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 961 min) they received, through the cannulae, 0.5 mu l microinfusions of saline or of 5.0 mu g of AP5 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.