Functional Dysregulations in CA1 Hippocampal Networks of a 3-Hit Mouse Model of Schizophrenia

Abstract

For a better translation from treatment designs of schizophrenia to clinical efficiency, there is a crucial need to refine preclinical animal models. In order to consider the multifactorial nature of the disorder, a new mouse model associating three factors (genetic susceptibility-partial deletion of the MAP6 gene, early-life stress-maternal separation, and pharmacological treatment-chronic Δ-9-tetrahydrocannabinol during adolescence) has recently been described. While this model depicts a schizophrenia-like phenotype, the neurobiological correlates remain unknown. Synaptic transmission and functional plasticity of the CA1 hippocampal region of male and female 3-hit mice were therefore investigated using electrophysiological recordings on the hippocampus slice. While basal excitatory transmission remained unaffected, NMDA receptor (NMDAr)-mediated long-term potentiation (LTP) triggered by theta-burst (TBS) but not by high-frequency (HFS) stimulation was impaired in 3-hit mice. Isolated NMDAr activation was not affected or even increased in female 3-hit mice, revealing a sexual dimorphism. Considering that the regulation of LTP is more prone to inhibitory tone if triggered by TBS than by HFS, the weaker potentiation in 3-hit mice suggests a deficiency of intrinsic GABA regulatory mechanisms. Indeed, NMDAr activation was increased by GABA_A receptor blockade in wild-type but not in 3-hit mice. This electrophysiological study highlights dysregulations of functional properties and plasticity in hippocampal networks of 3-hit mice, one of the mechanisms suspected to contribute to the pathophysiology of schizophrenia. It also shows differences between males and females, supporting the sexual dimorphism observed in the disorder. Combined with the previously reported study, the present data reinforce the face validity of the 3-hit model that will help to consider new therapeutic strategies for psychosis.

Keywords: NMDA receptor; functional plasticity; hippocampus; long term potentiation; psychiatric disorder.

Figure 1

Basal synaptic transmission in CA1 hippocampal area is not impaired in 3-hit mice. (A) Index of synaptic efficacy Ise (fEPSP/PFV ratio) in Wild Type (WT) and 3-hit mice, males (left) and females (right), with increasing stimulus intensity. (B) Paired pulse facilitation (PPF) ratio in WT and 3-hit mice, males (left) and females (right). In inserts, traces recorded are shown: WT (grey line) and 3-hit (black line) male (left) and a female (right) mouse. The number of slices used in each group is indicated in histogram bars. Represented data are Mean ± SEM.

Figure 2

Functional plasticity is altered in 3-hit mice according to stimulation protocols, sex and/or the timing of potentiation. (A) Time-course of high frequency stimulation (HFS)-induced long-term potentiation (LTP) in WT and 3-hit mice, males (left) (WT: n = 13 slices vs. 3-hit group n = 11 slices) and females (right) (WT: n = 13 slices vs. 3-hit: n = 12 slices). (B) Time-course of theta-burst stimulation (TBS)-induced LTP in WT and 3-hit mice, males (left) (WT: n = 26 slices vs. 3-hit group n = 13 slices) and females (right) (WT: n = 35 slices vs. 3-hit: n = 21 slices). In inserts, examples of traces recorded in a WT and a 3-hit mouse before (filled line) and 60 min after (dashed line) the conditioning stimulation. Represented data are Mean ± SEM; non-parametric permutation test (group and time effect), group effect ***: p < 0.001 (last 15 min). ANOVA with permutation tests for STP, group effect ###: p < 0.001 (first 10 min).

Figure 3

(A) NMDAr activation is specifically enhanced in female 3-hit mice. Superimposed traces of long lasting NMDAr-induced fEPSPs recorded in a slice from a male (left) and a female (right), WT (grey line) and a 3-hit (black line) mouse. (B) NMDAr-mediated synaptic efficacy ratio Ise (fEPSP/PFV ratio) in WT and 3-hit male (left) and female (right) mice with increasing stimulus intensity. Number of slices per group are indicated in histogram bars. Represented data are Mean ± SEM; ANOVA or ANOVA with permutation tests, ***: p < 0.001.

Figure 4

Responsiveness of NMDAr activation to bicuculline but not D-serine is altered in 3-hit mice. (A) Index of NMDAr-mediated synaptic efficacy Ise (fEPSP/PFV ratio) determined in slices from WT and 3-hit male (left) and female (right) mice, with increasing stimulus intensity, in control vs. D-serine supplemented artificial cerebrospinal fluid (aCSF) (males: WT: n = 18 slices vs. 3-hit: n = 20 slices; females: WT: n = 12 slices vs. 3-hit: n = 10 slices). (B) Index of NMDAr-mediated synaptic efficacy Ise (fEPSP/PFV ratio) determined in slices from WT and 3-hit male (left) and female (right) mice, with increasing stimulus intensity, in control vs. bicuculline supplemented aCSF (males: WT: n = 11 slices vs. 3-hit: n = 9 slices; females: WT: n = 13 slices vs. 3-hit: n = 12 slices). Represented data are Mean ± SEM; Paired t-tests or Wilcoxon paired t tests, *: p < 0.05; **: p < 0.01; ***: p < 0.001.