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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 [³H]N-propylnorapomorphine (NPA) and [³H]quinpirole binding in a dose-dependent manner to the DA D_2L,D_2S, and D₄ receptors. However, agonist binding to the D₁ and D₃ receptors and antagonist binding to the D_2L receptors by PLG were not significantly affected. Scatchard analysis of [³H]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 D₄ receptors. Analysis of agonist …