Time-dependent disruption of passive avoidance acquisition by post-training intra-amygdala injection of tetrodotoxin in rats.

The role of the amygdala (AM) in the consolidation of the passive avoidance reaction (PAR) has been examined with the tetrodotoxin (TTX)-induced functional block of this structure. Rats were trained in the step-through PAR and anesthetized with ketamine. TTX injected immediately after acquisition into both AM (2 x 10 ng) significantly reduced avoidance of the dark compartment in the retrieval test performed two days later. The amnesic effect was significant when the acquisition--TTX delay was prolonged to 90 min but not to 6 h or 24 h. Earlier research indicated that TTX blockade of the parabrachial nuclei caused similar PAR disruption when elicited 24 h but not 48 h after acquisition. Comparison of the TTX-induced retrograde amnesias indicates that the two structures play different roles in the formation of the PAR engram.