Kv4.2 knockout mice have hippocampal-dependent learning and memory deficits

Abstract

Kv4.2 channels contribute to the transient, outward K(+) current (A-type current) in hippocampal dendrites, and modulation of this current substantially alters dendritic excitability. Using Kv4.2 knockout (KO) mice, we examined the role of Kv4.2 in hippocampal-dependent learning and memory. We found that Kv4.2 KO mice showed a deficit in the learning phase of the Morris water maze (MWM) and significant impairment in the probe trial compared with wild type (WT). Kv4.2 KO mice also demonstrated a specific deficit in contextual learning in the fear-conditioning test, without impairment in the conditioned stimulus or new context condition. Kv4.2 KO mice had normal activity, anxiety levels, and prepulse inhibition compared with WT mice. A compensatory increase in tonic inhibition has been previously described in hippocampal slice recordings from Kv4.2 KO mice. In an attempt to decipher whether increased tonic inhibition contributed to the learning and memory deficits in Kv4.2 KO mice, we administered picrotoxin to block GABA(A) receptors (GABA(A)R), and thereby tonic inhibition. This manipulation had no effect on behavior in the WT or KO mice. Furthermore, total protein levels of the α5 or δ GABA(A)R subunits, which contribute to tonic inhibition, were unchanged in hippocampus. Overall, our findings add to the growing body of evidence, suggesting an important role for Kv4.2 channels in hippocampal-dependent learning and memory.

Figure 1.

Kv4.2 knockout (KO) mice showed impairment in spatial learning and memory. Wild-type (WT) and Kv4.2 KO mice were examined for differences in acquisition and retention of spatial memory in the Morris water maze. The mice were trained over eight blocks to find the hidden platform. (A) Kv4.2 KO mice showed impairment in the ability to learn to find the hidden platform. After eight blocks of learning were completed, the animals were tested for spatial memory retention by the probe trial. The Kv4.2 KO mice spent less time in the target quadrant compared with the WT mice (B) and had decreased crossings in the region that previously housed the hidden platform (target quadrant) (C). Data are shown as mean ± standard error of the mean. (*) P < 0.05; (**) P < 0.01; n = 15 (WT); n = 16 (KO).

Figure 2.

Kv4.2 knockout mice showed impairment in contextual learning in the fear-conditioning paradigm. Kv4.2 KO mice and wild-type (WT) mice were examined for differences in learning and memory through fear conditioning. (A) The Kv4.2 KO mice had decreased freezing in the contextual learning condition compared with the WT mice. This change was specific to contextual learning, since they did not show an alteration in freezing for the CS-only condition or in the new context condition (B). These data suggest that Kv4.2 mice have a specific impairment in hippocampal-dependent memory without a change in amygdala-dependent memory. Data are shown as mean ± standard error of the mean. (***) P < 0.001; n = 18 (WT), n = 15 (KO).

Figure 3.

Kv4.2 knockout mice did not show a change in anxiety compared with wild-type mice. WT and Kv4.2 KO mice were tested for differences in anxiety using the elevated-plus maze for 10 min. The amount of time spent in the closed arms (A) and open arms (B) of the elevated-plus test were not significantly different between WT and Kv4.2 KO mice. Data are shown as mean ± standard error of the mean. P > 0.05; n = 19 (WT), n = 14 (KO).

Figure 4.

Kv4.2 knockout mice showed normal acoustic startle and sensorimotor gating. The Kv4.2 KO and wild-type mice were examined for differences in acoustic startle and sensorimotor gating through prepulse inhibition of acoustic startle. (A) The maximum startle response amplitude to the acoustic startle stimulus was not different between the groups. (B) The Kv4.2 KO mice did not show any alterations in acoustic stimulus to varying levels of startle. Data are shown as mean ± standard error of the mean. P > 0.05; n = 24 (WT), n = 15 (KO).

Figure 5.

α5 and δ GABA_A receptor subunit levels were unchanged, and pharmacological suppression of tonic inhibition does not reverse the spatial learning and memory deficits in Kv4.2 knockout mice. The protein levels of α5 or δ GABA_AR subunits were assessed in the KO and wild-type mouse hippocampus. The hippocampus was removed from Kv4.2 KO mice and WT mice and total homogenates and crude synaptosomes were prepared. (A,B, top) Representative blots from these samples were probed with antibodies against total α5 and δ GABA_AR. The protein levels were densitized for α5 GABA_AR subunit and δ GABA_AR subunit. There was no significant change between the WT and Kv4.2 KO mice. P > 0.05; n = 7 (WT). WT and Kv4.2 KO mice were injected (intraperitoneally) with saline or 0.01 mg/kg of picrotoxin and then tested in the Morris water maze (MWM) for differences in acquisition and retention of spatial memory. The mice were injected for 3 d prior to training and throughout testing. (C) Kv4.2 KO mice show impairment in the ability to learn to find the hidden platform. However, administration of picrotoxin did not improve performance in Kv4.2 KO mice and had no effect on behavior in the WT mice. After the blocks of learning were completed, the animals were tested for spatial memory retention using the probe trial. The Kv4.2 KO mice spent less time in the target quadrant compared with the WT mice (D), but the picrotoxin-treated animals did not show any improvement compared with the saline-treated WT mice. Overall, the noncompetitive GABA_AR antagonist, picrotoxin, had no effect on the behavior of the WT or KO mice in the MWM. Data are shown as mean ± standard error of the mean. (*) P < 0.05; (**) P < 0.01; n = 7 (WT + sal), n = 8 (WT + picrotoxin), n = 6 (KO + saline), n = 9 (KO + picrotoxin).