Agmatine produces antinociception in tonic pain in mice.

Agmatine is an endogenous polyamine metabolite formed by decarboxylation of L-arginine. In this study, the effect of agmatine on tonic pain was compared to its effect on phasic pain by using the formalin and tail-flick (TF) tests in mice. When administered intraperitoneally (ip), agmatine (37.5-300 mg/kg) exhibited a decrease in nociceptive behaviours in the first and second phase of the formalin test, which is a tonic pain model. The alpha(2) adrenoceptor antagonist yohimbine blocked the effect of agmatine in Phase 2 but did not change its effect in Phase 1. In the TF test, there was no significant change in the behaviour of agmatine-administered (75-300 mg/kg) animals. As a result, agmatine appears to have an analgesic effect on tonic rather than phasic pain, and alpha(2) receptors seem partly to have a role in the antinociceptive effect of agmatine on tonic pain.