Agmatine sulfate exerts modulatory action at multiple molecular targets, such as neurotransmitter systems, ion channels and nitric oxide synthesis. It is an endogenous agonist at imidazoline receptor and a NO synthase inhibitor.
Agmatine binds to alpha 2-adrenergic and imidazoline receptors and stimulates release of catecholamines from adrenal chromaffin cells. Its biosynthetic enzyme, arginine decarboxylase, is present in brain. Agmatine, locally synthesized, is an endogenous agonist at imidazoline receptors, a noncatecholamine ligand at alpha 2-adrenergic receptors and may act as a neurotransmitter[1]. Agmatine is synthesized in the brain, stored in synaptic vesicles in regionally selective neurons, accumulated by uptake, released by depolarization, and inactivated by agmatinase. Agmatine inhibits nitric oxide synthase, and induces the release of some peptide hormones[2]. Agmatine, 4-(aminobutyl)guanidine, is produced by decarboxylation of L-arginine by the enzyme arginine decarboxylase. Agmatine is a competitive inhibitor of all NOS isoenzymes but not an NO precursor. Ki values are approximately 660 µM (NOS I), 220 µM (NOS II) and 7.5 mM (NOS III)[3]. Agmatine stimulates nitrite production three-fold above basal nitrite formation by endothelial cells. Agmatine displaces [3H]-idazoxan from endothelial cellmembranes and is found to induce transients in the cytosolic calcium of endothelial cells. The transients could be downregulated by repeated exposure to agmatine but are not affected by pretreatment with norepinephrine[4].
Agmatine produces an antidepressant-like effect when assessed in the forced swimming test and in the tail suspension test in mice (dose range 0.01-50 mg/kg, i.p.), without accompanying changes in ambulation in an open-field[5]. In ischemic stroke, agmatine protects the blood-brain barrier, which can be monitored in vivo by quantification of permeability by using dynamic contrast-enhanced MR imaging[6]. Agmatine substantially augments the antidepressant-like effect of MK-801, reinforcing the notion that this compound modulates NMDA receptor activation[7].
References:
[1]. Li G, et al. Agmatine: an endogenous clonidine-displacing substance in the brain. Science. 1994 Feb 18;263(5149):966-9.
[2]. Reis DJ, et al. Is agmatine a novel neurotransmitter in brain Trends Pharmacol Sci. 2000 May;21(5):187-93.
[3]. Galea E, et al. Inhibition of mammalian nitric oxide synthases by agmatine, an endogenouspolyamine formed by decarboxylation of arginine. Biochem J. 1996 May 15;316 ( Pt 1):247-9.
[4]. Morrissey JJ, et al. Agmatine activation of nitric oxide synthase in endothelial cells. Proc Assoc Am Physicians. 1997 Jan;109(1):51-7.
[5]. Zomkowski AD, et al. Agmatine produces antidepressant-like effects in two models of depression in mice. Neuroreport. 2002 Mar 25;13(4):387-91.
[6]. Ahn SS, et al. Effects of agmatine on blood-brain barrier stabilization assessed by permeability MRI in a rat model of transient cerebral ischemia. AJNR Am J Neuroradiol. 2015 Feb;36(2):283-8.
[7]. Neis VB, et al. Agmatine enhances antidepressant potency of MK-801 and conventional antidepressants in mice. Pharmacol Biochem Behav. 2015 Mar;130:9-14.