Identification and regional distribution of the dopamine D-1A receptor in the gastrointestinal tract

Dopamine (DA) is regarded as an important modulator of enteric function. Re cent experiments have suggested that newly cloned DA receptor subtypes are widely expressed in peripheral organs, including the gastrointestinal tract . In the present studies, the D-1A receptor subtype was identified in rat g ut regions through localization of receptor protein by means of light micro scopic immunohistochemistry and Western blot analysis and receptor mRNA by RT-PCR and in situ amplification and hybridization (3SR in situ). D-1A rece ptor immunoreactivity was shown to have a diverse distribution in the gastr ointestinal tract, being present in the gastroesophageal junction, stomach, pylorus, small intestine, and colon. The receptor has a transmural distrib ution present in both epithelial and muscle layers as well as in blood vess els and lamina propria cells of different gastrointestinal regions. Western blot analysis demonstrated a single 50-kDa band for esophagus, stomach, du odenum, jejunum, and colon. The in situ hybridization signal was localized to the same sites revealed by D-1A receptor immunoreactivity. RT-PCR reveal ed an appropriate sized signal in similar regions. This study is the first to identify expression of the central D-1A receptor throughout the normal m ammalian gastrointestinal tract.