Dopamine and BDNF Modulate LTP in the BLA Figure 9. TrkB receptor activation lowers the threshold of LTP induction in the BLA. A) Application of the TrkB receptor agonist, 7, 8- dihydroxyflavone (500 nM, n = 6) facilitated the induction of LTP by 2xHFS. B) Left, raw data showing a typical PPR response before, and at 10 and 30 min after 2xHFS in the presence of the TrkB agonist. Right, a bar chart illustrating the group data. C) Bar chart summarizing the group effects of TrkB receptor activation on LTP threshold using 2xHFS. Application of BDNF (100 ng/ml, n = 4) for 30 min, mimicked the induction of LTP by 2xHFS induced by 7, 8-dihydroxyflavone. Intracellular application of calcium chelator, BAPTA (10 mM, n = 5, hooped bar) as well as the PKA inhibitor, KT5720 (100 nM, n = 5, diagonal bar) or the selective MEK inhibitor, U0126 (10 mM, n = 4, hatched bar) fully blocked the 7, 8-dihydroxyflavone facilitated LTP. However, incubation with DETC (20 mM, n = 6, striped Bar) did not block this effect. ** P,0.01, * P,0.05. Error bars indicate SEM. doi:10.1371/journal.pone.0026065.g009 tically released zinc not only regulates LTP formation in the BLA but also fear memory formation [43,44]. Likewise, BDNF release and the subsequent activation of TrkB receptors in the BLA has been shown to play a critical role in regulating synaptic plasticity and fear memory formation [43,45]. In the present study, we have shown that LTP can be induced by either TBS or by spaced 5xHFS. Studies in the hippocampus have shown that spaced HFS and TBS-induced LTP are cAMP- PKA dependent [23,24,25]. Consistent with these data, we have shown that in BLA principal neurons TBS- and 5xHFS-induced LTP were both blocked by the D1 receptor antagonist SCH23390, as well as by intracellular application of adenylate cyclase and PKA inhibitors. Moreover, our paired pulse experiments showed that the PPR was not altered after LTP induction, suggesting that both TBS- and spaced 5xHFS-induced LTP may be dependent on activation of the same postsynaptic cAMP-PKA signaling cascade but independent of presynaptic modifications. In the hippocam- pus, Kim and colleagues have shown that LTP induced by 4xHFS at 40s intervals was not blocked by a PKA inhibitor [46]. The reason(s) for the difference between their results and ours are not immediately apparent but could be attributable to differences in the recording method; whole-cell patch clamp recording com- pared to field potential recording, or may simply reflect differences in the sensitivity to LTP induction between the BLA and hippocampus. We have shown that D1 receptors co-distribute with the NMDA receptor NR1 subunit in the spines of BLA principal neurons, where they function to modulate NMDA currents [8]. The co- activation of these two receptors has been shown to up-regulate the surface expression and incorporation of the GluR1 receptor subunit of the AMPA receptor at synaptic sites in hippocampus [47]. Thus, concurrent activation of D1 and NMDA receptors alone could functionally modulate the D1 receptor induction of LTP in BLA principal neurons. Moreover, in this study we have shown that inhibition of AC and PKA activity can block TBS- or 5xHFS-induced LTP as well as the facilitation of LTP induced by the dopamine transport inhibitor, GBR12783 using 2xHFS, suggesting that both the induction and facilitation of D1-induced LTP in BLA principal neurons is dependent on activation of the AC – cAMP -PKA signaling cascade. These results are consistent with the seminal studies of Schafe and LeDoux showing that fear memory consolidation in the amygdala is a cAMP- and PKA- dependent process [48,49], Significantly, Jentsch and Taylor have shown that stimulation of PKA activity in the amygdala facilitates PLoS ONE | www.plosone.org 10 October 2011 | Volume 6 | Issue 10 | e26065