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I am little confused here. I used the term adrenoagonist and sympatholytic to describe the compound. However, my teacher says that the correct term here is adrenomimetic -term.

My understanding of Terms

  • Adrenoagonist says that to bind to adrenoreceptor which prolongs the effect of NE in receptor.
  • Sympatholytic - inhibition of postganglionic fibers of sympathetic nervous system
  • Adrenomimetic - mimic effects of sympathetic nerve stimulation on pre- / postganglionic fibers.

I still think clonidine is sympatholytic i.e. adrenolytic (not adrenomimetic).

How can clonidine be adrenomimetic? Why not adrenolytic?

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Clonidine is an agonist on the α2 receptor... but then again norepinephrine is also an agonist on the α2 receptor. Then the physiologic ligand of the a2 receptor, norepinephrine is autocrine (meaning it is released from a cell then binds on a receptor on the same cell) causing negative feed back and it inhibits further release of norepinephrine from the neuron thus preventing overstimulation. Clonidine is quite specific to the α2 receptor and it doesn't really stimulate α1/β1/β2. Given a situation where only the α2 receptor is stimulated and none of the other epinephrine receptors, you would have a net decrease of sympathetic effects since the amount of norepinephrine being released is begin decreased by the stimulation of the α2 receptor on the presynaptic membrane of the sympathetic neuron. Hence clonidine is an α2 agonist that is sympatholytic

I have no clue what the difference are between these

  • agonist vs mimetic
  • adreno vs sympatho
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  • $\begingroup$ So I propose that clonidine is not adrenomimetic but adrenolytic (i.e. adrenonergic and sympatholytic in one word). Do you agree? - Agonist mean to bind etc adrenoagonist to bind to adrenoreceptor. Mimetic means positive ANS effect and/or mimicing the behaviour of the stimuli from ANS. Adreno - just adrenoreceptor where the neurotransmitter is acetylcholine. Sympatho is about if the drug has sympathetic effect and its nature (sympathopositive (-mimetic) vs sympathonegative (sympatholytic i.e. decreasing concentration of acetylcholine etc in case of adrenoreceptor in a synapse)). $\endgroup$ – Léo Léopold Hertz 준영 Dec 5 '14 at 21:45
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    $\begingroup$ I suppose I failed to mention this, but agonist means that it not only binds (as an antagonist binds as well, both agonist/antagonist bind to either the receptors active site or an allosteric Site, depending on if it's competative/noncompetative) but it stimulates the receptor in a way that mimics the endogenous ligands response. Now the a2 receptors response when stimulated is adrenolytic. $\endgroup$ – Jasand Pruski Dec 6 '14 at 23:16
  • $\begingroup$ Yes, you are right! Thank you for your answer! $\endgroup$ – Léo Léopold Hertz 준영 Dec 7 '14 at 8:13
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    $\begingroup$ I want to nuance the analysis presented above. Clonidine acts both directly on the alpha-2 receptors of blood vessels (triggering vasoconstriction), and then on the neurons alpha-2 receptors when it gets through the blood-brain barrier (inhibiting norepinephrine release). I think those definitions are too simplistic to fully qualify clonidine action. First reaction is sympathomimetic, prolonged action is sympatholytic. $\endgroup$ – Raoul Dec 7 '14 at 16:02
  • $\begingroup$ Wikipedia seems to confirm what you have said. The a2 receptors are found not only in the presynaptic membrane of adrenergic neurons (and in the CNS), but also the vascular smooth muscle. Then there are the 2nd messengers involved, a1 acts via Gq (increasing IP3 and DAG) and a2 acts via Gi (decreasing cAMP). As I understand it, cAMP levels have varied effects on vascular smooth muscle (vasoconstriction/vasodilation) depending about which place int he body we are talking about, but if a1/a2 both worked on cAMP but in contradictory capacities then I would say no, but one is cAMP one is IP3/DAG. $\endgroup$ – Jasand Pruski Dec 7 '14 at 16:44

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