Effects of post-training infusions of a mitogen-activated protein kinase kinase inhibitor into the hippocampus or entorhinal cortex on short- and long-term retention of inhibitory avoidance

Involvement of kinases in memory consolidation of inhibitory avoidance training

The inhibitory avoidance (IA) task is a paradigm widely used to investigate the molecular and cellular mechanisms involved in the formation of long-term memory of aversive experiences. In this review, we discuss studies on different brain structures in rats associated with memory consolidation, such as the hippocampus, striatum, and amygdala, as well as some cortical areas, including the insular, cingulate, entorhinal, parietal and prefrontal cortex. These studies have shown that IA training triggers the release of neurotransmitters, hormones, growth factors, etc., that activate intracellular signaling pathways related to protein kinases, which induce intracellular non-genomic changes or transcriptional mechanisms in the nucleus, leading to the synthesis of proteins. We have summarized the temporal dynamics and crosstalk among protein kinase A, protein kinase C, mitogen activated protein kinase, extracellular-signal-regulated kinase, and Ca2+/calmodulin-dependent protein kinase II described in the hippocampus. Protein kinase activity has been associated with structural changes and synaptic strengthening, resulting in memory storage. However, little is known about the molecular mechanisms involved in intense IA training, which protects memory from typical amnestic treatments, such as protein synthesis inhibitors, and induces increased spinogenesis, suggesting an unexplored mechanism independent of the genomic pathway. This highly emotional experience causes an extinction-resistant memory, as has been observed in some pathological states such as post-traumatic stress disorder. We propose that the changes in spinogenesis observed after intense IA training could be generated by protein kinases via non-genomic pathways.

Molecular insights from the crab Neohelice memory model

Memory acquisition, formation and maintenance depend on synaptic post-translational machinery and regulation of gene expression triggered by several transduction pathways. In turns, these processes lead to stabilization of synaptic modifications in neurons in the activated circuits. In order to study the molecular mechanisms involved in acquisition and memory, we have taken advantage of the context-signal associative learning and, more recently, the place preference task, of the crab Neohelice granulata. In this model organism, we studied several molecular processes, including activation of extracellular signal-regulated kinase (ERK) and the nuclear factor kappa light chain enhancer of activated B cells (NF-κB) transcription factor, involvement of synaptic proteins such as NMDA receptors and neuroepigenetic regulation of gene expression. All these studies allowed description of key plasticity mechanisms involved in memory, including consolidation, reconsolidation and extinction. This article is aimed at review the most salient findings obtained over decades of research in this memory model.

Contributions of extracellular-signal regulated kinase 1/2 activity to the memory trace

The ability to learn from experience and consequently adapt our behavior is one of the most fundamental capacities enabled by complex and plastic nervous systems. Next to cellular and systems-level changes, learning and memory formation crucially depends on molecular signaling mechanisms. In particular, the extracellular-signal regulated kinase 1/2 (ERK), historically studied in the context of tumor growth and proliferation, has been shown to affect synaptic transmission, regulation of neuronal gene expression and protein synthesis leading to structural synaptic changes. However, to what extent the effects of ERK are specifically related to memory formation and stabilization, or merely the result of general neuronal activation, remains unknown. Here, we review the signals leading to ERK activation in the nervous system, the subcellular ERK targets associated with learning-related plasticity, and how neurons with activated ERK signaling may contribute to the formation of the memory trace.

The Entorhinal Cortex as a Gateway for Amygdala Influences on Memory Consolidation

The amygdala, specifically its basolateral nucleus (BLA), is a critical site integrating neuromodulatory influences on memory consolidation in other brain areas. Almost 20 years ago, we reported the first direct evidence that BLA activity is required for modulatory interventions in the entorhinal cortex (EC) to affect memory consolidation (Roesler, Roozendaal, and McGaugh, 2002). Since then, significant advances have been made in our understanding of how the EC participates in memory. For example, the characterization of grid cells specialized in processing spatial information in the medial EC (mEC) that act as major relayers of information to the hippocampus (HIP) has changed our view of memory processing by the EC; and the development of optogenetic technologies for manipulation of neuronal activity has recently enabled important new discoveries on the role of the BLA projections to the EC in memory. Here, we review the current evidence on interactions between the BLA and EC in synaptic plasticity and memory formation. The findings suggest that the EC may function as a gateway and mediator of modulatory influences from the BLA, which are then processed and relayed to the HIP. Through extensive reciprocal connections among the EC, HIP, and several cortical areas, information related to new memories is then consolidated by these multiple brain systems, through various molecular and cellular mechanisms acting in a distributed and highly concerted manner, during several hours after learning. A special note is made on the contribution by Ivan Izquierdo to our understanding of memory consolidation at the brain system level.

ERK1/2: A Key Cellular Component for the Formation, Retrieval, Reconsolidation and Persistence of Memory

Extracellular regulated kinase 1/2 (ERK1/2) has been strongly implicated in several cellular processes. In the brain ERK1/2 activity has been primarily involved in long-term memory (LTM) formation and expression. Here, we review earlier evidence and describe recent developments of ERK1/2 signaling in memory processing focusing the attention on the role of ERK1/2 in memory retrieval and reconsolidation, and in the maintenance of the memory trace including mechanisms involving the protection of labile memories. In addition, relearning requires ERK1/2 activity in selected brain regions. Its involvement in distinct memory stages points at ERK1/2 as a core element in memory processing and as one likely target to treat memory impairments associated with neurological disorders.

L-type VDCCs participate in behavioral-LTP and memory retention

Although L-type voltage-dependent calcium channels (VDCCs) have been reported to display different even contrary actions on cognitive functions and long-term potentiation (LTP) formation, there is little information regarding the role of L-type VDCCs in behavioral LTP, a learning-induced LTP model, in the intact brain of freely behaving animals. Here we investigated the effects of verapamil, a non-selective blocker of L-type VDCCs, on behavioral LTP and cognitive functions. Population spikes (PS) were recorded by using electrophysiological methods to examine the role of verapamil in behavioral LTP in the hippocampal dentate gyrus (DG) region. Y-maze assay was used to evaluate the effects of verapamil on learning and memory. Electron microscope was used to observe the changes on synaptic ultrastructural morphology in hippocampal DG area. We found that intrahippocampal verapamil treatments had no significant changes on the PS amplitude during a 90min recordings period. However, intrahippocampal applications of verapamil, including pre- or post-training, reduced behavioral LTP magnitude and memory retention but did not prevent the induction of behavioral LTP and the acquisition of learning. The saline group with behaving trainings showed obvious increases in the number of smile synapses, the length of active zones and the thickness of postsynaptic density as compared to the baseline group, but verapamil with pre-training treatment almost returned these changes to the baseline levels except for the synaptic interface curvature. In conclusion, our results suggest that L-type VDCCs may only contribute to the magnitude of behavioral LTP and the memory maintenance with an activity-independent relationship. L-type VDCCs may be critical to new information long-term storage rather than acquisition in hippocampus.

The role of the dorsal hippocampus on the Ginkgo biloba facilitation effect of fear extinction as assessed with fear-potentiated startle

RATIONALE

Ginkgo biloba extract, EGb761, is widely used as herbal supplements throughout Western society. It has been used in the treatment of various common geriatric complaints including short-term memory loss. Our previous study has shown that acute systemic administration of EGb761 enhanced extinction of fear-potentiated startle (FPS) in rats. Little is known about the behavioral effects of hippocampally administered EGb761 on the extinction of FPS.

OBJECTIVE

The current study was performed to evaluate the involvement of the dorsal hippocampus (DH) in the EGb761 facilitation effect on the extinction of FPS.

METHODS AND RESULT

Male adult SD rats were used. EGb761 (28 ng/side, bilaterally) was infused into DH bilaterally 10 min prior to extinction training. Animals were then tested for FPS 24 h later. Results showed that intra-hippocampal infusion of EGb761 prior to extinction training facilitated extinction, which was not due to impairments of expression of conditioned fear. Intra-hippocampal injection of ERK1/2 inhibitor PD98059 partially attenuates the above EGb761 effect. Therefore, acute EGb761 administration modulated extinction of conditioned fear, which might be mediated by more than one signal cascade.

CONCLUSIONS

These results suggest that DH may participate in the facilitation effect of EGb761 on the extinction of conditioned fear. In addition to ERK1/2, another signal cascade may also be involved in the EGb761 facilitation effect on extinction.

NGF-Dependent activation of TrkA pathway: A mechanism for the neuroprotective effect of troxerutin in D-galactose-treated mice

D-galactose-(D-gal)-treated mouse, with cognitive impairment, has been used for neurotoxicity investigation and anti-neurotoxicity pharmacology research. In this study, we investigated the mechanism underlying the neuroprotective effect of troxerutin. The results showed that troxerutin improved behavioral performance in D-gal-treated mice by elevating Cu, Zn-superoxide dismutases (Cu, Zn-SOD) activity and decreasing reactive oxygen species levels. Furthermore, our results showed that troxerutin significantly promoted nerve growth factor (NGF) mRNA expression which resulted in TrkA activation. On one hand, NGF/TrkA induced activation of Akt and ERK1/2, which led to neuronal survival; on the other hand, NGF/TrkA mediated CaMKII and CREB phosphorylation and increased PSD95 expression, which improved cognitive performance. However, the neuroprotective effect of troxerutin was blocked by treatment with K252a, an antagonist for TrkA. No neurotoxicity was observed in mice treated with K252a or troxerutin alone. In conclusion, administration of troxerutin to D-gal-injected mice attenuated cognitive impairment and brain oxidative stress through the activation of NGF/TrkA signaling pathway.

Dose-dependent effects of post-training estradiol plus progesterone treatment on object memory consolidation and hippocampal extracellular signal-regulated kinase activation in young ovariectomized mice

Previous work from our laboratory has shown that the ability of estradiol to enhance object memory consolidation in young ovariectomized mice is dependent on dorsal hippocampal activation of the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) signaling pathway [Fernandez SM, Lewis MC, Pechenino AS, Harburger LL, Orr PT, Gresack JE, Schafe GE, Frick KM (2008) Estradiol-induced enhancement of object memory consolidation involves hippocampal extracellular signal-regulated kinase activation and membrane-bound estrogen receptors. J Neurosci 28:8660-8667]. However, it is unclear if estradiol modulates memory or ERK activation similarly in the presence of progesterone. Therefore, the present study investigated effects of combined estradiol and progesterone treatment on object memory consolidation and dorsal hippocampal ERK activation in young ovariectomized C57BL/6 mice. Object memory was tested in a novel object recognition task. Immediately after training, mice received intraperiotoneal (i.p.) injections of vehicle, 17beta-estradiol (E(2); 0.2 mg/kg), or E(2) plus 5, 10, or 20 mg/kg progesterone (P). Forty-eight hours later, mice receiving E(2) alone or E(2) plus 10 or 20 mg/kg P exhibited significantly enhanced memory for the novel object relative to chance, whereas those receiving vehicle or E(2) plus 5 mg/kg P spent no more time than chance with the novel object. Two weeks later, ERK phosphorylation was measured in the dorsal hippocampus 1 h after i.p. injection of vehicle, E(2), or E(2) plus P. Consistent with our previous work [Fernandez SM, Lewis MC, Pechenino AS, Harburger LL, Orr PT, Gresack JE, Schafe GE, Frick KM (2008) Estradiol-induced enhancement of object memory consolidation involves hippocampal extracellular signal-regulated kinase activation and membrane-bound estrogen receptors. J Neurosci 28:8660-8667], E(2) alone significantly increased phospho-p42 ERK protein levels in the dorsal hippocampus relative to vehicle controls. In contrast, no combination of E(2) and P affected dorsal hippocampal phospho-ERK levels. These data indicate that, unlike E(2) alone, the beneficial effects of combined E(2) plus P treatment on memory are not associated with ERK activation in the dorsal hippocampus 1 h after treatment, and suggest that E(2) alone and combined E(2) plus P may influence ERK activation in different time frames or enhance memory through different mechanisms.

Deletion of ERK2 mitogen-activated protein kinase identifies its key roles in cortical neurogenesis and cognitive function

The mitogen-activated protein (MAP) kinases ERK1 and ERK2 are critical intracellular signaling intermediates; however, little is known about their isoform-specific functions in vivo. We have examined the role of ERK2 in neural development by conditional inactivation of the murine mapk1/ERK2 gene in neural progenitor cells of the developing cortex. ERK MAP kinase (MAPK) activity in neural progenitor cells is required for neuronal cell fate determination. Loss of ERK2 resulted in a reduction in cortical thickness attributable to impaired proliferation of neural progenitors during the neurogenic period and the generation of fewer neurons. Mutant neural progenitor cells remained in an undifferentiated state until gliogenic stimuli induced their differentiation, resulting in the generation of more astrocytes. The mutant mice displayed profound deficits in associative learning. Importantly, we have identified patients with a 1 Mb microdeletion on chromosome 22q11.2 encompassing the MAPK1/ERK2 gene. These children, who have reduced ERK2 levels, exhibit microcephaly, impaired cognition, and developmental delay. These findings demonstrate an important role for ERK2 in cellular proliferation and differentiation during neural development as well as in cognition and memory formation.

Parallel memory processing by the CA1 region of the dorsal hippocampus and the basolateral amygdala

There is abundant literature on the role of the basolateral amygdala (BLA) and the CA1 region of the hippocampus in memory formation of inhibitory avoidance (IA) and other behaviorally arousing tasks. Here, we investigate molecular correlates of IA consolidation in the two structures and their relation to NMDA receptors (NMDArs) and beta-adrenergic receptors (beta-ADrs). The separate posttraining administration of antagonists of NMDAr and beta-ADr to BLA and CA1 is amnesic. IA training is followed by an increase of the phosphorylation of calcium and calmodulin-dependent protein kinase II (CaMKII) and ERK2 in CA1 but only an increase of the phosphorylation of ERK2 in BLA. The changes are blocked by NMDAr antagonists but not beta-ADr antagonists in CA1, and they are blocked by beta-ADr but not NMDAr antagonists in BLA. In addition, the changes are accompanied by increased phosphorylation of tyrosine hydroxylase in BLA but not in CA1, suggesting that beta-AD modulation results from local catecholamine synthesis in the former but not in the latter structure. NMDAr blockers in CA1 do not alter the learning-induced neurochemical changes in BLA, and beta-ADr blockade in BLA does not hinder those in CA1. When put together with other data from the literature, the present findings suggest that CA1 and BLA play a role in consolidation, but they operate to an extent in parallel, suggesting that each is probably involved with different aspects of the task studied.

The septic brain

Sepsis is a major disease entity with important clinical implications. Sepsis-induced multiple organ failure is associated with a high mortality rate in humans and is clinically characterized by pulmonary, cardiovascular, renal and gastrointestinal dysfunction. Recently, several studies have demonstrated that sepsis survivors present long-term cognitive impairment, including alterations in memory, attention, concentration and/or global loss of cognitive function. However, the pathogenesis and natural history of septic encephalopathy and cognitive impairment are still poorly known and further understanding of these processes is necessary for the development of effective preventive and therapeutic interventions. This review discusses the clinical presentation and underlying pathophysiology of the encephalopathy and cognitive impairment associated with sepsis.

ERK1/2 and CaMKII-mediated events in memory formation: is 5HT regulation involved?

Activity-dependent changes in neuronal efficacy underlie the formation and storage of new memories. Several studies indicate that modification of the phosphorylation/activation state of different protein kinases localized in the synapses or the nucleus plays a critical role in the induction and maintenance of plastic mechanisms and in the consolidation of long-lasting memories. Here we review some of the more recent findings concerning the regulation of two of the main protein kinase groups involved in memory processes and in neuronal plasticity: Ca2+/calmodulin-dependent protein kinase II (CaMKII), and the mitogen-activated protein kinase (MAPK) family. Since this issue of the journal is dedicated to serotonin (5HT) regulation of behavior, we will comment on the so far scanty, but significant, evidence for a role of 5HT in the regulation of CaMKII and MAPK.

JNK1 contributes to metabotropic glutamate receptor-dependent long-term depression and short-term synaptic plasticity in the mice area hippocampal CA1

Several recent reports implicate an important role played by c-Jun N-terminal kinases (JNKs) in long-term potentiation (LTP). However, little is known about how the isoforms of JNKs participate in synaptic plasticity. Here we showed that short-term synaptic plasticity was impaired in the hippocampal area CA1 of JNK1-deficient (JNK1-/-) mice; these mice showed normal LTP in response to a strong tetanus and no alteration of N-methyl-D-aspartate receptor-dependent long-term depression (LTD) in the hippocampus. However, LTD induced either by group I metabotropic glutamate receptors (mGluRs) agonist dihydroxyphenylglycine or by paired-pulse low-frequency stimulation was absent in both the JNK1-/- slices and in JNK inhibitor anthrax [1, 9-cd] pyrazol-6(2H)-1 (SP600125)-pretreated slices. Induction of mGluR-dependent LTD resulted in an increase in phosphorylation of JNK1 substrates, including p-c-Jun and p-ATF2 in wild-type (WT) mice, and these increases failed to occur in the JNK1-/- or SP600125-pretreated mice. These results demonstrated that JNK1 played a crucial role in the short-term synaptic plasticity and mGluR-dependent LTD, whereas hippocampus LTP was not affected by JNK1 deficiency.

Different molecular cascades in different sites of the brain control memory consolidation

To understand cognition, it is important to understand how a learned response becomes a long-lasting memory. This process of memory consolidation has been modeled extensively using one-trial avoidance learning, in which animals (or humans) establish a conditioned response by learning to avoid danger in just one trial. This relies on molecular events in the CA1 region of the hippocampus that resemble those involved in CA1 long-term potentiation (LTP), and it also requires equivalent events to occur with different timings in the basolateral amygdala and the entorhinal, parietal and cingulate cortex. Many of these steps are modulated by monoaminergic pathways related to the perception of and reaction to emotion, which at least partly explains why strong and resistant consolidation is typical of emotion-laden memories. Thus memory consolidation involves a complex network of brain systems and serial and parallel molecular events, even for a task as deceptively simple as one-trial avoidance. We propose that these molecular events might also be involved in many other memory types in animals and humans.

Early activation of extracellular signal-regulated kinase signaling pathway in the hippocampus is required for short-term memory formation of a fear-motivated learning

1. According to its duration there are, at least, two major forms of memory in mammals: short term memory (STM) which develops in a few seconds and lasts several hours and long-term memory (LTM) lasting days, weeks and even a lifetime. In contrast to LTM, very little is known about the neural, cellular and molecular requirements for mammalian STM formation. 2. Here we show that early activation of extracellular signal-regulated kinases 1/2 (ERK1/2) in the hippocampus is required for the establishment of STM for a one-trial inhibitory avoidance task in the rat. Immediate posttraining infusion of U0126 (a selective inhibitor of ERK kinase) into the CA1 region of the dorsal hippocampus blocked STM formation. 3. Reversible inactivation of the entorhinal cortex through muscimol infusion produced deficits in STM and a selective and rapid decrease in hippocampal ERK2 activation.4. Together with our previous findings showing a rapid decrease in ERK2 activation and impaired STM after blocking BDNF function, the present results strongly suggest that ERK2 signaling in the hippocampus is a critical step in STM processing.

The connection between the hippocampal and the striatal memory systems of the brain: A review of recent findings

Two major memory systems have been recognized over the years (Squire, in Memory and Brain, 1987): the declarative memory system, which is under the control of the hippocampus and related temporal lobe structures, and the procedural or habit memory system, which is under the control of the striatum and its connections (Mishkin et al., in Neurobiology of Learning by G Lynch et al., 1984; Knowlton et al., Science 273:1399, 1996). Most if not all learning tasks studied in animals, however, involve either the performance or the suppression of movement. Animals acquire connections between environmental or discrete sensory cues (conditioned stimuli, CSs) and emotionally or otherwise significant stimuli (unconditioned stimuli, USs). As a result, they learn to perform or to inhibit the performance of certain motor responses to the CS which, when learned well, become what can only be called habits (Mishkin et al., 1984): to regularly walk or swim to a place or away from a place, or to inhibit one or several forms of movement. These responses can be viewed as conditioned responses (CRs) and may sometimes be very complex. This is of course also seen in humans: people learn how to play on a keyboard in response to a mental or written script and perform the piano or write a text; with practice, the performance improves and eventually reaches a high criterion and becomes a habit, performed almost if not completely without awareness. Commuting to school in a big city in the shortest possible time and eschewing the dangers is a complex learning that children acquire to the point of near-perfection. It is agreed that the rules that connect the perception of the CS and the expression of the CR change from their first association to those that take place when the task is mastered. Does this change of rules involve a switch from one memory system to another? Are different brain systems used the first time one plays a sonata or goes to school as compared with the 100th time? Here we will comment on: 1) reversal learning in the Morris water maze (MWM), in which the declarative or spatial component of a task is changed but the procedural component (to swim) persists and needs to be re-linked with a different set of spatial cues; and 2) a series of observations on an inhibitory avoidance task that indicate that the brain systems involved change with further learning.

Glutamate uptake is stimulated by extracellular S100B in hippocampal astrocytes

1.S100B is a calcium-binding protein expressed and secreted by astrocytes, which has been implicated in glial-neuronal communication. Extracellular S100B appears to protect hippocampal neurons against toxic concentrations of glutamate. Here we investigated a possible autocrine role of S100B in glutamate uptake activity. 2. Astrocyte cultures were prepared of hippocampi from neonate Wistar rats. [(3)H] Glutamate uptake was measured after addition of S100B protein, antibody anti-S100B or TRTK-12, a peptide that blocks S100B activity mediated by the C-terminal region. 3.Antibody anti-S100B addition decreased glutamate uptake measured 30 min after medium replacement, without affecting cell integrity or viability. Moreover, low levels of S100B (less than 0.1 ng/mL) stimulated glutamate uptake measured immediately after medium replacement. 4. This finding reinforces the importance of astrocytes in the glutamatergic transmission, particularly the role of S100B neuroprotection against excitotoxic damage.

Differential patterns of extracellular signal-regulated kinase-1 and -2 phosphorylation in rat limbic brain regions after short-term and long-term inhibitory avoidance learning

Activation of the extracellular signal-regulated kinase-1 and -2 has been shown to be required for neural plasticity and memory. Previous pharmacological studies have demonstrated that inhibition of extracellular signal-regulated kinase-1 and -2 blocks inhibitory avoidance retention. The aim of the present study was to investigate the different neural substrates underlying short- and long-term inhibitory avoidance learning and memory in rats using phosphorylated extracellular signal-regulated kinase-1 and -2 labeling as an index of plasticity. Short- and long-term retention tests were given 10 min or 24 h after inhibitory avoidance training. A significant elevation in the number of phosphorylated extracellular signal-regulated kinase-1 and -2-immunoreactive neurons was observed in area 1 of anterior cingulate cortex, the secondary motor cortex, lateral orbital cortex, claustrum, and the medial amygdala nucleus after the short-term inhibitory avoidance test. After the long-term retention test, phosphorylated extracellular signal-regulated kinase-1 and -2-immunoreactive neurons were localized in area 1 of anterior cingulate cortex, prelimbic cortex, and the central nucleus of amygdala. This suggests that phosphorylated extracellular signal-regulated kinase-1 and -2-immunoreactivity may reveal different brain regions involved in the storage of short- and long-term aversive memories.

ERK phosphorylation is required for retention of trace fear memory

The extracellular signal-regulated kinase (ERK) has been previously associated with long-term memory formation. Earlier studies have demonstrated a role for phospho-ERK in delay fear conditioning and it has been shown to disrupt trace fear memory when inhibited after training. cAMP response element binding protein (CREB) is a key transcription factor that has been implicated in long-term memory formation across different species. It has also been shown to be modulated by ERK. In our study, we used the drug SL327 to prevent ERK phosphorylation. Two groups of Fischer 344 male rats (2-4 months) were injected intraperitoneally with 100% DMSO (2 ml/kg) or SL327 (100 mg/kg/2 ml dissolved in DMSO) 45 min before 10 trials of trace fear conditioning. Each trial consisted of a tone paired with a footshock with a 30-s interval separating the stimuli. Twenty-four hours later, rats were tested for fear to the tone. Our results showed that SL327-treated rats displayed memory deficits 24 h after training. Western blot analyses of total hippocampal protein revealed a significant increase in phosphorylated ERK immediately after training. There were also decreases in phosphorylated ERK at 45 and 90 min post-injection of SL327-treated rats as compared to DMSO-treated rats, but levels of phosphorylated CREB remained the same. These findings indicate that ERK phosphorylation is increased immediately after trace fear conditioning and inhibiting this increase is correlated with memory deficits in trace fear conditioning 24 h later. These findings support a role for ERK phosphorylation in the formation of trace fear memories.

Plasticity in the entorhinal cortex suppresses memory for contextual fear

Several studies have delineated a role for the hippocampus in fear conditioning. However, in this task, the role of the entorhinal cortex (EC), the main input-output structure for the hippocampus, is uncertain. The extracellular signal-regulated kinase (ERK) cascade has been shown to be a molecular correlate for long-term memory, and its activity is required for various types of memory storage, including fear memory. In this study, we show that ERK activity in the EC increased 90 min after fear conditioning. Post-training intra-EC infusion of ERK cascade inhibitors (PD098059, UO126) at 40 min, but not at 10 min, resulted in increased freezing to the context, but not to the tone, during a 48 hr retention test. Interestingly, both PD098059- and UO126-infused animals also demonstrated anticipatory freezing in the context, freezing maximally at the time the shock was given during training. This anticipatory behavior was also seen in naive animals receiving additional training. Together, these results suggest that ERK-mediated plasticity in the EC normally suppresses context-specific fear memory, especially the temporal nature of the freezing response, and that blocking this plasticity mimics the effects of additional training.

Short- and long-term memory are differentialy modulated by hippocampal nerve growth factor and fibroblast growth factor

Rats were implanted with cannulae in the CA1 area of the dorsal hippocampus and trained in one-trial step-down inhibitory avoidance. Two retention tests were carried out in each animal, one at 1.5 h to measure short-term memory (STM) and another at 24 h to measure long-term memory (LTM). The purpose of the present study was to evaluate the modulation on hippocampal nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) on short- and long-term memory. Immediately after training, animals received 5 microl of NGF (0.05, 0.5 or 5.0 ng), bFGF (1.25, 12.5 or 125 ng) or saline per side. At the higher dose, NGF blocked STM. In contrast, NGF at dose of 0.5 and 5.0 ng improved LTM. The bFGF infusion at a dose of 125 ng enhanced LTM. However, bFGF did not alter STM. These findings indicate that hippocampal NGF and bFGF modulate STM and LTM in a different manner.

Pretraining but not Preexposure to the Task Apparatus Prevents the Memory Impairment Induced by Blockade of Protein Synthesis, PKA or MAP Kinase in Rats

Adult male Wistar rats were trained and tested in a step-down inhibitory avoidance task (0.4 mA footshock, 24 h training-test interval). Fifteen minutes before or 0, 1.5 or 3 hours after training, animals received a 0.8 microl intrahippocampal infusion of the protein synthesis inhibitor anisomycin (80 microg), the PKA inhibitor Rp-cAMP (0.05 microg), the MAPK kinase inhibitor PD 098059 (50 microM solution) or vehicle (phosphate buffer in saline, pH 7.4). Anisomycin, Rp-cAMP and PD 098059 impaired retention test performance in animals injected at different times, prior and after training. Pretraining with a low footshock intensity (0.2 mA) 24 h before training prevented the amnestic effect of all drugs studied. However, simple preexposure to the inhibitory avoidance apparatus did not alter the amnestic effects of all drugs. The results suggest that memory processing requires hippocampal mechanisms dependent on protein synthesis, PKA and MAPK kinase at different times after training. These findings suggest that weak training must be sufficient to produce some lasting cellular expression of the experience so that the enhancement of consolidation of a previously acquired memory is not dependent on protein synthesis, PKA or MAPK.

One-trial aversive learning induces late changes in hippocampal CaMKIIα, Homer 1a, Syntaxin 1a and ERK2 protein levels

Most studies regarding altered gene expression after learning are performed using multi-trial tasks, which do not allow a clear discrimination of memory acquisition, consolidation and retrieval. We screened for candidate memory-modulated genes in the hippocampus at 3 and 24 h after one-trial inhibitory avoidance (IA) training, using a cDNA array containing 1176 genes. While 33 genes were modulated by training (respect to shocked-only animals), most of them were upregulated (27 genes) and only 6 were downregulated. To confirm and extend these findings, we performed RT-PCRs and analyzed differences in protein levels in rat hippocampus using immunoblot assays. We found several proteins upregulated 24 h after training: extracellular signal-regulated kinase ERK2, Ca2+/calmodulin-dependent protein kinase II alpha (CaMKIIalpha), Syntaxin 1a, c-fos and Homer 1a. The total level of none of these proteins were found to be altered when measured 3-h post-training. Several of the mRNAs corresponding to the upregulated proteins were changed at 3 h but not 24 h. Additionally, a number of other candidates were identified for the first time as modulated by learning. The results presented here suggest that single-trial tasks can expose previously unseen differences in dynamic regulation of gene expression after behavioral manipulations, both at the transcriptional and translational levels, and reveal a diversity of gene products modulated by this task, allowing deeper understanding of the molecular basis of memory formation.

Protein synthesis, PKA, and MAP kinase are differentially involved in short- and long-term memory in rats

We studied the involvement of hippocampal protein synthesis-, PKA-, and MAP kinase-dependent processes in short- (STM) and long-term memory (LTM) for inhibitory avoidance task. Fifteen minutes before or immediately after training rats received intrahippocampal infusions of vehicle, the protein synthesis inhibitor anisomycin, the PKA inhibitor Rp-cAMPs or the MAPKK inhibitor PD098059. The results show that STM recruits PKA and MAPK, whereas, LTM depends on PKA activity and protein synthesis during the early post-training period.

Inhibition of the phosphodiesterase 4 (PDE4) enzyme reverses memory deficits produced by infusion of the MEK inhibitor U0126 into the CA1 subregion of the rat hippocampus

Cyclic AMP-specific phosphodiesterase 4 (PDE4), which is an integral component of NMDA receptor-mediated cAMP signaling, is involved in the mediation of memory processes. Given that NMDA receptors also mediate MEK/mitogen-activated protein kinase (MAPK, ERK) signaling, which is involved in synaptic plasticity, and that some PDE4 subtypes are phosphorylated and regulated by ERK, it was of interest to determine if PDE4 is involved in MEK/ERK signaling-mediated memory. It was found that rolipram, a PDE4-selective inhibitor, reversed the amnesic effect in the radial-arm maze test of the MEK inhibitor U0126 administered into the CA1 subregion of the rat hippocampus. Consistent with this, rolipram, either by peripheral administration or direct intra-CA1 infusion, enhanced the retrieval of long-term memory impaired by intra-CA1 infusion of U0126 using the step-through inhibitory avoidance test. The same dose of rolipram did not affect U0126-induced reduction of phospho-ERK1/2 levels in the CA1 subregion. However, in primary cultures of rat cerebral cortical neurons, pretreatment with U0126 increased PDE4 activity; this was correlated with the U0126-induced reduction of phospho-ERK1/2 levels. These results suggest that MEK/ERK signaling plays an inhibitory role in regulating PDE4 activity in the brain; this may be a novel mechanism by which MEK/ERK signaling mediates memory. PDE4 is likely to be an important link between the cAMP/PKA and MEK/ERK signaling pathways in the mediation of memory.

A unified theory for systems and cellular memory consolidation

The time-limited role of the hippocampus for explicit memory storage has been referred to as systems consolidation where learning-related changes occur first in the hippocampus followed by the gradual development of a more distributed memory trace in the neocortex. Recent experiments are beginning to show that learning induces plasticity-related molecular changes in the neocortex as well as in the hippocampus and with a similar time course. Present memory consolidation theories do not account for these findings. In this report, we present a theory (the C theory) that incorporates these new findings, provides an explanation for the length of time for hippocampal dependency, and that can account for the apparent longer consolidation periods in species with larger brains. This theory proposes that a process of cellular consolidation occurs in the hippocampus and in areas of the neocortex during and shortly after learning resulting in long-term memory storage in both areas. For a limited time, the hippocampus is necessary for memory retrieval, a process involving the coordinated reactivation of these areas. This reactivation is later mediated by longer extrahippocampal connectivity between areas. The delay in hippocampal-independent memory retrieval is the time it takes for gene products in these longer extrahippocampal projections to be transported from the soma to tagged synapses by slow axonal transport. This cellular transport event defines the period of hippocampal dependency and, thus, the duration of memory consolidation. The theoretical description for memory consolidation presented in this review provides alternative explanations for several experimental observations and presents a unification of the concepts of systems and cellular memory consolidation.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Retrograde Amnesia Induced by Drugs Acting on Different Molecular Systems

The gamma aminobutyric acid-A (GABA-sub(A)) agonist, muscimol, the glutamate N-methyl-D-aspartate (NMDA) receptor antagonist, D-2-amino-5-phosphonopentanoic acid (AP5), and the inhibitor of the extracellularly regulated kinases (ERKs), UO 126, cause retrograde amnesia when administered to the hippocampus. In the present study, the authors found that they all cause retrograde amnesia for 1-trial inhibitory avoidance, not only when infused into the dorsal CA1 region of the hippocampus, but also when infused into the basolateral amygdala or the entorhinal, parietal, and posterior cingulate cortices. The posttraining time course of the effect of each drug was, however, quite different across brain structures. Thus, in all of them, NMDA receptors and the ERK pathway are indispensable for memory consolidation, and GABA-sub(A) receptor activation inhibits memory consolidation: but in each case, their influence is interwoven differently.

Gene expression during memory formation

For several decades, neuroscientists have provided many clues that point out the involvement of de novo gene expression during the formation of long-lasting forms of memory. However, information regarding the transcriptional response networks involved in memory formation has been scarce and fragmented. With the advent of genome-based technologies, combined with more classical approaches (i.e., pharmacology and biochemistry), it is now feasible to address those relevant questions--which gene products are modulated, and when that processes are necessary for the proper storage of memories--with unprecedented resolution and scale. Using one-trial inhibitory (passive) avoidance training of rats, one of the most studied tasks so far, we found two time windows of sensitivity to transcriptional and translational inhibitors infused into the hippocampus: around the time of training and 3-6 h after training. Remarkably, these periods perfectly overlap with the involvement of hippocampal cAMP/PKA (protein kinase A) signaling pathways in memory consolidation. Given the complexity of transcriptional responses in the brain, particularly those related to processing of behavioral information, it was clearly necessary to address this issue with a multi-variable, parallel-oriented approach. We used cDNA arrays to screen for candidate inhibitory avoidance learning-related genes and analyze the dynamic pattern of gene expression that emerges during memory consolidation. These include genes involved in intracellular kinase networks, synaptic function, DNA-binding and chromatin modification, transcriptional activation and repression, translation, membrane receptors, and oncogenes, among others. Our findings suggest that differential and orchestrated hippocampal gene expression is necessary in both early and late periods of long-term memory consolidation. Additionally, this kind of studies may lead to the identification and characterization of genes that are relevant for the pathogenesis of complex psychiatric disorders involving learning and memory impairments, and may allow the development of new methods for the diagnosis and treatment of these diseases.

Improvement in fear memory by histamine-elicited ERK2 activation in hippocampal CA3 cells

Consolidation of associative memories appears to require extracellular signal-related kinase2 (ERK2) activation, which is modulated by several factors, including neurotransmitter receptor stimulation. Here we show that in vitro stimulation of either H2 or H3 histaminergic receptors activates ERK2 in hippocampal CA3 pyramidal cells. In behaving animals, bilateral posttraining injections into the dorsal hippocampus of histamine H2 or H3 receptor agonists improve memory consolidation after contextual fear conditioning. Local administration of U0126, a selective inhibitor of ERK kinase, prevents memory improvements exerted by the agonists, without causing any behavioral effect per se. This is the first evidence of a positive correlation between ERK phosphorylation and memory improvement. Moreover, we demonstrate that the brain histaminergic system regulates hippocampal ERK cascade. Finally, our data indicate that early ERK2 hippocampal activation is not required for the expression of long-term fear memories.

[Memory consolidation and posttraumatic stress disorder]

Extensive evidence from animal and human studies has shown that memory formation is enhanced by an endogenous modulatory system mediated by stress hormones and activation of the amygdala. This system is an evolutionarily adaptive method of enhancing important memories. Under emotional stress, this system is activated promoting the formation of vivid, long lasting traumatic memories, which are the hallmark of PTSD. The understanding of the mechanisms underlying memory modulation might lead to an improved ability to assess and treat PTSD.

Can memory be improved? A discussion on the role of ras, GABA, acetylcholine, NO, insulin, TNF-alpha, and long-chain polyunsaturated fatty acids in memory formation and consolidation

It is proposed that long-chain polyunsaturated fatty acids when given from the perinatal period will ensure proper development and growth of the brain and maintain the activity and/or concentrations of ras, nitric oxide, insulin, and various neurotransmitters and cytokines at physiological level and thus, improve memory and prevent learning deficits.

Role of the amygdala in fear extinction measured with potentiated startle

Although much is now known about the neural basis of excitatory fear conditioning, much less is known about the neural basis of inhibitory conditioning. One type of inhibitory conditioning is extinction, a process in which stimuli that elicit fear by virtue of previous associations with aversive stimuli such as shock (excitatory fear conditioning) are now presented in the absence of the aversive stimuli (extinction training). As a result, the ability of the conditioned stimulus to elicit fear gradually diminishes. Extinction is different from forgetting and does not reflect an erasure of the original fear memory. Instead, extinction is an active form of inhibitory learning that competes with excitatory fear conditioning. Infusions into the amygdala (a brain area essential for excitatory fear conditioning) of either NMDA receptor antagonists or inhibitors of the NMDA-receptor-linked mitogen-activated protein kinase cascade block extinction learning. Conversely, the NMDA receptor agonist D-cycloserine facilitates extinction after either systemic administration or intra-amygdala infusion. Because therapeutic interventions based on extinction procedures are commonly used to treat fear disorders, and because D-cycloserine is a widely available and safe compound, D-cycloserine or similar agents might be usefully combined with traditional extinction-based approaches in the treatment of clinical fear.

Inhibition of hippocampal Jun N-terminal kinase enhances short-term memory but blocks long-term memory formation and retrieval of an inhibitory avoidance task

Learning initiates a series of plastic events the occurrence of which are required for the storage of information related to the training experience. Several lines of evidence indicate that, in the rat hippocampus, different members of the family of mitogen-activated protein kinases (MAPK) play a key role in the onset of such plastic events. Using SP600125, the newly developed inhibitor of the MAPK c-Jun amino-terminal kinase (JNK), we show a direct involvement of this protein kinase in mnemonic processes. The intra-CA1 infusion of SP600125, at a dose that in naïve animals significantly reduced the phosphorylation levels of c-Jun without affecting the activity of ERK1/2 or p38 MAPK, enhanced short-term memory (STM) but blocked long-term memory (LTM) formation and retrieval of an inhibitory avoidance learning task. No action of this drug on locomotor/exploratory activity or general anxiety state could be detected. The significance of these results is discussed in the context of others describing the independence of LTM from STM.

Memory retrieval and its lasting consequences

Many, if not all psychiatric diseases are accompanied by memory disturbances, in particular, the dementias, schizophrenia, and, to an extent, mood disorders. Anxiety and stress, on the other hand, cause important alterations of memory, particularly its retrieval. Here we discuss several new findings on the basic mechanisms of consolidation, retrieval and extinction of a prototype form of episodic memory in the rat: conditioned fear. The findings point the way for investigations on the pathology of these aspects of memory in health and disease. Emphasis is placed on the parallel processing of retrieval in several cortical areas, on the links between retrieval and the onset of extinction, on the fact that extinction involves new learning requiring gene expression, and on the differences between the retrieval of recent or remote long-term memories.

Molecular pharmacological dissection of short- and long-term memory

1. It has been discussed for over 100 years whether short-term memory (STM) is separate from, or just an early phase of, long-term memory (LTM). The only way to solve this dilemma is to find out at least one treatment that blocks STM while keeping LTM intact for the same task in the same animal. 2. The effect of a large number of treatments infused into the hippocampus, amygdala, and entorhinal, posterior parietal or prefrontal cortex on STM and LTM of a one-trial step-down inhibitory avoidance task was studied. The animals were tested at 1.5 h for STM, and again at 24 h for LTM. The treatments were given after training. 3. Eleven different treatments blocked STM without affecting LTM. Eighteen treatments affected the two memory types differentially, either blocking or enhancing LTM alone. Thus, STM is separate from, and parallel to the first hours of processing of, LTM of that task. 4. The mechanisms of STM are different from those of LTM. The former do not include gene expression or protein synthesis; the latter include a double peak of cAMP-dependent protein kinase activity, accompanied by the phosphorylation of CREB, and both gene expression and protein synthesis. 5. Possible cellular and molecular events that do not require mRNA or protein synthesis should account for STM. These might include a hyperactivation of glutamate AMPA receptors, ribosome changes, or the exocytosis of glycoproteins that participate in cell addition.

Facilitation of conditioned fear extinction by systemic administration or intra-amygdala infusions of D-cycloserine as assessed with fear-potentiated startle in rats

NMDA receptor antagonists block conditioned fear extinction when injected systemically and also when infused directly into the amygdala. Here we evaluate the ability of D-cycloserine (DCS), a partial agonist at the strychnine-insensitive glycine-recognition site on the NMDA receptor complex, to facilitate conditioned fear extinction after systemic administration or intra-amygdala infusions. Rats received 10 pairings of a 3.7 sec light and a 0.4 mA footshock (fear conditioning). Fear-potentiated startle (increased startle in the presence vs the absence of the light) was subsequently measured before and after 30, 60, or 90 presentations of the light without shock (extinction training). Thirty non-reinforced light presentations produced modest extinction, and 60 or 90 presentations produced nearly complete extinction (experiment 1). DCS injections (3.25, 15, or 30 mg/kg) before 30 non-reinforced light exposures dose-dependently enhanced extinction (experiment 2) but did not influence fear-potentiated startle in rats that did not receive extinction training (experiment 3). These effects were blocked by HA-966, an antagonist at the glycine-recognition site (experiment 4). Neither DCS nor HA-966 altered fear-potentiated startle when injected before testing (experiment 5). The effect of systemic administration was mimicked by intra-amygdala DCS (10 microg/side) infusions (experiment 6). These results indicate that treatments that promote NMDA receptor activity after either systemic or intra-amygdala administration promote the extinction of conditioned fear.

Lesions of the nucleus basalis magnocellularis induced by 192 IgG-saporin block memory enhancement with posttraining norepinephrine in the basolateral amygdala

Extensive evidence indicates that drugs and stress hormones act in the basolateral amygdala (BLA) to modulate memory consolidation. The BLA projects to the nucleus basalis magnocellularis (NBM), which sends broad cholinergic projections to the neocortex. NBM-cortex projections have been implicated in learning, memory storage, and plasticity. The current study investigated whether the cholinergic NBM-cortex projections are involved in BLA-mediated modulation of memory consolidation. Bilateral cholinergic cell lesions of the NBM were induced in rats with infusions of 192 IgG-saporin (0.1 microg/0.5 microl per side). Additionally, cannulae were implanted bilaterally in the BLA. One week after surgery, the rats were trained in an inhibitory avoidance task and, immediately after training, norepinephrine (0.3 microg, 1.0 microg, or 3.0 microg in 0.2 microl) or vehicle (PBS) was infused bilaterally into the BLA. Norepinephrine infusions produced a dose-dependent enhancement of 48-h retention (0.3 microg and 1.0 microg doses enhanced) in nonlesioned rats but did not affect retention in NBM-lesioned rats. Choline acetyltransferase assays of frontal and occipital cortices confirmed the NBM lesions. These findings indicate that cholinergic NBM-cortex projections are required for BLA-mediated modulation of memory consolidation.

Aversive experiences are associated with a rapid and transient activation of ERKs in the rat hippocampus

The extracellular signal-regulated kinases (ERKs) family of mitogen-activated protein kinases (MAPKs) has been shown to participate in memory formation. We recently found that a hippocampal ERK/MAPK cascade is required for memory formation of an inhibitory avoidance training in rats. Here we reported that this learning task is accompanied by a rapid increase in the phosphorylation of hippocampal p44 MAPK. A single mild foot shock produced a similar effect and three consecutive foot shocks provoked the activation of both p44 and p42 MAPKs. In contrast, a brief exposure to the training box or the habituation to an open field did not alter hippocampal ERK/MAPK levels. Together, these findings indicate that aversive behavioral experiences induced a rapid and transient activation of ERK/MAPKs in the hippocampus.

Simultaneous modulation of retrieval by dopaminergic D(1), beta-noradrenergic, serotonergic-1A and cholinergic muscarinic receptors in cortical structures of the rat

Retrieval of inhibitory avoidance has been recently shown to require intact glutamate receptors, protein kinases A and C and mitogen-activated protein kinase in the CA1 region of the rat hippocampus and in the entorhinal, posterior parietal and anterior cingulate cortex. These enzymatic activities are known to be modulated by dopamine D(1), beta-noradrenergic, 5HT1A and cholinergic muscarinic receptors. Here we study the effect on retrieval of this task of well-known agonists and antagonists of these receptors infused in the same brain cortical regions and into the basolateral amygdala, in rats. The drugs used were SKF38393 (D(1) agonist), noradrenaline, 8-HO-DPAT (5HT1A agonist), oxotremorine (muscarinic agonist), SCH23390 (D(1) antagonist), timolol (beta antagonist), NAN-190 (5HT1A antagonist) and scopolamine (muscarinic antagonist). All were studied at two different dose levels. The localised infusion of SKF38393, noradrenaline, NAN-190 and oxotremorine into any of the cortical structures mentioned 10 min prior to a 24-h retention test session of one-trial step-down inhibitory avoidance enhanced retention test performance. SCH2330, timolol, 8-HO-DPAT and scopolamine hindered retention test performance. In the basolateral amygdala only an enhancing effect of noradrenaline and an inhibitory effect of timolol were seen. Three hours after the infusions, retention test performance returned to normal in all cases. None of the treatments affected locomotion or rearing in an open field or behaviour in the elevated plus maze. Therefore, their effects on retention testing can be attributed to an influence on retrieval. In conclusion, memory retrieval of this apparently simple task requires the participation of CA1, entorhinal, posterior parietal and anterior cingulate cortex, and is strongly modulated by, dopaminergic D(1), beta-noradrenergic, muscarinic cholinergic and 5HT1A receptors in the four areas. The first three types of receptor enhance, and the latter inhibits, retrieval. Only beta-adrenoceptors appears to be involved in the modulation of retrieval of this task by the amygdala. The results bear on the well-known influence of emotion and mood on retrieval, and indicate that this involves many areas of the brain simultaneously. In addition, the results point to similarities and differences between the modulatory mechanisms that affect retrieval and those involved in the consolidation of the same task.

Mitogen-activated protein kinase/extracellular signal-regulated kinase induced gene regulation in brain: a molecular substrate for learning and memory?

The mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) pathway is an evolutionarily conserved signaling cascade involved in a plethora of physiological responses, including cell proliferation, survival, differentiation, and, in neuronal cells, synaptic plasticity. Increasing evidence now implicates this pathway in cognitive functions, such as learning and memory formation, and also in behavioral responses to addictive drugs. Although multiple intracellular substrates can be activated by ERKs, nuclear targeting of transcription factors, and thereby control of gene expression, seems to be a major event in ERK-induced neuronal adaptation. By controlling a prime burst of gene expression, ERK signaling could be critically involved in molecular adaptations that are necessary for long-term behavioral changes. Reviewed here are data providing evidence for a role of ERKs in long-term behavioral alterations, and the authors discuss molecular mechanisms that could underlie this role.

Pharmacological differences between memory consolidation of habituation to an open field and inhibitory avoidance learning

Rats implanted bilaterally with cannulae in the CA1 region of the dorsal hippocampus or the entorhinal cortex were submitted to either a one-trial inhibitory avoidance task, or to 5 min of habituation to an open field. Immediately after training, they received intrahippocampal or intraentorhinal 0.5-microl infusions of saline, of a vehicle (2% dimethylsulfoxide in saline), of the glutamatergic N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphono pentanoic acid (AP5), of the protein kinase A inhibitor Rp-cAMPs (0.5 microg/side), of the calcium-calmodulin protein kinase II inhibitor KN-62, of the dopaminergic D1 antagonist SCH23390, or of the mitogen-activated protein kinase kinase inhibitor PD098059. Animals were tested in each task 24 h after training. Intrahippocampal KN-62 was amnestic for habituation; none of the other treatments had any effect on the retention of this task. In contrast, all of them strongly affected memory of the avoidance task. Intrahippocampal Rp-cAMPs, KN-62 and AP5, and intraentorhinal Rp-cAMPs, KN-62, PD098059 and SCH23390 caused retrograde amnesia. In view of the known actions of the treatments used, the present findings point to important biochemical differences in memory consolidation processes of the two tasks.

Behavioural pharmacology and its contribution to the molecular basis of memory consolidation

Recent findings have significantly advanced our understanding the mechanisms of memory formation. Most of these advances stemmed from behavioural pharmacology research involving, particularly, the localized infusion of drugs with specific molecular actions into specific brain regions. This approach has revealed brain structures involved in different memory types and the main neurotransmitter systems and sequence of metabolic cascades that participate in memory consolidation. Biochemical studies and, in several cases, studies of genetically manipulated animals, in which receptors or enzymes affected by the various drugs were absent or overexpressed, have complemented the pharmacological research. Although most studies have concentrated on the involvement of the hippocampus, many have also investigated the entorhinal cortex, other regions of the cortex, and the amygdala. Behavioural pharmacology has been of crucial importance in establishing the major neurohumoral and hormonal systems involved in the modulation of memory formation. These systems act on specific steps of memory formation in the hippocampus and in the entorhinal, parietal, and cingulate cortex. A specialized system mediated by the basolateral amygdaloid nucleus, and involving several neuromodulatory systems, is activated by emotional arousal and serves to regulate memory formation in other brain regions. The core mechanisms involved in the formation of explicit (declarative) memory are in many respects similar to those of long-term potentiation (LTP), particularly in the hippocampus. However, there are also important differences between memory formation and LTP. Memory formation involves numerous modulatory influences, the co-participation of various brain regions other than the hippocampus, and some properties that are specific to memory and absent in LTP (i.e. flexibility of response). We discuss the implications of these similarities and differences for understanding the neural bases of memory.

Short- and long-term memory: differential involvement of neurotransmitter systems and signal transduction cascades

Since William James (1890) first distinguished primary from secondary memory, equivalent to short- and long-term memory, respectively, it has been assumed that short-term memory processes are in charge of cognition while long-term memory is being consolidated. From those days a major question has been whether short-term memory is merely a initial phase of long-term memory, or a separate phenomena. Recent experiments have shown that many treatments with specific molecular actions given into the hippocampus and related brain areas after one-trial avoidance learning can effectively cancel short-term memory without affecting long-term memory formation. This shows that short-term memory and long-term memory involve separate mechanisms and are independently processed. Other treatments, however, influence both memory types similarly, suggesting links between both at the receptor and at the post-receptor level, which should not be surprising as they both deal with nearly the same sensorimotor representations. This review examines recent advances in short- and long-term memory mechanisms based on the effect of intra-hippocampal infusion of drugs acting upon neurotransmitter and signal transduction systems on both memory types.

Role of hippocampal signaling pathways in long-term memory formation of a nonassociative learning task in the rat

Long-term habituation to a novel environment is one of the most elementary forms of nonassociative learning. Here we studied the effect of pre- or posttraining intrahippocampal administration of drugs acting on specific molecular targets on the retention of habituation to a 5-min exposure to an open field measured 24 h later. We also determined whether the exposure to a novel environment resulted in the activation of the same intracellular signaling cascades previously shown to be activated during hippocampal-dependent associative learning. The immediate posttraining bilateral infusion of CNQX (1 microg/side), an AMPA/kainate glutamate receptor antagonist, or of muscimol (0.03 microg/side), a GABA(A) receptor agonist, into the CA1 region of the dorsal hippocampus impaired long-term memory of habituation. The NMDA receptor antagonist AP5 (5 microg/side) impaired habituation when infused 15 min before, but not when infused immediately after, the 5-min training session. In addition, KN-62 (3.6 ng/side), an inhibitor of calcium calmodulin-dependent protein kinase II (CaMKII), was amnesic when infused 15 min before or immediately and 3 h after training. In contrast, the cAMP-dependent protein kinase (PKA) inhibitor Rp-cAMPS, the mitogen-activated protein kinase kinase (MAPKK) inhibitor PD098059, and the protein synthesis inhibitor anisomycin, at doses that fully block memory formation of inhibitory avoidance learning, did not affect habituation to a novel environment. The detection of spatial novelty is associated with a sequential activation of PKA, ERKs (p44 and p42 MAPKs) and CaMKII and the phosphorylation of c-AMP responsive element-binding protein (CREB) in the hippocampus. These findings suggest that memory formation of spatial habituation depends on the functional integrity of NMDA and AMPA/kainate receptors and CaMKII activity in the CA1 region of the hippocampus and that the detection of spatial novelty is accompanied by the activation of at least three different hippocampal protein kinase signaling cascades.

Time-dependent enhancement of inhibitory avoidance retention and MAPK activation by post-training infusion of nerve growth factor into CA1 region of hippocampus of adult rats

Several studies have demonstrated that chronic intracerebroventricular nerve growth factor (NGF) infusion has a beneficial effect on cognitive performance of lesioned as well as old and developing animals. Here we investigate: (i) the effect of post-training infusion of NGF into the CA1 region of hippocampus on inhibitory avoidance (IA) retention in rats; (ii) the extension of the effect, in time and space, of NGF infusion into CA1 on the activity of mitogen-activated protein kinase (MAPK, syn: ERK1/2, p42/p44 MAPK). NGF was bilaterally injected into the CA1 regions of the dorsal hippocampus (0.05, 0.5 or 5.0 ng diluted in 0.5 microL of saline per side ) at 0, 120 or 360 min after IA training in rats. Retention testing was carried out 24 h after training. The injection of 5.0 and 0.5, but not 0.05, ng per side of NGF at 0 and 120 min after IA training enhanced IA retention. The highest dose used was ineffective when injected 360 min after training. The infusion of 0. 5 microL of NGF (5.0 ng) induced a significant enhancement of MAPK activity in hippocampal microslices; this enhancement was restricted to a volume with 0.8 mm radius at 30 min after injection. The MAPK activation was still seen 180 min after NGF infusion, although this value showed only a tendency. In conclusion, localized infusion of NGF into the CA1 region enhanced MAPK activity, restricted in time and space, and enhanced IA retention in a time- and dose-dependent manner.