Induction of fear extinction with hippocampal-infralimbic BDNF

Abstract

The extinction of conditioned fear memories requires plasticity in the infralimbic medial prefrontal cortex (IL mPFC), but little is known about the molecular mechanisms involved. Brain-derived neurotrophic factor (BDNF) is a key mediator of synaptic plasticity in multiple brain areas. In rats subjected to auditory fear conditioning, BDNF infused into the IL mPFC reduced conditioned fear for up to 48 hours, even in the absence of extinction training, which suggests that BDNF substituted for extinction. Similar to extinction, BDNF-induced reduction in fear required N-methyl-D-aspartate receptors and did not erase the original fear memory. Rats failing to learn extinction showed reduced BDNF in hippocampal inputs to the IL mPFC, and augmenting BDNF in this pathway prevented extinction failure. Hence, boosting BDNF activity in hippocampal-infralimbic circuits may ameliorate disorders of learned fear.

Fig. 1

BDNF infused into the infralimbic cortex substitutes for behavioral extinction. (A) Rats’ freezing levels in response to tones that were paired with footshocks (Cond) or given alone during extinction (Ext) and Test sessions. BDNF infusions into the IL mPFC before extinction (arrow) reduced freezing on days 2 and 3 relative to saline-infused (SAL) controls (n = 8 per group). (B) A similar effect was observed when BDNF was infused in the absence of training on day 2 (SAL, n = 5; BDNF, n = 7). (C) Infusing BDNF 24 hours before conditioning had no effect (SAL, n = 9; BDNF, n = 7). Trials are shown in blocks of two. **P < 0.01, repeated-measures analysis of variance (ANOVA). Error bars represent SEM.

Fig. 2

Similar to extinction, the BDNF effect does not degrade the original fear memory and requires NMDA receptors. (A) Conditioned rats received BDNF or saline infusions into the IL mPFC on day 2 (SAL, n = 12; BDNF, n = 11). On day 3, both groups were extinguished, followed by two shocks, resulting in a complete return of freezing in the BDNF group. (B) IL infusion of BDNF was combined with a systemic injection of the NMDA antagonist CPP (CPP + BDNF, n = 8). Controls were infused with BDNF and given a saline injection (SAL + BDNF, n = 10) or were both infused and injected with saline (SAL + SAL, n = 10). On day 3, all groups underwent a single-tone extinction test. *P < 0.05, two-way repeated-measures ANOVA, main effect of drug; *P < 0.05, Student’s t test, SAL + SAL compared to SAL + BDNF.

Fig. 3

Hippocampal projections to the infralimbic cortex mediate BDNF-extinction. (A) One day after extinction training, rats were divided into two groups on the basis of their ability to retrieve extinction (Ext-Success, n = 6; Ext-Failure, n = 6; **P < 0.01). The following day, BDNF protein concentrations were determined by enzyme-linked immunosorbent assay (ELISA). (B) Ext-Success rats showed elevated levels of BDNF in the hippocampus but not in the mPFC or amygdala (*P < 0.05, Ext-Success versus Ext-Failure). Hippocampal BDNF levels in individual Success and Failure rats were nonoverlapping. (C) Conditioned rats were divided into three groups. Controls received a saline infusion into the IL mPFC followed by a saline infusion into the hippocampus [SAL(IL) + SAL(Hipp), n = 7]. Another group received a saline infusion into the IL mPFC followed by a BDNF infusion into Hipp [SAL(IL) + BDNF(Hipp), n = 8]. A third group received an infusion of a BDNF-sequestering antibody into the IL mPFC followed by BDNF infusion into the hippocampus [anti-BDNF(IL) + BDNF(Hipp), n = 9]. Infusion of BDNF antibody into the IL mPFC blocked the fear-reducing effects of hippocampal BDNF. *P < 0.05, SAL(IL) + SAL(Hipp) compared to SAL(IL) + BDNF(Hipp).