Early age-dependent impairments of context-dependent extinction learning, object recognition, and object-place learning occur in rats

Learning shifts the preferred theta phase of gamma oscillations in CA1

Hippocampal neuronal oscillations reflect different cognitive processes and can therefore be used to dissect the role of hippocampal subfields in learning and memory. In particular, it has been suggested that encoding and retrieval is associated with slow gamma (25-55 Hz) and fast gamma (60-100 Hz) oscillations, respectively, which appear in a nested manner at specific phases of the ongoing theta oscillations (4-12 Hz). However, the relationship between memory demand and the theta phase of gamma oscillations remains unclear. Here, we assessed the theta phase preference of gamma oscillations in the CA1 region, at the starting and junction zones of a T-maze, while rats were learning an appetitive task. We found that the theta phase preference of slow gamma showed a ~180° phase shift when animals switched from novice to skilled performance during task acquisition. This phase-shift was not present at the junction zone, where animals chose a right or left turn within the T-maze, suggesting that a recall/decision process had already taken place at the starting zone. Our findings indicate that slow gamma oscillations support both encoding and retrieval, depending on the theta phase at which they occur. These properties are particularly evident prior to cognitive engagement in an acquired spatial task.

Genetic Depletion of BDNF Impairs Extinction Learning of a Spatial Appetitive Task in the Presence or Absence of the Acquisition Context

Brain derived neurotropic factor (BDNF) supports neuronal survival, growth, and differentiation and is involved in forms of hippocampus-dependent and independent learning, as well as hippocampus-dependent learning. Extinction learning comprises active inhibition of no-longer relevant learned information, in conjunction with a decreased response of a previously learned behavior. It is highly dependent on context, and evidence exists that it requires hippocampal activation. The participation of BDNF in memory processing is experience-dependent. For example, BDNF has been associated with synaptic plasticity needed for spatial learning, and it is involved in acquisition and extinction learning of fear conditioning. However, little is known about its role in spatial appetitive extinction learning. In this study, we evaluated to what extent BDNF contributes to spatial appetitive extinction learning in the presence (ABA) or absence (AAA) of exposure to the acquisition context. Daily training, of BDNF^+/--mice or their wildtype (WT) littermates, to reach acquisition criterion in a T-maze, resulted in a similar performance outcome. However, extinction learning was delayed in the AAA, and impaired in the ABA-paradigm compared to performance in WT littermates. Trial-by-trial learning analysis indicated differences in the integration of the context into extinction learning by BDNF^+/--mice compared to WT littermates. Taken together, these results support an important role for BDNF in processes that relate to information updating and retrieval that in turn are crucial for effective extinction learning.

Rolipram treatment during consolidation ameliorates long-term object location memory in aged male mice

Normal aging is accompanied by cognitive and memory impairments that negatively impact quality of life for the growing elderly population. Hippocampal function is most vulnerable to the deleterious effects of aging, and deficits in hippocampus-dependent memories are common amongst aged individuals. Moreover, signaling networks such as the cAMP/PKA/CREB pathway, which are critical for memory consolidation, are dampened in healthy aged subjects. Phosphodiesterase (PDE) enzymes that break down cAMP are also affected by aging, and increased break down of cAMP by PDEs may contribute to reduced activity of the cAMP/PKA/CREB signaling network in the brain of aged individuals. Here, we report that the PDE4 inhibitor rolipram administered during consolidation of hippocampus-dependent object location memory improves aged-related spatial memory deficits in aged mice.

Functional Compartmentalization of the Contribution of Hippocampal Subfields to Context-Dependent Extinction Learning

During extinction learning (EL), an individual learns that a previously learned behavior no longer fulfills its original purpose, or is no longer relevant. Recent studies have contradicted earlier theories that EL comprises forgetting, or the inhibition of the previously learned behavior, and indicate that EL comprises new associative learning. This suggests that the hippocampus is involved in this process. Empirical evidence is lacking however. Here, we used fluorescence in situ hybridization of somatic immediate early gene (IEG) expression to scrutinize if the hippocampus processes EL. Rodents engaged in context-dependent EL and were also tested for renewal of (the original behavioral response to) a spatial appetitive task in a T-maze. Whereas distal and proximal CA1 subfields processed both EL and renewal, effects in the proximal CA1 were more robust consistent with a role of this subfield in processing context. The lower blade of the dentate gyrus (DG) and the proximal CA3 subfields were particularly involved in renewal. Responses in the distal and proximal CA3 subfields suggest that this hippocampal subregion may also contribute to the evaluation of the reward outcome. Taken together, our findings provide novel and direct evidence for the involvement of distinct hippocampal subfields in context-dependent EL and renewal.

Sex- and age-associated differences in episodic-like memory and transcriptional regulation of hippocampal steroidogenic enzymes in rats

The aim of this study was to evaluate the episodic-like memory (ELM) and the transcriptional regulation of the enzymes involved in hippocampal allopregnanolone synthesis in young adult and middle-aged male and female rats. Young adult males, but not middle-aged ones, showed a good performance in the ELM task. In contrast, neither young nor middle-aged females were able to discriminate the spatial order in which the objects were presented. In females, aging decreased the transcription of steroidogenic-related genes. In addition, the mRNA levels of 5α-reductase-1 were higher and the methylation of its promoter was lower in young adult females than in males, suggesting an epigenetic control. Further studies are needed to establish correlations between ELM and the transcriptional regulation of hippocampal steroidogenic enzymes. Our results contribute to the knowledge of sex differences in gene expression, methylation and memory during aging.

Environmental enrichment improves hippocampal function in aged rats by enhancing learning and memory, LTP, and mGluR5-Homer1c activity

Previous studies from our laboratory have shown that environmental enrichment (EE) in young rats results in improved learning ability and enhanced metabotropic glutamate receptor-dependent long-term potentiation (mGluR-dependent LTP) resulting from sustained activation of p70S6 kinase. Here, we investigated whether 1-month EE is sufficient to improve hippocampus-dependent learning and memory and enhance hippocampal LTP in 23-24 month-old Fischer 344 male rats. Aged rats were housed in environmentally enriched, socially enriched, or standard housing conditions. We find that aged rats exposed to 1-month of EE demonstrate enhanced learning and memory relative to standard housed controls when tested in the Morris water maze and novel object recognition behavioral tasks. Furthermore, we find that environmentally enriched rats perform significantly better than socially enriched or standard housed rats in the radial-arm water maze and display enhanced mGluR5-dependent hippocampal LTP. Enhanced hippocampal function results from activity-dependent increases in the levels of mGluR5, Homer1c, and phospho-p70S6 kinase. These findings demonstrate that a short exposure of EE to aged rats can have significant effects on hippocampal function.

A chronic increase of corticosterone age-dependently reduces systemic DNA damage from oxidation in rats

Stress and depression are associated with an acceleration of brain and bodily aging; effects which have been attributed to chronic elevations of glucocorticoids. We tested the hypothesis that a three week administration of stress-associated levels of corticosterone (CORT, the principal rodent glucocorticoid) would increase systemic and CNS DNA and RNA damage from oxidation; a phenomenon known to be centrally involved in the aging process. We also hypothesized that older individuals would be more sensitive to this effect and that the chronic CORT administration would exacerbate age-related memory decline. Young and old male Sprague-Dawley rats were non-invasively administered CORT by voluntary ingestion of nut paste containing either CORT (25mg/kg) or vehicle for a total of 22 days. CORT increased the 24h urinary excretion of the hormone to the levels previously observed after experimental psychological stress and caused a downregulation of the glucocorticoid receptor in the CA1 area of the hippocampus. Contrary to our hypothesis, 24h excretion of 8-oxodG/8-oxoGuo (markers of DNA/RNA damage from oxidation) was reduced in CORT-treated young animals, whereas old animals showed no significant differences. In old animals, CORT caused a borderline significant reduction of RNA oxidation in CNS, which was paralleled by a normalization of performance in an object location memory test. To our knowledge, this is the first demonstration that chronic stress-associated levels of CORT can reduce nucleic acid damage from oxidation. These findings contradict the notion of elevated CORT as a mediator of the accelerated aging observed in stress and depression.

Loss of Catecholaminergic Neuromodulation of Persistent Forms of Hippocampal Synaptic Plasticity with Increasing Age

Neuromodulation by means of the catecholaminergic system is a key component of motivation-driven learning and behaviorally modulated hippocampal synaptic plasticity. In particular, dopamine acting on D1/D5 receptors and noradrenaline acting on beta-adrenergic receptors exert a very potent regulation of forms of hippocampal synaptic plasticity that last for very long-periods of time (>24 h), and occur in conjunction with novel spatial learning. Antagonism of these receptors not only prevents long-term potentiation (LTP) and long-term depression (LTD), but prevents the memory of the spatial event that, under normal circumstances, leads to the perpetuation of these plasticity forms. Spatial learning behavior that normally comes easily to rats, such as object-place learning and spatial reference learning, becomes increasingly impaired with aging. Middle-aged animals display aging-related deficits of specific, but not all, components of spatial learning, and one possibility is that this initial manifestation of decrements in learning ability that become apparent in middle-age relate to changes in motivation, attention and/or the regulation by neuromodulatory systems of these behavioral states. Here, we compared the regulation by dopaminergic D1/D5 and beta-adrenergic receptors of persistent LTP in young (2-4 month old) and middle-aged (8-14 month old) rats. We observed in young rats, that weak potentiation that typically lasts for ca. 2 h could be strengthened into persistent (>24 h) LTP by pharmacological activation of either D1/D5 or beta-adrenergic receptors. By contrast, no such facilitation occurred in middle-aged rats. This difference was not related to an ostensible learning deficit: a facilitation of weak potentiation into LTP by spatial learning was possible both in young and middle-aged rats. It was also not directly linked to deficits in LTP: strong afferent stimulation resulted in equivalent LTP in both age groups. We postulate that this change in catecholaminergic control of synaptic plasticity that emerges with aging, does not relate to a learning deficit per se, rather it derives from an increase in behavioral thresholds for novelty and motivation that emerge with increasing age that impact, in turn, on learning efficacy.

Involvement of Dopamine D1/D5 and D2 Receptors in Context-Dependent Extinction Learning and Memory Reinstatement

Dopamine contributes to the regulation of higher order information processing and executive control. It is important for memory consolidation processes, and for the adaptation of learned responses based on experience. In line with this, under aversive learning conditions, application of dopamine receptor antagonists prior to extinction result in enhanced memory reinstatement. Here, we investigated the contribution of the dopaminergic system to extinction and memory reinstatement (renewal) of an appetitive spatial learning task in rodents. Rats were trained for 3 days in a T-maze (context "A") to associate a goal arm with a food reward, despite low reward probability (acquisition phase). On day 4, extinction learning (unrewarded) occurred, that was reinforced by a context change ("B"). On day 5, re-exposure to the (unrewarded) "A" context took place (renewal of context "A", followed by extinction of context "A"). In control animals, significant extinction occurred on day 4, that was followed by an initial memory reinstatement (renewal) on day 5, that was, in turn, succeeded by extinction of renewal. Intracerebral treatment with a D1/D5-receptor antagonist prior to the extinction trials, elicited a potent enhancement of extinction in context "B". By contrast, a D1/D5-agonist impaired renewal in context "A". Extinction in the "A" context on day 5 was unaffected by the D1/D5-ligands. Treatment with a D2-receptor antagonist prior to extinction had no overall effect on extinction in context "B" or renewal in context "A", although extinction of the renewal effect was impaired on day 5, compared to controls. Taken together, these data suggest that dopamine acting on the D1/D5-receptor modulates both acquisition and consolidation of context-dependent extinction. By contrast, the D2-receptor may contribute to context-independent aspects of this kind of extinction learning.

Beta-adrenergic receptors support attention to extinction learning that occurs in the absence, but not the presence, of a context change

The noradrenergic (NA)-system is an important regulator of cognitive function. It contributes to extinction learning (EL), and in disorders where EL is impaired NA-dysfunction has been postulated. We explored whether NA acting on beta-adrenergic-receptors (β-AR), regulates EL that depends on context, but is not fear-associated. We assessed behavior in an "AAA" or "ABA" paradigm: rats were trained for 3 days in a T-maze (context-A) to learn that a reward is consistently found in the goal arm, despite low reward probability. This was followed on day 4 by EL (unrewarded), whereby in the ABA-paradigm, EL was reinforced by a context change (B), and in the AAA-paradigm, no context change occurred. On day 5, re-exposure to the A-context (unrewarded) occurred. Typically, in control "AAA" animals EL occurred on day 4 that progressed further on day 5. In control "ABA" animals, EL also occurred on day 4, followed by renewal of the previously learned (A) behavior on day 5, that was succeeded (on day 5) by extinction of this behavior, as the animals realised that no food reward would be given. Treatment with the β-AR-antagonist, propranolol, prior to EL on day 4, impaired EL in the AAA-paradigm. In the "ABA" paradigm, antagonist treatment on day 4, had no effect on extinction that was reinforced by a context change (B). Furthermore, β-AR-antagonism prior to renewal testing (on day 5) in the ABA-paradigm, resulted in normal renewal behavior, although subsequent extinction of responses during day 5 was prevented by the antagonist. Thus, under both treatment conditions, β-AR-antagonism prevented extinction of the behavior learned in the "A" context. β-AR-blockade during an overt context change did not prevent EL, whereas β-AR were required for EL in an unchanging context. These data suggest that β-AR may support EL by reinforcing attention towards relevant changes in the previously learned experience, and that this process supports extinction learning in constant-context conditions.

Aging accelerates memory extinction and impairs memory restoration in Drosophila

Age-related memory impairment (AMI) is a phenomenon observed from invertebrates to human. Memory extinction is proposed to be an active inhibitory modification of memory, however, whether extinction is affected in aging animals remains to be elucidated. Employing a modified paradigm for studying memory extinction in fruit flies, we found that only the stable, but not the labile memory component was suppressed by extinction, thus effectively resulting in higher memory loss in aging flies. Strikingly, young flies were able to fully restore the stable memory component 3 h post extinction, while aging flies failed to do so. In conclusion, our findings reveal that both accelerated extinction and impaired restoration contribute to memory impairment in aging animals.

The metabotropic glutamate receptor, mGlu5, is required for extinction learning that occurs in the absence of a context change

The metabotropic glutamate (mGlu) receptors and, in particular, mGlu5 are crucially involved in multiple forms of synaptic plasticity that are believed to underlie explicit memory. MGlu5 is also required for information transfer through neuronal oscillations and for spatial memory. Furthermore, mGlu5 is involved in extinction of implicit forms of learning. This places this receptor in a unique position with regard to information encoding. Here, we explored the role of this receptor in context-dependent extinction learning under constant, or changed, contextual conditions. Animals were trained over 3 days to take a left turn under 25% reward probability in a T-maze with a distinct floor pattern (Context A). On Day 4, they experienced either a floor pattern change (Context B) or the same floor pattern (Context A) in the absence of reward. After acquisition of the task, the animals were returned to the maze once more on Day 5 (Context A, no reward). Treatment with the mGlu5 antagonist, 2-methyl-6-(phenylethynyl) pyridine, before maze exposure on Day 4 completely inhibited extinction learning in the AAA paradigm but had no effect in the ABA paradigm. A subsequent return to the original context (A, on Day 5) revealed successful extinction in the AAA paradigm, but impairment of extinction in the ABA paradigm. These data support that although extinction learning in a new context is unaffected by mGlu5 antagonism, extinction of the consolidated context is impaired. This suggests that mGlu5 is intrinsically involved in enabling learning that once-relevant information is no longer valid.