Opioid and non-opioid stress analgesia from cold water swim: importance of stress severity.

We have previously reported that stress severity plays an important role in determining the neurochemical basis of stress-induced analgesia from inescapable footshock. Increasing severity (duration or intensity of continuous footshock) causes a shift in mediation of the resultant analgesia from opioid to non-opioid. In this study, we find that stress severity plays a similar role in analgesia from cold water swim. More severe swims (longer duration or lower water temperature) produce stress analgesia insensitive to the opiate antagonist, naltrexone, whereas less severe swims produce analgesia significantly attenuated by this drug.