부산대학교 도서관

The present study was undertaken to investigate the role of the hypothalamic tripeptide l-prolyl-l-leucyl-glycinamide (PLG) and its conformationally constrained analog 3(R)-[(2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide (PAOPA) in modulating agonist binding to human dopamine (DA) receptor subtypes using human neuroblastoma SH-SY5Y cells stably transfected with respective cDNAs. Both PLG and PAOPA enhanced agonist [(3)H]N-propylnorapomorphine (NPA) and [(3)H]quinpirole binding in a dose-dependent manner to the DA D(2L),D(2S), and D(4) receptors. However, agonist binding to the D(1) and D(3) receptors and antagonist binding to the D(2L) receptors by PLG were not significantly affected. Scatchard analysis of [(3)H]NPA binding to membranes in the presence of PLG revealed a significant increase in affinity of the agonist binding sites for the D(2L), D(2S), and D(4) receptors. Analysis of agonist/antagonist competition curves revealed that PLG and PAOPA increased the population and affinity of the high-affinity form of the D(2L) receptor and attenuated guanosine 5'-(beta,gamma-imido)-triphosphate-induced inhibition of high-affinity agonist binding sites for the DA D(2L) receptor. Furthermore, direct NPA binding with D(2L) cell membranes pretreated with suramin, a compound that can uncouple receptor/G protein complexes, and incubated with and without DA showed that both PLG and PAOPA had only increased agonist binding in membranes pretreated with both suramin and DA, suggesting that PLG requires the D(2L) receptor/G protein complex to increase agonist binding. These results suggest that PLG possibly modulates DA D(2S), D(2L), and D(4) receptors in an allosteric manner and that the coupling of D(2) receptors to the G protein is essential for this modulation to occur.