Strain-dependent effects of shock-induced release of opioids: dissociation between analgesia and behavioral seizures.

Intermittent, but not continuous, footshock resulted in naltrexone-reversible analgesia in two strains of mice: C57BL/6 (C57) and BALB/c (BALB). While no strain differences were evident in relation to analgesia, a clear strain effect appeared when the protective action of intermittent footshock on electroconvulsive shock-induced seizures was considered. In fact naltrexone-reversible protection exerted by intermittent footshock on behavioral seizures was much higher in C57 than in BALB mice. The reason of this dissociation between the effects of endogenous opioids on analgesia and seizures is discussed.