endomorphin-1 and beta-proline

We previously described a novel endomorphin-1 analogue (Tyr-L-beta-Pro-Trp-Phe-NH(2); Endo1-beta-Pro) more resistant to enzymatic hydrolysis than endomorphin-1 that acts as a mu-opioid receptor agonist. In this study we report that Endo1-beta-Pro, s.c. injected in the mouse, is an effective antinociceptive agent in the tail flick (ED(50)=9.2 mg/kg) and acetic acid-induced abdominal constriction (ED(50)=1.2 mg/kg) tests. Moreover, s.c. Endo1-beta-Pro significantly decreases, in the mouse, the gastrointestinal propulsion measured as transit of an orally administered charcoal meal (ED(50)=10.0 mg/kg). Subcutaneous beta-funaltrexamine or a high dose of the mu(1)-opioid receptor-selective antagonist naloxonazine (50 mg/kg) prevents the antinociceptive and antitransit action of Endo1-beta-Pro; moreover, these effects are partially blocked by i.c.v. naloxone or by i.p. naloxone methiodide, this latter does not readily cross the blood-brain barrier. On the contrary, the kappa-opioid receptor antagonist nor-binaltorphimine or the delta-opioid receptor antagonist naltrindole are ineffective Thus, Endo1-beta-Pro may act, preferentially, through central and peripheral mu(2)-opioid receptors to produce antinociception and to inhibit gastrointestinal transit. Endo1-beta-Pro is among the first endomorphin-1 analogues showing antinociceptive activity after systemic administration. This compound will be extremely useful for exploring the pharmacological profile of endomorphins in vivo and confirms the potential therapeutic interest of endomorphin derivatives as novel analgesic agents.

Topics: Analgesics; Animals; Dose-Response Relationship, Drug; Male; Mice; Oligopeptides; Pain Measurement; Proline

In this paper we describe the synthesis and affinity toward the mu-opioid receptor of some tetrapeptides obtained from endomorphin-1, H-Tyr-Pro-Trp-Phe-NH(2) (1), by substituting each amino acid in turn with its homologue. The ability to bind mu-opioid receptors depends on the beta-amino acid, and in particular 4, which contains beta-L-Pro, has a K(I) in the nanomolar range. The peptides 4 and 5 are significantly more resistant to enzymatic hydrolysis than 1. The same compounds, as well as the mu-opioid receptor agonist DAMGO, produced a concentration-dependent inhibition of forskolin-stimulated cyclic AMP formation, thus behaving as mu-opioid agonists. These features suggest that this novel class of endomorphin-1 analogues may represent suitable candidates for the in vivo investigation as potential mu-opioid receptor agonists.

Topics: Animals; Binding, Competitive; Brain; Carboxypeptidases; Cathepsin A; CD13 Antigens; Chymotrypsin; Cyclic AMP; Hydrolysis; In Vitro Techniques; Oligopeptides; Proline; Radioligand Assay; Rats; Receptors, Opioid, mu; Stereoisomerism; Tumor Cells, Cultured