Unexpected antinociceptive potency of cyclic [D-Tca1]CTAP: potential for a novel mechanism of action.
This study tested the hypothesis that compounds which may bind simultaneously to delta and mu receptors may be more potent antinociceptive agents than would be predicted from their binding affinities at individual mu and delta opioid receptors. D-Tca-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 ([D-Tca1]CTAP) (where D-Tca is a cyclic D-tryptophan analogue) was synthesized and evaluated in radioligand competition assays, opioid bioassays, and in an antinociceptive assay (the tail-flick test in mice). Additionally, the metabolic stability of [D-Tca1]CTAP was evaluated in striatal and cerebellar tissue slices. In rat brain in vitro, [D-Tca1]CTAP competed weakly for sites labelled by [3H]D-Phe-Cys-Tyr-D-Trp-Om-Thr-Pen-Thr-NH2 ([3H]CTOP) (mu-ligand), and [3H][D-Pen2,pCl-Phe4,D-Pen5]enkephalin (delta-ligand); [D-Pen2,D-Pen5]enkephalin (DPDPE) (delta-agonist) was 6.5-fold less and 230-fold more potent, respectively, against these ligands. Additionally, in mouse isolated vas deferens and guinea pig isolated ileum smooth muscle preparations, [D-Tca1]CTAP proved to be weak as either a delta (IC50 of approximately 2 microM) or mu (IC50 > 8 microM) receptor agonist. Surprisingly, however, i.c.v. [D-Tca1]CTAP produced antinociception with potency similar to DPDPE. The antinociceptive actions of [D-Tca1]CTAP were apparently not due to a metabolite or the release of endogenous opioids, as this compound proved stable in both striatal and cerebellar tissue slices and its antinociceptive actions were not enhanced by the 'enkephalinase' inhibitor thiorphan. The suggestion that [D-Tca1]CTAP might be acting by binding simultaneously to mu and delta receptors to produce its antinociceptive effect is supported by the demonstrated antagonism resulting from mu receptor blockade with either beta-funaltrexamine (beta-FNA) or naloxonazine, or by delta receptor blockade by ICI 174,864 ([N,N-diallyl-Tyr1,Aib2,3,Leu5] enkephalin). Furthermore, the antinociceptive properties of [D-Tca1]CTAP were antagonized by (naltrindole-5'-isothiocyanate) (5'-NTII), an antagonist at the delta 2 opioid receptor subtype, but not by the delta 1 antagonist [D-Ala2,D-Leu5,Cys6]enkephalin (DALCE). Additionally, no antagonism was produced by nor-binaltorphimine (nor-BNI), a kappa antagonist. From these data, [D-Tca1]CTAP appears to bind to mu, and 5'-NTII-sensitive delta 2, opioid receptors, and may represent the first of a class of compounds which may act at an opioid receptor complex via 'self-potentiation'.
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