Autoradiographic mapping of dopamine-D2 ⁄D3 receptor stimulated [S]GTPcS binding in the human brain

Agonist stimulated [S]guanosine 5¢-c-thiotriphosphate ([S]GTPcS) binding autoradiography was established for the examination of dopamine-D2 ⁄D3 receptors in human brain sections. The distribution of G proteins activated by dopamine-D2 ⁄D3 receptors was studied in whole hemisphere cryosections. Dopamine stimulated [S]GTPcS binding in brain regions with high densities of dopamine D2-like receptors, i.e. putamen (23 ± 2%, mean ± SEM,% stimulation over basal binding), caudate (20 ± 0%) and substantia nigra (22 ± 2%), but also in regions with lower receptor densities such as amygdala (17 ± 8%), hippocampus (16 ± 6%), anterior cingulate (13 ± 3%), and thalamus (12 ± 2%). Dopamine stimulated [S]GTPcS binding to significantly higher levels in the dorsal than in the ventral part of the striatum. Dopamine caused low or very low stimulation in all cortical areas. Raclopride, a selective D2 ⁄D3 receptor antagonist, potently inhibited dopamine stimulated [S]GTPcS binding, whereas R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5tetrahydro-1H-3-benzazepine hydrochloride (SCH23390), a selective D1 antagonist, did not block the [ S]GTPcS binding response stimulated by dopamine. Hence, the stimulatory effect of dopamine was primarily mediated by D2 ⁄D3 receptors. Quinpirole stimulated [S]GTPcS binding in the same regions as dopamine. The maximal level of stimulation induced by dopamine and quinpirole was not significantly different. The present study demonstrates that agonist stimulated [S]GTPcS binding autoradiography could be a suitable technique for the examination of dopamine-D2 ⁄D3 receptors in the human brain. This functional assay could provide useful new information about dopamine receptor ⁄G protein coupling in the postmortem human brain, and reveal possible disease related alterations of the interaction between D2 ⁄D3 receptors and G proteins.