Dopamine and BDNF Modulate LTP in the BLA Figure 7. D1 receptor activation acts to reduce the threshold for LTP induction in BLA principal neurons. A) Application of the dopamine reuptake inhibitor GBR12783 (10 mM, n = 5), or B) the D1 receptor agonist SKF38393 (50 mM, n = 5) induced a robust and stable LTP in BLA principal neurons following 2xHFS. C) Raw data showing the effects of GBR12783 and SKF38393 on the PPR in BLA principal neurons. D) A bar chart illustrating the group effects of drug application on the PPR of EPSCs before, immediately after, and at 10 min and 30 min post 2xHFS. ** P,0.01. Error bars indicate S.E.M. doi:10.1371/journal.pone.0026065.g007 also blocked LTP induced by 2xHFS in the presence of GBR12783 (11465% of baseline, n = 5, 3 animals, P,0.05, Figure 8C, hatched bar), suggesting that activation of the BDNF- TrkB pathway may also play a role in the induction of LTP by 2xHFS in the presence of GBR12783. Consistent with this hypothesis, prior activation of TrkB receptors with either the TrkB receptor agonist, (7, 8-dihydroxy- flavone, 500 nM), or BDNF (100 ng/ml), reduced the threshold for LTP-induction in response to 2xHFS (7, 8-dihydroxyflavone: 14968% of baseline, n = 6, 5 animals, P,0.01, Figure 9A, red squares, and Figure 9C, grey bar; BDNF: 142 69% of baseline, n = 4, 4 animals, P,0.01, Figure 9C, black bar). Application of either the TrkB receptor agonist or BDNF alone caused no enduring changes in input resistance at any time point measured (7, 8-dihydroxyflavone: (F (10, 110) = 1.44, P = 0.1739; BDNF: (F (13, 112) = 0.17, P = 0.9995 Two-way ANOVA)). Similar to the GBR12783 response, intracellular application of BAPTA (10 mM) (111611% of baseline, n = 5, 3 animals, P,0.05, Figure 9C, hooped bar) or KT5720 (100 nM) (11264% of baseline, n = 5, 4 animals, P,0.01, Figure 9C, diagonal bar) fully blocked the 2xHFS induced LTP in the presence of the TrkB agonist. Hence, facilitation of LTP induction in BLA principal neurons by D1 and TrkB receptor activation appear to share a common pathway that is dependent on postsynaptic calcium influx and activation of the cAMP -PKA signaling cascade. Significantly, intracellular perfu- sion of the MEK inhibitor U0126 (10 mM) also prevented the facilitation of LTP by the TrkB receptor agonist, 7, 8-dihydroxy- flavone, (116 66% of baseline, n = 4, 3 animals, P,0.05, Figure 9C hatched bar), suggesting that activation of D1 and TrkB receptors may converge at the level of MEK to facilitate LTP. In contrast, application of the zinc chelator, DETC, failed to block the TrkB agonist effect (148 69% of baseline, n = 6, 5 animalsP,0.01, Figure 9C striped bar), suggesting that exoge- nous TrkB receptor activation can lower the threshold of LTP induction in the BLA through direct activation of the MEK cascade, and that this effect is independent of stimulus-induced zinc release. PLoS ONE | www.plosone.org 8 October 2011 | Volume 6 | Issue 10 | e26065