Rearing-environment-dependent hippocampal local field potential differences in wildtype and inositol trisphosphate receptor type 2 knockout mice

Abstract

Rearing in an enriched environment (ENR) is known to enhance cognitive and memory abilities in rodents, whereas social isolation (ISO) induces depression-like behaviour. The hippocampus has been documented to undergo morphological and functional changes depending on these rearing environments. For instance, rearing condition during juvenility alters CA1 stratum radiatum gamma oscillation power in rats. In this study, hippocampal CA1 local field potentials (LFP) were recorded from bilateral CA1 in urethane-anesthetized mice that were reared in either ENR or ISO condition. Similar to previous findings in rats, gamma oscillation power during theta states was higher in ENR group. Ripple events that occur during non-theta periods in the CA1 stratum pyramidale also had longer intervals in ISO mice. Since astrocytic Ca²⁺ elevations play a key role in synaptic plasticity, we next tested if these changes in LFP are also expressed in IP₃R2-knockout (KO) mice, in which astrocytic Ca²⁺ elevations are largely diminished. We found that the gamma power was also higher in IP₃R2-KO-ENR mice compared to IP₃R2-KO-ISO mice, suggesting that the rearing-environment-dependent gamma power alteration does not necessarily require the astrocytic IP₃/Ca²⁺ pathway. By contrast, ripple events showed genotype-dependent changes as well as rearing condition-dependent changes: ISO housing and IP₃R2 deficiency both lead to longer inter-ripple intervals. Moreover, we found that ripple magnitude in the right CA1 tended to be smaller in IP₃R2-KO. Since IP₃R2-KO mice have been reported to have depression phenotypes, our results suggest that ripple events and animals' mood may be broadly correlated.

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