Diurnal differences in opioid peptide levels correlated with nociceptive sensitivity

An increase in whole mouse brain total opioid levels, determined by mouse was deferens bioassay, was observed in mice sacrified in late afternoon (when they are least responsive to pain) compared to early morning (when they are most responsive to pain). There were no comparable increases in levels of met⁵- or leu⁵- enkephalin measured by RIA methodology. There were differences, however, in the effects of noxious stimuli on met⁵-enkephalin levels in the efternoon compared to the morning. In the afternoon, but not in the morning, the levels of met⁵-enkephalin in mice tested after hot plate stress were significantly increased compared to those of unstressed animals. Thus, there appears to be some correlation between activity in endogenous opioid systems and the ability of mice to withstand noxious stimuli.