Changes in beta-endorphin and stress-induced analgesia in mice after exposure to forced walking stress

In this study, we attempted to clarify the correlation between changes in t he level of b-endorphin (b-EP) in the mouse brain and the stress-induced an algesia (SIA) after exposing animals to forced walking stress. The immunohi stochemical distribution of b-EP after stress was first analyzed quantitati vely in mice and then more specifically using a microphotometry system with results showing that the fluorescence intensity of b-EP in the periaqueduc tal gray matter (PAG) and arcuate nucleus of the medial basal hypothalamus (ARC) was increased 6 h after exposure to forced walking stress. Further SI A was examined after exposing animals to the forced walking stress and the formalin test. At 6 h after forced walking stress, significant SIA was obse rved in the second phase (from 10 to 30 min after formalin injection) of th e formalin-induced, paw licking behavior; but not in the first phase (from 0 to 10 min after formalin injection). This SIA was antagonized by b-EP-(1- 27), an opioid e receptor antagonist. In nonstressed mice, the injection of b-EP produced a reduction in formalin-induced paw-licking in the second ph ase. A significant antinociceptive effect by b-EP was well antagonized by b -EP-(1-27). Thus, the present results suggest that the increase in P-EP lev els in PAG and/or ARC may be involved in SIA after exposure of mice to the forced walking stress. (C) 1999 Prous Science. All rights reserved.