Localization of dopamine D1 receptors and dopaminoceptive neurons in the chick forebrain

The distributions of dopamine D1 receptors, dopaminoceptive neurons, and catecholaminergic fibers were investigated in the forebrain of the domestic chick by using D1 receptor autoradiography and immunohistochemical detection of D1 receptor protein (D1rp), the dopamine- and cAMP-regulated phosphoprotein DARPP-32, and tyrosine hydroxylase (TH). Particular attention was paid to two forebrain regions, the mediorostral neostriatum/ hyperstriatum ventrale (MNH) and neostriatum dorsocaudale (Ndc), which have been shown to be crucially involved in filial imprinting. In general, there was a good, but not complete, correlation between the immunohistochemical pattern of DARPP-32 positive perikarya and the distribution of D1 receptors. Both, DARPP-32 positive neurons as well as D1 receptors were highly enriched in the striatal part of the basal ganglia including the lobus parolfactorius (LPO) and paleostriatum augmentatum. High to moderate densities were observed in the outer rind of the pallium. Low to moderate densities were found in the belt regions of primary sensory areas, whereas densities in the respective core regions were generally low. Labeling in the MNH and Ndc was heterogeneous. Whereas the neostriatal part of MNH displayed both, moderate DARPP-32 immunostaining and moderate D1 receptor densities, the hyperstriatal part showed also moderate D1 receptor densities but was only weakly labeled by DARPP-32. The rostral part of the Ndc was among the most intensely DARPP-32 labeled areas of the pallium, its caudal part revealed only moderate DARPP-32 immunostaining. By using D1 receptor autoradiography, a homogeneous labeling throughout the rostrocaudal extension of the Ndc was found. Double-labeling experiments with antibodies to DARPP-32 and TH revealed that TH positive fibers in the MNH, Ndc, and LPO were often closely related to DARPP-32 positive perikarya. At the ultrastructural level, both immunoreaction for D1rp and DARPP-32 in the MNH and Ndc were primarily found to be associated with postsynaptic elements. Whereas D1rp immunoreactivity was enriched at postsynaptic densities or in their vicinity, reaction product for DARPP-32 was present throughout the perikaryal cytoplasm, dendrites, and dendritic spines. These results indicate that DARPP-32 as well as D1 receptors in the avian forebrain reveal a distribution that is substantially similar to that of mammals.