Mechanisms of hippocampus‐related memory formation are time‐of‐day‐dependent. While the circadian system and clock genes are related to timing of hippocampal mnemonic processes (acquisition, consolidation, and retrieval of long‐term memory [LTM]) and long‐term potentiation (LTP), little is known about temporal gating mechanisms. Here, the role of the neurohormone melatonin as a circadian time cue for hippocampal signaling and memory formation was investigated in C3H/He wildtype (WT) and melatonin receptor‐knockout (MT1/2-false/-) mice. Immunohistochemical and immunoblot analyses revealed the presence of melatonin receptors on mouse hippocampal neurons. Temporal patterns of time‐of‐day‐dependent clock gene protein levels were profoundly altered in MT1/2-false/- mice compared to WT animals. On the behavioral level, WT mice displayed better spatial learning efficiency during daytime as compared to nighttime. In contrast, high error scores were observed in MT1/2-false/- mice during both, daytime and nighttime acquisition. Day‐night difference in LTP, as observed in WT mice, was absent in MT1/2-false/- mice and in WT animals, in which the sympathetic innervation of the pineal gland was surgically removed to erase rhythmic melatonin synthesis. In addition, treatment of melatonin‐deficient C57BL/6 mice with melatonin at nighttime significantly improved their working memory performance at daytime. These results illustrate that melatonin shapes time‐of‐day‐dependent learning efficiency in parallel to consolidating expression patterns of clock genes in the mouse hippocampus. Our data suggest that melatonin imprints a time cue on mouse hippocampal signaling and gene expression to foster better learning during daytime.
Laboratory mice are well capable of performing innate routine behaviour programmes necessary for courtship, nest-building and exploratory activities although housed for decades in animal facilities. We found that in mice inactivation of the clock gene Period1 profoundly changes innate routine behaviour programmes like those necessary for courtship, nest building, exploration and learning. These results in wild-type and Period1 mutant mice, together with earlier findings on courtship behaviour in wild-type and period-mutant Drosophila melanogaster, suggest a conserved role of Period-genes on innate routine behaviour. Additionally, both per-mutant flies and Period1-mutant mice display spatial learning and memory deficits. The profound influence of Period1 on routine behaviour programmes in mice, including female partner choice, may be independent of its function as a circadian clock gene, since Period1-deficient mice display normal circadian behaviour.
The adult, mature central nervous system (CNS) has limited plasticity. Physical exercising can counteract this limitation by inducing plasticity and fostering processes such as learning, memory consolidation and formation. Little is known about the molecular factors that govern these mechanisms, and how they are connected with exercise. In this study, we used immunohistochemical and behavioral analyses to investigate how running wheel exercise affects expression of the neuronal plasticity-inhibiting protein Nogo-A in the rat cortex, and how it influences motor learning in vivo. Following one week of exercise, rats exhibited a decrease in Nogo-A levels, selectively in motor cortex layer 2/3, but not in layer 5. Nogo-A protein levels returned to baseline after two weeks of running wheel exercise. In a skilled motor task (forelimb-reaching), administration of Nogo-A function-blocking antibodies over the course of the first training week led to improved motor learning. By contrast, Nogo-A antibody application over two weeks of training resulted in impaired learning. Our findings imply a bimodal, time-dependent function of Nogo-A in exercise-induced neuronal plasticity: While an activity-induced suppression of the plasticity-inhibiting protein Nogo-A appears initially beneficial for enhanced motor learning, presumably by allowing greater plasticity in establishing novel synaptic connections, this process is not sustained throughout continued exercise. Instead, upregulation of Nogo-A over the course of the second week of running wheel exercise in rats implies that Nogo-A is required for consolidation of acquired motor skills during the delayed memory consolidation process, possibly by inhibiting ongoing neuronal morphological reorganization to stabilize established synaptic pathways. Our findings suggest that Nogo-A downregulation allows leaning to occur, i.e. opens a ‘learning window’, while its later upregulation stabilizes the learnt engrams. These findings underline the importance of appropriately timing of application of Nogo-A antibodies in future clinical trials that aim to foster memory performance while avoiding adverse effects.
We evaluated the signalling framework of immortalized cells from the hypothalamic suprachiasmatic nucleus (SCN) of the mouse. We selected a vasoactive intestinal peptide (VIP)-positive sub-clone of immortalized mouse SCN-cells stably expressing a cAMP-regulated-element (CRE)-luciferase construct named SCNCRE. We characterized these cells in terms of their status as neuronal cells, as well as for important components of the cAMP-dependent signal transduction pathway and compared them to SCN ex vivo. SCNCRE cells were treated with agents that modulate different intracellular signalling pathways to investigate their potency and timing for transcriptional CRE-dependent signalling. Several activating pathways modulate SCN neuronal signalling via the cAMP-regulated-element (CRE: TGACGCTA) and phosphorylation of transcription factors such as cAMP-regulated-element-binding protein (CREB). CRE-luciferase activity induced by different cAMP-signalling pathway-modulating agents displayed a variety of substance-specific dose and time-dependent profiles and interactions relevant to the regulation of SCN physiology. Moreover, the induction of the protein kinase C (PKC) pathway by phorbol ester application modulates the CRE-dependent signalling pathway as well. In conclusion, the cAMP/PKA- and the PKC-regulated pathways individually and in combination modulate the final CRE-dependent transcriptional output.
It has been brought to our attention that the data analysis for figures 1 and 2 is not appropriate. The colleague who contacted us correctly remarked that we should have compared the mean frequencies of the two genotypes, instead of all calls of all animals in the genotypes, because the genotype is the decisive parameter ('The decisive number for calculating the mean + s.e.m. for each column is the number of animals, not the number of calls. Therefore, the calculations and the statistics are misleading'.).All ultrasound vocalization (USV) data regarding frequency and amplitude have therefore been re-analysed accordingly, the figures were revised and corrected and all frequency and amplitude data are now provided together with the statistical data and the analysis procedure. In brief, we first took the data from the automated analysis provided by AviSoft SASLabPro for USV parameters (start-, peak-, end-frequency, start-, peak-, end-amplitude, etc.) and calculated a mean value for each parameter of each individual mouse (see the electronic supplementary material, tables S1-S3). These mean values were then tested for Gaussian distribution (normality) with the Kolmogorov-Smirnov test with Lilliefors-modification. If the normality-test was passed, we compared the genotypes by Student's t-test, if not by an appropriate non-parametric test, e.g. Mann-Whitney U-test. All p-values are 'given per comparison'. Results (a) Ultrasound vocalization frequency and amplitudeWT (N ¼ 6) and Per1 2/2 (N ¼ 16) mice showed differences in the maximal and minimal peak frequencies (figure 1) and the peak amplitude of USV (figure 2) when confronted to a WT female. At day 2 (figure 1a), the mean maximal peak frequencies of each entire element (USV call) that was detected of the WT (81.1 + 8.0 kHz) differed from the Per1 2/2 (74.0 + 9.9 kHz) animals. The minimal peak frequencies were significantly different and, similar to the maximal frequencies, lower in Per1 2/2 compared to WT (WT: 68.0 + 4.7 kHz; Per1 2/2 : 60.7 + 7.3 kHz; p 0.02 t-test).
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