Dynorphin a (1-13) alleviated stress-induced behavioral impairments in mice

In this study we investigated whether κ-opioid receptor stimulation by dynorphin A (1-13), a potent fragment of endogenous peptide, attenuated repeated stress-induced behavioral impairments in mice. In order to reduce the motivation to escape, mice were preexposed to inescapable electric footshock (day 0), and then dynorphin A (1-13) was administered to mice prior to the stress from the next day for 4 d (days 1-4). Dynorphin A (1-13) (1500 pmol/5 µL intracerebroventricular (i.c.v.)) attenuated the repeated stress-induced escape failures from the shock, and this improvement was inhibited by the pretreatment of nor-binaltorphimine (4.9 nmol/kg subcutaneously (s.c.)), a κ-opioid receptor antagonist. In the neurochemical experiments, we detected an increase in 5-hydroxyindoleacetic acid (5-HIAA) content, but not in serotonin (5-HT) content, and an increase in the 5-HIAA/5-HT ratio was observed in the amygdala of the group with footshock compared with the group without shock. Additionally, the changes in 5-HIAA content and the ratio were reversed by dynorphin A (1-13). However, there were no differences in 5-HT or 5-HIAA content or their ratios in the hippocampus among the three groups. These results suggest that dynorphin might alleviate the stress-induced behavioral impairments accompanied by regulation of the 5-HTergic system in the brain.