Selective blockade of dopamine D-1 receptors by SCH 23390 discloses striatal dopamine D-2 receptors mediating the inhibition of adenylate cyclase in rats

l-Glutamate but not methyl-D-aspartate (NMDA) or quisqualate (Quis) (10^?6 M) in vitro with or without preincubation increased significantly the K_D value of the [³H]N-propylnorapomorphine ([³H]NPA) binding sites by 21 and 36% respectively in striatal membranes of rat without influencing the striatal [³H]spiperone binding sites. The number of striatal [³H]NPA binding sites was not changed by l-glutamate (10^?6 and 10^?5 M) in vitro. There may thus exist interactions between striatal glutamate receptors — not related to excitatory amino-acid receptors of the NMDA or the QUIS type - and high affinity striatal DA receptors.