Accelerating Maturation of Spatial Memory Systems by Experience: Evidence from Sleep Oscillation Signatures of Memory Processing

During early development, memory systems gradually mature over time, in parallel with the gradual accumulation of knowledge. Yet, it is unknown whether and to what extent maturation is driven by discrete experience. Sleep is thought to contribute to the formation of long-term memory and knowledge through a systems consolidation process that is driven by specific sleep oscillations (i.e., ripples, spindles, and slow oscillations) in cortical and hippocampal networks. Based on these oscillatory signatures, we show here in rats that discrete spatial experience speeds the functional maturation of spatial memory systems during development. Juvenile male rats were exposed for 5 min periods to changes in the spatial configuration of two identical objects on postnatal day (PD)25, PD27, and PD29 (Spatial experience group), while a Control group was exposed on these occasions to the same two objects without changing their positions. On PD31, both groups were tested on a classical Object Place Recognition (OPR) task with a 3 h retention interval during which the sleep-associated EEG and hippocampal local field potentials were recorded. On PD31, consistent with forgoing studies, Control rats still did not express OPR memory. By contrast, rats with Spatial experience formed significant OPR memory and, in parallel, displayed an increased percentage of hippocampal ripples coupled to parietal slow oscillation-spindle complexes, and a stronger ripple-spindle phase-locking during the retention sleep. Our findings support the idea that experience promotes the maturation of memory systems during development by enhancing cortico-hippocampal information exchange and the formation of integrated knowledge representations during sleep.SIGNIFICANCE STATEMENT Cognitive and memory capabilities mature early in life. We show here that and how discrete spatial experience contributes to this process. Using a simple recognition paradigm in developing rats, we found that exposure of the rat pups to three short-lasting experiences enhances spatial memory capabilities to adult-like levels. The adult-like capability of building spatial memory was connected to a more precise coupling of ripples in the hippocampus with slow oscillation-spindle complexes in the thalamo-cortical system when the memory was formed during sleep. Our findings support the view that discrete experience accelerates maturation of cognitive and memory capabilities by enhancing the dialogue between hippocampus and cortex when these experiences are reprocessed during sleep.

Possible Role of Polarized Light Information in Spatial Recognition in the Cricket Gryllus bimaculatus

Many insects are able to use skylight e-vector patterns to deduce their heading direction. Crickets have been well known to orient themselves to certain e-vector orientations to keep their walking direction. However, it is still unknown if crickets are able to utilize polarized light information for spatial recognition. Using an experimental paradigm similar to the Morris water maze for rodents, here we examine the possibility that the cricket Gryllus bimaculatus can utilize polarized light information to find the target place. Crickets were placed in a round arena with a heated floor, a portion of which was cooled, and a cross-shaped e-vector pattern was presented from the top of the arena so that the cricket could find the cool spot by walking along the e-vector direction. When the arrangement of the e-vector pattern and the cool spot were fixed throughout the experiments, the time and the walking distance to find the cool spot were significantly decreased with increasing trials, but not when the e-vector pattern was rotated between each trial. Moreover, a model selection indicated that the visual stimulus contributed to the decrease in time and distance. To investigate the cricket's exploration patterns in the arena, a test trial in which the whole floor was uniformly heated was performed before and after the training trials. In the test trial, the crickets trained with the positionally fixed e-vector pattern showed wall-following behavior for a significantly longer time than those untrained and those trained with random e-vector patterns.

PQBP1 promotes translational elongation and regulates hippocampal mGluR-LTD by suppressing eEF2 phosphorylation

Eukaryotic elongation factor 2 (eEF2) mediates translocation of peptidyl-tRNA from the ribosomal A site to the P site to promote translational elongation. Its phosphorylation on Thr56 by its single known kinase eEF2K inactivates it and inhibits translational elongation. Extensive studies have revealed that different signal cascades modulate eEF2K activity, but whether additional factors regulate phosphorylation of eEF2 remains unclear. Here, we find that the X chromosome-linked intellectual disability protein polyglutamine-binding protein 1 (PQBP1) specifically binds to non-phosphorylated eEF2 and suppresses eEF2K-mediated phosphorylation at Thr56. Loss of PQBP1 significantly reduces general protein synthesis by suppressing translational elongation. Moreover, we show that PQBP1 regulates hippocampal metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) and mGluR-LTD-associated behaviors by suppressing eEF2K-mediated phosphorylation. Our results identify PQBP1 as a novel regulator in translational elongation and mGluR-LTD, and this newly revealed regulator in the eEF2K/eEF2 pathway is also an excellent therapeutic target for various disease conditions, such as neural diseases, virus infection, and cancer.

Association of Lifelong Intake of Barley Diet with Healthy Aging: Changes in Physical and Cognitive Functions and Intestinal Microbiome in Senescence-Accelerated Mouse-Prone 8 (SAMP8)

Barley intake reportedly reduces the risk of cardiovascular disease, but effects on the systemic phenotypes during healthy aging have not yet been examined. Therefore, we examined the effects of barley on the lifespan; behavioral phenotypes, such as locomotor activity, and cognitive functions, and intestinal microbiome in the senescence-accelerated mouse-prone 8 (SAMP8) mouse. We prepared two mild high-fat diets by adding lard, in which the starch components of AIN-93G were replaced by rice or barley "Motchiriboshi." SAMP8 (four weeks old, male) mice were fed AIN-93G until eight weeks old, and then rice (rice group) or barley diet (rice: barley = 1:4, barley group) until death. Changes in aging-related phenotypes, object and spatial recognition, locomotor and balancing activities, and the intestinal microbiome were recorded. Moreover, plasma cholesterol levels were analyzed at 16 weeks old. Barley intake prolonged the lifespan by approximately four weeks, delayed locomotor atrophy, and reduced balancing ability and spatial recognition. Barley intake significantly increased the medium and small particle sizes of high-density lipoprotein (HDL) cholesterol, which is associated with a reduced risk of total stroke. The Bacteroidetes to Firmicutes ratio in the barley group was significantly higher than that in the rice group during aging. Thus, lifelong barley intake may have positive effects on healthy aging.

The Role of Astrocytic Calcium Signaling in the Aged Prefrontal Cortex

Aging is a lifelong process characterized by cognitive decline putatively due to structural and functional changes of neural circuits of the brain. Neuron-glial signaling is a fundamental component of structure and function of circuits of the brain, and yet its possible role in aging remains elusive. Significantly, neuron-glial networks of the prefrontal cortex undergo age-related alterations that can affect cognitive function, and disruption of glial calcium signaling has been linked with cognitive performance. Motivated by these observations, we explored the possible role of glia in cognition during aging, considering a mouse model where astrocytes lacked IP₃R2-dependent Ca²⁺ signaling. Contrarily to aged wild-type animals that showed significant impairment in a two-trial place recognition task, aged IP₃R2 KO mice did not. Consideration of neuronal and astrocytic cell densities in the prefrontal cortex, revealed that aged IP₃R2 KO mice present decreased densities of NeuN⁺ neurons and increased densities of S100β⁺ astrocytes. Moreover, aged IP₃R2 KO mice display refined dendritic trees in this region. These findings suggest a novel role for astrocytes in the aged brain. Further evaluation of the neuron-glial interactions in the aged brain will disclose novel strategies to handle healthy cognitive aging in humans.

The role of postnatal neurogenesis in supporting remote memory and spatial metric processing

In this study, we determined the contribution of juvenile neurogenesis to the performance of mice on a remote memory for temporally based association task and in a novelty based spatial pattern separation task. This was accomplished by mating homozygous DNMT1-loxP mice with heterozygous GFAP-Cre mice and comparing Cre+ (no postnatal neurogenesis) to Cre- (wild type) littermate offspring. The results indicate that Cre+ mice are impaired relative to Cre- mice in the remote memory for a temporal based association task and in a novelty based spatial pattern separation task. These results support the temporal integration model of Aimone et al., [(2006) Nat Neurosci 9:723-727] and provide further support for an important role for postnatally born neurons in spatial pattern separation. In contrast, Cre+ mice are not impaired relative to Cre- mice in an object-context recognition task and a spatial location recognition task. These latter data suggest that postnatally derived neurons in the dentate gyrus (DG) do not support all spatial and object recognition functions of the DG.

Impaired spatial memory after ketamine administration in chronic low doses

Ketamine is a noncompetitive antagonist of the NMDA-receptors, used as a dissociative anesthetic, presently included in the category of the psychoactive substances known as "club drugs". Ketamine administration was associated with impaired working memory and increased psychopathological symptoms, but there is a lack of information regarding the effects of chronic sub-anesthetic doses. Adult Wistar rats were administered ketamine, 5 and 10 mg/kg twice daily, subcutaneously for 14 days. One week later, rats were tested in an object recognition/object location task and in the open field arena. There was altered performance in both the object recognition/location and in the open field tests by the group chronically exposed to the lower dose of ketamine. These animals displayed a decreased discrimination index (p<0.05) in the object recognition task, were unable to recognize the displacement of a familiar object and displayed decreased activity across open filed sessions. Importantly, these alterations were not observed in animals administered a higher dose of ketamine. Collectively, these results consistently show that chronic administration of ketamine in sub-anesthetic doses may lead to decreased habituation and inability to update spatial representations.