Intense aversive training protects memory from the amnestic effects of hippocampal inactivation

There is extensive evidence that amnestic treatments are less effective, or ineffective when administered to subjects that have been overtrained or subjected to high foot-shock intensities in aversively motivated learning. This protective effect has been found with a variety of learning tasks and with treatments that disrupt activity in several regions of the brain, including the hippocampus, amygdala, striatum, and substantia nigra. Such findings have been interpreted as suggesting that the brain regions disrupted are not critical sites for the memory processes induced by these types of training. In most experiments investigating this issue the amnestic treatments were administered after training. Thus, it might be less amnesia was induced because the training accelerated memory consolidation and, thus, the maximum effect of the amnestic treatment occurred after memory of the learning experience was consolidated. This study investigated this issue by inactivating the hippocampus of rats bilaterally with tetrodotoxin (TTX) (10 ng/side) 30 min before one-trial inhibitory avoidance training using relatively low (1.0 mA), medium (2.0 mA), or high (3.0 mA) foot-shock intensities. Retention of the task was measured 48 h after training. TTX produced a profound retention deficit, a mild deficit, and no deficit at all in the 1.0, 2.0, and 3.0 mA groups, respectively. These data confirm the protective effect of training with relatively high foot-shock intensity against experimentally induced amnesia, and suggests that this protection is not due to accelerated consolidation. Rather, the findings suggest that strong training activates brain systems other than those typically involved in mediating memory consolidation.