Antidepressant like effect of selective serotonin reuptake inhibitors involve modulation of imidazoline receptors by agmatine

Recent findings demonstrated the dysregulation of imidazoline receptor binding sites in major depression and their normalization by chronic treatment with antidepressants including selective serotonin reuptake inhibitors (SSRIs). Present study investigated the role of agmatine and imidazoline receptors in antidepressant like effect of SSRIs and imipramine in mouse forced swimming test (FST) paradigm. The antidepressant like effect of fluoxetine or paroxetine was potentiated by imidazoline I1/I2 receptor agonist agmatine (5–10 mg/kg, ip), imidazoline I1 receptor agonists, moxonidine (0.25–0.5 mg/kg, ip) and clonidine (0.015–0.03 mg/kg, ip), imidazoline I2 receptor agonist, 2-(2-benzofuranyl)-2-imidazoline (5–10 mg/kg, ip) as well as by the drugs known to increase endogenous agmatine levels in brain viz., l-arginine, an agmatine biosynthetic precursor (40 μg/mouse, icv), ornithine decarboxylase inhibitor, difluoromethyl ornithine (12.5 μg/mouse, icv), diamine oxidase inhibitor, aminoguanidine (6.5 μg/mouse, icv) and agmatinase inhibitor, arcaine (50 μg/mouse, icv). Conversely, prior administration of I1 receptor antagonist, efaroxan (1 mg/kg, ip), I2 receptor antagonist, idazoxan (0.25 mg/kg, ip) and arginine decarboxylase inhibitor, d-arginine (100 mg/kg, ip) blocked the antidepressant like effect of paroxetine (10 mg/kg, ip) and fluoxetine (20 mg/kg, ip). On the other hand, antidepressant like effect of imipramine was neither augmented nor attenuated by any of the above drugs. Mice pretreated with SSRIs but not imipramine and exposed to FST showed higher concentration of agmatine in brain as compared to saline control. This effect of SSRIs on agmatine levels was completely blocked by arginine decarboxylase inhibitor d-arginine but not by imidazoline receptor antagonists, efaroxan or idazoxan. These results demonstrate that modulation of imidazoline receptors by agmatine are implicated in the antidepressant like effect of SSRIs and may be projected as a potential therapeutic target for the treatment of depressive disorders.