Duration of retrograde amnesia induced by tetrodotoxin inactivation of the parabrachial nuclei is inversely related to the intensity of footshock in rat's passive avoidance response.

Previous work has demonstrated that in rats post-trial bilateral functional blockade of the parabrachial nuclei by local injection of tetrodotoxin (10 ng in 1 microliter) partially disrupts retention of an overtrained passive avoidance response. The time-course of functional blockade disrupting effects on passive avoidance response has been studied by altering both the acquisition-tetrodotoxin injection interval and footshock intensity as independent variables. When week footshocks (0.8 mA) were delivered, functional ablation of the parabrachial nuclei was still effective when induced up to 8 days after acquisition training. On the other hand, when rats were shocked with the stronger stimuli (1.2 mA), 2 days after acquisition functional ablation was no longer effective. The results are discussed in terms of unconditioned stimulus (US) intensity and engram consolidation.