Antagonist properties of polyamines and bis(ethyl)polyamines at N-methyl-D-aspartate receptors

The polyamine spermine has both stimulatory and inhibitory effects on N-methyl-D-aspartate (NMDA) receptors. At recombinant NMDA receptors, effects of spermine are dependent on the subunit composition of the receptor. In the present work we have used voltage-clamp recording to examine the effects of polyamines and bis(ethyl)polyamines on recombinant NMDA receptors expressed in Xenopus oocytes. The compounds that were studied include several bis(ethyl)polyamines that may be clinically useful as antitumor agents. A number of pentaamines and bis(ethyl)pentaamines were found to act as potent voltage-dependent antagonists at heteromeric NR1A/NR2A and NR1A/NR2B receptors, but not at NR1A/NR2C receptors. Antagonism was more pronounced in oocytes voltage-clamped at -80 mV than at -20 mV. Some polyamine analogs also potentiated responses to glutamate at NR1A/NR2B receptors at membrane potentials of -20 to +40 mV, but this effect required higher concentrations of polyamines than did inhibition seen at hyperpolarized membrane potentials. At NR1A/NR2A receptors the block seen with pentaamines and bis(ethyl)pentaamines, but not with spermine or bis(ethyl)spermine, was maximal at a membrane potential of -100 mV and was relieved at more negative as well as at more positive membrane potentials. This suggests that the mechanism of inhibition of NMDA receptors by pentaamines is different from that of spermine. Pentaamines may permeate the ion channel of NMDA receptors at very hyperpolarized membrane potentials and may be useful for studying the structural properties of NMDA receptor channels.