Food restriction increases long-term memory persistence in adult or aged mice

Photoperiod, food restriction and memory for objects and places in mice

The suprachiasmatic nucleus (SCN) contains a population of cell-autonomous circadian oscillators essential for entrainment to daily light-dark (LD) cycles. Synchrony among SCN oscillators is modified by photoperiod and determines functional properties of SCN clock cycling, including its amplitude, phase angle of entrainment, and free running periodicity (τ). For many species, encoding of daylength in SCN output is critical for seasonal regulation of metabolism and reproduction. C57BL/6 mice do not show seasonality in these functions, yet do show photoperiodic modulation of SCN clock output. The significance of this for brain systems and functions downstream from the SCN in these species is largely unexplored. C57BL/6 mice housed in a long-day photoperiod have been reported to perform better on tests of object, spatial and fear memory compared to mice in a standard 12 h photoperiod. We previously reported that encoding of photoperiod in SCN output, evident in τ in constant dark (DD), can be blocked by limiting food access to a 4 h mealtime in the light period. To determine whether this might also block the effect of long days on memory, mice entrained to 18 h:6 h (L18) or 6 h:18 h (L6) LD cycles were tested for 24 h object memory (novel object preference, NOP) and spatial working memory (Y-maze spontaneous alternation, SA), at 4 times of day, first with food available ad libitum and then during weeks 5-8 of daytime restricted feeding. Photoperiod modified τ as expected, but did not affect performance on the NOP and SA tests, either before or during restricted feeding. NOP performance did improve in the restricted feeding condition in both photoperiods, eliminating a weak time of day effect evident with food available ad-libitum. These results highlight benefits of restricted feeding on cognitive function, and suggest a dose-response relationship between photoperiod and memory, with no benefits at daylengths up to 18 h.

The caloric value of food intake structurally adjusts a neuronal mushroom body circuit mediating olfactory learning in Drosophila

Associative learning enables the adaptive adjustment of behavioral decisions based on acquired, predicted outcomes. The valence of what is learned is influenced not only by the learned stimuli and their temporal relations, but also by prior experiences and internal states. In this study, we used the fruit fly Drosophila melanogaster to demonstrate that neuronal circuits involved in associative olfactory learning undergo restructuring during extended periods of low-caloric food intake. Specifically, we observed a decrease in the connections between specific dopaminergic neurons (DANs) and Kenyon cells at distinct compartments of the mushroom body. This structural synaptic plasticity was contingent upon the presence of allatostatin A receptors in specific DANs and could be mimicked optogenetically by expressing a light-activated adenylate cyclase in exactly these DANs. Importantly, we found that this rearrangement in synaptic connections influenced aversive, punishment-induced olfactory learning but did not impact appetitive, reward-based learning. Whether induced by prolonged low-caloric conditions or optogenetic manipulation of cAMP levels, this synaptic rearrangement resulted in a reduction of aversive associative learning. Consequently, the balance between positive and negative reinforcing signals shifted, diminishing the ability to learn to avoid odor cues signaling negative outcomes. These results exemplify how a neuronal circuit required for learning and memory undergoes structural plasticity dependent on prior experiences of the nutritional value of food.

Blueprints for measuring natural behavior

Until recently laboratory tasks for studying behavior were highly artificial, simplified, and designed without consideration for the environmental or social context. Although such an approach offers good control over behavior, it does not allow for researching either voluntary responses or individual differences. Importantly for neuroscience studies, the activity of the neural circuits involved in producing unnatural, artificial behavior is variable and hard to predict. In addition, different ensembles may be activated depending on the strategy the animal adopts to deal with the spurious problem. Thus, artificial and simplified tasks based on responses, which do not occur spontaneously entail problems with modeling behavioral impairments and underlying brain deficits. To develop valid models of human disorders we need to test spontaneous behaviors consistently engaging well-defined, evolutionarily conserved neuronal circuits. Such research focuses on behavioral patterns relevant for surviving and thriving under varying environmental conditions, which also enable high reproducibility across different testing settings.

The medial prefrontal cortex - hippocampus circuit that integrates information of object, place and time to construct episodic memory in rodents: Behavioral, anatomical and neurochemical properties

Rats and mice have been demonstrated to show episodic-like memory, a prototype of episodic memory, as defined by an integrated memory of the experience of an object or event, in a particular place and time. Such memory can be assessed via the use of spontaneous object exploration paradigms, variably designed to measure memory for object, place, temporal order and object-location inter-relationships. We review the methodological properties of these tests, the neurobiology about time and discuss the evidence for the involvement of the medial prefrontal cortex (mPFC), entorhinal cortex (EC) and hippocampus, with respect to their anatomy, neurotransmitter systems and functional circuits. The systematic analysis suggests that a specific circuit between the mPFC, lateral EC and hippocampus encodes the information for event, place and time of occurrence into the complex episodic-like memory, as a top-down regulation from the mPFC onto the hippocampus. This circuit can be distinguished from the neuronal component memory systems for processing the individual information of object, time and place.

Fasting enhances extinction retention and prevents the return of fear in humans

Fear is prone to return following extinction that is the basis of exposure therapy for fear-related disorders. Manipulations that enhance the extinction process can be beneficial for treatment. Animal studies have shown that fasting or caloric restriction can enhance extinction and inhibit the return of fear. The present study examined the effects of fasting on fear acquisition, extinction, and the return of fear in humans. One hundred and twenty-five male participants were randomized into a fasting group and food group and exposed to a Pavlovian fear conditioning paradigm. Changes in plasma cortisol and ghrelin levels were examined using enzyme-linked immunosorbent assays. One-night fasting had no effect on fear acquisition but enhanced fear extinction retention and prevented the return of fear, and this effect persisted for at least 6 months. This procedure was also effective for remote fear memory. Plasma ghrelin levels were elevated after fasting and had a negative relationship with the fear response in spontaneous recovery test. However, overnight fasting did not affect cortisol levels. These findings indicate that fasting enhances extinction retention and prevents the return of fear, without influencing fear memory formation. We propose that this novel procedure may open new avenues for promoting extinction-based therapies for fear-related disorders.

Youthful systemic milieu alleviates renal ischemia-reperfusion injury in elderly mice

The incidence of acute kidney injury (AKI) is high in elderly people, and is difficult to prevent and treat. One of its major causes is renal ischemia-reperfusion injury (IRI). A young systemic environment may prevent the senescence of old organs. However, it is unknown whether a young milieu may reduce renal IRI in the elderly. To examine this question, bilateral renal IRI was induced in old (24 months) mice three weeks after parabiosis model establishment. At 24 hours after IRI, compared to old wild-type mice, the old mice with IRI had significantly damaged renal histology, decreased renal function, increased oxidative stress, inflammation, and apoptosis. However, there was no increase in autophagy. Compared to old mice with IRI, old-old parabiosis mice with IRI did not show differences in renal histological damage, oxidative stress, inflammation, apoptosis, or autophagy, but did exhibit improved renal function. Compared to the old-old parabiosis mice with IRI, the old mice with IRI in the young (12 week)-old parabiosis showed less renal histological injury and better renal function. Renal oxidative stress, inflammation, and apoptosis were significantly decreased, and autophagy was significantly increased. Thus, a youthful systemic milieu may decrease oxidative stress, inflammation, and apoptosis, and increase autophagy in old mice with IRI. These effects ameliorated IRI injuries in old mice. Our study provides new ideas for effectively preventing and treating AKI in the elderly.

Motivational state, reward value, and Pavlovian cues differentially affect skilled forelimb grasping in rats

Motor skills represent high-precision movements performed at optimal speed and accuracy. Such motor skills are learned with practice over time. Besides practice, effects of motivation have also been shown to influence speed and accuracy of movements, suggesting that fast movements are performed to maximize gained reward over time as noted in previous studies. In rodents, skilled motor performance has been successfully modeled with the skilled grasping task, in which animals use their forepaw to grasp for sugar pellet rewards through a narrow window. Using sugar pellets, the skilled grasping task is inherently tied to motivation processes. In the present study, we performed three experiments modulating animals’ motivation during skilled grasping by changing the motivational state, presenting different reward value ratios, and displaying Pavlovian stimuli. We found in all three studies that motivation affected the speed of skilled grasping movements, with the strongest effects seen due to motivational state and reward value. Furthermore, accuracy of the movement, measured in success rate, showed a strong dependence on motivational state as well. Pavlovian cues had only minor effects on skilled grasping, but results indicate an inverse Pavlovian-instrumental transfer effect on movement speed. These findings have broad implications considering the increasing use of skilled grasping in studies of motor system structure, function, and recovery after injuries.

Aging and brain rejuvenation as systemic events

The effects of aging were traditionally thought to be immutable, particularly evident in the loss of plasticity and cognitive abilities occurring in the aged central nervous system (CNS). However, it is becoming increasingly apparent that extrinsic systemic manipulations such as exercise, caloric restriction, and changing blood composition by heterochronic parabiosis or young plasma administration can partially counteract this age-related loss of plasticity in the aged brain. In this review, we discuss the process of aging and rejuvenation as systemic events. We summarize genetic studies that demonstrate a surprising level of malleability in organismal lifespan, and highlight the potential for systemic manipulations to functionally reverse the effects of aging in the CNS. Based on mounting evidence, we propose that rejuvenating effects of systemic manipulations are mediated, in part, by blood-borne ‘pro-youthful’ factors. Thus, systemic manipulations promoting a younger blood composition provide effective strategies to rejuvenate the aged brain. As a consequence, we can now consider reactivating latent plasticity dormant in the aged CNS as a means to rejuvenate regenerative, synaptic, and cognitive functions late in life, with potential implications even for extending lifespan.