In the present study the neuroprotective effects of agmatine against neuronal damage caused by glucocorticoids were examined in cultured rat hippocampal neurons. Spectrophotometric measurements of lactate dehydrogenase activities, β-tubulin III immunocytochemical staining, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick-endlabeling assay (TUNEL) labeling and caspase-3 assays were carried out to detect cell damage or possible involved mechanisms. Our results show that dexamethasone and corticosterone produced a concentration-dependent increase of lactate dehydrogenase release in 12-day hippocampal cultures. Addition of 100 μM agmatine into media prevented the glucocorticoid-induced increase of lactate dehydrogenase release, an effect also shared with the specific N-methyl-D-aspartate receptor antagonist MK801 and glucocorticoid receptor antagonists mifepristone and spironolactone. Arcaine, an analog of agmatine with similar structure as agmatine, also blocked glucocorticoid-induced increase of lactate dehydrogenase release. Spermine and putrescine, the endogenous polyamine and metabolic products of agmatine without the guanidino moiety of agmatine, have no appreciable effect on glucocorticoid-induced injuries, indicating a structural relevance for this neuroprotection. Immunocytochemical staining with β-tubulin III confirmed the substantial neuronal injuries caused by glucocorticoids and the neuroprotective effects of agmatine against these neuronal injuries. TUNEL labeling demonstrated that agmatine significantly reduced TUNEL-positive cell numbers induced by exposure of cultured neurons to dexamethasone. Moreover, exposure of hippocampal neurons to dexamethasone significantly increased caspase-3 activity, which was inhibited by co-treatment with agmatine. Taken together, these results demonstrate that agmatine can protect cultured hippocampal neurons from glucocorticoid-induced neurotoxicity, through a possible blockade of the N-methyl-D-aspartate receptor channels or a potential anti-apoptotic property. Keywordsagmatine; hippocampus; glucocorticoids; apoptosis; lactate dehydrogenase; β-tubulin III Agmatine is an endogenous polyamine derived from enzymatic decarboxylation of Larginine (Tabor and Tabor, 1984) and is present in the brain and other tissues of mammals (Li et al., 1994;Lortie et al., 1996). In the past decade, accumulating pharmacological and physiological evidence has adumbrated a major role for agmatine in the CNS. In neuronal tissues, agmatine is present in axon terminals where it is associated with synaptic vesicles * Corresponding author. Tel: (Reis et al., 1998) and can be transported into synaptosomes via a Na + -independent system (Sastre et al., 1997). Agmatine has been reported to act as a ligand of the imidazoline receptor (Li et al., 1994). It inhibits all isoforms of nitric oxide synthase (NOS) (Galea et al., 1996), and blocks nicotinic receptors (Loring, 1990), voltage-gated Ca 2+ channels (Weng et al., 2003;Zheng et al., 2004) and N-methy...