Synthesis and dopamine receptor modulating activity of unsubstituted and substituted triproline analogues of l-prolyl-l-leucyl-glycinamide (PLG)

Triprolines Pro-Pro-Pro-NH2 (4), Pro-Pro-D-Pro-NH2 (5), Pro-Pro(trans-3-Me)-D-Pro-NH2 (6), and Pro-Pro(cis-3-Me)-D-Pro-NH2 (7) were made as conformationally constrained analogues of Pro-Leu-Gly-NH2. Triprolines 4-6 produced significant increases in the high- and low-affinity state ratio (RH/RL) of the dopamine receptor, but only 4 was found to increase apomorphine induced rotations in 6-hydroxydopamine-lesioned rats.