Autoradiographic localisation of 3H-apomorphine binding sites in rat brain

The best experimental conditions for a selective binding of 3H-apomorphine to dopamine receptors on cryostat sections were first selected by liquid scintillation quantification of the bound radioactivity. In the corpus striatum, a specific binding occurred with a half-maximal saturation concentration of about 1 nM and a maximal capacity of 180 fmol/mg of slice protein, both values in agreement with previous binding data on either membranes or slices incubated in a physiological medium. Inhibition with domperidone was clearly biphasic, indicating two classes of sites corresponding to the D-2 and D-3 sites as previously defined on membranes. When 3H-apomorphine was used at low concentrations (0.8-1.5 nM), a condition ensuring a preferential labelling of D-2 sites, rather well contrasted autoradiographic pictures were generated. The major dopaminergic projection fields in telencephalon (caudate-putamen, nucleus accumbens, olfactory tubercles) were visualised as well as other catecholaminergic regions such as the superficial gray layer of superior colliculi. Within the striatum, differences in density of these sites were observed in three perpendicular planes and confirmed by a computer densitometric image analysis. Labelling of areas of origin of the cerebral dopaminergic neurons in substantia nigra or ventral tegmental area were also observed. When a higher concentration of 3H-apomorphine (3.5 nM) was used in the presence of domperidone, another, but autoradiographically less distinct subclass of sites (D-3 sites) was demonstrated.