Different time course for the memory facilitating effect of bicuculline in hippocampus, entorhinal cortex, and posterior parietal cortex of rats

Several lines of evidence indicate that gamma-aminobutyric acid (GABA) type A (GABA(A)) receptors regulate memory consolidation. Here we studied the effect on consolidation of the selective antagonist of GABA(A) receptors, bicuculline, given into several regions of the cortex at different times after one-trial step-down inhibitory avoidance (0.5 mA, 2-s footshock). Rats were bilaterally implanted with cannulae aimed at the CA1 region of the dorsal hippocampus, entorhinal cortex or posterior parietal cortex, three areas known to be involved in the memory consolidation of this task. At different times after training, bicuculline (0.5 microg/side) was infused into the above mentioned structures. Bicuculline increased memory retention when administered either immediately or 1.5h after training into CA1, and both immediately and 3h after training in the entorhinal or parietal cortex. Thus, in agreement with previous findings using other drugs, the response was biphasic in these latter structures. This suggests that GABAergic mechanisms normally downregulate, memory processing by inhibiting on-going activities necessary for consolidation at the times in which bicuculline was effective in each structure. Based on previous findings, in the hippocampus, such activity involves a number of receptors and signaling pathways in the first 1.5h after training. In the entorhinal and parietal cortex memory-related activities include the participation of protein kinase A and extracellularly regulated kinase (ERK) twice, right after training and then again 3h later.     

Amnesia by post-training infusion of glutamate receptor antagonists into the amygdala, hippocampus, and entorhinal cortex.

The blockers of glutamate receptors, aminophosphonovaleric acid (AP5) (5.0 micrograms) and cyano-nitroquinoxaline-dione (CNQX) (0.5 microgram), were infused bilaterally into the amygdala, dorsal hippocampus, or entorhinal cortex of rats through indwelling cannulae 0, 90, 180, or 360 min after step-down inhibitory avoidance training. Animals were tested for retention 24 h after training. In the amygdala or hippocampus, AP5 was amnestic when given 0 min after training and CNQX was amnestic when given 0, 90, or 180 min after training. In the entorhinal cortex, AP5 was amnestic when given 90 or 180 min after training and CNQX had no effect. The results suggest that a phenomenon sensitive first to AP5 and then to CNQX in the amygdala and hippocampus, probably long-term potentiation (LTP), is crucial to post-training memory processing. LTP in these two structures could underlie their role in memory consolidation and could explain the late involvement of the entorhinal cortex in post-training memory processing.     

Time-dependent impairment of inhibitory avoidance retention in rats by posttraining infusion of a mitogen-activated protein kinase kinase inhibitor into cortical and limbic structures

Mitogen-activated protein kinase (MAPK) is abundantly expressed in postmitotic neurons of the developed nervous system. MAPK is activated and required for induction of long-term potentiation (LTP) in the CA1 area of the hippocampus, which is blocked by the specific inhibitor of the MAPK kinase, PD 098059. Recently it was demonstrated that MAPK is activated in the hippocampus after training and is necessary for contextual fear conditioning learning. The present work tests the role of the MAPK cascade in step-down inhibitory avoidance (IA) retention. PD 098059 (50 microM) was bilaterally injected (0.5 microl/side) into the CA1 region of the dorsal hippocampus or entorhinal cortex at 0, 90, 180, or 360 min, or into the amygdala or parietal cortex at 0, 180, or 360 min after IA training in rats using a 0.4-mA foot shock. Retention testing was carried out 24 h after training. PD 098059 impaired retention when injected into the dorsal hippocampus at 180 min, but not 0, 90, and 360 min after training. When infused into the entorhinal cortex, PD 098059 was amnestic at 0 and 180 min, but not at 90 and 360 min after training. The MAPKK inhibitor also impairs IA retention when infused into the parietal cortex immediately after training, but not at 180 or 360 min. Infusions performed into amygdala were amnestic at 180 min, but not at 0 and 360 min after training. Our results suggest a time-dependent involvement of the MAPK cascade in the posttraining memory processing of IA; the time dependency is different in the hippocampus, amygdala, entorhinal cortex, or parietal cortex of rats.     

A comparison of the effects of fimbria-fornix, hippocampal, or entorhinal cortex lesions on spatial reference and working memory in rats: short versus long postsurgical recovery period.

Using a radial maze task and different postoperative recovery periods, this experiment assessed and compared the reference and working memory performances of adult Long-Evans male rats subjected to entorhinal cortex, fimbria-fornix, and hippocampus lesions. Sham-operated rats were used as controls. In order to see whether the duration of the postsurgical recovery period would influence acquisition of the complex radial maze task, training began 1 month following surgery (Delay 1) for half the rats in each group, while for the other half training was started 6.5 months following surgery (Delay 2). The results indicated that at both recovery periods the entorhinal cortex lesions failed to affect either working or reference memory in the spatial task. Conversely, both fimbria-fornix and hippocampus lesions impaired both reference and working memory. While the reference memory deficit was generally similar in both fimbria-fornix and hippocampal lesion groups, analysis of the results for working memory indicated that at the longer delay rats with fimbria-fornix lesions were still impaired but in animals that had the hippocampus removed, working memory did not differ from that of controls. These results suggest that there was some recovery in those rats with hippocampal lesions (e.g., on the working memory task) but both hippocampal and fimbria-fornix animals were still impaired compared to controls when training was delayed 6.5 months following the operations.     

Memory-enhancing corticosterone treatment increases amygdala norepinephrine and Arc protein expression in hippocampal synaptic fractions

Considerable evidence indicates that glucocorticoid hormones enhance the consolidation of memory for emotionally arousing events through interactions with the noradrenergic system of the basolateral complex of the amygdala (BLA). We previously reported that intra-BLA administration of a β-adrenoceptor agonist immediately after inhibitory avoidance training enhanced memory consolidation and increased hippocampal expression of the protein product of the immediate early gene activity-regulated cytoskeletal-associated protein (Arc). In the present experiments corticosterone (3 mg/kg, i.p.) was administered to male Sprague-Dawley rats immediately after inhibitory avoidance training to examine effects on long-term memory, amygdala norepinephrine levels, and hippocampal Arc expression. Corticosterone increased amygdala norepinephrine levels 15 min after inhibitory avoidance training, as assessed by in vivo microdialysis, and enhanced memory tested at 48 h. Corticosterone treatment also increased expression of Arc protein in hippocampal synaptic tissue. The elevation in BLA norepinephrine appears to participate in corticosterone-influenced modulation of hippocampal Arc expression as intra-BLA blockade of β-adrenoceptors with propranolol (0.5 μg/0.2 μL) attenuated the corticosterone-induced synaptic Arc expression in the hippocampus. These findings indicate that noradrenergic activity at BLA β-adrenoceptors is involved in corticosterone-induced enhancement of memory consolidation and expression of the synaptic-plasticity-related protein Arc in the hippocampus.     

Post-training intra-basolateral amygdala infusions of norepinephrine block sevoflurane-induced impairment of memory consolidation and activity-regulated cytoskeletal protein expression inhibition in rat hippocampus

Considerable evidence indicates that the noradrenergic system of the basolateral amygdala (BLA) participates in the consolidation of various types of emotionally arousing memories. We previously reported that administration of an anesthetic-dose of sevoflurane immediately after continuous multiple-trail inhibition avoidance (CMIA) training impaired memory consolidation. This experiment investigated whether posttraining noradrenergic activation of the BLA is sufficient to reverse the memory impairing effect of sevoflurane. Adult male Sprague-Dawley rats received bilateral injections of norepinephrine (NE 0.3, 1.0, or 3.0 μg/0.5 μl) or normal saline (NS 0.5 μl) immediately after training in a CMIA paradigm. Subsequently, the rats were exposed to sevoflurane (2% inspired) or air for 2h. Norepinephrine produced a dose-dependent enhancement of memory consolidation on a 24-h retention test. The highest dose of NE tested (3.0 μg/0.5 μl) blocked sevoflurane-induced impairment of memory consolidation and reversed the inhibitory effect of sevoflurane on activity-regulated cytoskeletal protein (Arc) expression in the hippocampus 2h after training. These findings provide evidence that the mechanism mediating the memory-impairing effect of sevoflurane involves a network interaction between the BLA noradrenergic system and modulation of Arc protein expression in the hippocampus.     

Studies on retrograde and anterograde amnesia of olfactory memory after denervation of the hippocampus by entorhinal cortex lesions

The effect of hippocampal denervation on olfactory memory in rats was tested after interrupting the lateral olfactory tract projections at the level of the entorhinal cortex. When lesioned animals were trained to learn new odors, they showed no evidence of retention 3 h after acquisition. These results confirm earlier data on rapid forgetting in rats after hippocampal deafferentation and are in parallel to the anterograde amnesia typically found in humans with hippocampal damage. On the other hand, preoperatively learned information was minimally impaired after hippocampal deafferentation even if it was acquired within less than 1 h before the lesion. This finding differs from reports on humans as well as monkeys with hippocampal damage where memories formed during a critical time span of months or even years before the lesion are found to be impaired. This may suggest that the consolidation process in humans and rodents has different time scales or that the roles of the human and the rat hippocampal structure in memory formation are somewhat different.     

Increasing acetylcholine levels in the hippocampus or entorhinal cortex reverses the impairing effects of septal GABA receptor activation on spontaneous alternation

Intra-septal infusions of the γ-aminobutyric acid (GABA) agonist muscimol impair learning and memory in a variety of tasks. This experiment determined whether hippocampal or entorhinal infusions of the acetylcholinesterase inhibitor physostigmine would reverse such impairing effects on spontaneous alternation performance, a measure of spatial working memory. Male Sprague-Dawley rats were given intra-septal infusions of vehicle or muscimol (1 nmole/0.5 μL) combined with unilateral intra-hippocampal or intra-entorhinal infusions of vehicle or physostigmine (10 μg/μL for the hippocampus; 7.5 μg/μL or 1.875 μg/0.25 μL for the entorhinal cortex). Fifteen minutes later, spontaneous alternation performance was assessed. The results indicated that intra-septal infusions of muscimol significantly decreased percentage-of-alternation scores, whereas intra-hippocampal or intra-entorhinal infusions of physostigmine had no effect. More importantly, intra-hippocampal or intra-entorhinal infusions of physostigmine, at doses that did not influence performance when administered alone, completely reversed the impairing effects of the muscimol infusions. These findings indicate that increasing cholinergic levels in the hippocampus or entorhinal cortex is sufficient to reverse the impairing effects of septal GABA receptor activation and support the hypothesis that the impairing effects of septal GABAergic activity involve cholinergic processes in the hippocampus and the entorhinal cortex.     

Time-courses of Fos expression in rat hippocampus and neocortex following acquisition and recall of a socially transmitted food preference

The present study examined expression of the immediate-early gene, c-Fos, following acquisition, 48-h (recent) recall, and 1-week (remote) recall of a socially transmitted food preference (STFP) in multiple brain regions implicated in learning and memory. In comparisons with controls, trained Long-Evans rats had increased Fos immunoreactivity in the ventral hippocampus following acquisition and recent recall. In the parahippocampal cortices, Fos was increased in the lateral entorhinal cortex after acquisition. In the orbitofrontal cortex, increased Fos immunoreactivity was observed in the lateral orbital cortex following both recent and remote recall and in the ventral orbital cortex following remote recall, indicating a role for the orbitofrontal cortex in the remote recall of STFP memory. In contrast, in the medial prefrontal cortex, increased Fos-ir was found following acquisition in the prelimbic cortex and following recent recall in the prelimbic and infralimbic cortices. No differences in Fos expression were found between trained rats and controls in the dorsal hippocampus, posterior parietal cortex, or amygdala. The present findings support a time-limited role of the hippocampus in the acquisition and recall of STFP memory and implicate neocortical regions involved in STFP acquisition, recent, and remote recall.     

Infusion of protein synthesis inhibitors in the entorhinal cortex blocks consolidation but not reconsolidation of object recognition memory

Memory consolidation and reconsolidation require the induction of protein synthesis in some areas of the brain. Here, we show that infusion of the protein synthesis inhibitors anisomycin, emetine and cycloheximide in the entorhinal cortex immediately but not 180 min or 360 min after training in an object recognition learning task hinders long-term memory retention without affecting short-term memory or behavioral performance. Inhibition of protein synthesis in the entorhinal cortex after memory reactivation involving either a combination of familiar and novel objects or two familiar objects does not affect retention. Our data suggest that protein synthesis in the entorhinal cortex is necessary early after training for consolidation of object recognition memory. However, inhibition of protein synthesis in this cortical region after memory retrieval does not seem to affect the stability of the recognition trace.     

Spatial learning in the rat: impairment induced by the thiol-proteinase inhibitor, leupeptin, and an analysis of [3H]glutamate receptor binding in relation to learning

Rats were given continuous intraventricular infusion of saline or the thiol-proteinase inhibitor leupeptin, via subcutaneously implanted osmotic minipumps, while being trained on a spatial learning water task using spaced trials. Leupeptin caused overnight forgetting during training, but performance eventually reached asymptote in both groups. A retention test conducted 48 h later to assess spatial memory revealed no significant group differences, but did cause, in saline-treated rats only, a disruption of subsequent retraining back to the correct spatial location. The groups showed no differences in Cl-dependent [3H]glutamate receptor binding to hippocampal or entorhinal cortex membranes subsequent to training. In a second experiment, normal rats trained on the same task also showed no differences in Cl-dependent [3H]glutamate binding relative to rats exposed to the water task but given random spatial position training and handled controls. The results are discussed in relation to the hypothesis of Lynch and Baudry (Science (1984) 224, 1057-1063) that a calcium-dependent thiol proteinase is involved in memory formation through its ability to modify glutamate receptor distribution and dendritic spine shape.     

On the role of hippocampal protein synthesis in the consolidation and reconsolidation of object recognition memory

Upon retrieval, consolidated memories are again rendered vulnerable to the action of metabolic blockers, notably protein synthesis inhibitors. This has led to the hypothesis that memories are reconsolidated at the time of retrieval, and that this depends on protein synthesis. Ample evidence indicates that the hippocampus plays a key role both in the consolidation and reconsolidation of different memories. Despite this fact, at present there are no studies about the consequences of hippocampal protein synthesis inhibition in the storage and post-retrieval persistence of object recognition memory. Here we report that infusion of the protein synthesis inhibitor anisomycin in the dorsal CA1 region immediately or 180 min but not 360 min after training impairs consolidation of long-term object recognition memory without affecting short-term memory, exploratory behavior, anxiety state, or hippocampal functionality. When given into CA1 after memory reactivation in the presence of familiar objects, ANI did not affect further retention. However, when administered into CA1 immediately after exposing animals to a novel and a familiar object, ANI impaired memory of both of them. The amnesic effect of ANI was long-lasting, did not happen after exposure to two novel objects, following exploration of the context alone, or in the absence of specific stimuli, suggesting that it was not reversible but was contingent on the reactivation of the consolidated trace in the presence of a salient, behaviorally relevant novel cue. Our results indicate that hippocampal protein synthesis is required during a limited post-training time window for consolidation of object recognition memory and show that the hippocampus is engaged during reconsolidation of this type of memory, maybe accruing new information into the original trace.     

Posttraining activation of CB1 cannabinoid receptors in the CA1 region of the dorsal hippocampus impairs object recognition long-term memory

Evidence indicates that brain endocannabinoids are involved in memory processing. However, the participation of CB1 and CB2 cannabinoid receptors in recognition memory has not been yet conclusively determined. Therefore, we evaluated the effect of the posttraining activation of hippocampal cannabinoid receptors on the consolidation of object recognition memory. Rats with infusion cannulae stereotaxically aimed to the CA1 region of the dorsal hippocampus were trained in an object recognition learning task involving exposure to two different stimulus objects. Memory retention was assessed at different times after training. In the test sessions, one of the objects presented during training was replaced by a novel one. When infused in the CA1 region immediately after training, the non-selective cannabinoid receptor agonist WIN-55,212-2 and the endocannabinoid membrane transporter inhibitor VDM-11 blocked long-term memory retention in a dose-dependent manner without affecting short-term memory, exploratory behavior, anxiety state or the functionality of the hippocampus. The amnesic effect of WIN-55,212-2 and VDM-11 was not due to state-dependency and was completely reversed by co-infusion of the CB1 receptor antagonist AM-251 and mimicked by the CB1 receptor agonist ACEA but not by the CB2 receptor agonists JWH-015 and palmitoylethanolamide. Our data indicate that activation of hippocampal CB1 receptors early after training hampers consolidation of object recognition memory.     

Bilateral entorhinal cortex lesions impair acquisition of delayed spatial alternation in rats

Entorhinal cortex lesions induce significant reorganization of several homotypic and heterotypic inputs to the hippocampus. This investigation determined whether surviving heterotypic inputs after bilateral entorhinal lesions would support the acquisition of a learned alternation task. Rats with entorhinal lesions or sham operations were trained to acquire a spatial alternation task. Although the sham-operated rats acquired the task within about 3 weeks postsurgery, rats with bilateral entorhinal lesions failed to learn the task after 12 consecutive weeks of training despite heterotypic sprouting of the cholinergic septodentate pathway and the expansion of the commissural/associational fiber plexus within the dentate gyrus. Thus, heterotypic sprouting failed to ameliorate significantly the effects of bilateral entorhinal lesions. Rather, entorhinal lesions produced a persistent impairment of spatial memory, characterized by a mixture of random error production and perseverative responding.     

Systemic and intrahippocampal administrations of the glucocorticoid receptor antagonist RU38486 impairs fear memory reconsolidation in rats

Reconsolidation is the process by which previously consolidated memories are stabilized after retrieval. Several lines of evidence indicate that glucocorticoids modulate distinct phases of learning and memory. These effects are considered to be mediated by mineralocorticoid receptors and glucocorticoid receptors (GRs), which display a high concentration and distinct distribution in the hippocampus. The role of glucocorticoid system in fear memory reconsolidation is the subject of some controversy. Moreover, we found no studies that assessed the role of hippocampal GRs in fear memory reconsolidation. Here, we investigated the effect of GR blockade on fear memory reconsolidation in rats. Rats were trained and tested in an inhibitory avoidance task. Intrahippocampal or systemic administration of the GR antagonist RU38486 immediately following memory reactivation produced a deficit in post-retrieval long-term memory that persisted over test sessions, and memory did not re-emerge following a footshock reminder. These results indicate that hippocampal GRs are required for reconsolidation of fear-based memory.     

Inverse temporal contributions of the dorsal hippocampus and medial prefrontal cortex to the expression of long-term fear memories

Retrograde amnesia following disruptions of hippocampal function is often temporally graded, with recent memories being more impaired. Evidence supports the existence of one or more neocortical long-term memory storage/retrieval site(s). Neurotoxic lesions of the medial prefrontal cortex (mPFC) or the dorsal hippocampus (DH) were made 1 day or 200 days following trace fear conditioning. Recently encoded trace fear memories were most disrupted by DH lesions, while remotely encoded trace and contextual memories were most disrupted by mPFC lesions. These data strongly support the consolidation theory of hippocampus function and implicate the mPFC as a site of long-term memory storage/retrieval.     

Differential roles of hippocampal metabotropic glutamate receptors 1 and 5 in inhibitory avoidance learning

Group I metabotropic glutamate receptors (mGlu1 and 5) have been implicated in synaptic plasticity and learning and memory. However, much of our understanding of how these receptors in different brain regions contribute to distinct memory stages in different learning tasks remains incomplete. The present study investigated the effects of the mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and mGlu1 receptor antagonist, (S)-(+)-alpha-amino-4-carboxy-2-methylbenzene-acetic acid (LY 367385) in the dorsal hippocampus on the consolidation and extinction of memory for inhibitory avoidance learning. Male, Sprague-Dawley rats were trained in a single-trial step-down inhibitory avoidance task. MPEP, LY 367385 or saline were infused bilaterally into the CA1 region immediately after training or immediately after the first retention test which was given 24h after training. Rats receiving MPEP (1.5 or 5.0 microg/side) or LY 367385 (0.7 or 2.0 microg/side) infusion exhibited a dose-dependent decrease in retention when tested 24h later. MPEP was ineffective while LY 367385 significantly attenuated extinction when injected after the first retention test using an extinction procedure. These findings indicate a selective participation of hippocampal group I mGlu receptors in memory processing in this task.     

The temporal–hippocampal region and retention: The role of temporo-entorhinal connections in rats

Results from previous studies suggest that the entorhinal cortex may be involved in mnemonic processes. The present study was carried out to investigate whether disruption of fibre connections between the temporal cortex and lateral entorhinal area may impair retention of a pre-operatively acquired simultaneous brightness discrimination task. The lesion resulted in a severe impairment in retaining the discrimination task (Experiment 1). The retention deficit could not be traced into the hippocampal formation by making perforant path lesions or hippocampal lesions (Experiment 2). The results indicate that the lateral entorhinal cortex is more crucial for reference memory than the hippocampal formation.     

Growth points in research on memory and hippocampus.

We present an overview of two of our on-going projects relating processes in the hippocampus to memory. We are trying to understand why retrograde amnesia occurs after damage to the hippocampus. Our experiments establish the generality of several new retrograde amnesia phenomena that are at odds with the consensus view of the role of the hippocampus in memory. We show in many memory tasks that complete damage to the hippocampus produces retrograde amnesia that is equivalent for recent and remote memories. Retrograde amnesia affects a much wider range of memory tasks than anterograde amnesia. Normal hippocampal processes can interfere with retention of a long-term memory stored outside the hippocampus. We conclude that the hippocampus competes with nonhippocampal systems during memory encoding and retrieval. Finally, we outline a project to understand and manipulate adult hippocampal neurogenesis in order to repair damaged hippocampal circuitry to recover lost cognitive functions.     

The temporal context model in spatial navigation and relational learning: toward a common explanation of medial temporal lobe function across domains

The medial temporal lobe (MTL) has been studied extensively at all levels of analysis, yet its function remains unclear. Theory regarding the cognitive function of the MTL has centered along 3 themes. Different authors have emphasized the role of the MTL in episodic recall, spatial navigation, or relational memory. Starting with the temporal context model (M. W. Howard & M. J. Kahana, 2002a), a distributed memory model that has been applied to benchmark data from episodic recall tasks, the authors propose that the entorhinal cortex supports a gradually changing representation of temporal context and the hippocampus proper enables retrieval of these contextual states. Simulation studies show this hypothesis explains the firing of place cells in the entorhinal cortex and the behavioral effects of hippocampal lesion in relational memory tasks. These results constitute a first step toward a unified computational theory of MTL function that integrates neurophysiological, neuropsychological, and cognitive findings.