Control of Fear Behavior by Brain Derived Neurotrophic F

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Brain circuits controlling fear behaviors are well characterized, however molecular events taking place in those circuits during formation and expression of various types of fear are not well understood. Understanding of these events will be useful for development of pharmacological tools to control and modulate fears caused by psychiatric illnesses. Following generation of mice with forebrain restricted knockout of brain derived neurotrophic factor (BDNF), we have noticed that these animals express inadequately high levels of fear. This fear is registered as several features including elevated startle and freezing responses to various stimuli. These behavioral features make BDNF knockout mice a potential model of several psychiatric illnesses including post traumatic stress disorder, which is characterized by increased startle response. The general goal of this project is to understand how molecular changes in the fear circuits of the brain control innate and learned fear responses and to define potential therapeutic targets for correcting the abnormal fear. The specific goal is to determine how a reduction in the level of BDNF causes increased fear responses. The following aims are established to achieve this goal. 1) Identification the areas in the brain where absence of BDNF leads to elevated fear responses. To map those circuits we will try to rescue behavioral deficits by reintroducing or eliminating BDNF in various regions of the brain using lentiviral vectors expressing BDNF or Cre recombinase. The primary areas of interest include amygdala and bed nucleus stria terminal. 2) Characterization of changes in gene expression resulting from the absence of BDNF. To identify differentially expressed genes we are comparing gene expression profiles between the animals with and without BDNF gene. 3) Characterization of the electrical properties of neurons in the fear controlling areas of the brain. Single cell recording from both excitatory and inhibitory neurons in basolateral and central amygdala of mice with BDNF knockout. 4) Following identification of molecular and physiological changes caused by the lack of BDNF, rescue experiments will be performed to identify those molecular changes, which are directly responsible for the physiological alterations within the fear circuits. The following has been accomplished during the past fiscal year: 1) Amygdala was identified as a primary site mediating changes in fear responses in BDNF knockout animals. Amygdala lesions resulted in the elimination of the startle differences between the wild type and BDNF knockout animals. 2) We have created and tested lentiviral vectors expressing BDNF and Cre recombinase. These viruses will be used for identification of specific sites in the brain where BDNF is influencing expression of fear. The lentivirus Cre vector will be used for local deletion of BDNF gene, whereas the BDNF vector will be used to introduce BDNF into the brain of the knockout animals. 3) Proper conditions have been worked out for stereatactic injections of lentivirus into the amygdala. 4) Single cell electrophysiology system has been set up. Initial analysis of BDNF KO mice revealed increased inhibitory postsynaptic currents in non-accommodating neurons in the central nucleus of the amygdala. 5) Somatostatin has been identified as a gene reducing its expression following elimination of BDNF in the forebrain.