THz radiation is generated from topological insulators using femtosecond laser pulses. Two‐channel free carrier absorption with bulk and surface carriers is indispensable to explaining the strong dependence of THz emission power on the carrier concentration. The characteristics of THz emission provide valuable information regarding the fundamental properties of Dirac fermions.

There is a close correlation between long-term potentiation (LTP) in the synapses of lateral amygdala (LA) and fear conditioning in animals. We predict that reversal of LTP (depotentiation) in this area of the brain may ameliorate conditioned fear. Activation of group II metabotropic glutamate receptors (mGluR II) with DCG-IV induces depotentiation in the LA. The induction of depotentiation is independent of NMDA receptors, L-type Ca ++ channels, and calcineurin activity, but requires presynaptic activity and extracellular Ca ++ . (2S,2ЈR,3ЈR)-2-(2Ј,3Ј-dicarboxycyclopropyl)glycine (DCG-IV) depotentiation is accompanied by a decrease in the frequency but not the amplitude of miniature excitatory post-synaptic currents (mEPSCs) and could be mimicked by endogenously released glutamate. DCG-IV inhibited the release of glutamate evoked by 4-AP but not that evoked by ionomycin, suggesting that the effect of DCG-IV is not mediated by an action downstream of Ca ++ entry. Intra-amygdala infusion of mGluR II agonist blocks the consolidation of fear memory measured with fear-potentiated startle. Taken together, the present results characterize the properties of DCG-IV depotentiation and reveal a close parallel between depotentiation in the amygdala slice and the reduction of conditioned fear in animals.The metabotropic glutamate receptors (mGluRs) are G-proteincoupled receptors whose activation modulates neural activity via their linkage to multiple second-messenger systems (Pin and Duvoisin 1995;Anwyl 1999). Eight mGluRs have been cloned and classified into three groups based on their sequence homologies, signal transduction mechanisms, and agonist pharmacology (Nakanishi 1992; Pin and Duvoisin 1995). Group I (mGluR1 and mGluR5) receptors stimulate phosphatidylinositol hydrolysis, whereas group II (mGluR2 and mGluR3) and group III (mGluR4, mGluR6, mGluR7, and mGluR8) receptors are negatively coupled to cAMP production. The group II mGluRs are implicated as negative feedback autoreceptors to modulate neurotransmission by presynaptic mechanisms (Pin and Duvoisin 1995). In situ hybridization and immunohistochemical studies of rat brain have revealed that the intense mGluR II-like immunoreactivity was seen predominantly in the cortical and limbic regions including the amygdala (Neki et al. 1996;Schaffhauser et al. 1998), suggesting that these receptors may modulate higher CNS functions such as emotion and cognition. Indeed, mGluR II agonists have been shown to inhibit the release of glutamate in vivo (Battaglia et al. 1997) and exhibit potent anxiolytic effects in rats (Helton et al. 1998). These compounds also ameliorated the symptoms of nicotine withdrawal (Helton et al. 1997) and lactate-induced panic responses (Shekhar and Keim 2000). Moreover, mice lacking mGluR II display impaired hippocampal mossy fiber longterm depression (LTD) (Yokoi et al. 1996). In external capsule (EC)-basolateral amygdala synapses, it has been shown that lowfrequency stimulation (LFS) induced long-term potentiation (LTP), whereas brief high-frequ...

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