robin082006
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Stimulating Alpha 2 receptor in CNS will inhibit the release of NE. Clonidine act as an agonist at these receptors and thereofre called a centrally acting alpha agonist. It is easier to understand if you see the picture in FA or Kaplan.
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| star1
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As I know agonist bind to the receptor and activate it and antagonist bind to the receptor and inactivate it . So when clonidine bind to the receptor it activate the receptor and then this activation lead to the inhibition of norepinephrine release.
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| pearljam59
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No no I get it...but FA calls it a centrally acting alpha agonist and I thought it should be a centrally acting (still dont know what that means) alpha TWO agonist.
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| p53
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Clonidine is a partial alpha agonist (i.e. it acts on both alpha 1 & 2 receptors, though binds more tightly to alpha 2 than alpha 1 receptors). After intravenous injection it produces a brief rise in blood pressure due to direct stimulation of alpha 1 receptors in arterioles - this effect is leading on periphery because postsynaptic alpha 1 receptors (vascular) are more abundant than presynaptic alpha 2 (in cardiac adrenergic neurons).
This is followed by more prolonged hypotension due to centrall action in the medulla at neuronal alpha 2 receptors (not yet defined on pre- or postsynaptic - maybe on both) - alpha 2 receptors in CNS are more important than alpha 1 - this reduces sympathetic and increases parasympathetic tone, resulting in blood pressure lowering and bradycardia.
Edited by p53 on 03/14/06 - 11:28 PM
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| doc179
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centrally acting means acting in the CNS
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