Expression of D1 but not D2 dopamine receptors in striatal neurons producing neurokinin B in rats

Neostriatal projection neurons are known to be largely divided into two groups, striatoentopeduncular/striatonigral and striatopallidal neurons, which mainly express D1 and D2 dopamine receptors, respectively. Recently, a small population of neostriatal neurons have been reported to produce neurokinin B (NKB), and send their axons mainly to the basal forebrain regions. To reveal which type of dopamine receptors were expressed by these NKB‐producing neurons, we examined rat striatal neurons by combining immunofluorescence labeling for preprotachykinin B (PPTB), the precursor of NKB, and fluorescence in situ hybridization labeling for dopamine receptors. Fluorescent signals for D1 receptor mRNA were detected in 85–89% of PPTB‐immunopositive neurons in the neostriatum, accumbens nucleus and lateral stripe of the striatum, whereas almost no signal for D2 receptor was observed in PPTB‐positive striatal neurons. To further reveal intracellular signaling downstream of D1 receptor in PPTB‐producing neurons, we used a double immunofluorescence labeling method to study the localization of some substrates for protein kinase A (PKA), which was known to be activated by D1 receptor. Although only 3–7% of PPTB‐immunopositive striatal neurons displayed immunoreactivity for dopamine‐ and cAMP‐regulated phosphoprotein of 32 kDa, a well‐known PKA substrate expressed in the two major groups of neostriatal projection neurons, 60–64% of PPTB‐positive striatal neurons exhibited immunoreactivity for striatal‐enriched tyrosine phosphatase. These results suggest that NKB‐producing neostriatal neurons are similar to striatoentopeduncular/striatonigral neurons in the usage of dopamine receptor subtypes, but different from the two major groups of neostriatal projection neurons in terms of the downstream signaling of dopamine receptors.