Autoradiographic comparison of D1-specific binding of [3H]SCH39166 and [3H]SCH23390 in the primate cerebral cortex.

Quantitative autoradiography was used to compare the binding of the novel dopamine D1 receptor antagonist, [3H]SCH39166, with that of the widely used radioligand, [3H]SCH23390 (in the presence of ritanserin), in the primate cerebral cortex. Specific binding of both radioligands, determined using SCH23390 or cis-flupentixol as displacing agents, had very similar densities and distributions throughout the cortex. However, the specific binding of [3H]SCH39166 obtained with SCH39166 as a blank was significantly higher than that obtained using SCH23390 or cis-flupentixol as displacing agents in some layers of motor, somatosensory and occipital cortices. In addition, the non-specific binding of [3H]SCH39166 obtained in the presence of an excess of SCH23390 of cis-flupentixol displayed a complex laminar pattern very different from that of the specific binding. These observations suggest that [3H]SCH39166 may have a high affinity to more than the D1 receptor subtype bound by SCH23390 or cis-flupentixol. Also, these additional sites are likely to be different from 5-HT2 or 5-HT1C receptors since the latter sites were not displaced by 1 microM SCH23390.