Fear extinction and BDNF: translating animal models of PTSD to the clinic

Abstract

Brain-derived neurotrophic factor (BDNF) is the most studied neurotrophin involved in synaptic plasticity processes that are required for long-term learning and memory. Specifically, BDNF gene expression and activation of its high-affinity tropomyosin-related kinase B (TrkB) receptor are necessary in the amygdala, hippocampus and prefrontal cortex for the formation of emotional memories, including fear memories. Among the psychiatric disorders with altered fear processing, there is post-traumatic stress disorder (PTSD) which is characterized by an inability to extinguish fear memories. Since BDNF appears to enhance extinction of fear, targeting impaired extinction in anxiety disorders such as PTSD via BDNF signalling may be an important and novel way to enhance treatment efficacy. The aim of this review is to provide a translational point of view that stems from findings in the BDNF regulation of synaptic plasticity and fear extinction. In addition, there are different systems that seem to alter fear extinction through BDNF modulation like the endocannabinoid system and the hypothalamic-pituitary adrenal axis. Recent work also finds that the pituitary adenylate cyclase-activating polypeptide and PAC1 receptor, which are upstream of BDNF activation, may be implicated in PTSD. Especially interesting are data that exogenous fear extinction enhancers such as antidepressants, histone deacetylases inhibitors and D-cycloserine, a partial N-methyl d-aspartate agonist, may act through or in concert with the BDNF-TrkB system. Finally, we review studies where recombinant BDNF and a putative TrkB agonist, 7,8-dihydroxyflavone, may enhance extinction of fear. These approaches may lead to novel agents that improve extinction in animal models and eventually humans.

Figure 1

Simplified schematic diagram of BDNF modulators acting at the synaptic level during fear extinction processes. NMDA receptor, HDACi, CB1 receptor, 5-HT receptor and Pac1 receptor activation lead to epigenetic BDNF gene changes which affect BDNF synthesis. At a presynaptic level, BDNF is released from vesicles and its binding of the postsynaptic TrkB receptor leads to autophosphorylation. This causes activation of downstream signaling pathways (Pi3K/Akt, MEK/ERK and PKC), leading to gene transcription changes and synaptic plasticity. In addition, PACAP has been suggested to transactivate the TrkB receptor through Src and causing the activation of TrkB downstream signaling. Moreover, BDNF and TrkB modulate glutamate release and NMDA phosphorylation.