D2 dopamine receptor protein location: Golgi impregnation-gold toned and ultrastructural analysis of the rat neostriatum.

The neostriatal distribution of D2 dopamine receptor protein has been assessed using subtype-selective polyclonal antibodies generated against three unique polypeptide sequences of the receptor. The experimental tissues were processed by peroxidase based immunohistochemical procedures for routine light microscopy, Golgi impregnation-gold toned morphological characterization, and correlative light/electron microscopy. The results demonstrated a regional gradient of D2-like dopamine receptor expression in the neostriatum, where lateral portions in the nucleus exhibited more reactive cell bodies than medial portions. D2-like expression was detected in the three populations of neostriatal neurons, i.e., the medium-sized spiny projection neurons, and the medium- and large-sized aspiny interneuron types. Morphometric measurements of labeled neurons verified that medium and large diameter neurons expressed the D2-like receptor subtype. D2-like immunoreactivity was distributed throughout the cytoplasm in dendritic processes, and in presynaptic terminal boutons. Immunoreactivity for the receptor protein was also detected in small, thinly myelinated axons, suggesting the possibilities of anterograde transport of the receptor from cell bodies in the substantia nigra to their neostriatal terminal fields, as well as from local axon collaterals of neostriatal projections neurons. These findings provide evidence of widespread distribution of the D2-like receptor protein in neostriatal neurons, and showed that the presynaptic D2 receptors contain analogous epitopes to the postsynaptic receptor subtype.