Chronic treatment with milnacipran reverses the impairment of synaptic plasticity induced by conditioned fear stress

Rationale Recent studies have focused on neural plasticity at the cellular and molecular levels in the etiology and treatment of stress-related disorders; however, there are no reports concerning modulation of synaptic plasticity in the hippocampus underlying therapeutic effects of antidepressants and/or anxiolytics.Objectives To elucidate the functional interaction between the stress-induced alteration of synaptic plasticity and therapeutic effects, we examined the anxiolytic mechanism(s) of milnacipran, focusing on modulation of long-term potentiation (LTP) in the hippocampal CA1 field.Methods Rats that received footshock stimulation five times (intensity, 0.5 mA; duration, 2 s; shock interval, 30 s) for 5 days were treated with milnacipran (30 mg kg^–1, p.o.) or vehicle for 14 days. On the 15th day, rats were subjected to conditioned fear stress (CFS) to evaluate freezing behavior. Separate from the behavioral study, electrophysiological approach was performed to evaluate the synaptic efficacy under anesthesia.Results Exposure to CFS suppressed LTP in the CA1 field. Chronic treatment with milnacipran (30 mg kg^–1, i.p. after 30 mg kg^–1 day^–1, p.o. ×14 days), but not acute treatment (30 mg kg^–1, i.p. after vehicle 5 ml kg^–1 day^–1, p.o. ×14 days), reduced freezing behavior and reversed the impairment of LTP induced by CFS.Conclusion The present data suggest that a correspondence exists between fear-related behavior and synaptic plasticity in the hippocampus. In other words, anxiolytic mechanism(s) of chronic treatment with milnacipran may be explained by reversal effects on the psychological stress-induced impairment of synaptic plasticity.