The antinociceptive effects of synthetic dermorphin tetrapeptide analogs containing D-Arg in position 2, H-Tyr-D-Arg-Phe-Gly-NH2 and H-Tyr-D-Arg-Phe-beta-Ala-OH, were measured in mice by the tail-pressure test. The antinociceptive effect produced by intracerebroventricular (ICV), intrathecal (IT) and subcutaneous (SC) administration of either peptide was greater than that produced by morphine. Oral (PO) administration of the peptides showed approximately the same antinociceptive potency as morphine. In addition, the antinociceptive effect produced by SC and PO administration of either peptide was of longer duration than morphine. Pretreatment with naloxone resulted in nearly complete antagonism of the antinociceptive effects produced by ICV and IT administration of either peptide or morphine. Dose ratios (ICV/IT) of H-Tyr-D-Arg-Phe-Gly-NH2 and H-Tyr-D-Arg-Phe-beta-Ala-OH, which were calculated from the AD50 (Antinociceptive Dose = 50% MPE) values, were 5.8 and 6.2, respectively, whereas that of morphine was only 1.46. These results suggest that the mechanisms of the antinociceptive effects of [D-Arg2]-dermorphin tetrapeptide analogs differ from morphine, and that these peptides may possess higher affinities than does morphine for opioid receptors in the spinal cord.
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