Spermidine attenuation of volatile anesthetic inhibition of glutamate-stimulated [³H](5D, 10S)-(+)-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine ([³H]MK-801) binding to N-methyl-D-aspartate (NMDA) receptors in rat brain

The influence of spermidine, a polyamine agonist, on volatile anesthetic inhibition of N-methyl-D-aspartate (NMDA) receptor activation, as indicated by glutamate stimulation of [³H]MK-801 ([³H](5D,10S)-(+)-methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine) binding, was studied in rat brain. Spermidine reserved the inhibition caused by four volatile anesthetics (enflurane, halothane, methoxyflurane and chloroform) at the same concentrations (EC₅₀ ≈ 3 μM) at which it potentiated glutamate opening of the NMDA ion channel. The anesthetics had no effect on the direct stimulation of channel opening by spermidine, which occurred at concentrations of spermidine greater than 30 μM in the absence of receptor agonist. In these actions, spermidine closely resembled the allosteric co-agonist glycine. The present results suggest that anesthetic action on NMDA receptors involves a set of sites on the channel complex that is distinct from the recognition sites for glutamate, glycine, and channel blockers, and are consistent with the idea that blockade of NMDA receptors contributes to the development of the anesthetic state.