Social conflict-induced changes in nociception and β-endorphin-like immunoreactivity in pituitary and discrete brain areas of C57BL/6 and DBA/2 mice

The present study characterizes the time course of social conflict analgesia and its reversibility by opioid antagonist drugs in the C57BL/6 and DBA/2 inbred strains of mice and examines the relationship between alterations in brain and pituitary levels of β-endorphin-like immunoreactivity (β-ELIR) and the antinociception elicited by social stress. Data revealed statistically strain differences in regard to β-ELIR in control animals. The pituitary content of β-ELIR was higher in DBA/2, while the values in the periaqueductal grey (PAG) and in the amygdala were higher in C57BL/6 mice. No interstrain differences were found in the hypothalamus. Exposure to 50 attack bites resulted in a 6-fold higher analgesia in DBA/2 mice and in a strain-dependent fall of ⊝ELIR in pituitary (∼27%) and PAG (23%). PAG but not pituitary β-ELIR levels in C57BL/6 mice correlated positively with the increase in tail-flick latency after attack. Mere confrontation with a non-aggressive opponent failed to induce analgesia and was associated in C57BL/6 mice with a significant reduction in the β-ELIR content of both the pituitary and the PAG. The data are discussed in terms of genotype-dependent sensitivity of the β-endorphin system to stress and its relation to analgesia.