[L-Ala3]DPDPE: a new enkephalin analog with a unique opioid receptor activity profile. Further evidence of delta-opioid receptor multiplicity.

To investigate delta-opioid receptor topography near the 3-position of [D-Pen2,D-Pen5]enkephalin (DPDPE), a series of small-group 3-position analogs of DPDPE have been synthesized and assayed for binding potencies and in vitro biological activities. L-Amino acid substitutions at this position are highly favored over D-amino acid substitutions, with the smallest, [L-Ala3]DPDPE (DPADPE), being the most favored in the series investigated. [L-Ala3]DPDPE is nearly as delta-potent and more delta-selective in both rat brain binding (18 nM vs [3H] [p-ClPhe4]DPDPE and mu/delta = 610) and peripheral bioassays (12 nM in the MVD and GPI/MVD = 4500) when compared to DPDPE (8.5 nM, mu/delta = 73 and 4.1 nM, GPI/MVD = 1800, respectively). Whereas DPDPE is a potent analgesic when given icv, [L-Ala3]DPDPE is only a weak analgesic. However, [L-Ala3]DPDPE has been found to antagonize DPDPE, but not Deltorphin II, in a moderately potent (pA2 = 5.7) and selective fashion in vivo. Thus, [L-Ala3]DPDPE is a fairly potent agonist at peripheral delta receptors and is a moderately potent (mixed) antagonist of delta 1 receptors in the brain. It appears that [L-Ala3]DPDPE does not interact in any significant manner with delta 2 or mu receptors in the brain.