Contrasting the Amnesic Effects of Temporary Inactivation with Lesions of the Hippocampus on Context Memory
2016
Lesions and temporary inactivation of the hippocampus (HPC) in rodents
occasionally lead to discrepant amnesic effects. We directly compared and
contrasted the retrograde amnesic effects that small HPC lesions (~50% damage),
large HPC lesions (~80% damage), and combined dorsal and ventral HPC inactivation
using the sodium channel blocker tetrodotoxin (TTX) had on contextual fear
conditioning. Compared to control rats, large HPC lesions significantly reduced
freezing during retention testing, a behaviour consistent with retrograde
amnesia. In contrast, neither the small lesions nor the TTX inactivation
significantly reduced freezing. The extent of damage was significantly and
negatively correlated with retention performance (r(9) = -0.896, p < 0.001), suggesting that 70% or more of the HPC needed to be
damaged to observe deficits. Importantly, TTX inactivation disrupted spatial
memory in the Morris Water Task, confirming that our inactivation procedure did
impair one form of HPC-dependent memory. To assess the extent of the TTX inactivation,
immediate early gene expression was quantified in the HPC following the Morris
Water Task. However, despite the behavioural impairment, we did not find a
significant reduction in expression. We conclude that temporary inactivation of
the HPC may fail to impair context fear memory because this technique does not
sufficiently disrupt the HPC.
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