In a variety of species memory consolidation following different learning paradigms has been shown to be dependent on protein synthesis. However, it is not known whether modulation of protein synthesis is a critical component of the consolidation process, nor is the identity of any protein(s) subject to translational regulation, known. We report here that phosphorylation of eukaryotic elongation factor-2 (eEF2), an indicator for translational elongation attenuation, is correlated with input that produces taste memory consolidation in the relevant cortex of rat. The temporal pattern of eEF2 phosphorylation is similar to extra-cellular regulated kinase 2 (ERK2) activation and S6K1 phosphorylation, which are known to stimulate translation initiation. In addition, increased eEF2 phosphorylation and increased alphaCaMKII expression is detected in a synaptoneurosomal fraction made from taste cortex following memory consolidation. These results suggest that increased initiation rate together with decreased elongation rate, during memory consolidation, shift the rate-limiting step of protein synthesis, to produce a local switch-like effect in the expression of neuronal proteins.

The processes underlying long-term memory formation in the neocortex are poorly understood. Using taste learning, we found learning-related induction of PSD-95 in the gustatory cortex, which was temporally restricted, coupled to the learning of a novel, but not familiar, taste and controlled by ERK. Using temporally and spatially restricted RNA interference knockdown of PSD-95 in vivo, we found that PSD-95 induction is necessary for learning novel tastes, but not for the recollection of familiar ones.

Novel taste learning is a robust one-trial incidental learning process, dependent on functional activity of the insular (taste) cortex. In contrast to that of the cortex, the role of the hippocampus in taste learning is controversial. We set out to identify the time courses of the activation of mitogen-associated protein kinase (MAPK), transcription factor cAMP-response element-binding protein (CREB) and Akt/PKB (protein kinase B) in the insular cortex and hippocampus of rats subsequent to novel taste learning. Following taste learning, an early response (20 min) occurred at the same time in the insular cortex and the hippocampus. However, whereas MAPK was activated specifically in the insular cortex, CREB and Akt were phosphorylated in the hippocampus but not in the cortex. In addition, the immediate early gene, CCAAT/enhancer binding protein (C/EBPbeta) was induced in both the hippocampus and the insular cortex 18 h following taste learning. The results demonstrate, for the first time, correlative activation and gene expression in the hippocampus following novel taste learning. Moreover, the results suggest that different signal transduction cascades necessary for taste learning are activated in concert in different brain structures, to enable taste learning and consolidation.

Different forms of memories and synaptic plasticity require synthesis of new proteins at the time of acquisition or immediately after. We are interested in the role of translation regulation in the cortex, the brain structure assumed to store long-term memories. The mammalian target of rapamycin, mTOR (also known as FRAP and RAFT-1), is part of a key signal transduction mechanism known to regulate translation of specific subset of mRNAs and to affect learning and synaptic plasticity. We report here that novel taste learning induces two waves of mTOR activation in the gustatory cortex. Interestingly, the first wave can be identified both in synaptoneurosomal and cellular fractions, whereas the second wave is detected in the cellular fraction but not in the synaptic one. Inhibition of mTOR, specifically in the gustatory cortex, has two effects. First, biochemically, it modulates several known downstream proteins that control translation and reduces the expression of postsynaptic density-95 in vivo. Second, behaviorally, it attenuates long-term taste memory. The results suggest that the mTOR pathway in the cortex modulates both translation factor activity and protein expression, to enable normal taste memory consolidation.

Profound evidence indicates that GABAA receptors are important in the control of physiological response to stress and anxiety. The alpha subunit type composition contributes significantly to the functional characterization of the GABAA receptors. The alpha2, alpha3, alpha5 subunits are predominately expressed in the brain during embryonic and early postnatal periods of normal rats, whilst alpha1 are most prominent during later developmental stages. In the present study, we examined the long-term effects of juvenile stress on GABA alpha subunit expression in adulthood in the amygdala and hippocampus. We applied the elevated platform stress paradigm at juvenility and used the open-field and startle response tests to assess anxiety level in adulthood. Juvenile stress effects without behavioural tests in adulthood were also examined since previous studies indicated that the mere exposure to these tests might be stressful for rats, enhancing the effects of the juvenile exposure to stress. In adulthood, we quantitatively determined the level of expression of alpha1, alpha2 and alpha3 in the hippocampus and amygdala. Our results indicate that subjecting juvenile stressed rats to additional challenges in adulthood results in an immature-like expression profile of these subunits. To test for potential functional implications of these alterations we examined the effects of the anxiolytic (diazepam) and the sedative (brotizolam) benzodiazepines on juvenile stressed and control rats following additional challenges in adulthood. Juvenile stressed rats were more sensitive to diazepam and less sensitive to brotizolam, suggesting that the alterations in GABA alpha subunit expression in these animals have functional consequences.