic receptors in the PFC inhibits the action of downstream neurons by preventing the secretion of norepinephrine. [ 85 ] This mechanism is similar to the brain's physiological inhibition of PFC neurons by the locus ceruleus (LC), which secretes norepinephrine into the PFC.
It is weaker than clonidine in producing hypotension and sedation, has weaker pre-synaptic actions on the α 2A-adrenergic receptor than clonidine (10-fold less effective in decreasing locus coeruleus activity and norepinephrine release), and may have greater efficacy in activating post-synaptic α 2A-adrenergic receptors (as suggested by ...
Clonidine is now thought to decrease blood pressure via this central mechanism. Other nonselective agonists include dexmedetomidine, lofexidine (another antihypertensive), TDIQ (partial agonist), tizanidine (in spasms, cramping) and xylazine. Xylazine has veterinary use.
Clonidine and other imidazoline compounds have also been shown to reduce muscle spasms by their central nervous system activity. Tizanidine is perhaps the most thoroughly studied clonidine analog, and is an agonist at α 2-adrenergic receptors, but reduces spasticity at doses that result in significantly less hypotension than clonidine. [26]
However, there are exceptions: clonidine is an adrenergic agonist at the α2 receptor; since this receptor is located presynaptically, agonism at this receptor inhibits the presynaptic release of adrenaline and noradrenaline, preventing postsynaptic adrenergic receptor activation and downstream signaling.
[1] [4] Chemicals in this family can act either directly by stimulating the nicotinic or muscarinic receptors (thus mimicking acetylcholine), or indirectly by inhibiting cholinesterase, promoting acetylcholine release, or other mechanisms. [5] Common uses of parasympathomimetics include glaucoma, Sjögren syndrome and underactive bladder. [6]
Moxonidine is a new-generation alpha-2/imidazoline receptor agonist antihypertensive drug licensed for the treatment of mild to moderate essential hypertension. [5] [6] It may have a role when thiazides, beta-blockers, ACE inhibitors, and calcium channel blockers are not appropriate or have failed to control blood pressure.
Brimonidine is an α 2 adrenergic agonist. [4]Peripheral α 2 agonist activity results in vasoconstriction of blood vessels (as opposed to central α 2 agonist activity that decreases sympathetic tone, as can be seen by the medication clonidine).
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