Cross-tolerance to acute administration of mu and kappa opioid agonists at the spinal cord level in the rat

Analgesic tolerance and cross-tolerance to the bifunctional opioid/neuropeptide FF receptors agonist EN-9 and μ-opioid receptor ligands at the supraspinal level in mice

The chimeric peptide EN-9 was reported as a κ-opioid/neuropeptide FF receptors bifunctional agonist that modulated chronic pain with no tolerance. Many lines of evidence have shown that the effect of the κ-opioid receptor is mediated by not only its specific activation but also downstream events participation, especially interaction with the μ-opioid receptor pathway in antinociception and tolerance on most occasions. The present study investigated the acute and chronic cross-tolerance of EN-9 with μ-opioid receptor agonist EM-2, DAMGO, and morphine after intracerebroventricularly (i.c.v) injection in the mouse tail-flick test. In the acute tolerance test, EN-9 showed symmetrical acute cross-tolerance to DAMGO but no cross-tolerance to EM2. In the chronic tolerance test, EN-9 had no tolerance after eight days of repeated administration. However, EN-9 illustrated complete cross-tolerance to morphine and symmetrical cross-tolerance to EM2. In addition, inhibition of NPFF receptor could induce the tolerance development of EN-9. These findings indicated that supraspinal EN-9-induced antinociception contains additional components, which are mediated by the downstream μ-opioid receptor pathway both in acute and chronic treatment, whereas the subtypes of μ-opioid receptor or NPFF system pathway involved in antinociceptive effects induced by EN-9 are complex. Identifying the receptor mechanism could help design preferable bifunctional opioid compounds.

Characterization of the antinociceptive effects of intrathecal DALDA peptides following bolus intrathecal delivery

Background and aims We systematically characterized the potency and side effect profile of a series of small opioid peptides with high affinity for the mu opioid receptor. Methods Male Sprague Dawley rats were prepared with intrathecal (IT) catheters, assessed with hind paw thermal escape and evaluated for side effects including Straub tail, truncal rigidity, and pinnae and corneal reflexes. In these studies, DMT-DALDA (dDAL) (H-Dmt-D-Arg-Phe-Lys-NH2 MW=981), dDALc (H-Dmt-Cit-Phe-Lys-NH2 MW=868), dDALcn (H-Dmt-D-Cit-Phe-Nle-NH2 MW=739), TAPP (H-Tyr-D-Ala-Phe-Phe-NH2 MW=659), dDAL-TICP ([Dmt1]DALDA-(CH2)2-NH-TICP[psi]; MW=1519), and dDAL-TIPP (H-Dmt-D-Arg-Phe-Lys(Nε-TIPP)-NH2 were examined. In separate studies, the effects of approximately equiactive doses of IT DMT DALDA (10 pmol), morphine (30 nmol) and fentanyl (1 nmol) were examined on formalin-induced flinching at different pretreatment intervals (15 min - 24 h). Results (1) All agents resulted in a dose-dependent reversible effect upon motor function (Straub Tail>Truncal rigidity). (2) The ordering of analgesic activity (%MPE) at the highest dose lacking reliable motor signs after bolus delivery was: DMT-DALDA (80%±6/3 pmol); dDALc (75%±8/1 pmol); dDALcn (84%±10/300 pmol); TAPP (56%±12/10 nmol); dDAL-TICP (52%±27/300 pmol). (3) All analgesic effects were reversed by systemic (IP) naloxone (1 mg/kg). Naltrindole (3 mg/kg, IP) had no significant effect upon the maximum usable peptide dose. (4) Tolerance and cross-tolerance development after 5 daily boluses of DMT-DALDA (3 pmol) and morphine (30 nmol) revealed that both agents displayed a progressive decline over 5 days. (5) Cross-tolerance assessed at day 5 revealed a reduction in response to morphine in DMT-DALDA treated animal but not DMT-DALDA in the morphine treated animal, indicating an asymmetric cross-tolerance. (6) IT DMT-DALDA, morphine and fentanyl resulted in significant reductions in phase 1 and phase 2 flinching. With a 15 min pretreatment all drugs resulted in comparable reductions in flinching. However, at 6 h, the reduction in flinching after DMT-DALDA and morphine were comparably reduced while fentanyl was not different from vehicle. All effects on flinching were lost by 24 h. Conclusions These results emphasize the potent mu agonist properties of the DALDA peptidic structure series, their persistence similar to morphine and their propensity to produce tolerance. The asymmetric cross-tolerance between equiactive doses may reflect the relative intrinsic activity of morphine and DMT-DALDA. Implications These results suggest that the DALDA peptides with their potency and duration of action after intrathecal delivery suggest their potential utility for their further development as a spinal therapeutic to manage pain.