Cortical localization of dopamine D4 receptors in the rat brain - Immunocytochemical study

Using polyclonal antibody against dopamine D4 receptor we investigated cort ical distribution of D4 receptors, with the special emphasis on regions df the prefrontal cortex. Prefrontal cortex is regarded as a target for neurol eptic drugs, and engaged in the regulation of the psychotic effects of vari ous substances used in the experimental modeling of schizophrenia. Western blot analysis performed on samples from the rat cingulate, parietal, pirifo rm cortices and also striatum revealed that antibody recognized one main ba nd of approximately 40 kD, which corresponds to the predicted molecular wei ght of D4 receptor protein. In immunocytochemical studies we found D4 recep tor-positive neurons in all regions of prefrontal cortex (cingulate, agranu lar/insular and orbital cortices) and all cortical regions adjacent to pref rontal cortex, such as frontal, parietal and piriform cortex. Substantial n umber of D4 receptor-positive neurons has also been observed within the str iatum and nucleus accumbens. In general, a clear stratification of the D4 r eceptor-positive neurons was observed in the cortex with the highest densit y seen in layers II/III and V/VI. D4 immunopositive material was also found in the dendritic processes, particularly clearly visible in the layer II/I II. At the cellular level D4 receptor immunoreactivity was seen predominant ly on the periphery of the cell body, but a certain population of neurons w ith clear cytoplasmatic localization was also identified. In addition to co rtical distribution of D4 receptor-positive neurons we tried also to define types of neurons expressing D4 receptor protein. In double-labeling experi ments, D4 receptor protein was found in nonphosphorylated neurofilament H-p ositive, calbindin-D28k-positive, as well as parvalbumin-positive cells. Si nce, used proteins are markers of certain populations of pyramidal neurons and GABA-ergic interneurons, respectively, our data indicate that D4 recept ors are located on cortical pyramidal output neurons and their dendritic pr ocesses as well as on interneurons. Above localization indicates that D4 re ceptors are not only directly influencing excitability of cortical inter- a nd output neurons but also might be engaged in dendritic spatial and tempor al integration, required for the generation of axonal messages. Additionall y, our data show that D4 receptors are widely distributed throughout the co rtex of rat brain, and that their cortical localization exceeds the localiz ation of dopaminergic terminals.