Mineralocorticoid receptors in the ventral hippocampus are involved in extinction memory in rats

Fear extinction decreases conditioned fear responses that normally occur when a conditioned stimulus (CS) is repeatedly presented in the absence of an aversive unconditioned stimulus (US), which is the behavioral basis of exposure therapy for posttraumatic stress disorder (PTSD). However, knowledge about the neurobiology of extinction is insufficient. The present study investigated changes in the protein expression of mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs) in the dorsal hippocampus (DH)and ventral hippocampus (VH), induced by extinction and return of conditioned fear responses to an auditory signal. Western blot analysis indicated that shock stress enhanced the expression of MRs in only the DH, whereas extinction selectively increased the expression of MRs in the VH. The infusion of MRs antagonist spironolactone in the VH indicated that MRs in the VH did not affect the retrieval of fear and extinction memories, but facilitated the formation of extinction memory. However, no changes in GRs in either the DH or VH were observed in each phase of auditory fear conditioning. These results suggest that MRs in the DH and VH have differential functions in the extinction of auditory fear conditioning. MRs in the DH appear to be related to only stressful experiences, whereas MRs in the VH are involved in extinction formation. The enhancement of MRs in the VH might be necessary to improve PTSD.