Acutely increasing δGABA(A) receptor activity impairs memory and inhibits synaptic plasticity in the hippocampus

Abstract

Extrasynaptic γ-aminobutyric acid type A (GABA(A)) receptors that contain the δ subunit (δGABA(A) receptors) are expressed in several brain regions including the dentate gyrus (DG) and CA1 subfields of the hippocampus. Drugs that increase δGABA(A) receptor activity have been proposed as treatments for a variety of disorders including insomnia, epilepsy and chronic pain. Also, long-term pretreatment with the δGABA(A) receptor-preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) enhances discrimination memory and increases neurogenesis in the DG. Despite the potential therapeutic benefits of such treatments, the effects of acutely increasing δGABA(A) receptor activity on memory behaviors remain unknown. Here, we studied the effects of THIP (4 mg/kg, i.p.) on memory performance in wild-type (WT) and δGABA(A) receptor null mutant (Gabrd(-/-)) mice. Additionally, the effects of THIP on long-term potentiation (LTP), a molecular correlate of memory, were studied within the DG and CA1 subfields of the hippocampus using electrophysiological recordings of field potentials in hippocampal slices. The results showed that THIP impaired performance in the Morris water maze, contextual fear conditioning and object recognition tasks in WT mice but not Gabrd(-/-) mice. Furthermore, THIP inhibited LTP in hippocampal slices from WT but not Gabrd(-/-) mice, an effect that was blocked by GABA(A) receptor antagonist bicuculline. Thus, acutely increasing δGABA(A) receptor activity impairs memory behaviors and inhibits synaptic plasticity. These results have important implications for the development of therapies aimed at increasing δGABA(A) receptor activity.

Keywords: CA1; THIP; dentate gyrus; extrasynaptic GABAA receptors; long-term potentiation; memory; tonic inhibition; δ subunit.

Figure 1

THIP impairs spatial memory in the Morris water maze. (A) THIP increased the escape latencies on the third and fourth trials in WT but not Gabrd^−/− mice. (B) THIP decreased the preference for the goal quadrant formerly containing the escape platform in WT but not Gabrd^−/− mice. (C) THIP did not affect swim speed in WT or Gabrd^−/− mice. n = 16–19, ^*p < 0.05.

Figure 2

THIP impairs contextual but not auditory-cued fear memory. (A) THIP decreased the freezing score following the third tone-shock pairing in WT but not Gabrd^−/− mice. (B) THIP reduced the freezing score for contextual fear memory in WT but not Gabrd^−/− mice. (C) THIP did not affect the freezing score for auditory-cued fear memory in response to tone in both WT and Gabrd^−/− mice. n = 25–30, ^*p < 0.05.

Figure 3

THIP impairs novel object recognition. (A) Schematic diagram showing the protocol. (B) THIP decreased the preference for the novel object in WT but not Gabrd^−/− mice. (C) THIP had no effect on total interaction time in both WT and Gabrd^−/− mice. n = 12–21, ^*p < 0.05.

Figure 4

The effects of THIP, and the tonic inhibitory current revealed by BIC, in DG granule cells from WT and Gabrd^−/− mice. (A) Representative recordings show the effects of THIP, and the tonic current revealed by BIC (20 μM). (B) Quantified data. n = 6–7, ^*p < 0.05.

Figure 5

THIP inhibits long-term potentiation in the dentate gyrus. (A,B) THIP depressed LTP in the DG in slices from WT but not Gabrd^−/− mice. Upper panels: Representative traces before and after tetanic stimulation. Middle panels: Normalized slope of fPSPs following tetanic stimulation. Bottom panels: Summarized data showing the last 5 min of recording. Note that THIP depressed LTP in DG only in WT mice. n = 8–13, ^*p < 0.05.

Figure 6

BIC occludes the inhibitory effects of THIP on long-term potentiation in the dentate gyrus. (A,B) THIP does not impair LTP in the DG in BIC-treated slices. BIC (100 μM) was perfused throughout the recordings. Upper panels: Representative traces before and after tetanic stimulation. Middle panels: Normalized slope of fPSPs following tetanic stimulation. Bottom panels: Summarized data showing the last 5 min of recording. Note that THIP did not depress LTP in the DG in both WT and Gabrd^−/− mice. n = 9–10.

Figure 7

SR-95531 does not prevent THIP-mediated depression of long-term potentiation in the dentate gyrus. (A,B) THIP impairs LTP in the DG of SR-95531-treated slices from WT but not Gabrd^−/− mice. SR-95531 (1 μM) was perfused throughout the recordings. Upper panels: Representative traces before and after tetanic stimulation. Middle panels: Normalized slope of fPSPs following tetanic stimulation. Bottom panels: Summarized data showing the last 5 min of recording. n = 10–12. ^*p < 0.05.

Figure 8

THIP has no effects on baseline synaptic transmission or presynaptic function in slices from both WT and Gabrd^−/− mice. (A) Representative traces show fPSPs with increasing stimulus intensities. The input-output relationships beneath the traces, indicators of baseline synaptic transmission, were similar between genotypes, and between control and THIP treatment groups in either genotype. n = 9–10. (B) Sample traces show paired-pulse depression. Paired pulse ratios, indicators of presynaptic function, were also similar between genotypes, and between control and THIP treatment groups in either genotype. n = 9–10.

Figure 9

THIP inhibits long-term potentiation in the CA1 region. (A,B) THIP depressed LTP in CA1 in slices from WT but not Gabrd^−/− mice. Upper panels: Representative traces before and after tetanic stimulation. Middle panels: Normalized slope of fPSPs following tetanic stimulation. Bottom panels: Summarized data showing the last 5 min of recording. Note that THIP depressed LTP in CA1 only in WT mice. n = 8–9, ^*p < 0.05.