The small conductance Ca2+-activated K+ channel activator GW542573X impairs hippocampal memory in C57BL/6J mice

Abstract

Small conductance Ca²⁺-activated K⁺ (SK) channels, expressed throughout the CNS, are comprised of SK1, SK2 and SK3 subunits, assembled as homotetrameric or heterotetrameric proteins. SK channels expressed somatically modulate the excitability of neurons by mediating the medium component of the afterhyperpolarization. Synaptic SK channels shape excitatory postsynaptic potentials and synaptic plasticity. Such SK-mediated effects on neuronal excitability and activity-dependent synaptic strength likely underlie the modulatory influence of SK channels on memory encoding. Converging evidence indicates that several forms of long-term memory are facilitated by administration of the SK channel blocker, apamin, and impaired by administration of the pan-SK channel activator, 1-EBIO, or by overexpression of the SK2 subunit. The selective knockdown of dendritic SK2 subunits facilitates memory to a similar extent as that observed after systemic apamin. SK1 subunits co-assemble with SK2; yet the functional significance of SK1 has not been clearly defined. Here, we examined the effects of GW542573X, a drug that activates SK1 containing SK channels, as well as SK2/3, on several forms of long-term memory in male C57BL/6J mice. Our results indicate that pre-training, but not post-training, systemic GW542573X impaired object memory and fear memory in mice tested 24 h after training. Pre-training direct bilateral infusion of GW542573X into the CA1 of hippocampus impaired object memory encoding. These data suggest that systemic GW542573X impairs long-term memory. These results add to growing evidence that SK2 subunit-, and SK1 subunit-, containing SK channels can regulate behaviorally triggered synaptic plasticity necessary for encoding hippocampal-dependent memory.

Keywords: Contextual memory; Fear memory; K(Ca)(2+); Learning; Object memory; Rodent.

Fig. 1.. Influence of Systemic GW542573X on Object Memory Processes.

(A) Pre-sample session administration: mice given 4% DMSO vehicle (n = 18), GW542573X 7.5 mg/kg (n = 8) or GW542573X 15 mg/kg (n = 14) acquired the sample object exploration criterion in a similar latency. (B) During the test session GW542573X mice explored the novel object significantly less than did the vehicle mice, F(2,37) = 12.67, **P < 0.001; discrimination ratio scores of 7.5 mg/kg and 15 mg/kg treated mice were significantly lower than that of vehicle mice. Inset: Test session total object exploration was also significantly lower in the 7.5 mg/kg mice, overall analysis: F(2, 37) = 10.73; **P < 0.001. (C) Post-sample administration: all mice acquired the sample object exploration criterion in a similar latency and then received vehicle (n = 9), or 15 mg/kg GW542573X (n = 8). (D) All mice displayed equivalent test session discrimination. Inset: GW542573-treated mice exhibited significantly less object exploration compared to vehicle mice, t(15) = 2.21, *P < 0.05. (E) Pre-test administration: all mice, regardless of future treatment, acquired the sample object exploration criterion in a similar latency. (F) During the test session, vehicle mice (n = 10) and 15 mg/kg GW542573X (n = 14) displayed equivalent test session discrimination. Inset: All mice spent comparable time exploring the objects. Vertical red arrow in plots A, D and F indicate time of drug administration.

Fig. 2.. Influence of Local Infusion of GW542573X into CA1 of Dorsal Hippocampus on Encoding of Object Memory.

(A) Histological analyses confirmed the location of bilateral infusion sites within the CA1 region of dorsal hippocampus. Left: representative photomicrographs depicting the infusion site within the CA1 of one hemisphere. CA1 = CA1 layer; DG = dentate gyrus; white arrowheads indicate the location of the respective infusion track. Right: black dots on stereotaxic atlas plates indicate placement of the bilateral infusion sites within the CA1 region of the dorsal hippocampus. Numbers to the right of the atlas plates indicate the distance from bregma. (B) Mice that received bilateral vehicle (n = 8) or 0.15 μg/μl/side GW542573X (n = 8) pre-sample, acquired the sample object exploration criterion in a similar latency. Red arrow indicates when the bilateral local infusions were administered. (C) During the test session, GW542573X mice exhibited significantly less preference for exploring the novel object compared to vehicle mice, as inferred from the discrimination ratio, t(14) = 8.73, **P < 0.001. Inset: All mice exhibited equivalent total object exploration.

Fig. 3.. Influence of Systemic GW542573X on Encoding and Consolidation of Trace Fear Memory.

(A) Trace conditioning session: mice received pre-conditioning vehicle (n = 15), 7.5 mg/kg GW542573X (n = 6), or 15 mg/kg GW542573X (n = 14). Red arrow indicates when the systemic injection was administered. All mice acquired the trace conditioned freezing response equivalently. (B) During the trace fear memory test, all mice exhibited comparable levels of % freezing during the presentation of each non-reinforced CS and during the 15-s interval after each non-reinforced CS.

Fig. 4.. Influence of Systemic GW542573X on Contextual and Cued Fear Memory.

(A) Mice exhibited comparable exploration during context pre-exposure, t(18) = 1.44; n.s. (B) During conditioning, mice given vehicle (n = 10) or 15 mg/kg GW542573X (n = 10) acquired the conditioned freezing response equivalently. Red arrow indicates when the systemic injection was administered. (C) During the contextual fear memory test, GW542573X mice exhibited significantly less total % freezing compared to vehicle mice, t(18) = 2.50, *P < 0.05. (D) During the cued fear memory test all mice exhibited comparable levels of tone-elicited freezing.