Dopamine agonist radioligand binds to both D2High and D2Low receptors, explaining why alterations in D2High are not detected in human brain scans.

The features of schizophrenia are consistent with increased sensitivity to endogenous dopamine. Animal models of schizophrenia reveal an increase in the in vitro proportion of striatal dopamine D2 receptors in the high-affinity state for dopamine (i.e., D2High), as measured by dopamine/[(3) H]domperidone competition. However, in vivo studies did not reveal the dopamine agonist (11) CPHNO to be elevated in amphetamine-sensitized rats. Also, no increase was found in the in vivo binding of (11) CPHNO in schizophrenia patients. This work was done to resolve the contradictory findings. It was found that the in vitro density of rat striatal D2 receptors was 18 pmol/g for [(3) H]raclopride and 12 pmol/g for (3) HPHNO; most of the latter sites disappeared in the presence of guanine nucleotide. Using 2 nM (3) HPHNO (K(d) of 0.72 nM at D2) to label D2 receptors in the striata and the human D2 clone, 10 nM to 100 nM dopamine inhibited 10-20% of the (3) HPHNO bound, representing high-affinity binding of (3) HPHNO, with the remainder inhibited above 100 nM dopamine, representing low-affinity binding of (3) HPHNO. It was found that (+)PHNO and (-)NPA dissociated from the D2 clone with half-times of 96 and 600 s, respectively. These rates are slower than the reported sub-second dissociation of the G protein from a receptor, suggesting that these two ligands still occupy the D2Low receptor after the G protein has separated. Thus, the radio-agonist label for (+)PHNO is not selective for dopamine D2High receptors, but also binds to the D2Low state of the dopamine receptor.