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In the present study, seven novel dimeric analogues of endomorphin-2 with longer spacers were designed and synthesized. Through dimerization, their affinity for -opioid receptor was mostly increased, especially the -opioid receptor preferred dimeric analogue, DEM12. The results were confirmed by the in vitro bioassay. The structure-activity relationships were also discussed.

In the present study, it was found that intraperitoneal (i.p.) pre-injection of NG-nitro-L-arginine methyl ester (LNAME) significantly influenced the endomorphin-1 (EM-1) and endomorphin-2 (EM-2) induced antinociception. These effects could be inhibited or reversed by L-Arg or naloxone. Our results suggest that the modulatory effect of NO system on the mu-receptor evoked analgesia is different between the two mu receptor subtypes.

In the present study, it was found that intraperitoneal (i.p.) pre-injection of NG-nitro-L-arginine methyl ester (LNAME) significantly influenced the endomorphin-1 (EM-1) and endomorphin-2 (EM-2) induced antinociception. These effects could be inhibited or reversed by L-Arg or naloxone. Our results suggest that the modulatory effect of NO system on the mu-receptor evoked analgesia is different between the two mu receptor subtypes.