Differential activity of the endogenous antiopiate Tyr-MIF-1 after various intensities of stress.

Three forms of stress-induced analgesia (electric shock, forced water-swim and novelty) were used to examine the nature of the endogenous antiopiate system. It was hypothesized that a role of the antiopiate system may be to regulate the extent of antinociception within varying environments. The antiopiate properties of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), which were manifest by reduction of opiate analgesia in mice on a hot-plate, were best expressed within a defined range of intensities. In each of the 3 analgesic situations, pre-administration of Tyr-MIF-1 (0.1 mg/kg) resulted in an antinociceptive effect after low to moderate stress but not after more intense stress. These observations indicate that the antiopiate system can function differentially under various environmental conditions, thus ensuring that the organism's responses to its perception of the immediate environment are appropriate and specific.