Luciana A. Izquierdo

Mechanisms for memory types differ

The formation of long-term memory takes several hours, during which time memories rely on short-term systems,,,. For over 100 years, the main unanswered question of memory research has been whether short-term memory is a necessary step towards long-term memory,, or whether they are separate processes,. Here we report four treatments that block short-term memory while leaving long-term memory intact, showing that these memory systems are separate to some degree.

Separate mechanisms for short- and long-term memory.

It has been assumed for over a century that short-term memory (STM) processes are in charge of cognition while long-term memory (LTM) is being formed, a process that takes hours. A major question is whether STM is merely a step towards LTM, or a separate entity. Recent experiments have shown that many treatments with specific molecular actions given into the hippocampus, entorhinal or parietal cortex immediately after one-trial avoidance training can effectively block STM without affecting LTM formation. This shows that STM and LTM involve separate mechanisms. Some treatments even affect STM and LTM in opposite directions. Others, however, influence both memory types similarly, suggesting links between the two both at the receptor and at the post-receptor level. Drug effects on working memory (WM) were also studied. In some brain regions WM is affected by receptor blockers that alter either STM or LTM; in others it is not. This suggests links between the three memory types at the receptor level. The anterolateral prefrontal cortex is crucial for WM and LTM but is not involved in STM. The hippocampus, entorhinal and parietal cortex are crucial for the three types of memory, in some cases using different receptors for each. The amygdala is not involved in WM or STM, but it plays a key role in the modulation of the early phase of LTM.

Molecular pharmacological dissection of short- and long-term memory.

Abstract1. 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.

Different hippocampal molecular requirements for short- and long-term retrieval of one-trial avoidance learning.

Rats were trained in one-trial step-down inhibitory avoidance and tested either 3 h or 31 days later. Ten minutes prior to the retention test, through indwelling cannulae placed in the CA1 region of the dorsal hippocampus, they received 0.5 microl infusions of: saline, a vehicle (2% dimethylsulfoxide in saline), the glutamate NMDA receptor blocker, aminophosphonopentanoic acid (AP5) (5.0 microg), the AMPA/kainate receptor blocker, cyanonitroquinoxaline dione (CNQX) (0.25 or 1.25 microg), the metabotropic receptor antagonist, methylcarboxyphenylglycine (MCPG) (0.5 or 2.5 microg), the inhibitor of calcium/calmodulin-dependent protein kinase II (KN62) (3.5 microg), the inhibitor of cAMP-dependent protein kinase (PKA), Rp-cAMPs (0.1 or 0.5 microg), the stimulant of the same enzyme, Sp-cAMPs (0.1 or 0.5 microg), or the inhibitor of the mitogen-activated protein kinase (MAPK) kinase, PD098059 (10 or 50 microM). CNQX, KN62 and PD098059 were dissolved in the vehicle; the other drugs were dissolved in saline. All these drugs, at the same doses, had been previously found to affect short- and long-term memory formation of this task. Retrieval measured 3 h after training (short-term memory) was blocked by CNQX and MCPG, and was unaffected by all the other drugs. In contrast, retrieval measured at 31 days was blocked by MCPG, Rp-cAMPs and PD098059, enhanced by Sp-cAMPs, and unaffected by CNQX, AP5 or KN62. The results indicate that, in CA1, glutamate metabotropic receptors are necessary for the retrieval of both short- and long-term memory; AMPA/kainate receptors are necessary for short-term but not long-term memory retrieval, and NMDA receptors are uninvolved in retrieval. Both the PKA and MAPK signalling pathways are required for the retrieval of long-term but not short-term memory.

Molecular signalling pathways in the cerebral cortex are required for retrieval of one-trial avoidance learning in rats

Rats were implanted bilaterally with cannulae in the CA1 region of the dorsal hippocampus, the entorhinal cortex, anterior cingulate cortex, posterior parietal cortex, or the basolateral complex of the amygdala. The animals were trained in one-trial step-down inhibitory avoidance and tested 24 h later. Prior (10 min) to the retention test, through the cannulae, they received 0.5 microl infusions of a vehicle (2% dimethylsulfoxide in saline), or of the following drugs dissolved in the vehicle: the glutamate NMDA receptor blocker, aminophosphonopentanoic acid (AP5, 2.0 or 5.0 microg), the AMPA receptor blocker, 6,7-dinitroquinoxaline-2,3 (1H,4H)dione (DNQX, 0.4 or 1.0 microg), the metabotropic receptor antagonist, methylcarboxyphenylglycine (MCPG, 0.5 or 2.5 microg), the inhibitor of cAMP-dependent protein kinase (PKA), Rp-cAMPs (0.1 or 0.5 microg), the PKA stimulant, Sp-cAMPs (0.5 microg), or the inhibitor of the mitogen-activated protein kinase (MAPK), PD098059 (10 or 50 microM). All these drugs, at the same doses, had been previously found to alter long-term memory formation of this task. Here, retrieval test performance was blocked by DNQX, MCPG, Rp-cAMPs and PD098059 and enhanced by Sp-cAMPs infused into CA1 or the entorhinal cortex. The drugs had similar effects when infused into the parietal or anterior cingulate cortex, except that in these two areas AP5 also blocked retrieval, and in the cingulate cortex DNQX had no effect. Infusions into the basolateral amygdala were ineffective except for DNQX, which hindered retrieval. None of the treatments that affected retrieval had any influence on performance in an open field or in a plus maze; therefore, their effect on retention testing can not be attributed to an influence on locomotion, exploration or anxiety. The results indicate that the four cortical regions studied participate actively in, and are necessary for, retrieval of the one-trial avoidance task. They require metabotropic and/or NMDA glutamate receptors and PKA and MAPK activity. In contrast, the basolateral amygdala appears to participate only through a maintenance of its regular excitatory transmission mediated by glutamate AMPA receptors.

Participation of hippocampal metabotropic glutamate receptors, protein kinase A and mitogen-activated protein kinases in memory retrieval.

The ability to recall past events is a major determinant of survival strategies in all species and is of paramount importance in determining our uniqueness as individuals. In contrast to memory formation, the information about the molecular mechanisms of memory retrieval is surprisingly scarce and fragmentary. Here we show that pretest inhibition of the specific upstream activator of mitogen-activated protein kinase kinase, or of protein kinase A in the hippocampus, blocked retrieval of long-term memory for an inhibitory avoidance task, a hippocampal-dependent learning task. An activator of protein kinase A enhanced retrieval. Mitogen-activated protein kinase activation increased in the hippocampus during retrieval, while protein kinase A activity remained unchanged. Pretest intrahippocampal blockade of metabotropic glutamate receptors or alpha-amino-3-hydroxy-5-methyl-4-isoxazolone propionic acid/kainate receptors, but not N-methyl-D-aspartate receptors or calcium/calmodulin dependent-protein kinase II, impaired retrieval. Thus, recall of inhibitory avoidance activates mitogen-activated protein kinase, which is necessary, along with metabotropic glutamate receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazolone propionic acid/kainate receptors, and protein kinase A, for long-term memory expression. Our results indicate that memory formation and retrieval may share some molecular mechanisms in the hippocampus.

Differential involvement of cortical receptor mechanisms in working, short-term and long-term memory

Rats received, through bilaterally implanted indwelling cannulae, 0.5 µl infusions of 6-cyano-7-nitroquinoxaline2,3-dione (CNQX) (0.5 µg), D-2-amino-5-phophono pentanoic acid (AP5) (5.0 µg), muscimol (0.5 µg), scopolamine (2.0 µg), SCH23390 (2.5 µg), saline or a vehicle into the CA1 region of the hippocampus, or into the antero-lateral prefrontal (PRE), posterior parietal (PP) and entorhinal cortex (EC). The infusions were given 6min prior to one-trial step-down inhibitory avoidance training in order to measure their effect on working memory (WM), or immediately post-training in order to measure their effect on short-term (STM) and long-term memory (LTM), 1.5 and 24 h later, respectively. WM was inhibited by CNQX or muscimol given into any of the cortical areas, by SCH23390 given into CA1, PRE or PP, and by scopolamine given into PRE or EC. STM was unaffected by any of the treatments given into PRE, and was inhibited by CNQX or muscimol given into CA1, PP and EC and by scopolamine given into PP, and enhanced by SCH given into CA1. LTM was inhibited by CNQX, muscimol, scopolamine or SCH23390 given into PRE, by scopolamine given into PP, by SCH23390 given into the entorhinal cortex, and by AP5, CNQX, muscimol or scopolamine given into CA1. The results indicate a differential involvement of the various neurotransmitter systems in the three types of memory in the various brain areas, and a separation of the mechanisms and of the regions involved in each. In addition, some of the findings suggested links between WM and LTM processing in PRE, between WM and STM processing in EC and PP, and between all three types of memory in CA1.

Late and prolonged post-training memory modulation in entorhinal and parietal cortex by drugs acting on the cAMP/protein kinase A signalling pathway.

Rats implanted bilaterally with cannulae in the entorhinal or posterior parietal cortex or in the amygdaloid nucleus were trained in one-trial step-down inhibitory (passive) avoidance using a 0.3 mA footshock. At 0, 3, 6 or 9 h after training, they received localized 0.5 µl infusions into these areas of a vehicle, or of 8-Br-cAMP, forskolin (adenylyl cyclase activator), KT5720 (protein kinase A inhibitor), SKF38393 (dopamine D, receptor agonist), SCH23390 (Dt antagonist), norepinephrine hydrochloride, timolol hydrochloride (βblocker), 8-HO-DPAT (5-HT1A receptor agonist) or NAN-190 (5-HT1A antagonist) dissolved in 20% dimethylsulfoxide (DMSO) in saline (vehicle). Rats were tested for retention 24 h after training. 8-Br-cAMP, forskolin, SKF 38393 and norepinephrine caused memory facilitation and KT5720, SCH23390, timolol and 8-HO-DPAT caused retrograde amnesia when given into the entorhinal cortex 0,3 or 6 h but not 9 h after training. When given into the posterior parietal cortex 0, 3 or 6 but not 9 h after training, KT5720 was amnestic. When given into this structure 3 or 6 h but not 0 or 9 h after training 8-Br-cAMP, forskolin and norepinephrine caused memory facilitation and KT5720, SCH23390 and timolol caused retrograde amnesia. All treatments given into the amygdala 0,3 or 6 h after training were ineffective except for norepinephrine given at 0 h, which caused facilitation. The data point to a role of cAMP/protein kinase A-dependent mechanisms in memory formation in the entorhinal and parietal cortex, but not the amygdala, from 0 to 6 h after training, and to a strong modulation of these mechanisms by dopaminergic D1, β-noradrenergic and 5-HT1A receptors. The lack of effect of NAN-190 but not 8-HO-DPAT in both cortical regions suggests that 5-HT1A receptors do not play a physiological role but can be activated pharmacologically. The fact that SCH23390 was amnestic but SKF38393 had no effect when given into the parietal cortex suggests that D1 receptors may play a maintenance rather than a stimulant role in this area.

Melanin-concentrating hormone (MCH) modifies memory retention in rats.

The purpose of the present study was to evaluate the possible effect of melanin-concentrating hormone (MCH) on learning and memory by using the one-trial step-down inhibitory avoidance test in rats. The peptide was infused into hippocampus, amygdala, and entorhinal cortex. MCH caused retrograde facilitation when given at 0 or 4 h post-training into hippocampus, but only at 0 h into amygdala. From these results, it seems that MCH modulates memory early after training by acting on both the amygdala and hippocampus and, 4 h after training, on the hippocampus.

Molecular Mechanisms of Memory Retrieval

Memory retrieval is a fundamental component or stage of memory processing. In fact, retrieval is the only possible measure of memory. The ability to recall past events is a major determinant of survival strategies in all species and is of paramount importance in determining our uniqueness as individuals. Most biological studies of memory using brain lesion and/or gene manipulation techniques cannot distinguish between effects on the molecular mechanisms of the encoding or consolidation of memories and those responsible for their retrieval from storage. Here we examine recent findings indicating the major molecular steps involved in memory retrieval in selected brain regions of the mammalian brain. Together the findings strongly suggest that memory formation and retrieval may share some molecular mechanisms in the hippocampus and that retrieval initiates extinction requiring activation of several signaling cascades and protein synthesis.