Deletion of neuronal gap junction protein connexin 36 impairs hippocampal LTP

Abstract

In the mammalian CNS, deletion of neuronal gap junction protein, connexin 36 (Cx36), causes deficiencies in learning and memory. Here we tested whether Cx36 deletion affects the hippocampal long-term potentiation (LTP), which is considered as a cellular model of learning and memory mechanisms. We report that in acute slices of the hippocampal CA1 area, LTP is reduced in Cx36 knockout mice as compared to wild-type mice. Western blot analysis of NMDA receptor subunits indicates a higher NR2A/NR2B ratio in Cx36 knockout mice, indicating that there is shift in the threshold for LTP induction in knockout animals. Data suggest a possibility that learning and memory deficiencies in Cx36 knockout mice are due to deficiencies in LTP mechanisms.

Fig.1

LTP does not occur in the hippocampal CA1 region of Cx36 knockout mice. Averaged traces of normalized (relative to baseline) fEPSP slopes (a) and statistical analysis (b) are shown. In a, the time of HFS is indicated by a vertical arrow. In b, the pre-HFS (baseline) data are from 10-min-long background recordings obtained immediately before HFS; the post-HFS data are from recordings obtained between 50 and 60 min after HFS. In both panels, data are shown as mean ± SE; n = 5-6. Statistical analysis: one-way ordinary ANOVA with post hoc Tukey; ***P<0.001; **P<0.01; NS, non-significant. Insert in a: fEPSPs from pre-HFS recordings (gray) and from recordings obtained between 50 and 60 min post-HFS (black); left panel, WT; right panel, Cx36 knockout (Cx36 KO); vertical bar, 0.2 mV; horizontal bar, 1 ms.

Fig.2

The NR2A/NR2B ratio is higher in the hippocampus of Cx36 knockout mice than in WT mice. Data from western blot experiments are shown. a,b, Representative image (a) and statistical analysis (b) of the expression of NR2A and NR2B receptor subunits are shown. (c) Graph presents the analysis of the NR2A/NR2B ratio. Statistical analysis: paired (b) and unpaired (c) Students t-test; n = 6-7; mean ± SE. Optical density signals are normalized relative to tubulin. The westerns were done sequentially on one membrane.