Abstract
The Tyr-MIF-1 family (Tyr-MIF-1 's) of peptides includes MIF-1, Tyr-MIF-1, Tyr-W-MIF1 and Tyr-K-MIF-1, which have been isolated from bovine hypothalamus and parietal cortex of human tissue. They have opioid-like and antiopioid-like properties. All these peptides interact with opioid receptors and in addition bind to non-opiate sites specific for each of the peptides and are able to inhibit the expression of some forms of stress-induced analgesia (SIA) in various species. Stressors are potent modulators of opioid activities. Exposure to stressors causes an array of biochemical, physiological and behavioral changes. There is some evidence to indicate that stress elicits antinociceptive effects. This is an important phenomenon relevant to the perception and the response to pain, and to the modulation of behavioral responses. Stress-induced analgesia has been categorized into one of two broad categories, opioid and non-opioid. The aim of the present study was to examine the effects of the Tyr-MIF-1 peptides on immobilization stressinduced antinociception. Tyr-MIF-1 peptides were administered in male Wistar rats intraperitoneally after 1 hour restraint. The changes in the mechanical nociceptive threshold of the animals were measured by the Randall-Selitto paw pressure test. Injection of Tyr-MIF-1 peptides after immobilization decreased immobilization SIA. Data in the literature show that analgesia induced by immobilization may be due to a non-opioid substance acting peripherally or reaching the spinal cord via the systemic circulation. Anti-opioid properties of Tyr-MIF-1 peptides are able to reduce some of the acute (in particular antinociceptive) opioid effects, and permanently mask the effects of exogenous and endogenous opioids. Accordingly, in addition to pain inhibiting pathways, there are anti-analgesic neuronal peptides, which act by blocking pain inhibition in the central nervous system. Immobilization of the rats increased the pain threshold at least 1 h. Antinociceptive effects of peptides Tyr-MIF-1, Tyr-W-MIF-1 and Tyr-KMIF-1 were reduced after stress. We suggest that in this case peptides exerted antiopioid properties, and one of the mechanisms responsible for that is increased level of endogenous opioids under stress conditions and respectively decreased of ~x-receptor binding sites for Tyr-MIF1 's. In conclusion Tyr-MIF-1 peptides reduced the antinociception after immobilization stress.
Published Version
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