Analgesic activity of dipeptide Tyr-Pro

The effect of dipeptide Tyr-Pro, present in representatives of most families of opioid peptides, and two its analogs, Tyr-Pro-NH₂ and Tyr-Pro-OMe, on analgesic activity was studied in different tests (tail-flick test, tail pinch (Haffner’s) test, formalin test, and acetic acid writing test) describing different organization levels of pain sensitivity. Intraperitoneal administration of the dipeptide decreased the pain threshold in all above-mentioned tests. Coadministration of the dipeptide and naloxone or naloxone methiodide insignificantly decreased the dipeptide analgesic effect in the tail-flick and acetic acid writing tests. Its analogs Tyr-Pro-NH₂ and Tyr-Pro-OMe demonstrated a similar analgesic activity in the tail-flick test and a higher activity in the acetic acid writing test. Administration of individual amino acids (Tyr or Pro) or their mixture had no effect on the pain threshold.