ELECTRON-MICROSCOPIC ANALYSIS OF D1 AND D2 DOPAMINE-RECEPTOR PROTEINSIN THE DORSAL STRIATUM AND THEIR SYNAPTIC RELATIONSHIPS WITH MOTOR CORTICOSTRIATAL AFFERENTS

The precise localization of D1 and D2 dopamine receptors within striat al neurons and circuits is crucial information for further understandi ng dopamine pharmacology, We have used subtype specific polyclonal and monoclonal antibodies against D1 and D2 dopamine receptors to determi ne their cellular and subcellular distributions, their colocalization, and their differential connectivity with motor cortical afferents lab eled either by lesion-induced degeneration or by anterograde transport of biotinylated dextrans, D1 and D2 are primarily expressed in medium -sized neurons and spiny dendrites, Axon terminals containing D1 were rare whereas D2-immunoreactive axon terminals forming symmetrical syna pses with dendrites and spines were common, In 2 mu m sections, D1 was localized to 53% of neurons, and D2 to 48% of neurons, while mixing D 1 and D2 antibodies labeled 78%, By electron microscopy, D1 was locali zed to 43% of dendrites and 38% of spines while D2 was localized to 38 % of dendrites and 48% of spines, Combining D1 and D2 antibodies resul ted in the labeling of 88.5% of dendrites and 92.6% of spines, Using d ifferent chromogens for D1 and D2, colocalization was not observed, Ip silateral motor corticostriatal afferents were primarily axospinous an d significantly more synapsed with D1 than D2-positive spines (65% vs 47%), Contralateral motor corticostriatal afferents were frequently ax odendritic and no difference in their frequency of synapses with D1 an d D2 dendrites and spines was observed, These findings demonstrate dif ferential patterns of expression of D1 and D2 receptors in striatal ne urons and axon terminals and their differential involvement in motor c orticostriatal circuits.