QUANTITATIVE AUTORADIOGRAPHIC CHARACTERIZATION OF THE BINDING OF (+)-5-METHYL-10,11-DIHYDRO-5H-DIBENZO[A,D]CYCLOHEPTEN-5, 10-IMINE ([H-3]MK-801) IN RAT-BRAIN - REGIONAL EFFECTS OF POLYAMINES

The distribution and properties of N-methyl-D-aspartate (NMDA) receptors, labeled with [H-3](+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5 5, 10-imine (MK-801), were examined in rat brain. In sections of brain mash, the kinetics of association and dissociation of [H-3]MK-801 were monophasic in the presence of 100-mu-M glutamate and glycine, and Scatchard transformations of saturation isotherms resulted in linear plots. Inhibition of the binding of [H-3]MK-801 by other noncompetitive antagonists produced competition curves with Hill coefficients close to 1.0, consistent with a simple bimolecular interaction between the radioligand and the receptor. Scatchard plots based upon densitometric measurements of [H-3]MK-801 binding in serial sections of rat brain were also linear, with dissociation constant values ranging from 5.0 to 8.4 nM in different regions at the level of the hippocampus. The distribution of [H-3]MK-801 binding sites paralleled the distribution of NMDA displaceable L-[H-3]glutamate binding sites. One exception was the cerebellar granule cell layer, where the density of binding sites for [H-3]MK-801 was extremely low. The relative density of [H-3]MK-801 to NMDA displaceable L-[H-3]glutamate binding sites was approximately 1 to 2, consistent with the existence of two transmitter recognition sites per NMDA receptor. The modulatory effects of polyamines on [H-3] MK-801 binding were studied in washed brain sections. The polyamine agonists spermine and spermidine enhanced [H-3]MK-801 binding in all regions studied, with increases ranging from 18% in the thalamus to 106% in the ventromedial striatum. The effects of spermine and spermidine in these regions were highly correlated. Diethylenetriamine, which blocks the effects of spermidine, by itself produced decreases in the binding of [H-3]MK-801 in most regions ranging from 5 to 21% but increased binding in parts of the striatum by 3 to 22%. The decrease in binding produced by diethylenetriamine in different brain regions was negatively correlated with the increase in binding produced by the agonists, suggesting that variability in the residual concentration of endogenous polyamines contributes to the regional variability of agonist effects. The polyamine 1,10-diaminodecane produced decreases in [H-3]MK-801 binding (15-25%) in all regions studied, consistent with previous reports that it is an inverse agonist at the polyamine site. Under these experimental conditions there was no regional heterogeneity in the effects of 1,10-diaminodecane, in contrast to the regional heterogeneity in the effects of diethylenetriamine and the polyamine agonists.