Effects of an anti-ß-Endorphin on tonic immobility in rabbits

The involvement of endogenous beta-endorphin (beta-EP) in tonic immobility (TI) was evaluated in rabbits following the intracerebroventricular (ICV) administration of a specific antibody. Rabbits were tested twice with a 1-week intersession interval. For each animal, results of Session 1 were used as the baseline. Six hours before the beginning of Session 2, a bilateral ICV injection of specific anti-beta-EP serum or aspecific gamma-globulin (vehicle) was performed. Results showed that pretreatment with anti-beta-EP serum increased TI duration, whereas no change occurred in the vehicle group. In a parallel experiment we evaluated TI duration in the presence of formalin pain: TI increased in animals pretreated with aspecific gamma-globulins and decreased in the animals pretreated with anti-beta-EP serum. Results suggest that the beta-EP system acts to limit TI duration, but that this effect is reversed by persistent pain.