On the participation of hippocampal PKC in acquisition, consolidation and reconsolidation of spatial memory.

Memory consolidation involves a sequence of temporally defined and highly regulated changes in the activation state of several signaling pathways that leads to the lasting storage of an initially labile trace. Despite appearances, consolidation does not make memories permanent. It is now known that upon retrieval well-consolidated memories can become again vulnerable to the action of amnesic agents and in order to persist must undergo a protein synthesis-dependent process named reconsolidation. Experiments with genetically modified animals suggest that some PKC isoforms are important for spatial memory and earlier studies indicate that several PKC substrates are activated following spatial learning. Nevertheless, none of the reports published so far analyzed pharmacologically the role played by PKC during spatial memory processing. Using the conventional PKC and PKCmu inhibitor 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo[2,3-a]pyrrollo[3,4-c]carbazole (Gö6976) we found that the activity of these kinases is required in the CA1 region of the rat dorsal hippocampus for acquisition and consolidation of spatial memory in the Morris water maze learning task. Our results also show that when infused into dorsal CA1 after non-reinforced retrieval, Gö6976 produces a long-lasting amnesia that is independent of the strength of the memory trace, suggesting that post-retrieval activation of hippocampal PKC is essential for persistence of spatial memory.

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.

On the participation of hippocampal p38 mitogen-activated protein kinase in extinction and reacquisition of inhibitory avoidance memory.

Inhibitory avoidance (IA) learning relies on the formation of an association between stepping down from a platform present in a certain context (conditioned stimulus; CS) with an aversive unconditioned stimulus (US; i.e. a footshock). A single CS-US pairing establishes a robust long-term memory expressed as an increase in step-down latency at testing. However, repeated retrieval of the avoidance response in the absence of the US induces extinction of IA memory. That is, recurring presentation of the CS alone results in a new learning indicating that the CS no longer predicts the US. Although the signaling pathways involved in the consolidation of IA and other fear-motivated memories have been profusely studied, little is known about the molecular requirements of fear memory extinction. Here we report that, as happens with its consolidation, extinction of IA long-term memory requires activity of the p38 subfamily of mitogen-activated protein kinases (MAPK) in the CA1 region of the dorsal hippocampus. Moreover, we found that inhibition of hippocampal p38MAPK blocked memory reacquisition after extinction without affecting either the increase in IA memory retention induced by a second training session or animal's locomotor/exploratory activity and anxiety state.

Memory consolidation induces N-methyl-D-aspartic acid-receptor- and Ca2+/calmodulin-dependent protein kinase II-dependent modifications in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor properties.

The N-methyl-D-aspartic acid (NMDA) receptor-dependent activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) is necessary for induction of the long-term potentiation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated responses in the CA1 region of the hippocampus, a putative model for learning and memory. We analyzed the interplay among NMDA receptor, CaMKII and AMPA receptor during consolidation of the memory for an inhibitory avoidance learning task in the rat. Bilateral intra-CA1 infusion of the NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid (AP5) or of the CaMKII inhibitor 2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)] amino-N-(4-chlorocinnamyl)-N-methylbenzylamine) (KN-93) immediately after step-down inhibitory avoidance training hindered memory consolidation. Learning of the avoidance response induced the NMDA receptor-dependent translocation of alphaCaMKII to a postsynaptic density-enriched fraction isolated from dorsal CA1 and the autophosphorylation of this kinase at Thr-286. Step-down inhibitory avoidance training increased the quantity of GluR1 and GluR2/3 AMPA receptor subunits and the phosphorylation of GluR1 at Ser-831 but not at Ser-845 in CA1 postsynaptic densities. The intra-CA1 infusion of KN-93 and AP5 blocked the increases in GluR1 and GluR2/3 levels and the phosphorylation of GluR1 brought on by step-down inhibitory avoidance training. Our data suggest that step-down inhibitory avoidance learning promotes the learning-specific and NMDA receptor-dependent activation of CaMKII in the CA1 region of the dorsal hippocampus and that this activation is necessary for phosphorylation and translocation of AMPA receptor to the postsynaptic densities, similarly to what happens during long-term potentiation.

Learning twice is different from learning once and from learning more.

The rat hippocampus plays a crucial role in the consolidation of a variety of memories, including that for a one trial inhibitory avoidance learning task in which stepping down from a platform is associated with a footshock. Here we show that this is the case regardless of the intensity of the footshock used and hence, of the strength of the learned response. However, additional learning produced by a second training session in this task does not involve the hippocampus but, instead, the striatum. Memory consolidation of the second trial requires glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate, N-methyl-D-aspartate and metabotropic receptors, activation of signaling pathways, gene expression and protein synthesis in the striatum, as are required in the hippocampus during memory consolidation of the first trial.

Src kinase activity is required for avoidance memory formation and recall.

Using 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-D]pyrimidine (PP2), a specific inhibitor of the Src family of tyrosine kinases, here we show a direct involvement of these enzymes in memory formation and recall. When infused into the CA1 region of the dorsal hippocampus, immediately or 30 min after training rats in a one-trial inhibitory avoidance task, PP2 but not its inactive analog 4-amino-7-phenylpyrazol[3,4-D]pyrimidine (PP3), blocked short- (STM) and long-term memory (LTM) formation, as tested 2 or 24 h post-training, respectively. PP2 had no effect on STM when given at 60 min post-training or on LTM when administered at 60, 120 or 180 min after the training session, but blocked memory recall when infused into CA1 15 min before a LTM expression test. Hence, activity of the Src family of tyrosine kinases is required in the CA1 region of the rat dorsal hippocampus for the normal formation and retrieval of one-trial inhibitory avoidance memory.

Stress hormones enhance retrieval of fear conditioning acquired either one day or many months before.

It has been known for years that systemic administration of the stress hormones, adrenocorticotrophin (ACTH), lysine-vasopressin, adrenaline, or beta-endorphin, enhances retrieval of aversive behaviours acquired one or a few days before. Here we show that the pre-test i.p. injection of the hormones in rats can also enhance retrieval when given months after the original training. The effectiveness of the treatments changed with time. When animals were tested 3 months after training the hormones enhanced retrieval only at doses five times higher than those needed 1 day after training. Between 6 and 9 months from training the hormones either lost their effect (vasopressin, beta-endorphin) or actually inhibited retrieval (ACTH, adrenaline). The effects of the hormones cannot be explained by a decrease in locomotor activity: none of the treatments had such an effect, as measured in an open field. However, when the animals were tested between 12 and 19 months after training, the hormones once again became as effective as they had been 1 day after training. This was so in spite of the fact that control retention levels became very low with age, probably as a result of extinction. The oscillation of the sensitivity of retrieval to the hormones does not appear to depend on changes in anxiety levels with ageing or to effects of the hormones on locomotor activity.

Bupropion and sertraline enhance retrieval of recent and remote long-term memory in rats.

Wistar rats were trained in step-down inhibitory avoidance at the age of 3 months, and tested for retrieval either 1 day later or 3, 6, 9, 12, 15 or 19 months later, when the animals were 6, 9, 12, 15, 18 or 22 months old, respectively. Bupropion (20 or 60 mg/kg) and sertraline (3.3 or 10 mg/kg) given orally 6 or 3 h before retention testing, respectively, enhanced retrieval of this task at all training-test intervals, despite the fact that retrieval at the longest intervals was practically not seen in control animals. The effect cannot be explained by influences of the drugs on locomotor activity; the treatments had no effect on open field behaviour at the age of 3, 8 or 21 months. The findings may be relevant to the use of these drugs as cognitive enhancers in elderly subjects.

Modulation of working memory and of long- but not short-term memory by cholinergic mechanisms in the basolateral amygdala.

Male Wistar rats were exposed to one-trial step-down inhibitory avoidance training using a 0.5 mA footshock. Through bilaterally implanted indwelling cannulae, they received bilateral 0.5 microl infusions of saline, oxotremorine (0.06 or 0.3 microg) or scopolamine (0.25 or 2.0 microg) into the basolateral complex of the amygdaloid nucleus (BLA). Infusions were either 10 min before training (experiment 1) or 4 min after training (experiment 2). In experiment 1, the animals were tested three times: first for working memory (WM) 2 s after training, then for short-term memory (STM) 1.5 h later, and finally for long-term memory (LTM) 24 h later. Oxotremorine enhanced and scopolamine depressed WM and LTM while leaving STM unaffected. In experiment 2, the treatments were given after WM was presumably over. Again, oxotremorine again enhanced and scopolamine depressed LTM, and neither had any effect on STM. The results fit with the suggestion by Beninger and his co-workers that cholinergic synapses in the BLA regulate WM, generalize that finding to a different task, and show that this mechanism uses muscarinic receptors. In addition, they indicate that the well-known effects of intra-amygdala oxotremorine and scopolamine on LTM are independent of those that the drugs have on WM.

Effects of posttraining treatments in the posterior cingulate cortex on short- and long-term memory for inhibitory avoidance in rats.

Adult male Wistar rats were bilaterally implanted with indwelling cannulae in the caudal region of the posterior cingulate cortex. After recovery, animals were trained in a step-down inhibitory avoidance task (3.0-s, 0.4-mA foot shock) and received, right after training, a 0.5-microl infusion of vehicle (phosphate-buffered saline, pH 7.4), of the GABA(A) receptor agonist muscimol (0.1 or 0.5 microg), of the cAMP-dependent protein kinase (PKA) stimulant Sp-cAMPS (0.1 or 0.5 microg), or of the PKA inhibitor Rp-cAMPS (0.1 or 0.5 microg). Animals were tested twice, 1.5 h and, again, 24 h after training, in order to examine the effects of these agents on short- and long-term memory, respectively. Muscimol (0.5 but not 0.1 microg) hindered retention for both short- and long-term memory (p <.05). Rp-cAMPS (0.1 or 0.5 microg) hindered retention for short-term memory (p <.05). In addition, these animals showed lower, but not significantly lower, latencies than controls in the test session for long-term memory (p >.10). A trend toward an amnesic effect on long-term memory was also observed after Sp-cAMPS infusion at 0.1 microg (p <.10). These results show that strong stimulation of GABAergic synapses in the caudal region of the rat posterior cingulate cortex right after training impairs short- and long-term memory (the latter less dramatically). The same occurs by inhibiting PKA activity with regard to STM and possibly to LTM.

Retrieval of memory for fear-motivated training initiates extinction requiring protein synthesis in the rat hippocampus.

Evidence that protein synthesis inhibitors induce amnesia in a variety of species and learning paradigms indicates that the consolidation of newly acquired information into stable memories requires the synthesis of new proteins. Because extinction of a response also requires acquisition of new information, extinction, like original learning, would be expected to require protein synthesis. The present experiments examined the involvement of protein synthesis in the hippocampus in the extinction of a learned fear-based response known to involve the hippocampus. Rats were trained in a one-trial inhibitory avoidance task in which they received footshock after stepping from a small platform to a grid floor. They were then given daily retention tests without footshock. The inhibitory response (e.g., remaining on the platform) gradually extinguished with repeated testing over several days. Footshock administered in a different context, instead of a retention test, prevented the extinction. Infusions of the protein synthesis inhibitor anisomycin (80 microg) into the CA1 region of the hippocampus (bilaterally) 10 min before inhibitory avoidance training impaired retention on all subsequent tests. Anisomycin infused into the hippocampus immediately after the 1st retention test blocked extinction of the response. Infusions administered before the 1st retention test induced a temporary (i.e., 1 day) reduction in retention performance and blocked subsequent extinction. These findings are consistent with other evidence that anisomycin blocks both the consolidation of original learning and extinction.

Training in the step-down inhibitory avoidance task time-dependently increases cAMP-dependent protein kinase activity in the entorhinal cortex.

The cAMP/cAMP-dependent protein kinase (PKA) signaling pathway has been implicated in synaptic plasticity changes and memory consolidation. Several cortical structures are involved in the consolidation of memory for inhibitory avoidance. The aim of the present work was to observe the effects of training in the inhibitory avoidance task on the levels of PKA activity in the entorhinal, parietal and posterior cingulate cortex (EC, PARIET and PC), and the medial precentral area (Fr2) of the rat, at different post-training times (0, 1.5, 3 and 6h). PKA activity, assayed using [gamma-32P]ATP and kemptide, a selective substrate, increased in the EC 3 h after training, but no changes were observed in PARIET, PC and Fr2. These results suggest that the late phase of memory consolidation of inhibitory avoidance requires a functional PKA signaling pathway in the EC in a way that a 'peak' of PKA activity is observed.

Facilitation and inhibition of retrieval in two aversive tasks in rats by intrahippocampal infusion of agonists of specific glutamate metabotropic receptor subtypes.

RATIONALE: The generic antagonist of glutamate metabotropic receptors (mGlus), MCPG, blocks retrieval of inhibitory avoidance when infused into the CA1 area of rat hippocampus. It was considered important to study the effect of agonists of different types of mGlus on retrieval both of this task and of a related one, contextual fear. OBJECTIVES: To measure the effect of three mGlu agonists (3HPG, which is selective to mGlu1; LCCG, which binds to mGlu2 and mGlu3; and LAP-4, which binds to mGlu4 and mGlu6), infused bilaterally into CA1, on the retrieval of one-trial inhibitory avoidance and contextual fear in rats. METHODS: Rats bilaterally implanted with cannulae in the CA1 region of the dorsal hippocampus were trained in one-trial step-down inhibitory avoidance or in a contextual fear task and tested for retention 24 h later. The drugs 3HPG, LCCG and LAP-4 were infused into CA1 at different concentration levels 10 min before retention testing. In addition, we studied the effect of these drugs on locomotor and exploratory activity measured in an open field, and on pro- and anti-conflict behaviour in an elevated plus-maze. RESULTS: 3HPG hindered, and LCCG and LAP-4 enhanced, retrieval of the two tasks. In all cases the effects were dose-dependent. The drugs had no effects on open field or plus maze behaviour. CONCLUSIONS: Retrieval of one-trial inhibitory avoidance and of contextual fear is regulated by mGlus in the CA1area of the rat hippocampus. The results suggest that mGlu2s, mGlu3s, mGlu4s and mGlu6s are necessary for retrieval and that mGlu1s play an inhibitory role. The effects are not explainable by nonspecific influences on locomotor or exploratory activity or anxiety levels.

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.

Novelty enhances retrieval: molecular mechanisms involved in rat hippocampus.

Rats exposed to a novel environment just prior to or 1-2 h, but not 4 or 6 h, before retention testing exhibited an enhanced retrieval of a one-trial inhibitory avoidance training. The bilateral intrahippocampal infusion of PD098059, an inhibitor of mitogen-activated protein kinase (MAPK), the specific upstream activator of p42 and p44 MAPKs, given 10 min before the exposure to the novel environment, blocked the enhancing effect of novelty on memory retrieval. In addition, prenovelty infusion of DL-2-amino-5-phosphonovalerate (APV), an antagonist of glutamate NMDA receptors, produced similar effects. The exposure to the novel environment is associated with an activation of p42 and p44 MAPKs and an increase in the phosphorylation state of the transcription factor cAMP response element binding protein (CREB). No changes were observed in cAMP-dependent protein kinase (PKA) activity or in alpha-CAMKII activation. Taken together, our results indicate that novelty activates hippocampal MAPKs, which are necessary, along with glutamate NMDA receptors, for the enhancing effect of novelty on retrieval.

Molecular modeling and QSAR analysis of the interaction of flavone derivatives with the benzodiazepine binding site of the GABA(A) receptor complex.

A large number of structurally different classes of ligands, many of them sharing the main characteristics of the benzodiazepine (BDZ) nucleus, are active in the modulation of anxiety, sedation, convulsion, myorelaxation, hypnotic and amnesic states in mammals. These compounds have high affinity for the benzodiazepine binding site (BDZ-bs) of the GABA(A) receptor complex. Since 1989 onwards our laboratories established that some natural flavonoids were ligands for the BDZ-bs which exhibit medium to high affinity in vitro and anxiolytic activity in vivo. Further research resulted in the production of synthetic flavonoid derivatives with increased biochemical and pharmacological activities. The currently accepted receptor/pharmacophore model of the BDZ-bs (Zhang, W.; Koeler, K. F.; Zhang, P.; Cook, J. M. Drug Des. Dev. 1995, 12, 193) accounts for the general requirements that should be met by this receptor for ligand recognition. In this paper we present a model pharmacophore which defines the characteristics for a ligand to be able to interact and bind to a flavone site, in the GABA(A) receptor. closely related to the BDZ-bs. A model of a flavone binding site has already been described (Dekermendjian, K.; Kahnberg, P.; Witt, M. R.; Sterner, O.; Nielsen, M.; Liljerfors, T. J. Med. Chem. 1999, 42, 4343). However, this alternative model is based only on graphic superposition techniques using as template a non-BDZ agonist. In this investigation all the natural and synthetic flavonoids found to be ligands for the BDZ-bs have been compared with the classical BDZ diazepam. A QSAR regression analysis of the parameters that describe the interaction demonstrates the relevance of the electronic effects for the ligand binding, and shows that they are associated with the negatively charged oxygen atom of the carbonyl group of the flavonoids and with the nature of the substituent in position 3'.

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.

Anxiolytic-like behavior in rats is induced by the neonatal intracranial injection of apotransferrin.

To determine whether neonatal intracranial injection of apotransferrin (aTf), which increases myelin deposition, has behavioral effects in rats, 3-day-old rats were intracranially injected with 350 ng of aTf and tested at 25 and 60 days of age. An anxiolytic-like behavior was observed in aTf-treated rats, evidenced by an increase in the exploration of open arms in the plus maze test without changes in the locomotor activity. This behavioral profile persists until adulthood. Intraperitoneal injection of 0.75 mg/kg of picrotoxin, a GABA(A) receptor channel antagonist, abolished this anxiolytic-like behavior, indicating that neonatal aTf induces a long-lasting increase in GABA(A) receptor functionality.

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-æl infusions of saline, of a vehicle (2 percent 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 æg/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