Agmatine selectively blocks the N-methyl-D-aspartate subclass of glutamatereceptor channels in rat hippocampal neurons

We investigated in rat hippocampus neurons whether 4-(aminobutyl)guanidine (agmatine), formed by decarboxylation of L-arginine by arginine decarboxyla se and metabolized to urea and putrescine, can modulate the function of N-m ethyl-D-aspartate (NMDA) receptor channels. In cultured hippocampal neurons studied by whole-cell patch clamp, extracellular-applied agmatine produced a voltage- and concentration-dependent block of NMDA but not alpha-amino-3 -hydroxy-5-methyl-4-isoxazolepropionic acid nor kainate currents. Analysis of the voltage dependence of the block suggests that agmatine binds at a si te located within the NMDA channel pore with a dissociation constant of 952 mu M at 0 mV and an electric distance of 0.62. We also tested effects of s everal agmatine analogs. Arcaine (1,4-butyldiguanidine) also produced a sim ilar voltage-dependent block of the NMDA current, whereas putrescine (1,4-b utyldiamine) had little effect, suggesting that the guanidine group of agma tine is the active moiety when blocking the NMDA channel. Moreover, spermin e (an endogenous polyamine) potentiated the NMDA current even in the presen ce of blocker agmatine or arcaine, suggesting that the guanidine-containing compounds agmatine and arcaine interact with the NMDA channel at a binding site different from that of spermine. Our results indicate that in hippoca mpal neurons agmatine selectively modulates the NMDA subclass of glutamate receptor channels mediated by the interaction between the guanidine group a nd the channel pore. The results support other data that agmatine may funct ion as an endogenous neurotransmitter/neuromodulator in brain.