Dissociation of multiple memory systems by posttraining intracerebral injections of glutamate
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The role of glutamatergic transmission in the memory functions of the hippocampus and the caudate-putamen was examined using posttraining intracerebral injections of glutamate. Rats received an eight-trial (30-sec intertrial interval) training session on a hidden or visible platform task in a water maze. In the hidden platform task, a submerged escape platform was located in the same quadrant of the maze on all trials. In the visible platform task, a visible escape platform was located in a different quadrant of the maze on each trial. Following Trial 8 in either task, rats received a posttraining intrahippocampal or intracaudate injection of glutamate or saline. On a two-trial retention test session 24 h later, latency to mount the escape platform was used as a measure of memory. In the hidden platform task, the retention test escape latencies of rats receiving intrahippocampal injection of glutamate (2 g) were significantly lower than those of vehicle-treated rats, indicating an enhancement of memory. Intracaudate injection (2, 5, or 10 g) of glutamate had no effect on memory in the hidden platform task. In contrast, intracaudate injection of glutamate (5 g) enhanced memory in the visible platform task, whereas intrahippocampal injection (1, 2, or 5 g) had no effect. In both tasks and brain structures, intracerebral injections of glutamate had no effect on memory when delayed 2 h posttraining, indicating a time-dependent effect of glutamate on memory storage processes. The findings indicate a double dissociation of the roles of the hippocampus and the caudate-putamen in memory and suggest that glutamate modulates the distinct forms of memory mediated by these two brain structures.