Regional differences in reappearnce of D2-dopamine receptors in the rat caudate-putamen complex after irreversible inactivation

The reappearance of D2-receptors in the striatum of the rat was studied by autoradiography after in vivo labeling with [3H]N-n-propylnorapomorphine ([3H]NPA) at various time intervals after the inactivation of dopamine receptors by intraperitoneal administration of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Within two days after inactivation the labeling had decreased to 18% of controls. Thereafter, the label reappeared and after 8 days or more reached levels of 80% of that of untreated controls. Autography showed that 4 h after EEDQ treatment no preferential labeling of the striatum can be seen. Five days after EEDQ a slight difference in labeling density between the medial and lateral striatum was detected, whereas after 18 days a prominent lateromedial gradient in silver grain density was seen, resembling the gradient seen without EEDQ treatment. This silver grain gradient is not paralleled by the density of medium-sized neuronal cell bodies. This suggests a difference in synthesis rate of receptors either in other cells than the medium-sized neuron or, alternatively, in otherwise indistinguishable medium-sized neurons. Five days after EEDQ treatment, clusters of silver grains in the lateral striatum were seen. These clusters have a diameter of 150–400 μm and are separated from each other at 200–500 μm. Each cluster may represent newly synthesized receptors of a single neuron (e.g. cholinergic or somatostatinergic interneuron).