Antinociceptive and respiratory effects of intrathecal H-Tyr-D-Arg-Phe-Lys-NH2 (DALDA) and [Dmt1] DALDA

DALDA (H-Tyr-D-Arg-Phe-Lys-NH(2)) and [Dmt(1)]DALDA (H-Dmt-D-Arg-Phe-Lys-NH(2)) (Dmt = 2',6'-dimethyltyrosine) are potent and highly selective mu-opioid agonists (K(i)(delta)/K(i)(mu) > 10,000 and K(i)(kappa)/K(i)(mu) > 100). Both peptides carry a 3+ charge at physiological pH. Their antinociceptive and respiratory effects were compared with morphine (MOR) after intrathecal administration in rats. Both DALDA and [Dmt1]DALDA produced dose-dependent and naloxone-reversible antinociceptive effects with relative potencies of 14 and 3000x that of MOR. The antinociceptive potency of [Dmt1]DALDA far exceeded its affinity and potency at the mu-opioid receptor and may be explained by its ability to inhibit norepinephrine (NE) uptake in spinal cord synaptosomes. The antinociceptive response to [Dmt1]DALDA was significantly attenuated by the alpha(2)-adrenergic antagonist yohimbine. Thus, [Dmt1]DALDA may be regarded as a drug with dual actions, and its antinociceptive potency is better described by both its affinity and potency at mu-opioid receptors, and its potency at inhibiting NE uptake. The analgesic duration of an equipotent dose of MOR, DALDA, and [Dmt1]DALDA was 3, 7, and 13 h, respectively, and the long duration may be due to the hydrophilic nature of these peptide analogs. Respiratory effects were determined using whole body plethysmography at 3 and 30x the antinociceptive ED(50). A significant depression in minute ventilation was observed with the higher dose of morphine and both doses of DALDA, but not with either dose of [Dmt1]DALDA. Because of its high antinociceptive potency, long duration of action, and low propensity to induce respiratory depression, [Dmt1]DALDA is of interest as a drug candidate for intrathecal analgesia.