Amygdala BDNF signaling is required for consolidation but not encoding of extinction

Abstract

Brain-derived neurotrophic factor (BDNF) acting through the tyrosine kinase B receptor (TrkB) is thought to be a critical mediator of learning. As there are no available selective antagonists of TrkB, we used a lentivirus encoding a dominant-negative TrkB (TrkB.t1) to antagonize BDNF signaling during extinction of conditioned fear. Whereas TrkB.t1-infected rats showed normal within-session extinction, their retention of extinction was impaired, suggesting that amygdala TrkB activation is required for the consolidation of stable extinction memories.

Figure 1

Bdnf mRNA expression and assessment of lentiviral gene expression within the BLA. (a) Regulation of Bdnf mRNA following extinction. In situ hybridization was used to assess the amount of Bdnf mRNA (exon V) present at three time points following extinction. n = 6 per group. At right, a replication experiment in which some rats underwent the same extinction protocol as on the left, whereas others were simply placed in the extinction context without light presentations. Both groups were killed 2 h after the exposure. n = 8 per group. (b) At the 2-h time point, significant increases in Bdnf mRNA were found within the basolateral amygdala (BLA, arrow), as shown with dark-field optics. (c,d) Histological examination of viral infection in the BLA following behavioral studies. No amygdala damage was seen following infection (visualized with cresyl violet staining). LA, lateral amygdala. CeA, central amygdala. (e) Expression of TrkB.t1 was assessed using immunocytochemistry (ICC) for a hemagglutinin (HA) epitope tag incorporated into the TrkB.t1 coding sequence. (f) GFP expression was directly assessed by visualizing tissue under an epifluorescence microscope. *P < 0.05. Error bars represent s.e.m. Scale bar, 1 mm.

Figure 2

Lentivirus expression of TrkB.t1 in BLA blocks the consolidation but not the acquisition of extinction. (a) Level of fear-potentiated startle (FPS) following extinction training, when tested at 1 h or 48 h. TrkB.t1-infected rats showed significantly higher levels of fear 2 d after extinction, as compared to GFP-infected rats (n = 9 and 7 for TrkB.t1 and GFP groups, respectively). (b) TrkB.t1 impairs extinction retention. Averaging across all trials, TrkB.t1-infected rats showed a deficit in extinction as compared to GFP-infected rats. (c) Examining extinction within the testing session suggested that the TrkB.t1-infected rats had normal within-session extinction, but lacked extinction retention across the 2-d interval between tests (n = 9 and 10 for TrkB.t1 and GFP groups, respectively). Note that GFP-infected rats showed complete extinction by test 3 and were not tested thereafter. TrkB.t1-infected rats showed poor extinction retention even on day 4, as evidenced by high FPS during the first 5 trials of the test. (d) Normalization of FPS across the first 3 d (100% = mean of first 5 trials per rat per day) demonstrated that there was no difference in the rates of within-session acquisition of extinction between the two groups. *P < 0.05, **P < 0.01. Error bars represent s.e.m.