Marilene de Souza Zanatta

Sequential Role of Hippocampus and Amygdala, Entorhinal Cortex and Parietal Cortex in Formation and Retrieval of Memory for Inhibitory Avoidance in Rats

The hippocampus and amygdala, the entorhinal cortex and the parietal cortex participate, in that sequence, both in the formation and in the expression of memory for a step‐down inhibitory avoidance task in rats. Bilateral infusion of AP5 or muscimol caused retrograde amnesia when given O min after training into both hippocampus and amygdala, when given or 180 min after training into the entorhinal cortex, or when given 180 min after training into the parietal cortex. Therefore, memory formation requires the sequential and integrated activity of all these areas mediated by glutamate NMDA receptors in each case. Pre‐test administration of CNQX 1 day after training into hippocampus and amygdala, 1 or 31 days after training in entorhinal cortex, or 1, 31 or 60 days after training in the parietal cortex temporarily blocked retention test performance. Therefore, 1 day after training, all these brain structures are necessary for retrieval; 1 month later, the hippocampus and amygdala are no longer necessary for retrieval but the entorhinal and parietal cortex still are; and 60 days after training only the parietal cortex is needed. In all cases the mechanisms of retrieval require intact glutamate AMPA receptors.

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

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

Memory processing by the limbic system: Role of specific neurotransmitter systems

Experiments using localized infusions into selected brain structures of agonists and antagonists of various synaptic receptors, given before or after behavioral training, have led to the following conclusions: (1) Memory is processed shortly after training in the amygdala, medial septum and hippocampus by glutamatergic NMDA and AMPA receptors activated in that sequence. Cholinergic muscarinic receptors are activated concurrently with the former. GABAA receptors modulated by brain benzodiazepines and by beta-noradrenergic receptors inhibit the process. (2) The sequential involvement of NMDA and AMPA receptors suggests that long-term potentiation (LTP) of the synapses activated by the learning experiences in the hippocampus and/or amygdala and medial septum is the crucial event. Expression of this LTP at the time of testing is necessary for retrieval: AMPA receptor blockade in the hippocampus and amygdala at the time of testing hinders retrieval. This suggests that the LTP underlies the memory process itself. (3) The amygdala, medial septum and hippocampus mediate different types of memory and/or different components of memories. The entorhinal cortex, through mechanisms that require intact NMDA receptors and are inhibited by GABAA receptors, intervenes in post-training memory processing 90-180 min after the other limbic regions. The entorhinal cortex integrates consecutively acquired memories; this role could be maintained by the LTP that is generated after training in the amygdala, hippocampus and medial septum. Post-training intervention of the entorhinal cortex does not occur if this region is inhibited at the time of training.

CNQX infused into rat hippocampus or amygdala disrupts the expression of memory of two different tasks

The bilateral infusion of CNQX (0.5 or 1.25 micrograms) into the amygdala or dorsal hippocampus 10 min prior to a retention test partially blocked the expression of stepdown inhibitory avoidance in rats 24 h after training. When infused into both the amygdala and the hippocampus at a dose of 0.5 microgram. CNQX caused a complete blockade of the expression of that task. Retention test performance recovered 2 h after the infusions. In rats trained for habituation to a novel environment and tested 24 h later, pretest intrahippocampal CNQX (0.5 microgram) blocked the expression of retention at a dose of 0.5 microgram, and intra-amygdala CNQX (0.5 or 1.25 micrograms) had no effect. The data suggest that, up to at least 1 day after training, memory of the avoidance task depends on glutamate acting on non-NMDA receptors in both the hippocampus and the amygdala, whereas memory of the habituation task depends on non-NMDA receptor activity in the hippocampus but not the amygdala.

Memory expression is blocked by the infusion of CNQX into the hippocampus and/or the amygdala up to 20 days after training

Bilateral infusion of CNQX (0.5 microgram) into the amygdala and the dorsal hippocampus prior to a retention test blocked the expression of step-down inhibitory avoidance in rats 6, 13, or 20 days after training. Retention test performance recovered 90 min after the infusions. Pretest intrahippocampal CNQX (0.5 microgram) blocked the expression of habituation to a novel environment measured 20 days after training. The data suggest that memory expression depends on non-NMDA receptor-mediated mechanisms, perhaps the expression of LTP, up to at least 20 days after acquisition. These mechanisms operate in the hippocampus in both tasks and in the amygdala in the avoidance task.

Different brain areas are involved in memory expression at different times from training.

Rats were trained in a step-down inhibitory avoidance task and tested for retention 1, 31, or 60 days later. Three to 7 days prior to testing, they were bilaterally implanted with cannulae in the CA1 region of the dorsal hippocampus and in the amygdaloid nucleus (H + A), in the entorhinal cortex (EC), and in the posterior parietal cortex (PPC). Ten minutes prior to testing, the animals received, through the cannulae, 0.5-microliter microinfusions of vehicle (20% dimethylsulfoxide in saline) or of 0.5 microgram of CNQX dissolved in the vehicle. A second test session was carried out 90 min after the first. CNQX blocked retention test performance when given into H + A 1 day after training but not later; when given into EC 1 or 31 days after training, but not later; and when given into PPC 1, 31, or 60 days after training. In all cases performance returned to normal levels in the second test session. The data suggest that H and A are involved in memory expression for only a few days after acquisition; that EC is involved in memory expression for up to 31, but less than 60, days after acquisition; and that PPC is involved in memory expression for up to at least 2 months after acquisition.

Involvement of mechanisms dependent on NMDA receptors, nitric oxide and protein kinase A in the hippocampus but not in the caudate nucleus in memory.

The effects of the NMDA receptor antagonist AP5, the nitric oxide synthase (NO) inhibitor NO-arg or the protein kinase A (PKA) inhibitor KTS720 on memory were evaluated. Rats bilaterally implanted in the CA1 region of the dorsal hippocampus were trained and tested in a step-down inhibitory avoidance task, and rats unilaterally implanted in the left posteroventral region of the caudate nucleus were trained and tested in a cued water maze task. Previous findings from this and other laboratories had found that lesions or pharmacological treatments of these sites significantly altered memory of these two tasks. Immediately after training, animals received intrahippocampal or intracaudate 0.5 µl microinfusions of saline, AP5, NO-arg or KT5720. All three drugs impaired retention of inhibitory avoidance, but did not affect retention of the cued water maze. The findings suggest that NMDA receptor-, NO- and PKA-mediated processes in the dorsal hippocampus, but not in the caudate nucleus, are involved in memory.

Sequential involvement of NMDA receptor-dependent processes in hippocampus, amygdala, entorhinal cortex and parietal cortex in memory processing

Rats bilaterally implanted with cannulae in the CA1 region of the dorsal hippocampus and/or in the amygdaloid nucleus, in the entorhinal cortex, and in the posterior parietal cortex, were trained in a step-down inhibitory avoidance task. At various times after training (immediately, 30, 60 or 90 min) they received, through the cannulae, 0.5 μl microinfusions of saline or of 5.0 μg of APS dissolved in saline. A retention test was carried out 24 h after training. Retention test performance was hindered by AP5 given into hippocampus, amygdala, or both hippocampus and amygdala immediately but not 30 min post-training. The drug was amnestic when given into the entorhinal cortex 30, 60 or 90 min after training, or into the parietal cortex 60 or 90 min after training, but not at earlier times. The findings suggest a sequential entry in operation, in the post-training period, of NMDA-receptor mediated mechanisms involved in memory processing; first in hippocampus and amygdala, 30 min later in entorhinal cortex, and 30 min later in posterior parietal cortex.

Involvement of the hippocampus, amygdala, entorhinal cortex and posterior parietal cortex in memory consolidation

A total of 182 young adult male Wistar rats were bilaterally implanted with cannulae into the CA1 region of the dorsal hippocampus and into the amygdaloid nucleus, the entorhinal cortex, and the posterior parietal cortex. After recovery, the animals were trained in a step-down inhibitory avoidance task. At various times after training (0, 30, 60 or 90 min) the animals received a 0.5-microliter microinfusion of vehicle (saline) or 0.5 microgram of muscimol dissolved in the vehicle. A retention test was carried out 24 h after training. Retention test performance was hindered by muscimol administered into both the hippocampus and amygdala at 0 but not at 30 min posttraining. The drug was amnestic when given into the entorhinal cortex 30, 60 or 90 min after training, or into the parietal cortex 60 or 90 min after training, but not before. These findings suggest a sequential entry in operation, during the posttraining period, of the hippocampus and amygdala, the entorhinal cortex, and the posterior parietal cortex in memory processing.

CNQX infused into entorhinal cortex blocsk memory expression, and AMPA reverses the effect

Abstract Rats were trained in a step-down inhibitory avoidance task using a 0.8 mA foot shock and tested for retention 26 days later. Three to five days prior to the retention test they were bilaterally implanted with cannulae aimed at the etorhinal cortex. Ten minutes before testing they received an infusion, into the entorhinal cortex, of vehicle, ciano-nitro-quinoxaline-dione (CNQX; 0.5 μg), amino-hydroxy-methyl-isoxalone-propionate (AMPA; 1.0 or 2.5 μg), or AMPA (1.0 μg) plus CNQX (0.5 μg). Using blocked memory expression; the effect lasted less than 90 min. AMPA had no effect of its own, but at the lower dose level it counteracted the depressant influence of CNQX. It is not likely that the effect of CNQX could have been due to an influence on performance: In separate sets of experiments the bilateral intraentorhinal infusion of CNQX (0.5 μg) 10 min before training did not affect either acquistion or retention of the avoidance task of general activity during 3 min of free exploration in the training box. The results indicate that the integrity of AMPA receptors in the entorhinal cortex is nessary for memory expression.