3H-3-N-methylspiperone labels D2 dopamine receptors in basal ganglia and S2 serotonin receptors in cerebral cortex.

Detailed studies of the properties of 3H-3-N-methylspiperone (NMSP) binding in rat and human brain homogenates were performed at 37 degrees C. In homogenates of rat striatum and frontal cortex and human caudate and frontal cortex tissues, the specific binding was found to be saturable. Rat caudate contained 33.2 pmol/gm wet-weight tissue and displayed an equilibrium dissociation constant (Kd) of 8.7 X 10(-11) M; rat frontal cortex contained 18.5 pmol/gm wet-weight tissue and displayed a Kd of 1.5 X 10(-10) M. Human caudate contained 8.96 pmol/gm wet-weight tissue and displayed a Kd of 1.1 X 10(-10) M; human frontal cortex possessed 9.8 pmol/gm wet-weight tissue and a Kd of 4.4 X 10(-10) M. Kinetic studies revealed a very rapid rate of association in all the tissues studied. The rate of dissociation was relatively slow in all 4 tissue preparations; the dissociation rate was somewhat slower in rat striatum and human caudate relative to rat and human frontal cortex. This was consistent with the somewhat higher affinity, relative to frontal cortex, displayed by 3H-NMSP in rat striatal and human caudate tissue. The pharmacological properties of the specific binding in rat striatal and human caudate tissues were very similar and indicated the presence of brain D2 dopamine receptors. In rat and human frontal cortex tissue homogenates, the pharmacological characteristics of the specific binding indicated the presence of 5-HT2 receptors.