Abstract

The endogenous polyamine spermine has multiple effects on the N-methyl-D-aspartate (NMDA) receptor. These include an increase in the magnitude of NMDA-induced whole-cell currents that is seen in the presence of saturating concentrations of glycine ("glycine-independent" stimulation), an increase in the affinity of the receptor for glycine ("glycine-dependent" stimulation), and voltage-dependent inhibition. Although many of the properties of native NMDA receptors are seen with homomeric NR1 receptors expressed in Xenopus oocytes, we have found that the effects of spermine are differentially regulated by NR2 subunits in heteromeric NR1/NR2 receptors. Glycine-independent stimulation by spermine occurred at homomeric NR1A receptors, which lack the amino-terminal insert in NR1, and at heteromeric NR1A/NR2B receptors but not at heteromeric NR1A/NR2A or NR1A/NR2C receptors. Glycine-independent stimulation was not seen at homomeric NR1B receptors, which include the amino-terminal insert in NR1, or at heteromeric receptors containing NR1B. Thus, glycine-independent stimulation by polyamines requires the presence of an NR1 variant, such as NR1A, that lacks the amino-terminal insert, but the manifestation of the stimulatory effect is controlled by the type of NR2 subunit present in a heteromeric complex. Glycine-dependent stimulation was seen at NR1A/NR2A and NR1A/NR2B receptors and may therefore involve a second polyamine binding site distinct from that which produces glycine-independent stimulation. The voltage-dependent inhibitory effect of spermine, which is more pronounced at hyperpolarized membrane potentials, occurred with similar magnitudes at NR1A/NR2A and NR1A/NR2B receptors but was absent at NR1A/NR2C receptors. Thus, NR2 subunits control both the stimulatory and inhibitory effects of spermine at NMDA receptors. Stimulation but not inhibition by spermine was seen at NR1A/NR2B receptors in the presence of extracellular Mg2+. Stimulation, seen in the presence of physiological concentrations of Ca2+ and Mg2+, may be the predominant effect of polyamines at NMDA receptors in the intact nervous system.