Tolerance develops in spinal cord, but not in brain with chronic [Dmt 1]DALDA treatment

Abstract

Previously, we reported that H-2′,6′-dimethyltyrosine [Dmt 1]-D-Arg-Phe-Lys-NH 2 (DALDA), an analogue of the naturally occurring opioid peptide dermorphin, is a highly potent and selective mu receptor agonist with low cross-tolerance to morphine. In the present study, we investigated the effect of treating mice chronically with [Dmt 1]DALDA. The AD 50 of [Dmt 1]DALDA (s.c.) increased eight-fold in animals given this drug chronically; in contrast, the AD 50 increased two-fold in mice chronically treated with morphine. The AD 50 of morphine (s.c.) in these [Dmt 1]DALDA-treated animals was increased more than 120 times, while that of the more selective μ agonist [D-Ala 2-MePhe 4-Gly-ol 5] enkephalin (DAMGO) given intrathecally was increased more than 240 times. However, the AD 50 of DAMGO given intracerebroventricularly was essentially the same in animals treated chronically with [Dmt 1]DALDA as in naïve animals. The dose of naloxone required to precipitate withdrawal in [Dmt 1]DALDA-treated animals was 20 times lower than that in morphine-tolerant animals. Using real-time quantitative PCR, we found that expression of the μ opioid receptor, δ opioid receptor, preproenkephalin and preprodynorphin genes was upregulated in the brain by [Dmt 1]DALDA treatment. No significant changes in expression of opioid receptor or opioid peptide genes were detected in the spinal cord of [Dmt 1]DALDA-treated mice, nor in the brain or spinal cord of morphine-treated mice. We conclude that a high degree of tolerance to [Dmt 1]DALDA develops in the spinal cord but not brain, and cannot be accounted for by changes in expression of opioid receptors or opioid peptides in these tissues.