We have isolated various bioactive peptides acting by bindin

We have isolated various bioactive Sephin 1 acting by binding to the receptors of endogenous bioactive peptides such as opioid, bradykinin, neurotensin, and complements C3a and C5a etc. [24]. Rubimetide is the first example of an agonist peptide of the FPR2 of exogenous origin. Anxiolytic-like activity of rubimetide after icv administration was almost comparable to those of potent FPR2 ligands, humanin and MMK1, in spite that its affinity for the FPR2 was small. Rubimetide also has an inhibitor activity for angiotensin I-converting enzyme (ACE) (IC50=0.6μM) [21]. It also has antioxidative activity [24]. Especially, inhibitors of ACE such as captopril and SQ29,852 have been reported to reduce anxiety [3]. The ACE inhibitor activity of rubimetide might contribute to potentiate its anxiolytic-like activity mediated by the FPR2. The smallness in molecular size of rubimetide might also facilitate its central effect to occur after the peripheral administration. In previous study, we found that the anxiolytic-like activity of rubimetide was mediated by the PDG2-DP1 receptor system [26]. We also found that PGD2 itself exerted an anxiolytic-like activity, which was blocked by antagonists of the DP1, A2A and GABAA receptors [27]. Then, we investigated the mechanism underlying the anxiolytic-like activity of rubimetide downstream of the FPR2 and DP1 receptors. The anxiolytic-like activity of rubimetide was blocked by antagonists of the A2A and GABAA receptors (Fig. 4-A and -B). Furthermore, the anxiolytic-like activity of MMK1, a selective agonist of the FPR2, was also blocked by the DP1 receptor antagonist. Taken together, activations of the PGD2-DP1, adenosine-A2A and GABA-GABAA receptor systems downstream of the FPR2 receptor are involved in the anxiolytic-like activities induced by rubimetide, as well as the typical agonist peptides of the FPR2, as shown in Fig. 5. We found previously that complement C5a exerted an anxiolytic-like activity after icv administration in mice, which was also mediated successively by DP1, A2A, and GABAA receptors downstream of the C5a receptor [12]. FPR2 and C5a receptor have been reported to have a structural homology and might be evolutionaly related each other [1], [2]. It should be noted that both receptors mediate anxiolytic-like activities followed by essentially the same post-receptor mechanisms.
Acknowledgement This study was supported in part by a grant form Kagome Co. Ltd. (Tokyo Japan).