Involvement of sonic hedgehog activation and neurogenesis in the amygdala in fear memory formation and extinction

  • 洪 惠琪

Student thesis: Doctoral Thesis


It is known that neurogenesis occurs throughout the life mostly in the subgranular zone (SGZ) of the hippocampus and the subventricular zone (SVZ) of the lateral ventricle We investigated whether neurogenesis occurred in the amygdala and its function in fear memory formation Mice were injected intraperitoneally with 5-bromo-2'-deoxyuridine (BrdU) 2 h before receiving 15 tone–footshock pairings The number of BrdU+/DCX+ and BrdU+/NeuN+ cells was significantly higher in the conditioned mice suggesting that association of tone with footshock induced neurogenesis To determine the relationship between neurogenesis and memory formation mice were given cell proliferation inhibitor methylazoxymethanol acetate (MAM) MAM markedly reduced neurogenesis and impaired fear memory formation Similarly intra-amygdala infusion of cytosine arabinoside (Ara-C) which interferes with DNA synthesis decreased freezing responses Sonic hedgehog (Shh) its receptor patched1 (Ptc1) and transcription factor Gli1 protein levels increased at 1 day and returned to baseline at 7 days after fear conditioning Immunohistochemistry confirmed that Shh+ cells increased after conditioning Silencing Shh gene expression with small hairpin interfering RNA (shRNA) by means of a retrovirus vector encoding Shh shRNA (Retro-Shh-shRNA) which allowed us to knockdown Shh specifically in the mitotic neurons reduced the number of BrdU+/NeuN+ cells and decreased freezing responses These results suggest that fear learning induces Shh signaling activation in the amygdala which promotes neurogenesis and long-term memory formation We next investigated the relationship between neurogenesis in the amygdala and extinction of fear memory Mice received 15 tone-footshock pairings Twenty-four hours after training the mice were given 15 tone-alone trials (extinction training) once per day for 7 days Two hours before extinction training the mice were injected intraperitoneally with BrdU BrdU+/NeuN+ cells were analyzed 52 days after training A group of mice that received tone-footshock pairings but no extinction training served as controls (FC+No-Ext) The number of BrdU+/NeuN+ cells was significantly higher in the extinction (FC+Ext) than in the FC+No-Ext mice MAM or Ara-C reduced neurogenesis and retarded extinction Silencing Sonic hedgehog (Shh) gene with shRNA by means of a retrovirus expression system to knockdown Shh specifically in the mitotic neurons reduced neurogenesis and retarded extinction By contrast over-expression of Shh increased neurogenesis and facilitated extinction These results suggest that amygdala neurogenesis and Shh signaling are involved in the extinction of fear memory We conclude that activation of Shh signaling plays a critical role in promoting amygdalar neurogenesis support of LTM formation and facilitating of extinction
Date of Award2015 Jan 21
Original languageEnglish
SupervisorPo-Wu Gean (Supervisor)

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