The Tyr-MIF-1 family of peptides includes MIF-1, Tyr-MIF-1, Tyr-W-MIF-1 and Tyr-K-MIF-1, which have been isolated from bovine hypothalamus and human brain cortex. All these peptides interact with opioid receptors and in addition bind to non-opiate sites specific for each of the peptides. Data in the literature suggest that peptides of the Tyr-MIF-1 family (Tyr-MIF-1s) have antiopioid and opioid- like effects. It is known that some anti-opioid peptides (AOP) could reverse morphine-induced analgesia in rodents and men and able to inhibit the expression of some forms of stress-induced analgesia (SIA) in various species. We examined the effects of the Tyr-MIF-1 peptides (all in dose 1 mg/kg i.p.) in the male Wistar rats on morphine-induced analgesia in acute pain using the paw-pressure (PP) and the tail-flick (TF) tests and on immobilization stress-induced antinociception using the PP test. Our results showed that the Tyr-MIF-1 peptides significantly decreased the analgesic effect of morphine (1 mg/kg i.p.) in both tests used. Immobilization of the rats increased the pain threshold for at least 1 h. The Tyr-MIF-1 peptides reduced stress-induced antinociception in PP test. In conclusion, our findings indicate that Tyr-MIF-1s modulate the analgesic effects of morphine and SIA, which corresponds with the hypothesis about AOP mentioned above.
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