Distinctive expression and cellular distribution of dopamine receptors in the pancreatic islets of rats

Activation of the dopamine (DA) D2 receptor inhibits glucose-stimulated insulin secretion in isolated rodent islets in vitro; however, no information is available regarding the cellular localization of DA receptors (DRs, including D1-D5 receptors) in pancreatic islets in situ. We investigate the protein expression and cellular localization of five types of DRs in pancreatic islets by means of Western blotting and double-labeling immunofluorescence in both normal control and alloxan-induced type 1 diabetes model (T1DM) rats. In control rats, D1 immunoreactivity (-IR) was distributed in the core of the islet and co-localized with insulin-IR, D2-IR was peripherally distributed and found only in somatostatin-immunoreactive cells and D5-IR was co-localized with glucagon-IR and pancreatic polypeptide-IR. No IR for either the D3 or D4 receptor was observed in rat islets. The protein level of the D1 receptor was reduced in T1DM rats (D1/D-glyceraldehyde-3-phosphate dehydrogenase [GAPDH], 0.63 +/- 0.05 in control rats compared with 0.16 +/- 0.03 in T1DM rats, n = 8, P < 0.05) but no significant alteration was detected in the protein expression of either the D2 receptor (D2/GAPDH, 0.48 +/- 0.04 compared with 0.43 +/- 0.04, n = 8, P = 0.42) or the D5 receptor (D5/GAPDH, 0.50 +/- 0.04 compared with 0.47 +/- 0.04, n = 8, P = 0.58). The present study is the first clear demonstration of the protein expression and cellular localization of the D1, D2 and D5 receptors in rat pancreatic islets and provides crucial morphological evidence for further investigations of the underlying mechanism regarding the DA regulation of pancreatic endocrine function.