The activity-regulated cytoskeletal-associated protein (Arc/Arg3.1) is an immediate early gene that has been widely implicated in hippocampal-dependent learning and memory and is believed to play an integral role in synapse-specific plasticity. Here, we examined the role of Arc/Arg3.1 in amygdala-dependent Pavlovian fear conditioning. We first examined the regulation of Arc/Arg3.

The homeodomain transcription factors Cdx1, Cdx2 and Cdx4 play essential roles in anteroposterior vertebral patterning through regulation of Hox gene expression. Cdx2 is also expressed in the trophectoderm commencing at E3.5 and plays an essential role in implantation, thus precluding assessment of the cognate-null phenotype at later stages. Cdx2 homozygous null embryos generated by tetraploid aggregation exhibit an axial truncation indicative of a role for Cdx2 in elaborating the posterior embryo through unknown mechanisms. To better understand such roles, we developed a conditional Cdx2 floxed allele in mice and effected temporal inactivation at post-implantation stages using a tamoxifen-inducible Cre. This approach yielded embryos that were devoid of detectable Cdx2 protein and exhibited the axial truncation phenotype predicted from previous studies. This phenotype was associated with attenuated expression of genes encoding several key players in axial elongation, including Fgf8, T, Wnt3a and Cyp26a1, and we present data suggesting that T, Wnt3a and Cyp26a1 are direct Cdx2 targets. We propose a model wherein Cdx2 functions as an integrator of caudalizing information by coordinating axial elongation and somite patterning through Hoxindependent and -dependent pathways, respectively. DEVELOPMENT 4100 system to derive a conditional null allele. In agreement with previous studies (Chawengsaksophak et al., 2004), loss of Cdx2 in the postimplantation embryo resulted in axial truncation posterior to the forelimb. This axial truncation, together with the nature of many of the genes impacted by Cdx2 loss, suggested that precocious cessation of the generation of presomitic mesoderm (PSM) is the primary basis for this phenotype. To further define the role of Cdx2 and to elucidate the mechanistic basis for this phenotype, we sought to determine whether any of the affected genes were direct Cdx2 targets. Chromatin immunoprecipitation (ChIP) demonstrated occupancy of the Wnt3a, Cyp26a1 and T promoters by Cdx2 in vivo. Moreover, all of these promoters harbor functional Cdx response elements (CDREs) as determined by transfection analysis or transgenic assays. Thus, Cdx2 directly regulates the expression of multiple players essential for the development of the posterior embryo. Taken together with previous work, these findings suggest that Cdx2 is required to couple the generation of paraxial mesoderm through multiple Hox-independent mechanisms with Hoxdependent AP vertebral patterning. MATERIALS AND METHODS Gene targeting and the generation of Cdx2 -/-mutantsA 5 kb fragment of genomic Cdx2 sequence encompassing the first intron through to the 3Ј UTR was subcloned into pBluescript II KS + . A floxed thymidine kinase/neomycin resistance cassette (loxPGK-TK-Neolox) (Iulianella and Lohnes, 2002) was cloned into the BglII site in intron 1, and a single loxP site was inserted into the NruI site in intron 2, generating the targeting vector (see Fig. S1A in the supplementary material). R1 embryonic stem cells were electroporated with...

Recent studies have shown that nitric oxide (NO) signaling plays a crucial role in memory consolidation of Pavlovian fear conditioning and in synaptic plasticity in the lateral amygdala (LA). In the present experiments, we examined the role of the cGMP-dependent protein kinase (PKG), a downstream effector of NO, in fear memory consolidation and long-term potentiation (LTP) at thalamic and cortical input pathways to the LA. In behavioral experiments, rats given intra-LA infusions of either the PKG inhibitor Rp-8-Br-PET-cGMPS or the PKG activator 8-Br-cGMP exhibited dose-dependent impairments or enhancements of fear memory consolidation, respectively. In slice electrophysiology experiments, bath application of Rp-8-Br-PET-cGMPS or the guanylyl cyclase inhibitor LY83583 impaired LTP at thalamic, but not cortical inputs to the LA, while bath application of 8-Br-cGMP or the guanylyl cyclase activator YC-1 resulted in enhanced LTP at thalamic inputs to the LA. Interestingly, YC-1-induced enhancement of LTP in the LA was reversed by concurrent application of the MEK inhibitor U0126, suggesting that the NO-cGMP-PKG signaling pathway may promote synaptic plasticity and fear memory formation in the LA, in part by activating the ERK/MAPK signaling cascade. As a test of this hypothesis, we next showed that rats given intra-LA infusion of the PKG inhibitor Rp-8-Br-PET-cGMPS or the PKG activator 8-Br-cGMP exhibit impaired or enhanced activation, respectively, of ERK/MAPK in the LA after fear conditioning. Collectively, our findings suggest that an NO-cGMP-PKG-dependent form of synaptic plasticity at thalamic input synapses to the LA may underlie memory consolidation of Pavlovian fear conditioning, in part, via activation of the ERK/MAPK signaling cascade.Nitric oxide (NO) signaling has been widely implicated in synaptic plasticity and memory formation (Schuman and Madison 1991;Bredt and Snyder 1992;Chapman et al. 1992;Bohme et al. 1993;Zhuo et al. 1994;Bernabeu et al. 1995;Arancio et al. 1996;Doyle et al. 1996;Holscher et al. 1996;Suzuki et al. 1996;Zou et al. 1998;Ko and Kelly 1999;Lu et al. 1999). A highly soluble gas generated by the conversion of L-arginine to L-citrulline by the Ca 2+ -regulated enzyme nitric oxide synthase (NOS), NO is known to have a variety of effects both pre-and postsynaptically. One immediate downstream effector of NO, for example, is soluble guanylyl cyclase (sGC) (Bredt and Snyder 1992;Son et al. 1998;Denninger and Marletta 1999;Arancio et al. 2001). This enzyme directly leads to the formation of cyclic-GMP, and in turn, to the activation of the cGMP-dependent protein kinase (PKG). PKG, in turn, can have a number of effects, including targeting and mobilization of synaptic vesicles in the presynaptic cell, leading to enhanced transmitter release (Hawkins et al. 1993) and also to activation of protein kinase signaling cascades in the postsynaptic cell, leading to activation of transcription and translation that are critical for long-term synaptic plasticity and memory formation (Lu et al. 1999;Chi...

Even a motivationally neutral sensory stimulus, lacking detectable primary or secondary reinforcing properties, can facilitate self-administration of nicotine. Possibly, drug-paired stimuli provide a "response marker" or serve as a temporal bridge between the operant response and drug effect. Motivationally neutral stimuli may therefore serve to isolate primary reinforcing effects of nicotine.

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