Posttraumatic stress disorder (PTSD) is characterized by a hypermnesia of the trauma and by a memory impairment that decreases the ability to restrict fear to the appropriate context. Infusion of glucocorticoids in the hippocampus after fear conditioning induces PTSD-like memory impairments and an altered pattern of neural activation in the hippocampal-amygdalar circuit. Mice become unable to identify the context as the correct predictor of the threat and show fear responses to a discrete cue not predicting the threat in normal conditions. These data demonstrate PTSD-like memory impairments in rodents and identify a potential pathophysiological mechanism of this condition.

No abstract

We investigated the spatio-temporal dynamics of learning-induced cAMP response element-binding protein activation/phosphorylation (pCREB) in mice trained in a spatial reference memory task in the water maze. Using immunohistochemistry, we examined pCREB immunoreactivity (pCREB-ir) in hippocampal CA1 and CA3 and related brain structures. During the course of spatial learning over Days 1-9, pCREB-ir progressively increased in hippocampal neurons whereas its level in the dorsal striatum decreased. No significant changes were observed in the prelimbic cortex and lateral amygdala. Mice killed at various time points after the last training session demonstrated two waves of pCREB-ir in CA1 and an early transient CREB phosphorylation in area CA3, lateral amygdala, and prelimbic cortex. We show that CREB phosphorylation and downstream gene Zif268 activation remained sustained in CA1 and CA3 for at least 24 h after extended training (Days 8-9) but not during early training (Day 3). The present results indicate that the strong CA1 CREB phosphorylation observed immediately after training was not related strictly to learning or to memory. In contrast, at 15 min after training, the changes in CA1 CREB phosphorylation state were specifically related to individual learning capability. We suggest that hippocampal-learning specificity of CREB is reflected best by duration, rather than magnitude, of CREB phosphorylation.Memory can be divided into at least two distinct forms according to its temporal and biochemical properties: short term memory (STM), which lasts no longer than a few hours, and long-term memory (LTM), which lasts from several hours to days or even longer (McGaugh 1966;Davis and Squire 1984;Matthies 1989; Bozon et al. 2003a,b). Studies in rodents have provided evidence that STM is a labile state that depends on activation and/or posttranslational modifications of preexisting molecules whereas consolidation of LTM depends on a crucial phase of gene expression and de novo protein synthesis to transform newly learned information into a permanent and stable state (Davis and Squire 1984;Dudai 1996;McGaugh 2000). Numerous studies have shown that consolidation of many types of LTM in rodents requires phosphorylation/activation of the transcription factor CREB (cAMP response element-binding protein) on Ser 133 by cAMP-or Ca 2+ -dependent protein kinase (Bernabeu et al. 1997;Izquierdo and Medina 1997;Impey et al. 1998;Taubenfeld et al. 1999;Cammarota et al. 2000Cammarota et al. , 2005Vianna et al. 2000;Izquierdo et al. 2001;Stanciu et al. 2001;Kida et al. 2002;Desmedt et al. 2003;Zhang et al. 2003;Countryman et al. 2005;Trifilieff et al. 2006;Brightwell et al. 2007). We and others recently showed that spatial memory formation was associated with increased CREB phosphorylation within the hippocampus (Mizuno et al. 2002;Colombo et al. 2003;Goldbart et al. 2003;Martel et al. 2006;Moncada and Viola 2006; Porte et al.2008). Indeed, a role for CREB in hippocampus-dependent learning was strongly supported by oligonucleotide antis...

The multiple memory systems hypothesis posits that different neural circuits function in parallel and may compete for information processing and storage. For example, instrumental conditioning would depend on the striatum, whereas spatial memory may be mediated by a circuit centered on the hippocampus. However, the nature of the task itself is not sufficient to select durably one system over the other. In this study, we investigated the effects of natural and pharmacological rewards on the selection of a particular memory system during learning. We compared the effects of food-or drug-induced activation of the reward system on cue-guided versus spatial learning using a Y-maze discrimination task. Drug-induced reward severely impaired the acquisition of a spatial discrimination task but spared the cued version of the task. Immunohistochemical analysis of the phosphorylated form of the cAMP response element binding (CREB) protein and c-Fos expression induced by behavioral testing revealed that the spatial deficit was associated with a decrease of both markers within the hippocampus and the prefrontal cortex. In contrast, drug reward potentiated the cued learning-induced CREB phosphorylation within the dorsal striatum. Administration of the protein kinase A inhibitor 8-Bromo-adenosine-3',5'-cyclic monophosphorothioate Rp isomer (Rp-cAMPS) into the dorsal striatum before training completely reversed the drug-induced spatial deficit and restored CREB phosphorylation levels within the hippocampus and the prefrontal cortex. Therefore, drug-induced striatal hyperactivity may underlie the declarative memory deficit reported here. This mechanism could represent an important early step toward the development of addictive behaviors by promoting conditioning to the detriment of more flexible forms of memory.

Gravity has a structural role for living systems. Tissue development, architecture, and organization are modified when the gravity vector is changed. In particular, microgravity induces a redistribution of blood volume and thus pressure in the astronaut body, abolishing an upright blood pressure gradient, inducing orthostatic hypotension. The present study was designed to investigate whether isolated vascular smooth muscle cells are directly sensitive to altered gravitational forces and, second, whether sustained blood pressure changes act on the same molecular target. Exposure to microgravity during 8 days in the International Space Station induced the decrease of ryanodine receptor subtype 1 expression in primary cultured myocytes from rat hepatic portal vein. Identical results were found in portal vein from mice exposed to microgravity during an 8-day shuttle spaceflight. To evaluate the functional consequences of this physiological adaptation, we have compared evoked calcium signals obtained in myocytes from hindlimb unloaded rats, in which the shift of blood pressure mimics the one produced by the microgravity, with those obtained in myocytes from rats injected with antisense oligonucleotide directed against ryanodine receptor subtype 1. In both conditions, calcium signals implicating calcium-induced calcium release were significantly decreased. In contrast, in spontaneous hypertensive rat, an increase in ryanodine receptor subtype 1 expression was observed as well as the calcium-induced calcium release mechanism. Taken together, our results shown that myocytes were directly sensitive to gravity level and that they adapt their calcium signaling pathways to pressure by the regulation of the ryanodine receptor subtype 1 expression.

Centrifugation is a widely used procedure to study the impact of altered gravity on Earth, as observed during spaceflights, allowing us to understand how a long-term physical constraint can condition the mammalian physiology. It is known that mice, placed in classical cages and maintained during 21 days in a centrifuge at 3G gravity level, undergo physiological adaptations due to hypergravity, and/or stress. Indeed, an increase of corticosterone levels has been previously measured in the plasma of 3G-exposed mice. Corticosterone is known to modify neuronal activity during memory processes. Although learning and memory performances cannot be assessed during the centrifugation, literature largely described a large panel of proteins (channels, second messengers, transcription factors, structural proteins) which expressions are modified during memory processing. Thus, we used the Illumina technology to compare the whole hippocampal transcriptome of three groups of C57Bl6/J mice, in order to gain insights into the effects of hypergravity on cerebral functions. Namely, a group of 21 days 3G-centrifuged mice was compared to (1) a group subjected to an acute corticosterone injection, (2) a group receiving a transdermal chronic administration of corticosterone during 21 days, and (3) aged mice because aging could be characterized by a decrease of hippocampus functions and memory impairment. Our results suggest that hypergravity stress induced by corticosterone administration and aging modulate the expression of genes in the hippocampus. However, the modulations of the transcriptome observed in these conditions are not identical. Hypergravity affects per-se the hippocampus transcriptome and probably modifies its activity. Hypergravity induced changes in hippocampal transcriptome were more similar to acute injection than chronic diffusion of corticosterone or aging.

On earth, gravity vector conditions the development of all living beings by physically imposing an axis along which to build their organism. Thus, during their whole life, they have to fight against this force not only to maintain their architectural organization but also to coordinate the communication between organs and keep their physiology in a balanced steady-state. In space, astronauts show physiological, psychological, and cognitive deregulations, ranging from bone decalcification or decrease of musculature, to depressive-like disorders, and spatial disorientation. Nonetheless, they are confronted to a great amount of physical changes in their environment such as solar radiations, loss of light-dark cycle, lack of spatial landmarks, confinement, and obviously a dramatic decrease of gravity force. It is thus very hard to selectively discriminate the strict role of gravity level alterations on physiological, and particularly cerebral, dysfunction. To this purpose, it is important to design autonomous models and apparatuses for behavioral phenotyping utilizable under modified gravity environments. Our team actually aims at working on this area of research.

Although the importance of cAMP-response element binding protein (CREB) phosphorylation in long-term memory formation is well documented for hippocampus-dependent tasks, little is known about the changes in phosphorylated CREB (pCREB) that occur during the process of extinction. The purpose of this study was to characterize the temporal patterns of pCREB in the CA1 and the amygdala after the extinction of previously acquired spatial information in the water maze. Mice were trained to find a hidden platform located at a fixed position and then were given extinction sessions in which the platform was either absent (NoPF) or relocated every day (RandomPF). We show that water maze spatial training evoked a biphasic response of pCREB in the CA1, with two different peaks occurring 15 min and 8 h postacquisition. The extinction of the original spatial preference significantly reduced the two peaks of CA1 pCREB in both RandomPF and NoPF groups whereas CA1 pCREB at 60 min post-training remained unaffected. Moreover, the early and late phases of extinction training produced regionally dissociable effects on pCREB in the CA1 and the lateral nucleus of the amygdala. These findings provide new insights on the molecular dynamics and anatomical dissociations underlying spatial memory and extinction learning.