The Effect And Mechanism Of SKF83959and IGF2on Fear Extinction And Depressive-like Behavior Of Mice

Objective:SKF83959, which can be regarded as an agonist of Gq protein-coupled dopamine receptors, has been found to have many beneficial effects such as antidepressant-like effect and antiparkinsonian action. To date, it remains unknown whether SKF83959plays a role in regulating fear extinction in animals. On this basis, we herein attempt to investigate the potential effect of SKF83959on fear extinction.Methods:Fear conditioning paradigm was utilized to study the effect of SKP83959on fear extinction in mice; Open field test was used to determine the impact of SKF83959on locomotion and basal anxiety levels of mice; Drug microinfusion techniques and fear conditioning paradigm were employed to study the involvement of PLC/IP3/Ca2+signaling in amygdala in SKF83959-enhanced fear extinction in mice.Results:(1) SKF83959(1mg/kg, intraperitoneally (i.p.)) had the effect to enhance fear extinction and to reduce spontaneous recovery of fear, fear renewal and reinstatement in mice when administered for7consecutive days either before or after fear conditioning. When administered before conditioning, SKF83959reduced freezing levels of mice from60.9±5.1%to44.2±4.4%in test of spontaneous recovery (n=7-8/group, p<0.05vs vehicle), reduced freezing levels of mice from78.4±3.2%to67.5±3.2%in fear renewal test (n=8-10/group,p<0.05vs vehicle), and decreased freezing levels of mice from95.2±0.7%to78.3±2.3%in fear reinstatement test (n=10-13/group, p<0.01vs vehicle). When administered after conditioning, SKF83959reduced freezing levels of mice from52.0±7.0%to28.9±5.4%in fear renewal test (n=7/group,p<0.05vs vehicle) and reduced freezing levels of mice from90.9±3.1%to71.0±7.1%in fear reinstatement test (n=11/group, p<0.05vs vehicle).(2) Subchronic SKF83959treatments (once daily,1mg/kg/day for7consecutive days) did not affect locomotion and basal anxiety level of mice in an open field test.(3) SKF83959enhanced fear extinction in mice by activating the PLC/IP3/Ca2+signaling in amygdala.Conclusion:SKF83959has the effect to significantly enhance fear extinction in animals. The enhancing effect of SKF83959on fear extinction requires the activation of PLC/IP3/Ca2+signaling in amygdala. Part II The effect of IGF2on CSDS-induced depressive-like behavior of miceObjective:Insulin-like growth factor2(IGF2) is a member of insulin-like peptide family. Although it has been recently reported that IGF2has remarkable antidepressant-like effect, the role IGF2-mediated signaling in the central nervous systems (CNS) plays in the production of depressive-like behavior in animals is also largely elusive. Thus, we conduct studies aimed at providing further insights into the implications of IGF2signaling in the CNS in the production of depressive-like behavior in mice.Methods:Chronic social defeat stress (CSDS) was adopted to establish mouse models of depression; Western blotting was employed to determine the expression levels of IGF2protein in mice subjected to CSDS; shRNA-mediated RNA interference techniques, forced swim test and subthreshold social defeat stress were used to study the effects of IGF2knockdown in hippocampus on depressive-like behavior of mice and the susceptibility of mice to harmful stress; Drug microinfusion techniques were utilized to investigate the effects of exogenous IGF2on immobile behavior in the forced swim test and CSDS-induced depressive-like behavior in mice.Results:(1) The hippocampal IGF2expression in mice subjected to CSDS was negatively correlated with the severity of depressive-like behavior of mice. The more depressive phenotypes the mice had, the lower IGF2expression levels were observed.(2) IGF2knockdown in hippocampus not only directly induced depressive-like behavior in the forced swim test but also increased the susceptibility of mice to subthreshold social defeat stress. In the forced swim test, hippocampal IGF2knockdown led to an increase in immobile time of mice from131±26s to222±34s (n=13-15/group, p<0.05vs control). In the social interaction test, hippocampal IGF2knockdown resulted in a decrease in the interacting time from51.4±8.0s to29.5±6.7s in mice subjected to subthreshold social defeat stress (n=11-13/group, p<0.05vs control), whereas in the sucrose preference test, hippocampal IGF2knockdown resulted in a decrease in the sucrose preference levels from89.8±2.6%to71.3±6.4%in mice subjected to subthreshold social defeat stress (n=11-13/group, p<0.05vs control).(3) Hippocampal IGF2infusions produced antidepressant-like effects, as evidenced by reduced immobile time of naive mice acutely treated with IGF2(2.5ng/side) in the forced swim test and reduced depressive-like behavior in depressed mice having received repeated injections of IGF2(250ng/side for7consecutive days). Specifically, in the forced swim test, hippocampal IGF2infusion (2.5ng/side) resulted in a decrease in the immobile time of mice from94±11s to49±12s (PBS:n=21; IGF2(2.5ng/side):n=14; p<0.05vs PBS). Repeated IGF2infusions (250ng/side for7consecutive days) increased the interacting time of CSDS susceptible mice from14.6±4.1s to38.8±5.4s in the social interaction test (CSDS susceptible-PBS:n=10; CSDS susceptible-IGF2(250ng/side):n=10; p<0.05vs CSDS susceptible-PBS) and elevated the sucrose preference levels of CSDS susceptible mice from64.2±3.2%to85.7±3.0%in the sucrose preference test (CSDS susceptible-PBS:n=9; CSDS susceptible-IGF2(250ng/side):n=9; p<0.01vs CSDS susceptible-PBS). In addition, it was found that IGF2produced the antidepressant-like actions by selectively recruiting the IGF2receptors but not the IGF1receptors in hippocampus of mice.Conclusion:IGF2and IGF2receptor-mediated signaling in hippocampus of mice plays an important role in the regulation of depressive-like behavior in mice. Downregulation of IGF2expression in hippocampus enhances the susceptibility of mice to stress, whereas IGF2infusions in hippocampus can produce antidepressant-like effects by selective activation of the IGF2receptors instead of IGF1receptors.