Amygdala modulation of hippocampal-dependent and caudate nucleus-dependent memory processes.

These experiments investigated the effects, on memory, of injections of d-amphetamine (10 micrograms/0.5 microliter) administered into the amygdala, hippocampus, or caudate nucleus immediately after training in cued or spatial water-maze tasks. In experiment 1, rats received an eight-trial training session on one of the two tasks followed by injections of d-amphetamine or saline. Retention was tested 24 hr later. On the spatial task, intrahippocampal, but not intracaudate, injections of d-amphetamine facilitated retention. In contrast, on the cued task intracaudate, but not intrahippocampal, injections of d-amphetamine facilitated retention. Posttraining intraamygdala injections of d-amphetamine enhanced retention of both tasks. In experiment 2, lidocaine (2% solution; 1.0 microliter) injected intraamygdally prior to the retention test did not block the memory enhancement induced by posttraining intraamygdala injections of d-amphetamine. The findings (i) provide further evidence of a dissociation between the roles of the hippocampus and caudate nucleus in different forms of memory, (ii) indicate that the modulatory role of the amygdala is not limited to either of the two different forms of memory represented in spatial and cued discriminations in a water maze, and (iii) are consistent with previous findings indicating that amygdala influences on memory storage are not mediated by lasting neural changes located within the amygdala.