Figure 10. Synergistic activation of D1- and TrkB receptors lowers the threshold for LTP induction in BLA principal neurons. A) Preincubation with SCH23390 (10 mM) blocked the induction of LTP by 2xHFS in the presence of 7, 8-dihydroxyflavone (500 nM). B) Preincubation with TrkB/Fc blocked the induction of LTP by 2xHFS in the presence of GBR12783. C) Bar chart illustrating the synergistic interaction between D1 and TrkB receptors to reduce LTP threshold in BLA principal neurons. ** P,0.01. Error bars indicate S.E.M. doi:10.1371/journal.pone.0026065.g010 reward-related learning [50], further suggesting that aversive and appetitive learning may share common sub-cellular substrates in the BLA. Although our results suggest that D1 receptor activation alone may be sufficient to enable the induction of LTP in principal neurons, synaptic release of zinc together with activation of D1 and TrkB receptors might not occur independently of each other. Hence, stimulation-induced BDNF activation of TrkB receptors appears to be critically dependent on synaptically released zinc, concurrent activation of D1- and TrkB receptors would then act synergistically to markedly facilitate the induction of LTP in the BLA. Indeed, in cortical networks cross-talk between the D1 system and the BDNF system may be the rule, rather than the Dopamine and BDNF Modulate LTP in the BLA exception. Hence, Cammarota and colleagues have reported that a similar interaction regulates long-term memory (LTM) forma- tion in the hippocampus. Here, infusion of the D1 receptor antagonist, SCH23390, blocked training-induced increase in BDNF and prevented LTM. Conversely, infusion of the D1 receptor agonist, SKF38393, increased BDNF levels and facilitat- ed LTM. Significantly, the D1 mediated effect was blocked by infusion of a PKA inhibitor, and infusion of a BDNF function- blocking antibody impaired the SKF38393-caused increase in LTM [19], suggesting that hippocampal-dependent LTM requires a synergistic interaction between these two systems and activation of the AC-cAMP-PKA cascage. Our results could provide insight into the cellular substrate mediating this cross-talk. Moreover, we have shown that a key component potentially linking synaptically released zinc and concurrent activation of D1- and TrkB receptors is the functional expression of extracellular matrix metalloproteinase (MMPs) by principal neurons of the BLA. MMPs are not constitutively active under basal conditions, and require synaptically released zinc to bind as a co-factor before they become fully active [37,39]. Synaptic concentrations of zinc can reach micromolar levels following HFS [50], suggesting that extracellular MMPs in the BLA may be preferentially activated only in response to strong (salient) sensory input. A similar activity- dependent conversion of pro-BDNF to mBDNF has been reported in cultured hippocampal neurons [51]. Here, low-frequency stimulation was reported to release proBDNF [51]. In contrast, following HFS substantial amounts of mBDNF were detected [29], suggesting that a co-factor may be released during HFS that facilitates the conversion of proBDNF to mBDNF. Significantly, local release of zinc is restricted to a subset of glutamatergic afferents arising from cortical/limbic structures, and not from sub-cortical structures such as the thalamus, whose terminals are devoid of synaptic zinc [50]. Recently, Humeau and colleagues reported that simultaneous stimulation of cortical and thalamic afferents onto neurons of the lateral amygdala (LA) induced heterosynaptic LTP at cortical, but not at thalamic, inputs [27]. Consistent with this observation, we were unable to induce robust LTP in the thalamic pathway using stimulation parameters that consistently induced LTP in the cortical input onto BLA principal neurons. However, a presynaptic form of LTP in the thalamic input onto LA neurons has been reported using a prolonged period of paired-pulse stimulation [52]. Further studies are required to determine if a similar mechanism exists in the thalamic input onto principal neurons of the BLA. Nonetheless, our data suggest that concurrent activation of cortical and thalamic inputs into the BLA, as occurs during conditioned- (CS) and unconditioned-stimulus (US) pairing in fear conditioning, may selectively restrict BDNF-dependent LTP to the site of cortical, sensory, input. Thus, presynaptic zinc release from the cortical input may contribute to the maintenance of a precise sensory topography for LTP induction (synaptic tagging) even within the dendritic arbor of an individual principal neuron, thereby preventing sensory generalization. Synaptically released zinc has been shown to affect multiple postsynaptic receptors and ion channels [50], and is able to transactivate TrkB receptors in the hippocampal mossy fiber-CA3 pyramid synapse [53]. Hence, it is possible that the inhibition of LTP observed following chelation of zinc by DETC may result from processes that were independent of the inhibition of MMP activation. However, the similarity of the response to DETC, gallardin, and TrkB/Fc application, together with the inability of DETC pretreatment to block the response to exogenous TrkB receptor agonist application strongly suggests that DETC was acting to directly block the activation of MMPs and, hence, PLoS ONE | www.plosone.org 11 October 2011 | Volume 6 | Issue 10 | e26065