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Memories are initially stored in a labile state and are subject to modification by a variety of treatments, including disruption of hippocampal function. We infused a sodium channel blocker (or CNQX) to inactivate the rat dorsal hippocampus reversibly for 1 week following training on a task of spatial memory (the water maze). Previous work with conventional lesions has established that the dorsal hippocampus is essential for both the acquisition and expression of memory in this task. The question in the present study was whether chronic disruption of neuronal activity in the dorsal hippocampus after training would abolish memory or whether memory would survive extended disruption of hippocampal activity. As expected from earlier work, we found that performance was impaired during the infusion period. The critical test occurred 1 week after the lesion was reversed. We found that retention of the water maze recovered to control levels. Accordingly, sustained hippocampal activity following training is not obligatory for either the maintenance of long-term spatial memory or its subsequent retrieval., (Copyright © 2009 Wiley-Liss, Inc.)

Damage to the hippocampus typically produces temporally graded retrograde amnesia, whereby memories acquired recently are impaired more than memories acquired remotely. This phenomenon has been demonstrated repeatedly in a variety of species and tasks. It has also figured prominently in theoretical treatments of memory and hippocampal function. Yet temporally graded retrograde amnesia has not been demonstrated following hippocampal damage in spatial tasks like the water maze. We have assessed recent and remote spatial memory following hippocampal lesions in three different tests of spatial memory: (1) the standard water maze; (2) the Oasis maze, a dry-land version of the water maze; and (3) the annular water maze, where training and testing occur within a circular corridor. Training protocols were developed for each task such that retention of spatial memory could be expressed after very long retention intervals. In addition, retention in each task was assessed with single probe trials so that the assessment of remote memory did not depend on the ability to relearn across multiple trials. The findings were consistent across the three tasks. In the standard water maze (Experiment 1), spatial memory was impaired after training-surgery intervals of 1 day, 8 weeks, or 14 weeks. Similarly, in the Oasis maze (Experiment 2), spatial memory was impaired after training-surgery intervals of 1 day and 9 weeks. Finally, in the annular water maze (Experiment 3), spatial memory was impaired after training-surgery intervals of 9 weeks and 14 weeks. Dorsal hippocampal lesions impaired performance to the same extent as complete lesions. The impairment in remote spatial memory could reflect disruption of previously acquired spatial information. Alternatively, it is possible that in these tasks hippocampal lesions might produce an impairment in performance that prevents the expression of an otherwise intact spatial memory., ((c) 2004 Wiley-Liss, Inc.)

Damage to the hippocampus typically produces temporally graded retrograde amnesia, whereby memories acquired recently are impaired more than memories acquired remotely. This phenomenon has been demonstrated repeatedly in a variety of species and tasks, and it has figured prominently in theoretical treatments of memory and hippocampal function. A striking exception to the finding of temporally graded retrograde amnesia comes from studies with rodents using spatial tasks like the water maze. In these studies, recent and remote memory were similarly impaired following hippocampal lesions. In contrast to work with rodents, studies of patients with medial temporal lobe lesions, including complete hippocampal lesions, indicate that remote spatial memory can be intact. One difference between studies in humans and studies in rodents is that spatial memory in animal studies is acquired during a limited period of time when the animals are adults. In contrast, the spatial memory studied in humans was acquired beginning at an early age and learning continued for a considerable period of time. We initiated training in a standard water maze immediately after rats had been weaned at 21 days of age and continued training until the rats were young adults (90 days old). Large hippocampal lesions were made 100 days after the completion of training. After recovery from surgery, control rats exhibited good retention on the first retention probe trial, but rats with hippocampal lesions performed at chance. Thus, even after extended training beginning early in life, and with a prolonged training-surgery interval, hippocampal lesions impair performance in the water maze task. Possible reasons for these findings are discussed in the context of the specific performance requirements of the water maze task.