SCOP/PHLPP1β mediates circadian regulation of long-term recognition memory

Circadian neurogenetics and its implications in neurophysiology, behavior, and chronomedicine

The circadian rhythm affects multiple physiological processes, and disruption of the circadian system can be involved in a range of disease-related pathways. The genetic underpinnings of the circadian rhythm have been well-studied in model organisms. Significant progress has been made in understanding how clock genes affect the physiological functions of the nervous system. In addition, circadian timing is becoming a key factor in improving drug efficacy and reducing drug toxicity. The circadian biology of the target cell determines how the organ responds to the drug at a specific time of day, thus regulating pharmacodynamics. The current review brings together recent advances that have begun to unravel the molecular mechanisms of how the circadian clock affects neurophysiological and behavioral processes associated with human brain diseases. We start with a brief description of how the ubiquitous circadian rhythms are regulated at the genetic, cellular, and neural circuit levels, based on knowledge derived from extensive research on model organisms. We then summarize the latest findings from genetic studies of human brain disorders, focusing on the role of human clock gene variants in these diseases. Lastly, we discuss the impact of common dietary factors and medications on human circadian rhythms and advocate for a broader application of the concept of chronomedicine.

The memory impairment by hypothyroidism in mice is dependent on time-of-day and sex

Hypothyroidism is an endocrine-metabolic disorder, and as such it compromises a wide range of physiological functions. Memory deficits and, the most recently described, circadian rhythm disruption are among the impairments caused by thyroid dysfunctions. However, although highly likely, there is no evidence connecting these two effects of hypothyroidism. Here, we hypothesized the time-of-day interferes with the memory deficit caused by hypothyroidism. C57BL/6 J mice from both sexes were subjected to novel object recognition (NOR) task during the rest and active phases, corresponding to ZT 2-4 and 14-16, respectively (ZT: Zeitgeber time; ZT 0: lights on at 07:00 am). First, we showed that neither sex nor ZT altered object recognition memory (ORM) in euthyroid mice. Next, animals were divided into control (euthyroid) and hypothyroid [induced with methimazole (0.01%) and perchlorate (0.1%) treatment in the drinking water for 21 days] groups. Under euthyroid conditions, male and female mice recognized the novel object regardless of the time-of-day. However, hypothyroidism impaired ORM at rest phase (ZT 2-4) in both sexes. Surprisingly, in the active phase (ZT 14-16), the hypothyroid males performed the NOR, though a longer time to execute the task was required. In contrast, female hypothyroid mice showed a greater impairment in ORM. Our results suggest that hypothyroidism may disrupt the circadian rhythm in brain areas related to mnemonic processes since in euthyroid condition ORM is not affected by the time-of-day. Furthermore, our findings in an animal model indicate a pronounced deleterious effect of hypothyroidism in women.

The clock gene Per1 is necessary in the retrosplenial cortex-but not in the suprachiasmatic nucleus-for incidental learning in young and aging male mice

Aging impairs both circadian rhythms and memory, though the relationship between these impairments is not fully understood. Circadian rhythms are largely dictated by clock genes within the body's central pacemaker, the suprachiasmatic nucleus (SCN), though these genes are also expressed in local clocks throughout the body. As circadian rhythms can directly affect memory performance, one possibility is that memory deficits observed with age are downstream of global circadian rhythm disruptions stemming from the SCN. Here, we demonstrate that expression of clock gene Period1 within a memory-relevant cortical structure, the retrosplenial cortex (RSC), is necessary for incidental learning, and that age-related disruption of Period1 within the RSC-but not necessarily the SCN-contributes to cognitive decline. These data expand the known functions of clock genes beyond maintaining circadian rhythms and suggests that age-associated changes in clock gene expression modulates circadian rhythms and memory performance in a brain region-dependent manner.

The circadian clock in the piriform cortex intrinsically tunes daily changes of odor-evoked neural activity

The daily activity in the brain is typically fine-tuned by the circadian clock in the local neurons as well as by the master circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus. In the olfactory response, odor-evoked activity in the piriform cortex (PC) and olfactory behavior retain circadian rhythmicity in the absence of the SCN, yet how the circadian rhythm in the PC is achieved independently of the SCN remains elusive. Here, to define neurons regulating the circadian rhythm of the odor-evoked activity in the PC, we knocked out the clock gene Bmal1 in a host of specific neurons along the olfactory circuit. We discovered that Bmal1 knockout in the PC largely abolishes the circadian rhythm of the odor-evoked activity. We further showed that isolated PC exhibits sustained circadian rhythms of the clock gene Per2 expression. Quantitative PCR analysis revealed that expression patterns of multiple genes involved in neural activity and synaptic transmission exhibit circadian rhythm in the PC in a BMAL1-dependent manner. Our findings indicate that BMAL1 acts intrinsically in the PC to control the circadian rhythm of the odor-evoked activity in the PC, possibly through regulating expression patterns of multiple genes involved in neural activity and transmission.

Diurnal variation in declarative memory and the involvement of SCOP in cognitive functions in nonhuman primates

Cognitive functions depend on the time of day in various organisms. Previously, we found that 24-h recognition memory performance of nocturnal mice changes diurnally through SCOP protein-dependent regulation. It remains unknown whether diurnal change and SCOP-dependent regulation of memory performance are conserved across species with diurnal/nocturnal habits. We tested whether the memory performance of diurnal Japanese macaques depends on the time of day. The memory association between bitter taste of drinking water and the nozzle color of the water bottle was established during the light period of the day to evaluate of memory performance for macaques. Here we found diurnal variation of declarative memory in Japanese macaques. The middle of the daytime is the most effective time for memory performance during the light period. To assess whether SCOP is involved in declarative memory performance, we interfered with SCOP expression by using lentiviral vector expressing shRNA against Scop in the hippocampus of Japanese macaques. Scop knockdown in the hippocampus abrogated the memory performance in the middle of the daytime. Our results implicate that SCOP in the hippocampus is necessary for the diurnal rhythm of the memory system and that the SCOP-dependent memory regulation system may be conserved in mammals.

Circadian regulation of dentate gyrus excitability mediated by G-protein signaling

The central circadian regulator within the suprachiasmatic nucleus transmits time of day information by a diurnal spiking rhythm driven by molecular clock genes controlling membrane excitability. Most brain regions, including the hippocampus, harbor similar intrinsic circadian transcriptional machinery, but whether these molecular programs generate oscillations of membrane properties is unclear. Here, we show that intrinsic excitability of mouse dentate granule neurons exhibits a 24-h oscillation that controls spiking probability. Diurnal changes in excitability are mediated by antiphase G-protein regulation of potassium and sodium currents that reduce excitability during the Light phase. Disruption of the circadian transcriptional machinery by conditional deletion of Bmal1 enhances excitability selectively during the Light phase by removing G-protein regulation. These results reveal that circadian transcriptional machinery regulates intrinsic excitability by coordinated regulation of ion channels by G-protein signaling, highlighting a potential novel mechanism of cell-autonomous oscillations.

Diazepam induces retrograde facilitation of object recognition and object location memory in male mice

Benzodiazepines are widely prescribed for patients suffering from anxiety and insomnia. Although amnesic effects of benzodiazepines are commonly known as side effects, it has also been reported that these drugs improve memory for information learned before drug intake, a phenomenon called retrograde facilitation. However, the retrograde effects of benzodiazepines on cognitive performances in rodents remain controversial. It should be considered that studies on diazepam-induced retrograde facilitation in humans have been conducted using a recall paradigm focused on short-term memory, whereas these studies in rodents have been conducted using memory tasks that mainly target long-term memory and/or require negative or positive reinforcers. In the current study, we investigated whether diazepam, a benzodiazepine, induces retrograde facilitation for object recognition memory and spatial memory in mice, using a novel object recognition test and an object location test, respectively. These tests are available for short-term memory and do not require any reinforcer. The mice treated with diazepam retained object recognition memory for at least 180 min and spatial memory for at least 150 min. In contrast, vehicle-treated control mice retained object recognition memory for 120 min but not 150 min or longer, and spatial memory for 90 min but not 120 min or longer. These data clearly demonstrated diazepam-induced retrograde facilitation for both object recognition and spatial memories in mice. The present study is expected to contribute to the elucidation of the neural basis of retrograde facilitation.

Circadian rhythms as modulators of brain health during development and throughout aging

The circadian clock plays a prominent role in neurons during development and throughout aging. This review covers topics pertinent to the role of 24-h rhythms in neuronal development and function, and their tendency to decline with aging. Pharmacological or behavioral modification that augment the function of our internal clock may be central to decline of cognitive disease and to future chronotherapy for aging-related diseases of the central nervous system.

Emerging roles of PHLPP phosphatases in the nervous system

It has been more than a decade since the discovery of a novel class of phosphatase, the Pleckstrin Homology (PH) domain Leucine-rich repeat Protein Phosphatases (PHLPP). Over time, they have been recognized as crucial regulators of various cellular processes, such as memory formation, cellular survival and proliferation, maintenance of circadian rhythm, and others, with any deregulation in their expression or cellular localization causing havoc in any cellular system. With the ever-growing number of downstream substrates across multiple tissue systems, a web is emerging wherein the central point is PHLPP. A slight nick in the normal signaling cascade of the two isoforms of PHLPP, namely PHLPP1 and PHLPP2, has been recently found to invoke a variety of neurological disorders including Alzheimer's disease, epileptic seizures, Parkinson's disease, and others, in the neuronal system. Improper regulation of the two isoforms has also been associated with various disease pathologies such as diabetes, cardiovascular disorders, cancer, musculoskeletal disorders, etc. In this review, we have summarized all the current knowledge about PHLPP1 (PHLPP1α and PHLPP1β) and PHLPP2 and their emerging roles in regulating various neuronal signaling pathways to pave the way for a better understanding of the complexities. This would in turn aid in providing context for the development of possible future therapeutic strategies.

Impaired Morris water task retention following T21 light dark cycle exposure is not due to reduced hippocampal c-FOS expression

Circadian rhythms influence virtually all aspects of physiology and behavior. This is problematic when circadian rhythms no longer reliably predict time. Circadian rhythm disruption can impair memory, yet we don’t know how this fully works at the systems and molecular level. When trying to determine the root of a memory impairment, assessing neuronal activation with c-FOS is useful. This has yet to be assessed in the hippocampi of circadian rhythm disrupted rats in a hippocampal gold standard task. Rats were trained on the Morris water task (MWT), then received 6 days of a 21-h day (T21), 13 days of a normal light dark cycle, probe trial, and tissue extraction an hour later. Despite having impaired memory in the probe trial, compared to controls there were no differences in c-FOS expression in hippocampal sub regions: CA1; CA3; Dentate gyrus. These data confirm others in hamsters demonstrating that arrhythmicity which produces an impairment in spontaneous alternation does not affect c-FOS in the dentate gyrus. The current study indicates that the memory impairment induced by a lighting manipulation is likely not due to attenuated neuronal activation. Determining how the master clock in the brain communicates with the hippocampus is needed to untangle the relationship between circadian rhythms and memory.

Handling, task complexity, time-of-day, and sleep deprivation as dynamic modulators of recognition memory in mice

Sleep is essential for optimal cognitive functioning. Although we lack a complete understanding of the role of sleep in memory consolidation, we know that various factors that disturb sleep or sleep quality have consequences for cognitive performance. Such factors can be unintended components of behavioral experiments on rodents and other experimental animals that generate differing results from different labs. These experimental variables include habituation to handling, intended or unintended sleep deprivation, task complexity, time of testing, and environmental features. We have examined how these variables impact recognition memory in C57BL/6 mice. Handled mice outperformed their non-handled counterparts across different combinations of delay phase duration and lighting conditions. Results also suggest that simple task recall is more resistant to diurnal variation and the impairing effects of sleep deprivation than is complex task recall. This study underscores the role of protocol and environmental factors in recognition memory and in conflicting results from different laboratories.

Circadian regulation of memory under stress: Endocannabinoids matter

Organisms ranging from plants to higher mammals have developed 24-hour oscillation rhythms to optimize physiology to environmental changes and regulate a plethora of neuroendocrine and behavioral processes, including neurotransmitter and hormone regulation, stress response and learning and memory function. Compelling evidence indicates that a wide array of memory processes is strongly influenced by stress- and emotional arousal-activated neurobiological systems, including the endocannabinoid system which has been extensively shown to play an integral role in mediating stress effects on memory. Here, we review findings showing how circadian rhythms and time-of-day influence stress systems and memory performance. We report evidence of circadian regulation of memory under stress, focusing on the role of the endocannabinoid system and highlighting its circadian rhythmicity. Our discussion illustrates how the endocannabinoid system mediates stress effects on memory in a circadian-dependent fashion. We suggest that endocannabinoids might regulate molecular mechanisms that control memory function under circadian and stress influence, with potential important clinical implications for both neurodevelopmental disorders and psychiatric conditions involving memory impairments.

Control of complex behavior by astrocytes and microglia

Evidence that glial cells influence behavior has been gaining a steady foothold in scientific literature. Out of the five main subtypes of glial cells in the brain, astrocytes and microglia have received an outsized share of attention with regard to shaping a wide spectrum of behavioral phenomena and there is growing appreciation that the signals intrinsic to these cells as well as their interactions with surrounding neurons reflect behavioral history in a brain region-specific manner. Considerable regional diversity of glial cell phenotypes is beginning to be recognized and may contribute to behavioral outcomes arising from circuit-specific computations within and across discrete brain nuclei. Here, we summarize current knowledge on the impact of astrocyte and microglia activity on behavioral outcomes, with a specific focus on brain areas relevant to higher cognitive control, reward-seeking, and circadian regulation.

Circadian clocks, cognition, and Alzheimer's disease: synaptic mechanisms, signaling effectors, and chronotherapeutics

Modulation of basic biochemical and physiological processes by the circadian timing system is now recognized as a fundamental feature of all mammalian organ systems. Within the central nervous system, these clock-modulating effects are reflected in some of the most complex behavioral states including learning, memory, and mood. How the clock shapes these behavioral processes is only now beginning to be realized. In this review we describe recent findings regarding the complex set of cellular signaling events, including kinase pathways, gene networks, and synaptic circuits that are under the influence of the clock timing system and how this, in turn, shapes cognitive capacity over the circadian cycle. Further, we discuss the functional roles of the master circadian clock located in the suprachiasmatic nucleus, and peripheral oscillator populations within cortical and limbic circuits, in the gating of synaptic plasticity and memory over the circadian cycle. These findings are then used as the basis to discuss the connection between clock dysregulation and cognitive impairments resulting from Alzheimer's disease (AD). In addition, we discuss the conceptually novel idea that in AD, there is a selective disruption of circadian timing within cortical and limbic circuits, and that it is the disruption/desynchronization of these regions from the phase-entraining effects of the SCN that underlies aspects of the early- and mid-stage cognitive deficits in AD. Further, we discuss the prospect that the disruption of circadian timing in AD could produce a self-reinforcing feedback loop, where disruption of timing accelerates AD pathogenesis (e.g., amyloid deposition, oxidative stress and cell death) that in turn leads to a further disruption of the circadian timing system. Lastly, we address potential therapeutic approaches that could be used to strengthen cellular timing networks and, in turn, how these approaches could be used to improve cognitive capacity in Alzheimer's patients.

Polysaccharides from Polygonatum cyrtonema Hua Reduce Depression-Like Behavior in Mice by Inhibiting Oxidative Stress-Calpain-1-NLRP3 Signaling Axis

Polysaccharides from Polygonatum cyrtonema Hua (PSP) exert antioxidant, anti-inflammatory, and antidepressant effects. Production of reactive oxygen species (ROS) and activation of the calpain system and the NOD-like receptor protein 3 (NLRP3) inflammasome are closely related to the pathogenesis of depression. However, the relationships among those pathways and the protective effects of PSP have not been characterized. In this study, lipopolysaccharide (LPS) and chronic unpredictable mild stress- (CUMS-) induced depression models were used to evaluate the protective mechanisms of PSP against depression. ROS levels were measured in HT-22 cells using flow cytometry. Brain tissues were collected to determine the levels of oxidation-related indicators and inflammatory cytokines. The protein levels of calpain-1, calpain-2, calpastatin, phosphatase and Tensin Homolog deleted on Chromosome 10 (PTEN), suprachiasmatic nucleus circadian oscillatory protein (SCOP), nuclear factor-erythroid factor 2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1), NLRP3, apoptosis-associated speck-like protein (ASC), caspase-1, cleaved-caspase-1, ionized calcium binding adapter molecule 1 (Iba1), phosphorylation of extracellular signal-regulated kinase (p-ERK), nuclear factor-kappa B (NF-κB), interleukin-1β (IL-1β), and glial fibrillary acidic protein (GFAP) were measured using western blotting or immunofluorescence. In cellular experiments, we showed that PSP attenuated LPS-induced production of ROS in HT-22 cells. In animal experiments, we found that LPS increased the expression of calpain-1, NLRP3, ASC, caspase-1, cleaved-caspase-1, Iba1, p-ERK, NF-κB, and GFAP and reduced the expression of calpastatin, PTEN, SCOP, and Nrf2. Administration of PSP reversed these changes. N-Acetyl-L-cysteine (NAC) administration also inhibited oxidative stress and activation of the calpain system and the NLRP3 inflammasome. Furthermore, PSP, calpeptin, MCC950 (a selective NLRP3 inflammasome inhibitor), and NAC reduced LPS-induced proinflammatory cytokine release. We also showed that PSP prevented CUMS-induced changes in the calpain system and the Nrf2 and NLRP3 signaling pathways and reduced depression-like behavior. These results indicate that PSP exerts antidepressant effects through regulation of the oxidative stress-calpain-1-NLRP3 signaling axis.

Circadian disruption of hippocampus in an early senescence male mouse model

Age-related cognitive decline and disruptions in circadian rhythms are growing problems as the average human life span increases. Multiple strains of the senescence-accelerated mouse (SAM) show reduced life span, and the SAMP8 strain in particular has been well documented to show cognitive deficits in behavior as well as a bimodal pattern of circadian locomotor activity. However, little is known about circadian regulation within the hippocampus of these strains of mice. Here we test the hypothesis that in this early senescence model, disruption of the molecular circadian clock in SAMP8 animals drives disrupted behavior and physiology. We found normal rhythms in PER2 protein expression in the SCN of SAMP8 animals at 4 months, despite the presence of disrupted wheel-running activity rhythms at this age. Interestingly, a significant rhythm in PER2 expression was not observed in the hippocampus of SAMP8 animals, despite a significant 24-h rhythm in SAMR1 controls. We also examined time-restricted feeding as a potential strategy to rescue disrupted hippocampal plasticity. Time-restricted feeding increased long-term potentiation at Schaffer collateral-CA1 synapses in SAMP8 mice (compared to SAMR1 controls). Overall, we confirm disrupted circadian locomotor rhythms in this early senescence model (as early as 4 months) and discovered that this disruption is not due to arrhythmic PER2 levels in the SCN; however, other extra-SCN circadian oscillators (i.e., hippocampus) are likely impaired with accelerated aging.

The Effects of Light and the Circadian System on Rhythmic Brain Function

Life on earth has evolved under the influence of regularly recurring changes in the environment, such as the 24 h light/dark cycle. Consequently, organisms have developed endogenous clocks, generating 24 h (circadian) rhythms that serve to anticipate these rhythmic changes. In addition to these circadian rhythms, which persist in constant conditions and can be entrained to environmental rhythms, light drives rhythmic behavior and brain function, especially in nocturnal laboratory rodents. In recent decades, research has made great advances in the elucidation of the molecular circadian clockwork and circadian light perception. This review summarizes the role of light and the circadian clock in rhythmic brain function, with a focus on the complex interaction between the different components of the mammalian circadian system. Furthermore, chronodisruption as a consequence of light at night, genetic manipulation, and neurodegenerative diseases is briefly discussed.

Chronic circadian phase advance in male mice induces depressive-like responses and suppresses neuroimmune activation

Altered working and sleeping schedules during the COVID-19 pandemic likely impact our circadian systems. At the molecular level, clock genes form feedback inhibition loops that control 24-hr oscillations throughout the body. Importantly, core clock genes also regulate microglia, the brain resident immune cell, suggesting circadian regulation of neuroimmune function. To assess whether circadian disruption induces neuroimmune and associated behavioral changes, we mimicked chronic jetlag with a chronic phase advance (CPA) model. 32 adult male C57BL/6J mice underwent 6-hr light phase advance shifts every 3 light/dark cycles (CPA) 14 times or were maintained in standard light/dark cycles (control). CPA mice showed higher behavioral despair but not anhedonia in forced swim and sucrose preferences tests, respectively. Changes in behavior were accompanied by altered hippocampal circadian genes in CPA mice. Further, CPA suppressed expression of brain-derived neurotrophic factor (BDNF) and pro-inflammatory cytokine interleukin-1 beta in the hippocampus. Plasma corticosterone concentrations were elevated by CPA, suggesting that CPA may suppress neuroimmune pathways via glucocorticoids. These results demonstrate that chronic circadian disruption alters mood and neuroimmune function, which may have implications for shift working populations such as frontline health workers.

Dysregulated clock gene expression and abnormal diurnal regulation of hippocampal inhibitory transmission and spatial memory in amyloid precursor protein transgenic mice

Patients with Alzheimer's disease (AD) often have fragmentation of sleep/wake cycles and disrupted 24-h (circadian) activity. Despite this, little work has investigated the potential underlying day/night disruptions in cognition and neuronal physiology in the hippocampus. The molecular clock, an intrinsic transcription-translation feedback loop that regulates circadian behavior, may also regulate hippocampal neurophysiological activity. We hypothesized that disrupted diurnal variation in clock gene expression in the hippocampus corresponds with loss of normal day/night differences in membrane excitability, synaptic physiology, and cognition. We previously reported disrupted circadian locomotor rhythms and neurophysiological output of the suprachiasmatic nucleus (the primary circadian clock) in Tg-SwDI mice with human amyloid-beta precursor protein mutations. Here, we report that Tg-SwDI mice failed to show day/night differences in a spatial working memory task, unlike wild-type controls that exhibited enhanced spatial working memory at night. Moreover, Tg-SwDI mice had lower levels of Per2, one of the core components of the molecular clock, at both mRNA and protein levels when compared to age-matched controls. Interestingly, we discovered neurophysiological impairments in area CA1 of the Tg-SwDI hippocampus. In controls, spontaneous inhibitory post-synaptic currents (sIPSCs) in pyramidal cells showed greater amplitude and lower inter-event interval during the day than the night. However, the normal day/night differences in sIPSCs were absent (amplitude) or reversed (inter-event interval) in pyramidal cells from Tg-SwDI mice. In control mice, current injection into CA1 pyramidal cells produced more firing during the night than during the day, but no day/night difference in excitability was observed in Tg-SwDI mice. The normal day/night difference in excitability in controls was blocked by GABA receptor inhibition. Together, these results demonstrate that the normal diurnal regulation of inhibitory transmission in the hippocampus is diminished in a mouse model of AD, leading to decreased daytime inhibition onto hippocampal CA1 pyramidal cells. Uncovering disrupted day/night differences in circadian gene regulation, hippocampal physiology, and memory in AD mouse models may provide insight into possible chronotherapeutic strategies to ameliorate Alzheimer's disease symptoms or delay pathological onset.

Time-restricted feeding rescues high-fat-diet-induced hippocampal impairment

Feeding rodents a high-fat diet (HFD) disrupts normal behavioral rhythms, particularly meal timing. Within the brain, mistimed feeding shifts molecular rhythms in the hippocampus and impairs memory. We hypothesize that altered meal timing induced by an HFD leads to cognitive impairment and that restricting HFD access to the "active period" (i.e., night) rescues the normal hippocampal function. In male mice, ad-lib access to an HFD for 20 weeks increased body weight and fat mass, increased daytime meal consumption, reduced hippocampal long-term potentiation (LTP), and eliminated day/night differences in spatial working memory. Importantly, two weeks of time-restricted feeding (TRF) at the end of the chronic HFD protocol rescued spatial working memory and restored LTP magnitude, even though there was no change in body composition and total daily caloric intake. These findings suggest that short-term TRF is an effective mechanism for rescuing HFD-induced impaired cognition and hippocampal function.

Suppression of Circadian Timing and Its Impact on the Hippocampus

In this article, I describe the development of the disruptive phase shift (DPS) protocol and its utility for studying how circadian dysfunction impacts memory processing in the hippocampus. The suprachiasmatic nucleus (SCN) of the Siberian hamster is a labile circadian pacemaker that is easily rendered arrhythmic (ARR) by a simple manipulation of ambient lighting. The DPS protocol uses room lighting to administer a phase-advancing signal followed by a phase-delaying signal within one circadian cycle to suppress clock gene rhythms in the SCN. The main advantage of this model for inducing arrhythmia is that the DPS protocol is non-invasive; circadian rhythms are eliminated while leaving the animals neurologically and genetically intact. In the area of learning and memory, DPS arrhythmia produces much different results than arrhythmia by surgical ablation of the SCN. As I show, SCN ablation has little to no effect on memory. By contrast, DPS hamsters have an intact, but arrhythmic, SCN which produces severe deficits in memory tasks that are accompanied by fragmentation of electroencephalographic theta oscillations, increased synaptic inhibition in hippocampal circuits, and diminished responsiveness to cholinergic signaling in the dentate gyrus of the hippocampus. The studies reviewed here show that DPS hamsters are a promising model for translational studies of adult onset circadian dysfunction in humans.

A local circadian clock for memory?

The master clock, suprachiasmatic nucleus, is believed to control peripheral circadian oscillators throughout the brain and body. However, recent data suggest there is a circadian clock involved in learning and memory, potentially housed in the hippocampus, which is capable of acting independently of the master clock. Curiously, the hippocampal clock appears to be influenced by the master clock and by hippocampal dependent learning, while under certain conditions it may also revert to its endogenous circadian rhythm. Here we propose a mechanism by which the hippocampal clock could locally determine the nature of its entrainment. We introduce a novel theoretical framework, inspired by but extending beyond the hippocampal memory clock, which provides a new perspective on how circadian clocks throughout the brain coordinate their rhythms. Importantly, a local clock for memory would suggest that hippocampal-dependent learning at the same time every day should improve memory, opening up a range of possibilities for non-invasive therapies to alleviate the detrimental effects of circadian rhythm disruption on human health.

The melatonin metabolite N1-acetyl-5-methoxykynuramine facilitates long-term object memory in young and aging mice

Melatonin (MEL) has been reported to enhance cognitive processes, making it a potential treatment for cognitive decline. However, the role of MEL's metabolites, N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) and N1-acetyl-5-methoxykynuramine (AMK), in these effects are unknown. The current study directly investigated the acute effects of systemic MEL, AFMK, and AMK on novel object recognition. We also analyzed MEL, AFMK, and AMK levels in hippocampus and temporal lobe containing the perirhinal cortex following systemic MEL and AMK treatment. AMK administered post-training had a more potent effect on object memory than MEL and AFMK. AMK was also able to rescue age-associated declines in memory impairments when object memory was tested up to 4 days following training. Results from administering AMK at varying times around the training trial and the metabolism time course in brain tissue suggest that AMK's memory-enhancing effects reflect memory consolidation. Furthermore, inhibiting the MEL-to-AMK metabolic pathway disrupted object memory at 24 hours post-training, suggesting that endogenous AMK might play an important role in long-term memory formation. This is the first study to report that AMK facilitates long-term object memory performance in mice, and that MEL crosses the blood-brain barrier and is immediately converted to AMK in brain tissue. Overall, these results support AMK as a potential therapeutic agent to improve or prevent memory decline.

High-Fat and High-Sucrose Diets Impair Time-of-Day Differences in Spatial Working Memory of Male Mice

OBJECTIVE

This study aimed to investigate both the long-term and short-term impacts of high-fat diets (HFD) or high-sucrose diets (HSD) on the normal diurnal pattern of cognitive function, protein expression, and the molecular clock in mice.

METHODS

This study used both 6-month and 4-week feeding strategies by providing male C57BL/6J mice access to either a standard chow, HFD, or HSD. Spatial working memory and synaptic plasticity were assessed both day and night, and hippocampal tissue was measured for changes in NMDA and AMPA receptor subunits (GluN2B, GluA1), as well as molecular clock gene expression.

RESULTS

HFD and HSD both disrupted normal day/night fluctuations in spatial working memory and synaptic plasticity. Mice fed HFD altered their food intake to consume more calories during the day. Both diets disrupted normal hippocampal clock gene expression, and HFD reduced GluN2B levels in hippocampal tissue.

CONCLUSIONS

Taken together, these results suggest that both HFD and HSD induce a loss of day/night performance in spatial working memory and synaptic plasticity as well as trigger a cascade of changes that include disruption to the hippocampal molecular clock.

Circadian Modulation of Neurons and Astrocytes Controls Synaptic Plasticity in Hippocampal Area CA1

Most animal species operate according to a 24-h period set by the suprachiasmatic nucleus (SCN) of the hypothalamus. The rhythmic activity of the SCN modulates hippocampal-dependent memory, but the molecular and cellular mechanisms that account for this effect remain largely unknown. Here, we identify cell-type-specific structural and functional changes that occur with circadian rhythmicity in neurons and astrocytes in hippocampal area CA1. Pyramidal neurons change the surface expression of NMDA receptors. Astrocytes change their proximity to synapses. Together, these phenomena alter glutamate clearance, receptor activation, and integration of temporally clustered excitatory synaptic inputs, ultimately shaping hippocampal-dependent learning in vivo. We identify corticosterone as a key contributor to changes in synaptic strength. These findings highlight important mechanisms through which neurons and astrocytes modify the molecular composition and structure of the synaptic environment, contribute to the local storage of information in the hippocampus, and alter the temporal dynamics of cognitive processing.

Memory and the circadian system: Identifying candidate mechanisms by which local clocks in the brain may regulate synaptic plasticity

The circadian system is an endogenous biological network responsible for coordinating near-24-h cycles in behavior and physiology with daily timing cues from the external environment. In this review, we explore how the circadian system regulates memory formation, retention, and recall. Circadian rhythms in these memory processes may arise through several endogenous pathways, and recent work highlights the importance of genetic timekeepers found locally within tissues, called local clocks. We evaluate the circadian memory literature for evidence of local clock involvement in memory, identifying potential nodes for direct interactions between local clock components and mechanisms of synaptic plasticity. Our discussion illustrates how local clocks may pervasively modulate neuronal plastic capacity, a phenomenon that we designate here as circadian metaplasticity. We suggest that this function of local clocks supports the temporal optimization of memory processes, illuminating the potential for circadian therapeutic strategies in the prevention and treatment of memory impairment.

Interaction Between Circadian Rhythms, Energy Metabolism, and Cognitive Function

The interaction between meal timing and light regulates circadian rhythms in mammals and not only determines the sleep-wake pattern but also the activity of the endocrine system. Related with that, the necessity to fulfill energy needs is a driving force that requires the participation of cognitive skills whose performance has been shown to undergo circadian variations. These facts have led to the concept that cognition and feeding behaviour can be analysed from a chronobiological perspective. In this context, research carried out during the last two decades has evidenced the link between feeding behaviour/nutritional habits and cognitive processes, and has highlighted the impact of circadian disorders on cognitive decline. All that has allowed hypothesizing a tight relationship between nutritional factors, chronobiology, and cognition. In this connection, experimental diets containing elevated amounts of fat and sugar (high-fat diets; HFDs) have been shown to alter in rodents the circadian distribution of meals, and to have a negative impact on cognition and motivational aspects of behaviour that disappear when animals are forced to adhere to a standard temporal eating pattern. In this review, we will present relevant studies focussing on the effect of HFDs on cognitive aspects of behaviour, paying particular attention to the influence that chronobiological alterations caused by these diets may have on hippocampaldependent cognition.

Hippocampal clock regulates memory retrieval via Dopamine and PKA-induced GluA1 phosphorylation

Cognitive performance in people varies according to time-of-day, with memory retrieval declining in the late afternoon-early evening. However, functional roles of local brain circadian clocks in memory performance remains unclear. Here, we show that hippocampal clock controlled by the circadian-dependent transcription factor BMAL1 regulates time-of-day retrieval profile. Inducible transgenic dominant negative BMAL1 (dnBMAL1) expression in mouse forebrain or hippocampus disrupted retrieval of hippocampal memories at Zeitgeber Time 8-12, independently of retention delay, encoding time and Zeitgeber entrainment cue. This altered retrieval profile was associated with downregulation of hippocampal Dopamine-cAMP signaling in dnBMAL1 mice. These changes included decreases in Dopamine Receptors (D1-R and D5-R) and GluA1-S845 phosphorylation by PKA. Consistently, pharmacological activation of cAMP-signals or D1/5Rs rescued impaired retrieval in dnBMAL1 mice. Importantly, GluA1 S845A knock-in mice showed similar retrieval deficits with dnBMAL1 mice. Our findings suggest mechanisms underlying regulation of retrieval by hippocampal clock through D1/5R-cAMP-PKA-mediated GluA1 phosphorylation.

Circadian regulation of membrane physiology in neural oscillators throughout the brain

Twenty-four-hour rhythmicity in physiology and behavior are driven by changes in neurophysiological activity that vary across the light-dark and rest-activity cycle. Although this neural code is most prominent in neurons of the primary circadian pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus, there are many other regions in the brain where region-specific function and behavioral rhythmicity may be encoded by changes in electrical properties of those neurons. In this review, we explore the existing evidence for molecular clocks and/or neurophysiological rhythms (i.e., 24 hr) in brain regions outside the SCN. In addition, we highlight the brain regions that are ripe for future investigation into the critical role of circadian rhythmicity for local oscillators. For example, the cerebellum expresses rhythmicity in over 2,000 gene transcripts, and yet we know very little about how circadian regulation drives 24-hr changes in the neural coding responsible for motor coordination. Finally, we conclude with a discussion of how our understanding of circadian regulation of electrical properties may yield insight into disease mechanisms which may lead to novel chronotherapeutic strategies in the future.

NSP-C contributes to the upregulation of CLOCK/BMAL1-mediated transcription

The bHLH-PAS transcription factors clock circadian regulator (CLOCK) and brain and muscle ARNT-like protein 1 (BMAL1) play essential roles in the generation of circadian gene expression rhythms through the activation of E-box-mediated transcription. Importantly, circadian transcriptional rhythms mediated by CLOCK/BMAL1 are observed in peripheral tissues as well as in the suprachiasmatic nucleus and contribute to tissue-specific functions. These findings suggest that CLOCK/BMAL1 have roles in many biological phenomena by interacting with various cellular regulators. In the present study, to understand the mechanisms underlying the multiple functional roles of CLOCK, we tried to identify new proteins that interact with CLOCK using a yeast two-hybrid system. We identified neuroendocrine-specific protein (NSP)-C, which is highly expressed in the brain, as a positive regulator of CLOCK/BMAL1-mediated transcription. We found that NSP-C interacted with CLOCK in mammalian cells. Co-expression of NSP-C with CLOCK/BMAL1 enhanced the transcriptional activation by CLOCK/BMAL1. Furthermore, knockdown of endogenous NSP-C by small interfering RNA (siRNA) suppressed E-box-mediated transcription, while this reduction of transcription was rescued by the expression of NSP-C protected from the action of siRNA. These observations suggest that NSP-C contributes to the upregulation of CLOCK/BMAL1-mediated transcription.

Sleep/Wake Disruption in a Mouse Model of BLOC-1 Deficiency

Mice lacking a functional Biogenesis of Lysosome-related Organelles Complex 1 (BLOC-1), such as those of the pallid line, display cognitive and behavioural impairments reminiscent of those presented by individuals with intellectual and developmental disabilities. Although disturbances in the sleep/wake cycle are commonly lamented by these individuals, the underlying mechanisms, including the possible role of the circadian timing system, are still unknown. In this paper, we have explored sleep/circadian malfunctions and underlying mechanisms in BLOC-1-deficient pallid mice. These mutants exhibited less sleep behaviour in the beginning of the resting phase than wild-type mice with a more broken sleeping pattern in normal light-dark conditions. Furthermore, the strength of the activity rhythms in the mutants were reduced with significantly more fragmentation and lower precision than in age-matched controls. These symptoms were accompanied by an abnormal preference for the open arm in the elevated plus maze in the day and poor performance in the novel object recognition at night. At the level of the central circadian clock (the suprachiasmatic nucleus, SCN), loss of BLOC-1 caused subtle morphological changes including a larger SCN and increased expression of the relative levels of the clock gene Per2 product during the day but did not affect the neuronal activity rhythms. In the hippocampus, the pallid mice presented with anomalies in the cytoarchitecture of the Dentate Gyrus granule cells, but not in CA1 pyramidal neurones, along with altered PER2 protein levels as well as reduced pCREB/tCREB ratio during the day. Our findings suggest that lack of BLOC-1 in mice disrupts the sleep/wake cycle and performance in behavioural tests associated with specific alterations in cytoarchitecture and protein expression.

Modulation of learning and memory by the genetic disruption of circadian oscillator populations

While a rich literature has documented that the efficiency of learning and memory varies across circadian time, a close survey of that literature reveals extensive heterogeneity in the time of day (TOD) when peak cognitive performance occurs. Moreover, most previous experiments in rodents have not focused on the question of discriminating which memory processes (e.g., working memory, memory acquisition, or retrieval) are modulated by the TOD. Here, we use assays of contextual fear conditioning and spontaneous alternation in WT (C57Bl/6 J) mice to survey circadian modulation of hippocampal-dependent memory at multiple timescales - including working memory (seconds to a few minutes), intermediate-term memory (a delay of thirty minutes), and acquisition and retrieval of long-term memory (a delay of two days). Further, in order to test the relative contributions of circadian timing mechanisms to the modulation of memory, a parallel set of studies were performed in mice lacking clock timing mechanisms. These transgenic mice lacked the essential circadian gene Bmal1, either globally (Bmal1 null) or locally (floxed Bmal1 mice, which lack Bmal1 in excitatory forebrain neurons, e.g. cortical and hippocampal neurons). Here, we show that in WT mice, retrieval (but not working memory, intermediate-term memory, or acquisition of long-term memory) is modulated by TOD. However, transgenic mouse models lacking Bmal1 - both globally, and only in forebrain excitatory neurons - show deficits regardless of the memory process tested (and lack circadian modulation of retrieval). These results provide new clarity regarding the impact of the TOD on hippocampal-dependent memory and support the key role of hippocampal and cortical circadian oscillations in circadian gating of cognition.

Acute Physiology and Neurologic Outcomes after Brain Injury in SCOP/PHLPP1 KO Mice

Suprachiasmatic nucleus circadian oscillatory protein (SCOP) (a.k.a. PHLPP1) regulates long-term memory consolidation in the brain. Using a mouse model of controlled cortical impact (CCI) we tested if (1) brain tissue levels of SCOP/PHLPP1 increase after a traumatic brain injury (TBI), and (2) if SCOP/PHLPP1 gene knockout (KO) mice have improved (or worse) neurologic outcomes. Blood chemistry (pH, pCO₂, pO₂, pSO₂, base excess, sodium bicarbonate, and osmolarity) and arterial pressure (MAP) differed in isoflurane anesthetized WT vs. KOs at baseline and up to 1 h post-injury. CCI injury increased cortical/hippocampal SCOP/PHLPP1 levels in WTs 7d and 14d post-injury. Injured KOs had higher brain tissue levels of phosphorylated AKT (pAKT) in cortex (14d post-injury), and higher levels of phosphorylated MEK (pMEK) in hippocampus (7d and 14d post-injury) and in cortex (7d post-injury). Consistent with an important role of SCOP/PHLPP1 on memory function, injured-KOs had near normal performance on the probe trial of the Morris water maze, whereas injured-WTs were impaired. CA1/CA3 hippocampal survival was lower in KOs vs. WTs 24 h post-injury but equivalent by 7d. No difference in 21d cortical lesion volume was detected. SCOP/PHLPP1 overexpression in cultured rat cortical neurons had no effect on 24 h cell death after a mechanical stretch-injury.

miR-132 couples the circadian clock to daily rhythms of neuronal plasticity and cognition

The microRNA miR-132 serves as a key regulator of a wide range of plasticity-associated processes in the central nervous system. Interestingly, miR-132 expression has also been shown to be under the control of the circadian timing system. This finding, coupled with work showing that miR-132 is expressed in the hippocampus, where it influences neuronal morphology and memory, led us to test the idea that daily rhythms in miR-132 within the forebrain modulate cognition as a function of circadian time. Here, we show that hippocampal miR-132 expression is gated by the time-of-day, with peak levels occurring during the circadian night. Further, in miR-132 knockout mice and in transgenic mice, where miR-132 is constitutively expressed under the control of the tetracycline regulator system, we found that time-of-day dependent memory recall (as assessed via novel object location and contextual fear conditioning paradigms) was suppressed. Given that miRNAs exert their functional effects via the suppression of target gene expression, we examined the effects that transgenic miR-132 manipulations have on MeCP2 and Sirt1-two miR-132 targets that are associated with neuronal plasticity and cognition. In mice where miR-132 was either knocked out, or transgenically expressed, rhythmic expression of MeCP2 and Sirt1 was suppressed. Taken together, these results raise the prospect that miR-132 serves as a key route through which the circadian timing system imparts a daily rhythm on cognitive capacity.

Circadian Regulation of Hippocampal-Dependent Memory: Circuits, Synapses, and Molecular Mechanisms

Circadian modulation of learning and memory efficiency is an evolutionarily conserved phenomenon, occurring in organisms ranging from invertebrates to higher mammalian species, including humans. While the suprachiasmatic nucleus (SCN) of the hypothalamus functions as the master mammalian pacemaker, recent evidence suggests that forebrain regions, including the hippocampus, exhibit oscillatory capacity. This finding, as well as work on the cellular signaling events that underlie learning and memory, has opened promising new avenues of investigation into the precise cellular, molecular, and circuit-based mechanisms by which clock timing impacts plasticity and cognition. In this review, we examine the complex molecular relationship between clock timing and memory, with a focus on hippocampal-dependent tasks. We evaluate how the dysregulation of circadian timing, both at the level of the SCN and at the level of ancillary forebrain clocks, affects learning and memory. Further, we discuss experimentally validated intracellular signaling pathways (e.g., ERK/MAPK and GSK3β) and potential cellular signaling mechanisms by which the clock affects learning and memory formation. Finally, we examine how long-term potentiation (LTP), a synaptic process critical to the establishment of several forms of memory, is regulated by clock-gated processes.

Stereotaxic Surgery for Suprachiasmatic Nucleus Lesions in Mice

Site-specific lesions are invaluable methods for investigating the function of brain regions within the central nervous system and can be used to study neural mechanisms of behaviors. Precise stereotaxic surgery is required to lesion small regions of the brain such as the suprachiasmatic nucleus (SCN), which harbors the master circadian clock. In this protocol, we describe stereotaxic surgery optimized for bilateral lesion of the mouse SCN by loading electric current. Success of the SCN lesion is verified histologically and behaviorally by monitoring arrhythmic locomotor activity. The SCN-lesioned mouse allows for the evaluation of behavioral, biochemical, and physiological consequences of ablation of the master circadian clock.

Ras Activity Tunes the Period and Modulates the Entrainment of the Suprachiasmatic Clock

The small GTPase Ras is a universal eukaryotic cytoplasmic membrane-anchored protein, which regulates diverse downstream signal transduction pathways that play an important role in the proper functioning of neurons. Ras activity is a central regulator of structural and functional synaptic plasticity in the adult nervous system, where it channels neuronal responses to various extracellular cues allowing the organism to adapt to complex environmental stimuli. The suprachiasmatic nucleus (SCN) is the principle pacemaker of the circadian clock, and the circadian and photic regulation of Ras activity in the SCN is an important modulator of the clockwork. We have generated transgenic mouse expressing constitutively active V12-H-Ras selectively in neurons via a synapsin I promoter (synRas mice), which serves as a suitable model to study the role of neuronal Ras signaling. Modulation of Ras activity affects ERK1,2/CREB signaling and glycogen synthase kinase-3 beta expression in the SCN, which in turn modify the photoentrainment of the clock and the fine tuning the circadian period length. The main focus of this review is to offer an overview of the function of Ras signaling in the circadian rhythm and its potential role in learning and memory consolidation.

Fine-Tuning Circadian Rhythms: The Importance of Bmal1 Expression in the Ventral Forebrain

Although, the suprachiasmatic nucleus (SCN) of the hypothalamus acts as the central clock in mammals, the circadian expression of clock genes has been demonstrated not only in the SCN, but also in peripheral tissues and brain regions outside the SCN. However, the physiological roles of extra-SCN circadian clocks in the brain remain largely elusive. In response, we generated Nkx2.1-Bmal1^−/− mice in which Bmal1, an essential clock component, was genetically deleted specifically in the ventral forebrain, including the preoptic area, nucleus of the diagonal band, and most of the hypothalamus except the SCN. In these mice, as expected, PER2::LUC oscillation was drastically attenuated in the explants of mediobasal hypothalamus, whereas it was maintained in those of the SCN. Although, Nkx2.1-Bmal1^−/− mice were rhythmic and nocturnal, they showed altered patterns of locomotor activity during the night in a 12:12-h light:dark cycle and during subjective night in constant darkness. Control mice were more active during the first half than the second half of the dark phase or subjective night, whereas Nkx2.1-Bmal1^−/− mice showed the opposite pattern of locomotor activity. Temporal patterns of sleep-wakefulness and feeding also changed accordingly. Such results suggest that along with mechanisms in the SCN, local Bmal1–dependent clocks in the ventral forebrain are critical for generating precise temporal patterns of circadian behaviors.

SCOP/PHLPP1β in the basolateral amygdala regulates circadian expression of mouse anxiety-like behavior

While disruption of the circadian clock triggers a spectrum of affective abnormalities, how the clock regulates mammalian emotionality remains unclear. Here, we characterized the time-of-day-dependent regulation of mouse anxiety-like behaviors. We show that anxiety-like behaviors are expressed in a circadian manner in mice and demonstrate that the clock machineries in the dorsal telencephalon (dTel) are required for the time-of-day-dependent regulation of anxiety-like behaviors. We identify suprachiasmatic nucleus circadian oscillatory protein (SCOP/PHLPP1β) as an essential intracellular signaling molecule mediating this temporal regulation downstream of the clock. Using viral-mediated, basolateral amygdala (BLA)-specific knockout of Scop, we demonstrate that deletion of SCOP in the BLA exerts anxiolytic effects on the elevated plus maze at early subjective night, thereby blunting the circadian variation in the anxiety-like behavior. We conclude that the circadian expression of SCOP in the BLA plays a key role in generating circadian rhythmicity in the anxiety-like behavior. Our results demonstrate SCOP as a regulator of anxiety-like behaviors and reveal its key roles in the anxiogenic functions of the BLA.